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Author Topic: Idle hissing from Crackatwoa  (Read 510 times)

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Offline luckme10

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Idle hissing from Crackatwoa
« on: December 18, 2020, 12:53:07 PM »
Hi Everyone,

I've been tinkering with a crackatwoa for awhile now and finally was able to get a working multimeter to run some tests. Granted I'm not an engineer, but everything generally looked within perimeters of the guide to me. So I turned it on for the first time and noticed some idle hissing with only the amp and the headphones. The hissing does not amplify with increase to the gain. I've tried running a few sets of headphones through the amp just to test it and have gotten the same hiss.

I've eventually replaced all the tubes trying to figure out what could be causing the idle hiss and have noticed no differences in the idle hiss.

I made a few upgrades and modifications during assembly:
2x Dayton Audio DMPC-100 100uF 250V Polypropylene Capacitors from parts express

Copper RCA connectors
6 x WBT-0210 Cu Topline Nextgen RCA socket from hificollective

SOST-22: Neotech Solid Silver Wire, 1/0.65mm running only through the rca sockets to connector switch and onwards between the balance and gain controllers from hificollective.

I had to replace the  power switch from radio shack. I had to replace this when i dropped the unit during the earlier stages of assembly.

For reference, here's my readings/ comparisons to the expected results in voltage and resistance tests:
https://docs.google.com/spreadsheets/d/1ZJBpG_B9ZdoGuOIm9WlCp3dtR-X8LhLpc6lBBsZye1c/edit#gid=0


Offline Paul Birkeland

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Re: Idle hissing from Crackatwoa
« Reply #1 on: December 18, 2020, 02:33:55 PM »
The 93V on the output terminal of the center low current C4S board is concerning.  On the 9 pin socket, do both LEDs glow about at the same brightness?  I would suspect a solder joint on that board isn't quite right. 

What kind of headphones are you using to evaluate the amp?  It wouldn't necessarily surprise me if there was some hiss on Grados, but there should be on Sennheiser 580/600/650/800 or the other high impedance headphones.

Can you post some build photos?
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline luckme10

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Re: Idle hissing from Crackatwoa
« Reply #2 on: December 18, 2020, 03:15:06 PM »
"On the 9 pin socket, do both LEDs glow about at the same brightness?  I would suspect a solder joint on that board isn't quite right. "

It appears that both LEDs glow about the same brightness levels on the 9 pin. I've attached pictures below.

Primarily using it to power an hd800 with an hd800 super dupont resonator mod. Have tested lower impedance stuff that i have lying around like 598s and a hd25 i ii just to test the hiss.

Just for clarification, there should be some idle hiss on high impedance headphones?

I know my soldering skills can use some work. More photos are in link: 
https://photos.app.goo.gl/7hsMQE8NAepSpiB57
« Last Edit: December 18, 2020, 03:20:12 PM by luckme10 »

Deke609

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Re: Idle hissing from Crackatwoa
« Reply #3 on: December 18, 2020, 03:36:19 PM »
This may be unrelated to the hiss issue, but in the 2nd pic (pcb), it looks like you're not using enough solder, or not enough heat, or both. I can't see a single solder joint where the solder has been drawn up to the topside of the board. 

I can't see the solder joints elsewhere in the amp, so I have no idea whether the pcb soldering is reflective of a similar issue elsewhere.  But flaky joints could cause noise.

Subject to what PB says, you might want to post some detailed pics of the soldering in other parts of the amp.

cheers and good luck, Derek

Offline Paul Birkeland

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Re: Idle hissing from Crackatwoa
« Reply #4 on: December 18, 2020, 04:25:04 PM »
I would agree that reflowing all of the PC board joints other than the LEDs would be a good plan.
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline luckme10

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Re: Idle hissing from Crackatwoa
« Reply #5 on: December 18, 2020, 04:32:09 PM »
Alright, I guess that's a good place to start. Appreciate all of the feedback guys!