boosting low audio signal

[Tomek]

Hi

In my home music studio one of devices I use
is signal processor (multieffect) Lexicon MPX100
It is fantastic machine however there is one problem
- signal level on output is very low
There is input knob and output knob
Input one I can't turn more - clip.
Output is on max and final signal still very low
even turned up on mixer to maximum.
It makes me a lot of mixing problems.
Comparing to signals from other instruments/processors
on the mixer it is very low

I was just wondering if I could (with your help)
make some simply modification on PCB
I have soldering/desoldering experience
but no theoretical knowledge

is it possible to boost signal by simply replacing some of components
like resistors? I don't think I will be able to replace whole opamp
or output potentiometer itself (I think it is R97) but maybe R88/98?
or R102/104 (I'm simply guessing)

I will appreciate any tips and help

schematics and pic of unit attached

 

[ronsimpson]

Does the "output level" have any effect?
I see there is a "mute" is it on?
Check your cable and wiring. Switch Right and Left.
This thing should drive your mixer very hard.

 

[Tomek]

there is effect on output depend on mix knob
mute is not muted, cables etc ok
I use it since long time
all ok but just signal is low from beginning

 

[crutschow]

You could reduce the values of R102 and R104 to increase the gain of the output op amps.
To try it you can add a resistor in parallel with these to reduce the equivalent value without having to un-solder the resistors.

You could also increase the values of R101 and R103 to increase the gain but that would affect the de-emphasis rolloff frequency.

 

[Tomek]

You could reduce the values of R102 and R104 to increase the gain of the output op amps.
To try it you can add a resistor in parallel with these to reduce the equivalent value without having to un-solder the resistors.

You could also increase the values of R101 and R103 to increase the gain but that would affect the de-emphasis rolloff frequency.

I will try both options
Im not sure - does de-emphasis affect signal/noise ratio?
I mean can it be then more noisy?

 

[audioguru]

De-emphasis affects frequency response. Earlier in the circuit there is pre-emphasis that boosts high frequencies and at the output the de-emphasis returns the frequency response back to normal and at the same time it reduces noise that occurs in the middle of the circuit. De-emphasis must perfectly cancel pre-emphasis.

 

[ronsimpson]

Before you make changes you can't undo;
The manual said, that the output can drive a headphone direct. There should be plenty of signal. Try a headphone. If you can hear anything then the answer is simple.

 

[JimB]

I have no suggestions other that to agree with Crutschows suggestion to change resistor values.
As AG comments, this will of course alter the de-emphasis characteristics, which may or may not be a problem for you.

I am surprised that the designers of this thing did not ensure that it had a 1:1 through gain, or maybe they did and there are some odd effects taking place.
Looking at the picture, there is a BYPASS switch, presumably this removes all electronic processing of the signal.
Are you still lacking output when bypass is selected?

Also, is it possible to disconnect this unit from the rest of your system?
If you can run with the thing disconnected, is there enough signal from the source to drive the amplifier?

JimB

 

[crutschow]

I have no suggestions other that to agree with Crutschows suggestion to change resistor values.
As AG comments, this will of course alter the de-emphasis characteristics, which may or may not be a problem for you.
...............................

Changing the value of R102 and R104 should have little effect on the de-emphasis characteristics.

 

[ronsimpson]

Before you make changes you can't undo;
The output IC RC4556 may be dead. The signal is just leaking through the part.
The mute is stuck on. The mute is not very good. Some signal will slip through.
Connector or cable problems.
This thing can deliver 2.5Vrms of signal. It was designed OK. (the CS4327 will deliver 2.83Vp-p or 1V rms) The filter has a gain of 1.
The output buffer has a gain of 2.5x.
If your mixer is seeing mV of signal then something is wrong. Not the design. You need to fix something before you modify.