AMB Laboratories DIY Audio

[Bolivar]

Hi.

I've just finished building this amp. All checks completed and quiescent current set to 30mA. However I'm getting a hum through the amp when any interconnect cable is connected to the input. Doesn't matter if there's any source connected or not. The hum gets louder when turning up the volume. Also, the hum mostly comes through the left channel, unless I disconnect the left channel input, the hum switches over to the right. And I tried playing music through the amp and it sounds very distant and washed out.

[amb]

The pics aren't very clear, but it looks like you used white and red for the left and right input wires, and green for ground, which then goes to the ε27. What order are the wires connected on the ε27's input? And what order are the wires at the ε27 output? And at CK²III's input connectors?

How are the CK²III output wires connected to the headphone jack?

You also have a green wire going from the ground hole over the front panel. What is that for?

[Bolivar]

Allright, some info on the wiring:

orientation as in last pic(front end upwards)

ε27 inputs top to bottom = left, ground, ground, right

ε27 outputs top to bottom = left, ground, ground, right

CK²III input left top to bottom = left, ground

CK²III input right top to bottom = right, ground

Output wires to headphone jack = OL -> tip, OR -> ring, OG -> sleeve. And the green wire is a temporary ground wire to the volume pot chassis.

[amb]

The description is all correct, but a "distant and washed out" sound is almost always a wiring error (reversed phase on one channel). I would look carefully at the headphone jack to make sure what you think is the tip, ring and sleeve terminals are really what they are.

As for hum, if you don't have a source connected, then as you turn up the volume, you'll have floating inputs and that could pick up hum and noise. That's not a normal mode of operation anyway. The close proximity of the power transformer to the input wiring makes this situation worse. If you still have hum with a source connected, then check your interconnect cable.

What source and headphones are you using and is your amp built to the default gain of 9x? That might be too much gain for some low impedance headphones.

[Bolivar]

You're a genius! My headphone jack is wired wrong. What a rookie mistake, feeling kinda dumb now....

[Bolivar]

Now this is weird. I corrected the headphone jack wiring, but the hum and phase problems persist.

My source is a y1 dac, I'm using Shure SRH-840 (44 ohm) headphones and I've set the gain to 4.1x. I've tested several interconnect cables. I'm utterly confused now.

[amb]

Now this is weird. I corrected the headphone jack wiring, but the hum and phase problems persist.

If you still have a phase problem then the wiring is still not correct... Is your headphone jack a "switched" type (what jack did you use)? Did you by any chance connect the amp output and ground wires to the "switch" pins on the jack instead of the TRS contact pins?

My source is a y1 dac, I'm using Shure SRH-840 (44 ohm) headphones and I've set the gain to 4.1x. I've tested several interconnect cables. I'm utterly confused now.

Did you try a different source (i.e., CD player, iPod, whatever) just to rule out a problem with the source? Is the hum level volume knob position dependent? Does it hum when the volume is turned to minimum?

[Bolivar]

Ok, I've figured it out. My RCA input jacks had a bad ground connection. Took them apart and reassembled and now the amp works beautifully. Thanks for your help.