diyAudio

diyAudio (https://www.diyaudio.com/forums/index.php)
-   The diyAudio Store (https://www.diyaudio.com/forums/the-diyaudio-store/)
-   -   Starving Student II Headphone Amplifier (https://www.diyaudio.com/forums/the-diyaudio-store/335476-starving-student-ii-headphone-amplifier.html)

mediumroast 2nd April 2019 04:27 PM

Hi Pete, Thank you for the quick reply!

I'll check on it tonight.

MEPER 2nd April 2019 06:47 PM

2 Attachment(s)
I finished my Staving Student and it is playing music! I use CCS setting to begin with. Before inserting tubes I measured at pin 3 on tube sockets and there was exactly 19.0 VDC.

I have an issue with hum. With Vol. turned down and no input hum gets worse if my hands comes close to the tubes. If I touch chassis it gets worse but if I touch Gnd on either input or output RCAs hum is almost gone. So it must be a grounding issue. Is chassis floating or where is the signal Gnd to chassis supposed to be?

Is it the nut on the vol. pot that makes the gnd to chassis?
I noticed that the cutouts in the front panel is black but 6L6s was pure aluminium so he may get a better galvanic connection.

If I turn up the vol to about middle the hum change from a buzz hum to a more clean not so loud hum and at max vol. it is back to a buzz hum sound.

At vol. set to about 9 o'clock it plays quite loud. My DAC probably has large output. Probably 2Vpp or more.

If I make a connection using a wire (just by hand) from a chassis screw to the gnd of one of the RCA output......it is silent. So now I just have to find out where the "official" Gnd to chassis should be?

I tested using a DMM the connection from the pot nut to chassis and there is no connection. Pot nut has connection to Gnd so I am quite sure if I scrap off the black "paint" or find a shim which can cut through the black layer the hum is gone……..

aljordan 2nd April 2019 08:16 PM

Not that this is an answer, but I have bare aluminum on mine and it is quiet as a mouse.

MEPER 2nd April 2019 08:49 PM

1 Attachment(s)
I was able to get electrical contact between pot nut and chassis and this reduced the hum a bit and hum is not dependent of vol setting anymore. But I have still a bit of hum. If I lay hands on chassis and wrap my hands around the tubes without touching them hum is gone so I guess the tubes pick up some hum?

I have no Earth ground in the 230 VAC outlets.

I tried to screen one of the tubes and that made it almost silent in both channels. But only if I connect the end of the solder to be chassis. If I screened both tubes like you sometimes see in old tube amps it may make it 100% silent. Seems there is something very high impedance capacitive coupling from the surroundings (eg from my hands)?

I also just checked the two LEDs inside the chassis which controls the CCS. They also lights up nicely.

pmillett 2nd April 2019 10:54 PM

Quote:

Originally Posted by MEPER (https://www.diyaudio.com/forums/showthread.php?p=5748774#post5748774)
I tried to screen one of the tubes and that made it almost silent in both channels. But only if I connect the end of the solder to be chassis. If I screened both tubes like you sometimes see in old tube amps it may make it 100% silent. Seems there is something very high impedance capacitive coupling from the surroundings (eg from my hands)?

The plate circuit of the tube is a very high impedance node, especially when using the CCS. So it is susceptible to picking up environmental noise. You are a big antenna, so bringing your hand near the tube couples line voltage noise into the plates.

I think you figured out that the pot frame is connected to ground. The issue with black anodize is that it is non conductive, so to get good contact you may have to remove some of the anodizing around the volume control shaft.

Bonding the chassis to a real ground might help things, but it sounds like you may not have a real ground easily available.

Pete

mediumroast 3rd April 2019 03:43 AM

Quote:

Originally Posted by pmillett (https://www.diyaudio.com/forums/the-diyaudio-store/335476-starving-student-ii-headphone-amplifier-post5748483.html#post5748483)
The most likely error is that the grounded end of R11 isn't really soldered to ground. It's easy to get a solder joint that looks good, but actually has not flowed onto the grounded pad. It's connected to a large copper area that sinks heat away, so it is difficult to solder.

If this happens, the gate of Q3 is biased to 48V, so the source (and VT1 pin 3) goes to 48V - Vgs(th) or about 43V.

With power off, use an ohmeter to confirm that R11 is 200k, and R15 is 221k. And measure from the grounded end of R11 (on the resistor lead) to ground to see if it is connected.

Also check for solder bridges on Q3.

Pete

After reflowing R11, VT1 pin3 to headphone ground now measures 20.5 volts.

Plugged in the tubes and headphones but there's a lot of loud crackling after the 20 second turn on. The tube in VT1 glows bright orange while VT2 tube I barely see any orange.

MEPER 3rd April 2019 06:42 AM

Quote:

Originally Posted by pmillett (https://www.diyaudio.com/forums/the-diyaudio-store/335476-starving-student-ii-headphone-amplifier-post5748861.html#post5748861)
The plate circuit of the tube is a very high impedance node, especially when using the CCS. So it is susceptible to picking up environmental noise. You are a big antenna, so bringing your hand near the tube couples line voltage noise into the plates.
I think you figured out that the pot frame is connected to ground. .

Pete


Thank you for the answer. Yes, I removed some of the black "layer" and got electrical contact to the pot. I will experiment a bit more with grounding and also change from CCS to resistor to check this out. A shield around the tubes could also help.

I was thinking of adding a grounding screw to the chassis for easy experiments with grounding. Like on some phono pre-amps for grounding the turntable. First I will switch to resistor "load". Then the 47.5 k should "ground" the plate through the PSU I guess and then reduce the plate impedance. I learned many years ago that a CCS has infinite impedance :-) …..it is a good learning project also. That is the funny part of DIY.

I switched from CCS to resistor and in this mode hum is only in one channel (in CCS mode I also think hum was much worse in one channel). It follows the tube. If I switch tubes hum also switch channel. So some tubes seems to be more sensitive for picking up hum than others? I think I may prefer this mode for playback. But I may also "hunt" for a shielding solution of the tubes.


Some of the "hardware" they sell here will probably work?
https://www.surplussales.com/Tubes-S...Shields-1.html

The sound is very good from this amplifier used as headphone amp. The subjective impression is that bass seems deeper than when I use the Phone output on my DAC. I found shields from my "local" store in Germany. Think it will look good when installed. Will post pictures and results when installed. It only requires two M3 holes for each shield. For now I use my "solder roll" on the humming tube. 60/40 solder.....have not tried using solder with silver :-)

pmillett 3rd April 2019 04:46 PM

Quote:

Originally Posted by mediumroast (https://www.diyaudio.com/forums/the-diyaudio-store/335476-starving-student-ii-headphone-amplifier-post5749022.html#post5749022)
After reflowing R11, VT1 pin3 to headphone ground now measures 20.5 volts.

Plugged in the tubes and headphones but there's a lot of loud crackling after the 20 second turn on. The tube in VT1 glows bright orange while VT2 tube I barely see any orange.

Can you check the voltage on pin 3 with the tubes installed? Both should be around 19-20V.

The crackling is again usually indicative of a cold solder joint somewhere...

Pete

mediumroast 4th April 2019 01:46 PM

Quote:

Originally Posted by pmillett (https://www.diyaudio.com/forums/the-diyaudio-store/335476-starving-student-ii-headphone-amplifier-post5749515.html#post5749515)
Can you check the voltage on pin 3 with the tubes installed? Both should be around 19-20V.

The crackling is again usually indicative of a cold solder joint somewhere...

Pete

With the tubes installed, VT1 pin3 is varying 29.5v to 25v while VT2 pin3 also increased to 24.5v.

I'll reflow everything when I get a chance.

Centurion53 4th April 2019 06:38 PM

Some overall observations from the build. This was probably one of the more difficult boards I have ever worked on. Some of the pads were so small it was difficult to get the solder to flow well. I tried using Cardas as well as Kester 44 solder, neither was easy to work. Was using a Hakko iron with a fine pointed tip. When completed and ready for the smoke test, no LED's lit up. I went over the solder joints and tried again and success! I checked the voltage from ground to pin 3 of both tubes and got about 18.95 volts. I plugged the tubes in and fired it up and was getting about 17. 5 on one and 17.72 on the other. I have hooked it up to my ACA and ran it for about a half hour using my phone as an audio source. Everything sounded very good, no noise at all.


All times are GMT. The time now is 01:29 PM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2020 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 17.65%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2020 DragonByte Technologies Ltd.
Copyright ©1999-2020 diyAudio

Wiki