CMOY Amp, slightly odd application, help and advice.

Hi folks.

I've just build myself a CMOY style amp using the Tangent tutorial, but its for a slightly unusual application.

My car stereo head unit has a "line in", however i'm needing an amplifier as the levels it seems to expect from the line in are quite a bit higher than the levels most portable devices seem to put out, resulting in me having to really crank up the levels on the stereo to get a sensible volume, which invariably leads to you forgetting and switching sources without turning the volume down.

So my questions:

I've built it using the standard resistor devider powersupply, however i seem to be getting an uneven split, without any load on the supply at all. I carefully checked both resistors were the same, and they both measured 4.69k, but when powered up the board seems to split a 12v supply to around 7v and 5v. I've rebuilt the entire circuit with new components on a new board and its exactly the same. Soldering two resistors together off the board and sticking 12v across them gives the expected 6v split.

I've mostly resigned myself to rebuilding it with a TLE2426, but i was wondering if anyone had any ideas?

Next and more importantly, i've been wondering about the car audio somewhat. The Cmoy amp references the audio ground against the virtual ground created by the power supply, the car stereo however is presumably going to reference ground as Battery negative... I suspect this will mean that when i plug the audio lead in, its going to do funny things to the circuit, as its basically creating a short between the V- input to the opamp and the virtual ground?

Any thaughts would be much appreciated.

I'll try and get some pics, Although i think i've just had a "D'oh" moment. I installed both electrolytics with their "-" pin pointing towards virtual ground, but it looks as though one of them should be the other way round, which will probably explain whats wrong with my splitter.

On the car side of things, I've done some reading, and i think if i can get the opamp running off a single supply, ie 12v and 0v rather than +6 and -6v, then i should be able to reference the audio signals against real ground, but its going to need some extra components to "AC couple" the output to ensure any DC offset is removed...

http://courses.cit.cornell.edu/bionb440/datasheets/SingleSupply.pdf

The "non-inverting" schematic on page 6 of that PDF seems to be what i want? Although it does seem to be missing the AC coupling capacitors for the output. That way the audio ground is referenced against the real car ground and the smoke stays inside the wires!

Any thaughts?

any ideas how they'll fare driving a high impedence (c.10k) load rather than a pair of headphones?

I know the OPA2132 is happy running a 10k load, but the LM4880/1 appear to be power amps and the specs only seem to mention low impedence loads.

FWIW swapping the capacitor has fixed the divider, but thats not going to help with the bigger issue.

How will the 2132 fare if driven like the schematic in the Ti pdf above?

Ok well i decided i'd try it.

Moved the audio grounds over to the battery negative pin, and soldered a pair of 680uF electrolytics in the output wires, and it seems to work.

Anyone care to comment on whats actually going on here?

I've also noticed some background hiss when theres nothing playing now, any ideas on getting rid of that?

Cheers.

Hmm, ok it gets more interesting.

Just hooked it upto the car stereo with the ipod as source.

It worked, kinda. I got the volume on the ipod to around 3/4 max and begun to get some crackling, any more and it started cracking and distorting really badly. The point at which it starts clipping is still quieter than playing a CD on the head unit, although it IS much better than using the ipod alone.

I wondered if it was being caused by me having moved the grounds, so i moved the audio ground back to the virtual ground, bridged out the two electrolytics and powered it with a 9v battery, exactly the same result.

So where do i look now? Is it likely to be the 2132 thats clipping? i cant see it being the head unit, since its still quieter than the internal radio and CD modes?

Cheers

Kev