Compatibility of Dynaco PAS with VTA ST70, Subwoofers, and other power amps -- INPUT IMPEDANCE discussion

peterh wrote:
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Oh, that was a misunderstanding. I have a vta70, between the pas and vta is my CF (cathode follower).
The FET device will be tested instead of the CF. So will a 5670-based board from aliexpress.
My CF is more then CF, it also contains a remote volume and a power sequencer for the vta70 and
in addition a second CF that buffers tape-out connected to a MAC soundcard.
it's unlikely that this box will be replaced.

Sounds like fun. If I decide I really like the PAS sound, I may get that FET buffer to make a second output RCA pair specifically for driving an optional subwoofer. Then I would adjust the PAS resistors to best match the VTA70 on the existing outputs.

Incidentally, How did you settle on the PAS? What do you like/dislike about it? Did you have an opportunity to compare it with any other preamps?

One resistor at less than a buck vs. anything else.

Peter W. wrote:One resistor at less than a buck vs. anything else.

Agreed. For any given input impedance of power amp > 51.7 K ohms a single resistor can be chosen to properly match the PAS output to that amp.

I was thinking I might do this for the input to the VTA70, and then use the FET buffer for the input to the subwoofer which has around a 10K input impedance.

I tried an experiment last night in which I hooked up a buffer to drive the subwoofer and it made a significant difference in the sound.

For the buffer I used one of these Schiit Loki tone control devices. Good stuff.

http://www.schiit.com/products/loki

Normally it is used for EQ, but with all controls centered it makes a nice unity gain buffer. Its input impedance measures 50K. Not ideal but better than my 10K sub.

I split the PAS 3X outputs using Y adapters and sent half to the VTA70 and the other half to the Loki, which then went to the sub.

The Loki has a switch that bypasses the unit entirely using an internal relay. Perfect!

I turned the sub to mute for first test. So listening only to main speakers.

All PAS tone controls centered (bypassed)

Results:
With Loki active (sub is buffered), music sounds great.
With Loki bypassed (sub is not buffered), music sounds not so great. Lacking punch/definition is best I can describe it.

So essentially what I am hearing here is the difference between a 10K load and a 50K load on the PAS when tone controls are out of the circuit.

The X tone controls were in their center (defeated) positions. It was very easy to find the defeat positions while listening, as there is a very distinct position where the load has far less effect on the sound. This is the whole purpose of their design, of course. Just nice to see it in action.

If I rotate the bass knobs even just a little counterclockwise to move the control off of bypass, the difference between the 10K and 50K load is huge! In addition to the lack of punch mentioned above, there is also a significant loss of bass with the unbuffered 10K load.

Conclusion:
(1) The PAS X tone controls work very well at bypassing the tone circuits, and in this position there is far less sensitivity to output load. i.e., they work exactly as expected.
(2) Even when the X tone controls are in bypass, the PAS output sound quality still degrades slightly when a 10K load is applied (subwoofer amp). It is not good enough to use under these conditions in my opinion.
(3) A 50K load sounds much better. I did not yet thoroughly compare the degree of further improvement going from 50K to the intended 500K, but from first impressions it is far less than the improvement moving from 10K to 50K.

So I suppose the take away here for future readers is this: Be very careful using a Y adapter to connect a subwoofer to a PAS amplifier *even* if you remove the tone controls. If the sub input impedance is very low you may loose fidelity from your main speakers.

EDIT: I can't tell from looking at the schematic why a low 10K Ohm load would reduce fidelity. Maybe someone else can explain this.

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Yeah, I read that Audio Regenesis study too, and the numbers do seem to say it should be ok to drive loads at least down to 20KOhm.

http://audioregenesis.com/documents/PAS_Line.pdf

(I assume this is with tone controls bypassed or removed). I cannot explain why my experience is so different with a 10K load. I can tell you that what I heard did not seem to be merely a lack of bass. Something was not "healthy" sounding about the signal. The overall volume was slightly reduced, and it seemed to lack punch even if I attempted to compensate for the drop in volume. Perhaps this is an example of the futility of evaluating preamps by measurements we have been discussing? Dave's measured frequency response says all is well, but it doesn't sound well to my ears.

Also, I cannot say for certain that the sub input impedance is 10K. I cannot find any specs for it. (It is a Klipsch SW311). I measured the 10K using a LCR meter at 100Hz and 1KHz while it was on to get my number. Maybe it is actually much lower?

Whatever the case, it is taxing the amp into not-good-fidelity land by some mechanism.

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rjpjnk wrote:Yeah, I read that Audio Regenesis study too, and the numbers do seem to say it should be ok to drive loads at least down to 20KOhm.

http://audioregenesis.com/documents/PAS_Line.pdf

(I assume this is with tone controls bypassed or removed). I cannot explain why my experience is so different with a 10K load. I can tell you that what I heard did not seem to be merely a lack of bass. Something was not "healthy" sounding about the signal. The overall volume was slightly reduced, and it seemed to lack punch even if I attempted to compensate for the drop in volume. Perhaps this is an example of the futility of evaluating preamps by measurements we have been discussing? Dave's measured frequency response says all is well, but it doesn't sound well to my ears.

Also, I cannot say for certain that the sub input impedance is 10K. I cannot find any specs for it. (It is a Klipsch SW311). I measured the 10K using a LCR meter at 100Hz and 1KHz while it was on to get my number. Maybe it is actually much lower?

Whatever the case, it is taxing the amp into not-good-fidelity land by some mechanism.

When the tubes is loaded to high they will start to distort as the feedback no longer can
compensate for errors.

And even if PAS3X can drive 50k with the tonecontrols centered i cannot hope for nice sound
when tonecontrols are off center ( which is the whole purpose of them )
A buffer will solve this ( but now i am repeating myself )

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peterh wrote:
...A buffer will solve this ( but now i am repeating myself )

I agree. The purpose of my posting my experimental results is to underscore this point.
Thank you for suggesting the buffer!

On my PAS3, I had the Dynaco Doctor Joe Curcio add a cathode follower.  Then I used a Energy EAC active crossover. Worked Great.

Here's the pictures. As you see he used a small onboard transformer & redid the power supply. The CF used a 12AT7, I used a 6201 tube in mine.

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PeterCapo wrote:

rjpjnk wrote:
...
http://www.schiit.com/products/loki
...
So essentially what I am hearing here is the difference between a 10K load and a 50K load on the PAS when tone controls are out of the circuit.
...
EDIT: I can't tell from looking at the schematic why a low 10K Ohm load would reduce fidelity. Maybe someone else can explain this.

This has got my interest.  The Loki's Z-IN specified on the website is 47KΩ.  47KΩ || Latino amp 75KΩ ≈ 28.9KΩ, correct?  And, a 10KΩ subwoofer || Latino amp 75KΩ ≈ 8.8KΩ.  So, at the risk of splitting hairs, and if my math is right, then you're hearing the difference between ≈ 8.8KΩ load vs. ≈ 28.9KΩ load.  On the face of it, it seems like kind of a narrow range for the PAS to have such a serious transition from sounding great to lackluster, but what do I know?  Still musing, though, on the role of the current divider the Y-connector sets up when the load drops into the range of ≈ 10KΩ.  PAS might still do fine driving just one input as low as ≈ 10KΩ, I don't know.

Interesting discussion.  If I can find out anything else, I'll report back.

You figures are correct for total load. I didn't include the 75K VTA my values. One thing to keep in mind is we do not know for sure the input impedance of the subwoofer. It measured around 10K with LCR, but I'm not sure if this is a valid way to determine input impedance.

I agree it seems suspicious that a load change from 28,9K to 8.8K *with tone controls bypassed* would be sufficient to cause a noticeable decrease in audio quality. I will repeat the test to confirm.

Thanks for looking into this. I look forward to hearing anything you may find.

I'll toss in my two cents worth.  Maybe too lacking in real EE knowledge.  

I measured my VTA ST70 with my DMM and the resistance is 231k Ohms assuming there aren't any reactive impedances involved.  But with 231k Ohms paralleled with 10k Ohms of your subwoofer amp, the PAS 3x would be seeing something like 9.5 k Ohms into the sub and basically an open into the ST70.

Have you tried powering the sub amp with the speaker-level inputs?

dave

Dave, I assume based on your measurement that your VTA ST70 does not have the optional stepped attenuator in place. The VTA itself has either 230 or 280 K internal impedance, but the stepped attenuators are typically 100K in parallel with this, which brings the overall input impedance down to the 75K number we were discussing (actually 69-73).

The PAS develops its output voltage across whatever total load is connected, and both the subwoofer and the VTA70 see this same signal.

My sub does not have speaker level inputs. I could make some adapters to make this work though. I may give that a try.

Might I suggest a simple integrated amp for the sub-woofers? Driven by the tape-output of the PAS? This allows for the subs to be entirely independent of the mid/highs. Unless you are constantly fiddling with the volume, once you make the (relative) settings, you can then adjust to any level of bass you wish.

Keeping in mind that the subs are not the biggest users of energy in the system, and that the 70 is a rather modest amp in any case, you really do not need any sort of massive amp - an SCA35 would do nicely.

rjpjnk, when you build your SP12 make sure you include a 2nd pair of outputs. I would think it would be able to drive all of your amps.

Yes, that second pair of outputs will be perfect for this application.

Peter W. wrote:Might I suggest a simple integrated amp for the sub-woofers? Driven by the tape-output of the PAS? This allows for the subs to be entirely independent of the mid/highs. Unless you are constantly fiddling with the volume, once you make the (relative) settings, you can then adjust to any level of bass you wish.

Keeping in mind that the subs are not the biggest users of energy in the system, and that the 70 is a rather modest amp in any case, you really do not need any sort of massive amp - an SCA35 would do nicely.

Yes, that would definitely work as long as the integrated amp has a high enough input impedance. Ideally though, I would prefer to have the sub and mains controlled by a single volume control.

At this point, I'm not even sure I like the subwoofer in the mix, so this discussion is more of a "what if" science experiment just for fun, and an opportunity to discuss the PAS's peculiar output impedance needs.

If I do decide to use the sub permanently I like peterh's idea of adding the small FET buffer inside the PAS best so far. SInce the sub amp is already solid state how could a few FETs mess it up?

rjpjnk wrote:Dave, I assume based on your measurement that your VTA ST70 does not have the optional stepped attenuator in place. The VTA itself has either 230 or 280 K internal impedance, but the stepped attenuators are typically 100K in parallel with this, which brings the overall input impedance down to the 75K number we were discussing (actually 69-73).

The PAS develops its output voltage across whatever total load is connected, and both the subwoofer and the VTA70 see this same signal.

My sub does not have speaker level inputs. I could make some adapters to make this work though. I may give that a try.

If you go for an external buffer, you might install a volume pot and 2 rca outlets
for your subwoofer. The other output goes to the vta.
The pot might even be an adjustment pot as you don't need to adjust it once it
has been adjusted.
What remains is how to powercontrol your buffer, but if you have a original PAS
you also have switched outlets at the back of it. Alternatively if you already
power on your vta with a powerbar, then the buffer can be connected there too.
( i do not recommend to power the vta from the PAS switched outlet, the power
switch is ( in my opinion) not good enough)

Wima caps and ceramic sockets are fine, but -- ouch!--passing a line level or phono signal through those "tin" RCA jacks is bad juju. Time to order Kevin's rear panel kit and some shielded wire!

j beede wrote:Wima caps and ceramic sockets are fine, but -- ouch!--passing a line level or phono signal through those "tin" RCA jacks is bad juju. Time to order Kevin's rear panel kit and some shielded wire!

That was done a long time ago. Actually it was given the complete VTA upgrade, 12X4 rectifier tube power supply & all.  This happened just before Holger took over the PAS part of the VTA biz.
It had the ZMOD board in it 1st, then I missed the tone controls, so it has the TCLA board in it now.

The ZMOD board got stuffed into a PAT-4 chassis with the old PA211 power transformer. Roy hooked me up with a PAT-4 gold I-O board (last one) for that pre & shared with me the Front Panel Express template so I could make it look nice.

We ran that until we got the DS-2 version SP-14.

"j beede"]Wima caps and ceramic sockets are fine, but -- ouch!--passing a line level or phono signal through those "tin" RCA jacks is bad juju. Time to order Kevin's rear panel kit and some shielded wire!

Why?  

a) Tin plated bronze happens to offer better contact conditions than hard gold plating, if kept clean.
b) The only advantage gold has over any other metal is its resistance to corrosion. However, poorly plated items can actually be far worse than those same components unplated.
c) One should re-make ones connections - of all sorts - once per year in any case. It is just good practice.
d) Shielding has its uses - but is not needed everywhere. And, a twisted pair, shield-pair grounded, is as good as shielded wire for the application shown. And with less handling difficulty.
e) And some of the replacement banks are "some assembly required" - each of which is a soldering point with an opportunity to fail.

All and at the same time, the OEM connector banks on any of David Hafler's equipment all came from the lowest bidder, and Hafler was known to dive pretty deep to find them. So, replacement with more rugged options is not a bad thing if one is so-minded and there is enough wear to justify the work. Just that gold is neither a necessary nor a sufficient reason to do so.

Yes, that would definitely work as long as the integrated amp has a high enough input impedance. Ideally though, I would prefer to have the sub and mains controlled by a single volume control.

Given that each source is slightly different, and each engineer has a different idea of how to mix down to two channels, your idea of a single VC to both is apt once you have balanced to *that* source. However, I suspect that much, if not all, of your uncertainty of the sub-woofer mix is because the sub(s) are not optimized to the rest of the signal. And, that is the idea of using an integrated amp. To achieve that optimization over a fixed relationship. Listening under those (un-optimized) conditions will lead to a level of listener-fatigue as well.

And, irrespective of how the subs are powered, do be mindful of phasing - and experiment along those lines.

Any option taken eliminates other options by necessity. As with all such things, I generally suggest the most open-ended choices involving the least number of limiting decisions.

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