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Tube Preamplifier
Jerry, the Phono Stage
Last modified: 14-Sep-2003
This is going to be my 4th tube-based phono amp (and I made a few with opAmps too). And of course it is my intention to make it the best sounding phono amp ever (duhh). Well, on the other hand humm and noise are difficult to eliminate in a phono preamp.
As described on the introduction page of Tom&Jerry I had not yet decided on the design of this phono amp. Several designs were considered such as:
In a three stage amplifier design it is easy to split the RIAA filter in two and put the halves in between two stages. In a 2-stage amp it is less easy to split the filter unless the input impedance of the next (pre)amplifier is high and there are no parasitic capacitances from cabling etc. This means that such a design is only feasible where phono and pre-amp are integrated. Hmm, I still prefer to build the filter in between the two stages of the phono amp (read more about RIAA filters on the background page).
The mu-Follower does look like a modified SRPP circuit, it does however have better gain and lower output impedance Ro as the upper valve behaves as a Cathode amplifier.
There are some things to take into account when using the mu-Follower design: First of all the voltage dropped over the extra resistor (39k in my case) in the cathode of the upper tube. How larger the resistor how better the design would work, but also the voltage dropped over the resistor becomes significant. in my case the 39k value seems to work well (22k would work too) and results in a voltage drop of 20 Volts. Therefore, the power supply of jerry needs to have a higher B+ than used for Lookie and Tweety. I'm shooting at a value of about 300V. This is my thinking about the design so far:
I've decided that I do not want to bypass the cathode resistors of both lower tubes. I'll loose some gain and a lower Ro, but I do not want to use caps when not necessary. It will mean a little less gain and a slightly higher output impedance. But there is enough left, and it keeps the cicuit simple. The Cathode resistor itself needs to be chosen well and not too small else the grid voltage won't be sufficient. A value of 2.2k Ohms is OK.
This is a picture of the inside of the phono amp. The yellow caps are the Auricaps, the black ones are Audyn caps.
When connecting the amp, everything worked first time. But unfortunately there is still too much humm and noise left, I expect a motorboating problem. The solution could well be a further decoupling of the first and second stage so that is what I'll try shortly.
I would like to insert a choke between 1st and 2nd stage, but since the chokes I have are not insulated they probably won't be used on top of the amplifier. The alternative is of course a pi filter, but I'll better ask Wil of AE-Europe to find me a solution with a choke.