Leach amp has hum only when turned off!

[kiwi_abroad]

Hi all,
I have just got one channel of my Leach amplifier up and running. It seems to be fine, DC offset is 12mV, bias is stable, music is clean and free from obvious problems (o critical listening yet), no hum at all, and only a very faint hiss when my ear is right against the loudspeaker. All in all I'm very pleased...

But when I turn it off, because of the filter caps, it takes a long time to discharge them. All is fine until the Rail voltage drops below 40V, when a moderate hum/hash noise is generated. It takes about another minute of this before the voltage drops enough to silence the amp. I'm quite annoyed by this, and I can find no mention of this problem.

Can anyone suggest a cause for this, and possible solutions.
All I can think of is a relay and a power resistor to discharge the filter caps quickly on turnoff to reduce the time the noise is produced, but I'd much perfer to find the source of the problem.

Adrian

[Nick Walker]

Hi mate,

I guess all sorts of thing can happen if the big electrolytics are discharging at different rates - are they all OK? It's quite common to see a resistor across the last reservoir capacitor on each rail to ensure the caps drain themselves at shutdown. 10K 1W should do it, I've never seen a relay involved.

Nick

[slowhands]

Use bleeder resistors, 2.2K 2W, on each voltage to ground. Will discharge the caps in a shorter time.

As to why your two amps are different, I suspect a poorly matched input pair in the noisy one. It should go down quietly if things are balanced.

[peranders]

I don't want to be too negative but your pcb laypout may cause the problems which is oscillations. A gilden rule is to get all pcb traces short, especially between VAS/buffer stage to the power transistors. You can't get an oscilloscope for determine your problems?

[pooge]

Turn your signal source off first.

[5th element]

I also have a similar problem. When I turn off my 200watt fet amps they hum after the power has been switched off. I have bleeder resistors. They hum AFTER music has ceased to play becuase the caps ARE drained. This is confusing lol as you can imagine. Nothing to worry about really as I rarely turn them off, and yes the problem does go away when the source is deactivated.

[Tony]

Originally posted by pooge
Turn your signal source off first.

it makes good practice to turn off the source first before powering off your amp.....


hi slowhands,
just wondering, if you don't mind, i would like to know if you are the same one using that handel at www.pinoydvd.com? salamat po!!!

[kiwi_abroad]

OK. Update

On further investigation...

If the souce signal is off, then the noise is produced only about half of the time, otherwise is silent.
The noise only occurs if both the source interconnect AND the mains cable is plugged in.

Most interestingly, like 5th Element, the noise continues even after the voltage rails have dropped to 0 volts. ie I don't know where the power is coming from. It must be coming down the interconnect from the DAC (DIY DAC, the output buffer is an AD797, which can in theory deliver 50mA), or perhaps I have found a source of free power.

I think more investigations are needed...

On listening, I don't think I've got any problems with oscillations when the amp is fully powered up. I don't have a scope, but the music is clean clear and detailed, no harshness, grain etc. which would be indicitave of problems.
Adrian

[peranders]

You may have problems with oscillation even when the amp is on but you have to have an oscillscope for this.

[Tony]

if you mean the interconnect to the leach amp input and source, and the mains cable of the preamp or signal source, then it is most likely ground loops...

you can try reversing the ac power plug of the leach amp, if possible and see what happens!!!