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Old 17th September 2014, 04:38 PM   #1
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Default Modified Dynaco PAS Phono

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.
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Last edited by denny9167; 17th September 2014 at 04:42 PM.
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Old 17th September 2014, 04:48 PM   #2
rayma is offline rayma  United States
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Originally Posted by denny9167 View Post
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. I was really impressed With the bass performance.
What kind of circuit did you end up with? The Dyna's line stage certainly leaves a lot to be desired, though.
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Old 17th September 2014, 05:16 PM   #3
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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.
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Old 17th September 2014, 05:41 PM   #4
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Do you retain Dyna's positive feedback resistor (going from cathode-to-cathode, I think it was 100k or so) used to make up for insufficient open loop gain?
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Old 17th September 2014, 07:17 PM   #5
rayma is offline rayma  United States
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Originally Posted by denny9167 View Post
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.
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Old 17th September 2014, 07:18 PM   #6
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Originally Posted by denny9167 View Post
The line stage is a 12au7 common cathode stage coupled to a cathode follower, with the gain of 8~10dB in the line stage.
Are you using feedback in the line stage? Not much gain there.
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Old 17th September 2014, 08:23 PM   #7
rayma is offline rayma  United States
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Originally Posted by denny9167 View Post
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.
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.
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Old 17th September 2014, 08:24 PM   #8
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Originally Posted by SY View Post
Do you retain Dyna's positive feedback resistor (going from cathode-to-cathode, I think it was 100k or so) used to make up for insufficient open loop gain?
Yes Sir, I used 47k.
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Old 17th September 2014, 08:27 PM   #9
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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.
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.
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Old 17th September 2014, 08:30 PM   #10
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Are you using feedback in the line stage? Not much gain there.
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.
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