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Old 24th June 2008, 08:31 AM   #11
oshifis is offline oshifis  Hungary
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BTW I also use a passive RIAA circuit, but with 3 gain stages and the eq network is divided in 2 sections. Each gain stage is an SRPP. The gain is sufficient, noise is low and it is easier to calculate the values of the passive circuit. I described my circuit here:

Vishay Audio Resistors
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Old 24th June 2008, 08:32 AM   #12
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Hi Dieter


What oshifis probably means is that some cap values seem an order of magnitude off the mark using the common formulas. Which does not necessarily mean they are wrong. I tried calculating the response of your riaa and it seems indeed wrong - 3db down at 2kHz and -6db at 5kHz and above. Of course it's best to simulate the riaa network but i'll only get time to do that tomorrow.

A set of safe values, with guaranteed response are the following:

R1 = 950k
R3 = 96k3 (95k3 can be retained with small error)
C4 = 3n3
C5 = 1n13

These assume that R8 is in place as shown (and that i haven't made a silly mistake).

The gain should probably be raised from the second stage. I haven't given this much thought.


Seems like i posted at the same time as oshifis
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Old 24th June 2008, 09:24 AM   #13
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thank you so much, both of you!
your calculations seem to correspond with what I hear: Dull and lack of treble.
I'll be patiently waiting for your simulation.

Will changing the Bias for increasing the gain have any impact on the RIAA network values?
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Old 24th June 2008, 10:41 AM   #14
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I did the math according to the formulas provided by oshifis and came up with the following numbers:

R1 663K
R3 96,5K
C4 3,3n
C5 1,13n

plate resistance has not been factored in. Do I have to deduct 16K or 8K (2 triodes in parallel) or add?

cheers
Dieter
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Old 24th June 2008, 10:56 AM   #15
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Don't worry about plate resistance - it's too low to affect anything. And it's neither 16 nor 8k Your R1 is however wrong - there is 2M2 in parallel and that's why i suggest 950k.

Changing the gain of either stage won't affect riaa much. The first stage will have a negligible effect and the second none.
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Old 24th June 2008, 11:12 AM   #16
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sorry, but I don't see the 2,2M parallel. Which R do you mean? remember please, the brackets hold the actual values.
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Old 24th June 2008, 11:22 AM   #17
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R8 + R14
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Old 24th June 2008, 11:30 AM   #18
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could I take this one (R8+R14) out? Is it required?
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Old 24th June 2008, 11:49 AM   #19
oshifis is offline oshifis  Hungary
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A grid resistor is required, and as analog_sa wrote it affects the accuracy of the equalizer circuit. A trick which I use is that I put the coupling capacitor C1 before the RIAA network and the grid resistor right after C1 (plate - C1 - R14 to GND - R1 - ...). Then the grid resistor does not load the equalizer network and it can be 1M or less.
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Old 24th June 2008, 12:05 PM   #20
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Oshifis

Not sure if i got the picture right but it makes absolutely no difference where you put the cap or the grid resistor - it is still in parallel with the series combination of R1 and whatever is the output impedance of the driving stage. There is one potential advantage in having C1 preceding the riaa though - you can use low voltage riaa caps.
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