Leach Amp troubleshooting

Hello everybody!

I built a Leach Amp quite a while ago and reactivated it this week. When I power it up, it does quite a "plopp" in the speakers which doesn`t disturb me.

What I don`t understand is the fact, that while it plopps, the membrane of the left channels speaker moves outwards, while the membrane of the right speaker moves inwards...

Does anyone here has an idea what might be the reason for this behaviour?

Sound of the amp is quite good. The Amp is completely dual mono, with 2 PSU (also 2 transformers).

I checked all the connections and polarity, but did not find anything. One idea I have is the polarity of the rectifiers, but i don`t know if that might be the reason.

I would be glad if someone could help!

Ian Finch

Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
The Leach amplifier is a symmetrical, mirror-image topology. That suggests any pulse generated as power supplies charge/discharge
at switch-on/off can be of either polarity. Speaker relay switch-on delays are used on most amplifiers specifically to remove any such "plopps"
which are present in all basically similar amplifiers, to some degree.

Assuming this problem was not there when you built the amplifier, you might consider that the problem is in the preamplifier, which is often
the case with integrated amplifiers, since relay power and other circuits may use the same power supplies and thus cause rails to charge/
discharge differently. To remove any doubt, you could disconnect the preamp and short the amplifier inputs either directly or via 1-10k resistors.

Otherwise, ageing electrolytics likely have made any existing power supply rail problem worse. Check the parts and swap them
if you are interested to find out how sensitive the amplifiers are during switch-on.

joe mobile

2006-09-25 3:28 pm
Hi Ian,

thank you for your reply. It confirms nattawas answer he gave in the other thread I posted my question. I was concerned that the moving of the speaker membranes in opposite directions might indicate something is wrong with my amp, but since yours and nattawas replies I`m not worried anymore.

The plopp itself doesnt bother me, and so I dont think Ill intall speaker relays. Swapping the caps is a good idea, I think Ill give that a try and see how things change, just in order to know.

Thank you very much for your help!