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Old 26th December 2014, 08:01 PM   #1
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Default Help for quiescent current adjustment

Hello to all community, i have constructed an amplifier, the design is mine. Is bootstraped, BC546 for differential amp, MJE350 for VAS, BC556 for current mirrors, BC546 for BIAS Servo, MJE340-350 for drivers and BD911-912 for output devices.
The usage is for home at 70-80wRMS @ 8ohms and 4ohms some periods,
The rail voltage is 42vDC +/-.
My problem is about quiescent current, i have set it about 25mA per pair (I measure the voltage drop on the pins of the emitter resistors. (about 5.5mV)) but i have a little elevated distortion.
I'm afraid to increase the quiescent current because last month i've blow up all output devices from too much quiescent current.

The circuit is working, but i would appreciate the appreciation and your help in case of error.
Thanks for all, Happy holidays and merry christmas!
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Old 26th December 2014, 10:12 PM   #2
djoffe is offline djoffe  United States
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It looks like a pretty reasonable design (just as a quick look-see).

I'm a bit concerned about stability given the lack of the damped L at the output. It would probably measure fine, but a real speaker might make it oscillate. That could have even been a contributing factor to the failure you mentioned.

Is the Vbe multiplier attached to the heatsinks for the output devices? That can be useful to have thermal feedback to help bias stability.

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Old Yesterday, 04:28 AM   #3
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reduce the value of R21, try to increase your quescent current and see if it works.
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Old Yesterday, 04:43 AM   #4
AJT is offline AJT  Philippines
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reducing the value of R21 reduces idle current more....
you are running close to class B, running it class AB will help a bit.
make R11 higher, say 2.2k and try again...
go for at least 50mA per pair....
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Old Yesterday, 09:51 AM   #5
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Quote:
Originally Posted by djoffe View Post
It looks like a pretty reasonable design (just as a quick look-see).

Thank you very much!!

I'm a bit concerned about stability given the lack of the damped L at the output. It would probably measure fine, but a real speaker might make it oscillate. That could have even been a contributing factor to the failure you mentioned.

maybe, i dont know right now.... But it have good sound quality!


Is the Vbe multiplier attached to the heatsinks for the output devices? That can be useful to have thermal feedback to help bias stability.

yes, the vbe is attached to main heatsink with cables...

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Old Yesterday, 09:55 AM   #6
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Quote:
Originally Posted by clovi_kate View Post
reduce the value of R21, try to increase your quescent current and see if it works.
in multisim, seems to work better, with 100R from 220, i do some tests today in real life!

Quote:
Originally Posted by AJT View Post
reducing the value of R21 reduces idle current more....
you are running close to class B, running it class AB will help a bit.
make R11 higher, say 2.2k and try again...
go for at least 50mA per pair....
I will test it with TIP30550/2955 today for output devices....
My opinion was 50-60mA per pair, just wanted to make sure for this...




I'm a hobbyist and always need help if I'm not sure at what I do.
results will have the evening, stay tuned!!
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Old Yesterday, 11:46 AM   #7
AndrewT is offline AndrewT  Scotland
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Set the output bias by voltage (Vre), not current.

The voltage across the output device emitter resistors should be around 26mVre.
But this Voltage includes the transistor's internal voltage drop across it's intrinsic emitter resistance (to any AC guru: Is that a correct way to explain this?)

As the external emitter resistance is lowered the proportion of the total emitter resistance is increased in the internal side and reduced in the external side.
At 0r22, one is starting to see a reduction in Vre. Try for maybe around 20mVre to 22mVre.
At 0r1 many are finding that 18.5Vre is best for lowest crossover distortion.
Whereas for 0r47 usually 25mVre to 26mVre is good.

The bias currents (quiescent current when zero signal is present) are usually set by applying a fixed voltage across pre-set resistance.
R9 sets bias for Q1, 2, 3, & 4.
R13+14 for Q6.
R21 for 8 & 9.
R12:R11 for Q10, 11, 12, & 13.
(note that Vr13+14 is NOT a fixed voltage: it depends massively on how the supply rail voltage changes during amplifying operation)

Check the currents in all the transistors and see where they lie on the hFE vs Ic curve and on the fT vs Ic curve. If you are way off to one side of the Ic values given in the datasheet curves, then your transistors will not perform as well. And performance begins to depend on which transistor operates least well. i.e. the transistor parameter begins to determine the amplifier performance, (not good).
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Last edited by AndrewT; Yesterday at 11:51 AM.
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Old Yesterday, 11:48 AM   #8
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Quote:
Originally Posted by AJT View Post
reducing the value of R21 reduces idle current more....
you are running close to class B, running it class AB will help a bit.
make R11 higher, say 2.2k and try again...
go for at least 50mA per pair....
then your making it too risky..
increasing vas will result to high Voltage and current across 220R.
one way is to compensate the the VAS current without increasing the voltage across R21.
lets say you have 1V across 220R, then I=(1/220r) = 4.5mA
then if you reduce to 100R at same current,you would probably get 0.45V. Increase your VAS current until it reaches 1V across 100R and you will have 10mA.
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Old Yesterday, 11:53 AM   #9
AndrewT is offline AndrewT  Scotland
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You do not show any decoupling capacitors on any of the supply rails !

This omission can create terrible distortions and terrible oscillations.
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Old Yesterday, 11:55 AM   #10
AndrewT is offline AndrewT  Scotland
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You show C5 connected to the Power Ground.
You also show R2 and C2 connected to Power Ground.
All three components must connect to Signal Return/Ground.
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