Radiotron/Eli Duttman phono change C1 to .00391? - diyAudio
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Old 11th October 2015, 12:36 AM   #1
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Default Radiotron/Eli Duttman phono change C1 to .00391?

What would changing C1 to .00391 do?

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Old 11th October 2015, 12:57 AM   #2
rayma is offline rayma  United States
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Quote:
Originally Posted by lugnut View Post
What would changing C1 to .00391 do?
That would affect the RIAA correction, probably reducing the higher frequencies by several dB for one thing.

Last edited by rayma; 11th October 2015 at 01:01 AM.
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Old 11th October 2015, 12:58 AM   #3
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I realize that, but how would the sonics be changed?
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Old 11th October 2015, 01:03 AM   #4
rayma is offline rayma  United States
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Quote:
Originally Posted by lugnut View Post
I realize that, but how would the sonics be changed?
Similar to turning the treble tone control down part way.
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Old 11th October 2015, 01:07 AM   #5
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Would changing R1 to 20K and C1 to .00391 restore the original top end performance?
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Old 11th October 2015, 01:18 AM   #6
rayma is offline rayma  United States
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Quote:
Originally Posted by lugnut View Post
Would changing R1 to 20K and C1 to .00391 restore the original top end performance?
If you want to define C1 as 3.91 nF, the RIAA circuit can be redesigned, but all of the other parts in the RIAA will change their values.
Don't forget the source impedance of the first tube stage, which affects the value of R6.
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Old 11th October 2015, 06:02 AM   #7
hego is offline hego  Europe
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Quote:
Originally Posted by rayma View Post
Similar to turning the treble tone control down part way.
Yes, C1 affects mainly at 75uS.

and remember... C4 = 3 x C1.

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Old 11th October 2015, 10:32 AM   #8
Koonw is offline Koonw  Malaysia
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I just finished the simulation of RCA photo with MOSFET buffer stage you attached and found few issues:

1) With zero cathode resistance on the 2nd triode, distortion is as high as 5%, min value I found is 5 ohms.

2) For output coupling cap of 0.1uF the output impedance below 100Hz is very high, 15K compared with less 1K above 1Khz. So I increase the value to 15uF and the output impedance is as low as 1K down to 10Hz.

3) 20 Meg bias resistor to compensate for bottom end? There is stability concern for current more 1mA. So I mod that too.

Here are some snapshots for sharing.
Attached Images
File Type: png rca-phono-12ax7-mod-1.png (275.0 KB, 226 views)
File Type: png rca-phono-12ax7-mod-2.png (242.9 KB, 220 views)
File Type: png rca-phono-12ax7-mod-3.png (214.9 KB, 218 views)
File Type: png rca-phono-12ax7-mod-4.png (298.3 KB, 217 views)
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Old 11th October 2015, 02:43 PM   #9
kevinkr is offline kevinkr  United States
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Koonw the second stage uses grid leak bias, and most tube models I am aware of don't model grid current behavior correctly - why not build the thing as designed; I think you will find it performs quite a bit differently from your predictions.

That 0.1uF cap could be increased, but 15uF is way beyond reasonable, you don't really want to be amplifying the arm/cartridge compliance resonance any more than you have to. I'd say 0.47uF is at or beyond the point of diminishing returns.
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Old 11th October 2015, 02:56 PM   #10
kevinkr is offline kevinkr  United States
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Here's a purely mathematical way to implement an inverse RIAA in LTSpice using the LaPlace transform. (Convert behaviors specified in terms of time domain i.e poles at 3180us and 75us, and a zero at 318uS into frequency domain)
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File Type: png laplace transform for Inverse RIAA.PNG (47.5 KB, 66 views)
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