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    PAS-2 tone control bypass

    billinrio
    billinrio

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    Post by billinrio on Wed Jul 01, 2020 2:52 pm

    I was never satisfied with the tone control performance of my PAS-2. On my system it did little for the high frequencies and
    made the lower ones tubby.  Therefore, I always left them "flat".  But since the PAS-2 isn't a PAS-3x the controls were never really out of
    the circuit. So, I decided to remove them.  Of the various instructions that I found online, I decided to go with the Audio Regenesis mod, since it
    includes using 1µF caps in order to prevent DC voltage at the outputs.  I just finished the mod and I'm stunned by the improvement
    in clarity and imaging. Moreover, both trebel and bass reproduction are greatly improved, and I now have more usable volume below 12:00 It's as if a veil has been removed from in front of my speakers. This mod is truly a no-brainer.

    PeterCapo, MechEngVic and Midwestside like this post

    Midwestside
    Midwestside

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    Post by Midwestside on Wed Jul 01, 2020 4:40 pm

    billinrio wrote:I was never satisfied with the tone control performance of my PAS-2. On my system it did little for the high frequencies and
    made the lower ones tubby.  Therefore, I always left them "flat".  But since the PAS-2 isn't a PAS-3x the controls were never really out of
    the circuit. So, I decided to remove them.  Of the various instructions that I found online, I decided to go with the Audio Regenesis mod, since it
    includes using 1µF caps in order to prevent DC voltage at the outputs.  I just finished the mod and I'm stunned by the improvement
    in clarity and imaging. Moreover, both trebel and bass reproduction are greatly improved, and I now have more usable volume below 12:00 It's as if a veil has been removed from in front of my speakers. This mod is truly a no-brainer.

    I did this to my Pas 3, it is indeed a big improvement.

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    PeterCapo
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    Post by PeterCapo on Wed Jul 01, 2020 4:41 pm

    Having followed along in Dynaco-related forum discussions for about twenty years, I can say that your experience of bypassing the PAS tone controls is not uncommon.

    For anyone in a similar situation, I think it's worth mentioning that getting the tone controls "flat" in the non-X PAS means orienting the knobs as instructed in the manual.  Specifically, flat response for the non-X tone controls is achieved when the knobs are installed so that there is a certain asymmetrical rotation relative to the center mark on the face-plate.

    The improvement in sound you hear is probably largely a result of bypassing what are old, worn-out electronic parts in the circuit.  It is doubtful that the PAS with tone controls in-circuit sounded that bad when it came off of the assembly line in the 1960s.  But if there are problems with the tone control pots presently, given their age and irreplaceability, bypassing the entire tone control circuit is probably the best option.

    Don't recall what you have for a power amplifier, but with your tone controls now bypassed, you should be able to drive low input impedance power amplifiers.  Just a few days ago I hooked my PAS (tone controls bypassed) back up to my Parasound Halo A21 with input impedance of 33KΩ.  They sound great together, and there's no problem with bass roll-off.

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    billinrio
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    Post by billinrio on Wed Jul 01, 2020 8:27 pm

    PeterCapo wrote:

    ..... Don't recall what you have for a power amplifier, but with your tone controls now bypassed, you should be able to drive low input impedance power amplifiers.  Just a few days ago I hooked my PAS (tone controls bypassed) back up to my Parasound Halo A21 with input impedance of 33KΩ.  They sound great together, and there's no problem with bass roll-off.

    In my office I use the PAS-2 with a pair of Heath W5M amps that I alternate once in a while with a pair of Dyna MKIV. I don't know what the input impedence is for these amps. They easily drive a pair of AR2ax speakers.
    PeterCapo
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    Post by PeterCapo on Wed Jul 01, 2020 9:24 pm

    Their input impedance is a good deal higher than the 33KΩ of my A21.  Point is, when the tone controls are bypassed, the PAS then has better drive capability into lower input impedance amplifier loads. IOW, the PAS can then be used more freely with solid state power amps.  You can do the same while retaining the tone controls, but you'd need to do a little calculation and adjust the value of a resistor at the PAS output, accordingly.
    billinrio
    billinrio

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    Post by billinrio on Wed Jul 01, 2020 9:44 pm

    Here in Brazil, there's a Parasound Halo A21 (black) being offered at an online auction site for US$2,500 + shipping.  Worth it?
    Midwestside
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    Post by Midwestside on Wed Jul 01, 2020 10:02 pm

    PeterCapo wrote:Having followed along in Dynaco-related forum discussions for about twenty years, I can say that your experience of bypassing the PAS tone controls is not uncommon.

    For anyone in a similar situation, I think it's worth mentioning that getting the tone controls "flat" in the non-X PAS means orienting the knobs as instructed in the manual.  Specifically, flat response for the non-X tone controls is achieved when the knobs are installed so that there is a certain asymmetrical rotation relative to the center mark on the face-plate.

    The improvement in sound you hear is probably largely a result of bypassing what are old, worn-out electronic parts in the circuit.  It is doubtful that the PAS with tone controls in-circuit sounded that bad when it came off of the assembly line in the 1960s.  But if there are problems with the tone control pots presently, given their age and irreplaceability, bypassing the entire tone control circuit is probably the best option.

    Don't recall what you have for a power amplifier, but with your tone controls now bypassed, you should be able to drive low input impedance power amplifiers.  Just a few days ago I hooked my PAS (tone controls bypassed) back up to my Parasound Halo A21 with input impedance of 33KΩ.  They sound great together, and there's no problem with bass roll-off.

    I actually got some Erhard audio clone boards and built them up. Cleaned my tone controls and it still sounded better bypassed. It still sounded good with the new components, but just seemed more opened up in every way with the controls bypassed. That being said, my tone controls still had the black cats connected. I had wondered about being able to switch between bypassed and regular with one of the unused switches.
    PeterCapo
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    Post by PeterCapo on Thu Jul 02, 2020 9:22 am

    Some thoughts...

    I would think that replacing the Black Cats with polypropylenes could make a significant difference, depending on the condition of the pots.

    Some research I have done indicates that cleaning vintage pots could make them worse off than if there hadn't been an attempt at cleaning them.  Even if they can be cleaned without being damaged, after decades of use the pots are probably not what they once were.  The carbon track wears thin.  They may have been compromised by being sprayed over the years.  I would think that to get closer to what they originally sounded like, you'd need to find NOS pots as well as other NOS tone control parts, or a PAS with very low miles, all stored in a controlled environment.

    In any case, I would agree that bypassing the tone controls should usually result in an improvement to one degree or another, probably greater than lesser.  However, for any readers wanting to retain the tone controls, there is hope, but the situation would need some thoughtful assessment.  IMO, I'd refrain from spraying or otherwise treating vintage pots unless they are already showing signs of failure, IOW, as a last resort.

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    PeterCapo
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    Post by PeterCapo on Thu Jul 02, 2020 9:48 am

    billinrio wrote:Here in Brazil, there's a Parasound Halo A21 (black) being offered at an online auction site for US$2,500 + shipping.  Worth it?

    Uhh... well I guess that seems like it's a little high for a used A21. Then again, I have no idea what the market is in Brazil for used audio gear. You might also inquire about the shipping cost, because the A21 is pretty heavy. But, before jumping on it, I'd explore other options, if you want to go the solid-state route in a power amp. For example, do you need the kind of power the A21 produces? What else is available for less money and would it be sonically synergistic with a PAS? One of the most important things I can think of would be to have return privileges if you don't end up liking the way it sounds with your PAS - cannot stress this enough.
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    rjpjnk

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    Post by rjpjnk on Thu Jul 02, 2020 10:15 am

    I think it does make sense to bypass the tone controls on the non-X PAS. Having a frequency response that depends on the output loading is not a good thing.
    PeterCapo
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    Post by PeterCapo on Thu Jul 02, 2020 11:17 am

    rjpjnk wrote:I think it does make sense to bypass the tone controls on the non-X PAS. Having a frequency response that depends on the output loading is not a good thing.

    Absolutely, bypassing the tone controls solves era-related issues, IOW, any drive issues with the original PAS configurations are a result of technology that appeared after they were designed.  Since the PAS was never expected to be used with the much lower input impedances of later solid-state power amps, bypassing the tone controls can result in impressive benefits.

    Worth mentioning for anyone interested, especially any newcomers, is that it is possible to use the PAS with solid-state amps and retain the tone controls, too, but it requires calculating a change in resistor value at the PAS outputs.  IIRC, the total load the bass pots need to see is 50KΩ, made up of certain resistor(s) at the PAS outputs as well as the input impedance of the power amplifier.  You just do a product/sum calculation to find the resultant resistor value change needed at the PAS outputs so that the bass pots see a 50KΩ load - it's actually pretty easy to figure out - and then you're good to go.  Here's a great technical article that explains it http://www.audioregenesis.com/documents/PAS%20Compatibility%20Issues.pdf
    peterh
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    Post by peterh on Thu Jul 02, 2020 11:49 am

    An alternative to tampering with the PAS is to install a buffer amp that isolates the PAS
    from the loading of the poweramp. Optionally another buffer can make the tape out
    connector insensitive of external loading, good for connection with computer soundcards.
    ebay seller  re-engineer  sold a nice board mounted inside the PAS, powered by the filament psu
    that buffers the line outputs.
    Another buffer sold as kit on ebay : https://www.ebay.com/itm/6N3-5670-Tube-buffer-Audio-Preamplifier-Pre-AMP-board-For-DIY-Amplifier-board/132249408707?hash=item1ecaadd4c3:g:isEAAOSwvflZWyGX

    Another kit that seems ok : https://www.ebay.com/itm/JSDZ-DIY-Kit-6N3-Tube-Buffer-Amp-Audio-PreAmp-Amplifier-Board/362869932971?hash=item547cbbc3ab:g:EE0AAOSwW89eDrL6

    External buffers ( and it should be cathode followers !) is an alternative.
    I myself built one for buffering both tape and line outputs + extra features.
    Visit  http://n.manet.nu/fm1/index.html  if you are interested.
    billinrio
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    Post by billinrio on Thu Jul 02, 2020 7:51 pm

    PeterCapo wrote:Some thoughts...

    I would think that replacing the Black Cats with polypropylenes could make a significant difference, depending on the condition of the pots.

    I don't know if "Black Cat" is a capacitor brand, or a generic term for some of the caps used in Dynaco equipment.
    Mine are labeled "John E Fast".


    Last edited by billinrio on Thu Jul 02, 2020 8:09 pm; edited 2 times in total
    billinrio
    billinrio

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    Post by billinrio on Thu Jul 02, 2020 7:56 pm

    PeterCapo wrote:
    billinrio wrote:Here in Brazil, there's a Parasound Halo A21 (black) being offered at an online auction site for US$2,500 + shipping.  Worth it?

    Uhh... well I guess that seems like it's a little high for a used A21.  Then again, I have no idea what the market is in Brazil for used audio gear.  You might also inquire about the shipping cost, because the A21 is pretty heavy.  But, before jumping on it, I'd explore other options, if you want to go the solid-state route in a power amp.  For example, do you need the kind of power the A21 produces?  What else is available for less money and would it be sonically synergistic with a PAS?  One of the most important things I can think of would be to have return privileges if you don't end up liking the way it sounds with your PAS - cannot stress this enough.

    Excellent advice.  The seller is local, so no freight.  
    PeterCapo
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    Post by PeterCapo on Thu Jul 02, 2020 8:40 pm

    billinrio wrote:
    PeterCapo wrote:Some thoughts...

    I would think that replacing the Black Cats with polypropylenes could make a significant difference, depending on the condition of the pots.

    I don't know if "Black Cat" is a capacitor brand, or a generic term for some of the caps used in Dynaco equipment.
    Mine are labeled "John E Fast".

    Black Cat was a brand or model designation for some of the original capacitors.  IIRC, John E. Fast was another, but I'd need to see a photo to be more sure.  Pyramid was yet another.

    Thinking about it some more, I can't say how much it would help to replace those original capacitors, considering the pots themselves. Your bypass did a lot for the sound and drive capability, freeing you to easily experiment with different amplifiers, if you want to.
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    rjpjnk

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    Post by rjpjnk on Thu Jul 02, 2020 11:52 pm

    peterh
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    Post by peterh on Fri Jul 03, 2020 3:00 am

    From wikipedia :
    The 6N3P (Russian: 6Н3П) is a Russian-made direct equivalent of the 2C51 medium gain dual triode vacuum tube. It may be used as an amplifier, mixer, oscillator or multivibrator over a frequency range AF through VHF. The Russian tube is slightly larger in size than the American tube. ... populated with the 6N3, which is said to be the Chinese version of the 6N3P.
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    rjpjnk

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    Post by rjpjnk on Fri Jul 03, 2020 10:07 am

    I'd like to find some reviews of how is sounds in audio circuits. Looks like it is pretty popular so shouldn't be too difficult.
    peterh
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    Post by peterh on Fri Jul 03, 2020 10:14 am

    rjpjnk wrote:I'd like to find some reviews of how is sounds in audio circuits. Looks like it is pretty popular so shouldn't be too difficult.

    a tube does not sound. Circuits do. A proper cathode follower adds or subtracts
    very little. That's why i recommend a buffer based on a cathode follower.

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