D2 law amp – please help to set - diyAudio
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Old 23rd March 2006, 10:32 AM   #1
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Default D2 law amp – please help to set

Hello,
This circuit I was made last time, but it is really hard to set up, for me!
In general I got too high bias.
From a simulation software I found, that P1 should set the bias at right level. In this case when P1 resistance is higher, the higher is the bias.
In reality R1 is 10k potentiometer, to set “zero” at the output. As its resistance is lower, the bias gets higher.
The problem is: after setting zero with R1, I set P1 at zero resistance and got too high bias (Vgs for Q1=4,22V, what gives idle current ~2A). More, after setting P1 at max, nothing changes in reality... I wish to set the idle current at ~200mA.
I spend few days on trying to set this amp, without glory.
However, I plugged the music at the input, and for such a simple unit, the sound was very promising. But idle current has to be lowered, because of the smoke from inside)).
I believe that some of you did try this schematic and will help me to set it, please (but do not advice to rise R1).
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Old 23rd March 2006, 12:06 PM   #2
ilimzn is offline ilimzn  Croatia
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You might want to try increasing R7/R8, or R1 and R4. However, i don't see how this amp can be thermally stable.
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Old 23rd March 2006, 12:24 PM   #3
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Originally posted by ilimzn
i don't see how this amp can be thermally stable.

so adding source resistors for the hexfets will be necessary?
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Old 23rd March 2006, 01:42 PM   #4
ilimzn is offline ilimzn  Croatia
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Actually no - what you have there are complementary CFP pairs with gain. The fact that the actual output device is a MOSFET is not as important. While the CFP has feedback for AC, it does not have feedback for idle current change. Think of it like this: if both MOSFETs were ideally symetrical, any change in bias current does not move the output up or down, hence the feedback network sees no change, and cannot compensate for bias current variation.
There are several things you could try:
1) Put the input transistors on the heatsink. This approach is not entirely correct as the BJTs actually compensate each other thermally, however, you may get a negative tempco due to the output stage gain.
2) Insert low value resistor between each MOSFET _DRAIN_ and output. Keep the feedback ressitors connected to MOSFET drains, do NOT connect them to the output of the amp ('after" the low value resistors). This approach has the disadvantage that output impedance increases.
Keep in mind I'm just giving these as possibilities, to be certain, some work would be required witha simulator first, and then of course with the real thing.
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Old 23rd March 2006, 04:59 PM   #5
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Another posibility is to use thermally compensated LM317/LM337 instead of zener diodes. This should also give lower rail induced noise. Look how A. Holton uses it in his mosfet amp:
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Old 24th March 2006, 05:38 AM   #6
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Originally posted by darkfenriz
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Another posibility is to use thermally compensated LM317/LM337 instead of zener diodes.

Hej, dzięki!
I was thinking about DC-servo too...
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Old 24th March 2006, 09:32 AM   #7
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The problem you are facing is that the mosfets are thermally unstable. Even if Vgs is fixed, the Id will grow out of control because their transconductance increases with Tj at the Id you are trying to set.

I don't see how you can overcome this without finding a way to reduce Vgs as Tj increases so as to keep Id constant.

An easy compensation is to use source resistors.
Another is ilimzn (2).

If you are determined to avoid series resistors in the output then you could try using thermistors for R6/R9 with a negative temperature coefficient...resistance decreases with temperature. There is some calculation involved to choose the right values and coefficients. The thermistors need to be mounted on the heatsink very close to the mosfets.
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Old 24th March 2006, 01:18 PM   #8
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Originally posted by traderbam
An easy compensation is to use source resistors.

Thank you,
what value of those resistors will cause reliable compensation?
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Old 24th March 2006, 05:27 PM   #9
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Try between 0.2 and 0.5 ohms.

The value depends on the rate of change of Vgs with Tj, and this will be affected by the size of your heatsink and the quality of the thermal connection between transistor case and heatsink. I'm not sure what these are so you'll have to find the right value by trial and error.

If you don't succeed, then try Ilimzn's method (2) with 0.2 ohms.
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