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Old 11th November 2009, 05:22 PM
John Caswell John Caswell is offline
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Join Date: Dec 2005
Location: Wokingham, Berkshire
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Default Re: Phono 3 with 5751 valves

Hi all,
Been watching this thread since the beginning and had a quick look at my Phono II/III with 5751 tried in output and input positions.

Test setup:
AWA very low distortion oscillator > Hagerman inverse RIAA filter > Phono II/III with Sowter 9575 20:1 stepup Tx > AWA distortion/noise analyser > HP180 50mHz scope.
Figures are compared to my standard PhonoII/III with Philips 12AX7WA input valves and Brimar 12AX7 output valve

Firstly - output position. As far as I could measure there was negligible to no change in either frequency response or gain when used in this position, which is not surprising as it is a cathode follower! There was no need to adjust the variable capacitor in the phono III unit to re-correct the square wave shape

Secondly - input position. Once again as far as I could measure negligible frequency response errors(Less than 0.1dB - measuring error?). Gain however was down -2dB to -3dB depending on valves so, as Richard M says, could be useful if you have an over sensitive pre/main amp.

Thirdly - Both positions with 5751s. Negligible frequency response changes but gain down -2dB to -3dB.

The only advantages I can see are lower gain possibly lower noise floor and possibly lower microphonics.

I will do some further tests using my phono II without the transformers as they will undoubtably have some effect on the frequency response.

There are also some other valves that may be suitable but I think are pretty rare/expensive e.g. 12AY7, 12AD7.

John - Dr John
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