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Old 5th February 2006, 05:39 PM   #1
pieroh is offline pieroh  Germany
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Default quiescent current mosfet amplifier

Hi,
I need some help!
please have a look at the circuit diagramm below.
This is a fully assembled amp module I bought in Bangkok some years ago. I built an stereo amp with two of them completed by a PSU +31V/-31V.
When I switch on the power supply there is no quiescent current through the mosfets, the audio signal is distorted.
what could be wrong?
Peter
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Old 5th February 2006, 05:47 PM   #2
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Check the voltage between collector and emitter of Q9.
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Old 5th February 2006, 06:59 PM   #3
pieroh is offline pieroh  Germany
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Default Transistor Q9?

Hi Jorge,
UBE at the BD139/BD140 transistor ist around 0,6V, they seem to be intact. Voltage from collector to ground at the BD140 is -1,5V at the BD139 -2,5V.
Peter
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Old 5th February 2006, 07:20 PM   #4
lineup is offline lineup  Sweden
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The important voltage here
is voltage from Collector to Emitter of Q9 = TIP 29 C.

This voltage sets bias current in output stage.
Bias current in output can be measured across one of the .5 Ohm resistors.
A normal bias current, 100 mA, should give 0.050 Volt ( 50 mV ) across 0.5 ohm.

What looks strange in schematic is the value of resistors
that set the voltage across TIP 29 C.
330 - 390 - 180 Ohm
Instead of the 330 Ohm resistor, I would want a 4k7 resistor,
or something like that.


You could replace 330 Ohm resistor temporary with a 10 kOhm potentiometer.

Start with a resistance of this pot set a low value.
Turn on power.
While measuring the voltage across one 0.5 Ohm resistor in output
increase the resistance of potentiometer slowly,
until you get 50 mV across output resistor.

Then turn power off.
Measure the resistance of the potentiometer
and replace it with a resistor with equal resistance.
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Old 5th February 2006, 07:24 PM   #5
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What we need to know , is the voltage between collector and emitter of Q9 ( the VBE transistor).

As the threshold voltage of the IRF141 is higher than 2 volts , the voltage between collector and emitter of Q9 must be ~ 4 Volts.

There is a fatal error in the schematic , the resistor between the collector and base of Q9 must be 3.3 KOhms , not 330 as in the schematic and the 390 Ohms resistor that is connected with the base of Q9 , must be changed to a trimmer pot for adjusting bias...
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Old 5th February 2006, 07:34 PM   #6
mikeks is offline mikeks  United Kingdom
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Default Re: quiescent current mosfet amplifier

Quote:
Originally posted by pieroh
Hi,
I need some help!
please have a look at the circuit diagramm below.
This is a fully assembled amp module I bought in Bangkok some years ago. I built an stereo amp with two of them completed by a PSU +31V/-31V.
When I switch on the power supply there is no quiescent current through the mosfets, the audio signal is distorted.
what could be wrong?
Peter

Probably unstable to boot!
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Old 5th February 2006, 07:51 PM   #7
MikeB is offline MikeB  Germany
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Looks like a design only fully functional with +/-37v as indicated...
If you're lucky, one of the 330(3.3k)/390/180 ohms in the vbe multiplier is a trimpot. If not, you need to reduce the 180ohms.
There is another error, q8 is drawn reversed...

Mike
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Old 5th February 2006, 08:10 PM   #8
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Quote:
Originally posted by MikeB
Looks like a design only fully functional with +/-37v as indicated...
As the tails of the LTP are not feed by a CCS , but only with a humble resistor, any difference in the voltage of the rails will be reflected , as an alteration in the currents in the LTP and next stages...
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Old 6th February 2006, 05:15 AM   #9
pieroh is offline pieroh  Germany
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Default modification

Quote:
Originally posted by lineup
The important voltage here
is voltage from Collector to Emitter of Q9 = TIP 29 C.

This voltage sets bias current in output stage.
Bias current in output can be measured across one of the .5 Ohm resistors.
A normal bias current, 100 mA, should give 0.050 Volt ( 50 mV ) across 0.5 ohm.

What looks strange in schematic is the value of resistors
that set the voltage across TIP 29 C.
330 - 390 - 180 Ohm
Instead of the 330 Ohm resistor, I would want a 4k7 resistor,
or something like that.


You could replace 330 Ohm resistor temporary with a 10 kOhm potentiometer.

Start with a resistance of this pot set a low value.

Hi you all,
thanks a lot for your very helpfull advices!
I will try it this evening and send a report.
Peter

Turn on power.
While measuring the voltage across one 0.5 Ohm resistor in output
increase the resistance of potentiometer slowly,
until you get 50 mV across output resistor.

Then turn power off.
Measure the resistance of the potentiometer
and replace it with a resistor with equal resistance.

Quote:
Originally posted by Tube_Dude
What we need to know , is the voltage between collector and emitter of Q9 ( the VBE transistor).

As the threshold voltage of the IRF141 is higher than 2 volts , the voltage between collector and emitter of Q9 must be ~ 4 Volts.

There is a fatal error in the schematic , the resistor between the collector and base of Q9 must be 3.3 KOhms , not 330 as in the schematic and the 390 Ohms resistor that is connected with the base of Q9 , must be changed to a trimmer pot for adjusting bias...
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Old 6th February 2006, 06:37 AM   #10
pieroh is offline pieroh  Germany
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Default I will try it with different resistor values!

Hi,
thank you all for your advices!
I will try it this evening!
Peter
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