SX 1565 Bass Amp On/Off Pop

[Palbert]

Hey all,

I have been using this little sx bass amp that came with my first bass. Whenever I turn it on, I get a pop, and when I turn it off, I get a louder pop. Here is a link to the schematic:

https://www.electro-tech-online.com/custompdfs/2013/02/ba1565.pdf

I tried putting 20V TVS diodes across the secondary sides of the transformer thinking that opening the switch was causing large transients from the transformer but they didn't help. I'm not sure where the popping would be coming from. Any ideas?

 

[dougy83]

The pop is likely due to the voltage on some capacitors taking their time to approach their steady-state voltage. While the cap voltage is transitioning, it will be injecting the voltage into the amplifier, which will be heard at the speaker.

The only thing I can see that would be causing this is the capacitors in the 'equaliser'/tone-control section (all other caps operate at close to 0V DC across them). This can be tested by:
1) reducing the volume control to minimum. If the pop is still occurring, then the preamp has no part in the pop.
2) shorting out R21 (which effectively connects the TDA2030A pin 1 to GND). If the pop stops, then it is the equaliser section causing the issue.

If the above points the problem at the tone section, removing the pop can be done by shorting out R21 on power-up and power-down using perhaps a JFET or two.

 

[diy didi]

Hi. Replace U2!!

 

[dougy83]

Hi. Replace U2!!

With what?

 

[Palbert]

The pop is likely due to the voltage on some capacitors taking their time to approach their steady-state voltage. While the cap voltage is transitioning, it will be injecting the voltage into the amplifier, which will be heard at the speaker.

The only thing I can see that would be causing this is the capacitors in the 'equaliser'/tone-control section (all other caps operate at close to 0V DC across them). This can be tested by:
1) reducing the volume control to minimum. If the pop is still occurring, then the preamp has no part in the pop.
2) shorting out R21 (which effectively connects the TDA2030A pin 1 to GND). If the pop stops, then it is the equaliser section causing the issue.

If the above points the problem at the tone section, removing the pop can be done by shorting out R21 on power-up and power-down using perhaps a JFET or two.

Thank you for your help dougy. I tried turning the volume down and I still heard the pop. I will try shorting out R21 tonight to bypass the tone stack.

 

[Nigel Goodwin]

Thank you for your help dougy. I tried turning the volume down and I still heard the pop. I will try shorting out R21 tonight to bypass the tone stack.

This is a simple amp, has it always done this? - it's probably quite normal, there's no protection against pops when turning ON or OFF (it's common to have a delay circuit that doesn't connect the speaker until after the pops have finished).

 

[Palbert]

I just shorted out R21 and it got rid of most of the pop. There was still some, but nowhere near as pronounced so the main culprit must be the tone stack. Time for a soft start circuitry adventure. Thanks dougy.

This is a simple amp, has it always done this? - it's probably quite normal, there's no protection against pops when turning ON or OFF (it's common to have a delay circuit that doesn't connect the speaker until after the pops have finished).

Yep it has always done this so I didn't think it was a problem, I was just curious as to how to get rid of it for the learning experience more than anything.

 

[Nigel Goodwin]

For one example of doing this, check out:

**broken link removed**

 

[dougy83]

Time for a soft start circuitry adventure.

If you haven't come up with an idea for this yet, there's an untested quick sketch attached. Sorry about the quality, my camera's not the best.

 

[KeepItSimpleStupid]

The turn-off pop is an easy one to fix. Just connect a relay coil to the mains after the power switch and place the Normally Open contacts in series with the speakers.

It may fix your turn-on pop too because of the 20 ms or so it takes the relay to engage. That's what I would do for starters. Find a relay with a longish pull-in time.

If there are turn-on pops left, I can probably fix that with a couple of JFET's and a dozen or so other parts, but I would need to know what B+ and B- is.

 

[Palbert]

Thanks for the suggestions. I'll have to try that one out.