Aleph 60 bias problem - diyAudio
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Old 24th February 2002, 08:07 AM   #1
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Location: Riyadh, Saudi Arabia
Unhappy Aleph 60 bias problem

Hello to everyone. I have just competed my Aleph 60 and plug it in. I took measurements according to the schematic. It seemed fine except voltage across R46-51 which read 0.68v. Voltage rading across R40-45 is 0.4v. The other voltage reading are:
Vce of Q4-4.36
Vce of Q5-4.13
Q3 drain voltage-3.64
R11 voltage across-5.36
R17 voltage across-12.7
R18 voltage across-12.3
R19(56k) voltage across-15.5
I've searched all the threads and they mention voltage across source resistors must all be the same. I am puzzled why the voltage reading on the amp side (Q12-17) is almost 0.3v higher than current source side (Q6-11).
Btw, I used 2-1.8ohm in parallel (makes 0.9ohm) as my source resistors in all my 12 IRFP240.
I haven't connected any speaker to check the sound, as I'm still not satisfied if it's stable.
Any help will be appreciated.

explorer
Riyadh, KSA
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Old 24th February 2002, 11:08 AM   #2
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I had a similar problem with an ALEPH 3.

Check Aleph3 problem to see if this helps.

Compare the equivalent resistor values.

I'll check back later to see if you solved the problem.

Regards
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Old 24th February 2002, 03:35 PM   #3
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Are you reading any DC offset?

Grey
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Old 24th February 2002, 07:45 PM   #4
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rtrion,
Thank you for your reply. I've checked the thread you indicated and it seemed that increasing the value of R19 will also increase the bias current. I tried to change R17 to 4.75k and R19 to 121k only to find that the voltage across R40-45 has increased from 0.4v to 0.6v and R46-51 became 0.9v. This is way up against the schematic.
I return the original value of R17 to 1.5k and changed R19 to 47k. R40-45 voltage became 0.36 and R46-51 became 0.55v.

Grey,
Also thanks for the reply. The output offset voltage is 10mv.

I tried to connect a speaker and inject an audio signal. The sound is clear in the highs, but a bit distorted in the lows and mids. I'm planning to change R19 back to 56k, because this gives the correct R40-45 voltage of 0.4v, but this will probably increase the R46-51 voltage proportionately.

I need help.
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Old 24th February 2002, 08:19 PM   #5
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Check the thread : Aleph 5 / aleph 30 hybrid ?
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Old 25th February 2002, 06:46 PM   #6
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I would carefully check the values of each of the
R46-51. If they are truly the same as R40-45,
then the extra current has to be coming from somewhere,
not the current source.
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Old 27th February 2002, 03:46 PM   #7
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I rechecked my PCB and my wiring. They all seemed fine. After 5 retries I got the same reading. So, I decided to rewire everything. I did not use a PCB for the MOSFETs because I attached each MOSFET to a heatsink which are bolted together. Each of the MOSFETs are 5 to 7 inches apart and wired in chain. Is this ok?
After rewiring and thorough checking, I fired it up and I got the correct (almost) voltage reading in the source resistor. As Mr Pass said, the extra current must be coming from somewhere else in the circuit. But I still don't know where it came from. I got reading of 0.372v to 0.425v. Is this the correct voltage? It is suppose to be all the same, right? And my dc offset value is 100mV when the amplifier is heated.
When I connected a speaker and tried some music, the sound is distorted and there is a loud hum at the output which is really annoying. I don't know if my amp is oscillating or not ? (I don't have an oscilloscope)

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Old 28th February 2002, 09:55 AM   #8
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Default Aleph 60

Explorer.

Firstly,

Take a break from the project, often contant focus can create a blind spot .. go play with chickie bab for a while.

When you come back draw the complete circuit on a notwe pad from wire wiring and actual board WITHOUT reference to the schematic. Check also fet insulation, orientation and fet integrity and potential damage to other active devices in the previous launch.

I'm sure the problem will become clear.

regards

macka
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Old 28th February 2002, 12:14 PM   #9
Koy is offline Koy  Czech Republic
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I have read it in a hurry but .... having distorded sound in lows Iit means I thing not enough current through endMOSFets ( like low bias in AB amps).Check the bias current once more.
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Old 1st April 2002, 06:39 AM   #10
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Thanks everyone for your replies.
I finally decided to downsize my aleph as I am only starting to hifi diy and have limited knowledge. I built my aleph from aleph60 with 12 fets to aleph3 with 4 fets. Aleph3's 4 fets can be managed easily than taking care of 12!!!.
So, instead of placing all the fets in separate sinks wired in series, I designed my pcb to accomodate all components and bolted the fets in a bigger sink. Although the heatsink is still not enough to dissipate the heat, I used fans, as I am still saving some bucks for a bigger heatsink.
The current flowing accross the fets are now almost identical with difference of just 20mV. (Is this OK???) I used metal film in parallel for the source resistors. I also used larger capacitors in the power supply, 60000 uF and 22000uF in parallel to dampen hum. I had a bad experience with the capacitance multipler I first used.
My jaw dropped when I hear the sound. The sound of this design is amazing, detailed and crystal clear.
I have a problem with the input connection from my preamp (MF X-Pre) When I connect the preamp to the aleph, the hum seemed to increase a bit. Is it ground loop?. Any comment on this???
Jose
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