mus fid preamp 3 or 8

[rmgvs]

Can somebody help me with the circuit diagram of the preamp 3a or preamp 8 of musical fidelity? I'm interested in the line stage. In the preamp 3a it is build around a quad n-channel mosfet array vq1000j, in the preamp 8 it is the same circuit (I guess) but this time using the zvn4206e quad array.

Also the musical fidelity A1000 uses this same circuit in the preamp section. It even says 'preamp 8' on the circuit board. On the interesting website AMP�C—��H–[ Amp Repair Studio good quality pictures of the pcb can be found, but for me the puzzle is too complex to get the circuit out of it.

Contrary to what some people belief this preamp is not using a integrated circuit like the TL084 (A1, A100). It looks like an IC but in reality it uses 4 discrete mosfet's in one package.

Power supply is a regulated +30 Volt one, using some kind of TIP transistor to regulate.

The circuit delivers some hiss (more than usual) but this is due to the use of mosfets in amplifying stages (I think). The sound is, contrary to using j-fets (as far as my experience goes), warmer and more 'tube-like' than many other circuits.

[rmgvs]

I have made some efforts to ease the discussion. I blew up the pictures and mirrored the back-side of the pcb. Now you have the traces and components as if you are looking a the circuit from above.

I did not succeed in making the diagram fitting the traces. That is: a diagram that makes sense to me.

Please give it a try: it will take you 15 minutes maybe, for those who have more experience in this than me it can't be too difficult?

[jez]

The VQ1000 and ZVN are basically the same. The vq became unavailable from what I recall and so it was replaced by the zvn. It uses a SRPP type arrangement of the mosfets. From what I recall the preamp 8 was basically a cheapened version of the 3a internally.

[rmgvs]

Thanks for this. Mosfets 1 and 4 form a srpp together it seems. Can't figure out what the other 2 mosfets are doing. Especially one capacitor is puzzling me.

Looking at the picture, we see that starting from the source of mosfet number 1 (a resistor to ground, that's clear) it goes to a polyester cap that goes straight to the +30V of the power supply (and to the drain of mosfet number 4 that goes to the +30V itself). I can't see what this cap is doing.

[jez]

Having checked my records..... The phono stage uses 2 mosfets in srpp which then drive a common source amplifier which is constant current loaded by another mosfet. The riaa is in a feed back loop around both stages and there is a bipolar transistor preamp before it. The MVX uses the same stage for just the riaa bit! The line stage is more conventional and does not use srpp but has an extra stage.