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Millett "Starving Student" hybrid amp - Page 410

post #6136 of 6806
Quote:
Originally Posted by qusp View Post

I doubt thats enough to cause the scratching; its certainly not ideal, but unless your amp is DC coupled I dont think it would be a problem

 

I'm still unsure which is "ac coupled" and which is "dc coupled" I know the MSSH has no capacitor on it's input, so that makes it what, dc coupled?

Quote:
Originally Posted by qusp View Post

 

no, the impedance of the 470uf cap remains the same (well not really, but i'm not going to do the math wink.gif and dont know enough about the circuit to try), but the impedance at higher frequencies that can pass through the 0.22uf film cap is much lower and the path more linear. throwing randomly chosen bypass caps in parallel is very unlikely to improve matters. the frequency able to pass through a 0.22uf film cap into what? what impedance are your headphones? very low anyway; is likely to be up with the bats, so no wonder you didnt hear anything =)

 

I just calculated. The .22µf cuts off at 17khz with the SRH440 and 11khz with the Q701. It's high, but it should be audible on certain tracks. At least it didn't make things worse, so it can't hurt to bypass I suppose.

post #6137 of 6806

I feel like this THREAD needs a sticky! So much information!

 

Im going to build a SSMH-12AU7 and was wondering if there is a roundup anywhere of some performance/feature modifications before I start ordering parts. Things like adding a DAC, support for multiple inputs, reducing noise and hum, increasing longevity, yada yada yada. I dont know if this information is all in one place somewhere but if anyone can post some links that would be appreciated!

post #6138 of 6806
When I put 12AX7s in my MSSH I experienced similar issues (scratchy pot, hiss) SQ was off the chart otherwise. 12AU7s eliminated the noise problems. With my 600 ohm AKG 240M cans the MSSH is dead silent. With my MAudio Q40's I can hear some hiss even with the volume at normal listening levels (and a wee bit of pot noise). The casing of my pot is not grounded though and I'm sure that is contributing to the problem.

It's my opinion that the 12AX7s amplify and exaggerate ANY noise in the amp... just too much gain I suppose. 12AU7s are more forgiving.
post #6139 of 6806
Quote:
Originally Posted by bengoshi2000 View Post

When I put 12AX7s in my MSSH I experienced similar issues (scratchy pot, hiss) SQ was off the chart otherwise. 12AU7s eliminated the noise problems. With my 600 ohm AKG 240M cans the MSSH is dead silent. With my MAudio Q40's I can hear some hiss even with the volume at normal listening levels (and a wee bit of pot noise). The casing of my pot is not grounded though and I'm sure that is contributing to the problem.
It's my opinion that the 12AX7s amplify and exaggerate ANY noise in the amp... just too much gain I suppose. 12AU7s are more forgiving.

 

I know this is not the place to talk about that, but what did you mean by "SQ was of the chart otherwise"?

 

I'm just gonna get some NOS 12AU7... tube rolling, here I come! k701smile.gif

 

Just for kicks, I tried only the .22µf MKP as output caps. There is a lot of audio going trough them, I'd say half of what the 470µf electrolytic pass trough. So I installed the MKP permanently. I had them anyways, so why not? I don't know if it's placebo or what, but once they were soldered in, I could ear the difference. I'm guessing the reason why I didn't hear anything when just touched the wires together was that the ESR of the electrolytic was way lower than the MKP + the two solder-less connections.

 

I also calculated that my SSMH burns around 25W of power. Less efficient amplifier I've ever seen. biggrin.gif I'm actually using the MOSFET's heat sink as cup warmer. Keeps my tea warm for hours.

post #6140 of 6806
SQ = sound quality.

Also with the 12AX7s, I could only use about the first 10-15 degrees of the volume pot before it was just too darn loud. I found that 12AT7s were a reasonable compromise with my AKGs, but still too much gain to use with the Q40s.

The heatsinks do get hot! My laser thermometer shows them running at about 130*F!
post #6141 of 6806
Quote:
Originally Posted by bengoshi2000 View Post

SQ = sound quality.
Also with the 12AX7s, I could only use about the first 10-15 degrees of the volume pot before it was just too darn loud. I found that 12AT7s were a reasonable compromise with my AKGs, but still too much gain to use with the Q40s.
The heatsinks do get hot! My laser thermometer shows them running at about 130*F!

 

Yes, I know that SQ means Sound Quality. But I still don't get what you meant by "Sound quality was off the chart otherwise". Were you saying that the 12AX7 had a much higher SQ than the AU? Or that it was just awful?

post #6142 of 6806
Umm... "off the charts" In this context would mean "exceptional" and/or "superior"... clean, crisp, transparent highs; full-bodied mids; tight, deep bass.
post #6143 of 6806

Well, I gave the 12AU7 version a shot and its pretty nice. I etched myself a board and built it using some old GE tubes...

1a52865d.jpg

 

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DSC_0098.jpg

 

3db3dd9c.jpg


Edited by vixr - 7/2/12 at 3:26pm
post #6144 of 6806

Great job!

 

The schematic in the first picture shows the LED connected across the tube heater. Is that the way it's connected in the Beezar PCB, or is it your addition? I thought of doing this, but wondered if it would affect the circuit, or if the LED would flash like a VU meter. It's probably more efficient than powering the LED from the 48v and dropping all this voltage trough a resistor, anyways.

 

Do you have an enclosure for it? Don't forget to post pictures when it's fully assembled. biggrin.gif

post #6145 of 6806
Quote:
Originally Posted by KimLaroux View Post

Great job!

 

The schematic in the first picture shows the LED connected across the tube heater. Is that the way it's connected in the Beezar PCB, or is it your addition? I thought of doing this, but wondered if it would affect the circuit, or if the LED would flash like a VU meter. It's probably more efficient than powering the LED from the 48v and dropping all this voltage trough a resistor, anyways.

 

Do you have an enclosure for it? Don't forget to post pictures when it's fully assembled. biggrin.gif

KimLaroux,

    I dont remember where I got this schematic but the LED addition was already there. The LED isnt very bright and it doesn't flash...I was sorta leaning towards a clear enclosure like the one you used (fabulous) but this amp was a prototype. I am hoping to do some tweaks to the board now that I know the traces are all good. I am really infatuated with this amp right now, I built the 19J6 version when it first came out, but this version is more desirable in my humble opinion. I will put up some pics of the optimized board as soon as I etch one...thank you for the kind comments.

 

DSC_0091.jpg

 

I changed a few things...gonna try to etch this soon.

 

MHSS.JPG


Edited by vixr - 7/3/12 at 8:54am
post #6146 of 6806

I found this aluminum case in my stash along with the heat sink from something I gutted. The board I etched fit perfect inside...I still have to cut the holes in the lid for the tubes but I will do that at work tomorrow...I just need to polish up the outside a little.

 

DSC_0099.jpg

 

DSC_0101.jpg

 

DSC_0102.jpg

 

DSC_0104.jpg

post #6147 of 6806

That's a serious case. It would take quite something to break it. Not like mine...* Though if I were you, I'd drill holes into the fins of the heat sink, from top to bottom. Because heat goes up, and placing the fins horizontally like you did is not very efficient.

 

*Yesterday I tried to clean the flux from the case with alcohol. Some solvent got into a hole with a screw and it cracked the plastic. I learned that PMMA will crack that way if solvent get on a part that is under stress.

 

Tried to clean the enclosure with alcohol. Learned that PMMA cracks when solvent get in contact with parts of it that is under stress.

 

You can see the cracks in the plastic right of the tubes. So yeah, building an enclosure out of PMMA is not a good idea. I don't know what I'll do at this point. It's cracked so badly that there's no way I can tighten the screw without shattering the plastic into pieces.

 

These are the new NOS 12AU7 tubes I received yesterday. I'm still testing them. The gain is indeed lower than the AX, but not as dramatically lower as I expected. I added 100K resistors before the 100K pot, something I omitted originally. At least now the pot has an useful range. I tried a few other pots, and also cleaned them, but I still get pot scratching noise. Though the scratching noise is only audible when the pot is turned to max, which is just too loud anyways.

 

I still have a low hissing noise in the background, but low enough to be acceptable. I have two new parasitic noises though. One seems to be a ground loop hum. I don't see why I get such a hum though since I took extreme cares with the grounds and used only two star grounds : one for the PSU and one for the amplifier. I guess I'll have to experiment some more, but I honestly don't know what to try next. I connected the 0v/ground to earth using a 100 ohms resistor. Any opinions on this? Should I just connect them directly together, or use a 10 ohms resistor?

 

The second new noise is interesting, to say the least. I can hear my laptop and my router talking to each others! blink.gif That's something I really had not expected. I'm blaming it on the plastic enclosure, which cannot shield the amplifier from EMI. I'd really like some clarification here. Anyone else experienced such problem? Was your enclosure metal or something else? I'm starting to regret building an enclosure out of plastic. But then some people used wood... so the problem could be somewhere else.

post #6148 of 6806

KimLaroux,

    sorry to hear about the cracks in the enclosure...It is one of the coolest I've seen here. If i understand the grounding issue (prolly not), there are two common problems...ground loops and ground imbalances. Look for any turns or partial turns in the transformer wiring and get the signal wires as far away as possible from mains wiring...Long wires act like antennas...Do you have hum before the music source is connected? Is the imbalance caused by your computer being plugged into the same mains circuit as the amp? Seems like two star grounds could cause trouble...I found (using a toroidal transformer) that turning the transformer can reduce hum...if you rebuild the enclosure, you could try setting the transformer on the case, listen, rotate 90 degrees, listen...etc. The noise from the laptop is common. I always use my iphone as source with no hiss, so try running the computer on batteries only? or change sources to a portable type for a test?... My old cell phone would put all kinds of buzzing and clicking on my amps if it were too close to the amp.


Edited by vixr - 7/5/12 at 3:11pm
post #6149 of 6806

Finished!!! Now for a good polish... And a better knob.

 

DSC_00962.jpg

post #6150 of 6806

I just did a few tests to isolate the cause of the noises.

 

I noticed that if I lifted the amplifier in the air, the hum stopped. So I placed it on a metal pan, which made things worse. I then grounded the pan, and all the hum just muted.

 

Placing the amplifier on a grounded metal pan muted the hum.

 

But I could still loudly hear the interference of my wifi network. So I placed the other part of the pan, which is actually the cover for a box, on top of the amplifier and grounded it. This eliminated this interference.

 

Completely enclosing the amplifier in a grounded metal shield eliminated both the hum and the interference from my Wi-fi network.

 

Now there's just a hiss if I crank the volume to max. That's probably just random EMI picked by the open RCA connectors. This isn't an issue when listening to music.

 

Can I now conclude that building the starving student in a plastic enclosure is an epic failure? I mean the results of this experiment are hard to argue against: enclosed in this grounded shield, the amplifier is dead silent at any listenable level. Still I've seen plenty of such amplifiers built in plastic or wood enclosures, and I haven't heard their owner talking about such noises problems. I'll have to go trough this thread again and see if I can dig up anything related.

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