Mute function stops working after removing DC blocking caps?

[Stabby]

I'm working on a Philips CD610 player and I'd like to keep the mute function but get rid of the DC blocking caps. However, when I remove the DC blocking caps there are popping sounds when turning on/off which indicates the mute circuit is not working.

I tried to put a resistor in place of the caps but it seems that some capacitance is required in order for muting to work. Both polar and non-polar caps fix the problem.

I want to get rid of these caps though, I like the sound much more without them. Is there any way to have no caps without affecting the mute circuit?

 

[jcx]

schematics often help with this sort of question

muting circuits I've seen use bjt Q and have to have caps to make the single polarity sw work with audio

 

[Stabby]

This is the muting part from the schematic (4.7uF 50V DC blocking cap). I'm not very familiar with how it works and how I could fix it without the need for a cap. I've tried 4.7uF Silmic II's (polar) and a 4.7uF Panasonic polypropylene film cap (non-polar). Mute function works fine with both but I want no cap at all.

 

[DF96]

The popping sounds without the caps are a clear sign that the mute circuit is working. However, mute circuits (like all audio switches) can only work silently if there is no DC present. The cap ensures this; removing the cap guarantees that DC will be present. You can reduce the DC by reducing the offset at the output of the opamp, either by adding offset trimming or a DC servo.

The caps do no harm. Assuming a 10k load, 4.7uF gives LF rolloff at 3.4Hz which I suspect is significantly lower than your speakers (and ears?). However, you could try putting a slightly larger cap, say 22uF? Don't go too big, as the cap has to finish charging on power-on before the mute lifts.

 

[wa2ise]

I've bypassed mute circuits like this in CD players, and had nothing bad happen. Had nothing at all. When I modified the analog section of CD players with vacuum tube circuits. There might be a thump when you power up a CD player, but the tubes take time to warm up, and I hear no thump. I don't hear anything when changing CDs or skipping a track on a CD.

 

[stephensank]

I very strongly suspect that you have significant DC offset voltage on the opamp output lines. Did you measure for DC? I don't recall on these TDA1543 units, but on any TDA1541 based unit from Philips, there's always a couple of volts of DC on the final opamp output pins, which can only be zero'd out by adding an offset trim circuit. Otherwise, you MUST have dc blocking caps in the path.

 

[Mooly]

You can't remove the caps without drastic modification. Correctly sized and the caps have no effect on the audio.

 

[Stabby]

Thanks for the insight. I did not measure DC offset on the player but my amp has coupling caps at the inputs so I didn't have to worry about DC reaching my speakers.

Leaving in the Pana film cap for now, it sounds pretty good.

You can't remove the caps without drastic modification. Correctly sized and the caps have no effect on the audio.

Well I did try a 4.7uF Silmic II and it sounded a bit harsh and unpleasant compared to the current film cap that's in.

 

[Mooly]

Well I did try a 4.7uF Silmic II and it sounded a bit harsh and unpleasant compared to the current film cap that's in.

Well that's boutique parts for you 😉 Stick to good commercial grade.

 

[Chivvyp]

If the 610 uses the 7210/7220 chip combination you could try connecting the 7220 pin 23 (MUSB) to 7210 pin 11. That should let the decoder do the muting rather than the original muting circuit.

Regards

Pete

 

[Willi Studer]

That will non prevent on/off popping. It is DC that appears at audio outputs before the muting circuit comes alive. Also, when turning the player off, the muting transistor will turn off before the DC bleeds off.