Could someone please post the final SquareWave schematics (or at least a picture of the PCB). I see some discrepancies in schematics/BOM/silkscreen published at http://tee8tee4388.blogspot.ru/2013/07/squarewave.html, e.g.:

1. Schematics uses two opamps, but BOM and silkscreen have only one.
2. Schematics and silkscreen have 402 ohm resistor, but BOM lists a 49.9 ohm one.
3. BOM contains a 1n914 diode, but schematics and silkscreen do not use it.

this is the final and correct schematic

http://gilmore.chem.northwestern.edu/squarewaveproduction.pdf

 

if you are careful and match the fets, you can get rid of the 1500uf caps.

 

the original board had the servo inverted, hence the extra opamp

to invert the output.

 

latest stereo board

http://gilmore.chem.northwestern.edu/sqwave9.jpg

 

pair of boards installed in icariums SS1

http://gilmore.chem.northwestern.edu/sqwave10.jpg

 

the corrected mono board is also available.

 

R16 really should be 100k because the servo is too slow with 1M

Edited June 22, 201412 yr by kevin gilmore

this is the final and correct schematic

http://gilmore.chem.northwestern.edu/squarewaveproduction.pdf

 

Thanks. To sum it up:

1. Two opamps.
2. 402 ohm resistor.
3. 1n914 is not needed.
4. Two 1500uF caps can be replaced with one 1000uF.
5. R16 can be changed to 100K from 1M.

Wouldn't I need to change C4 (for 100K R16) to keep the same LPF behavior? Also, it seems that only R7 and R8 need to be 1/4W, am I right?

you really want 100k. with 100k resistor, the servo gets it under 1mv in 30 seconds.

otherwise its a few minutes.

 

r7 and r8 can be up to .2 watt.  if you short the output, about .5 watt

 

and its a dual opamp in a single package

 

you can get rid of both electrolytics, or the one electrolytic in the latest board

by maching the fets and moving the left side of R21 to the junction of R1 and R2

and then shorting the bottom of R17 to ground.

 

Then its fully dc coupled, but you have to go thru a bunch of fets to get

the matching right.

Edited June 23, 201412 yr by kevin gilmore

More SquareWave questions:

 

1. Can the DC offset be stabilized with the output shorted to ground, e.g. closed relay in parallel with the load?

2. What parameters should the JFETs be matched for, Idss, Vp, Vgs(Ids)?

3. I'm designing my own SMT PCB (smaller 2-channel board). Can 2N4393 be replaced BF862 or 2sk209?

 

Thanks.

1) of course not. and it is usually a bad idea to short out any amp to ground

2) idss

3)  both of those should work.

Edited July 27, 201412 yr by kevin gilmore

Too bad. I was using a mechanical switch to short Bottlehead Crack output before powering it up to avoid the turn-on thump (as suggested by one of the designers on the BH forum). Guess I'll go with relay in series with the load and make SquareWave look into 300Ohm resistor for the first 30s.

Is the Squarewave power up sequence that wild? I mean it should take a few secs to stabilize and then an open load is perfectly fine.

servo is taken from the output.

can't possibly stabilize when the output is shorted to ground.

in fact, probably makes it much worse.

nm

Edited July 27, 201412 yr by luvdunhill

So any otl tube amp with a capacitor coupled output is going to have one

very large turn on thump. except for Mikhail's disasters that left the output

cap charged forever (black gates stay charged for months)

That turn on thump can be 50 volts or more. Not a good idea.

And the original cap coupled square wave probably had a 10 volt or more

turn on thump to charge the 2 x blackgates on the output.

Would using a 0.1uf 250vac rated mkp be a good move to suppress any in burst charge on power up on the switch? I've have some tube amps and some high voltage ss amps that use this on the switch (my Stax SRM1 has it).

Edited July 29, 201412 yr by DefQon

The Stax amps have a surge suppressor which is a cap and a resistor in series inside one box.  It's more to protect the switch than anything else. 

True true just read the samething elsewhere.

Here is the layout I came up with. It's my first SMT board and I would appreciate some tips. The board is 85x100mm, traces are 30 (power), 20 and 12mil.

SquareWave.pdf

that looks good to me, but I would never attempt to build something that small