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Trying to figure a topology to use a grounded grid amp stage with a moving coil cartridge as input. I figured the impedance match should be pretty good, and grounded grid circuits have some nice qualities. But I just can't figure a way to do it without running dc through the cartridge, which seems like a really bad idea. All I've ended up with is my brain on a rack.
It wouldn't have to have a very high turns ratio. Jensen used to make one and still could be approached to wind some more if not currently in their catalog. I'm sure other transformer makers like Sowter and Lundahl could also abblige. Ray Hughes
"I take you as you are
And make of you what I will,
Skunk-bear, carcajou, bloodthirsty
Lord, let me die but not die out." THE LAST WOLVERINE by James Dickey
1/ The rough solution: Eli talks about it above and VTL did this with their "Ultimate" preamp - a BIG cap coupling the cartridge into the cathode. The disadvantage is obvious - passing the ultra fine detail signal off a good MC through a big elko cap just makes me cringe!
Plus there could be enough leakage in such a cap to "bias" the cartridge, or at least slowly magnetise it.
2/ The elegant solution: place the cartridge between the cathodes of a (double) current sourced diff pair of MATCHED input tubes. The match has to be perfect of course, so there is no current through the carridge - but that would work perfectly! I have often wanted to try this, and my RTP3 input stage would be the ideal test bed, but a slight disadvantage is that if any little thing goes wrong - let alone a tube failure - the megabuck cartridge is toast...
Thanks for your thoughtful answer. I've thought of both of those solutions, and like you, rejected the capacitor. I can't figure out how to build the latter in a practical way, with real world parts. Also, I can't get away from some of the signal being "eaten" by the CCS or other current supply. The problem is supplying the cathode(s) with electrons without affecting the cartidge signal or loading.
> I can't figure out how to build the latter in a practical way, with real world parts <
Parts are not the problem - the problem and it's MAJOR, is that the cartridge will BLOW with any problem - and problems are easily found in a tube stage such as this.
> Also, I can't get away from some of the signal being "eaten" by the CCS or other current supply <
Not a problem - the impedance of the cartridge is measured in ohns - as is the cathode impedance of any suitable tube - but the impedance of a good CCS is measured in megohms or even higher.
> The problem is supplying the cathode(s) with electrons without affecting the cartidge signal or loading <
As above - not a problem.
Just work out how to keep both cathodes at PRECISELY the SAME voltage for longer than minutes, hours or days and I might even try it myself. But until then I'm more than happy enough with my current voltage input solutions. My cartridges are all far to valuable to risk them fusing...
And I agree - no transformers!
do you have any idea of what the grid impedance is of the tube in question (pick a tube) at 0V bias? Unless i'm reading you wrong your thought is to match that impedance to the load your cartridge wants thus killing two birds with one stone.
I have been thinking about this too. I have actually one as far as thinking about an A2 input tube where the grid current biases the core of the SUT.
sure he whole thing would have to be tuned to a specific combination, but for some reason i think something magical could happen with this approach.
You have use a LARGE capacitance to block the DC. A Black Gate will never break in at these miniscule signal levels. Bypass a high quality Nichicon 'lytic with a metallized film part. Bypass the bypass with a MultiCap.
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