Silence from wet channel in clean blend circuit

[marcelomd]

Hi,

I've been working on a Box of Rock inspired design with a clean blend plus a new tonestack. It's a learning exercise and part of the challenge is to use only MOSFETs and no opamps.

Right now I'm having trouble with  the wet/dry blender. Here is the relevant part of the circuit:



The problem is that I get no wet sound at the blender output. The dry sound blends in and out just like it should. Both signals are ok at their coupling caps, C11 and C12, so I know the problem is at the blender.

I'm guessing it is some kind of impedance mismatch, but I was expecting at least a weak signal, not complete silence from the wet side.

I got the values from the panning article by RG.

Bonus question: I expect a big volume difference between channels, what is the best way to make them more even? Simple voltage divider in the strongest side?

Thanks a lot!

[Redvers]

I'm confused, isn't that a circuit with one input and one output?

[marcelomd]

This is just a fragment of the whole circuit. My question is about the blending circuit in the middle; the other parts are there for context =)

In more detail:
The input buffer/booster is to the left (not shown). The signal is split into two. The upper part (from C6 to C12) has two high gain overdrive stages and the lower part (from C7 to C11) is just a clean buffer.
These two signals are then blended with R21-R24. The MIX pot moves the ground from side to side, blending that side in and out.
Then there is a volume pot (because I dislike the volume being the last thing in a circuit) and another buffer that drives the following stages (tonestack, not shown).

Thanks!

[ElectricDruid]

Is this on a breadboard or a PCB?

The reason I ask is that the first thing I'd try would be swapping the dry and wet inputs over, so wet goes in R23 and Dry goes in R21. Since the stage is symmetrical, if it's an impedance problem, it'll behave exactly the same. If something changes, there's probably a fault somewhere, and what you're looking at isn't the same as the circuit diagram.

HTH,
Tom

[Kipper4]

Seems like maybe the wiper of the mix pot should go to a series 10k to lug3 of vol?

otherwise isnt it just bypassing the mix pot? (2x 15k)

It still might not be brilliant. for the amount of parts you have, I'd go active mixer stage.

[ElectricDruid]

Seems like maybe the wiper of the mix pot should go to a series 10k to lug3 of vol?

No, this is a fairly common balance/pan circuit that you see in mixers and such like. It has inherent losses, so usually it's followed by make-up gain (and that may be the problem here). As you shift the Mix pot, the divider on one side approaches 15K/0K (so no output) and the one on the other side sees 15K/10K (so 2/5ths output - hence the need for make up gain). In the centre, both channels see 15K/5K, so you get equal amounts (1/4) of each signal.

Here's an example:



Oh look! Same values! What a surprise! Note the 51/15 = x3.4 gain on the op-amps to maintain the levels.

From http://www.next.gr/audio/mixers/two-channel-panning-circuit-l12498.html

T.

[PRR]

> no wet sound at the blender output.

The plan is fine. This suggests a wiring error.

[marcelomd]

Is this on a breadboard or a PCB?

The reason I ask is that the first thing I'd try would be swapping the dry and wet inputs over, so wet goes in R23 and Dry goes in R21. Since the stage is symmetrical, if it's an impedance problem, it'll behave exactly the same. If something changes, there's probably a fault somewhere, and what you're looking at isn't the same as the circuit diagram.

HTH,
Tom

> no wet sound at the blender output.

The plan is fine. This suggests a wiring error.

Had only 5 minutes to play with this circuit this weekend. It's still on the breadboard. I disassembled the mixer and put it together again. No change. Then I inverted the inputs and got the same result: dry channel is ok, and the wet channel is silent (only the action is inverted).

I'll try to simulate it when I can.

Thanks for your responses.

[marcelomd]

Well.

It works in LTspice. Meh, let's start again.

Thanks!