humm and buzz

[Koonw]

It is not that simple. The single resistor solution is just not good. Take the example from post 14. It does very little to prevent ground loop currents, plus the input and output interconnects are parallel to the resistor. The audio signal will return via the interconnects (and PE) instead of the resistor.
You should not just insert resistors and believe all is good just because the hum is reduced. You need to know at what cost.

Hello, agreed, I really appreciate your insight as it does not prevent ground loop currents as such, in my case I have zero hum at chassis ground. I just wait for someone to point that out. Remember that is only chassis ground really need a really profession ground lift big bridge, 5-10 resistor to the PE (earth). The question I raise because since I have zero hum at chassis, and I have removed right channel (so cross channel looping is out of issue), I still have hum when I plugged in both channels RCA cable, even both screen are tied exactly the same spot so single channel earth loop is also out of issue, the working channel just be able to pick up more hum and buzz. So I feel this has nothing to do with cross channel loop as pointed out in Daniel Joffe's article. If you remove the screen connection of one channel, it is dead quiet (as I have zero ground loop hum as a pre-condition), it appears inserting 1 ohm or less resistor is as good as disconnecting one of screen cable to ground. It also appears the the looped screen cable is not due to ground loop hum but another kind of hum due magnetic induction into the end of rca cable screen, the AC potential across each screen is different therefore hum is injected in signal. The potential difference is even greater and therefore louder hum if added the joint connect cable or wire to active device input such as grid input. It appeared that the added resistor is doing something to minimise the potential difference, but lifting both the screen and pre-amp appears most effective in removing this induced hum. Daniel Joffe's article does not discuss screen cable, he barely touched it, so I hope my fix is not a hack but based on scientific proof.

Edit: There is induced hum on first 100K grid bias when the amp power off and zero hum on ground chassis.

Edit can you review your option after reading this? "You should not just insert resistors and believe all is good just because the hum is reduced. You need to know at what cost."

 

[AndrewT]

I'm not a tube/valve man, so here are a few questions.
Post60 sch:
Where does the INPUT circuit end?
Where does the OUTPUT circuit start?
Is the "join" (or voltage reference link) the bit of wire from the bottom of R13 to the bottom of R19?

If so, then that sch can be compared to the SS version shown by D.Joffe.
The 1r0 for R20 is NOT HBRR

If the voltage reference link between input and output is that R13 to R19 wire then HBRR goes into that link.
and HBRL goes into the wire link from R3 to R10

The added resistors now attenuate the loop current such that the interference voltage fed in via the interconnect screen is also attenuated.

Measure before and after to confirm.

 

[Mark Whitney]

Koonw, as you have explained, increasing the resistance of the interconnect shields also increases the ground loop voltages at the inputs of the amps and therefore probably not recommended by D.Joffe.

From what I understand of your explanation is that you have connected everything with one channel switched off. This will not change the ground loops or hum of the working channel. The only way to break all ground loops is to use only one channel, one channel interconnects and no more than one PE connection. If you still have hum then look for bad layout like signal and return loops as shown in post #20.

 

[Koonw]

Koonw, as you have explained, increasing the resistance of the interconnect shields also increases the ground loop voltages at the inputs of the amps and therefore probably not recommended by D.Joffe.

From what I understand of your explanation is that you have connected everything with one channel switched off. This will not change the ground loops or hum of the working channel. The only way to break all ground loops is to use only one channel, one channel interconnects and no more than one PE connection. If you still have hum then look for bad layout like signal and return loops as shown in post #20.

The ground of other channel is removed (I will confirm this later only power amp bias is still grounded).

Btw any one can share insight as to why 1 ohm when already shunt to few mill-ohms still can reduce hum just the same? I mentioned that before the other channel ground is solid wire, not difference if both 1 ohm. 1 ohm resistor can be on on both sides, so only 1 resistor and a solid link, hum reduction just the same. Of course it really boil down to keep both channels cable exactly identical without the need of additional resistors. In practice one never would be able to keep both cable identical for many reasons.

 

[Mark Whitney]

As I pointed out before, the audio current that should go though the 1 ohm resistor now takes other routes, the 4 interconnects and PE, this cant be good.

It is not that difficult to apply D.Joffe's work to a source->pre->amp system.

 

[Koonw]

As I pointed out before, the audio current that should go though the 1 ohm resistor now takes other routes, the 4 interconnects and PE, this cant be good.

It is not that difficult to apply D.Joffe's work to a source->pre->amp system.

As I pointed out before, the audio current that should go though the 1 ohm resistor now takes other routes, the 4 interconnects and PE, this cant be good.

It is not that difficult to apply D.Joffe's work to a source->pre->amp system.

No, of course not. Go to post #14 you should look at the attached drawing to left the OTL amp I mod before reading D.Joffe's work. I don't dispute it but because I want to make ground resistance as low as possible and implemented Class 2 or additional ground lift to PE. This is only my preference not to dispute with idea in the article. If screen connection is part of ground resistance requirement it must be very low, maybe not even be 1 ohm not to mention doubled to 2 ohms. In fact a better solution would a single screen cable with 2 signal wires, because it is double loop of screen causing the problem.

The sch to left Chopchip's build, only shows the signal/chassis ground, never the the PE but CC connected with PE not me．It's a bit of challenge to redo the PE on ST70 and no PE on preamp and yet stay safe, when means Class 2 equipment for preamp with no ground, double insulated just like most CD player. In the OTL amp the preamp/driver and power amp is integrated in one unit I can have full control.

 

[Koonw]

Cardas 4x24 snaked through copper braid shield should be better than RCA unbalanced cable.
Vinyl Asylum

 

[TonyTecson]

there are two ways for this amp to hum and buzz,
thru the filaments and the B+....
not a simple matter of putting a single resistor to ground...