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Old 20th August 2009, 09:53 PM   #1
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Default Yet another monoblock

A very boring amplifier, the usual circuit:

Click the image to open in full size.

Note: miller cap is actually 1000pF(!) right now. Supply is actually +/-25V, the 20 is a typo.

Departures from usual:
- CCS bias is temperature compensated (not overcompensated, as the usual two-diode bias is, and not uncompensated as a naked voltage divider is). The 10k and 220 basically form a voltage divider referenced to emitter voltage, which means bias (and therefore overall performance, such as peak output current and slew rate) is exactly proportional to supply voltage. Not really anything special, but it does seem to have nicer turn-off behavior (that is, as the power supply caps discharge).

- MJ11030 and -31 are BEASTLY darlingtons, rated for 50A peak. And they'll do it with only 50mA base current. This circuit could potentially source (but probably not sink) 100A peak into the load. This is why I'm using it for a bench power supply (that it does audio is a fringe benefit; think "bench supply with external input").

Because the darlingtons are so stocky, and the supply is +/-25V, 5A, the ideal load impedance is unusually low: easily 20V, 10A per side (200W square wave, continuous, forever), with peaks up to 50A easily (should do 20V peak sine wave into a 1 ohm load fairly easily = 200W).

Photographs attached. I actually laid out this PCB on computer and toner transferred it. Worked not too badly, but toner leaves a lot to be desired. The output stage is laid out point-to-point (notice the terminal strips!). Chassis is cut from scrap sheet metal. Without a consistent paint job, it looks something like Frankenstein's monster, doesn't it?

Tim
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Old 20th August 2009, 10:07 PM   #2
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Thats a dangerous bias cct.
If the pot wiper becomes detached you will short out the supply through the output transistors.
I always put the pot in the lower leg and a resistorr in the upper leg to ge taround that problem.
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Old 21st August 2009, 12:07 AM   #3
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No. If the pot goes open, the 10k pulls it to 0.7V (more or less). This connection is equivalent to a shunt style adjustment.

What's more dangerous is, there seems to be enough thermal resistance and mass between transistors and Vbe that, if the output stage starts putting out the watts, the transistors locally heat up more than the massive heatsink, and the Vbe in turn lags behind that. When I first turned it on, it was biased at about 1A, which climbed exponentially to 4A (and rising) after a few minutes. It seems to be stable in the 0.2A range. The Vbe transistor definitely needs to be thermally coupled to one of the output transistors, directly.

Tim
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Old 21st August 2009, 06:58 PM   #4
jaycee is offline jaycee  United Kingdom
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why such a large miller cap?
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Old 22nd August 2009, 12:00 AM   #5
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Because the diffamp is running fairly hot (~2mA, whereas I usually set it, oh maybe 1/10 that?), and the output, well it's just darlingtons. Still goes out to ~60kHz (full amplitude -3dB), which is plenty for my purposes. Stability with a capacitive load is most important.

Tim
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