Midiman Mini Mixer mod - advice needed

[Marco Pancaldi]

Found old Midiman Mini Mixer. Very small, simple, unbalanced, but (near) perfect for routing my Jam Man loopers in parallel with my live guitar rig (some analog stomps to Pod Live X3). The mixer is a stereo, two 4-to-1 sections  using a TL062, with very simple circuit.

I have not a schematic, sorry, but I try to make it in simple words:

- input jacks to opamp inverting inputs via 100K res. and mylar .22 cap. in series
- feedback res. 100K (unity gain)
- non-inverting inputs biased to 1/2 V with couple 100K res. in series + a .22 mylar filter cap between 1/2 V and ground
- opamp output to out jacks via 100R and 5uF bipolar cap. (two back-to-back 10uF electros), plus 100K to ground in parallel to output jacks
- internal 9V battery, no power filtering, no external power socket.

The external powering will be probably 12V (from a pedalboard power supply) so I will need to add a power input with some filtering (inspired from some Jack Orman schematics, I hope a diode for polarity protection and a 100R + 100uF will be enough).
Pods put out a rather large level in line out mode, so I need to optimize the headroom and limit hiss to the minimum.

Using a bipolar opamp (33078?) is a possibility, but I will probably need to reduce the resistors value, and increase the input caps to compensate, using in this case electros. A 2132 is another possibility to keep the fet inputs and 100K resistors. Third, a rail-to-rail opamp may be necessary to optimize headroom.
In general - I know it sounds funny - an opamp that can distort gracefully on some very fast (occasional) transient will be another good idea.


Any advice welcome, many thanks
m.

 

[PRR]

You want 4-into-1, no controls, from sources with large levels?

Four 22K resistors from four input jacks to one output jack. (Duplicate for stereo.)

This has loss, but for hot sources it may not be a problem. Hiss will surely be lower than 100K resistors and an '062. Power supply is a non-issue.