The Sonic Role of a Preamplifier

[Msegal]

Originally Posted by parkcaka

2 words. Impedance matching.

+1

[trponhunter]

Here is an interesting test for a pre amp if you have an open reel tape deck with a variable output. Play the tape deck directly to the amplifier, then insert a pre amp between the tape deck and the amp. Listen to the loss of resolution that the pre amp will introduce. Compare 2 pre amps against each other in this capacity to see which one changes the sound of the tape deck least - that's the winner.

[cma29]

Originally Posted by ylee

For most of my audiophile life, I viewed preamplifiers as complex switching devices that allowed easy selection of source components with minimal degradation of the source's signal. About a decade ago, my belief in this principle led me to develop an interest in passive preamplifiers that by design had minimal impact on the sonic character of the source components. I bought a Placette passive preamp which did an excellent job save for its susceptibility to EMI. The noise of this particular passive preamp led me to buy an active one - a Classe CP-800, which seemed about as neutral as any preamp I've ever heard in that it added nothing and subtracted nothing. In recent years, I noted with interest as reviewers observed that better preamplifiers over the last decade not only avoided degrading the source signal, but somehow improved the sonic qualities where tonality, soundstaging, and even resolution were unambiguously improved. Examples of such preamps I read about include Ayre's KX-R and KX-R Twenty, PS Audio's BHK Preamp, and Audio Research's Ref 6. Based on my own experience in recent days, I would describe the Rogue Audio RP-7 as having this quality as well.

There is a running debate about how a preamp improves on a source's signal. How is such an effect possible? Does the preamp add something that wasn't in the recording? Does it somehow unpack the source's signal in a way that an amplifier can't? Are tubes especially good at this? Perhaps not with Ayre and PassLabs preamps as examples.

My Totaldac d1-twelve SE DAC is regarded as one of the best digital sources in the world. Without a preamp, it's sound is musical and extremely well detailed. It has a buffered output which can drive my system without a preamp. But with the RP-7 in place, the soundstage depth is improved and individual instruments and voices have a greater sense of dimensionality/body. Because of this, individual voices and instruments are somehow easier to follow in the midst of complex counterpoint, which symphonic and operatic music have plenty of. As good as my system sounds without a preamp, it sounds even better with the RP-7.

Perhaps engineers will be able to explain just what good preamps do to enhance a hi-fi system’s sound quality. There may be definitive explanations about this phenomenon already and I’m simply unaware of it. But this topic to me is one of the more fascinating aspects of our hobby that I would love to see explained in engineering (measurement) terms.

Nice topic of discussion, Yong.

I've thought about these issues as well and that's how I discovered the passive preamplifier I use in my system. Here is the principle behind its design:

The unit's autoformers keep signal power (from the DAC) constant as the gain to the amp is attenuated. So as the voltage to drive the amps goes down, the current increases and this works to effectively lower the output impedance of the preamp to maintain the integrity of the low-level signal that drives the amplifiers. Most volume controls are resistive; when you turn down the volume a proportion of the voltage and current is thrown away along with dynamic contrasts.

I wrote about it here.

[ylee]

Nice write-up of the AVC-1 Carlos. I can attest in having visited your home that your system is transparent and beautiful sounding. The former having a lot to do with your passive preamp and the latter probably more because of the Pass amplifiers.

My DAC has a 69-bit digital volume control (implemented via FPGA I believe). I don't know how lossy it is when attenuating the signal (turning down the volume). Given the price of the AVC-1, especially when bought as a DIY kit, it's cheap enough I may buy it just to play with it. That said, it's some of the other properties/behaviors of a preamp I speak of in regard to musicality. Transparency is an interesting notion, with people defining it in a variety of ways. Is it emphasis on leading edge transients or tonality/bloom/body of what follows the onset of the note/sound one hears? Is it the reverb/echoes of the studio and concert hall? Is it the richness of timbre? For me it's a balance of all of the above. My new preamp gives me more information in terms of tonality and reverb. The more balanced sound with the active preamp can make the leading edge of notes sound not quite as accentuated, but that doesn't mean that audible information isn't there. It's just there is more of everything that follows that instance in time that is present to a greater degree.

I find it interesting that a reputable and well-respected designer such as Bascom King say he can't definitively explain why his BHK components sound better with the preamp than without with his DAC driving his amps directly. Did he engineer it that way just to make more money? Maybe. But this phenomenon has been observed by more than one reviewer with various other brands. And I can state that I am certain I'm hearing more musical information in my recordings with my Rogue preamp than without, and likewise compared to my previous active linestage - the Classe CP-800. I'm also fairly sure it goes beyond impedance matching given my experience with the Placette even though I agree that is part of the equation of what a good preamp brings to the table.

At the end of the day, I always believe there is a scientific explanation behind everything we experience. Sometimes science lags behind engineering (the Romans built bridges that have stood for 2 millennia long before equations were formulated that describes their ability to carry load). I think some day physicists and engineers will be able to explain why a Stradivarius sounds the way it does or why a preamplifier improves the sound of a stereo system. Until then, it's fun to debate and argue it out

p.s. - I just checked Bent Audio's website and he's evidently been so busy with OEM work that he's not selling DIY or fully built units.

[doggiehowser]

Ayre has implemented one of the most interesting volume controls with the VGT.

It addresses one of the challenges an analog preamp has - that at low levels they lack dynamics and punch and life.

From what I gather - the problem stems from using a variable resistor to control volume which has varying impedance presented. The difference is very clear to me in some of my combinations - a high gain EMM XDS1 player with a high gain Plinius SA Reference power amp makes my ARC Ref5SE quite unusable - pumping out almost full volume at less than 10 on the volume dial. Even with my PS Audio DS DAC, I’ve found a big improvement going to low gain on the DAC and using the Ref5SE at higher levels but maintaining the same SPL.

Ayre uses a variable gain transistor which maintains the same impedance throughout.

So I was really pleased with the BHK Signature Pre which uses tube gain and variable gain to achieve a similar result.

[cma29]

Originally Posted by ylee

Nice write-up of the AVC-1 Carlos. I can attest in having visited your home that your system is transparent and beautiful sounding. The former having a lot to do with your passive preamp and the latter probably more because of the Pass amplifiers.

My DAC has a 69-bit digital volume control (implemented via FPGA I believe). I don't know how lossy it is when attenuating the signal (turning down the volume). Given the price of the AVC-1, especially when bought as a DIY kit, it's cheap enough I may buy it just to play with it. That said, it's some of the other properties/behaviors of a preamp I speak of in regard to musicality. Transparency is an interesting notion, with people defining it in a variety of ways. Is it emphasis on leading edge transients or tonality/bloom/body of what follows the onset of the note/sound one hears? Is it the reverb/echoes of the studio and concert hall? Is it the richness of timbre? For me it's a balance of all of the above. My new preamp gives me more information in terms of tonality and reverb. The more balanced sound with the active preamp can make the leading edge of notes sound not quite as accentuated, but that doesn't mean that audible information isn't there. It's just there is more of everything that follows that instance in time that is present to a greater degree.

I find it interesting that a reputable and well-respected designer such as Bascom King say he can't definitively explain why his BHK components sound better with the preamp than without with his DAC driving his amps directly. Did he engineer it that way just to make more money? Maybe. But this phenomenon has been observed by more than one reviewer with various other brands. And I can state that I am certain I'm hearing more musical information in my recordings with my Rogue preamp than without, and likewise compared to my previous active linestage - the Classe CP-800. I'm also fairly sure it goes beyond impedance matching given my experience with the Placette even though I agree that is part of the equation of what a good preamp brings to the table.

At the end of the day, I always believe there is a scientific explanation behind everything we experience. Sometimes science lags behind engineering (the Romans built bridges that have stood for 2 millennia long before equations were formulated that describes their ability to carry load). I think some day physicists and engineers will be able to explain why a Stradivarius sounds the way it does or why a preamplifier improves the sound of a stereo system. Until then, it's fun to debate and argue it out

p.s. - I just checked Bent Audio's website and he's evidently been so busy with OEM work that he's not selling DIY or fully built units.

Thanks, Yong.

I noticed that John Chapman is no longer supporting the production of the AVC-1 because he's doing so much work for others. I think his main customer is Vinnie Rossi.



I see a resemblance between this preamp and the the AVC-1.

[crwilli]

I appreciate the question was posed as the ‘sonic’ benefits of a PreAmp but it should be stated that switching sources is one of the most important functions of a PreAmp.

[ylee]

Originally Posted by crwilli

I appreciate the question was posed as the ‘sonic’ benefits of a PreAmp but it should be stated that switching sources is one of the most important functions of a PreAmp.

Definitely. My acquiring a preamp was in part motivated by the desire to add a phono preamp in the future along with a turntable. Other digital sources of interest may be added too as the years ago by. I may also have my dad's reel-to-reel purchased in 1977 repaired so I can listen to it from time to time.

[Karl Maga]

Originally Posted by trponhunter

Theoretical versus practical when talking about the digital resolution loss versus the analog loss when using a pre amp. From my experience, the loss introduced by the analog pre amp is much greater. As I mentioned, I have done the direct a to b comparison ( in the same room, at the same time, on the same system, at the same volume) for over 15 years - done at a minimum of 30 times (most likely many more times than that). I have also yet to find anyone I have done the comparison for or have a different opinion as to the outcome. If using a proper recording that is simply miked and not processed - I have yet to find a pre amp addition improve things. I have used pre amps up to $30,000 when doing these comparisons. The most you can hope for is as little change as possibly when you insert the pre amp into the signal path. I know there are a lot of people who will disagree with these statements, but they are my opinion based on the comparisons I have experienced - not a theory of what will happen.

While the number of iterations in my experience is far less than yours, I’ve found the exact opposite experience; my Mark Levinson No. 523 is far superior at attenuation of my Bel Canto 2.7 DAC signal than the Bel Canto is. It’s not even close.

I do believe you about the results of your comparisons, and it’s intriguing that we have had the opposite results.

I bought my ML No. 523 for source switching and HT Bypass, the preference for its sound was unexpected.

[trponhunter]

Originally Posted by Karl Maga

While the number of iterations in my experience is far less than yours, I’ve found the exact opposite experience; my Mark Levinson No. 523 is far superior at attenuation of my Bel Canto 2.7 DAC signal than the Bel Canto is. It’s not even close.

I do believe you about the results of your comparisons, and it’s intriguing that we have had the opposite results.

I bought my ML No. 523 for source switching and HT Bypass, the preference for its sound was unexpected.

I also find it interesting as to why you and others have not had the same experience I have when eliminating the pre amp. I have a few ideas , but cannot be sure unless we were both listening together to the same system at the same time. Here's a few ideas:
1) not all digital gain stages are identical - some are better than others
2) the interface between gain stage, amplifier and speaker sensitivity needs to work. Generally going much below-18db or so can lead to enough digital loss that it may become an issue.
3) When I do these comparisons, I do not use multi miked, studio type recordings. If these are used, it becomes much more difficult to determine better versus preferences. I think this is a very big point - if I tried to make decisions on less than great recordings it becomes much more difficult or even can lead to different conclusions. Also, I don't generally agree that it all comes down to preference. Some things are simply better than others, and when talking about a pre amp - it is simply which one changes the sound of the source less.
4) The winner on what you would consider your best, most truthful recording is the winner. I listen for information retrieval, image specificity, volume of recorded venue and micro and macro dynamics. The winner on the very best minimally miked recordings is the winner - even if i prefer the alternative with different recordings.
5) I am listening solely for "truthfulness" to the source - not a preference. Of course, this is still to some degree subjective, but I have never had anyone else listening with me at the same time come to contrary conclusions - better is better - less is more.Generally, I find the elimination of the pre amp simply puts less between you and the music.

Of course I know many others may not agree or have had the same experiences that I have had. If so, I would suggest listening or doing the comparison as highlighted above if you are interested - if not - no issues - we all need to be happy at the end of the day!