Aleph X strange behaviour

[JBL4435]

Hello Aleph X experts.

I finished another pair of Aleph X Monos. When I first hokked them up to the speakers I was surprised how quiet they are. Almost no noise from my very efficient speakers (97dB).

Next I connected my preamp via RCA. One of the X Amps started a 100Hz Hum. Only one! It's not a huge hum but its disturbing and it comes from only one amp.
I changed the RCA's left/right same power amp humming.
I disconnected everything from the preamp to avoid ground loops. Same.
I connected another preamp. Same.
I connected the third preamp I own via XLR balanced. Same.

When I disconnect either of the preamps it's quiet.

Has anyone ever had a similar problem? It seems that one of my X-Amps do not like being connected to whatever.
Do I have a problem in the input stage?

Otherwise the amps sound fine.

[macka]

You have an earth loop or wiring fault in that channel.

Try measuring from the amp chassis to amp earth for ohms and mv or ma and your preamp chassis or earth for ohms and mv or ma and compare to the good channel.



iMac

[JBL4435]

Thanks for answering macka.

Amp ground to earth ground is connected only via a 0,47uF capacitor in my amps.
I can try to measure mv.

The several preamps are all isolated designs. Means chassis are not connected to earth ground.

[macka]

Originally posted by JBL4435
Thanks for answering macka.

Amp ground to earth ground is connected only via a 0,47uF capacitor in my amps.
I can try to measure mv.

The several preamps are all isolated designs. Means chassis are not connected to earth ground.

Well this is not right if I read you correctly it is very dangerous.

The amp chassis must been grounded to earth.

If you have access to a Pass Aleph schematic you will be able to follow the standing earthing practise. If not I can send you one.

It involves a thermister CL60 from the ampliifier circuit earth to chassis ground and has about 5 ohms nominal DC resistance. If a problem occurrs or significant current flows the thermister DC resistance drops.

"TH1 is a power thermistor used to connect the circuit and chassis ground to the AC outlet
ground. It will normally operate at 5 ohms, suppressing ground loops in the system, but will
drop to a low impedance if significant current is passed through it."

[JBL4435]

Thanks Ian, I know all this.

The cap is a special X2 cap means it will short circuit if something is wrong.
The Amp chassis is of course connected to earth. The circuit ground is connected via this cap to earth.

All this doesn't help.

[macka]

You have made a mistake somewhere as I said earlier.

iMac

[Nelson Pass]

Originally posted by JBL4435
When I disconnect either of the preamps it's quiet.

One thing to remember is that the amps have low gain without
a source impedance, in other words, when there is no source
connection.

If you want to evaluate the amplifier's own noise without a
connection to a preamp, I suggest that you use shorting plugs
to see to it that both inputs (+ and -) are grounded.

[lhowing884]

I have the same problem, When a RCA is connected, the amp has a 60Hz hum. The chassis and everything is grounded correctly. When there is no input connected, (even just a cable with no connection at the other end) the amps are very quiet. I tried experimenting with grounding techniques between the preamp and the amps. After fiddling with it for a while, i connected a wire from the amp chassis to the preamp chassis. The hum has gone down quite alot, however, it is still not as quiet as when there is no input connected. BTW, all equipments have earth ground and are running off the same AC outlet. The hum is still annoying me, even at this low level. Any ideas? I have not tried inserting a small resistor between earth and chassis.

[Blues]

I had the same problem with the right channel of my amp. I disconnected the preamp/amp interconnect and shorted the amp input...silence...so the hunt was on outside the amp. The culprit...cable TV.

I removed the co-ax from receiver to TV and the hum went away. Using RCA interconnects instead of the coax brought the hum back but now I can disconnect easily from the TV front input whenever I turn my amp on. BTW, turning the receiver off does not remove the hum.

[Zen Mod]

Originally posted by Blues
I had the same problem with the right channel of my amp. I disconnected the preamp/amp interconnect and shorted the amp input...silence...so the hunt was on outside the amp. The culprit...cable TV.

I removed the co-ax from receiver to TV and the hum went away. Using RCA interconnects instead of the coax brought the hum back but now I can disconnect easily from the TV front input whenever I turn my amp on. BTW, turning the receiver off does not remove the hum.

you need this between cable and any TV and/or receiver in house

cable just rarely have it's ground on same potential as safety ground , and that's whats inducing hum , if you have these two potentials galvanic connected

(you don't know that just because you're wastin' too much time on that crazy music )