Differential to single-ended converter question

[DaveTV]

I've been working on building a digital reverb pedal using the Alesis AL3201-B chip and the circuit I'm using is based on the schematic shown in the data sheet for the chip. The basic signal path is:

Input==>A/D converter==>reverb chip==>D/A converter==>Output

The digital-to-analog chip I'm using outputs both an in-phase signal and an inverted phase signal and both signals are fed into a differential to single-ended converter opamp circuit at the output stage of the circuit.

I'm having trouble getting this opamp circuit to work. I can get signal coming out of the D/A converter, but I can't get anything out of the opamp. Here's the schematic for the opamp stage:



Do the values look okay for this type of circuit? I've been assuming that R24 is 4.7k ohms and not 47k ohms. Any help is appreciated.

[Transmogrifox]

Yes, it's all just fine.  make sure that you are using 4.7 k for R24 because it's hard to see "." in the schematic so it looks kinda like 47 k.

You need to be using +/- supplies as well, say +/- 9V.

The other option is to make a V/2 divider decoupled and connect the V/2 divider to where ground is shown on the schematic at R24.

It is also good to capacitively couple the output for DC blocking.  Use a 10uF or something.

[DaveTV]

Thanks for the reply, Trans. How important is it to use the 11k resistors in this circuit as opposed to just using the more common 10k resistors? Would that effect the filtering much in this circuit?

[DaveTV]

The other option is to make a V/2 divider decoupled and connect the V/2 divider to where ground is shown on the schematic at R24.

And is this right? Connect at R24? What about at C39? Should that also be connected to V/2?

[Transmogrifox]

C39 is better left connected to ground.  The reason is that the capacitor does not effect the steady-state DC conditions, which are important.  

The point is just to keep the operating range well above and below the negative and positive supply rails so that clipping does not occur.

...and about 10k vs. 11k:
I think it is fine to sub this value.  The purpose of the design is to take off the real high frequency (like digital) noise.  The change wouldn't have an audible effect unless you changed to 100k or something.

[DaveTV]

Thanks for all the great info, Trans. I decided that I didn't know enough about op-amps and carefully read through the TI Single-Supply Op-Amp Circuit Collection. Very informative.

After reading through the section on filters, I can confidently say that the above schematic is a multiple feedback Butterworth-style 2-pole low pass filter that is connected in a differential amplifier format.

For the sake of simpifying things for experimentation, I was wondering about how to wire a single-pole low pass filter in the same differential amplifier format. For the inverting input, I imagine a resistor connected to the - input, with a second resistor connected between the - input and the output, and a capacitor in parallel with the second reisistor.

My question then is, how do you wire the non-inverting input? Is it as simple as a resistor connected to the + input, a second resistor connected from the + input to V/2, and a capacitor connected from the + input to ground?