On the use of a preamp to improve sound quality with VTA70

Peter W. wrote:

By having the capacity to overdrive the amp, you now have the capacity to utilize it to its fullest extent. Without this capacity, you will not realize transients and/or the lowest signal may be below the noise floor. It is this last issue that contributes to the belief that pre-amps add color. Good ones do not.

Technical enough?

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Yes, this is technical, but also irrelevant. As I tried to explain in our PM sessions, this suggestion of daisy chaining preamp and power amp merely to get more overall gain is essentially no different that just buying a larger single amp in the first place. There is no change to the dynamic range. I am looking for a benefit of a preamp *other* than just making the power amp louder.

We are talking about input signals already at line level here.

rjpjnk, please don't make the mistake of placing engineering theory above listening. Many of us have been doing this stuff for a very long time, and have generally concluded the theory is a good starting point, but nowhere near the entire answer.

audiobill wrote:rjpjnk, please don't make the mistake of placing engineering theory above listening.  Many of us have been doing this stuff for a very long time, and have generally concluded the theory is a good starting point, but nowhere near the entire answer.

Put another way, the "entire answer" will have a firm basis in engineering theory, but selective use of engineering theory will always interfere with the "entire answer".

I'm just checking in now for the first time in about a month due to my move from the Seattle area to Las Vegas area (retiring to warmer (HOT) weather).
Anyway, the posts by AudioBill and Monkuboy on July 21 echo my own experience.   It seems that if you have enough gain, that passive is the way to go,
but my music has ALWAYS sounded MUCH better using an active preamp, in fact I think the preamp is THE most important component in the entire chain
(well, the source and the speakers are really) and the amp is the less important component.  Whether you need the gain or not, in my experience having
a preamp increases the dynamics of the sound, and is very much worth having no matter the price.

I suspect this topic is being underthought (if such a word exists).

Some random thoughts in no particular order:

-Clipping is easily detected when the input signal is of the form y=Asin(wt+φ). This time domain.
-The spectral content of an ideal sine wave is single frequency, there are no harmonics in the frequency domain.
-The spectral content of a clipped sine wave includes odd (and potentially even) harmonics.
-The spectral content of a transient or impulse includes multiple frequencies, the narrower the pulse, the wider the range of frequencies in the spectral content.

My personal experience with using a preamp versus "running straight in" surprised me. The result of an A/B comparison was that the preamp configuration sounded fine... but running straight in simply sounded wrong--in a subtractive sense. Everyone in the room, including me, expected running straight in with an ALPS stereo attenuator) to be better. Everyone in the room agreed that it wasn't even close. Running straight in was described as "broken".

Marantz CD63 (0dB)
PAS 2 preamp v ALPS 100KAX2
Dynaco MkIII
Quad ESL

Vocals and instruments sounded fine with or without the preamp. Transients (e.g. rim shots, knuckles rapping guitar soundboard, flamenco heel taps, non-standard percussion (ACopeland, et al) were not correctly re-produced when running straight in.

My hypothesis is that the spectral content will help explain the difference. My LCD 'scope does fast Fourier transforms. I will start looking for a programmable waveform generator to download.

I suggest you buy or borrow a preamp if you are without. Discussing this topic without having experienced familiar music played both ways is limiting.

j beede wrote:I suspect this topic is being underthought (if such a word exists).

Some random thoughts in no particular order:

-Clipping is easily detected when the input signal is of the form y=Asin(wt+φ). This time domain.
-The spectral content of an ideal sine wave is single frequency, there are no harmonics in the frequency domain.
-The spectral content of a clipped sine wave includes odd (and potentially even) harmonics.
-The spectral content of a transient or impulse includes multiple frequencies, the narrower the pulse, the wider the range of frequencies in the spectral content.

My personal experience with using a preamp versus "running straight in" surprised me. The result of an A/B comparison was that the preamp configuration sounded fine...  but running straight in simply sounded wrong--in a subtractive sense. Everyone in the room, including me, expected running straight in with an ALPS stereo attenuator) to be better. Everyone in the room agreed that it wasn't even close. Running straight in was described as "broken".

Marantz CD63 (0dB)
PAS 2 preamp v ALPS 100KAX2
Dynaco MkIII
Quad ESL

Vocals and instruments sounded fine with or without the preamp. Transients (e.g. rim shots, knuckles rapping guitar soundboard, flamenco heel taps, non-standard percussion (ACopeland, et al) were not correctly re-produced when running straight in.

My hypothesis is that the spectral content will help explain the difference. My LCD 'scope does fast Fourier transforms. I will start looking for a programmable waveform generator to download.

I suggest you buy or borrow a preamp if you are without. Discussing this topic without having experienced familiar music played both ways is limiting.

Thank you for this post jbeede. I was beginning to think I was the only one in the world seeking some manner of technical explanation for why premps improve sound. I hope you get some resolution in your spectral analysis approach. I look forward to reading about anything you discover.

Your experimental testing of pre vs direct sounds like exactly the kind of test I would do. I like that you approached it with the mentality that it has no reason to sound better, but it did. That makes the outcome all the more believable.

I wish I could borrow a preamp from a friend, but I don't know of anyone with a preamp locally. I am looking at a used PAS 3x now (about 500.00) or possibly the VTA SP9 (about 1000.00), but these both seem a little expensive just to "try". Still, I agree it is really the only way I can get the experience you describe of A/B-ing the direct vs preamp approach so that I can see for myself the magical difference these marvelous boxes can make.

I don't really mind if I never discover what makes the preamp improve the sound, but it would be more satisfying if I could. I suspect you feel similarly based on your experiments and obvious technical inclination. I suppose in a way it may be like asking for a technical explanation as to why one cello sounds better than another. There is nothing we can measure, it simply does. Or rather I should say, we do not understand what we should measure in order to discern the difference. But our ears tell us one is better.

How do you like the PAS2 you listed? Is it all stock? That seems like it might be the easiest first step to get in the game.

Thanks again.

Ok, I bought a PAS 3x. Let the fun begin

Now I need to read up on what sort of maintenance and mods need to be done before putting it into service I suppose. It appears to be in excellent condition.

peterh wrote:

rjpjnk wrote:Ok, I bought a PAS 3x. Let the fun begin

Now I need to read up on what sort of maintenance and mods need to be done before putting it into service I suppose. It appears to be in excellent condition.

Keep it original !  
If anything needs replacement then go for a power board, there is several of these that
replaces filament rectifiers, filament caps and B+ caps ( and if wanted 12x4 replaces with Si diodes)
The world is full of munged PAS , originals are more scarce !

See an analysis of PAS : http://www.audioregenesis.com/
it shows that at least a few of those "improvements" are actually destructive.

100% endorsement on the power-board. Taking the 12X4 off the transformer lightens its load, and that is one of the few weak-spots with this unit. You also get rid of the selenium diode, and add a fuse in most case.

It is also a good idea to verify that it is truly an X version - many have been called as such by virtue of having the aluminum faceplate and/or wishful sellers. X tone controls are neutral at the null position (essentially out of circuit). There is a gap in the element for the treble control and a dual-wiper for the base control. Verify this with a VOM. It is clear on the schematic.

100% endorsement on leaving the rest of it alone. I have been known to upgrade a cap or six, and verify resistor values, but that is about it.

100% endorsement for not leaving it alone. A stock PAS is a curious thing: crosstalk, odd volume taper, low qulaity switches/potentiometers, waxed circuit boards, and inadequate output buffering. The rear panel RCA jacks are seriously flawed. The good news is that these preamplifiers are easy to work on and parts are readily available. Kevin at Dynakitparts.com is your "friend".

The easiest way to determine whether it is a (stock) PAS 3 or PAS3X is to simply rotate each tone control away from the 12:00 position while the preamp is powered up and connected. Hear a pop or thump? It's a 3X. Another way to tell is that the 3X tone controls have about 240° of rotating range versus the PAS 3's ~300°.

It is definitely an X. The markings on the tone control pots match Dave Gilespie's pictures, and it has the 1uf caps in place.

http://www.audioregenesis.com/documents/Dynaco%20PAS%203%20or%203X.pdf

Sad to hear you dislike it so much jbeede. One of the reasons I chose the PAS was because it was what you used in your A/B testing described above. I want to repeat this same test to see that preamps make things sound "right". Was your PAS significantly modified?

I already know Kevin

Best advice is to sell the PAS asap and get one of Roy's SP designs - far, far superior imo.

j beede wrote:100% endorsement for not leaving it alone. A stock PAS is a curious thing: crosstalk, odd volume taper, low qulaity switches/potentiometers, waxed circuit boards, and inadequate output buffering. The rear panel RCA jacks are seriously flawed. The good news is that these preamplifiers are easy to work on and parts are readily available. Kevin at Dynakitparts.com is your "friend".

The easiest way to determine whether it is a (stock) PAS 3 or PAS3X is to simply rotate each tone control away from the 12:00 position while the preamp is powered up and connected. Hear a pop or thump? It's a 3X. Another way to tell is that the 3X tone controls have about 240° of rotating range versus the PAS 3's ~300°.

I have kept my 3X for about 10 years now. No such pops or thumps. And, it is an X.
There are a LOT of poorly maintained, bundged, badly assembled and otherwise spavined PAS amps out there that meet every critical reference as may be.

But, a couple of hours of basic housekeeping, cap replacement, resistor checking and control cleaning makes it as meek and mild a pre-amp as ever there is.

Wow, pretty awesome rebuild thread here. Looks so easy to work on and so many parts are readily available. He specifically addresses the crosstalk issue as due to proximity of unshielded wiring. I have the same green circuit boards fwiw.

https://www.audiocircle.com/index.php?topic=58900.0

audiobill wrote:Best advice is to sell the PAS asap and get one of Roy's SP designs - far, far superior imo.

Probably true. I'd love to have the SP9, but it is more than twice the price and doesn't have the resale potential PAS has.

To buy one of Roy's amps I have to first know I want to keep it.

This one is a test to see if preamps make a difference and to have some fun tinkering

I neither like nor dislike the PAS series of preamps. I was trying to be objective in describing the PAS's quirks in stock configuration. As long as I can remember people have been modifying these preamps... with or without the "help" of Trevor Lees, Frank Van Alstine, Joe Curcio, The Audio Amateur, etc. I suppose there is a reason for that.

If you re-review the list of concerns I shared earlier you may notice that none of them are necessarily showstoppers. Just avoid driving low input impedance amplifiers and don't connect more than one source at a time, for now.

Replacing/rewiring the rear panel jacks with Kevin's kit and replacing the nasty input selector with a modern switch is well worth doing in my opinion. I used shielded wire and routed it near the floor of the chassis. Part of the phono stage is tangled in that selector... once you start updating the PAS it can become a bit of a "hobby" if you know what I mean.

...j

Whats wrong with the input selector anyway ?
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Mostly, they are fragile if not kept clean and properly lubricated - and, how often do you think that really happens?

Otherwise, if the unit is properly assembled in the first place, and due provision is made for wire routing, there is nothing at all wrong with the input selector other than user-error.

I have been known to soak the phenolic wafers in ultra-thin superglue (Zap Brand), very carefully, to reduce the tendency for them to split down the middle. Repeat: VERY carefully. This is all part of the basic housekeeping.

Seems to be a lot of great advice out there on fixing these up. I like the one about taking the selector switch completely apart for detailed cleaning and lubricating and then reassembling with either shielded cables or by nice bundled twisted pairs.

Btw, what is the weird looking component I see in some pictures on the input selector around the second wafer? Some kind of old fashion capacitor? mica maybe? Some PAS don't seem to have these (like the one I just bought). Does that mean it has been replaced?

The weird component is  a cap/resistor combination made on a thick-film substrate. It is briefly
described in the manual, it was a way to fabricate components with low tolerances.
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AKA = PEC = Packaged Electrical Circuit.

There is a brisk aftermarket in these devices, as you will find out.

Taking the thing apart is playing with fire as it is genuine unobtanium. And as also noted the newer units do not short unused inputs.

Remember that even though the OEM was from CenterLab (the Yugo of controls-makers), it was designed to switch NAB-balanced Tape-Head inputs, as well as microphone inputs. Thereby being by necessity more robust then the replacement options.

peterh wrote:

Shielded cables is totally unnecessary as there is no ac fields inside the PAS. On the contrary
shielded cables increases the treble loss by their inherent capacitance.

From what I read the shielding or twisting is for input source crosstalk reduction, not protection from internal fields.

peterh wrote:
The weird component is  a cap/resistor combination made on a thick-film substrate. It is briefly
described in the manual, it was a way to fabricate components with low tolerances.

No, that is on the phono board. I am familiar with those RC packages. The thing I was referring to is a two-terminal device actually mounted on the input selector switch and turned out to be a 750pf capacitor in some unusual looking large flat package.  There are actually two of them. The assembly instructions refer to them as "precision" caps.

PAS after "housekeeping. 12x4 and "can cap" are no longer used but left in place for no particular reason. The selenium stack was not left in place as that space is occupied by the cap board.

I had been using Bottlehead preamps for years.  Many of their kits are great bargains and fun to build but their choice of tubes sometimes leaves a lot to be desired.  Plus I always thought resolution could be better.  

I almost went down the PAS road.  I looked at all of the various upgrade boards out there because I didn't want a stock PAS, I wanted great performance.  Ultimately I was able to build Roy's SP13 for around the same cost.  

Within seconds of playing music through the SP13 I heard way more detail with a bigger soundstage and knew I made the right choice.  Changing the output caps the Clarity CMR and a couple of the power supply caps to Clarity TC upped the resolution a few more notches.  I saved a little money building it into a relatively cheap aluminum Hammond box.


VTA SP13 tube preamplifier