Re: output 'boost' biases to class a ?

[pstamler]

[quote author="buttachunk"]http://www.vintagedesign.halmstad.net/diy/B338_Schem.pdf

see note for pin 4; 39 ohm resistor for 300ohm O/P.

does this bias the output into class a ?[/quote]

Class-A is a relative thing; the unit is single-ended with a constant-current-source bias, so to work as anything but a fuzzbox it has to be Class-A. But Class-A into what? It looks like TR5 biases the output to about 16.75 mA if my arithmetic is correct; into a 600-ohm load that will give you almost exactly 10V peak. That's about +22dBu, or about 18dB of headroom over +4dBu nominal level, a reasonable amount. But if you try to drive a 300-ohm load you only get 5V peak, or about +16dBu, which is only 12dB of headroom over nominal +4dBu. Not enough.

So they give you the option of attaching a 39-ohm resistor in parallel with the 150-ohm one. Now, if my arithmetic's corred, you get a whopping 80mA of current through the output device, which is 24V peak. Well, you can't really get there, because that's the rail voltage and there are a couple of p/n junctions in the way, but you certainly get enough current

Well, possibly I'm misinterpreting what's going on, but that's what it looks like at this end.

Peace,
Paul

 

[PRR]

> It looks like TR5 biases the output to about 16.75 mA if my arithmetic is correct

I bet it is "+24V" and "0V", not +/-24V as you seem to have assumed (if my arithmetic is correct). Note the connection and polarity of C1 C2. Yes, it is curious that no output cap is shown.

Taking the +24V guess gives 8 or 9mA, plus about 4mA through R8 R10 local feedback network. Say 12mA peak, at not even 12V peak.... 600 ohms is plausible. Actually, R1 R2 bias TR1 emitter to more like 9V, which is probably the output DC level, and TR5 is not going to pull down lower than 1V, so 9V-1V= 8V peak, at 11mA, is 7-something hundred ohms. Fudge a bit, it will pump around 5V RMS in 600 ohms.

Since that is only +16dBm, and +18dBm is a popular spec, they offered a 300 ohm option. R11= 39||150 gives perhaps 40mA (from the general style, I'm guessing this is antique and TR5 may be low Beta). That seems good to 200 ohms, though we may be fighting transistor losses in these old-school parts.

And as Paul says: it runs Class A or it fuzzes real bad.

 

[pstamler]

[quote author="PRR"]> It looks like TR5 biases the output to about 16.75 mA if my arithmetic is correct

I bet it is "+24V" and "0V", not +/-24V as you seem to have assumed (if my arithmetic is correct). Note the connection and polarity of C1 C2. Yes, it is curious that no output cap is shown.[/quote]

Duh, whoops. Yes; I'd let my late-night brain get confused by the "B-" into reading it as being an equal but opposite supply, but I'm sure you're right, and it's single-ended. Your numbers make a lot more sense.

Peace,
Paul