Musical Fidelity 3A Pre-amp background hiss

[m1ke]

I've had this Pre from new and done quite a number of mods (500VA transformer, Panasonic Pot, Multicap Rtx and Sonicap Gen 1 caps) which have all improved the sound considerably. One problem with this pre-amp is noise from the Quad N-Channel mosfets ( ZVN4206E ( VQ1001J ) ). In an effort to eradicate this I bought some 2sk170's. At first I just did push fit into the left channel's i.c. socket. The good news was that the hiss was gone and volume level was just the same as the right channel. Unfortunately there was a slight hum/burble noise. At first I put this down to local pick-up due to lead length.
Next I cut back the lead lengths and soldered them direct into the pcb, left and right channels.
OK, so no background hiss, but I was left with what I would call a motor-boat type hum/burble equal to about "20 to" on the volume control. Turning up the volume produced a very clear sound except of course for this background hum.

Can anyone suggest what may be causing this?

My background is electronic repair not design.

Many thanks in advance.

Mike

[raymondo]

i too have this preamp. its very noisy yes. i rebuilt the mc stage using some paralleled ssm devices a few yrs ago, that cct is now very quiet.
can u tell me a bit more about the mods you have done? i too replaced all the electrolytics as they were old and leaky, but the design looks all ac coupled and uses a stack of expensive looking poly caps .
i have tried in vain to find those zvn4206 devices but couldnt.....
did you draw that diagram yourself? never been able to find schemis for much MF stuff. a very helpful guy on here has been helping me fix my power amp though :-)

[rmgvs]

Mike, I sent you a private message. The post is rather old but I am still interested. I think the diagram has some errors, so I'm curious if you actually tested or simulated the circuit or that you have drawn the diagram by looking at the pcb. Furthermore, I like to hear in what way you've changed the amp, what worked for you and what didn't?

By the way: the preamp hisses somewhat and that is by the nature of the mosfets themselves (in relation to topology used). Changing to BS170 won't change this, I actually tried it myself. Your motorboating issue is probably related to the feedback capacitor of 100p. Try to fiddle with this (make it bigger or smaller, try to leave it out altogether).

[raymondo]

I measured the distortion on this pre recently , it was horrendous .
Amazingly it sounds ok.

[rmgvs]

This phenomenon (measuring badly, sounding okay) happens all the time and seems to be a hallmark of some of the older MF products. The A1 measures like a bad tube amp but it sounds like magic, so who cares. All Tim de Paracivini designs by the way. By contrast take the MF MVT preamp, which is designed by Dr Thomas (MVT for MV Thomas?) and of the same era (and very expensive at the time!), I don't know about the measurements, but it sounds more hifi and less euphonic. I have both original preamp at my disposal so I check this out every now and then.

[raymondo]

yes i know this can be the case but i was shocked at just how bad it was for a preamp and at the ugly waveform on my scope (output from the distortion analyser).
inspired me to build a better one

[rmgvs]

Now you make me curious Raymondo! Would you share your journey to the better preamp with us? Did you find something that sounds at least as good (= somewhat euphonic I guess) but measures the lot too?