Just updated my table, now looking at the phono stage in my preamp. I changed the biasing to led a while back. The attached circuit was a trace of board as the maker has not released the diagram. I am interested in comments. Quality of components of course affects the performance. Any possible suggestion, parts or circuit changes?
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Would need more data to do a complete analysis of the circuit: What is the Vf of the LED? What is B+?
At first blush, it looks like you are running the first gain stage at fairly low current (0.75mA with a B+ of 300VDC) and the second stage fairly hot (9mA-9.5mA depending on the LED). The total gain will only be ~36dB and the output impedance is rather high (~1K8).
At first blush, it looks like you are running the first gain stage at fairly low current (0.75mA with a B+ of 300VDC) and the second stage fairly hot (9mA-9.5mA depending on the LED). The total gain will only be ~36dB and the output impedance is rather high (~1K8).
Thanks for the quick response. From memory, B+ is around 220v. Your correct, I believe that the total stage draws about 7ma. I realize that the 6ma thru the second stage is minimal for the LED noise factor, but this is what the original resistor/bypass cap also had. It is a MM stage. There is enough gain for my use at the moment using a high output MC. This stage runs into another ecc88 stage for additional gain.
Why is the input terminated with 28 kohm rather than the usual 47 kohm?
For an ECC88, 0.6 mA is well below the optimal bias current for minimal RIAA- and A-weighted noise. The relatively high feedback impedance, the 2 kohm at the input and the lower-than-normal input termination resistor also add noise. It is probably negligible if you only care about noise with a record playing, though, as record surface noise is usually quite dominant.
For an ECC88, 0.6 mA is well below the optimal bias current for minimal RIAA- and A-weighted noise. The relatively high feedback impedance, the 2 kohm at the input and the lower-than-normal input termination resistor also add noise. It is probably negligible if you only care about noise with a record playing, though, as record surface noise is usually quite dominant.
Yes the original 47k resistor was replaced by the 28 k by me. I use a Denon DL 110 moving coil cartridge at this time. I am experimenting with load resistors. This is a commercialy designed phono stage that I am looking to improve. Should the 2k input series resistor and the 47k(28k) load resistor have a optimal ratio? I assume the first stage is acting as a filter not as a gain stage as the second stage is, is this correct?
It looks to me like a two-stage amplifier with frequency-dependent feedback around it.
The 2 kohm resistor should be large enough to prevent parasitic oscillations, but not so large that it degrades noise significantly. I don't see any reason why there would be an optimal ratio with the input termination resistor.
The 2 kohm resistor should be large enough to prevent parasitic oscillations, but not so large that it degrades noise significantly. I don't see any reason why there would be an optimal ratio with the input termination resistor.
All grid stoppers generate a rolloff point and filtering, but it's nominally above the audio range of interest. A good conservative design for a phono stage, where the source is exposed to who-knows-what level of modern RF trash, will try to damp down the trash without impinging on the audio band too badly. Yours looks like a well planned design.
All good fortune,
Chris
All good fortune,
Chris
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