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Old 12th June 2011, 04:31 PM   #11
SY is offline SY  United States
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You can reduce C8/C9 by the sum of the Miller capacitance and grid to cathode of the second stage. Miller C will equal second stage gain (likely to be 25 or so) times the grid to plate capacitance (you can find that on the data sheet).
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Old 12th June 2011, 06:26 PM   #12
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Thank you SY

I will look it up.
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Old 12th June 2011, 06:51 PM   #13
merlin el mago is offline merlin el mago
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Many thanks for help SY.
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Old 12th June 2011, 06:55 PM   #14
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For E88CC I found the following values (Hope these are ok as my German is very bad) :

Grid to Cathode 3.1pF
Grid to Plate 1.4pF
Miller 25 x 1.4 = 35pF
Total = 38.1pF

This is a very small value and will not introduce any nasties in the C8//C9 combo.

Anyway we can use C9//C8 = 2.7nF // 106pF // 180pF // .0381pF = 3.024nF


Now we just need input about first stage output impedance.
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Old 12th June 2011, 07:17 PM   #15
merlin el mago is offline merlin el mago
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Quote:
Originally Posted by RCruz View Post
For E88CC I found the following values (Hope these are ok as my German is very bad) :

Grid to Cathode 3.1pF
Grid to Plate 1.4pF
Miller 25 x 1.4 = 35pF
Total = 38.1pF

This is a very small value and will not introduce any nasties in the C8//C9 combo.

Anyway we can use C9//C8 = 2.7nF // 106pF // 180pF // 38pF = 3.024nF


Now we just need input about first stage output impedance.
So I only have to add 218pF in each channel

Last edited by merlin el mago; 12th June 2011 at 07:19 PM.
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Old 12th June 2011, 07:42 PM   #16
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I would determine that resistor value experimentally- in that design, the ECC83's output resistance is very significant and is likely to vary quite a bit from tube to tube. Measure the output of the preamp at 50Hz and 1kHz. If the difference is less than 20dB, add a bit more resistance. If it's more than 20dB, reduce the resistance.

As the preamp tubes age, the RIAA conformance will drift. That's a serious issue with any design that has a high rp tube feeding a passive RIAA filter.
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Old 12th June 2011, 08:56 PM   #17
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Quote:
Originally Posted by merlin el mago View Post
So I only have to add 218pF in each channel
No

You only need to add 180pF because the 38pF are coming from the second stage Miller.
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Old 12th June 2011, 09:09 PM   #18
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Quote:
Originally Posted by SY View Post
I would determine that resistor value experimentally- in that design, the ECC83's output resistance is very significant and is likely to vary quite a bit from tube to tube. Measure the output of the preamp at 50Hz and 1kHz. If the difference is less than 20dB, add a bit more resistance. If it's more than 20dB, reduce the resistance.

As the preamp tubes age, the RIAA conformance will drift. That's a serious issue with any design that has a high rp tube feeding a passive RIAA filter.
I agree with you but just as a starting point, what would be the 1st stage output impedance ? (Let us know your best guess please).

How can we make the first stage less dependent of tube ageing ?

How can we make this design less dependent of this tube rp ?

PS: reading this: TUBES: its RP, not MU, that's important! [Archive] - The Gear Page gave me a starting point relative to rp importance.
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Old 12th June 2011, 09:25 PM   #19
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Well, what's the current in that stage? rp is a VERY strong function of that. It could be anywhere from 50k to 250k.
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Old 12th June 2011, 10:27 PM   #20
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Well, those values are so high that if I try to include them im my simulator (excel calculator), It produces big errors to riaa.

The highly tuned design in attach is very similar to the Puresound, appart from the plate resistor that is 5k against the 120k in the Puresound.

It seems to me this schematic also disregards the effect of the the first stage output impedance so I would have a go with the values already determined.

Merlin... The circuit you need to tune is already built ?

We need some subjective listening impressions now.
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