Using tube amp with electrostatic speakers.

Normally you would use a higher tap. This is assuming the amplifier uses loop negative feedback, which most do.

The feedback will help the amp to boost its output into the lower impedance. Since there is almost nothing for power requirements, it should be able to do this easily. So its really whether you use the 4 or 8 ohm tap and I would start with the 8 ohm tap.

Ralph-  I've been using a MacIntosh MC275 with a set of custom Quad 2805's-  the chinese panels are not well made and I had Kent McCollum rebuild using UK-produced 63 panels, then add all his upgrades. Measurements on the Quad 63 & 2805 show 4 ohm as the low impedance. Measurements on the MC275 show good linearity and very low distortion (for a tube amp) when using a tap where the load equals or is higher than the tap utilized. While I originally ran the Quads at 8 ohms, I switched to the 4 ohm tap and it is very nice. I crossover at the appropriate point for the Quads and supplement the bottom with a Velodyne SW. The PA is a one-off SS design which is very clean and has almost nothing in the signal path- made by Ron Sutherland for me. Does the 4 ohm tap in this case make sense to you?

BTW, I have an unusual collection of ELS's including the original ML CLS pair. I don't mean one of the production models, I mean the original prototypes. Gayle Sanders voiced these to match a set of contemporary STAX ELS headphones as closely as possible. They really are spectacular but a difficult load to say the least. The production models were tamed down and never sounded as good at the original prototypes. Back in the day, I drove them with a Yamaha B-1, a rare beast that could handle very difficult loads and was quite an amp in the late 70's-  all vertical Static Induction Transistors in the output stages.  Speaking of Stax's, I also have a pair of 8x's- another superb and very rare electrostatic. 

Gary Osoba

I realize this is an old thread but it has a lot of information related to the issues of using a tube amp with electrostatic speakers.  I have a question related to the information provided.  It seems the issue is the impedance variation from high frequency hf (low impedance) to low frequency lf (high impedance) of an electrostatic speaker.  SS amps output a voltage for a given input so they will emphasize the hf vs the lf at a given power output because of the impedance variation of the electrostat over the audio spectrum.  From what I gather tube amps deliver constant power over the spectrum but you can be limited at low impedance (if the amp can't drive the load) or at high impedance if the output voltage does not swing high enough.  Please let me know if my understanding is incorrect.
Should you always use the lowest tap on your amplifier (e.g. 2, 4, 8 ohms) corresponding to the lowest impedance of the electrostat?  What is the trade off at low frequency high impedance using the 2 ohm tap.  If you went with the autoformer and it gives you the ability to drive the 2 ohm load what is the tradeoff at low frequency high impedance (e.g. driving 30 or 60 ohms).  Any insight would be appreciated. 

Best Regards,
Gary

It may seem that I am trying to avoid your actual question, but that is not the case, really.  First, most transformer-coupled tube amplifiers provide a choice of 4, 8, or 16 ohm taps, not 2, 4 and 8 ohms.  Second, the very low impedance at very high frequencies exhibited by an ESL is not so much of a problem for the amplifier, because very little energy is required at those high frequencies.  (For example, I made impedance vs frequency curves for my Sound Lab ESLs after eliminating the crossover components; impedance does not drop off significantly until 5kHz and goes to 2 ohms only above 10kHz. But every ESL will be different from every other one, in this respect.) I suggest that you try each pair of output taps for yourself and see what you prefer.  There is no right or wrong answer to your question, except to say that most amplifier energy will be required in the bass and midrange frequencies.

Nordicnorm, M-L candidly admits that they design their speakers to favor SS amplifiers.  (See also my post of 11-02-16.) This means that there is circuitry at the input that dramatically lowers the impedance seen by the amplifier, well below the native impedance of the speaker itself.  Thus your finding is not surprising. This is why I sold my last pair of M-L ESLs about 20 years ago and never looked back.  No way I would give up my OTL tube monoblocks just to suit one particular speaker.

lewm,

Thanks for the response.  I have an old set of Soundlab speakers (A1 and B1S) that I have been working on and upgrading.  I am currently driving the system with SS amps (Innersound ESL 800) but would like to try a couple of tube amps I have laying around (both are 60W).  One of the tube amps does have a 2 ohm tap (it is a Margules U280C).  I would use one amp to drive the A1s and the other to drive the B1S (with a crossover).  I have the luxury of optimizing the frequency band for HF and LF by using the B1S as my bass speaker.  Thus my question about using the 2 ohm tap for the A1s if bandwidth limited.  Would there be any benefit driving the B1S with a tube amp at the high impedance or would this work equally well with a SS amp.  Once again, thanks I really appreciate your input.

Also, since you seem to know Soundlabs, would there be an optimal frequency band for using the A1s from an impedance standpoint (e.g. minimize the wild impedance swings that the amp sees).

Regards,
Gary

I am fairly sure that the A1s would look like my 845PX with respect to impedance, which is "very high" up to 100Hz (50 to 100 ohms), then around 4-5 ohms up to around 1kHz to 2kHz where there is a sharp notch dip to 2 ohms or so, then back up to more reasonable values, then back down at 5kHz and above, to 2 ohms or less at 20kHz.  This is from memory.  (I could REALLY hear an improvement in vocals when I got rid of that 2-ohm dip; see below.)

You can probably assume, if you don't want to or cannot measure, that the B1S will present a fairly high impedance to the amplifier, and in that respect it should be an easy load for any tube amp, but the question would remain how much amplifier power you need to make it sing.  You might ask the folks at Sound Lab.  

One option you didn't mention but which works quite well is to bi-amplify your A1s.  I've never done it, but you can experiment with that idea.  See if you can find the old "Sound Lab Owners Group" archives for some hints.

And finally, you might do what I did with my 845PXs: (1) Acquire a full-range ESL step-up transformer with a turns ratio of 1:75 up to 1:90.  Plitron makes a nice one, toroidal at 1:75, but there are several others. I used a EI type transformer made in Australia that is unfortunately no longer available.  (2) Remove the existing treble transformer in your A1s and also remove the RC network that effects a high pass filter feeding the treble transformer. That RC network is the problem; it creates the low impedance dip right around the crossover point, because of the value of the R that SL chose to use. Also, the R (typically 5 to 8 ohms in value depending on how old your speakers are) soaks up amplifier power, because it is in parallel with the panel at all frequencies. The intrinsic impedance of the panel is mostly way higher than 5 to 8 ohms, so most amplifier energy flows through that R.
(3) Install the full-range transformer in place of the existing treble transformer AND the RC network, so there is no longer an RC at all.  You can retain the inductor that feeds the bass transformer.  Make sure the two transformers are "in phase".  And then your A1s can be driven by your 60W tube amplifier all day long. The resulting impedance is going to be 20-30 ohms from about 100Hz to 5kHz, going down from there but fairly constant between 100 Hz and 2kHz. And the efficiency is ridiculously improved.  I doubt my speakers draw much more than 20W, steady state.

Or finally, talk to Roger West about upgrading your treble transformer to his latest device, which is more nearly full range. That allows for a lower crossover point and also gets rid of that midrange dip in impedance, according to them. With that option you keep the crossover (but change the values according to his instructions). Plus, this mod is factory sanctioned, unlike what I did.

Does the 4 ohm tap in this case make sense to you?

@garyosoba Yes.

Lew, I've heard the new Sound Lab backpanels a number of times and its pretty obvious that Dr. West got it right with this one. Its much easier to drive (we can do with an MA-1 what took the MA-2s before) and sounds better too.

Ralph,
Your information about the new Sound Lab treble transformer ("back panels") is consistent with what I have heard too, from other Sound Lab owners.  They all seem to like it, and they all sense the improvement in sound compared to the old set-up. Even the one or two guys who first tried "my" approach and who were not satisfied, are now happier with the new toroidal treble transformer.  No doubt that is the simplest thing to do, if one owns an older set of panels.  

Note to others that getting rid of the crossover entirely was not originally my idea; it was communicated to me by "Will" in Australia, via SLOG.  Will also helped me and others to obtain the Australian-made full-range 1:90 step-up that he used too to modify his 945PXs. I bought the last pair of the original run of those transformers, but the vendor had more of them made due to the demand from among SL owners. There may have been something odd or different about that second batch of production units, because I know of at least two guys who were not happy with them. We'll never know why, with certainty.  This is why I would recommend the Plitron 1:75 full range toroid, not the Australian transformer, to anyone who wants to try the mod I describe above.  I personally have never thought twice about adopting the new SL toroids to replace what I've got.  Anyone in the DC area is welcome to stop by my home for a listen.  

To be more clear, the Plitron toroid is a full-range step-up that one might choose to install if one wants to go with no treble crossover at all.  SL sells a toroid that requires a high pass filter in front of it; it's not designed to run full range.  But with the SL toroid, the hi-pass crossover point is down around 200Hz, which does dramatically improve the impedance curve compared to original and the efficiency of the speaker.

Lewm,

Yes, I did ask many questions with Dr. West who was more than gracious with his time and help.  He told me that a 300W SS amp was more than enough to drive the B1s.  I am currently using a Crown K2 (350W @ 8ohms) on the B1s.

I am trying to absorb and understand your modification.  My speakers did not work initially so I had new mylar installed by Soundlab on the A1s and the B1s.  My panels had the Toroid l in it.  A few years ago, I found someone selling upgrade parts that they did not install in their system which I purchased.  The upgrade parts were a large 36uf capacitor (pio) and the Toroid ll transformer (the latest version).  I consulted Dr. West who confirmed the parts.  I removed the brilliance control and replaced it with a four ohm resistor (which was about the setting I had it at).  Dr. West told me I could remove the inductor coupled to ground in the circuit but I have a 12 or 14 ohm low inductance resistor to ground.  I believe the resistor was larger than what was in the circuit but Dr. West said that the Toroid ll can extend lower in frequency and that it would be fine.

A couple things mentioned by Dr. West.  He said the B1S can be used as a full range speaker which I found interesting.  He said that when they used the B1S with the A1 panels they would crossover at 500hz.  Running the A1 at 500hz would essentially be operating the A1 with only the toroid ll.  I tried it but I couldn't get it to integrate correctly (I will likely try it again in the future). It sounded better at an 80hz or less crossover.

Once again, thank you for your knowledge and help.  It sounds that I can drive the A1 with the tube amp with the 2 or 4 ohm tap.  I question whether I will have the voltage swing with a 60w amp for the midbass if the impedance gets high from 100hz to 1khz.

Best Regards,
Gary

One other thing.  Have you ever tried the zero transformer?  It sounds like it would help especially with low power amps that could have trouble driving the difficult load.  The only thing that could be at issue is the lower frequencies that get multiplied up to 40-200 ohms.  I don't know if this would be an issue for some amps.

Gary, Mea culpa.  I forgot that not only were there several different combinations of R and C used by SL over the years, but also at one point they did introduce an inductance in the hi-pass filter, which creates a 12db/octave or second-order slope.  I think that's the inductor you talk about.  I briefly experimented with that part (after purchasing a pair from SL), long before I made the major modification I described in my recent post.  For reasons I no longer can recall, but likely because it sounded bad, I ditched the inductor in the hi-pass filter and reverted to the simpler RC filter, which gives a 6db/octave slope.  I also forgot about the "brilliance" control, which is anything but brilliant.  Ditching the brilliance control was one of the first things I ever did to improve my speakers; I replaced it with nothing, no resistor even.

One must keep in mind that Dr. West is an engineer first and foremost.  Thus it seems he is more concerned with measurements than with how certain modifications affect perceived sound quality.  This means he does not like impedance peaks within the audio band, for example, but he seems less allergic to impedance dips.  At the same time, I regard him as a very fine person in all respects and someone who knows much more than I do about the technical aspects, of course. (Goes without saying, actually.) He communicated with me and Will, when we posted our findings with the AU transformer on SLOG.  Subsequently, he had the open-mindedness to replace the toroid he was using for treble with one that has a lower low end response, in recognition of what we were trying to "fix" in the first place.

For one example of his EE emphasis, Dr. West pointed out to me that my "solution", using two transformers in parallel where one is full range, results in a sharp impedance peak at 250 Hz. (I think he found this by computer modeling, not by actually measuring one of his speakers driven by the Australian 1:90 transformer that I use, because he never purchased one.)  When I made my measurements, I took impedance readings at 200 Hz and 500 Hz , thus completely missing the impedance peak.  (I used a log progression: 20, 50, 200, 500, etc) But the peak (assuming it is there in the real world) does absolutely no harm to the capacity of the Atma-sphere OTL to drive the speaker; there is certainly no discontinuity perceivable.

Just out of curiosity, what value of R does SL use with the latest toroid, if they use 36uF of capacitance?

Lewm,

Congrats to you and Will on the work you have done in bringing forth a significant improvement to the Soundlab speakers.  I can attest that the transformer upgrade was a significant improvement over the prior toroid l transformer.  It is not very often that a user pioneers improvement that results in commercial changes.  Kudos!

Yes the inductor was in parallel with the resistor.  I only have the resistor now (inductor removed) per Dr. West (another thing you probably had your hand in).  I do not know what they are using now for a resistor value but I believe it is the same (36uf and 10 ohms (4-2.5ohm potted resistors)).  As mentioned, Dr. West stated that the resistor value could be increased with the Toroid ll but not by a significant amount (I have 12 or 14 ohms).

What are the consequences of completely removing the brilliance control that is in series with the transformer?  Does it make the speaker much brighter?  Does it have some effect on the transformer at high frequencies or is it reliability issue? 

I would also like your final thoughts on the topic of the thread which is using tube amps with electrostats and more specifically Soundlab electrostats.   The Soundlabs obviously have wild impedance swings from HF to LF.  From the thread I assume that a tube amp should be selected to be able to handle the HF low impedance (with the zero transformer being a nice option to aid the tube amp).  Are there any issues you know of on why a tube amp cannot drive a high impedance load such as 30-200 ohms?

Once again, thank you for all your help.  Your knowledge has aided me tremendously.  I really like the fact that even old Soundlab speakers (like mine) can be upgraded and made to sound great.

Regards,
Gary

4 of those 2.5-ohm "coffin"-type resistors?  They are probably wired in series/parallel, but do you know the net resistance of the bundle?  Could be 10 ohms or could be 2.5 ohms.  Neither value is gonna make your OTL any happier than mine was before I chucked the crossover. (Really, give all credit to Will; all I did was follow his lead because I was so unhappy with the sound I was getting from my 845PXs.)  You are definitely better off with 12-14 ohms than 10 ohms.  However, I am a little surprised at the values; they are identical to what I was using with my "old" toroid (10 ohms/36uF), before the big modification (removing the crossover entirely).  The -3db point of the high pass filter is inversely related to the product of R times C, where C is in Farads and R in ohms.  For a 10-ohm resistance with 36uF, I calculate a crossover point of 440 Hz.  I would have thought they could use a much lower crossover point with the new toroid, but I bear in mind that Dr. West is very conservative in his design so as to favor reliability.  From my experience making measurements, the intrinsic impedance of the speaker is much higher than 10 ohms at all frequencies up to somewhere between 2kHz and 5kHz. At 5kHz, it's about 8 ohms for my 845PXs.  (Bear in mind, measurements were made with NO crossover, driving the SL factory bass transformer and the full-range Australian transformer in parallel with each other, directly with an audio frequency generator.) So, as you can imagine, when you interpose the RC network, at low and mid frequencies, much more of the energy is going through the resistor than through the speaker (R is smaller in value than speaker Z at those frequencies, so R dominates). This may be why some guys on SLOG discovered long ago that very high wattage resistors seemed to sound better.  There is almost no limit to the wattage ratings they tried, more than 100W, even.  Before my epiphany, I was using an enormous 10-ohm resistor hand-made by one of the British contributors to SLOG.  It could tolerate at least 500W. (One benefit of bi-amplification is that only the treble amplifier then "see"s that R in parallel with its output; the bass amplifier drives the bass transformer only through the inductor.  Thus paradoxically, your SS amplifier might work best on the treble (because it can handle the power demand and the low impedance created by the R, with your tube amp on the bass).

If you go up to 20 ohms, the crossover point would be ~220 Hz. (The beauty of the equation is that once you know the values for one set of parameters, you can calculate in your head what will happen if you change R or C.  If 10 ohms and 36uF give you 440Hz, then 20 ohms and 36uF give you 220Hz, etc.)  I would have thought that the new toroid would be quite happy at around 300 Hz, not too far from where you are at if you are using 14 ohms now.

There is no difference between bypassing or removing the brilliance control and setting it to zero. But I preferred to remove it; it's a cheap part that muddies up the signal path. You will hear an improvement. No, the speaker is not too bright without it, in my opinion.  The easiest thing to do is to re-solder the input and output wires to the brilliance control together on one binding post of the control.

As far as impedance variations of the speaker per se, the ESL can be thought of as a giant capacitor.  It has very high impedance at very low frequencies and then gradually falls off in a fairly linear manner to very low impedance at very high frequencies.  I wrote here earlier that in my opinion, very low impedance at 10kHz or 20kHz is not a real problem, because the energy requirements are very low at those frequencies.  What makes for problems is the circuitry that comes before the panel, e.g., that 2-ohm impedance dip at about 1kHz that came with the old toroid due to its crossover.  Nearly every company does something to make ESLs more friendly to SS amplifiers, which in turn makes it hard on tube amplifiers. 

Lewm,

     The resistors are in series (10 ohms).  That is what I had in my circuit when I changed to the Toroid ll.  I do not know what they are using in the new Toroid ll backplate circuit.  I did exactly the same thing (you described) when I replaced the resistors. I have large ceramic non-inductive wire wound resistors (hi wattage).  I will definitely look into increasing the resistance to around 20 ohms.  I am not sure what transformer saturation is but that is what concerned Dr. West when increasing the resistor value (and lowering the croosover point for the Toroid).
     From your suggestion I might just stick with my SS amps.  They have ample power (Innersound 800 ESL monoblocks (800W@8 ohms)).  My journey to this thread was based on understanding if a lower power tube amp could be used on a speaker such as Soundlabs.  I believe I understand the tradeoffs you have outlined.
    As usual, thanks for all the information, it is much appreciated.  I try completely removing the brilliance control from the circuit.  I will also try lowering the crossover point of the Toroid ll.  I will then try crossing over to my B1S somewhere between 350hz to 80hz and see what works best for me (when I get the time of course).  Merry Christmas and Happy Holidays Lewm.  I have learned a lot.

Regards
Gary

My wonderful Quad ESLs(57) on 16ohm taps of rebuilt 60's EICO HF35 monos work great. 

Yep. Quad 57s, KLH 9s, early Acoustats, Janszen tweeters.  They were all made with tube amplifiers in mind. For a real treat you might look into the Quad mods made by Dave Slagle, for driving them with tubes. Fantastic results.

Gary, Most of what I have done is to make the speakers sound optimal with my OTL tube amplifiers, which I genuinely believe are superior, and I did once borrow a pair of Innersound amplifiers for comparison. Since you already own a tube amplifier as well as your Innersounds, you might want to try bi-amplification, with the tube amps running the bass transformer and the Innersounds running the treble.  You would have to make allowances for balancing the output, by attenuating either the Innersounds or the tube amp, depending upon what you hear.

On the other hand, with the 20 ohm resistance and the same 36uF capacitance, you may like your tube amplifier, full range, best.

Lewm

I will definitely try both of those configurations.  On my list is to remove the resistor I used to replace the brilliance control and increase the resistance.  Thank you for all the help.

Best Regards,
Gary

i have always used krell ss amps to drive m-logans....and a nice tube preamp such as audible illusions and ARC.....

very sweet combo imho

@lewm Thanks. Looked up Slagle mods. $36k. 
https://www.stereophile.com/content/capital-audio-fest-herbs-saturday-highlights#4scCzpwA1Ah3Rpyq.97

Thanks, Noromance.  I had no idea the cost would be so high.  In the photo (from 2015) they show a single pair of Quads. This year (and I think last year) EMIA ran a double pair of Quads. Believe me, with Dave's mods, there is no lack of bass response in the range where music lives.  The  speakers on demo the last three years of the show actually belong to a local aficionado who is a long time friend and customer of Dave's.  At that gentleman's house, he runs THREE pair of Quad 57s. I need to go hear that.  What's money, after all?

  From your suggestion I might just stick with my SS amps. They have ample power (Innersound 800 ESL monoblocks (800W@8 ohms)). My journey to this thread was based on understanding if a lower power tube amp could be used on a speaker such as Soundlabs. I believe I understand the tradeoffs you have outlined.

On a set of Sound Labs, a 200 watt tube amp can do what an 800 watt solid state can do. 

I would like to mention that my Linear Tube Audio ZOTL40 (David Berning designed) is absolute magic with my Janszen electrostats. If you can live with 40 watts or get them mono blocked for 80, I can’t recommend them highly enough. My Janszen zA2.1s are 87 dB efficient and I still have plenty of headroom.

ESLs on paper have low efficiency, but that is because the standard measurement is made at 1 meter.

If the ESL is a large panel, a good deal of the sound might go past the microphone without being picked up. So in reality many ESLs (and planars in general) are much more efficient than their specs suggest.

Hi Lewm,

I'm also interested in the transformer mod - I'm using first gen VTL MB450s for my M1 panels, so I'm interested in the transformer mod for the same reasons you mention in the thread of increased efficiency and of course improved sound. I would really like to hear everything my system can offer without over taxing my amps, with flexibility to try OTL or lower powered tube amps in the future. I sold my 160w joule electra amps because they were struggling with the panels.

I hadn't realized that plitron offered a suitable step up - do you have any tips to install them into the backplates? Im looking forward  to doing this in the new year.

Happy Holidays,

Andy 

@jazzdude99  Andy, If you want to get an idea of what the Sound Labs can do with the new backplate, there is a resistor that can be removed from the old toroidal backplate that gets you about 90% of the way there.

The resistor is usually composed of a bunch in series/parallel to get the wattage up- the result is a resistor with a 200 watt rating. Think about how much amplifier power is being absorbed to need to be rated that high to survive!

Anyway, in the older units that resistor can be removed and an instant improvement is heard with any amp and the speaker is easier to drive. Our customers feedback is that they can do with our MA-1 (140 watts) what used to take a set of MA-2s (220 watts) before. Its more than just an efficiency improvement, its also a load that is a lot easier for any amp. 

That's brilliant - thank you Ralph! I'll try this weekend.

Regards,

Andy

Ralph, Are you referring to the resistor "R" in the RC network?  I totally agree that R is a big problem in the original design, but if you remove it entirely, that would make a dramatic change in the crossover point.  For the example given above where 10 ohms and 36uF values of R and C give you a X-over point of ~440 Hz, removing the R entirely would give you a high pass -3db point at ~4400 Hz.  This would leave a big gaping hole in the midrange, because the bass transformer does not work much above 2kHz, based on my earlier measurements of my speaker.  I am sure you are aware of all this, which is why I think I might be misunderstanding the nature of your recommendation.  You could remove R and preserve the x-over point, if you add more C, in this example.  I would suggest increasing the value of R so that less current flows through it and the crossover point is thereby lowered to a frequency more in the bass range.  This needs to be done with caution because of the limitations of the toroid itself, which we don't really know for sure (because SL is a bit secretive in this regard).  But I bet you could go to 20 ohms, giving a hi-pass at 220 Hz (assuming 36uF of capacitance).  That's actually what I thought SL was doing with their new toroid, lowering the hi-pass X-over point.

Jazzdude, Plitron do (or did) make a "full-range" ESL step-up toroid with a 1:75 ratio, and in theory this would work.  However, bear in mind that I cannot personally say I have tried it.  There are also other companies that make EI type transformers with a step-up in the needed range (1:75 up to 1:90).  The Plitron looks to be pretty robust, and it would be my choice, if I was starting from scratch, is all I meant to say.  If you go to some other brand and to an EI type, just be sure to pick a "big" substantial transformer that is rated for full-range use.  Some of the aftermarket ESL transformers look like toys and would not be up to the job.

So, if you try the Plitron, I wondered whether it could mount on the very same bracket used to mount the existing toroid.  If the diameter is too large for it to fit in that bracket, probably the bracket can be modified to accommodate a larger diameter toroid.  This is what I was thinking.

Thanks for the info Lewm!

Andy

To summarize what Lewm said above (that he and Will have tried in their speakers) :
1. You can remove the brilliance control (or short the terminals) which is in series with the Toroid transformer (I removed the brilliance control and put in discrete resistors but will remove it entirely per Lewm's recommendation).
2. You can increase the shunt resistor value (mine had 10 ohms 4 - 2.5 ohm resistors in series) to lower the frequency of the pole (with the 36uf capacitor) from 440hz to a lower value.
3. Dr. West said it would not be an issue to increase the value from 10 to about 12-14 ohms with the toroid ll (I'm sure this is very conservative) to move the frequency pole lower.  Lewm suggested 20 ohms.  From my recollection, the concern that Dr. West had about increasing the shunt resistance was saturating the toroid transformer.  The toroid ll transformer offered by Soundlab is much larger than the toroid l.  I believe Dr. West said that it can handle an increased resistance without saturating.
4. You can use a better quality shunt resistor and it may add some minor benefits.

Regards,
Gary

Gary, That's a pretty accurate summary.  The notion to remove or bypass the brilliance control is actually older than the ideas to modify the backplate that came from Will and me.  

I remain puzzled that Sound Lab would upgrade to a larger toroid with presumably a better low frequency response and then continue to use the same values of R and C, thereby maintaining the same old crossover point (at round 440Hz for the values 10 ohms and 36uF).  But I don't actually recall what values SL used as OEM standard with the "old" original toroid, so it could be that the old values were actually something less than 10 ohms with something less than 36uF capacitance, which together or separately would give a higher crossover point.  Therefore, the values of 10 and 36 are "new" to Sound Lab.  As I now recall, the old values might have been 6-8 ohms (depending upon year of manufacture) and maybe 32 to 34uF.

Lewm,

For some reason, I did not ask the question of what resistor value is currently being used in the updated back plates with the Toroid ll when I discussed my upgrade.  Dr. West did confirm what you disclosed that the Toroid ll will not saturate as early as the Toroid l so you can increase the resistor value (I just don't know what they are using now, maybe someone else will chime in).

I have always wondered what the hot rod upgrade is for the Soundlab backplate.  Do you know what this upgrade is?

Atma-sphere - I agree that I am limited in power that can be delivered in the bass region of the speaker due to the high impedance of the speaker and the voltage that can be delivered.  Dr. West told me that the wattage claims for Soundlab speakers were to give people an idea of what kind of amplifier to buy (but he really mean't the wattage to infer what the power supply rails would be to adequately provide bass).

Regards,
Gary

I always loved the large Sound Labs speakers so when they announced their new 545 series I thought great! Got them in and after trying them with several different amps over 30 days (including my Atmasphere Novacrons) they just didn't have the magic that the larger Sound Labs  panels had. And yes I tried them with Zero Autoformers still without much success. To bad because they were made beautifully, got them in Birdseye maple. 

(Dealer disclaimer) 

Just to be clear-er, the Brilliance control is an "inexpensive" L-pad that in my opinion and that of many others colors the sound quite audibly.  Removing or bypassing it has absolutely no effect on the crossover point.  If one were to find that the treble is too bright with no Brilliance control in the circuit, one could add a high quality, high wattage, fixed value resistor in place of the L-pad.  The way to do that might be to set the speaker so you like the tonal balance with the L-pad in place.  Then measure the resistance across the L-pad.  Then replace the L-pad with that much resistance.  In my case, for my room, my ears, and my modifications, I am quite happy with no resistor at all.  If anything, I'd like to boost the treble a tiny bit, ideally.

For the example given above where 10 ohms and 36uF values of R and C give you a X-over point of ~440 Hz, removing the R entirely would give you a high pass -3db point at ~4400 Hz.  This would leave a big gaping hole in the midrange, because the bass transformer does not work much above 2kHz, based on my earlier measurements of my speaker.  I am sure you are aware of all this, which is why I think I might be misunderstanding the nature of your recommendation.  You could remove R and preserve the x-over point, if you add more C, in this example.

The issue here is that the old toroid was not set up right. As you know, the crossover is a bit of a moving target because the impedance of the transformer is on a slope with respect to frequency (increasing as frequency goes down, which means that the existing capacitor in the crossover is actually allowing it to go lower than 500Hz- the resistor was there to sort of 'enforce' that there was a set crossover frequency). In practice (rather than theory) removing the resistor on the older original toroidal backplates (which were replaced by the toroidal 2 about 4 years ago) works out quite well. The big concern Sound Lab had was low frequency saturation of the toroid, but in practice its actual impedance was so far off that this isn't a problem with most amps. We have a number of customers that have done this and been very happy. No-one has reported a gap in the midrange because its not there.

I agree the fact that you or your customer(s) have tried it, and it sounds good, would trump my purely mathematical analysis.  And you are correct in noting that the formula I used does not take into account the third variable, which is the intrinsic change in speaker impedance vs frequency (although my off the cuff thought on that is that the actual -3db point with no resistor would be boosted above the calculated 4400Hz by a greater degree than is the calculated vs the actual -3db point with the 10 ohm resistor, because speaker Z is going down as frequency goes up, and the -3db point is inversely related to total Z).  

Something in my memory is telling me that both Will and I did try removing the resistor from the RC network in front of the old toroid, before proceeding on to Will's ultimate solution.  And the sound did improve when we did that, although not to the degree that we heard/hear with the replacement of the treble-only transformer with a full-range one and no R OR C in the circuit.  Anyway, I am happy that everyone else is happier, no matter how they got there.

@lewm 

Yes, the tricky bit is that the transformer impedance increases as frequency goes down. So with a given capacitance with increasing impedance, the crossover point moves lower- there is no set crossover point! This is why the resistor is there, so that the crossover point is not a moving target- the resistor dominates the formula of the -3db point. Removing it theoretically would open the transformer up to saturation caused by too much bass, but in practice I've not heard of that happening.

At any rate Dr. West seems to have it sorted with the newer Toroidal 2 version, and thanks goes to you for pointing out the problem with his simulation from years earlier.

atmasphere

Thank you for this explanation as it gets to the heart of the matter.  I just want to make sure I understand this correctly.  What occurs to the sound of the speaker when the transformer saturates or begins to saturate?  Is transformer saturation a voltage or current issue?

Are you saying:
1. That the resistor is not needed for either the Toroid l or Toroid ll?
2. That the concern of transformer saturation does not occur (at higher input signals) because:
     a) at higher frequencies the music power requirements are typically not high (so the Toroid will not/does not saturate); and    
     b) the Toroid impedance increases at lower frequencies to a level that the Toroid cannot saturate even with a higher voltage signal (in other words the Toroid by it's impedance nature is self-limiting to prevent saturation?).

Regards,
Gary

What occurs to the sound of the speaker when the transformer saturates or begins to saturate? Is transformer saturation a voltage or current issue?

Distortion- Its pretty obvious. Its a power issue- both current and voltage (of which neither can exist without the other).

Are you saying:
1. That the resistor is not needed for either the Toroid l or Toroid ll?

Not needed for the Toroid 1, needed for Toroid II

2. That the concern of transformer saturation does not occur (at higher input signals) because:
    a) at higher frequencies the music power requirements are typically not high (so the Toroid will not/does not saturate); and    
    b) the Toroid impedance increases at lower frequencies to a level that the Toroid cannot saturate even with a higher voltage signal (in other words the Toroid by it's impedance nature is self-limiting to prevent saturation?).

We're talking here mainly about the Toroid 1:

Bass energy can saturate the core of the part, because it was built with intention to optimize higher frequency performance. In practice it does not saturate with low frequency energy simply because its impedance is so high at lower frequencies (and there is a capacitor in series with its input) that most amps can't make any power at those impedances.

The Toroid II:

The crossover of the resistor and capacitor prevent lows from entering the input of the transformer; otherwise it too would saturate because its core is too small just like the Toroid 1.

Atmasphere,

Thank you very much for that explanation.

Best Regards,
Gary

HELP?

In your opinions, would a pair of Balanced Audio Technology (BAT) Rex II mono blocks be powerful enough to make the Martin Logan Neoliths sing?

I'm currently using a pair of Krell 575 mono blocks, which is powerful enough, but I'd like a warmer sound with more harmonics. I mostly listen to old standard jazz LPs from 50's to 70's (at night club sound level) and smooth jazz CDs (at concert levels).

BAT: 

 

Neolith: 

 

Dear  @milt808  : " a warmer sound with more harmonics. "

 

First the harmonics are developed by the recording source we are listening it and those harmonics have changes through all the signal path where that signal must travels in our room/system till goes in our ears/brain/body. A good room treatment and fine tunning the subwoofers SPLs can help for that warmer sound  you are looking for that again depends on the signal source. The issue is not about tube amplifier because your speakers impedance at a critical 20khz frequency is down to 0.43 ohms and I don't know yet a tube amp that can handled with applomb that speaker impedance.

In my opinion this is the amp for your system and I think you can't go wrong with:

 

https://parasound.com/jc1+.php

 

Regards and enjoy the MUSIC NOT DISTORTIONS,

R.

Unless Martin Logan  says otherwise, I would assume that they have not abandoned their stated policy of designing their speakers to accommodate solid-state amplification best of all. That really means the speakers are likely to have a low nominal impedance. You might check this out with Martin Logan. If I am correct, then you may be best off with a solid-state amplifier, but there certainly is no harm in trying those BAT amplifiers, if you have them on hand. If Raul is correct about the impedance at 20 kHz, that squeaky low impedance could be vexatious for any amplifier, solid-state or tube type. However, the saving grace is that very little amplifier power is needed at such very high frequencies.

I see now in the advert that the nominal impedance is 4 ohms, going down to 0.43 ohms at 20kHz, per Raul's statement too.  That's not a great situation for any sort of tube amplifier, but it could be made to work I guess.  Using Autoformers from Anti-Cables would help.

That's not a great situation for any sort of tube amplifier, but it could be made to work I guess.  Using Autoformers from Anti-Cables would help.

I can confirm that is the case. Once the impedance issue is corrected, the MLs are really fairly efficient; 100 Watts is plenty of power.