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iko 12th October 2008 12:45 PM

Low-V simple phono stage + tube buffer
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Hi everyone,
I've been playing a bit with this idea; an opamp based phono stage, balanced input, and tube buffer.

On the attached schematic you'll see what I mean. The schematic is exactly what I was simulating in tina-ti, so if anyone would like to play with it, just let me know.

The layout is very simple. The first stage is build around the ina163 opamp to allow for balanced input and voltage offset compensation. Please ignore L1 and R4, which happen to be values for my current cartridge. R11, R12, and C8 are used to load the cartridge and most often are 47kohm (R11+R12) and 100pF, but I use values which are more appropriate for my system. R1 sets the gain in the first stage.

The little circuit above the ina163, based on the opa227 does voltage offset compensation. Can be excluded if the output bias voltage is dealt with in a different way.

There is a plain RIAA filter in the middle of the two stages; passive.

The second stage is based on the opa134; the gain is set by R10.

Following the second stage is a buffer based on a couple of easy to find tv tubes, the 6at8a (or 6at8); pentode in a triode strapped mode. I am already using these exact tubes in this exact configuration as the input buffer of a lm3886 power amp I have, and they work well at the 18+18 volts that comes from the dual rail.

Any suggestions, feedback would be appreciated.

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