Single RIAA passive eq design for differential DC coupled tube circuit - diyAudio
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Old 30th January 2013, 08:26 PM   #1
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Default Single RIAA passive eq design for differential DC coupled tube circuit

I'm going to put together a differential phono preamp with cap coupled cathodes a la Shoog/B.Beck . In searching out how others have done one of the two most recommended all in one go passive eq networks (the one shown in the KAB calculator linked here) I have found three distinctly different ways of doing it, apparently all with good results according to their posters.

One is to take the basic circuit and simply mirror it.

Another is, beacuse mirroring will rail to rail effectively double R values and halve C values, simply do this beforehand when selecting the components and then instead of taking the "bottom" of the circuit to ground, take it to the plate to grid circuit of the opposite tube in the pair, resulting in the same combination of R1, R2, C1, C2 spanning from plate to plate but with one extra resistor in the R1 position on the second tube.

Third way is to take the basic circuit, halve the R values, double the C values and then build two full circuits mirrored, presumeably connected at the "center" to a common rail at their junction, as in the sketch.

I've searched for some broader discussion of the subject but haven't been able to find anything here. Just wondering of there are any out there who can speak to this.

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Old 11th February 2013, 01:09 AM   #2
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OK, ten days later, I've been fooling around with this. The basic circuit is pretty good, following the RIAA curve well and getting something just over 40dB gain.

It is interesting to see how any changes to the circuit yield such easily audible results. Everything I've tried til today has been with the standard passive eq., mirrored and with the bottoms of the two single ended eq circuits still connected. (ie. connection between C1 string and C2 string at centre.)

Most of the experiments were with the output stage, trying R/C out , L/C out , output transformer out , SVCS/Cap out , Individual Cathode CCSs , long tail pair , LTP cathode resistor, Independant cathode resistors. So far the best has been SVCS /Cap output with individual Rk.

Regulated top and bottom was troublesome (oscillation) and setting things up with the LTP was also either too finicky or just over my head - not sure which.

The big surprise though came today in pulling out the centre connection of the eq. The gain went up so much I yelled (you know what!) and the speaker wasn't too sure about it either. Haven't done measurements yet. I'll have to divide it down a little before looking at the curve.
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Old 26th June 2013, 02:06 AM   #3
JZatopa is offline JZatopa  United States
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Location: Chicago
Are you using a transformer on the front end of this?
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Old 26th June 2013, 03:06 PM   #4
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Yes, a Lundahl LL1690.
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