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Old 17th June 2007, 08:46 PM   #1
microp is offline microp  Latvia
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Default Need pro advice!

Hi! Created my first amp! Sounds good at +-25V, but i will increase it to +-60V! I have some noise like "raining" in midrange! May be must change some transistors or capacitators or resistors ... I am newbie in electronics. I created center gound, a rectangle in center of PCB. Transistors are cold even at full load (+-25V 8ohms ). Everything seems to be fine just this noise!!! Please help!

Thanks a lot!
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Old 17th June 2007, 09:21 PM   #2
microp is offline microp  Latvia
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Here is my amp !
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Old 17th June 2007, 09:21 PM   #3
microp is offline microp  Latvia
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Here is PSU with 2x33000mkF capacitors. Don't know what is wrong
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Old 17th June 2007, 09:22 PM   #4
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2 possibilities....... 1, noisy transistor in the diff amp, or 2, one or both of your zeners are noisy
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Old 17th June 2007, 09:31 PM   #5
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oh, and a 3rd possibility...... you need an o-scope for this one..... it could be oscillating since you have no compensation capacitance.......
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Old 17th June 2007, 10:03 PM   #6
microp is offline microp  Latvia
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Thanks for answers! As I told I am newbie in this. Compensation capacitance are these resistors (picture)? I have o-scope... but I don't know how to work with it... sry. I created my amp only because reading these forums
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Old 17th June 2007, 11:00 PM   #7
microp is offline microp  Latvia
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If I connect two more pairs of FET's... will it be more powerful? And bc337 and bc327 will handle them?

Sorry for stupid questions!
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Old 18th June 2007, 01:50 AM   #8
jaycee is offline jaycee  United Kingdom
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I would say running this circuit as is from 60V is asking to destroy the small transistors due to exceeding Vceo and maximum dissipation!

IMO, do the following:

* Replace R4 with a proper current source
* Use a cascode in the first stage to decrease dissipation in Q1, Q2. Try with a 30V zener as the reference.
* Use another transistor instead of D5, forming a current mirror
* Use MJE340/350 instead of BC327/337

If you want to use more MOSFETS, add a driver stage.

R1 should really be equal to R7 to avoid offset issues.

If it's oscillating, try compensation capacitors around Q3 and Q4, from base to collector. Start with 47pF.

edit: take a look at Bora's Sigma amplifier
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Old 18th June 2007, 02:07 AM   #9
Leolabs is offline Leolabs  Malaysia
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Resistor bias network with HEXFETs????Are you kidding me???
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Old 18th June 2007, 05:56 PM   #10
jaycee is offline jaycee  United Kingdom
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Good point. With the IRF mosfets, you need a VBE multiplier to regulate bias. If you don't have one, your amp will be thermally unstable and will blow the outputs.
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