single supply opamp balancing stage needs more gain

[Sleeper]

Hi All.
So I built an acoustic360 which works great but I need to squeeze a little more gain from the balanced output circuit. Problem is it's single supply and this is just outside of my knowledge. I've searched and I tried to apply what I do know, but I can't figure this one out.

here's the original:

I tried some changes and the amp was quieter with the lower impedances.
I've tried changing some of the resistor values like you would in a ground referenced amp, but as I increase the gain the relationship between out+ and out- isn't equal anymore, I get why this is happening but I can't figure out how to solve the problem. 

I'm thinking maybe there isn't a solution except to add a gain stage in front of the 50k pot.
I'm out of ideas. little help?
Thanks
Sleeper

 

[PRR]

> here's the original:

Yes and no.

Re-draw it like I have done. Duh! Top triangle is a simple unity gain follower, Bottom one is an inverter tied to the OUTput of the folower.

The two drawings are topologically equivalent; but your sketch leads the eye astray.

Now that you have your eye straight, the fix is clear. You want to change the top triange into a stage with gain. The bottom triangle MUST be left alone: unity-invert is the correct function. I've drawn gain-of-2, 6dB up from what it was; you can reduce the "~~~" resistor for higher gain (though if you go crazy, you may have DC trouble).

The single-supply hardly enters into it, except all "signal grounds" return to "Vg", not the general ground. This is (or ought to be) dead-zero for changing signals plus a +12V DC shift to keep everything midway between +25V and zero rails.

Lower impedance -may- be a little quieter. More to the point: is "Vg" really two large resistors and an opamp, no filter caps? I would expect a chunky cap (470uFd) right AT Vg, isolated from the opamp by say 470 ohms. There are other ways to do it, but hey.... it's only a stage-amp.

 

[Sleeper]

Hi PRR,
It's 98degrees here and this cooled me right down. Thanks I've got work to do in the morning instead of beating my head against the wall.
when you said:

The single-supply hardly enters into it, except all "signal grounds" return to "Vg", not the general ground. This is (or ought to be) dead-zero for changing signals plus a +12V DC shift to keep everything midway between +25V and zero rails.

  I get it NOW.
I'm not sure if it's that I have a solution AND a problem in front of me or that you've explained it simpler and with more precision than any of the articles I was looking at: but I get it.  Start working on that book soon.
Thanks
Kelly

 

[Sleeper]

Reporting in. This works perfectly 

As per PRR's advice I did add a 470ohm resistor after the opamp and a 220uf filter cap from VG to ground near the opamp.  I've put a 5532 in there,  10k resistors all around except the one PRR underlined in red... where I ended up using a 39K  (100K worked but it was way more gain than I needed).
I used 300ohm buildout resistors instead of 1k
Running into the 600ohm transformer on my test amplifier I was able to push about 10Vrms (20dbv) before a 1khz sine wave started getting all bent.
Woo.
I've got a compressor on my bass rig that feeds either house or a recorder and now I can hit that mofo as hard as I damn well please.  yes.

Thanks again PRR
Kelly

 

[PRR]

> Running into the 600ohm transformer

Then increase the 1uFd output caps to 100uFd.

0.5uFd (two 1u in series) against 600 is 530Hz!

The "600 ohm" winding is more than 600 ohm reactance at 500Hz, dropping below 600 at 50Hz or 10Hz depending what it is. The net result is a big resonance somewhere in mid-bass. It is not easy to eliminate the resonance, but you can shift it well below the audio band.

 

[Sleeper]

Good eye.
Oops, I forgot to mention that I used 100uf bipolars in place of the 1uf caps on the output.
K
 

 

[PRR]

Since you have a solid +12.5V DC on each output, there is no compelling reason to run bipolars. (And no reason to take them out either.)

The 1uFd is clearly intended to drive >10K loads. And they probably believed the future was entirely transformerless. It seemed a good bet at the time.

An "honest" +23dBm in 600 ohms is near the limit of what a 5532 can do for current. Near transformer bass roll-off, current increases. Broadcasters who had to demonstrate low THD at low frequency into transformers sometimes ran four units (two 5532) series-parallel for this kinda level.

However for "hit that mofo ..damn well" stage-amp use, this should do fine. If not, your next step is a W.E. Repeat Coil hanging on the speaker tap. Keep a fire extinguisher near the compressor.

 

[Sleeper]

I have a big bag of Nichicon Muse's that I use for this kind of coupling. I swear I don't get all audiophiley about much, but that's one of the few situations where I could hear a real difference when playing around with caps.

I'll have to see how much oomph this thing has under actual use, but it's clearly going to be plenty.  For bass amps I've always liked to go a bit overboard with headroom throughout the system. I WILL bring a fire extinguisher along just in case.

Kelly