Excellent article SY, thanks.

It got me thinking about the possiblity of an MM version, as mentioned a few pages back. Having done a few back-of-an-envelope calculations, I reckon that replacing the D3a with another ECC88/6DJ8 gain stage (identical to the second stage) would work quite nicely.

The equivalent noise resistance is 300 Ohms, giving a 1st stage SNR (assuming no transformer and a 100 Ohm grid stopper) of -79dB, cf. the -81dB figure that you get from a 630 Ohm cartridge.

Adding in a Sowter 1:1 600-600 Ohm transformer (1.9dB insertion loss, 150 Ohm secondary resistance) gives a SNR of -76dB, but no doubt a significant common-mode improvment.

The input capacitance works out to be about 56pF (including the 5pF for strays) although that number varies by 2.7pF between sections of the 6DJ8 - probably better to use the section on pins 6,7, and 8 for the input on both channels for the lower value. I have no experience of de-ringing transformers, but I'd expect that to be low enough to get decent results.

The 1st stage output signal level is about 165mV, and the output impedance is about 2K6 Ohms - perhaps different enough from the D3a's to make re-calculating the RIAA to R1 = 27k worthwhile, to keep the dependance on the tube's Ra down.

The output with the 5mV output of many MM carts would be ~0.54V at 1kHz.

Other possible tubes are the 6DX4 (Ra = 2K7, Mu = 30), or the 6DY4A (Ra = 2K5, Mu = 28) - but they seem to be on the rare side. I haven't looked into the input capacitance or noise resistance, partly because datasheets are thin on the ground.

Does anyone have any immediate thoughts?

Specifically, I'm unsure as to whether the 1:1 transformer should be of the 600 Ohm or 10K variety. My instinct tells me that the lower-impedance variety is the one to go for, as you might want to keep the winding impedances around the value of the cartridge imppedances - but OTOH, the cartridge load is 47K, so should it be the higher-impedance version. TBH, I'd probably build without the input transformer to start with, and then add one as funds become available.

I thought about this and I thought about what it takes to make a really good 50k:50k transformer, and I leave that path to others.

As a pentode, the D3a still gives pretty excellent performance in the first stage (the distortion spectra at these small signals are pretty clean), and if I were to be willing to give up balanced, that would be a high priority option. I'm not, so I've gone another route for MM...

D3a in pentode will give a wad of gain, easy to squeeze out 50-60dB.

Thanks for the feedback guys. I suppose the 50k:50k transformer value makes sense, although as you say it would be difficult in practice.

"so I've gone another route for MM..."

You've got me intrigued now. I suppose that one brute-force method would be to throw more active devices at it - maybe a balanced-input cathode follower, with a balanced-to-single ended transformer between that and the first gain stage.

Excellent - I look forward to it.

In the meantime I might put together the ecc88 input stage (without transformer) and see how it goes - not sure it can be worse than the phono stage in my integrated amplifier, which is really noisy.

are the boards for this project available? I am most interested in getting them... thanks!