[Phloodpants]

I recently finished an amp I dubbed "Phonehenge". Taking a couple design cues from Sheldon Stoke's design, I designed an amp on an octagonal base. The base is made of cobote wood with bloodwood corners.

It's based on the BUF634 chip, along with an OP275.

I'm really proud of the way it looks and it sounds terrific!

(this pic is from before I mounted the controls and jacks)

But it hums!!

Now, I had suspected this might be a problem from the beginning, so before getting the whole thing assembled, I tested the circuit out. Everything was kosher. No hum, no noise, nada, zip! Last night I try it out and it hums pretty badly. Then I take it to work with me and the problem is less, but it's still there. If I turn on a flourescent light, it gets worse.

So basically, i think what happened is the day I tested it, the power was unusually clean to my house. Today it is noisier and the problem rears it's ugly head.

So it's picking up hum from the transformer, which is mounted very close to the op-amps and input circuitry.

I've tried bypassing the volume control entirely, and that doesn't have any effect.

I've also tried running power right to the transformer, bypassing the fuse and power switch. No difference.

Anybody have any more ideas? I would hate to scrap this project and put it in a plain project box! I'm out of ideas...

Hmm... what if I put a resistor on the input to lower the input impedance?

 

[PRR]

> So it's picking up hum from the transformer

Unlikely. If it were, it would hum all the time. Anyway, that transformer is semi-shielded.

> mounted very close to the op-amps and input circuitry

Is there any shielding on the chips, or are they out in the open? What about the signal wires from jack to pot to chip? Can you post a close-up?

Is it on the same power-outlet as the source? If not, try that.

> If I turn on a flourescent light, it gets worse.

Suggesting that it is picking up hum on the input or feedback leads right out of thin air.

> what if I put a resistor on the input to lower the input impedance?

Taking out the volume-pot made no difference. Unless you are feeding with some very high impedance source, loading down the input any reasonable amount should make no difference.

If it does clear up suddenly at some magic value of input loading, you have hypersonic oscillations.

-PRR

 

[Phloodpants]

Well, my volume pot is actually being driven by a buffer stage.

Signal chain is like this...

Input ---> Unity buffer ---> 10K Volume pot ---> 5x gain stage ---> BUF634 ---> Out

The chips are all out in the open.

I've tried different outlets, even an isolation transformer.

Well, I think you're right about the high source impedance. I'm running that first buffer at about 150K Ohms, which I now realize is mighty high! I just tried shorting the input and all noise goes away!

I'm not at home right now, so I can't try different resistor values yet...

Thanks PRR... this is helpful.

 

[Phloodpants]

Oh, and what would be a standard input impedance? Would 10K be too low? I don't want to load the preamp down to the point it doesn't drive properly. Also, I want to be able to run a 15 foot patch cord sometimes.

 

[wab]

I don't know about the humming problems but I wanted to say your amp looks awesome. Please post more pics if you can!

 

[Phloodpants]

Thanks man! I hope to make it more than just pretty! It'll be a good doorstop otherwise!

I'll take a couple more pics when it's all together...

 

[PRR]

> I'm running that first buffer at about 150K Ohms

And unshielded. An exposed 150K node will pick up all the hum in the room. I can get away with it in my recording closet, but it would hum on my workbench (fluorescent lamps). A friend in NYC can't get away with anything, due to radio, TV and power crap permiating the air.

I assume this is all low-voltage. Put your finger near, not on, various parts. The one that hums loudest from finger-induced hum is where you should start.

Post or point-to your schemetic, maybe I can suggest alternate values.

And that is a GREAT construction. Leaving the low-level op-amps exposed is a risky choice, but may amps live in plastic boxes and work fine. There has to be a way to make it less hum-hungry.

-PRR

 

[Phloodpants]

Well, admittedly, this is a fetish object for me and I have placed the emphasis on appearances, assuming I could do it without compromising the audio quality much.

The schematic is basically as you see on the BUF634 datasheet, except with less gain.

http://www-s.ti.com/sc/ds/buf634.pdf

Figure 5 in the applications section...

Thanks, I'll try lowering the input impedance when I get home.

Still wondering what a practical lower limit is... 10K alright?

 

[Phloodpants]

OK, fixed! I lowered the input impedance to 10K and upped the input capacitor to 2mFd to compensate. (should get me to 8hz I believe)

No hum at all...

Interesting note... one of the channels circuitry is on the opposite side of the volume control, so the wires from the volume control have to run all the way to the other side. I had been running these wires (coax) around the inside perimeter of the octagon, avoiding the transformer as much as possible. This actually caused a little hum in that channel. (besides the input impedance problem) The fix was to run that cable straight down the middle of the transformer! My theory on this is that there is less inducted into the wire this way because it is recieving out of phase inducted currents this way.

Sometimes the way out it through...

I'll be posting a complete report on this amp when I get some cosmetic touches done...

Thanks guys!

 

[aos]

Another way is to filter the input by adding a low-pass filter that will shunt all ultrasonic & radio garbage to the ground. Basically a resistor or two and a small capacitor. You can pick up those ideas from Jan Meier's Corda (see Headwize project library).

Another and simpler to try is to use ferrite clamps on the input cables (or even on cables inside amp). These things are cheap and commonly used everywhere. It will have the similar low-pass filter effect without you needing to change the circuit.

Btw, it does look great. Original design and great execution of the idea.