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Old 24th February 2003, 04:30 PM   #1
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Default Probs with getting DoZ to work

Hi!

I have build a DoZ (just one side for now), using the PCB from ESP, 2N3055 transistors, a 225 VA 30 - 0 - 30 toroid, and the suggested supply with four BYW29E soft recovery diodes and two 10000 uF BHC SlitFoil caps.

First part of testing (with 10 ohms resistor in series to V+, measuring the output voltage at the + pin of C3) went OK, I got about half the supply voltage (about 19.8 V)

But then the second part of testing does not work. Connecting a 1 ohm resistor in series to V+, measuring voltage across the resistor in order to get the quiescent current, showed no current at all. And the amp does not heat up at all. And all the time the DC voltage at the output at the - pin of C3 (where the speaker should be connected) stays at about half the supply voltage... adjusting VR1 and VR2 does not seem to have any great affect...

Is maybe one of my transistors (also the driver transistors) broken? Is there an easy way to measure this? I do not think I made a mistake stuffing and soldering the amp, since it was very easy to build, and I checked everything three times before powering up...

Any Ideas?

Thanks in advance,

Arndt
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Old 24th February 2003, 07:55 PM   #2
halojoy is offline halojoy  Sweden
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Click the image to open in full size.

Figure 5 - The Complete DoZ Schematic (With Iq Stabilisation)
DOZ - ESP Project 36
--------------------------------------------------------------------

As you get the correct half voltage
there is no fault in the transistors and probably not
in any part.
You should check your wiring. Once again.
Especially around the pot/resistors that set
the Output Stage current.
Are the 3 connections to the pot, wired in the right way
to the circuit?

If you have connected the 1 ohm resistor correctly
you should have some small voltage drop over it,
at least 20-50 mV.
Because obviously the circuit get current, as the output is at 19.8V.

Check the wiring not only once or twice,
but 5 times.
Always a good advice!

Especially when you use circuits partly or totally without PCBs.

/halo
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Old 24th February 2003, 08:40 PM   #3
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Hi again!

There is the smal possibility that I mounted VR1 the wrong way around. In the construction article, R. Elliot showed both pots with the adjustment screw facing "up", that is with the connections to the output transistors to the right.

But I think that VR1 should be mounted with the adjustment screw facing "down", since the pin under the screw is pin "A" or "1", and according to schematics pin A of VR1 should be connected to GND, which would lead to mounting it the way I did. R. Elliot did not answer my second email with this statement yet, but I think he probably will...

But am I mistaken when I think that after C3 almost no DC offset should be measurable?

And the voltage drop across the testing resistor, do you measure it by calculating the difference between the voltage before and after the resistor, or by simply attaching the measuring pins to both sides of the resistor? Boy, seems I should have paid more attention to my electrotechnical courses at university (one reason why I dropped out )...

Anyway, thanks,

/Arndt - who just discovered that the testing heatsink for the DoZ would be absolutely ideal for a multi-channel GC using the OPA548T which arrived today, since they are so very small - TO220 -, and the sink already is pre-tabbed for that...
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Old 24th February 2003, 09:21 PM   #4
halojoy is offline halojoy  Sweden
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- simply attaching the measuring pins to both sides of the resistor.

Yes, that is the way.
Start on 2 VDC range. If nothing go downto 200mV and 20 mV DC.
-------------------
- I think that after C3 almost no DC offset should be measurable.

C3 would be the output Cap.And there should be no DC-voltage
on the output side to ground. That is where the 2 leads to Loudsp will connect.
---------------------------

I can't help get thinking there is some connections
you have not done correctly.

And as no change in output voltage, 19.8V,
take place the pot VR1 does not work.
VR1 is used to adjust the Voltage to half supply voltage

And VR2 sets the Current in output stage,
and should make the voltage drop over an 1 ohm resistor change. (1 Volt = 1 Ampere)
If it worked alright.
--------------------------------------------

You will find the fault(s), in time.
Just keep checking.

/halo
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Old 24th February 2003, 09:41 PM   #5
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Hi,
I have build the Doz and my version didn't work because I had 100k ohm resistors for R5 and R8 instead of 100 ohm.
If you have placed the two BD 139 wrong it will give you half the supply voltage but it will not work (I am not going to admit that I had those wrong too ).
I also measured some dc at the - pin of C3 but after connecting a resistor (4 ohm) at the output the DC was gone...
good luck!

menno
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Old 24th February 2003, 10:04 PM   #6
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Default Doz circuit!

Hi Guys!

Funny circuit this one!

Q1 work with a current of +- 0.37 mA...the gain of Q4-Q5 is aproximately2000.so the output current in the load will be

0,37 mA X 2000=740mA

In a 8 Ohms load it wil put 2 W in the load...and less than 8 (+-6)volts...and all of this with a 40 volts power supply and 15 amps transistores...

That i call overkill...one more gain stage will have good efect!!

Cheers

Jorge Santos
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Old 24th February 2003, 10:05 PM   #7
halojoy is offline halojoy  Sweden
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Quote:
Originally posted by Menno-de-Penno
Hi,
I have build the Doz
-------------------
I also measured some dc at the - pin of C3 but after connecting a resistor (4 ohm) at the output the DC was gone...
good luck!
menno
You do not have to use such low value resistor.
Any resistor below 1000 Ohms will do. It is connected like the LSP should been.
In less then 10 secs the Big Output cap
will be charged.

/halo
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Old 25th February 2003, 03:54 PM   #8
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Hi!

Thanks for the input.

I will definitely try charging C3...

As for the driver transistors, I hope I did not put them in the wrong way around. I have put them in with the heatsink pointing towards the input...

Oops.. think that could be wrong... for example, I connected the base of the BD139 to R4... should be the emitter...

Maybe checking the datasheets would be an obvious thing to do prior to soldering... But I already used the BD139 on the DoZ pre, and there it works... maybe my mind is just twisted...

I heat up my soldering station and try it now...

Bye,

Arndt
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Old 25th February 2003, 04:24 PM   #9
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Hi!

Ahem, actually the orientation of the driver transistors was right...

So it must be something else.

Maybe I will just build the second board, and try that one...

But that will have to wait till weekend...

Bye,

Arndt
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Old 6th June 2003, 01:36 AM   #10
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I'm having trouble w/ dc offset as well. I was running the amp on a variac for testing, and the output was at almost 10v before the C3 and at the actualy speaker output after c3 there is 9v of DC. Is this oscillation? I'll ask Rod and see what he says, but it sounds like I'm not the only one w/ this problem. Anyone else?

-Matthew K. Olson
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