Completed Project Need volume control on power amplifier

Thread Starter

Alex Cristian R

Joined Feb 16, 2019
22
Hi!

I have an old EKO Tiger P61 electric organ which has a builtin amplifier and speakers. The problem is it doesn't have a master volume control! I don't completely understand how this amplifier works but i gave it a shot.

I have tried to put a pot on the signal path (66) but i get strange results:
- 100k pot -> lowest volume position is not complete silence; catches radio signal
- 1M pot -> a lot of distortion as i reduce the volume (similar to an overdrive effect); catches radio signal

Sure i do something wrong, probably that's not the right place for the pot. I can't figure out where to place it.


In the small picture the op-amp sums multiple sources of signal to a single one (66)
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BobTPH

Joined Jun 5, 2013
9,149
Break the connection from R64 to the opamp.

Replace R64 with the two outer terminals of a 200K pot. Connect the slider to the opamp.
 

Irving

Joined Jan 30, 2016
3,983
how are you wiring the pot? can you show more of the preamp....

@BobTPH while i'm sure that willwork, its a hgh-impedance location so any length of wire to the pot will be susceptible to noise and radio pickup. The output of the preamp is low impedance (or should be) and is the better location IMHO.
 

Thread Starter

Alex Cristian R

Joined Feb 16, 2019
22
@Irving This is how i attempted to connect the 100k/1M pot the first time.

The preamp D is a CA3280.

Alwo i should have mentioned that on the headphone jack path to ground i have placed a 10k resistor because it was too loud and distorted.

1703226693088.png
 

Irving

Joined Jan 30, 2016
3,983
@Irving This is how i attempted to connect the 100k/1M pot the first time.

The preamp D is a CA3280.

Alwo i should have mentioned that on the headphone jack path to ground i have placed a 10k resistor because it was too loud and distorted.

View attachment 310561
Wrong way round..... needs to be like this else you're shorting the output of the opamp. I'd keep the pot resistance <5k as it feeds a 1uF capacitor in the power amp, which creates a high-pass filter at 15Hz. a higher value pot will adversely effect the bass response. If the leads to/from the pot are long I'd recommend audio screened cable.

BTW how old is this amp? The A4136 opamp is a 47y old product, obsolete since 2002.

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Thread Starter

Alex Cristian R

Joined Feb 16, 2019
22
@Irving I just got it working with the first solution @BobTPH suggested with a 100k pot. This time i have also used a home made shielded cable and it did get rid of any RF signal. The volume control works just fine now. Also in this case the cable is only 15cm.

I will try with a 5k pot with @Irving 's solution when i get better shielded cables.

It's old, the schematics are dated 1980. Probably that's why it also has some other issues, that i will have to address some other time.
 

BobTPH

Joined Jun 5, 2013
9,149
A 100K pot cuts your max output in half, which may or may not be a problem. I am glad to hear that it works okay, though.
 
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