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Old 16th August 2005, 09:38 PM   #1
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Default El Cheapo Phono Pre problem

hi

I've built this up to but not including the output caps, feeding it with 15V supplies and a lot of decoupling and it sounds lovely.

But for one thing: grossly overloading either channel (eg with a big scratch) makes that channel latch up to +11V and stay there til the power is reset. Has anyone else seen this? Could it be something to do with the enormous DC gain of the circuit?

thanks!
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Old 17th August 2005, 11:43 AM   #2
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The op-amp (I'm assuming there's one) might have phase reversal on overload, a common problem, or just plain latchup when it hits the rails. A diode clamp in the right place might fix it, or a different amp might not display this problem.
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Old 17th August 2005, 11:53 AM   #3
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Quote:
Originally posted by scott wurcer
The op-amp (I'm assuming there's one) might have phase reversal on overload, a common problem, or just plain latchup when it hits the rails. A diode clamp in the right place might fix it, or a different amp might not display this problem.
Yes, it's an OPA637. I was wondering whether anyone else has had the same problem (it's a pretty popular design it seems) and if not why not.

I'll try a pair of diodes across the input tonight, though I doubt it takes as much as 0.6V to cause the problem. Might have to roll off the gain below 20Hz but, like I said, I'd like to know why no-one else has needed to.
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Old 17th August 2005, 12:06 PM   #4
Previously: Kuei Yang Wang
 
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Default Re: El Cheapo Phono Pre problem

Konnichiwa,

Quote:
Originally posted by bremen nacht
But for one thing: grossly overloading either channel (eg with a big scratch) makes that channel latch up to +11V and stay there til the power is reset. Has anyone else seen this? Could it be something to do with the enormous DC gain of the circuit?
It is probably the specific Op-Amp used and maybe too much input from the cartridge. You might want to reduce gain a little. I never encountered this myself.

Sayonara
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Old 17th August 2005, 12:13 PM   #5
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cheers, KYW.

Output is about a volt, surely that's not too much? it's being driven by an Audio Technica MC cartridge and their step-up transformer btw.

How would I go about adjusting gain without upseting the EQ?
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Old 17th August 2005, 01:43 PM   #6
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Another thing you could do is get it latched up and probe the voltages in the circuit and find where it doesn't make sense. Or in other words makes sense only if something is wrong like the amplifier is in a state of having a huge bias current. I actually observed this in a preamp once. The transformer on the input can make some circuits do funny things at DC. Do you have a link to the schematic?
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Old 17th August 2005, 01:51 PM   #7
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Quote:
Originally posted by scott wurcer
Another thing you could do is get it latched up and probe the voltages in the circuit and find where it doesn't make sense. Or in other words makes sense only if something is wrong like the amplifier is in a state of having a huge bias current. I actually observed this in a preamp once. The transformer on the input can make some circuits do funny things at DC. Do you have a link to the schematic?

http://www.diyaudio.com/forums/attac...postid=109194&

I haven't used the fet input stage, or the output cap arrangement (just using a 1uF polyprop cap).

It behaves the same with or without transformer input ; when it's not connected to any input or output, dabbing a finger to the input makes the output latch up.

I'll also try rolling off the LF gain with a pair of electrolytics (sorry!) in series with the 475R.

Thanks for all the replies!
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Old 17th August 2005, 01:58 PM   #8
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Konnichiwa,

Quote:
Originally posted by bremen nacht
Output is about a volt, surely that's not too much?
Depends, is it 1V for 5cm/s? if so it's too much.

Quote:
Originally posted by bremen nacht
How would I go about adjusting gain without upseting the EQ?
Increase (adjust) the resistor to ground from the inveerting input.

Ciao T
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