STAX SRM323 schematic - balanced input

[purrin]

Does anyone have a schematic for the SRM323? I can't seem to find one through Google.

@Spritzer: I assume it's possible to do a balanced input on it? At least that's what I inferred and seen from other designs - the STAX amps are inherently balanced.

[kevin gilmore]

http://gilmore.chem.northwestern.edu/srm323aorig.pdf

balanced no problem. But you would have to change out the input pot, or

bypass completely.

[purrin]

http://gilmore.chem.northwestern.edu/srm323aorig.pdf

balanced no problem. But you would have to change out the input pot, or

bypass completely.

Thanks - it looks pretty straightforward.

Now I gotta find a 4-gang pot.

[spritzer]

Odds of finding a split section pot like the one Stax uses are zero so you have to get a new knob as well or do some drastic mods to the current one.

[purrin]

Odds of finding a split section pot like the one Stax uses are zero so you have to get a new knob as well or do some drastic mods to the current one.

LOL, I didn't even realize it was a split pot (balance control). I might glue or find a means of securing the two knob sections together. At least STAX makes it easy to get access to the board with a removable top cover and bottom plate.

I haven't had a chance to trace the circuit, but a quick glance at the board, I do see an R2 and C2 labeled but unpopulated, which I assume are the for other leg of the balanced input (R1 and C5 respectively).

[nikongod]

Balanced shunted 2 deck pot.

All you need is a few 1% resistors, match them if you feel like it. Or dont, your still better off than 90% of 4-deck pots. The Pot You have now will work fine. I would start with resistors at 1/2 the nominal impedance of the pot - if you have a 10Kohm pot try 5Kohm resistors to start. There is LOTS of room for experimentaion in that. Buy some other values too (10k & 15Kohm). Its still cheaper than buying another pot.

Configuration A is what everyone does. Configuration B is what I would suggest.

Advantages:

Minimal cost.

Better phase matching.

Keeps the split pot in this case.

No drilling, finding knobs, etc.

If the knob is switched you can keep that.

Disadvantages:

Its cheap

CAN get bitchy with floating output transformers.

Real men dont need balance controls.

You loose the switches and knobs race.

Depends on perspective:

This wastes some gain. IMO most amps have too much gain, others disagree.

Edited March 18, 2011 by nikongod

[purrin]

Holy crap! I never would have thought of that in a million years and I just need to head down to Frys. I wasn't exactly looking forward to locating a 4-gang pot.

B seems like the plan. I don't have output transformers to worry about, and I need to lose some gain after I go balanced anyways.

Edited March 19, 2011 by purrin

[spritzer]

Pfft... real men custom order 4-gang pots just for the hell of it...

[nikongod]

When you put it together make sure that when the volume is set to minimum there is very little resistance between the 2 pins you use.

Volume minimum=impedance minimum.

I usually spend 3 times as long as I should thinking about this & measuring a bunch to make up for the 1 time I didnt. Once bitten, twice shy.

[purrin]

When you put it together make sure that when the volume is set to minimum there is very little resistance between the 2 pins you use.

Volume minimum=impedance minimum.

Got it. I was already doing the math / voltages in my head. Although it's simple, it still gives me a headache since I haven't done this since college.

I usually spend 3 times as long as I should thinking about this & measuring a bunch to make up for the 1 time I didnt. Once bitten, twice shy.

Yeah, understood, I blew up a GS1000 driver and one channel of a DAC's output stage by accident once.

Edited March 18, 2011 by purrin

[kevin gilmore]

pretty sure the default pot is 50k. So use a series 25k resistor.

Should work for any solid state source, but may not work well with

a source that has a tube output stage. An input transformer won't

help with the impedance, but will cause other grief.

Also notice that very few pots actually go all the way to zero, so

there will be some minimum volume.

You are still going to have to punch holes in the back panel

for the xlr jacks.

[spritzer]

Not necessarily, the male Rean mini-xlr's should fit in the current holes:

You still need to add a switch if you want to use both types of inputs though...

Edit: The stock pot is 50K

Edited March 19, 2011 by spritzer

[purrin]

OK. I traced the board. Just a reality check before I proceed. I still need to get parts anyways.

Here is a photo of the relevant portion of the board. Fortunately, it looks like STAX set this up for a balanced input in the first place. It appears the negative leg of the balanced input can simply be soldered to the unmarked open hole at the top (below the "L"):

Here is what I intend to do. Additions and modifications are in pink. Note: there are some updates (C5/C1 and R1) from the schematic provided by Kevin Gilmore:

1. Replace R4 with a 470K resistor
   
2. Populate C2 with 240pF cap
   
3. Populate R2 with 2000ohm resistor
   

Finally, unless I am missing something, there is no need from anything to be connected to the ground - what's marked "G" on the board.

Edited March 19, 2011 by purrin