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M³ amplifier build discussions thread - Page 7  

post #91 of 828
3/10/05 at 5:43pm
Quote:
Originally Posted by amb
Nice list, although for this amp I would not recommend using any 1/8" jacks (either for input or output).
why are 1/8" jacks not recommended for this amp?
post #92 of 828
3/10/05 at 6:11pm
Thread Starter 
Quote:
Originally Posted by escher
why are 1/8" jacks not recommended for this amp?
Because 1/8" jacks are typically compromised in quality. They are used on portable gear to save space, and space is not an issue on the M³. Also, I have not come across any 1/8" jack that is isolated, and you must use an isolated jack for the output if the front panel is metal.

For home audio equipment, the standard is still RCA input jacks and 1/4" headphone output jack(s).
post #93 of 828
3/10/05 at 6:18pm
Thread Starter 
Quote:
Originally Posted by jamont
I only matched mine to < 0.1%, and I have L and G < 0.1 mV, but R = 1.6 mV. Could a slight mismatch in the output transistors account for what MisterX and I see?
On my prototype I didn't even bother with matching anything. I just used arbitrary MOSFETs and RN55D resistors from my piles, so maybe it's dumb luck .

Anyway, try swapping the opamps between the L and R channels and see if the DC offset moves with the opamp. That might be telltale.
post #94 of 828
3/10/05 at 9:14pm
Quote:
Originally Posted by amb
On my prototype I didn't even bother with matching anything. I just used arbitrary MOSFETs and RN55D resistors from my piles, so maybe it's dumb luck .

Anyway, try swapping the opamps between the L and R channels and see if the DC offset moves with the opamp. That might be telltale.
Yes, this was the obvious thing to try and it worked! I now have all offsets < 0.3 mV, and I am happy.
post #95 of 828
3/11/05 at 6:15am
I swapped the opamps amongst the channels and it got worse on the left side... said screw this and dropped in some OPA637 B grade opamps and of course the offset dropped considerably.

.09mV on the right.
. 12 mV on the left.

Replaced the OPOS637s with some 8610s and everything was good the first time.
Go figure huh?

FYI Gain = 8.5 but they showed no other signs of instability.

Quote:
MisterX, what meter did you use, and how many resistors?
Fluke 8840AF
# of resistors depends on the value.
The 1 meggers and 100 ohmers were the worst
(it took close to 50 100 ohmers to get a good matched set at that tolerance)
post #96 of 828
3/11/05 at 6:25am
Thread Starter 
Quote:
Originally Posted by MisterX
(it took close to 50 100 ohmers to get a good matched set at that tolerance)
I salute your insanity.
IMHO the only ones worth matching in this amp are the gain resistors in the feedback loop, but with 1% resistors the worse you're going to do without matching is a very small fraction of a dB difference between channels, which is probably much smaller than the tolerances of any headphone.
post #97 of 828
3/11/05 at 6:52am
Quote:
IMHO the only ones worth matching in this amp are the gain resistors in the feedback loop
Agreed.... but no matter what I/we think it all comes back to the whole "can I upgrade those green looking caps" thing again.
post #98 of 828
3/11/05 at 11:14am
Quote:
Originally Posted by MisterX
Agreed.... but no matter what I/we think it all comes back to the whole "can I upgrade those green looking caps" thing again.
I hate that for you, man.
I do feel your pain.

Nice looking work, guys. I'm still gathering some parts for mine, so nothing of interest to report here. I just ordered the board this morning (doh!), but have about 3/4 of the other parts on hand.
post #99 of 828
3/11/05 at 1:08pm
I'm almost done now, spent about 3hrs today and got everything on bar the output transistors and electrolytics.

I'm 200 miles from home and won't be back until Sunday, my Cerafines are probably waiting on my door mat now

Oh the agony...I guess I'll just have to stare at it for a little while longer imagining what it will sound like

Build went fine but I there is a small mistake in Amb's parts list..

R2, R3, R4G, R6 | 8 | 1/8W 1% metal film resistor 1KΩ

I used 9 resistors, so either I've put one somewhere I shouldn't or Amb counted one short
post #100 of 828
3/11/05 at 3:04pm
Thread Starter 
Quote:
Originally Posted by Magsy
R2, R3, R4G, R6 | 8 | 1/8W 1% metal film resistor 1KΩ

I used 9 resistors, so either I've put one somewhere I shouldn't or Amb counted one short
Thanks, yep, was a mistake and it's fixed now.
post #101 of 828
3/11/05 at 3:29pm
I'm using a pot for bass boost, so should I populate rbb? I read that it should be equal to the pot, so 50k in mycase but I'm sure I read elsewhere on the site to not use rbb if you have the pot control..
post #102 of 828
3/11/05 at 3:31pm
Thread Starter 
Quote:
Originally Posted by Magsy
I'm using a pot for bass boost, so should I populate rbb? I read that it should be equal to the pot, so 50k in mycase but I'm sure I read elsewhere on the site to not use rbb if you have the pot control..
Don't populate Rbb if you're using a bass boost pot.
post #103 of 828
3/11/05 at 3:34pm
Quote:
Originally Posted by amb
Don't populate Rbb if you're using a bass boost pot.
Thanks mate!
post #104 of 828
3/11/05 at 7:49pm
I am waiting for the board (a long way to Italy), in the meanwhile some questions (maybe silly).
If I don't want the bassboost option I just need to bridge across sbb. Right?
What the purpose of C2? Can I use some 1uF BG non polar caps I have laying in my drawer?
Why polypropilene for C6? As long as they are PS bypass caps, why not cheaper polyester? (Off course I have this stuff lying in my drawer too).
Thanks everybody.
post #105 of 828
3/11/05 at 8:07pm
You can use polyester film. The only caps worthy of polypropylene are Cbb and perhaps C2, although that is debatable since it doesn't really do much. C2 bypasses the variable impedence of Q2, the Vbe multiplier, but we were unable to hear or measure a difference with or without C2. Perhaps better ears and test equipment than ours will reveal a difference.
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