Bottlehead Forum

Bottlehead Kits => Crack => Topic started by: jt on July 09, 2014, 12:28:08 PM

Title: Padding volume induces hum
Post by: jt on July 09, 2014, 12:28:08 PM

I am running a preamp and have removed the pot and padding the input as mentioned on other posts.
On each RCA jack I have 100k resistor from center pin to ground tab and another 100k resistor in line on signal wire.
This results in hum.
Also tried with 33k resistor inline with same result.
If i remove the inline resistors the hum goes away.
How can I troubleshoot this?

Thanks, Jay.

Title: Re: Padding volume induces hum
Post by: Paul Birkeland on July 09, 2014, 12:30:08 PM

Quote from: jt on July 09, 2014, 12:28:08 PM

On each RCA jack I have 100k resistor from center pin to ground tab and another 100k resistor in line on signal wire.

That's not quite how it should be wired.

You need a 100K resistor soldered to the pin, another 100K solder resistor soldered to the tab, solder the free ends of those two resistors together, and that junction will be your signal out to the Crack.

-PB

Title: Re: Padding volume induces hum
Post by: jt on July 10, 2014, 11:39:18 AM

PB,
I just tried the resistor setup you detailed in the previous post but I am still getting hum.
Any ideas?
Thanks, Jay.

Title: Re: Padding volume induces hum
Post by: Paul Birkeland on July 10, 2014, 11:48:04 AM

Try installing the same resistor setup at the potentiometer instead of the RCA jack.

Title: Re: Padding volume induces hum
Post by: jt on July 10, 2014, 11:52:43 AM

I am running with a preamp so I removed the pot.

Title: Re: Padding volume induces hum
Post by: Paul Birkeland on July 10, 2014, 12:45:40 PM

Ah, well, one thing that can cause a ton of hum is if the black wire attached to the two RCA jack shells doesn't run right to terminal 3.  If you have any resistors in that path, you may end up with hum.

-PB

Title: Re: Padding volume induces hum
Post by: jt on July 13, 2014, 12:37:04 PM

I replaced the signal wire from RCA jack to socket.
Also replaced ground wire from RCA tabs to terminal 3.
Still have hum.
Anything else I can try?

Title: Re: Padding volume induces hum
Post by: Paul Birkeland on July 13, 2014, 01:21:39 PM

Can you post some photos of what you've constructed?

Title: Re: Padding volume induces hum
Post by: jt on July 13, 2014, 04:01:49 PM

I removed the resistors so I could use the amp.
But here is a pic of the resistor setup.
Twisted end connected to signal wire.
Other ends connected to center pin and tab of RCA jack.
Amp works fine with resistors removed.

Title: Re: Padding volume induces hum
Post by: Paul Birkeland on July 13, 2014, 05:26:31 PM

Those are each 200K resistors.

Try two 47K resistors.  Also consider a metal or carbon film type. 

-PB

Title: Re: Padding volume induces hum
Post by: jt on July 13, 2014, 06:19:39 PM

This is what I purchased.
They are supposed to be Riken 100k carbon resistors??
http://www.ebay.com/itm/4-X-MADE-IN-JAPAN-RIKEN-OHM-RM2-100K-5-2W-AUDIO-GRADE-CARBON-RESISTOR-/301053913699?pt=Vintage_Electronics_R2&hash=item4618362a63

Title: Re: Padding volume induces hum
Post by: Paul Birkeland on July 13, 2014, 06:25:26 PM

Red black yellow gold is 200K 5%.  You could double check by measuring them.

Title: Re: Padding volume induces hum
Post by: jt on July 13, 2014, 06:37:44 PM

Just measured them and they are 100k.

Title: Re: Padding volume induces hum
Post by: Paul Birkeland on July 13, 2014, 07:01:25 PM

Hmm, must be a very reddish brown.

Anyway, give a pair of 47 resistors a shot.  Otherwise, you might consider throwing a volume pot back in.

-PB

(PS - 1/16 Watt resistors would work well, 2W is overkill)