Samson mpl 1502 mixer op amp upgrade

[r3dback]

Hello, I'm new to the site and a bit of a noob. Anyway, I recently got a Samson mpl 1502 secondhand for $20. But some of the channels have a lot of noise on them. So I pulled it apart and had look, all the caps and traces are fine so I'm assuming the op amps are damaged. There are 28 njm4560l's from what I read fairly run of the mill. So I was thinking about replacing all of them with some better ones like njm2068's or njm4556a's or would I be better just replacing damaged ones with new njm4560l? Any help would be appreciated.

 

[abraxalito]

Noisy channels isn't very likely to be caused by opamps 'failing'. The opamps will be the most reliable parts of the mixer. Look to the pots first, then connectors then electrolytics.

If you're looking to upgrade the transparency the power supplies would be a great place to start.

 

[r3dback]

Thanks, I'll take a look at the pots, also the first two mic inputs are significantly weaker than the others and have an annoying hum.

 

[Enzo]

I'm assuming the op amps are damaged.

There is no reason to assume that. Individual op amps might go noisy. But when an op amp goes noisy, or a transistor or whatever, that is a failing part, replacing them with "lower noise" parts is different. Those can fail noisy just as easily. A failed op amp is not noisy because it isn't a low noise type, it is noisy because it failed. And all that assumes the trouble is an op amp in the first place, it could be otherwise. There is little point to changing parts in channels that are working well.

Your bad MIC inputs sound from here like those are not running on all cylinders so to speak. In other words unbalanced due to one of the signal pin circuits not working. I don't have your schematic, if you do, post it. But looking at other Samson mixers, like the PL1602 or XM610, I suspect your mic input feeds transistors rather than op amps. I could me making a faulty assumption though. Are these also the noisy channels?

On your noisy channels, does the noise remain or go down as you turn the channel volume down? The mic channels have insert jacks, have you used them as sends and returns to see if the noise is before or after the jacks?

 

[r3dback]

Yes, the weaker channels have the hum/noise it's always there regards if something plugged into it or not. But it goes away when I turn down the volume on the channel and it uses sony 2SA920 PNP transistors. I can't find them or an equivalent on ebay and I don't have a copy of the schematic.

 

[Enzo]

OK, so the noise is before the channel fader. You have insert jacks. They are after the input stages, but before a bunch of the channel circuit. They are TRS jacks. I forget which is send and which return, easy to figure out. So connect the insert send over to a clear channel. Turn the noise channel fader to zero. Now the noise from pre-insert on the noise channel will come out the clear chanel. Any noise?

Likewise, plug a sorting plug into the insert. Does it kill the noise or not? If not, the noise is after the insert.

If before, as I suspect due to your mics being weak, I suspect the transistors first.

2SA920? Sure that isn't 2SA970? That is the PNP they used on other mixers.

Most any audio quality PNP small signal transistor will work there.

2SA970:
B&D has them
Search

But if you look for subs for that type instead of 2SA920, I bet you will have better results.