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Old 20th May 2004, 07:47 PM   #1
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Unhappy Aleph 3 bass/noise problems

Hello all. I have one channel of my Aleph 3 up and running but I am encountering some problems with some misc. static noise and flabby bass. The biggest problem is the bass, which is very distorted. The high end seems to sound ok. The amp is completely stock; straight C power supply, all components on the PCB are the correct values and the input FET pair has been matched. I am using a star ground in the middle of the amp which everything runs to, including a wire grounding the chassis. When the chassis grounding wire is removed a decently lowd humm is output by the speaker. I didnt expect grounding the chassis to make such a big difference but it does, could this be some sign of a problem? The only thing that is a little off about the amp is that the power supply rails are at +/- 22.5 V instead of 25. In Mr. Pass' schematic he shows 0.5V across the power resistors, in my amp I am measuring about 0.3V across each. My DC offset varies between 5mV and 15mV at all times. I have been racking my brain and trying different things to eliminate these problems but so far I have been unsuccessful. If anyone has any ideas on what the problem may be I would greatly appriciate hearing them.
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Old 20th May 2004, 08:20 PM   #2
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Your bias current is 40% low. Try looking at the circuits
driving the positive current sources.
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Old 20th May 2004, 10:36 PM   #3
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Mr. Pass,

I cheched the current source circuit and all of the traces are going to the right places on the PCB and all of the values of the parts are correct. Would you suspect that the lower supply voltage is the problem or is it more likley that it is a bad part/something on PCB? Thanks for your input.
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Old 21st May 2004, 07:56 AM   #4
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I suppose a more appropriate way to approach the problem would be to ask what specific parts of the current source could be the problem and what could be modified to increase the bias current. Could the 0.47 resistors be reduced to draw more current? What about increasing R113? Another thing worth mentioning is the fact that I designed my own PCBs. Is it possible that the thickness of the traces is limiting the high current flow? I thought I made them reasonably thick but Im not 100% sure, this being my first PCB design. The high current traces are about 2mm wide. If anybody had any suggestions please help me out. I am eager to get the Aleph working correctly! Thanks guys.
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Old 21st May 2004, 08:52 AM   #5
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bigmernisher
Increasing R113 will increase the bias.
What about the other voltages?
VCE Q105?
Could it that the protection circuit is active? Try removing Q104
for testing.
Could you post some drawing of the PCB perhaps?

/Hugo
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Old 22nd May 2004, 06:15 PM   #6
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Thanks for your intrest Hugo.
Vce of Q105=3.8V.

Here are some other measurments I took as well:
V across R108 (390 ohm) = 3.5V. (Mr. Pass specs 4-5V)
V at drain of Q103 = 3.8V. (4V is the spec)
V across Z103 (9.1 V diode) = 8.44V.
V across R106 = 4.56V. (in line with spec)
VGS of Q106 and Q107 = 3.5V.

voltages across the power resistors:
R120: 0.3V
R121: 0.3V
R122: 0.36V
R123: 0.27V

Click on the link below to view the PCB. The size is 5x5 inches. The only two parts that are not labled are C101 and R102 which are on the left side near the ground line. I hope this sheds some light on things.

http://www.rose-hulman.edu/~miernija/Aleph3%20JAM.BMP
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Old 22nd May 2004, 06:57 PM   #7
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As Mr. Pass pointed out, the bias is running low.

You can solve this by increasing R113 to 100k. After this, repeat
your measurements. If still to low increase to 150k.
Increasing the resistor is easy. Add another 47k or 100k
in series with the existing resistor.

I have used 220k succesfully for increased bias on more
than one occassion. Watch the dissipation though.

The static could point at instabality of the Aleph CS. I have
seen this more than once as well. You can solve this
by adding a 1nF capacitor accross the B and C of Q105 (MPSA18)

Regards
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Old 23rd May 2004, 12:35 PM   #8
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Also, your input current source is a little lean and you may be outside of the linear region on the diff pair and causing a lot of DC NFB. I think you have a bad (wrong voltage) Z103. You could change R106 to 221*(8.44/9.1) to compensate.
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Old 23rd May 2004, 01:36 PM   #9
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I think that rtirion gave the solution with R113 (and 1nF cap).

By the way, before replacing R113 with the increased value, I would measure voltage across R120 when R113 is cut off. If the measured voltage is far less than 0.66, MPSA18 must be wrong. If so, the problem might come from MPSA18.

Your rail voltage is less than 25V. Then, if the amp works in right order, I think the voltage across R120 is to be not less than 0.5, but to be greater. Isn't it?

Good luck.

JH
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Old 24th May 2004, 01:39 AM   #10
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Yes to possible oscillation.
Here's where you need a scope. Check all areas for oscillation. My headphone amp had an oscillation in the diff pair that distorted the sound, but the output looked OK.
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