Power amp in (RETURN) jack for Musicman

[Leevi]

I'm adding a power amp input to Musicman 112RD.


The connection point is before the C24 where I have connected a mono jack including switch.
I have also cut the foil in order to separate the preamp when power amp input is plugged.


https://drtube.com/schematics/musicman/rd50a.gif


It works with any effect box but if I plug a guitar to the input I get hum and other noise.


I suppose there is a question about  impedance?


/Leevi

[Latole]

No, not impedance issue, non shieded wiring or grounding issue.

Best is to show your wiring, input jack and switch

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" It works with any effect box but if I plug a guitar to the input I get hum and other noise."
-Leevi

What do you mean ?
What effect box can you plug in a power amp input jack ? Or I'm wrong ?
Do you plug guitar in power amp input jack ??

[Leevi]

Effect box output is a line level signal that will be amplified in power amp. That was only a test. Normal case is of course that some preamp is connected to the power amp input.


Plugging a guitar was a test as well.

/Leevi

[PRR]

YES you need a resistor there. The opamp is inverting. The input is a "virtual earth". In stock connection, gain is R43/R42, 150k/15k, about 10. If you just stuff signal it with "zero" resistance it wants to make "infinite" gain. It also shorts-out the source (<10 Ohms). Between the two you probably have tons of hum and almost no guitar tone.

Hack below will be suitable for hot preamps (3V output). Lowering the new resistor makes the amp more sensitive but reduces its damping; also 22k is already awful low impedance for most guitarist's stage-cord devices.

It really wants a buffer or significant hacking of the reverb recovery stage to make it a line input.

[Leevi]

YES you need a resistor there. The opamp is inverting. The input is a "virtual earth". In stock connection, gain is R43/R42, 150k/15k, about 10. If you just stuff signal it with "zero" resistance it wants to make "infinite" gain. It also shorts-out the source (<10 Ohms). Between the two you probably have tons of hum and almost no guitar tone.Hack below will be suitable for hot preamps (3V output). Lowering the new resistor makes the amp more sensitive but reduces its damping; also 22k is already awful low impedance for most guitarist's stage-cord devices

Actually I added a 800 Ohm  resistor there already but the question is if it is too low? It improved the situation with guitar. The problem with a bigger resistor is that it changes the setup if power amp input is not plugged. There is already a 15K serial resistor before the connection point. Of course I can implement the input using a switch that eliminates the extra resistor when input is not plugged.


/Leevi

[Leevi]

In stock connection, gain is R43/R42, 150k/15k, about 10.

How does the R44 (47K) affect the gain?


/Leevi

[Leevi]

I did some tests and ended up to the following setup.


2.2K resistor is a limiting value when the ground hum disappears. If I drop it to for instance 1.5K
the system starts to hum even the signal source is off.

Also noticed that the Music Man power amp is pretty weak so the signal level must be high.

/Leevi

[pdf64]

With a tweak to c20 / r30, the reverb return jack, recovery stage etc could be used for a power amp in.

[Leevi]

Do you mean that the recovery input can be used as power amp input?
Then the reverb pot would work as a level control?
/Leevi

[pdf64]

Yes

[PRR]

.

[Leevi]

Yes

It works but the sound becomes very distorted if you increase a bit more volume i.e. turn the reverb pot open.


/Leevi

[Leevi]

.

Thanks PRR, this is the SOLUTION!
Furthermore I get free the level control for the input.

/Leevi