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Project: M³ home studio speaker amp (now finished!! pics at page 10) - Page 9

post #121 of 153
1/29/10 at 7:26pm
Quote:
Originally Posted by Jaypee View Post
started from zero, then +10 turns initially, and +10 more while powered and measuring, still no sign of life across R9.

The voltage across C2 (after the initial 10 turns) is 1.65VDC and increases as BIAS is turned clockwise.
Check to make sure you used the correct value resistors for R6 and R7. Other than that, I think you simply need to turn the trimpot some more. Something would be wrong if you reached the end of the trimpot's travel and still cannot get any voltage across the R9s.
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post #122 of 153
1/29/10 at 7:33pm
Quote:
Originally Posted by ShinyFalcon View Post
Are we troubleshooting the ground channel? Have you tried working with one of the other channels?

Edit: This is a balanced amp?
No ground channel. He's building the "Balanced output with unbalanced input" configuration as shown at the M³ website "Other options" section.
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post #123 of 153
1/29/10 at 8:47pm
Thread Starter 
Quote:
Originally Posted by amb View Post
Check to make sure you used the correct value resistors for R6 and R7. Other than that, I think you simply need to turn the trimpot some more. Something would be wrong if you reached the end of the trimpot's travel and still cannot get any voltage across the R9s.
I'll check those tomorrow, righ now I need to (at least TRY to) study for the exam I have tomorrow.. Tribology is fun ..

Queston for tomorrow, when re-check this: You mention on the website that I should "feel a slight click" when the trimpot's stop is reached..
How slight is this click to be exact (I'm having a hard time finding the stop)?

And how bad is it if the trimpot is turned further after the click (meaning, will the trimpot break if turned too many times)?
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post #124 of 153
1/30/10 at 12:15am
The "feel" when a trimpot is turned past its end of travel varies a bit from model to model, but it should be fairly obvious when you get there. As long as you don't force things too hard it shouldn't be damaged.
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post #125 of 153
1/30/10 at 7:56am
Thread Starter 
AMB, you're a true gem!

I'm ashamed to confess that I accidentally had placed a 10kohm resistor as R6, whereas it should be 1kohm ... I've noted in my parts list that R6=10kohm, so I somehow got it wrong from the beginning.

Well, there are new resistors in place now, and I'm getting the results I want!!

Couple of questions:
In my configuration, what do you think I should set the quiesent current through MOSFETs? I've set it now at 70mA

Another thing I noticed, I'm getting ~4-5mA between OR and IG but almost double (~10mA) between OL and IG. Is this normal in bridged conf?
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post #126 of 153
1/30/10 at 10:13am
Quote:
Originally Posted by Jaypee View Post
In my configuration, what do you think I should set the quiesent current through MOSFETs? I've set it now at 70mA
That's fine.

Quote:
Another thing I noticed, I'm getting ~4-5mA between OR and IG but almost double (~10mA) between OL and IG. Is this normal in bridged conf?
I think you mean mV (millivolts). Each opamp will exhibit slightly different DC offset due to tolerances. The DC offsets you have are negligible, especially for speakers, so don't worry about it.
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post #127 of 153
1/30/10 at 4:59pm
Thread Starter 
I got a chance to get a first listen on the amp, I'm really pleased with it's output volume levels, and overall ambience of the sound. I'll haveto do a more thorough listen when I get decent speaker cables and a clean source.

Problems:
The right channel burnt a fuse right after I boxed it. I tried it out of the chassis and it worked fine, but I think the potentiometer (or it's wiring) might have short circuited with the chassis or the pot pins might have bended somehow to cause it to break (had grounding problems with the pots even out of the chassis, since they're almost completely made of plastic). Now the right channel doesn't output audio. Checked the voltages across R5 and R9 and they're ok, so is there a way to see if only my pot is fried/broken?
It would be even more comforting, if someone could give me a measurement method to make sure no other component on the board is fried?

...anyways, I'm ordering alps pots on monday, these cheap PIHER ones didn't really please me..
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post #128 of 153
1/30/10 at 5:06pm
Quote:
Originally Posted by Jaypee View Post
It would be even more comforting, if someone could give me a measurement method to make sure no other component on the board is fried?
Only the values from the initial setup, really. Voltages at the input terminal block, voltages at the OPAMP, OPAMP bias, MOSFET bias......

Of course, the fuse popping then might just be a coincidence. What value and type of fuse are you running, and what is the total VA of all your transformers?
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post #129 of 153
1/30/10 at 6:14pm
Thread Starter 
Quote:
What value and type of fuse are you running, and what is the total VA of all your transformers?
I used a fast 2,5A AC mains fuse (am planning on putting a smaller one there), and the trafo is 100VA (I have separate fuses for left and right channels)

I'll go over the initial setup list tomorrow, to make sure everything is ok.

I need to mount the pots to the panel, and wire them to the boards, so what value AVG (or mm^2) should the wires be? I'm asking this cause now I have really hefty wires, which are difficult to bend in the box, and would like to lighten them up a bit..
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post #130 of 153
1/30/10 at 6:39pm
Fast blow is always risky to randomly pop fuses from the transformer inrush current, so you really should try a slow blow fuse. My completely unscientific rule of thumb is to use something double the transformer rating, which for 115V supply and 100VA is 1.15*2=2.3A. So start low at 2A, and work up if you are still having problems.

For the volume pot, it doesn't need to be heavy at all; 22-26AWG is fine. Just make sure all the pairs are twisted to minimise noise that sneaks in.
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post #131 of 153
1/30/10 at 7:05pm
Congrats Jaypee, I’m looking forward to some final pics.
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post #132 of 153
1/30/10 at 11:07pm
Your amp looks great. On the subject of balanced m3's.

If you were to use a balanced attenuator on 2 M3 boards your two options are to either wire it before the inputs or just wire it to the l/r channels where the alps would usually sit in the board?

Also if something like Cinemag transformers were to be used for SE to balanced conversion Cetoole had mentioned needing to use resistors to get the terminating impedance right. On the site it lists the input impedance as 47.6KΩ. Is that the impedance you would want to be shooting for by using resistors after the transformers? Thanks.
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post #133 of 153
1/31/10 at 7:51am
Quote:
Originally Posted by stew1234 View Post
If you were to use a balanced attenuator on 2 M3 boards your two options are to either wire it before the inputs or just wire it to the l/r channels where the alps would usually sit in the board?
Just wire it to the original PCB pot pads.

Quote:
Also if something like Cinemag transformers were to be used for SE to balanced conversion Cetoole had mentioned needing to use resistors to get the terminating impedance right. On the site it lists the input impedance as 47.6KΩ. Is that the impedance you would want to be shooting for by using resistors after the transformers? Thanks.
I don't have personal experience with Cinemags, but many such transformers need an RC snubber network on the "secondary" side to ensure flat response. Of course the values of such a network is influenced by the input impedance of the amp, which in this case is dominated by the volume pot.
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post #134 of 153
1/31/10 at 10:52am
Thread Starter 
Pictures (only of the σ11, the M³ is still undergoing some changes):

I'm going to chance the outer screws with cooler ones, as soon as I get them from a friend.





Front panel, I really like the "heavy" toggle swich (goes well with the overall look) but the LED is still a little too bright for my taste.





Rear panel featuring IEC- main AC input, connector for the DC-cord and AC-mains fuses for right and left channels.





Inside, the heatsinks are handmade, as are the corner pieces.





Detail pic of how the FET's are mounted to the sinks.

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post #135 of 153
1/31/10 at 11:10am
Great work! The casing and heatsinking is just phenomenal
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