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Starving Student II Headphone Amplifier
Starving Student II Headphone Amplifier
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Old 2nd May 2019, 01:18 AM   #81
aspringv is offline aspringv  Australia
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Starving Student II Headphone Amplifier
Here in Australia, eBay is pretty much the only source for tubes... And the pickings for that particular tube are pretty slim, hence me having a bit of a scrounge about now for alternatives.

Cheers!
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Old 5th May 2019, 07:00 PM   #82
olen is offline olen  United States
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Starving Student II Headphone Amplifier
Default Microphonic tube

I just finished building the kit this morning and after passing the smoke test I put the board in the chassis and plugged in my sennheiser 595s. The volume was all the way down to start. The first thing I noticed was a lot of noise on the right channel when I touched the volume knob. Even a light touch caused a lot noise. The noise level was independent of the volume position.
Iím running the tubes with the plate resistor load because I swapped out the blue LEDs for amber ones. I also replaced the resistors with Dale 1% RN55/RN60s, I use the same values and verified each with my DMM. There is no noise unless I touch the chassis or anything near the chassis, it seems clearly to be a microphonic tube. Would I be able to get a replacement tube?

Thanks,
Olen
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Old 5th May 2019, 07:16 PM   #83
MEPER is offline MEPER  Denmark
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Does the vol. pot has good electrical contact with front panel?
My front panel was black anodized inside the cut out for vol. knob. So I had to sand the "hole" to get good electrical contact to vol. pot. I think this is the only place where PCB gets its ground contact to chassis. This could remove the vol. knob noise.
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Old 5th May 2019, 11:16 PM   #84
olen is offline olen  United States
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Starving Student II Headphone Amplifier
Thanks for the tip. I did not sand the anodization from the hole, but the shaft on the pot is plastic so I’m not sure that will solve it. The noise sounds microphonic/mechanical more than electrical. I also get the noise when tapping on the chassis.
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Old 6th May 2019, 07:11 AM   #85
MEPER is offline MEPER  Denmark
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Ok.....have you tried to switch tubes (same tubes but L=R and R=L) to see if the "error" follows the tube?
I think it would be good not to have the chassis electrical floating. It may not solve the current problem you have but may reduce noise. It is a shame that the cut outs is not without the anodization like it seems some of the first production chassis was.
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Old 6th May 2019, 03:38 PM   #86
pmillett is offline pmillett  United States
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Quote:
Originally Posted by olen View Post
I just finished building the kit this morning and after passing the smoke test I put the board in the chassis and plugged in my sennheiser 595s. The volume was all the way down to start. The first thing I noticed was a lot of noise on the right channel when I touched the volume knob. Even a light touch caused a lot noise. The noise level was independent of the volume position.
Iím running the tubes with the plate resistor load because I swapped out the blue LEDs for amber ones. I also replaced the resistors with Dale 1% RN55/RN60s, I use the same values and verified each with my DMM. There is no noise unless I touch the chassis or anything near the chassis, it seems clearly to be a microphonic tube. Would I be able to get a replacement tube?

Thanks,
Olen
As was noted - swap tubes from one channel to the other. If the problem switches channels, it would be related to the tube and we can get you a replacement.

Just touching the amp generally will not induce enough vibration to make noise in a microphonic tube. This sounds more like a grounding issue.

The volume control has a plastic shaft, but the bushing and body are metal, and are connected to ground on the PCB. It helps if you remove the black anodizing from the inside (rear) surface of the front panel, where the volume control contacts it. Also, your AC line connection should have a ground - if not you may need to come up with some other way to ground the amp.

Also double check the solder joints on the volume control. An open ground can also cause this sort of problem.

Pete
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Old 18th May 2019, 02:50 PM   #87
olen is offline olen  United States
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Starving Student II Headphone Amplifier
I swapped the tubes and the noise followed the tube to the other channel. I ordered 10 tubes from radio electric supply for $7.50 with $10 shipping (they appear identical to Raytheon tubes that came in the kit), since I wanted backup tubes anyway. I tried out 3 tubes until I found one that nearly immune to microphonics. With the new tube (and one original) I can tap directly on the tubes without any noise on the output. I’m also trying out the SSII as a preamp so I’m trying to avoid that reverb effect you sometimes get with a microphonic tube.

I didn’t need to remove any of the anodization on the chassis, since I don’t have any electrical noise issues. The noise floor is very quiet! I measured the power supply and I was surprised that the switcher negative output is tied to earth ground. I’m used to two-prong wallwarts with floating outputs, so I had assumed his was always the case. Thanks for the great kit!
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Old 24th May 2019, 09:05 PM   #88
patkhan1 is offline patkhan1  Chile
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My DCPP amp is getting old and is making noise. I replaced it with a couple of ACA monoblocks (early set). The pairing is outstanding, very deep and firm bass and very musical upper range.
Played through Zu speakers the output is more than loud enough, if not louder, it's tighter than with the tube amp.
It's like rediscovering both the pre and the power, and even the speakers.
A gift that keeps on giving.
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Old 24th May 2019, 09:56 PM   #89
aljordan is offline aljordan  United States
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You are using the SSII as the preamp with the ACAs? I'll have to try that. I find the ACAs a little tough to drive with tube preamps, I'm guessing because of the low input impedance. Are you using the CCS setting in the SSII?
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Old 27th May 2019, 01:34 PM   #90
patkhan1 is offline patkhan1  Chile
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Yes, I'm using the SSII as a preamp to drive the ACAs. I'm using it with the CCS setting and it drives them with no struggle at all. Maybe it's because of the high efficiency speakers, but the ACAs seem to be a very easy load, there is plenty of power and the sound is very transparent.
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