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
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
Attachments
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
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
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.
pooge said:
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!!!
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
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
5th element said:
This thump is odd. Maybe you have some DC offset in your source, and/or a serious ground loop somewhere. The hum situation sounds like a ground loop. Pull one of the interconnects off from one of the channels into your amp to see if it goes away. If it does, try correcting it by the method Leach suggests on his web site.
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