Preamp & buffer

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This is a preamp & buffer project in the '80s by Suono, the Italian magazine.

There are two versions for preamp and buffer, but I just redraw the preamp. The article xerox copy I have doesn't allow scanning the page.

The good thing of this project is that there are lot of measurements, specs and scope photos to see how it behaves.




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This looks like a good solid design, though the choiceof the dual FET for U40t is important, particularly since it's using local degeneration, which implies the use of a high Gm part. But it isn't specified. If this is run DC coupled with DC gain, the offset stability and gain of U406 is critical.

The input stage is cascoded with a floating cascode reference, and the ouput stage is combines a VAS buffer and output cascode; some don't like the extra parts, but you could do a lot worse than to build this design, and I expect it would measure fairly well. A dual emitter follower for the output seems a little over the top, but it will certainly minize output loading on the cascode final stage. The cascode output stage gain is limited by the 12K resistor tied to ground- Douglas Self and others of his inclination believe that limiting loop gain this way is pointless, but I've found I do prefer it, and my own non loop feedback designs actually use the collector resistor in setting the final stage gain.

The major difference between this and preamp circuits I designed in the late 70's was using a full differential second stage combining to a pushpull drive at the output, and using four transistor current mirrors. But this should work quite well. The lead compensation employed in the feedback loop might be a tad touchy; there's a lead/lag network in the first stage, it would be interesting to analyze the composite resopnse, as the output gain stage is run at relatively low gain, without a dominant pole compensation.

As is or with some tweaking it could certainly be the basis of fairly "blameless" preamp, as Douglas Self might say. Particularly if one has some of the parts or comparable types on hand; for example, I might substitute Zetx devices like the 696B/796A for the outputs.


Thanks for your comments, Jon.

Like you I also feel this is a good design, probably "blameless" as you say.

A dual FET should be essential too, but that is not a problem nowadays. Perhaps a 2SK389 might be a better option than the U406.

In fact, if we follow what most DIYers do here, pairing transistors might help improve things further.

One thing I commented on another thread the Bryston does, which might be applied here, is pairing certain key passive parts between channels.

Why go or not to go fully differential is something I'm not quite clear on. Recent single ended designs seem to be a depart from the full differential princliple though, so maybe there's something there. Perhaps a slight resemblance with tube designs.

Gain can be set as per the table on the left lower corner. That also picks the zobel network on 1st stage.

Picking different output transistors might be the icing on the cake, even MOSFETs perhaps.

Whatever the parts are, my inclination would also be for best audio quality. That's the main "blameless" I would care for.

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