Ptownkid

Preamp cuts out at high gain
« on: June 28, 2011, 06:54:32 PM »
I'm racking 8 q8 micpre cards for a client and I'm getting something I'm not familiar with...at about 3/4 gain, it abruptly goes silent and a little above that it starts to crackle if you make noise, and a little above that it makes all sorts of alien noises...

There's 8 channels of ssm2015 based pres, with 8 TL072 balancing cards...I'm using a 50VA xfmr, (36VCT @ 1.39A). Even when only one card is hooked up it does this, and 2 of the 8 don't exhibit this behaviour.

Any ideas?
« Last Edit: June 28, 2011, 07:09:05 PM by Ptownkid »


tchgtr

Re: Preamp cuts out at high gain
« Reply #1 on: June 28, 2011, 07:43:07 PM »
Does the ssm2015 need a gain set cap? Is this with a p48 powered mic on the input?
Resistance is not futile. It is voltage divided by current.  (thanks, Bill)

Ptownkid

Re: Preamp cuts out at high gain
« Reply #2 on: June 28, 2011, 07:58:50 PM »
Not sure about the gain set cap...looking at the datasheet right now. These are supposedly "known working" cards from a westar console. Just using a sm58 for testing.

abechap024

Re: Preamp cuts out at high gain
« Reply #3 on: June 28, 2011, 08:05:07 PM »
They use a resistor to set gain I believe.

Sounds like they are possibly oscillating.
But that is strange cause they seem like stable chips.
Maybe they are touchy on how they are grounded.

Also have you isolated the problem to the ssm chips?
AC Sound - some DIY circuitboards

Ptownkid

Re: Preamp cuts out at high gain
« Reply #4 on: June 28, 2011, 08:22:04 PM »
My first thought was also oscillation...going to check that in the morning. Is there a specific cap that takes care of that, my knowledge in this area is limited...

http://www.ka-electronics.com/images/SSM/SSM2015.pdf

bruno2000

Re: Preamp cuts out at high gain
« Reply #5 on: June 29, 2011, 09:45:05 AM »
I'm racking 8 q8 micpre cards for a client and I'm getting something I'm not familiar with...at about 3/4 gain, it abruptly goes silent and a little above that it starts to crackle if you make noise, and a little above that it makes all sorts of alien noises...

There's 8 channels of ssm2015 based pres, with 8 TL072 balancing cards...I'm using a 50VA xfmr, (36VCT @ 1.39A). Even when only one card is hooked up it does this, and 2 of the 8 don't exhibit this behaviour.

Any ideas?

Transformerless?  Leaky Phantom blocking caps?
Best,
Bruno2000

JohnRoberts

Re: Preamp cuts out at high gain
« Reply #6 on: June 29, 2011, 10:17:56 AM »
Generally higher gain should be more resistant to oscillation as the NF is padded down, but not that topology, where closed loop gain interacts with open loop gain so remains relatively constant.

 One thought, if there is a DC issue, the higher gain could be driving the output into saturation at one rail? While symptoms do sound like oscillation.

Also check that the compensation caps are correct for the bias current and in place.

JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

Ptownkid

Re: Preamp cuts out at high gain
« Reply #7 on: June 29, 2011, 02:58:26 PM »
As it turns out...it really is all 8 channels, just that two of them only exhibit the behaviour when gain is all the way up and the pad off.

John...what do you mean by DC issue? They are all getting +/-18V

abechap024

Re: Preamp cuts out at high gain
« Reply #8 on: June 29, 2011, 03:07:53 PM »
are the 2 physically farther away from the power supply? Maybe the rails need to be reinforced?
Glad your getting it worked out!
Cheers
AC Sound - some DIY circuitboards

Ptownkid

Re: Preamp cuts out at high gain
« Reply #9 on: June 29, 2011, 03:19:49 PM »
no...it's totally random...3 and 8 are the least f'd up and 8 is closest to the psu.

Not the supply...tried my bench supply with the same result.
« Last Edit: June 29, 2011, 03:23:53 PM by Ptownkid »


Ptownkid

Re: Preamp cuts out at high gain
« Reply #10 on: June 29, 2011, 04:26:31 PM »
Ok, so it turns out it's being caused by the balancing pcb i made... can anyone see why?

http://sound.westhost.com/project51.htm

JohnRoberts

Re: Preamp cuts out at high gain
« Reply #11 on: June 29, 2011, 04:28:15 PM »
As it turns out...it really is all 8 channels, just that two of them only exhibit the behaviour when gain is all the way up and the pad off.

John...what do you mean by DC issue? They are all getting +/-18V

If there is a DC voltage present across the input, as the gain gets turned up that DC times the gain could exceed the rail voltage... Not common but a possible fault.

Maybe look at pin voltages between good/bad channels. Or with gain low/hi.


JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

Ptownkid

Re: Preamp cuts out at high gain
« Reply #12 on: June 29, 2011, 04:31:34 PM »
It's being caused by the balancing circuit...i used the one in the link in my last post (figure.1)

Should that work...or am I out to lunch...
« Last Edit: June 29, 2011, 05:27:47 PM by Ptownkid »

gemini86

Re: Preamp cuts out at high gain
« Reply #13 on: June 29, 2011, 09:18:43 PM »
are there dc blocking caps between the pre output and your balancer? Is there DC there when you crank the gain? it could be slamming the balancer with DC and that's no bueno.
- Rodney

"...you better call Kenny Loggins, 'cause you're in the danger zone."

Ptownkid

Re: Preamp cuts out at high gain
« Reply #14 on: June 29, 2011, 10:32:04 PM »
No there are not blocking caps...did not know they were required there  :-[ what kind of value? I'll check for DC in the morning.

Rochey

Re: Preamp cuts out at high gain
« Reply #15 on: June 29, 2011, 11:02:28 PM »
DC blocking caps would be used just in case there's any DC offset in the system.

Their value will be based on the input impedance of the next stage... As it'll create a high pass filter.

the larger the value, the closer to DC you can move the -3dB high pass filter point.

However, other's might comment on the side effects.


Good luck with this. I'm watching to see what the resolution is.
Expat Audio Home: http://www.expataudio.com

Ptownkid

Re: Preamp cuts out at high gain
« Reply #16 on: June 30, 2011, 08:42:47 AM »
With no gain there's about 135mV rms at the output of the pre...and it looks like HF noise, with full gain it goes up to about 230mV rms.

Ptownkid

Re: Preamp cuts out at high gain
« Reply #17 on: July 05, 2011, 09:18:51 AM »
So I experimented with various DC blocking caps between the output of the preamp and input of the balancing circuit with no real change in result.

Next?

Here's the pre http://www.mediafire.com/?dgb7oda55ijlk4d

Rochey

Re: Preamp cuts out at high gain
« Reply #18 on: July 05, 2011, 09:41:03 AM »
There's already a 100uF cap in the schem. That should take care of any bias.

Sounds like the issue is in your differental line driver. Have you tried a single chip line driver? Such as the drv134/5?



 
Expat Audio Home: http://www.expataudio.com

Ptownkid

Re: Preamp cuts out at high gain
« Reply #19 on: July 05, 2011, 09:46:11 AM »
Not yet...I was hoping to solve it using the existing pcbs and parts. I'm going to move on to the THAT chip or TI chip soon, but it means that my customer will have to pay an extra $5 per channel and I'll have to eat the cost of the tl072 pcbs and parts...


 

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