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"WHAMMY" Pass DIY headphone amp guide
"WHAMMY" Pass DIY headphone amp guide
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Old 19th January 2019, 07:06 PM   #1011
AllanMarcus is offline AllanMarcus  United States
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Quote:
Originally Posted by 6L6 View Post
Yes, I do.

Itís internally wired, has only one fuse drawer, is switched, filtered, and, the most important, has 3 clearly labeled tabs on the exterior for Live, Neutral and Earth. It covers all the requirements for most countries mains wiring, and happens to be a really nice IEC.

Of course, if it doesnít fit the enclosure, save it for another project and use something that works better for your build. This is DIY after all.
Thanks for the explanation. I really appreciate the response.

I think I will get it to work. It does look like a very good quality receptacle.
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Old 20th January 2019, 12:27 AM   #1012
AllanMarcus is offline AllanMarcus  United States
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Just curious. There is a screw hole in the center of the transformer and in the PCB. Is it necessary to secure the transformer to the PBC with that screw, or is the 8 points of solder sufficient?
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Old 20th January 2019, 02:28 AM   #1013
wayne is offline wayne  United States
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The solder is fine unless you want to ship it poorly packed.
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Old 20th January 2019, 06:49 PM   #1014
mark60615 is offline mark60615
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Quote:
Originally Posted by 6L6 View Post
Connect your RCA ground buss to the output ground. As far as I can tell, your inputs are not connected to ground except through the capacitor...
Thanks @6L6!!! I know this was meant for @Hook, but it worked for me as well!

To summarize my earlier post, my symptoms were:

- loud hum/buzz in left ear
- amp was silent with AD opamp wasn't working at all, TI opamp at least produced music but also the loud buzz. At first I thought I had fried the AD opamp somehow.
- Additional discoveries from last night: music was actually coming through in both ears, but the buzz was overwhelming it in the left. There was a quieter buzz in the right ear, so it was easier to hear the music.

Fix:

I took the RCA ground wire out of the hole behind the Alps pot and touched it directly to the output ground. Eureka, the buzz disappeared! I reconnected the AD opamp, and it was also working fine once the ground connection was fixed.

When the RCA ground wire is not connected to anything, the buzz is equally loud in both ears. When it's connected to the ground hole behind the Alps pot, it's much louder in the left ear. Does anyone know what would explain these symptoms? I did cut out a corner of the PCB near the C28 capacitor, and it looks like a trace connecting the PCB grounds was running along that corner. Would that explain the problem?

Is it ok if I keep the RCA ground connected to the output ground near R24? Would that cause any ground loop problems?
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Old 20th January 2019, 07:54 PM   #1015
AllanMarcus is offline AllanMarcus  United States
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A little sanity check here. As I’m building the amp, I plan on jumping C26 and C27 and then checking my DC offsets. If low enough, I will forgo those caps. Those are the filter caps, right? Does that sound like a sound plan?

Last edited by AllanMarcus; 20th January 2019 at 07:59 PM.
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Old 21st January 2019, 02:46 AM   #1016
wayne is offline wayne  United States
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The caps block DC and make the gain unity at DC. If you short them you get the same AC and DC gain.
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Old 22nd January 2019, 12:58 AM   #1017
AllanMarcus is offline AllanMarcus  United States
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Thanks wayne.

One (hopefully last) question. What gauge wire is recommended / safe to use between the IEC and the transformer. Is 20AWG solid core too small?
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Old 22nd January 2019, 03:37 AM   #1018
6L6 is offline 6L6  United States
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"WHAMMY" Pass DIY headphone amp guide
At the lengths required in even the biggest chassis, 20AWG will be fine for the wiring.
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Old 22nd January 2019, 03:46 AM   #1019
AllanMarcus is offline AllanMarcus  United States
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Thanks! I appreciate the advice.
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Old 22nd January 2019, 08:10 PM   #1020
nano35 is online now nano35  Hong Kong
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So I took away the input capacitors!

For both channels, the input DC offset(at input RCA jack) is less than 0.5mV; while the output DC offset(at headphone jack) is less than 1mV, depending on the op amp I use.
I tried different op amps and the worst DC offset in one channel is only 2.xmV. The best is 0.0mV.
(by accident, I turned on the amp once without an op amp installed. DC offset at output went up to 56mV!)

The SQ does improve compared to my vintage oil caps:
More clarity, more bass; treble is rendered more naturally.
Vocal is more upfront and everything has an extra layer of realism.

I believe "around 1mV" DC offset is a very good value?
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