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Millett "Starving Student" hybrid amp - Page 435

post #6511 of 6853
Quote:
Originally Posted by jarscar View Post

*Edit I just did a quick thread search, tons of heat is apparently normal. I'm seriously giddy over how awesome this thing sounds though.

 

 

 

It does indeed get hot, that's perfectly normal for this amp. How hot it gets is also a function of how large the heatsinks you used are. Bigger heatsinks cool things better.

post #6512 of 6853
Well I managed to sort my 'mono' problem. After re wiring my whole amplifier to achieve the same result. mad.gif

I have a faulty 'iDevices' phono lead. It's band new ffs! I did learn a lot more about grounding and have improved any hum issues no end.

Casually enjoying some stereo sound now. smily_headphones1.gif
post #6513 of 6853

I had to take my amp apart to fix an issue I was having with the paint job, upon putting it back together one of my MOSFETs is getting well over 40v from the source pin instead of ~13v

I didn't have to change any wiring when I took it apart, aside from having to disconnect the RCA jacks and the power plug because those disconnect from inside the case instead of outside, I've checked and re-checked 4 times, I guess I'll try replacing that MOSFET 

I took the tube out and left the other in, I'm getting a crackle on start up from the empty line, I'm assuming that's the capacitor reacting with the excessive voltage from the source. The other MOSFET works fine and the sound coming out is normal.

post #6514 of 6853

What voltages do you get at the gates of the MOSFETs? It could be that the voltage divider no longer connects to ground, which means the gate got pulled to the 48 V line.

post #6515 of 6853
Quote:
Originally Posted by KimLaroux View Post

What voltages do you get at the gates of the MOSFETs? It could be that the voltage divider no longer connects to ground, which means the gate got pulled to the 48 V line.

  I'm getting around 34v at the gate for the MOSFET acting up and 15v for the one working fine

post #6516 of 6853

You're getting 40 V at the source, but 34 V at the gate? Sounds like a burned MOSFET. Or you're messing up the source and the drain. 

 

You could try to disconnect R2/R8 of the misbehaving fet and see what it does. Just beware that if you disconnect the wrong resistor (R4/R10), I can guarantee you you'll blow a tube. Been there, done that.

 

If it's not a blown mosfet, then I'm guessing you have a cold joint on one of the pull-down resistor, R4 or R10.

post #6517 of 6853
Quote:
Originally Posted by KimLaroux View Post

You're getting 40 V at the source, but 34 V at the gate? Sounds like a burned MOSFET. Or you're messing up the source and the drain. 

 

You could try to disconnect R2/R8 of the misbehaving fet and see what it does. Just beware that if you disconnect the wrong resistor (R4/R10), I can guarantee you you'll blow a tube. Been there, done that.

 

If it's not a blown mosfet, then I'm guessing you have a cold joint on one of the pull-down resistor, R4 or R10.

Thanks a ton, that tells me enough to play it safe and just rebuild that section of the amp, I've got enough spare parts so there's no reason not to

post #6518 of 6853
Quote:
Originally Posted by KimLaroux View Post

You're getting 40 V at the source, but 34 V at the gate? Sounds like a burned MOSFET. Or you're messing up the source and the drain. 

 

You could try to disconnect R2/R8 of the misbehaving fet and see what it does. Just beware that if you disconnect the wrong resistor (R4/R10), I can guarantee you you'll blow a tube. Been there, done that.

 

If it's not a blown mosfet, then I'm guessing you have a cold joint on one of the pull-down resistor, R4 or R10.

Edit: I turned it off and walked away, I guess I shouldn't jump to conclusions, the voltages are all correct now, I guess I should have unplugged everything then tested again, but thank goodness it was just user error and not mechanical failure. Also I'm buying a new multimeter, and not one from Harbor Freight this time, I spent the longest time thinking my power supply was dead, turns out I had to solder the positive lead back on in my brand new multimeter, that should teach me to stop buying anything but a hammer from there.

 

Well I replaced the MOSFET and all the resistors for that side, I switched it on and was getting the correct 15v at pin 1, 48v at pin 2 however I was getting 18v at pin 3, went back to my work table and double checked things.

Then when I was double checking my measurements my multimeter probe slipped and connected pins 2 and 3 for a split second, now I'm getting 33v at pin 3, so there goes another MOSFET I guess.


Edited by jarscar - 5/13/13 at 9:28pm
post #6519 of 6853

I have a question to everyone that built a student.

Has anyone tried to measure a crosstalk? I am getting quite bad result:

 

 

 

The lowest my sound card is able to measure is about -75db so I don't think that it's a problem in here.

post #6520 of 6853
What pot are you using? As far as I'm aware this part has the greatest impact on crosstalk. I know my starving student doesn't have any audible crosstalk. I'll measure it when I get home on Friday to compare.
post #6521 of 6853

Can't say much about the pot except that it is 50k stereo.

I think that the crosstalk may be caused  by the cable mess in the case.

post #6522 of 6853

Hi guys.

I've been visiting for a while and keeping tabs on this thread. First I'd like to thank everyone who put time and effort into this project, not least Mr. Millett.

I prototyped my version on stripboard and put it all into an ugly little case pending my finding a more aesthetically pleasing enclosure. Whilst not exactly pleasing to the eye it worked first time. I haven't ever listened to music with an amp of this kind so my aim was to build it fast and cheaply to get a feel for how much effort I should put in. The amp has all of the additional parts suggested on the 12AU7 schematic. I didn't get a pair of matched valves (they were very cheap though).

I'm glad to say that I really enjoy the sound through my Sennheiser 595's fed from a Nokia N900. However, some minor grounding issues remain and the voltage at pin 4 is closer to 16V on one channel. I'm hoping there's an easy fix for this so I'm going to re-trawl the thread.

Many thanks.

post #6523 of 6853

How's It going guys?  So I have recently embarked on this project and am stoked to see how it will turn out.  Having recently finished the one and only circuits class I will ever need to take, I am not the most astute electrical engineer, but one of the reasons I started this project was to learn more about circuits and theory and also I'm a bit of an audiophile and this is the perfect project for me considering I am indeed a starving student.  

 

SO my question regards the Tube LED's.  I have drilled out the tube sockets, easy, but I do not know where to attach the LED and led resistor in the circuit?   Could some one shed some light on this for me?

 

Also, I using the specs on a 3mm LED I have and this website I calculated I need a 1kohm resit or rated for 2.5 watts.

 

So ya thanks for the help Ill be posting build pictures along the way, I wight even construct a small bass boosting circuit as well who knows!!


Edited by audi0lurker - 5/25/13 at 11:42am
post #6524 of 6853

Connect each led + resistor between Vin and ground. You can use this calculator to determine the appropriate resistance value:

http://ledcalculator.net/

post #6525 of 6853
Quote:
Originally Posted by audi0lurker View Post

Also, I using the specs on a 3mm LED I have and this website I calculated I need a 1kohm resit or rated for 2.5 watts.

That doesn't sound right, how much current you gonna be passing through it? A 1k ohm will give over 40mA and fry the average LED.

A typical LED has a rating of 20mA, but at higher temps (such as in a tube socket) you will need to de-rate this down to a lower value.

Also LED brightness is proportional to current, the resistor sets this current.

 

Most LEDs these days are fairly bright, you probably don't want to light your tube socket up so brightly as to completely overpower the tube glow.

I'd probably aim for a current ~2mA (22k 1/2W resistor). Anywhere between 10k (brighter) and 100k(dimmer) would also work.

 

Also, if you prefer, you can string both tube LEDs in series, saving yourself one resistor and a small amount of power draw from the supply.

48V(+) ---- (+)LED1(-) ---- (+)LED2(-) ---- Resistor ---- 0V/ground

The resistor can go anywhere in the series circuit you like, on the positive side, on the ground side, or in between the LEDs.

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