Hum in AE K502 or K12m

[SY847]

Hey,

My AES K502 kit came about 2 weeks ago, and I assembled it last week. I've been testing it for the past few days. Right now, I have it hooked up to a pair of headphones (without anything to attenuate the headphones or change the impedance). I'd hook it up to my speakers, but I don't have them with me.

There's a lot of hum in my amp, and I was wondering what the best solutions for this were. I've been to Voltsecond's site, and have read the threads on a few forums about reducing the hum. So far, I've only applied Voltsecond's .01uF transformer HV mod, and I plan on bypassing the etchings to the filament that run under the tubes with hookup wire.

Has anyone modded the amp to DC tube heating? Would this get rid of the hum completely? Thanks.

also, i noticed that when I touch the potentiometer with my finger, the hum increases dramatically. i plan on upgrading to an ALPS pot, but why does the hum increase?

[gmilitano]

Sounds like a grounding problem. IIRC, the K-12M and later have an additional grounding point on the PCB near the POT connections. There is some information regarding grounding in the kit instructions.

[Sherman]

Make sure that the body of the volume pot is grounded. This isn't usually a problem if you have it mounted in a metal chassis but if you have it on the piece of wood then it can be an issue. You can use a spade lug type connector or just loop a wire around the front of the pot and tighten the nut on it. Take the other end of the wire to either the ground point near the pot (if your kit has one) or to one of the input grounds.

I have one of these amps that I build a few years ago and it is quiet. AC heaters are perfectly fine.

[SY847]

Hey,

thanks for the replies. I checked most of the ground connections on the board, and my kit did come with the ground lug on the pot. when I checked the connection between the lug to the yellow ground on the transformer, its showed a lot of resistance. but looking at the PCB, the hole for the ground lug on the PCB doesn't connect directly to ground.


I hooked up a 22awg wire from the lug to the ground on one of the inputs (the outside of the input?), but the hum wasn't reduced at all. and for some reason, when I tried hooking up the lug ground to the input while music was playing, the music on the left channel would dissappear.

I read on another forum that an ALPS blue velvet pot does wonders for the hum and LR balance on this amp, and I think im going to try that out as soon as I get a real enclosure for this thing.

If I want to mount it in a metal enclosure, would I have to desolder all the caps and metal-oxide resistors, and resolder on the reverse side? or could I mount the PCB on the bottom of the enclosure and buy new tube sockets that mount on the surface of the enclosure and connect them to the PCB with hookup wire? Thanks

[SY847]

ok, I hooked up the amp to my speakers today (~88db/w efficiency)

the hum is no where as noticeable on real speakers as on headphones. It almost dissappears, but I can still hear it when the music stops.

Music quality:

I only have a Yamaha RX596 commercial amp to compare it with. The K502 has a smoother sound than my Yamaha, but I feel like it doesn't have as much clarity as my Yamaha. I can't believe how loud this thing can go on only 8 watts per channel with such a simple design, but I felt it lacked the bass that my RX596 can produce (out of my 6" speakers). Cymbals don't sound as good on this tube amp either. It doesn't have that sharp clear wack that I get from my yamaha. However, the soundstage is more forward with this amp. I was shocked at how well the amp played a really poor quality MP3 of Scott Mckenzie's San Francisco. The drums and guitars sound much clearer on the K502.
I still prefer my commercial SS amp over the K502. It just doesn't have the bass I like. The difference between the 2 amps reminds me of the difference between silk dome tweeters and metal dome tweeters.

[SY847]

I listened to some more songs on the K502, and I'm defn. seeing how tubes can sound better. The smoothness of the amp is really starting to grow on me.

[Neville]

SYS847,

I also recently bought and assembled the kit. When I had it on the "breadboard" and the lug connected between the pot and ground hold near the pot I still had a hum in my left channel. I called up S5 Electronics and he suggested I sand off more from the underside of the stock Transformers which also did nothing for me. I then bought a Hammond chassis but while moving parts to the underside of the PCB I must have damaged the pot so I ordered another pot but that was twice the size and 1/2 watt, not sure if the original is that value as well. I have not had a chance to attach it yet but when I did try it out in my Hammond I still had the hum.

I also got two output Transformers (Hammond 1609) and that also did not help the hum. I also tried to use the two black wires that are supposed to be connected to the underside of the transformer and connected them directly to the Ground connection in the IEC receptacle but this also did not resolve it.

Keep us posted ...

[SY847]

I'm thinking of moving the power supply off the board when I have time, and rectifiying the tube heaters. I'm also going to beef up the power supply caps.

If i don't rectify the tube heaters, I'll try something I saw on another forum where someone recommended covering the heater wires with copper mesh.

Im busy with school now so I won't get these changes done for a while but I figure these will isolate the amp from AC hum.

[gmilitano]

The stock power transformer runs hot as it is. Going to DC may require a different power transformer.

Also, make sure you twist all AC from the transformer.

[udailey]

I noticed you said that you lost the left channel. I had trouble with the pot as well and decided to replace it (after turning it on only one time). I replaced the pot with a radio shack A100KX2. Its a 100kOhm .5W Dual pot. I grounded it with the lug. I turned on the amp and had lost the left channel totally, except that it still pops when powered up. All tubes glow. I replaced the pot with an exact match to the B100K that it came with. Still the left channel is missing.
I got out the multimeter and tested every resistor and cap. I think the multimeter is bad (its a chinese cheapy from ebay). It read many caps and resistors but some of them it would not read AT ALL. I figure a bad cap will still give some reading and a bad resistor should still have some resistance.
I have yet to hook it up to the transformers.
So far, any suggestions? Has anyone dealt with this before?