Adapter For Using Series Effects Loop........

[The Ballzz]

Hi All, Newbie Here,
What I'm looking to build or buy is basically a 2 channel stereo mixer with no extra bells/whistles to simply utilize an existing amplifier's serial effects loop in a parallel fashion. Battery powered or installable into the amp utilizing some of the amp's power for a supply would be ideal. Please see this thread at the Marshall forum to get an idea of my goals:

http://www.marshallforum.com/workbench/74778-really-out-there-effects-loop-question.html

As you can see, I've already come up with a "workable" solution, but would like to trim it down to be no more than what is needed. I'm fairly certain the solution/design/circuit is truly simple, I simply need some advice and direction.
TIA,
Gene

[PBE6]

I really don't know much about the inner workings of tube amps, but according to this site:

http://www.koch-amps.com/blog/effects-loops-explained-by-dolf-koch-series-vs-parallel-effects-loops

built-in effects loops handle level matching and impedance matching in addition to signal routing, so it seems like a safe bet to tap the signal from the "send" jack.

A circuit like this should work:

http://www.runoffgroove.com/splitter-blend.html

Since you're only looking for one loop, make the following simplifications:

- delete the 1M resistor next to Red SEND (bottom left of the diagram)
- delete the 220n cap, the 1M resistor, opamp U2B and the 10u cap from Red RETURN (everything on the bottom right of the diagram)
- connect the output of the 10u cap from Red SEND directly to the 25k-B BLEND pot

Plug the "send" jack on your amp into the input, Green SEND/Green RETURN to/from your effects rack, and the output into the "return" on your amp. The blend knob will allow you to change the dry/wet balance and the polarity switch can correct out-of-phase errors (or create fun new ones!).

NOTE: If you have a quad opamp or two dual opamps, you can simplify further and save parts by replacing the Q1 buffer stage with an opamp buffer stage.

[The Ballzz]

PBE6,
That looks pretty simple, cheap and easy to try out! I truly appreciate your reply to this newbie! Your suggestions are nicely detailed, clear and easy to understand. I'll gather some parts and post any questions I may have.
Thank You,
Gene

[The Ballzz]

PBE6,
i have modified the schematic as suggested and have a few questions. What would be a suitable "quad" opamp for this purpose and how would I make the connections to it in replacing Q1 with one of the opamps? I see 3 connections to U2A (in addition to the power) whereas on Q1 I only see 2 connections, plus the power. Am I correct in assuming that if I were to use U2B, for example, in a dual opamp, I would simply need to connect #2 & #1 together as the output, use #3 (+) as my input and connect ground to #4? Would a quad only use one power in and one ground or does it require two of each?

Solid state devices are foreign to me, but it sort of appears that they connect and operate much like a tube! Hmmm. I find no way to upload attachments from my computer to private messages and did not think it appropriate to post schematics. I will PM my personal email if you would be willing to communicate with me in that manner. I'm pretty much ready to go on this project, but need a little guidance/advice before ordering components. It does seem that a quad would make all the construction a bit more convenient, as I already have a supply of 16 pin sockets on hand, along with some Veroboard.
All Help Appreciated,
Gene

[PBE6]

Quad opamps are much the same as dual opamps, except  each chip has four opamps built in and only one set of power/ground pins (4x3 + 1x2 = 14 pins total). Your 16 pin sockets will work just fine, just make sure you check the layout of your particular chip before you solder in the socket to make sure they match.

There are numerous choices for the type of quad opamp, but the TL074 is cheap, plentiful and well-suited to a number of applications. An LM324 would also be a good choice.

When replacing Q1 make the following adjustments:

- keep the 220n cap next to Green RETURN, but remove Q1, both 4k7 resistors and the 1M resistor
- connect the output of the 220n cap next to Green RETURN to the non-inverting input (v+ pin) of your opamp
- connect a 1M resistor to the input (v+) as well, and connect the other end of the resistor to Vref
- connect the inverting input (v- pin) to the output (vout pin) to form the buffer
- connect the output (vout) to the input of the second 220n cap

When you're done, the wiring should look identical to U1A and U1B.