poping soundings

[leander]

My amp seems to work fine at low volumes, however when cranking the volume up there is a point where lowed pop noises are heard. I realy don`t have any idea af what is happening. Maybe some sort of oscillations.

The amp is this one http://www.ampslab.com/c300.htm
and had to replace r21 to 400ohms and R23 to 50 ohms to achieve bias range

I did some testing signal and speaker are disconnected and an ac volmeter is hocked up to the output. The voltmeter reads 0v rms when the amp is initialy turned on, however is I touch the signal input with my finger, the output reads 54v rms and stays moving up and down between 54v to 12v even if I do not touch the signal any more.

Anyone have any ideas of what may be the cause of the problem?

Thanks
Leander

[MikeB]

Hmm, sounds like triggered oscillation... Have you built the amp exactly as in the schematic ?

Mike

[leander]

yes i did and rechecked it many times.

[MikeB]

If you power down and again up, is it 0volts again ?
While this effect happens, can you measure the vbe of q12 ? And the voltage across r17 ?

Mike

[wintermute]

Sounds like a problem that I had when I was testing my amp without the chassis bolted together properly. The problem went away when the heatsinks were properly earthed.

Tony.

[leander]

wintermute, which heatsink should be earthed?

Power trasistors are electricaly insulated from the heatsink, should I earth the heatsink to mains earth ?

[MikeB]

Originally posted by leander
should I earth the heatsink to mains earth ?

No, connect it to power-gnd and try to keep away mains earth.
The transistors are isolated from the heatsink, but have a capacitive connection.

Mike

[wintermute]

Hi Leander,

I'm referring to the heatsink that the power transistors are mounted on. knowing how a lot of us test things before everything is properly bolted together I thought I better mention my experience!

It depends a bit on the design of your chassis I guess, but if the chassis is earthed and the heatsinks are electrically connected (via screws) to the chassis it should be ok.

If the heat sinks are insulated from the main chassis then you could try getting an aligator clip, and connecting it to the heat sink and also to the chassis, and see if it fixes the problem.

I don't think it really matters where the heatsink is connected to earth as this is a sheilding issue rather than an electrical one. edit: ok MikeB beat me to it disregard this sentence

I had problems because my Heatsinks form the sides of the case, and the mains earth is connected to the bottom. panel... I didn't have the screws in which attach the bottom to the heatsinks (as it made accessing the PS much quicker) unfortunately for me it caused oscillation like you are describing, when I cranked up the volume pot (even with no signal) this resulted in my zobel resistors smoking After that I decided that installing and removing a few screws each time I wanted access to the PS wasn't such a big deal afterall


here is an extract from an article on Rod Elliots site on trouble shooting

2.3 Oscillation
The zobel resistor in most amps is at the output, and is in series with a cap - typically 100nF, but this varies. If the resistor goes up in smoke and/or the amp gets hot fast, either the amp is oscillating, or you are trying to amplify too high a frequency.

Oscillation is caused by using the wrong value compensation cap (typically between 47 and 220pF), or (and more likely) having input wiring too close to speaker wiring. Input cables to power amps should always be shielded, and kept as far away as possible from DC power leads, mains transformers and wiring, speaker leads and connectors, etc. In some cases, it may be necessary to provide shielding between the input circuits and power amp.

During testing, the heatsink may not be earthed to the power supply common. In some cases, this can cause oscillation because the heatsink acts as an antenna - as does the input lead if it is not shielded. Always earth the heatsink - even for just a quick test.

full article available here ---> http://sound.westhost.com/troubleshooting.htm#comm3

Tony.

[MikeB]

Originally posted by wintermute

I don't think it really matters where the heatsink is connected to earth as this is a sheilding issue rather than an electrical one. edit: ok MikeB beat me to it disregard this sentence

No need for beating !
I just suggested to keep mains earth completely out, it is always asking for problems...

Mike

[wintermute]

your technical knowledge far surpases mine Mike I'd rather retract a statment I made when I feel someone more knowlegeable has spoken on the matter, than have conflicting information floating around.

cheers,

Tony.