"Gain Clown" Composite Amp - diyAudio
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Old 22nd March 2009, 11:15 PM   #1
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Default "Gain Clown" Composite Amp

Attached is a schematic for my "Gain Clown", a composite amp using the OPA541 and OPA627. I actually have the boards on a heat sink and running - the schematic here was used in a simulation to do some final tweaking on the compensation. Simulations were very similar to the results I obtained at work using an HP4194A gain-phase analyzer. The response shows some gain peaking for smaller compensation values (100-150 pF). A compensation capacitor of 220p is about right, giving a nice, flat response curve.

In reality, the amp runs the OPA541 off of +/-30V rails, and knocks that voltage down to +/- 15V for the OPA627 using a pair of zeners.

When I get the thing wrestled into a box, I'll do some more tests, and more importantly, find out how it sounds. If anyone is interested, I'll put together a more detailed schematic.
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Old 22nd March 2009, 11:18 PM   #2
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Here is the frequency response curve. This amp came out of some idle musings regarding the relatively slow response of the OPA541, and how it might sound in a composite amp with a high quality JFET opamp to help out with some of the dirty work.
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Old 24th March 2009, 05:49 AM   #3
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Another thing I'm considering (it's a relatively easy add) is to put a current sink between the OPA627 output to its minus rail to shift the opamp into class A operation, especially considering that its output load is the high impedance (+) input of the OPA541, a JFET-input device. This will be most likely postponed until I can audition the basic setup as-is.
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Old 9th April 2009, 04:55 AM   #4
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419 views and no comments - lightweights...
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Old 9th April 2009, 07:43 AM   #5
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Probably, because everybody is waiting for the auditioning results.
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Old 9th April 2009, 08:09 AM   #6
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Probably not too long now, I have everything mounted in a case, but I want to change the output lugs that interface the boards to the output binding posts. I'll then bring up the power supply, and t if that looks ok, hook up the amps and let rip. The amp modules have already been tested on a bench supply. Right now I'm looking at 40kuF per supply of filter caps, using 100V TO-220 Schottky rectifiers. The amp modules have 1200 uF low impedance caps (UCC LXZ series) and 2 uF polycarbonate film caps mounted right next to the power chips. I also want to change out the 220 pF compensation caps. The polystyrene units I'm using now came from my junk box, and they're pretty flimsy physically. I found some more robust caps at a local surplus store.
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Old 9th April 2009, 12:56 PM   #7
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kF?
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Old 9th April 2009, 01:29 PM   #8
AndrewT is offline AndrewT  Scotland
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but he is from silicon valley
But that is a lot better than misusing mF where m is supposed to mean micro but is confused with the milli multiplier.

I call it mF when I mean milli Farads.

Maybe he still wears sleeved shirts.
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Old 10th April 2009, 03:02 AM   #9
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I'm reluctant to use the "m" prefix for anything except voltage. Even then, PSpice doesn't like or understand it. I'm also a slow typer, so I'm allergic to lots of zeros, especially late at night.

I was going to make a joke about stuffing the case with 40 sets of old pants kufs for damping, but there's enough silliness on this list, intentional and otherwise....

The amp will probably be completed this weekend, especially as I have tomorrow off from work. I'm kinda ambivalent as to what what I'll get, as the OPA541 seems to have been set to run with a bare minimum of bias current, but then this whole project was sort of a lark and a whim to begin with. I have a discrete amp with heavy Class A bias waiting for reassembly after some tweaks, and I wouldn't be at all surprised if it sounded better.
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Old 10th April 2009, 09:24 AM   #10
AndrewT is offline AndrewT  Scotland
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m is the standard 10^-3 multiplier.
It should be used where appropriate.
m 10^-3 milli
u 10^-6 micro
n 10^-9 nano
p 10^-12 pico
f 10^-15 femto

notice they are all lower case, the same as k kilo 10^3
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