SSM2017 to SSM2019 Upgrade--Noisy

[beginwhereUR]

Hi Friends,

I've got an older DBX 286A Project 1 mic preamp in which I've changed out the the preamp chip from an SSM2017 to an SSM2019. The problem is, the 2019 chip is much noisier -- sounds mostly like 60Hz hum.

I was under the impression the 2019 chip was a plug & play replacement for the 2017, but that doesn't seem to be the case here. BTW, I'm sure it's the 2019 chip. I installed a socket, and both 2019s I've got hum the same, whereas the 2017 sounds fine when I put it back in.

I'm hoping someone here can help me understand and solve the problem. Any help much appreciated.

Schematic similar to my unit can be found in a Zipfile here:

http://www.dbxpro.com/vintage_download.php?product=286A%20%28PROJECT%201%29

Regards,
Joe

 

[arigy]

How do the preamp works with SSM2019 installed? Do it have any unwanted effects other that 60 Hz hum, like distortion?.. Could you also check power supply rails for unusual noise?

 

[beginwhereUR]

Hi and thanks for your reply.

With the 2019 installed, the preamp seems to work fine other than the noise--the same amount of gain, no noticeable distortion, just a terrible background noise/hum.

As for power supply noise, wouldn't a problem like that be evident with the original 2017 installed?

BTW,  the preamp actually works fine with the 2017--no problems at all.  I chose to upgrade it just for the sake of it; plus I need the leftover 2017 to repair a friend's Joe Meek preamp, which needs to match the other of his pair.

Thanks again for your help.

Joe

 

[abbey road d enfer]

I tend to suspect some oscillation. What happens when you load both legs of the input?

 

[beginwhereUR]

If it's not too ignorant of me, may I ask you to explain how I should best do that?  I'll follow your instructions and then let you know the results.  Thanks.

 

[Denyle Guitars]

You could try increasing the 220pf across the inputs to 470pf.

 

[JohnRoberts]

The data sheets look like the parts should be interchangeable.

Do you have a scope?

Check that PS voltages are clean, and chip is stable.

JR

 

[Denyle Guitars]

Another thought. If the circuit was designed originally for a INA217, there might be no cap across the inputs and the ssm2019 reveals a problem that the ssm2017 didn't.

 

[arigy]

You could try increasing the 220pf across the inputs to 470pf.

Just looked on Rev.A1 of 286 Project 1 schematics, they have 470 pF capacitor across the inputs.