Preamp and impedance question

So are you sunshine, so are you, in more ways than one.

Yes it was

Ha ha, that's pretty funny.

atmasphere
I think its pretty obvious this isn’t about EPDR...

Yes it was, as this is what I answered to below, saying a 4ohm transformer would be a very good idea as that speaker "5ohm is a nominal figure" you can bet it goes down to 4ohm or even lower, and when combined with any -phase angle (epdr) could be even lower again.

gte357s 2) the transformer is designed for 8 ohm. If I want it to support 4 ohm, I need to upgrade the transformer.

The OP needs a 4ohm tap if not 8ohm also end of story!!!!!!!!!
That MQ300 Lux had one, you two are dreaming to think he only needs an 8ohm tap with that amp he’s getting built, and will have better resale value with both taps.

@roberttdid  I think its pretty obvious this isn't about EPDR...

George,

Everyone reading this thread can see that I and atmasphere talk in very specific terms and in detail when we talk about EPDR. You are not fooling anyone but perhaps yourself.

Since you are such an expert on EPDR, tell everyone, specifically, what was inaccurate about atmasphere's post about tube amplifiers and EPDR. Here, I am reposting the relevant section:

Tubes are a bit different from transistors in a number of ways as we all know :) One of the ways they are different is the Safe Operating Area (SOA). With tubes you can exceed the SOA without damage to the tube if the tube is allowed to cool off afterwards. A tube dramatically overtaxed, such as in a loss of bias, can turn cherry red from heat, but if allowed to cool off, and the problem corrected, can continue to give normal service.

Like I said anyone to say EPDR (combination of impedance and -phase angle) "does not matter", is dreaming.

The odds are very against you sunshine. Over and out👎

I think for someone to make this statement, they have to understand how EPDR impacts an amplifier. By all indications from what you have posted, you do not.

Taken on board Ralph, with a yawn.
But for anyone to say EPDR (combination of impedance and -phase angle) "does not matter", is dreaming.

Taken on board Ralph, with a yawn.
But for anyone to say EPDR (combination of impedance and -phase angle) "does not matter", is dreaming.

Your M60 mono blocks "could not drive the bass" of the JBL 1400 Array’s a 90db speaker without obvious stress playing the bass notes, even at "medium levels" and they were 100% fully functional.
And nowhere! near!! the EPDR load that a Wilson Alexia would present.

https://img.usaudiomart.com/uploads/large/1922864-fcdeac94-atmasphere-m60-mk-33-monoblock-otl-tube-a...

https://www.stereophile.com/floorloudspeakers/jbl_synthesis_1400_array_bg_loudspeaker/index.html

I shouldn’t have use his amps as an example that couldn’t drive those JBL’s 1400 which IS fact and not fiction, "which even he agreed on" way back on the 9-17-2019 and here’s the proof! Ralph's answer https://forum.audiogon.com/posts/1804702 to my post before it https://forum.audiogon.com/posts/1803629
Your slipping Ralph, "one should always remember what one said no matter how old before saying the opposite" Confucius.

@georgehifi 

Uh, George, the M-60 is **not** one of our

big Atmasphere’s monoblocks

as you put it in your post above. The M-60 is our **smallest** monoblock and the S-30 is the only amp we make that is smaller. The MA-1, MA-2 and MA-3 are all bigger than the M-60. I'm not slipping or in 'protection mode',  simply instead insisting on facts. The M-60 won't drive a 2 or 3 ohm load, but our 'big monoblocks' can.


It would be correct in your post above to have said

the **small** Atma-Sphere monoblocks that my friend had, could not drive the bass of the "so say" easy load, high efficiency (90db), easy impedance load (8ohms) of the JBL 1400 Array speakers, because of the added -phase angle of the EPDR combination.

-however to be really accurate you could just drop the bit about EPDR and simply state that the M-60 can't drive a 2 ohm load but we could have told you that, and did. BTW, the efficiency of a 2 ohm loudspeaker with a 90dB efficiency is 84dB- not a good choice with **any** 60 watt amp. 90dB is not 'high efficiency BTW; that's a moderate efficiency. High efficiency would be in the high 90s or higher. 

I believe this statement to be entirely fictional
Obviously this phenomena isn’t limited to our amps
So this Stereophile article seems
I believe this statement to be entirely fictional.

Ralph’s gone into product protection mode.
I shouldn’t have use his amps as an example that couldn’t drive those JBL’s 1400 which IS fact and not fiction, "which even he agreed on" way back on the 9-17-2019 and here’s the proof! Ralph's answer https://forum.audiogon.com/posts/1804702 to my post before it https://forum.audiogon.com/posts/1803629
Your slipping Ralph, "one should always remember what one said no matter how old before saying the opposite" Confucius.

And these words "phenomena" "I believe" "seems" are also words that snake oiler’s, and voodoo’ist use, so as not to be held to account, a certain boutique $$$$$ fuse company use them a lot

And this is the reason why Ralph Karsten’s big Atmasphere’s monoblocks that my friend had, could not drive the bass of the "so say" easy load, high efficiency (90db), easy impedance load (8ohms) of the JBL 1400 Array speakers, because of the added -phase angle of the EPDR combination.

I believe this statement to be entirely fictional. Our bigger amps (MA-2 and MA-3) drive that speaker with ease, although I would hesitate to call it an 8 ohm load. A customer of mine has that speaker and having spent a week at his house, I can say the two work very well together. He owns the MA-3s, but the MA-3 has a unique bank-switched power tube setup that allows you to shut off 1/3 or 2/3rds of the power tubes (such that they are not even lit up) thus allowing the amp to run with much less power. The speaker and the amps worked fine together in all three modes.


Tubes are a bit different from transistors in a number of ways as we all know :) One of the ways they are different is the Safe Operating Area (SOA). With tubes you can exceed the SOA without damage to the tube if the tube is allowed to cool off afterwards. A tube dramatically overtaxed, such as in a loss of bias, can turn cherry red from heat, but if allowed to cool off, and the problem corrected, can continue to give normal service. You simply can’t do this with semiconductors- Very Bad Things happen- exceed the SOA and the device is likely damaged immediately! Now the output section of the MA-1 amplifier (140 watt monoblock) can sustain enough current in a pinch that it can easily knock out a 15 amp circuit breaker in the wall. Of course we fuse the amplifier in such a way that this can’t happen in practice, but the interesting bit is that in this situation, while the tubes are obviously outside their SOA, they can survive this sort of thing easily.

During WW2, To get more power out of an 807 (which is basically a 6L6 with a plate cap to prevent arcing), some transmitters placed the tube in a water jacket to cool it- and thus got an astonishing amount of power out of it (around 500 watts- but keeping in mind it was also class C). So this concept is well-known and been used to advantage for a long time.


This simple fact is why a tube amp can drive loads that you would not expect possible (which is why the later Wilsons, despite their low impedance in the bass, seem to work fine with our gear). I’ve had to learn it through experience myself- when I was first approached decades ago to see if our original M-50 (immediate predecessor to our M-60) would drive a Quad ESL63, I thought the amp would fall flat on its face. Turned out to do the job quite well- better than an RM-9 owned by that customer.


Obviously this phenomena isn’t limited to our amps- all power tubes have this aspect of a ’soft’ SOA. So this Stereophile article seems to have more relevance to solid state amps, where semiconductors are simply going to punish you with failure if you don’t observe their limits.

I have not even once changed what I have wrote about EPDR. I have addressed your specific arguments in every case, while I have yet to see you make even one deep technical analysis w.r.t. the discussion  you raised on EPDR.  I have also used my own words, while you have only linked to articles.  Would you like some Maple Syrup with those waffles? Perhaps it will go well with those sour grapes.

georgehifi7,226 posts06-10-2020 1:21amJust keep waffling on,

Just keep waffling on, because you still can’t see the forest for the trees.
https://i.pinimg.com/originals/7b/df/d9/7bdfd904dbe2ff73b9de8b8f7d84563d.jpg

Savvy how? I am sure I can match them in patents and AES papers. Really doesn't matter, since they would all agree with ME, not with you. You don't even realize that you have not made any arguments to support your claim and have done nothing to show the impact of EPDR on a tube amp. You just keep repeating the talking points that illustrate either a lack of understanding of EPDR or of amplifier architecture and operation. I really couldn't care less that what you believe, but when you come on here and give bad information, I will point it out.  I called out to @atmasphere as he appears to have a solid understanding of amplifier operation.

Here is what @atmasphere said,

**Any** 60 watt OTL will be uncomfortable with a load like that (see JA's comments in the review at the link). This is a great example of how *not* to do it- you always want a tube amp to play loads for which it is intended!

I don't know atmasphere's OTL specs, but normally they have a somewhat high output impedance. The huge resonant spike with voltage drive on the JBL would cause a significant level change at that frequency.

We also noted that particular customer was going through power tubes faster, which is no surprise since a lower impedance will cause more of the power generated by the output section to be dissipated in the output section- and that's hard on tubes. We finally got him to install a pair of ZEROs and then the tubes stopped failing, plus he had more power. The Wilsons traditionally have been fairly efficient so none of this should come as any surprise. The national sales manager at Wilson (John Giolas) had our amps for several years.

Note this is --average-- impedance, not EPDR minimums, which will increase the instantaneous power dissipation in the output section of a tube amplifer (exactly as I said) based on music content, which does not have much impact on tube life at all (also what I stated). 

Here is another point, at the same volume level, whether you use the 4 ohm or 8 ohm tap, the instantaneous power dissipation in the amplifier output section will be almost exactly the same at the EPDR minimum.  Then again, I believe the MQ-300 is ClassA, in which EPDR does not have any meaning.

The proofs above from guys infinitely more savvy than you, like Matti Otala, Eric Benjimin, Keith Howard 

Atmasphere stick his neck out for you. That’s a low percentage ask.

Atmasphere was the one that said the JBL 1400 are a difficult load for the tube amps, when we were talking about the Wilson Alexia’s 0.9ohm load with his amps, because I stated the Alexia’s are even worse than the JBL’s.
do a search and find it.

@georgehifi , all you did is regurgitate what you already said, without in any way relating EPDR to a tube amplifier, except in the way you used it, it is wrong. EPDR is not impedance and-does-not impact frequency response w.r.t. output damping factor. That is not how it works ... at all.

How does EPDR specifically impact a tube amplifier??  This is a discussion on tube amplifiers. Just saying "It could not drive" is at best an opinion based on your friends listening preference, but without any hard data ....     EPDR does not impact current delivery, it impacts power dissipation in the amplifier. Do you refute that? The highest current will be delivered at the impedance minimum. Do you refute that?

@atmasphere may wish to jump in based on the comments made about his amplifier.

gte357s OP
@georgehifi Please excuse my limited knowledge, I don’t fully understand the phase angle and stuff.

Don’t listen to the conspiracy theorists here, just read this article in Stereophile "How Loudspeakers Torture Amplifiers" used by Stereophile in all speaker measurements they do, with comments always on "how combined impedance with -phase angle (EPDR)" is the real total load that the amplifier will see.

Look nearer the bottom of page 1 with the paragraph that starts of with

" But loudspeaker loads are not, in general, resistive. They comprise complex impedance’s with both resistive and reactive (capacitive and inductive) elements,"

https://www.stereophile.com/reference/707heavy/index.html

Then go to page 2 where they get right into EPDR
https://www.stereophile.com/content/heavy-load-how-loudspeakers-torture-amplifiers-page-2

Then also page 3
https://www.stereophile.com/content/heavy-load-how-loudspeakers-torture-amplifiers-page-3

And this is the reason why Ralph Karsten’s big Atmasphere’s monoblocks that my friend had, could not drive the bass of the "so say" easy load, high efficiency (90db), easy impedance load (8ohms) of the JBL 1400 Array speakers, because of the added -phase angle of the EPDR combination.
I circled In the red circle https://ibb.co/SKbkF4t

JBL 1400 Array. Its minimum modulus of 4.9 ohms occurs at 92Hz. Its minimum EPDR is 1.9 ohms!!!

Then read about the B&W 802D’s EPDR load to the amps

Cheers George

@georgehifi , Maybe you could explain to gte357s, exactly how this EPDR is "taxing" a tube amplifier knowing that being frequency based, and real music, the time at any of these EPDR minimums is very short.

@gte357s , changing the tap from 8 to 4 ohms will have minor changes on the frequency response, distortion, and damping primarily of the woofer (which also shows as frequency response). There are likely as many who would like the 8 ohm tap as the 4 ohm tap, but of course best to use the one you like.

Zu Omen Def is actually 6 ohm nominal! No wonder I hear better sound with the 4 ohm tap!

And if that’s nominal, you can bet you b****s, they dip down even further than that.! And even further again when you bring in the -phase angle into the equation as well if it’s high.
So what does that tell you? get the optional 4ohm tap on the MQ-300 output transformer.

Here again is something JA of Stereophile says about combined low impedance with -phase angle (EPDR)
This is on the Emotiva Airmotiv T2 review just put up to read on the web.   Nominal impedance: 4 ohms by the manufacturer. 
https://www.stereophile.com/content/emotiva-audio-airmotiv-t2-loudspeaker-measurements

John Atkinson : Stereophile
  Fig.1 shows that the T2's impedance magnitude does average close to 4 ohms, though the minimum value is 2.85 ohms at 112Hz, and there is an amplifier-taxing combination of 4 ohms and –53° electrical phase angle at 78Hz.

Cheers George

Guys, I did more research and realized the Zu Omen Def is actually 6 ohm nominal!  No wonder I hear better sound with the 4 ohm tap!  

Mistake, get over it, and now the conspiracy theorist!.

Why did you make a post, delete it, then create a reply to it like this?  I don't understand.

Post removed Jun 08, 2020georgehifi7,214 posts06-08-2020 4:58pm

Post removed Jun 09, 2020

That's right.
Clean up aisle 4.

Post removed Jun 09, 2020

That's right.
Clean up aisle 4.

This "idiot" obviously knows a heck of a lot more about amplifier design than you do. Do you even know what EPDR means? It is a measure of the voltage delivered / current delivered at a given frequency. Why does that matter? .... It matters because that comes into play for power dissipation in an amplifier in the linear region. Keep in mind, we are talking real music here, not continuous pure tones, so low EPDR only increases the dissipation a bit dependent on music material. EPDR raises the power dissipation at the device level, and for BJT devices, that can lead to BJT secondary breakdown. This is from a Toshiba app note.

Secondary breakdown is a failure mode in bipolar transistors in which negative resistance (current concentration) occurs under high-voltage and high-current conditions. Current concentration causes local heating, resulting in a small hotspot. The impedance of the hotspot decreases, causing further current concentration. This cycle called thermal runaway leads to device degradation and destruction.

MOSFETs and Tubes do have secondary breakdown effects, but their safe operating areas and secondary breakdown mechanisms are different and hence they don’t experience failure in the same way a BJT amplifier will from short term thermal events such as those that could be induced by low EPDR. Amplifier power dissipation at a given power output will go up with phase angle (assuming not pure class-A), but you don’t get the catastrophic failure of a BJT amplifier.

EPDR does not mean your amplifier has to delivery higher peak currents. An EPDR of 2 ohms, does not mean the current drawn is higher than an impedance minimum of say 3 ohms. What it means is the amplifier power dissipation is a maximum at the EPDR minimum. It would be rare that it would be at the impedance minimum (if at all), so the impedance minimum does represent the maximum current delivered by the amplifier.

Having a 4 ohm tap isn’t going to make your amplifier more robust to a lower EPDR than the impedance peak. It is quite meaningless within the framework of impedance matching of the amplifier, or the output impedance of an amplifier. EPDR just tells you for a given speaker what the potential maximum power dissipation will be in your amplifier output devices, assuming a particular amplifier architecture.

gte357s OP
@georgehifi Please excuse my limited knowledge, I don’t fully understand the phase angle and stuff.

EPDR is the -phase angle (expressed in -degrees) combined with the low impedance measured, and represents a "far more severe loading" to the amplifier than just the impedance itself, don’t listen to any idiots that says otherwise.

Here is the Wilson Alexia, it's one of the worst bass loads I’ve seen, and so thinks HNRR and Stereophile, it has an EPDR of 0.9ohm!!!!. And the reason why you should get the 4ohm transformer tap as well as 8ohm.

https://ibb.co/stQ44Ry

https://ibb.co/C8d2Zny

Cheers George

The phase element, if Georgehifi is relating it to EPDR is not relevant. It relates to thermal dissipation in output devices which tubes don't have an issue with.

@georgehifi Please excuse my limited knowledge, I don’t fully understand the phase angle and stuff.  But to clarify, my X5 has one 12” Eminence driver and one 12” powered Eminence subwoofer by Hypex Fusion NCore plate amp.  

The cross over frequency is 90Hz. AMT - 1 kHz and up
12” mid range - 90Hz to 1 kHz
12” powered subwoofer - 90Hz and below
But that 90Hz to 1 kHz is exactly where the impedance dips to 7 ohm.

actually, I can do a test on my current amp which supports 4 ohm.  And I have been connecting my X5 to 4 ohm.  I did a brief test earlier and I remember there is no difference on my X5.  But I remember there is an improvement to use the 4 ohm tap on my Zu Omen Def.  So, it seems this really depends on the speaker.  Maybe the phase angle is less relevant in a open baffle speaker?  
But I may change speaker or I may sell this.  Definitely having a 4 ohm tap is more future proof, but I am not sure if I want to spend 20% more for the unknown future.  If I happened to buy a new speaker, maybe I should just buy another better matching amp .... decisions,  decisions .... 

thats good news that I can just add the volume pots. =)

If the amp is a stereo amp, then a balance control is a really good idea. If they are monoblocks and each has its own volume, you have to manually adjust the volume for each channel- that gets old fast!

EPDR is not a meaningful term for this topic. That is a term more applicable to solid state amplifiers operated in the linear region and is most applicable to a discussion on thermal dissipation, not drive capability or optimization for frequency response and THD.

"And that if it say -50 degrees in the bass, which is not uncommon, combined that with the "5ohm" of what the manufacture "says" the speaker is, "could" represent a combined EPDR ("equivalent peak dissipation resistance") load even down to 2ohms depending on what the -phase angle is."

I know the X5 use a 12” Eminence driver. I look up their web site, and found the model and curve below. It seems it dips down to 7 ohm

This is definitely not what it will be when it’s in the speaker, as xover will come into it also, plus the -phase angle what ever that might be.
And that if it say -50 degrees in the bass, which is not uncommon, combined that with the "5ohm" of what the manufacture "says" the speaker is, "could" represent a combined EPDR ("equivalent peak dissipation resistance") load even down to 2ohms depending on what the -phase angle is.
Be safe and get the 4ohm tap also, or you could end up with a thick ill defined bass, which will effect the lower mids also. 

To give you an idea the 12" Eminence driver originally in my ML Monolith III’s, was an 8ohm driver but in the Monolith’s they measured with the 3rd order xover a combined EPDR down to 2ohms presented to the amp below 100hz

Cheers George

Thank you very much for all the inputs.  I need some time to digest the explanations.  I don’t understand all of them, but I feel this thread has the answers that I have been searching on the internet.  For now, I am good!  I will add only the volume pots, and not upgrade the transformer.  

@gte357s
No, what I meant was that if you have both 8-ohms and 4-ohms on the secondary, using 4-ohms will not be a problem as long as you don’t expect the maximum power the amp is capable of. The secondary will draw more current and if the xfmr and the tubes can handle it, then the amp will perform to spec.

If you are specifying a new transformer, then the above is moot. What you do is look at the impedance graph of your speaker for the entire audio range and look for the average impedance.

For example, my Thiel 3.6 speakers are rated at 4 ohms, however they are at 3 ohms from 100 hz to 10khz. If I were to spec a transformer, I would first see where the tube operates at the optimum power and THD. Say that the tube load is optimum at 3,000 ohms. I would then have the transformer wound to reflect a 3,000 ohm on the primary with a 3-ohm load on the secondary. Then I know what VA rating I need. With those specs in hand, I don’t need to worry about the transformer being able to drive the speaker because it is optimized for most of the impedance range. But in the lower frequencies when the impedance climbs to 8 ohms, the tube will be taken out of its power sweet spot but may still remain relatively low in THD. It's a tradeoff I'll take because it is, after all, a tube amp.

If it only dips to 7ohm, then it will be just fine but the speaker is only part of the equation. The crossover will have an influence too. I don't know what they speaker does at the frequencies for the subwoofer. There will be some influence from the circuit for the mid. I expect there is some loading at low frequencies, but would be highly unlikely to be much less than 8 ohms.

@roberttdid 
You bring up a good point.  I know the X5 use a 12” Eminence driver.  I look up their web site, and found the model and curve below.  It seems it dips down to 7 ohm around 200Hz.  So, only 1ohm lower than the nominal 8ohm.  In this case, I guess having only an 8ohm tap is ok?

And for the power subwoofer on the speaker, I guess the output impedance of the amp is irrelevant, right?

https://www.eminence.com/speakers/speaker-detail/?model=KL3012CX_8

The output impedance of tube amplifier with a transformer is a function of the (output impedance of the amplifier section multiplied by the turns ratio) ^2  (squared).  The 4 ohm tap has less total turns, so it has lower output impedance (higher damping factor).  On the other side, changing the tap, changing the turns ratio, reflects a different load back to the amplifier, and coupled with the transformer, that can have some impacts on the flatness of the frequency response at the ends of the audio spectrum and the phase. All depends on the overall design.

In terms of selling the amplifier, "clones", especially something that is a production clone, are of limited resale value, so keep that in mind when looking at what you spend. The transformer is a huge part of the implementation. You are building something "like" a Luxman mq-300, but certainly not a Luxman mq-300.

You can ask Spatial audio if they can supply an impedance graph/chart. There really is no excuse for not being able to supply one. From there, you could make a decision at least on this speaker. They claim 8 ohm, and there is no reason to doubt that. Rather large mid-bass, so the impedance could dip, but 8 ohms is still likely best for overall performance.  Different speakers, of course, will behave differently.

paying extra to get a 4 ohm tap will not have audible improvement over the 8 ohm tap, am I correct?

Yes it will the bass will be tighter/controlled with the 4ohm than the 8ohm tap, if the speakers are 4-5ohms
Go the $500 extra you can't go backwards once it’s done, and the amp will have far better resale value down the road.
Cheers George

@gs5556 
So, my understanding is that, as long as I listen in low to medium volume, paying extra to get a 4 ohm tap will not have audible improvement over the 8 ohm tap, am I correct?

You can put a volume pot on the amplifier, the only reason most amps do not have one is it limits their market to customers with only one source. What you have to take into consideration is what input impedance your source wants to see. Anything above 10k ohms should work fine. I would go with a shunt volume control as it only puts one (series) resistor in the signal path and I would use a series resistor of 15K ohms, high enough for a respectable input impedance through the volume steps and low enough to keep out noise. You could also include a balance control if you want, just place it upstream of the volume pot.

If your amp has a 4-ohm tap, you can use that with no problem. The power output does not change, but the current draw will be higher as the output xfmr tries to maintain voltage at the lower reflected impedance (power is consumed at the speaker, NOT the amplifier). Any limitations on the amplifier will show up here in the form of voltage drop and distortion as the tubes are asked to work harder. However, it should not affect anything if you listen at reasonable volumes.

While reviewers tend to prefer 4 Ohm taps, the reason for these taps is to ensure equal power output. My mind is hazy, but I think the main reason for this is that without the tap your amp’s power output drops at lower impedances.  @atmasphere is the real tube/transformer expert here.

Best,
E

@georgehifi yes, can’t find the curve for the X5.  Actually, the current price without upgrade only have 8ohm tap.  In your opinion, is 8 ohm ok, or is it worthwhile to add $500 to upgrade the transformer, which will then have a 4 ohm tap and likely will improve the sound?  

Spatial Audio X5

Na, nothing measured on these, but just by the looks of of them, I'd say a 4ohm tranie tap will be fine for these, btw the real Lux has 16-8-4ohm taps on it's transformer.
Cheers George

@georgehifi 
thats good news that I can just add the volume pots. =)

my current speaker is a Spatial Audio X5.  It has a power subwoofer.  When I say my speaker dips to 5 ohm at 30Hz, it is for my old Zu Omen Def.  And that is based on my memory, so, number may not be exact.  I just use that as an example, easier to give some numbers.

and I remember when I use my Zu, I feel the 4 ohm tap gives more bass.  But it can also be a psychological impression, nothing scientifically measured.

@erik_squires Currently, he will use Tango XE-20s transformer.  I hope it is a good quality transformer.  But if I want to have a 4 ohm speaker output, I need to add $500 to upgrade to Tango FC-30-3.5s.  So, for the additional $509, I get a 4ohm output and likely better sound, but don’t know how significant it is.  The ultimate question is, should I do this upgrade .... I think I don’t need 4 ohm.  My current speaker has a power subwoofer.  So, it seems the dip in impedance at low frequency is not a concern for now.  but I may change my speaker, or I may sell the amp.  To ask the question a different way, when you buy an amp, is it important to you for the amp to output 4 ohm?

gte357s
Can I just add a volume pot to a power amp schematic?

Of course you can, the Lux is 240kohm!! input impedance with 0.49v input sensitivity!!! so absolutely no need for any active preamp with even more gain here.
Just get your tech to use a 20kohm stereo Alps (Blue Velvet) volume pot on the input of the Lux.

The transformer is designed for 8 ohm.  If I want it to support 4 ohm, I need to upgrade the transformer.  I know that the speaker impedance curve can dip to 5 ohm at 30Hz.  Is it important to have 4 ohm support?

A 4ohm tap would be better, you'll halve your wattage though.
Depends on what -phase angle is associated with that 5ohms, what speakers are they? Hopefully there's a phase graph done on them.


Cheers George

Can I just add a volume pot to a power amp schematic? I did some research about passive preamp, but it seems even a passive preamp has a circuit and a transformer. Is it better to buy a separate passive preamp?

Depends on what’s driving it. Passive preamps are not all transformer based, some are resistive.

If you end up with a suitably high resistance for the source, sure.

I warn you that the magic in those old amps is in the quality of the transformer. Rather than attempt a clone, you might be better off with some other kit with suitably matched output transformers.