Fun little P-P input stage

[SY]

I got slightly distracted while working on my DD ESL amp, and have been playing around with this input stage; it's a variant of a design I've been using in amps for the past 20 years or so. Main difference is the incorporation of an input transformer (there actually is no center tap on the transformer, that's a mistake on my drawing). In any case, it's good for a gain of about 20, will swing about 150V p-p, and has an f₃ of about 80kHz. The balance is better than I have the capability to measure when the input resistors are well-matched.

Matching of the 2N5462s is a good idea; I used a simple test jig to match I_d with a 1K resistor between source and ground. With FETs thus matched plugged into the circuit, I got plate voltages within 5 volts of one another.

Variations: for feedback, one can add 100 ohm resistors in series with each input resistor to the ground point, then run symmetrical feedback resistors from the 16 and 0 ohm taps of the output transformer secondary, grounding the 4 ohm tap. And you can get more gain and a bit more swing by bypassing the 1K cathode resistors.

This would make a dandy input stage for an ST-70.

[SY]

BTW, at the expense of esthetic symmetry and galvanic isolation, you can get rid of the transformer, take the input to one of the FET gates, and etiher ground the other one or use it as a feedback point.

And to get enough gain for feedback, you can also direct couple this stage to a 5692-based long tail diff amp- that's pretty much the circuit I've been using in most of my push-pull amps over the years.

[DrG]

Great looking circuit, SY. It's similar to something I've been toying with a "balanced" input for a hybrid amp, using bipolars.

[SY]

It's the bastard child of Dave Berning and Bill Johnson. Cross-coupled circuits are very useful for situations where one may have to use both balanced and unbalanced inputs.

[planet10]

SY,

Wouldn't a CCS on the cathodes be of benefit?

dave

[SY]

Pardon me for being dumb, but I don't get what you mean. Where would I put the current source? (I know you can draw! )

[planet10]

I still don't know what names go with what on a FET, so i just call it a cathode... wouldn't the -24V like to be from a current source?

dave

[SY]

No, that's the FET's drain (plate), not the source connected to the minus rail; note that the FETs are configured as source followers, where you want the drains to be at signal ground.

[Pjotr]

Maybe a current mirror is meant at the anodes? But then you end up single ended out ......

[SY]

Pjotr, no, I suspect it's reflex on Dave's part; you work with tubes too long and your brain always thinks of the minus rail going to cathodes. Easy mistake to make. I suspect he looked at the circuit and thought, "Cascode!" Which it isn't at all.

If we only had positronic tubes...