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Hello,

I am really new to this, so please don't flame me for any stupidity on my part.

I am building a single supply inverting op amp circuit to slightly increase the gain of the output of a DAC. The DAC I'm using has an output of ~ 1.2v rms, and I'm trying to double that to ~ 2.4v rms. So, I know that I need a gain of 2. The problem that I'm having is that with the circuit connected, the output sounds like a digital reverb effect (similar to what would happen if you turned on the "hall" setting on your receiver). The output voltage is also lower than the input signal. I've triple checked the signal, and experimented with different resistors and capacitors, with little effect.

Any help is appreciated!

Here's the circuit:

I'm not at all sure why your real circuit is misbehaving.

Have you put something like a 100nF capacitor directly across the supply pins of the opamp to decouple it? If not, it may be oscillating and what you are hearing is an aliasing effect of the DAC sample rate and the opamp oscillation.

How tidy is your layout? Is it on breadboard or a PCB? Again you may have crosstalk causing oscillation or pickup from elsewhere in the circuit.

Not that it affects your real circuit but I note that in your CL opamp the slew rate is set to 0.00002 V/us.

I don't know if this is a CL or your typo: it should be 20 V/us. Double click on the opamp to see these parameters.

Thanks for your reply, signality.

The circuit is currently built on a breadboard. It was fairly tidy, but I completely rebuilt it so that it would be as clean as possible. One of the things I did was cut all the component leads as short as possible, especially the feedback resistor. I also added 100nf cap across the supply.

I'm still hearing the reverb/echo sound, but subjectively I don't think it sounds as bad. I've read that testing op amp circuits for audio on a breadboard can lead to weird problems, so I think my next step will be to build the circuit on a PCB.

As far as measurements go, I measure 7.5v DC across the +, -, and output pins for each amp, which is what I would expect. I tried to measure AC voltage on the input and output, and measured in the 100mV range on both, which doesn't seem right. However, I'm using a relatively cheap multimeter from RS, so that may be the problem.

One last thing with regards to the grounds that I'm using in the circuit. The ground I am using for the voltage divider and for the - voltage pin in the op amp (pin 4) is the negative output from the regulator. The ground I'm using on the output signal is the chassis of the source device (DAC). Does that sound correct?

Thanks again for your help!

"The ground I am using for the voltage divider and for the - voltage pin in the op amp (pin 4) is the negative output from the regulator."

If you mean that you're using a positive voltage regulator and so your ground is on the negative (ground) side of that regulator, then that is correct.

"The ground I'm using on the output signal is the chassis of the source device (DAC). Does that sound correct?"

Only if the DAC out ground (chassis) is very firmly and directly connected to the opamp circuit ground. Otherwise you have a big loop of wire back to the opamp power supply, possibly via capacitive coupling if that PSU is not earthed through the mains back to the DAC chassis.

That is asking for hum pickup in that loop.

You could do worse than download yourself a free copy and have a read of this:

http://www.analog.com/library/analogdialogue/archives/43-09/linear_circuit_design_handbook.html