B-5 hum question

[coils]

Greetings all,

I have been using a B-5 for bi-amping my new diy open Baffles. The trebles are Lowther EX-4's driven by a diy F-3, the bass are Emenence DeltaPro 15's driven by a diy X-SOZ, the pre is A diy X-BZLS. I have been extremely impressed, The sound is great and there is plenty of bass for all program material. I am having a great time tweeking the crossover.

But... when I connect the B-5 rca's to the amps, I get hum from bass amp and treble amp. When the B-5 is removed and the amps are individually connected to the pre, there is not a whisper of hum from either.

I have tried different interconnections on the mains side with no change. Since there is no hum with the individual amps, I am thinking my ground topology in the amps is ok. It seems like a ground loop rather than ripple or induction or injection.

I am running the X-SOZ unbalanced with pins 1 and 3 shorted. Lifting one of those shorts doubles the audible level of hum.

Only when I connect the rca's from the B-5 do I hear hum.

Does anyone have any suggestions on how to trouble shoot this problem.

[6L6]

Are the RCA sockets touching the chassis or floating/isolated on their nylon shoulder washers?

[coils]

all of the rca's are isolated from the chassis

[Nelson Pass]

The RCA's on the B5 are connected to the chassis.

You very likely have a ground loop, which is typical when
two power amps each with its own earth AC ground are
connected in common to a source. This would be confirmed
if the hum disappeared when one of the amps is unplugged
from the wall.

Usually the cure is to float one of the amplifiers, allowing
the other to do the earth grounding duty.

[Itsmee]

Originally Posted by Nelson Pass

Usually the cure is to float one of the amplifiers, allowing
the other to do the earth grounding duty.

Under NO circumstance defeat the protective earth (earth wire from mains socket to chassis), Nelson is suggesting lifting the signal ground from the chassis.

Go a step further after lifting the signal ground, and fit a 10R - 22R resistor between signal ground and chassis, also add a 100nF in parallel with the resistor.

[Itsmee]

Oops, just noted post #3.

Disconnect the screen in the phono plug at the input of one channel of one of the amplifiers, if it still hums, disconnect the screen to one channel to both amplifiers. Disconnecting the screens of both channels to one amplifier will probably stop it working.

[Melon Head]

Originally Posted by Itsmee

Under NO circumstance defeat the protective earth (earth wire from mains socket to chassis), Nelson is suggesting lifting the signal ground from the chassis.

Go a step further after lifting the signal ground, and fit a 10R - 22R resistor between signal ground and chassis, also add a 100nF in parallel with the resistor.

Why not just do what Nelson proposes and float one of the amps (ie lift signal ground from protective chassis earth).
If that fixes the problem then he can do as you propose (Thermistor would be safer) or use a bridge rectifier as a permanent solution.

Actually the best solution would be to add a switch to switch between signal float and earthed on the amp. Lab equipment always include this as a standard feature.

[Itsmee]

Originally Posted by Melon Head

Why not just do what Nelson proposes and float one of the amps (ie lift signal ground from protective chassis earth).

Thought i said that in post #5.

Maybe the confusion was my post in #6, noting that the connectors are isolated (post #3) I assumed that the amplifier was built as prescribed (stereo, not mono block) and a thermistor was fitted between 0V (signal ground) and chassis earth, hence my comments about disconnecting one screen.

[Bksabath]

As amplifiers and pre are connected to ground troug the earth safety cable cut the screen on the Phono plug going in to the amplyfier

The screens are still connected to the plugs on the source (Preamp/ Crossower )and still do the job


why should it not work?
Is still connected to the earth troug the mains cable.

and Is just snip snip away and if it does not work Just solder it back .

I would cut the lot of the screens on the amplifier sides and then solder them back one at the time listen out for hum between adding connections

[coils]

thanks for the feedback, I am still investigating the isue. Will let you know what I find out.