Adding a DI output to a mini mixer pedal.

[DFischer]

I have a DIY mini mixer built on perfboard that I'd like to add a DI output to send the mixed signal to a PA. I'm not much more than an electronics hobbyist, so I could use some help with a couple details.

The mini mixer is built from on RG Keen's design:

mini mixer

There's no room in my box for a transformer, so I plan on using a simple dual op-amp balancer. The one I chose is from my bass amp that seems to work well:



The Keen mixer works off a single rail power supply while the DI uses a bipolar power supply. Since I already have the mixer built into a pedal, I'd like to run the DI off the same power supply. My questions are:

1. How do I bias the positive op-amp inputs for the DI circuit? I sometimes see bias voltages applied through resistors and sometimes directly from VBias (1/2Vcc). I'm also confused where to inject it in reference to the voltage divider at the input to pin 3. Since the output of the final op-amp in the mini mixer should be at bias voltage, can I just take the input to R95 from just before the coupling cap at the mixer's output?
2. Am I correct to assume that I will need to add capacitors with reverse polarity before C63 and C68 to render them effectively non-polar due to bias voltage showing up at the op-amp outputs?
3. C63 and C68 are rated at 25V. Is this enough in the event that phantom power is applied to the XLR?

 

[Newmarket]

I have a DIY mini mixer built on perfboard that I'd like to add a DI output to send the mixed signal to a PA. I'm not much more than an electronics hobbyist, so I could use some help with a couple details.

The mini mixer is built from on RG Keen's design:

mini mixer

There's no room in my box for a transformer, so I plan on using a simple dual op-amp balancer. The one I chose is from my bass amp that seems to work well:

View attachment 132975

The Keen mixer works off a single rail power supply while the DI uses a bipolar power supply. Since I already have the mixer built into a pedal, I'd like to run the DI off the same power supply. My questions are:

1. How do I bias the positive op-amp inputs for the DI circuit? I sometimes see bias voltages applied through resistors and sometimes directly from VBias (1/2Vcc). I'm also confused where to inject it in reference to the voltage divider at the input to pin 3. Since the output of the final op-amp in the mini mixer should be at bias voltage, can I just take the input to R95 from just before the coupling cap at the mixer's output?
2. Am I correct to assume that I will need to add capacitors with reverse polarity before C63 and C68 to render them effectively non-polar due to bias voltage showing up at the op-amp outputs?
3. C63 and C68 are rated at 25V. Is this enough in the event that phantom power is applied to the XLR?

You are giving yourself the "Tascam Problem". For simplest solution I'd recommend a simple Impedance Balanced solution.
I'd also suggest increasing both the value and voltage of the output DC blocking capacitors.

 

[DFischer]

Thanks Newmarket. I'm reading up on Impedance Balancing as time permits. I like how Brian Roth summed up the Tascam Problem in another thread:

""Tascam" designs would cause one poor little opamp to SCREAM when it's destination went to "ground" if the destination was unbalanced."