Return to Aikido Hum issues

[cbutterworth]

Sorry to bug everyone again, but the Aikido hum saga continues:

After disconnecting the tube delay circuitry, which was close to the signal wiring, I still get hum on my power amps. For the time-being, I am not concerned with cathode stripping on my really cheap 6sn7's (some folk reckon that it isn't even a problem below 1kV).

I even mounted my jacks and isolated them from the chassis, and mounted my new Goldpoint. Still get hum.

Now, I have noticed that whenever, I switch on the preamp, I get a quiet, but audible hum from the preamp, simply by listening close to the amp. This surely must be some sort of transformer or choke resonance. My transformers and chokes are enclosed units and are mounted to the top plate of the amp. I have grounded the chassis of each trans/choke to the top-plate of the preamp via a star washer on one of the mounting bolts on each trans/choke. Should I mechanically isolate the trans/chokes from the preamp chassis using neoprene washers?

Sy, recommended that I try a cheater plug on the preamp, to test for ground loops. I did this and still got hum.

Another thing that I need to look at is my chassis layout with regard to star earth. I have a wooden case, that hold a steel top-chassis containing transformers/chokes, volume control, Aikido PCB (although the PCB can be optionally connected to the top-chassis). The IEC, mains fuse and jacks are mounted to a rear chassis, which is not directly connected to the top-chassis.

At present, Mains Earth is connected using star washer to rear-chassis, via one of the IEC connector bolts (I should really use a chunkier bolt to do this). I can braid connect from the top chassis to the rear plate via the Mains Earth/rear chassis bolt. I have tried connecting PCB GND to both Mains Earth and top-chassis GND.

Based on the complex options above, my next attempt may be to create a "star earth" on the top chassis. It would connect the grounds from PCB and the rear-chassis at one point. From here, a single GND would connect to IEC Earth.

The hum is fairly low-level, but audible with quiet music. My test amp has its own volume control, and the hum is only evident, when I turn that amp's volume way-up. On my AKSA, there is no volume control, so it is audible all the time.

Please help..........

Charlie

[jrevillug]

Should I mechanically isolate the trans/chokes from the preamp chassis using neoprene washers?

YES!

Have a look here:
Corian Turntable Fun

Post 253 is the one you are looking for.

My test amp has its own volume control, and the hum is only evident, when I turn that amp's volume way-up.

sounds like the hum is from the pre, rather than the power amp.

Do you get hum from the power amp, without the pre? If not, I suspect that the microphonic tubes in your pre are picking up this transformer vibration.

Kind regards,
James

Ps I am even more of a newbie than you, but I thought that I would at least link the other thread.

EDIT: post no is 253, not 235.

[cbutterworth]

James,

I'll certainly add the neoprene washers, maybe even rubber grommits would be better. The tubes are cheap and seem to be very microphonic, so they could simply be picking up the transformer hum. Should I still ground the transformer chassis?

The hum is certainly preamp, as my main amp has no detectable hum at all.

Thank-you (from a fellow Brit.)

Charlie

[pedroskova]

If you isolate any transformers/chokes from the psu, run a drain wire from transformer chassis to chassis ground just in case your transformer were to go open.

[jrevillug]

Thank-you (from a fellow Brit.)

You are very welcome.

James

P.s.I am sorry for any offence caused by "I am even more of a newbie than you" - I really don't know how much experience you have. I just wanted to say that I was a complete newbie.

[cbutterworth]

James,

No offence taken because I am very newbie (do not misread as nubile !!!!).

Charlie

[cbutterworth]

Pedroskova,

Yes, I was thinking that I could mechanically isolate the transformers and chokes using either neoprene washers or rubber grommits, while still maintaining a safety contact between trans/choke chassis and ground. I would simply do this via one of the mounting bolts on the transformer. I can remove a little paint on the bolt side of the mount, then the order would be bolt - star washer - transformer - neoprene washer - preamp chassis - star washer - nut. This should give both a safety ground and mechanically isolate.

Thank-you,
Charlie

[jayme]

I am using ElectroHarmonix 6SN7s. 2 of the original 4 that I got hummed in my Aikido. I had to order more, and swap them in as replacements.

Dead quiet now, with the current tubes...

Try swapping the tubes around between inputs/outputs to see if it changes the hum level. Hum was much louder for me when the hummy tubes were in the input stage...

[kevinkr]

I noted that your pre-amplifier is built on a metal top plate and is installed in a wood sub-chassis? Anything else nearby that might be inducing a hum field into the signal wiring of your pre-amp? (Most of it is unshielded.)

Are your filaments wired using tightly twisted wire and what have you done about filament wiring ground?

Do you have more than one ground connection between the steel top plate and the pcb?

Consider adding some electrostatic shielding inside the chassis, this can take the form of copper or aluminum foil glued to the insides and grounded to the top plate. Make a partition to go between the powe supply section and the audio section as well. These may not solve your current problem but help with external interference. (EMI)

The filaments should not be left floating, either the center tap should be grounded, if there is no center tap create a pseudo center tap using a pair of 100 ohm resistors in series across the filament supply and ground the node between the two resistors. Better still would be to elevate the filaments above ground by dividing down the B+ with a resistive divider. This should be bypassed to ground with a suitable electrolytic to ground.

Some Russian made 6SN7 are extremely microphonic and mechanical vibration from the transformer could be contributing.

I am not sure why so many people here are buying EH 6SN7 when good NOS and used 6SN7 are abundant on eBay, and often at relatively low prices for common brands. I prefer Sylvania types personally, they are abundant, and amongst the best sounding regardless of cost or rarity.

Edit: How about some pix of the insides and the whole thing?

[kittikun]

Hi all,

I have a completed assembled PSU board. The high voltage ground and heater dc regulated ground is connect together on the print circuit board.

Do I need to separate the heater ground from the high voltage ground and and re-connect it to High voltage with a resistive divider? If I directly connect both ground together, is there any problems?

Thanks in advance for all helps.

regards, Kittikun