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Dynaco PAS2 phono low frequency filter

tahoe_snow

tahoe_snow

New Member
I’ve been working on a Dynaco PAS2. I upgraded the power supply with a board from triode electronics. I was going to call it done as the remaining components appear to be within spec, but I’m having a problem with the phono section. In the left channel there is a very low frequency rumble. It appears that the RIAA circuit isn’t functioning correctly. Any ideas where I might start trouble shooting the low frequency filter?

Thanks-
 
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tahoe_snow said:
I’ve been working on a Dynaco PAS2. I upgraded the power supply with a board from triode electronics. I was going to call it done as the remaining components appear to be within spec, but I’m having a problem with the phono section. In the left channel there is a very low frequency rumble. It appears that the RIAA circuit isn’t functioning correctly. Any ideas where I might start trouble shooting the low frequency filter?

Thanks-
Click to expand...
See George rennenkaps excellent article http://www.audioregenesis.com/documents/PAS_Phono.pdf
discussing PAS riaa.
Possibly your 4.7 M resistor has got more resistance .
 
gadget73 said:
If the phono section has not been tampered with,I'd probably be looking for noisy plate or cathode resistors if there is a constant noise coming from it.
Click to expand...

The sound isn’t constant. It only happens when playing. To me it seems like the RIAA response is off and not equalizing low frequencies correctly
 
have an inverse RIAA network? that would be the easiest way to prove whether or not its functioning correctly.
 
BD75D67B-3F2C-42C1-BE12-1EE475DBE1B6

BD75D67B-3F2C-42C1-BE12-1EE475DBE1B6


I just want to note, I was wrong about the problem being in the phono section. I hooked up a scope and realized the problem existed in the line stage as well, it just wasn't as easy to reproduce. The left channel has way too much low frequency gain as shown by the picture above. I temporarily bypassed the tone controls (using the PAS 3x neutral implementation), and that seemed to resolve the problem. Today I'm going to take a harder look at the tone controls
 
gadget73 said:
not sure what image you posted but I get an error trying to look at it
Click to expand...

It is a picture of my scope showing the amplitude of the left channel at 20hz vs the right channel. Not sure why you can't see it. As much as I love the content on AK, the software running the site is getting long in the tooth.
 
Assuming your PAS 2 is in stock form other than the power supply board, check to make sure the 62K resistors on the PC-5 board are in place and measure correctly. These are required to properly load the bass pots for flat response when set to center. Without them, there would be about 10dB boost at 20Hz. If you take a resistance measurement at the output jacks you should see about 50K ohms, considering that there is also a 510K and the bass pot in parallel with the 62K.
 
it was giving some permissions error. Looks like it was uploaded to the gallery instead of just dropped into the post. Its working now.

as forums go this one actually runs fairly modern software. Everywhere else I post is using stuff that has far less features.

might be a failure in the bass pot itself, or some wire meddling. Whatever it is, seems like you have no or minimal feedback at the bottom end.
 
audioregen said:
Assuming your PAS 2 is in stock form other than the power supply board, check to make sure the 62K resistors on the PC-5 board are in place and measure correctly. These are required to properly load the bass pots for flat response when set to center. Without them, there would be about 10dB boost at 20Hz. If you take a resistance measurement at the output jacks you should see about 50K ohms, considering that there is also a 510K and the bass pot in parallel with the 62K.
Click to expand...

That's really good to know! I've been reading your docs on the PAS. Really interesting! Thanks!
 
audioregen said:
Assuming your PAS 2 is in stock form other than the power supply board, check to make sure the 62K resistors on the PC-5 board are in place and measure correctly. These are required to properly load the bass pots for flat response when set to center. Without them, there would be about 10dB boost at 20Hz. If you take a resistance measurement at the output jacks you should see about 50K ohms, considering that there is also a 510K and the bass pot in parallel with the 62K.
Click to expand...
I’m not sure if it is my problem, but both 62k resistors measure under 50k. I don’t have any 62k resistors here, but I’m surprised left isn’t significantly lower than right.
 
Values under 62K would result in a loss of low end response and above 62K a boost, with the bass pots at center. If the 62K resistor had been the problem, the one for the left channel would have been significantly higher than 62K. Thus, the resistors themselves are likely not the cause of your particular problem, but you may want to follow the traces from each end of the 62K on the left channel to check for broken traces, or a bad solder joint on the PCB. Check for a 50K ohm reading right at the related end of the bass pot to ground. Since bypassing the tone controls fixes the problem, it would seem a problem exists somewhere in the bass control circuitry for the left channel.
 
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