T/R startup voltage and hum

[NoobGuy]

Hi guys

So I finally have my Crack up and running. All the resistances and voltages check out a-okay, except the tip / ring voltage when powering up without headphones attached. The voltage spikes to around 25 (way over the expected 9V), but goes down to a few millivolts after a while.

After letting it sit for a few minutes (making sure that jack voltage stayed close to 0), I plugged in some cheapo headphones, and an old ipod as source. And it works! Super chuffed.

The other problem I've encountered is hum. There is no hum up until around 30% volume, then it increases with volume. With no source attached, the increase is constant. With a source attached, the increase is constant until 90% volume, thereafter the hum drops quickly until there is none audible at max volume. Weird. This occurred with the cheapo 'phones as well as HD650s.

Mine is the 240V version. Changes made:
Installed shielded signal wiring from RCAs to volume pot
Alps blue pot
Neutrik locking jack
Increased value of output capacitors for possible Grado usage later
Mullard 12AU7
GE 6080
Edit: Belton tube sockets

Sooo... any suggestions?

[JamieMcC]

Your most probably picking up the hum try rerouting you interconnect cables clear of any mains leads twisting together the rca leads that go from dac to crack helped with mine its quick and easy to try.

[Doc B.]

 How is the shielded cable connected?

[Natural Sound]

Hi guys

So I finally have my Crack up and running. All the resistances and voltages check out a-okay, except the tip / ring voltage when powering up without headphones attached. The voltage spikes to around 25 (way over the expected 9V), but goes down to a few millivolts after a while.

After letting it sit for a few minutes (making sure that jack voltage stayed close to 0), I plugged in some cheapo headphones, and an old ipod as source. And it works! Super chuffed.

The other problem I've encountered is hum. There is no hum up until around 30% volume, then it increases with volume. With no source attached, the increase is constant. With a source attached, the increase is constant until 90% volume, thereafter the hum drops quickly until there is none audible at max volume. Weird. This occurred with the cheapo 'phones as well as HD650s.

Mine is the 240V version. Changes made:
Installed shielded signal wiring from RCAs to volume pot
Alps blue pot
Neutrik locking jack
Increased value of output capacitors for possible Grado usage later
Mullard 12AU7
GE 6080

Sooo... any suggestions?

Can you give a little more detail about how you hooked up the shield on the signal wiring? You may have created a ground loop.

EDIT: Doc jumped in before me while I was typing.

[NoobGuy]

 

I connected the cable the same way I've made interconnects before.

The cable is dual coaxial, so centre conductors to signal points on RCA jacks and potentiometer, and spiral shield connected to the RCA and pot grounds

Edit: so the two centre conductors replace the red and white wires of the original, and the shields replace the black wire.

[Paul Birkeland]

Increased value of output capacitors for possible Grado usage later

That is causing your voltage spike.

Properly driving Grado headphones takes more than just changing out a capacitor.  If this was the case, don't you think we would've put the proper capacitors in the box to begin with?

I would strongly recommend putting the factory caps back in for the safety of all your other headphones.

-PB

[NoobGuy]

Paul, I would never presume to know better than you guys . I simply followed the recommendation in item no. 7 in the Crack FAQ right here on the forums.

Plus it was just an easy-to-implement mod for flexibility. Almost all listening will be done via high-impedance 'phones.

[Paul Birkeland]

With the 1K resistors, you're still getting 25V at startup?

I'll have to amend the recommendation if this is the case.

[NoobGuy]

Not 1kΩ exactly, but 1.1kΩ which I had in my parts box. These fellas: http://uk.rs-online.com/web/p/through-hole-fixed-resistors/0165804/

I had no idea that 0.1k would make that much of a difference - I'll swap em out right away.

The 25V is *extremely* brief, it falls away very quickly. But still enough to warrant power up without headphones plugged in

[Paul Birkeland]

If you have grados plugged in while the amp is warming up, the spike is going to be tiny. 

Do note that the 1K resistors marginalize the performance of the amp with high impedance headphones plugged in.

-PB

[NoobGuy]

Hmm. Okay, thanks Paul! In that case I'll swap back to the stock cap and resistor values.

Any advice on the hum/buzz?

[Doc B.]

We have seen this particular behavior on occasion and it seems to have something to do with stray capacitance of the cable interacting with the volume pot. You might try changng to the stock input wiring. You might also try disconnecting one end of one shield, maybe at the pot end. That would eliminate a ground loop between the shields and still keep them both grounded and with continuity. Simply touching up the solder joints on the connections might help too.

[skypig]

The voltage spikes to around 25 (way over the expected 9V), but goes down to a few millivolts after a while.

Mine does this too, after Speedball install. All stock parts.
Very briefly, flicked to 21v (so quick it's hard to read. Repeated a few times with similar results)
Back to mV quickly. (Volume all the way down)

It sounds great, am I risking my HD650s?

[NoobGuy]

Changed the caps and resistors to stock, and the voltage spike is way better but still a little high - it reaches around 12V briefly. Is it okay to leave the HD650s plugged in on power up?

Doc, here is a pic of my signal cabling. Not a separate screen, but rather the ground is doing double duty.

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi.imgur.com%2FcDCxBtG.jpg&hash=81cb5e8c844689ac713bef40d19c262a76955a85)

[Paul Birkeland]

Yeah, the voltage spike is a bit higher with Speedball, but very brief compared to the stock circuit.  With any headphones plugged in, the spike is reduced significantly, and I've never hesitated to leave my HD-800's plugged in during warmup. 

On that coax input wiring, you can try lifting the grounds on the RCA jack side, then running a separate grounding wire up to the pots, and leaving the coax shield connected on that end. 

Also, loose 220uF/250V caps in the amplifier will leave a lot of residual power supply noise, and can contribute to a hum problem without giving any indication in measured voltages.

-PB