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east electronics 27th July 2008 10:19 AM

help wanted in this schematic
2 Attachment(s)
i made a test board for this schematic and it doesnt work ..... powering it up from a variac with current limmiter but keep the 15-15 volts for the op amp always on then after you supply more than 15+15 volts to the amp the current limiter goes on ....

the circuits is exactly as shown in the scematic only i used 2sa 1943 and 2sc5200 for the transistors ....

any ideas ????

lineup 27th July 2008 10:39 AM

Start your test your circuit
with having R12-R15 = 1 Ohm

And having R22 and R23 a bit smaller .. say 22/33 Ohm., I do not know exactly.
Then you control how much current you get in output.
(Measure across one those 1 Ohm resistors, for example R12)

When is alright, you can put R12-R15 back 0.22 Ohm
... and if need you put R22 and R23 a bit bigger again .. for your normal current in output.


When you get too much Current, the current limiter will BREAK.
Be happy it did!
We do not want transistor to BURN in smoke ;)

east electronics 27th July 2008 10:53 AM

just did
reduced from 51 R to 25 R has absolutely no effect .....

amplifier actually working you have no dc in the out and aslo audio signal but draws a hell of a lot of current

east electronics 27th July 2008 10:58 AM

one other i think i forgot
since its sunday and NE 5534 wasnt available in my shop i put insted TL071 i presume its lower quality but other wise ok

east electronics 27th July 2008 11:10 AM

and then another thing
i disconected from the circuit R6 and R7 that goes to the base of the driver transistors and then R9 with C5 that brings the feedback ..... menaing that only the output section is connected to the rail .....

i think that by this way there shouldnt be any current draw correct or not ?????

woody 27th July 2008 12:24 PM

Where did the circuite come from? I don't see how that bias
circuite works with R6 and R7 different values and as low a
value as 130 ohm ?

east electronics 27th July 2008 12:38 PM

it is supposed to be
the hill audio chamaleon amplifier but the original of course doesnt work with 1943+5200 outs

space2000 27th July 2008 02:25 PM


i am not good in electronics but i had exp about TL071, i also made few times some circuit using this but dsn't works, so i got fdup at the end and used others and its works...i beleve that TL071 have some problem even you perchanse new. i dn't know what problem, its not easy to use TL071. may be its not original...

this is my own xpc..

lineup 27th July 2008 04:37 PM

Re: just did

Originally posted by sakis
reduced from 51 R to 25 R has absolutely no effect .....

amplifier actually working you have no dc in the out and aslo audio signal but draws a hell of a lot of current

I think you have an unstable circuit.
There are oscillation going on!
Is the current draw symmerical? = same current in both V+ and V-

If putting this into Simulation with Spice
you may find a way to stabilize it.
Using some compensation capacitors.

This is way too unknown and critical design, to be tested in live.
Besides, changing from the original can easy make many amplifier unstable.
Because some capacitor values were chosen to make the original stable.

1. You have any DUMMY Load 8 Ohm connected?
You should not run it unloaded.

2. Also PLEASE connect a ZOBEL filter at output.
You know those: 100nF in series with 10 Ohm

3. Also put Base Stoppers resistors in those 4 power transistors.
10 Ohm close to & going into the Base pin on each.

There are a number of reasons this amplifier can be un- stable.
As I can see from schematic.

This type of output stage is not always easy to get stable.
C10 and C11 is a try to make it more stable.
You can also try making C5 a bit bigger: 47 / 100 pF
10 pF is not much.

lineup 27th July 2008 04:46 PM

this amplifier is NOT suitable to test with variac.
As I can see.

Because of R27 - R6 - R7 - R26.
That line of 4 resistors needs to be active at correct voltages
to set the Q1 & Q2 into proper work.

I would do some math to see what those 4 resistors would be
at for example +- 30 Volt supply.

When you get this amplifier to work into lower voltages supply.
then you can do some math again to see what resistor values to use for +- 80 Volt.

But I never build such high voltage power amplifiers.
I do not think those 4 power transistor
will be able to put out power much higher than 40-50 Volt.

+-80 Volt is way too high for such a simple amplifier.

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