Help needed with Hum please!

[ZakB]

Dear All,

I have a hum issue that I'm struggling to understand and wonder if any of you might be able to help.

My recently rebuilt K5881 MkII (a MK1 convert to Mk2)is very quiet.....until I plug it into my Cathode follower converted Pre2. As soon as the signal leads are connected I experience significant hum. The hum goes away if I connect the chassis ground posts together (PSU and Pre posts), or bypass the ground lift resistor in the power supply (R6 in the attached diagram).

Could any of you knowledgeable people help me understand what is going on here and if these symptoms suggest I've wired something incorrectly?

Many thanks,

Zak

[Richard]

Hi Zak, you have a ground loop.

K5881 doesn't have an ground lift resistor so its chassis and signal are at mains earth.

Pre PSU signal ground is lifted by 100R but its chassis post is a solid connection to mains earth.

Pre2 "earth" post is actually signal ground (so is lifted 100R when connected to PSU).

Check these with your meter and visualize or draw the signal grounds sitting inside the mains earthed boxes for each unit and you'll understand what's happening. Get used to thinking of signal ground (0V) and mains earth as 2 separate things.

"The hum goes away if I connect the chassis ground posts together (PSU and Pre posts), or bypass the ground lift resistor in the power supply (R6 in the attached diagram)."

This is because you're bypassing ground lift 100R R6 in both cases and making the signal ground the same potential as the signal ground in K5881 (ie mains earth). It may be a solution and is fine if it's quiet enough but I suspect that although it's an improvement it may still hum a little.

The best way would be to separate signal ground and mains earth in K5881 by 10R as was done for later kits. This probably means insulated RCA phono sockets and reconnecting the signal ground to the chassis via 10R instead of a solid link. When done you should be able to confirm 10R between signal ground and the amp chassis mains earth.

[ZakB]

Hi Richard,

Many thanks for your clear reply. When I get home from work tonight I'll draw out the ground paths as you suggest.

My apologies, but I didn't mention in my initial post that when I converted the K5881 from Mk1 to Mk2, I incorporated a 10R ground lift resistor as you suggested in your 2013 post:

http://www.world-designs.co.uk/forum...?t=6918&page=3

Rgds,

Zak

[Richard]

Hi Zak, gosh I read your link but couldn't remember posting it, ah well consistent advice anyway

Yes, worth re-checking, so remove all connections and check K5881 for 10R signal lift, the PSU for 100R signal lift and that the Pre case is at signal ground (ie 100R).

If so all should be well so is a source plugged in the culprit? If a source has its signal ground connected to mains earth it may cause this when used with the amp system raised (and it would go quieter when you remove the amp's lift).

[ZakB]

Hi Richard,

Well, I've found the culprit this evening and it's a schoolboy error!

When I converted the amplifier to the MK2 version and fitted insulated Phono sockets, I failed to connect the socket earths to the ground lift resistor. So, the input stage of the amp was not earthed until the interconnect to the pre-amp was plugged in - hence the ground loop. A link soldered directly from the socket grounds to the earth lift resistor has solved the issue.

I now have 1.2mV of hum on the right channel and about 1.8mV on the left, which is a big improvement. I think the left may be slightly worse because of the presence of the switch wiring on the left side of the case. I have twisted the switch leads together and run them in the chassis corner but because my speakers are sensitive (~95dB), some hum is still audible. I am still trying to find ways of improving it though!

Many thanks for all your help,

Zak

[Richard]

Excellent A better place to earth the inputs may be 2 individual earth wires to the ground ends of the 100K first valve grid leaks thus separating them from any high currents in the ground circuit.

Connect high current parts (op stage cathodes and neg speaker posts) directly to the earthiest part (usually the neg of the large smoothing caps) and connect the input grounds at the opposite end of the ground chain.

The tiny differences in relative impedance are the reason for "bus bar" layout order of connections and is shown on the circuit though it may not be obvious that it matters.

[Richard]

Disregard the word "input" and just read ground as the connection to the smoothing cap neg on the drawings btw. (It refers to the original construction, where the chassis was ground and mains earth, and an input socket carried the ground connection.)

[Phil Y]

Hi Zak,
I don't know if this would suit you but I also thought I was getting a little hum from the mains switch wiring on my KiT88. I removed it and fitted a small toggle switch on the back, next to the mains inlet.

Phil.