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Please first consider the below schematic. It is the balanced output part of an existing hifi preamp. Not my design, it's confidential so I can only reproduce the relevant part.

balanced output section

Things that might be of importance before I ask my question:

  • 'IN' signal is the output from a discrete gain stage.
  • 'MUTE' signal is -7V when inactive and positive when active
  • C2 and C3 are 16V bipolar types: ELNA RBD series

Measurements:

  • OUTP measures 0V with 0V input
  • OUTN measures -2.0V with 0V input
  • muting abruptly shorts OUTN (and causes strong audio pops) and C2 recharges within 1 sec to the same level as before.
  • There are 2 instances of the above schematic (obviously for L and R channels) and both charge to a different voltage (one to -2.0V, the other to -1.6V). This suggests it is related to capacitor leaking.
  • Opamp power supply is +/-15V, nicely symmetrical and locally decoupled.

Question:

Why does C2 charge to a small negative voltage level?

Next question might be how to solve it, but if I know why, I can probably solve it myself.

EDIT: Most likely cause of this problem seems to be the muting transistor. According to specs they shouldn't cause any problems, but since they are directly at the output pins (with no added ESD protection), they could have been ESD damaged. I've ordered some new ones and I should have them by the end of this year. It's a pain to disassemble and take out those transistors, so I might as well have replacements ready and do it only once.

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  • \$\begingroup\$ What does the simulation result indicate? \$\endgroup\$ – Andy aka Nov 11 '20 at 16:26
  • \$\begingroup\$ when replacing symbols with voltage sources and opamp and NPN with generic types, I get a few 100's of nV on OUTP and OUTN No change when I add parasitic parallel resistance of 100k to C2 \$\endgroup\$ – gommer Nov 11 '20 at 16:38
  • \$\begingroup\$ muting causes strong audio pops - consider adding capacitors in parallel with Q1, Q2 base. \$\endgroup\$ – Reinderien Nov 11 '20 at 21:15
  • \$\begingroup\$ Not sure about that suggestion. Muting will become slow and it's used often when switching inputs or when digital inputs pause. There should not be any DC present to begin with. \$\endgroup\$ – gommer Nov 12 '20 at 9:03
  • \$\begingroup\$ It's precisely because your mute is too fast that there's a pop. "Too slow" is subjective - if there's a 10ms delay, will it be noticeable to the user? \$\endgroup\$ – Reinderien Nov 12 '20 at 17:01
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MUTE should not go below -5V with normal transistors, or else they may suffer reverse base-emitter voltage breakdown.

Which might pull an unloaded output to -2V (with MUTE off) or so... charging the capacitor to give you a surprise when operating MUTE.

Check the datasheet for this transistor (it MIGHT specify a higher voltage), and if necessary, replace them and take steps to reduce the -ve swing on Mute.

EDIT : Datasheet DOES specify Vebo = 25V max, so this ought to not be a problem. This is unusual, perhaps these transistors are designed for this specific role. But check for substituted parts or possible counterfeit parts, and/or test a few samples for possible out of spec leakage (at say, 10V)

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    \$\begingroup\$ Valid points for 'normal' transistors. But 2SC2878 is something special. Discussed here: electroschematics.com/… Furthermore, the capacitor charges only after the inverting opamp, not in other places with the exact same muting circuit. \$\endgroup\$ – gommer Nov 11 '20 at 17:20
  • \$\begingroup\$ @gommer : that's why I said check : having a spare moment I did, and your comment/my edit crossed paths. \$\endgroup\$ – user16324 Nov 11 '20 at 17:24
  • \$\begingroup\$ Sure, was not meant to gripe. Also added the link to muting transistor page for the general reader. It's apparently a special breed of transistors that I didn't know of myself before this issue. \$\endgroup\$ – gommer Nov 11 '20 at 17:37
  • \$\begingroup\$ No worries. But we're looking for about 40uA. Have you pulled a transistor from a faulty output and checked for leakage? Does the cap still charge without it? (If so, cap datasheet might be useful) \$\endgroup\$ – user16324 Nov 11 '20 at 17:59
  • \$\begingroup\$ cap datasheet: hfc-fs.s3-eu-west-1.amazonaws.com/s3fs-public/elna_rbd_data.pdf \$\endgroup\$ – gommer Nov 11 '20 at 18:07

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