Who is using passive preamps and why?

Thanks Almarg. This was from Sam Tellig's column on the Halcyon passive pre.
He recommended .5m max from the source to pre and 1m max from pre to power amp. Then again I don't understand why you would store e and electronic component in a wine cellar either.

Given the wide array of monitoring positions and speaker locations used by mixing engineers while twiddling the pan pots, it would seem logical that reproducing any perspective of depth and width would vary based upon these unknowable factors.

I'm starting to understand some of this through my friends here, as well as the effect of miking. I'm going to change my speakers out in the near future so I can listen to my Tonian Labs TL-D1's for a bit. At that time I will try the near wall, or even corner placement.

Mine cost a little extra because I had George wire it with 2 volume controls. He referred me to Allied Electronics for a Stancor regulated (very important) power supply. Mine is also wired with the center pin negative on the PS since that is the way the Stancor is wired. I've always been curious as to the battery PS, but this time I think I'll leave well enough alone.

http://www.alliedelec.com/search/productdetail.aspx?SKU=9289895

PhD, for some reason, George sold it to me for $415, but you do have to buy a 12DVC 400mA AC/DC adapter, which seem to only be available in Canada, ended costing $30 for the unit plus shipping.

Clio09,

I think there is something to the perceived soundstage depth argument postulated by your sound engineer friend. Much like finding the "correct" volume for any given recording, I often find myself shifting my chair slightly forward or back (maybe 3-4" either way) to find the correct listening perspective for any particular recording where center images snap into focus, and the soundstage expands nicely in all three axes. Given the wide array of monitoring positions and speaker locations used by mixing engineers while twiddling the pan pots, it would seem logical that reproducing any perspective of depth and width would vary based upon these unknowable factors (not that stereo isn't a complete contrivance anyway). While I don't agree with shoving speakers against a wall unless they're designed for it, the discussion is an interesting one.

Rrog, well its true I thought the thread outlived its usefulness I am glad it didn't stop there otherwise I wouldn't have learned about the Lightspeed which in my opinion is extremely unique in its design. Thus the thread has produced what I orinally was seeking to begin with, something new and innovative.

I know this whole issue is a matter of taste, and will never be resolved, but I will say this for the passive argument. I had this long discussion with Roger Modjeski of Music Reference and RAM Labs - that is, a fella known as one our our great designers of equipment and a tube lover. I asked him, point blank, when I was considering buying his $135 Pot-in-a-Box if it were not possbile for any active preamp to be better, he said no. Now I know there are many of us that love our actives, I do, but he is one pretty qualified person to talk about preamps since he can design whatever he wants, and he does love tubes, but in his view a passive does what a preamp should do IF it is in a good source to amp environment. It at least made me feel that a passive isn't just a cheap solution to volume control, but for some might be as good as it gets for a preampifier (in the right system).

I am using a Diy Passive at the moment as I am comparing different designs of Power Cables on my CD Player. Although I enjoy listening with my active, it is warm and colored. It is however, refreshing to have the clarity,transparency and detail of the passive at present. It is a much more elegant sound.
Cambridge Audio CD Player Output 2 Volts
.5 meter Virtue Audio Nirvana IC to Passive
Passive is wired with OCC-OFC Silver Wire and signal goes straight to a Dact-type 10K Attenuator.
Same 1 meter Nirvana IC from passive to an Audio Research VT50 input 100K.
Speakers are modified monitors and Powered Sub.
Active preamp is a heavily modified Audio Research SP 14.

I agree and I guess it comes down to how we answer the question, what is the best preamplifier? The one does nothing )very little) to the signal or the one that sounds best to us for whatever reason. In discussing this with Ken Stevens of CAT, he felt a preamp should have the tone color of water - none; that is the job of the preamp, if you want tone color, find it somewhere else in the system or you end up with band aids to complement the various colorations within the system, but the preamp should be as neutral as possible, that's why I think the passive approach is a step in the right direction, if it works nice with your equipment. The Goldpoint is such a piece, and frankly, I don't sense any rolloff or lack of dynamics compared with my Joule and Atma-sphere pres, but it does not sound like them either.

I remember a conversation I had with Kevin Carter when he was building me a TVC, my first passive. I had made some comment to the effect that passives have the reputation of leaning out the sound. It took him less than a second or two (meaning he didn't even have to think about it) to reply that its active preamps that add something to the sound.

I'm leaning towards the opinion these days that a lot is in the engineering and mixing and that active preamps are more like tone controls. I was hanging out with some audio engineers this past weekend and one guy liked having his speakers against the wall to improve the bass response. I commented that to me that would hinder depth of sound stage and that is why I prefer having my speakers out in the room. His comment was that much of the depth in the sound stage comes from how the recording was done, not where the speakers are placed. I'm not sure I buy that one yet, but considering the number of fine speakers out there that are corner/wall placed, maybe there is something to it.

I agree with the argument that its the lack of distortion that creates the notion that something is missing in the sound. A tube amp should provide all the distortion one needs.

What frequencies can "meat on the bone" be found? And are those flesh tones in the source signal, or added later? Or are active tube stages just keeping something from being lost that simply gets sucked up by essentially straight wire connection. Maybe we have a long addiction to distortion, and what it is gone something just seems to be missing.

Anthony, if the Lightspeed is as good as it seems, I think it and the RM10 would put ALOT of $10-20K pre/amp combos to shame - IF 35 watts is enouuh power. Looking at the diminutive RM10 (size of a hardcover book) reminds me of that old Steinway piano ad about how they laughed as the fella sat at the piano, till he started to play. Can't wait to give the Lightspeed a try, but the Goldpoint ain't chopped liver either.

I think you will like it. Definitely makes you think about what it takes these days to get high quality sound. As an example, the combined cost of my Lightspeed and RM-10 MkII is $1450. That's a lot of sound per dollar in my opinion.

$450 shipped from Australia; it takes two weeks or so before shipping. I get the feeling the Stereophile review is making him busy. It will be interesting to compare with the Goldpoint and then I'll sell one or the other. Either way, these passives are a really "inexpensive" way into some pretty darn good sound. I don't know if I will like them better than my Joule or Atma-sphere preamp, it just may be different, but worth having in the mix.

I want to try the lightspeed Attenuator as well. Pubul57, how much does it go for?

just ordered one from george.

It has been a long time since I read Stereophile, but Sam's Corner in the February issue has a write up of the old Halcyon PVA and the Lightspeed Attenuator that I own. The article prompted me to put the Lightspeed back in my system for a whirl. I'm not regretting it. This opto coupler designed passive is truly remarkable. As usual careful system matching applies. I believe minimum amp Zin is 50k ohms and high sensitivity is best. Here is some additional information for anyone interested:

Lightspeed Attenuator

Single input/single output only. I have a version with dual attenuators. One of the designers comments is that this type of design removes the sonic effect of the switch from the equation.

Almarg: The higher the output impedance of the source component, the greater the losses that would result from that transformer impedance.

While I only quoted part of your response Al, Jack did mention DC resistance and sound degradation, particularly in the bass the higher the output impedance of the source. The PVA is really designed for opamp driven sources and when I asked if I could use my 600 ohm Zout Otari tape deck with the PVA I was told that I would get some bass distortion and potentially other losses due to the mismatch.

With my Kenwood KT-8300 tuner and digital set-up the sound is fine.

Cdc: Why does the I/C from the source to passive pre have to be as short as possible while from the passive pre to power amp can be longer?

Actually, the reverse is true. The high output impedance of a passive preamp will form an RC low pass filter in conjunction with the capacitance of the cable that is connected to its output. The longer that cable is, and the higher the capacitance per unit length of the cable, the more likely it is that significant upper treble rolloff will occur.

The relatively low input impedance of a passive preamp will produce a minor increase in the amount of current flowing through the cable between source and preamp, thereby perhaps increasing some subtle cable effects if the cable is long, but those effects will be minor in comparison to the effects of excessive capacitance in the cable connecting passive to power amp.

Clio09:Question: Jack tells me the transformers are wound for 100 ohm impedance, meaning the source must have an output impedance of 100 ohms or less. Any of this make sense?

Transformer impedance ratings, as distinguished from the reflected impedances corresponding to the connected source and load impedances and the turns ratio, have always been something of a mystery to me. Perhaps someone else will comment more knowledgeably. But I believe that the rated impedance of the transformer itself corresponds to how lossy the tranformer is in terms of flux leakage and dc resistance, and represents the amount of current limiting that would occur in the primary with the secondary short circuited (and a voltage source with near-zero output impedance connected to the primary). The higher the output impedance of the source component, the greater the losses that would result from that transformer impedance.

Best regards,
-- Al

Why does the I/C from the source to passive pre have to be as short as possible while from the passive pre to power amp can be longer?

For comparison, the S&B 102 has a Zin 0f 10.4k and Zout of 1.4k @ 1kHz at it's worst case of zero attenuation. Zin halves for within each of the next few -3dB steps and Zout doubles. My setup never approaches the end of the dial.

http://www.stevens-billington.co.uk/page102.htm

Al is correct, not a TVC but a slightly different animal. Considering I have spent a lot of time with Jack Elliano the last few weekends (we are Las Vegans) I can say the guy knows his stuff and his SET amps sound great (with which he uses a PVA with 1:8 ratio). However, I think in this case Al and I are in agreement. I've fiddled around with this and a 1:1 or 1:2 ratio would be best. My main problem right now is I can't get the volume control past 9 o'clock, which supports the case for a lower ratio IMO.

Question: Jack tells me the transformers are wound for 100 ohm impedance, meaning the source must have an output impedance of 100 ohms or less. Any of this make sense?

Agreed, in that case it is. I was looking at the bigger picture. I'm not sure where that falls into the equation but according to ears I trust they say best is

.....autoformer at 1:1 or less

.....then autoformer with a small amount of voltage gain

.....then true transformer at 1:1 or less

It is wild conjecture on my part but logically following that sequence a step up followed by a pot would seem to be the next lower desired configuration.

I'll probably never try it but it would be an interesting experiment.

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Herman -- As I understand it, it's not a TVC. It's a step-up transformer followed by a resistive attenuator.

Best regards,
-- Al

big caveat, I've never tried this but those who's ears I trust, like Dave Slagle and Jeffrey Jackson tell me performance is compromised when you go with transformer ratios in TVCs that exceed 1:1. In other words trying to get voltage gain from a transformer volume control deteriorates the sound. No proof from my end but these guys have yet to steer me wrong.

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Hi Clio,

I took a look at it. I see that several different transformer ratios are offered. 1:1 and 1:2 will certainly be no problem for low impedance sources. The other two ratios, 1:8 and 1:13, will, as I speculated earlier, result in an overall input impedance that is extremely low. 1:8 will divide the 10K attenuator impedance down to 156 ohms. 1:13 will divide it down to 59 ohms. Addition of a 100K amplifier load may reduce those numbers slightly further, depending on the volume control setting.

Whether or not that heavy a load will result in good sound is dependent, of course, on how well the source component can handle having to supply relatively high currents, and on how flat its output impedance vs. frequency curve is. But in general I don't think that those two ratios can be counted on to perform well.

Also, I don't understand the statement about output impedance remaining constant as a function of attenuation setting. If the attenuation is set for minimum volume, the output impedance looking back from the amplifier into the pva will be zero or very close to it. If the attenuator is set fully clockwise (max volume), the output impedance will be 10K in parallel with the source component's output impedance times the square of the transformer's step-up ratio. As I indicated in one of my earlier posts, if the attenuator is set to the middle of its resistance range, the output impedance will be around 2.5K (the worst case), assuming the source component's output impedance and the transformer's step-up ratio are small.

BTW, moving the transformer to the output side of the pot, as you may realize, would most likely not be helpful, because it would probably raise the pvc's output impedance to levels that would be too high relative to the amp's input impedance. Also, it would increase the range of amplitudes over which the transformer would have to operate, which MIGHT compromise its performance to some small degree (I'm not knowledgeable enough in that area to be able to say).

The bottom line, imo: Assuming (as I do) that parts quality is good, it looks like an excellent product at 1:1 or 1:2 transformer ratios, but performance at 1:8 or 1:13 will be highly dependent on the characteristics of the source component, and good performance at those ratios cannot be counted on.

Best regards,
-- Al

Here is a schematic of what I built:

http://www.electra-print.com/pva.php

Here is some more information:

http://www.electra-print.com/techblog.php?blogid=1

I know it is a good thread when I understand half of it. On the issue of impedance matches and best pot ratings, I see the theoretical, and perhaps measureable difference, but are they audible (to most people)? I suspect that in my setup, whether the pot is 10, 25, or 50 I would ne hear the difference, though my test equipment might.

Is it safe to assume that the Goldpoint is shunt-type?

No, they are series types. From the Goldpoint website:

Goldpoint Level Controls used to offer Ladder and Shunt type stepped attenuators too. The fact is that with the very low noise, 0.1%, thin film resistors we are now using, we believe that there is no real advantage to Ladders and Shunts - and so have ceased offering them.

And a schematic of their Mini-V series attenuator:

http://www.goldpt.com/schm_ser.html

Best regards,
-- Al

Just to confuse things...

http://diyaudio.co.kr/wwwboard1/data/board1/compare.pdf

Is it safe to assume that the Goldpoint is shunt-type?

Almarg: In your case, with a 10K pot and a 100K amp Zin, at minimum volume control setting the input impedance (not output impedance) looking into the input side of the pot would be 10K. With the control turned all the way up, the impedance would be 10K in parallel with 100K, which is 9.1K.

The source component would see a load impedance equal to that value (dependent on the volume control setting) factored by square of the ratio of transformer primary turns to secondary turns (assuming the transformer is "ahead" of the volume control).

To make sure it's clear, I should add that if you have a transformer ahead of the volume control which steps UP the incoming voltage from the source component, the source component will see a load impedance that is REDUCED from the 9.1K or 10K number, in proportion to the square of the turns ratio.

For example, if the transformer provides a 4x (12db) voltage step-up, the source component would see a load impedance of only 568 ohms to 625 ohms, depending on the volume control setting. Obviously that gets into worrisome territory.

Best regards,
-- Al

Also, and I think that I have read this above, just want to make sure I understand, with a 10k pot the output impedance rises as the knob is rotated clockwise until it reaches 10k. Did I get this right?

No, Herman's relevant statement was:

In my example they were both 10K so it is 1/2 of the 10K. With the volume close to zero it rises to about 10K. and falls as the volume goes up.

In your case, with a 10K pot and a 100K amp Zin, at minimum volume control setting the input impedance (not output impedance) looking into the input side of the pot would be 10K. With the control turned all the way up, the impedance would be 10K in parallel with 100K, which is 9.1K.

The source component would see a load impedance equal to that value (dependent on the volume control setting) factored by square of the ratio of transformer primary turns to secondary turns (assuming the transformer is "ahead" of the volume control).

As I indicated a couple of posts ago, the OUTPUT impedance of the passive, which must be kept low in relation to the amp input impedance, will be at its worst case maximum when the volume control is set to the mid-point of its resistance range (assuming the source component's output impedance is small). Apart from the presence of the transformer, with a 10K pot and a 50 ohm source impedance, that output impedance would be 2.5K. The step-up transformer will raise that value a little, by stepping up the (very small) contribution of the 50 ohms. But for any reasonable turns ratio the resulting output impedance is likely to still be well under 5K, and therefore fine in relation to the amp's 100K Zin.

BTW, calculating the resistance of a parallel combination of two resistances is easy. It's just the product (multiplication) of the two numbers, divided by their sum.
If more than two resistances are in parallel, it's a little more difficult, the result being equal to the reciprocal of the sum of the reciprocals of the individual resistance numbers. (Reciprocal = the number divided into 1).

Best regards,
-- Al

I think I answered my own question. Using the link below, a 25k pot and 100k amp Zin equals 20k.

Parallel Resistor Calculator

Looking at this from Roger Modjeski's pot-in-the-box concept, which he recommends for the RM-10 MkII, and I have seen him use with the RM-200, the specifications are as follows:

Input Impedance 50K Ohms
Output Impedance 0 - 25K Ohms
Frequency Response DC - 100K Hz

Do I assume correctly that the Noble pots are the 25k version? What does this do in my case where Zout of source is 50 ohms and Zin of amp is 100k ohms.

At full volume (worst case) the combination of amp Zin and passive should be at least 10X source Zout but let's go with 100X. Pubul's 50 ohm source should see at least 5K so a 10K passive looks to be a great match. A 10K passive in parallel with the 100K amp is 9K.

My source is 50 ohms Zout and the amp is 100k ohms Zin. The passive I just built uses transformers to step up the signal from the source. These transformers are wound for sources that are ideally 100 ohms Zout or less (but could go as high as 300 ohms). Also, the sources must be opamp driven, as many CD/DACs are, not transformer driven. The passive also uses 10k Alps pots.

So basically, the 9k number listed above would apply giving me about a 200:1 ratio. Also, and I think that I have read this above, just want to make sure I understand, with a 10k pot the output impedance rises as the knob is rotated clockwise until it reaches 10k. Did I get this right?

I too wish like AA pictures can be posted here. If you would like to email me offline with one that would be fine.

...if the impedances and cable capacitance are right, so that a transformer doesn't really solve a problem, wouldn't a resistor based passive, like my Goldpoint, perform better than a TVC, especially in terms of bandwidth?

Theoretically it should and that is what I found in my comparison of the Slagle autoformers and Lightspeed attenuator. However, something about the sound of a transformer or autoformer type passive is still very appealing.

Yes, that strikes me as a good way to look at it, Herman.

I notice that at this page of the Goldpoint site the following statement is made, which I believe is incorrect and misleading:

When choosing the stepped attenuator value for an in-line level control or "passive preamp" (such as the Goldpoint Level Control Boxes), the attenuator value is chosen to match the input impedance of whatever it will be controlling. example: If the amplified monitor speakers or power amplifier you will connect the output of your your passive preamp to has an input impedance of 20K, then order a 20K stepped attenuator for that application.

Although they then qualify that with this statement:

You can usually use a level control value which is LESS than the rated input impedance of the gear it will be controlling.... 25K is usually a good choice for both vacuum tube and solid-state equipment

Best regards,
-- Al

Can't argue with your math but I think there is a simpler way to look at it, something I should have brought up before. You need to consider 2 factors.

To make a match first look at ratio of source Zout to amp Zin. Convention says 1:10 minimum, I say closer to 1:100 is much better. In Pubul's case with 50 Zout and 100k Zin he is way above minimum at 1:2000.

Then pick a passive as low as you can get away with. At full volume (worst case) the combination of amp Zin and passive should be at least 10X source Zout but let's go with 100X. Pubul's 50 ohm source should see at least 5K so a 10K passive looks to be a great match. A 10K passive in parallel with the 100K amp is 9K.

For the reasons your math points out going with a passive much greater than required can cause problems just like going too low with too low being worse. That's why you see Placette building 9K passives. In Pubul's case many would argue a 50K pot is too high.

.

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so 2 meters of Cardad Golden Reference 12pf/ft should be no problem for the passive approach, with with a 2 meter IC?

Yes, that is excellent, due to the very low cable capacitance per unit length. Xc calculates to 101K at 20 kHz for the 2 meter length, which I calculate will result in a roll-off of 0.066 db at 20kHz, in combination with the worst case source impedance of 12.5K. In other words, completely negligible.

Best regards,
-- Al

so 2 meters of Cardad Golden Reference 12pf/ft should be no problem for the passive approach, with with a 2 meter IC?

Pubul57: Does the 50kohm of my attenuator address that issue, or does it cause problems on the other end into my 100kohm amp load?

Herman's comments accurately addressed the impedance seen by the cdp looking into the resistive volume control, which as he explained is dependent on the rvc, it's setting, and the amplifier input impedance.

A separate issue, which I think is what the quoted question is addressing, is the relation between amplifier input impedance and the output impedance seen looking back from the amplifier into the output of the rvc. That will be dependent on the rvc, it's setting, and the source component's output impedance (if it is high enough to be significant).

With the volume control turned all the way down, the impedance looking into the output of the rvc (I'll call it Zo) will be essentially 0. With the volume control all the way up, it will be the parallel combination of the volume control's end-to-end resistance (in this case 50K) and the output impedance of the source component. If the source component has a low output impedance, such as 50 ohms, then the impedance of the combination will be essentially the same as the source component's output impedance.

Zo will be at a maximum when the volume control is set to the mid-point of its resistance range (which is NOT the 12 o'clock position; it will be just a few steps down from the maximum volume position). At that setting Zo in this case, where the rvc has an end-to-end resistance of 50K, equals the parallel combination of 25K with (25K + 50 ohms), or about 12.5K.

The ratio of your amp's input impedance (100K) to the 12.5K worst case output impedance that is driving it is 8:1. That is slightly short of the so-called 10x rule that you are no doubt familiar with, but considering how close it comes to satisfying that rule, and that the ratio will be better at any other volume control setting, that all seems comfortable.

The other criterion that Zo should satisfy is that it should be considerably lower than the capacitive reactance (which is an impedance, measured in ohms) of the cable that connects the rvc to the amp, at the worst case frequency within the audio band, which is 20kHz.

That number can be calculated by taking the capacitance per foot of the cable, multiplying by the number of feet, and plugging into the formula Xc (capacitive reactance) = 1/(2 x pi x f x C), where f is 20,000 and C is capacitance in farads. The resulting Xc will be in units of ohms.

For example, a 5 foot cable having a low capacitance of 20 pf (picofarads) per foot results in Xc = 1/(2 x 3.14 x 20000 x 5 x 20exp-12) = 79.6K, which would be a satisfactory result in relation to Zo = 12.5K.

Best regards,
-- Al

Only to the extent that they offer pots with different ratings. Goldpoint has 10, 20, 25, 50, and 100kohm attenuators, and they'll recommend the right one for your system source and amp.

Do manufacturers of passive preamps and attenuators assist in matching?

Clio, I wish they would allow pictures so I could post a drawing, but you are looking at a series-parallel circuit with the Zin of the amp in parallel with part of the passive. When the volume is all of the way up the whole passive is in parallel with the amp Zin so you use the math for parallel resistors. In my example they were both 10K so it is 1/2 of the 10K. With the volume close to zero it rises to about 10K. and falls as the volume goes up.

Pubul, yes, on paper the resistor is better but many ears say differently, If you look at the results with test equipment in the audio band it is pretty much a wash anyway. What measures better does not always sound better.

.

I once did have the Placette RVC and then tried the Placette Active, and what I notice was more articulate bass, I now wonder if the issue was the the RVC simply has too low an input impedance to be optimal in my setup - but it sure was transparent.

Herman, if the impedances and cable capacitance are right, so that a tranformer doesn't really solve a problem, wouldn't a resistor based passive, like my Goldpoint, perform better than a TVC, especially in terms of bandwidth? Anthony?

Many passives have a Zin of 10K or less and this is further reduce by the Zin of the amp. At full volume they are in parallel so a 10K passive and a 10K amp would result in a 5K load for the pre. That is problematic for many CD players and phono stages no matter how much voltage they can generate with no load applied.

Reviewing my response I see that my point was not clearly stated. What I meant was that in your example it would be obvious to me based on my own personal criteria that an amp with 10k input impedance would not be a good match for a passive. I'm not saying that you a stating otherwise. I understand you were just using the numbers to illustrate an example, which is of interest to me and why I was wondering about the formula you used to come up with the calculation.

hmmmm, can't find where I said 10K was a good match. Which original example are you referring to?

.

Herman,

What formula are you using for the calculations.

From your original example, I would have never thought an amp with !0k in put impedance to be a good match for a passive. More like 100k ohm in my mind.