I just finished a line/phono stage, and For the phono section
I took the Dynaco PAS and tweeked it, and must say
I was pleasantly surprised at its performance. For evaluation
I used Wilson Audiophile's recording of a James Welch organ concert
Which is known for its dynamics and accurate rendering
Of the acoustics at the Air Force Academy Chapel. I was really impressed
With the bass performance.
I took the Dynaco PAS and tweeked it, and must say
I was pleasantly surprised at its performance. For evaluation
I used Wilson Audiophile's recording of a James Welch organ concert
Which is known for its dynamics and accurate rendering
Of the acoustics at the Air Force Academy Chapel. I was really impressed
With the bass performance.
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What kind of circuit did you end up with? The Dyna's line stage certainly leaves a lot to be desired, though.
So far as the feedback in the phono stage, I used a 100k in parallel w/ a 820pf cap connected to a 4.7M in parallel to a 2700pf cap caps are TDK multi-layer ceramic, and everything else is the same except for slightly larger coupling caps.
The line stage is a 12au7 common cathode stage coupled to a cathode follower, with the gain of 8~10dB in the line stage.
I believe you'll need to recalculate the RIAA parts; these seem to be the original values, and likely won't give a flat response.
Have you tried taking the RIAA feedback directly coupled from the second plate, instead of after its coupling capacitor?
Not only do you eliminate one of the two series output caps, but also you get DC bias on the RIAA caps.
All I can say is the sound was right on, maybe it has to do with the tolerance of the caps used, not sure. I ran the design through Spice and it looked good to me.
No feedback,just lowered the value of the plate resistor, and raised the value of the cathode resistor, I'm at about 4 to 5mA of current for the gain stage.
"right on"? as rayma hinted, while you may very well like the sound but the original PAS does not meet the RIAA standard. One of the most popular mods for the PAS is to correct the RIAA playback error.
You can build an inverse RIAA network and use a square wave input to get a feel for the response with even a simple scope.
Precise data require more elaborate equipment, though.
For the PAS I've always had the notion to add a ZVN0545A to the output, ala the Duttman upgraded RCA phono stage. Of course the begs the question, FB loop (for the Dyna type circuit) before or after the follower.
And of course I would have to have a PAS lying around
Edit: Curcio has a quick upgrade to get a better RIAA response from a stock PAS.
And of course I would have to have a PAS lying around
Edit: Curcio has a quick upgrade to get a better RIAA response from a stock PAS.
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All I can say is that the values I calculated, and simulated in B2 Spice had the smoothest response. I've listened to many live organ events, as well as many recordings. All the upper registers, to the lowest of pedal notes were captured with true realism! Maybe my ears are off, I don't know but I appreciate the constructive criticism.
For the PAS I've always had the notion to add a ZVN0545A to the output, ala the Duttman upgraded RCA phono stage. Of course the begs the question, FB loop (for the Dyna type circuit) before or after the follower.
And of course I would have to have a PAS lying around
Edit: Curcio has a quick upgrade to get a better RIAA response from a stock PAS.
I have also considered a mosfet buffer on a pas... a good starting point would be the Koren pas circuit (replace the cathode follower with a source follower) but I don't know if it would be a drop in replacement without recalculating the eq network. You might need a device with higher current capability to drive the feedback as well as the output.
Not questioning your hearing nor personal preference at all - just the smoothness as you put it. In this case, it is easy to compare the results since the RIAA standard is readily available. On my simulator, here are the frequency responses of the Dynaco and a modified version, played through the inverse RIAA network:
An externally hosted image should be here but it was not working when we last tested it.
Thanks, I will work with the curcuit some more, I could have missed something in the simulation.
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