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OPA1656: High-Performance CMOS Audio Op Amp
OPA1656: High-Performance CMOS Audio Op Amp
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Old 15th September 2021, 12:53 AM   #571
JRA is online now JRA  United States
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Quote:
Originally Posted by Salas View Post
Was it reverse polarized and built up pressure? That white thick cable to its left looks like a minus PSU rail.
Indeed it was, sir. More current photos attached.
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Old 15th September 2021, 01:08 AM   #572
Salas is offline Salas  Greece
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OPA1656: High-Performance CMOS Audio Op Amp
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Old 15th September 2021, 01:12 AM   #573
simon7000 is offline simon7000  United States
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John

He hears a pop. The gain on a change of some sub millivolt offset to zero will make a pop. The DC blocking capacitor is 100 uF, that gives enough time constant to allow a substantial popping noise.

The input 47K resistor and the cartridge DC resistance may of course lower the voltage created by the offset current. Which actually may be a bad thing if it was offsetting the voltage. Of course the input offset current is typical 10 pA which if compensated would require a 50 megohm resistor across the dc blocking capacitor and leakage current from the card and or capacitor makes that insignificant.

Adding any small film capacitor will show if the half millivolt offset is the problem. (Or 5 volts at the output!)

Last edited by simon7000; 15th September 2021 at 01:37 AM.
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Old 15th September 2021, 01:15 AM   #574
JRA is online now JRA  United States
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To be clear to all: It's a potential cone-bottoming pop.
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Old 15th September 2021, 06:01 AM   #575
MarcelvdG is offline MarcelvdG  Netherlands
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Sounds like some sort of oscillation issue that depends on the source impedance.

The cable that's shorted at the other end will behave as a transmission line resonator that apparently gets undamped by the op-amp. Chances are that an RC series network across the input can damp the resonance and fix the issue.

How long is the cable from the turntable to the phono preamplifier and what dielectric does it have?

Last edited by MarcelvdG; 15th September 2021 at 06:20 AM.
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Old 15th September 2021, 06:15 AM   #576
jean-paul is offline jean-paul  Netherlands
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Maybe I am nitpicking but designing a PCB with both DIL and SMD layout for the opamp would make it better (apart from the issues). I miss some capacitance at the input.
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Last edited by jean-paul; 15th September 2021 at 06:37 AM.
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Old 15th September 2021, 08:22 AM   #577
MarcelvdG is offline MarcelvdG  Netherlands
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Assuming you have a cartridge that requires a very low load capacitance (does it?), you could try something like 82 ohm in series with 47 pF across the input. You could also connect it as an RC low-pass filter between the input and the op-amp, that will give some extra rejection of picked up RF signals at the expense of a very small increase of the noise.
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Old 15th September 2021, 08:39 AM   #578
jean-paul is offline jean-paul  Netherlands
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Old designs of decades ago had RF filtering. Today the aether is chockfull of RF and even modern designs by known designers lack RF filtering (add power on/off muting and DC protection to that). Modern silicon is way more wideband too. Some designs are proudly presented with plots going in MHz range…

Contradictory.
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Old 15th September 2021, 03:02 PM   #579
johnc124 is offline johnc124  United States
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Quote:
Originally Posted by simon7000 View Post
John

He hears a pop. The gain on a change of some sub millivolt offset to zero will make a pop. The DC blocking capacitor is 100 uF, that gives enough time constant to allow a substantial popping noise.

The input 47K resistor and the cartridge DC resistance may of course lower the voltage created by the offset current. Which actually may be a bad thing if it was offsetting the voltage. Of course the input offset current is typical 10 pA which if compensated would require a 50 megohm resistor across the dc blocking capacitor and leakage current from the card and or capacitor makes that insignificant.

Adding any small film capacitor will show if the half millivolt offset is the problem. (Or 5 volts at the output!)
I don't disagree with any of this, I was simply clarifying that at steady state the gain applied to the op amp input offset voltage is 1. I'm still suspicious that an input transient is forward-biasing the back-to-back diodes across the inputs of the OPA1656 causing the input offset current to be several orders of magnitude higher than the typical value momentarily.
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Old 15th September 2021, 03:05 PM   #580
johnc124 is offline johnc124  United States
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Quote:
Originally Posted by jean-paul View Post
Old designs of decades ago had RF filtering. Today the aether is chockfull of RF and even modern designs by known designers lack RF filtering (add power on/off muting and DC protection to that). Modern silicon is way more wideband too. Some designs are proudly presented with plots going in MHz range…

Contradictory.
This is very true. I was able to produce some extremely loud pops by placing my phone directly on top of a commercial phono preamp.
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