2 Channel Unity Gain Mixer

[astronomicum]

First, I am not a solid state design guy but can find my way around simple circuits. I am trying to bring 200mV attenuated wet and dry signals together without signal loss. I have breadboarded a buffer using a BC547, voltage divider bias, and 9V supply, with good results. Bringing the signals together into the base of a single buffer generates signal loss. Using two buffers and bringing the emitters together distorts the signals. I have searched but I have not had any luck so far on how to bring the outputs of two BJT buffers together. I am working on a schematic which I will post when complete. If anyone has any ideas in the meantime, it would be appreciated. Thanks.

[PRR]

Are these 'strong' signals, like from electronics? Or passive guitar pickups?

At 200mV, you may have to think about hiss. This means boosting the signal (at least 3X), then mixing, then padding-down if needed.

Opamps are VERY valuable here.

[astronomicum]

Are these 'strong' signals, like from electronics? Or passive guitar pickups?

PRR, Thanks. This is a signal from a guitar pickup. What I am doing is sampling the wet and dry signals within a Crybaby a providing a "blend" control. I want to remain at unity to keep the blend and bypass signals fairly equal.

At 200mV, you may have to think about hiss. This means boosting the signal (at least 3X), then mixing, then padding-down if needed.

I was not thinking about noise so your point is well taken. Overcomes the signal loss in the mixer as well. I will give this a try.

Opamps are VERY valuable here.

I will keep this in mind. I have a bunch laying around but never worked with any of them. I will need to learn the basics.

[pdf64]

The basic wet and dry signals will be in opposing polarity.
But the wet signal will probably have a lot of phase shift (up to 90°) either side of the passband.

[astronomicum]

The basic wet and dry signals will be in opposing polarity.

Not sure what this means.

But the wet signal will probably have a lot of phase shift (up to 90°) either side of the passband.

Thanks pdf64. Yes. I see this now with dual trace. Somewhere between 60-90°. I had wondered about this at the start but had not investigated further :/ Will an RC work to adjust the phase?

The circuit is nearly complete. I have a single input running about 2.5X gain, an attenuator, and the output at unity with room to boost it a bit to compensate for any loss in phase correction (if possible). Just need to add the second input channel. I intend to finish the circuit and see how it sounds with the shift (assuming it works). I am curious.

[astronomicum]

The circuit is fully functional. Testing with a signal generator and line level program music all looks and sounds good. Good blend with a stereo input. Just started the testing through the pedal and discovered the pot in the Crybaby is bad  Jury is out till a replacement arrives :)

[pdf64]

The basic wet and dry signals will be in opposing polarity.

Not sure what this means.

How about 'the signals are inverted with respect to each other'?
Or 'the signals are mirror images of each other'?
As one goes up, the other goes down, and visa versa.

But the wet signal will probably have a lot of phase shift (up to 90°) either side of the passband.

Thanks pdf64. Yes. I see this now with dual trace. Somewhere between 60-90°. I had wondered about this at the start but had not investigated further :/ Will an RC work to adjust the phase?

Not really.
Frequency and phase are 2 sides of the same coin, one can't be manipulated whilst leaving the other unaffected.

[astronomicum]

Thanks. Looking at the schematic I see that the wet signal would be inverted.

With some additional research, I understand. I am going to sweep through some frequencies so I can see the phase changes. The 60-90° I had observed was at 600Hz.

[astronomicum]

I have completed and tested this circuit. I achieved unity gain along with good balancing of the wet and dry outputs. IMO, the mix sounds rich and well balanced, and I prefer a mixed output over the original straight wet signal, even with the phase shift. By changing the footswitch, the circuit provides bypass in both modes. The only downside, if you could call it that, is the mix circuit draws a lot more current that the original circuit resulting in shortened battery life (if you use one). The toggle switch disconnects the DC from the mix circuit when "original" mode is selected. Thanks everyone for the input.