Zerozone class A dc offset adjustment?

N101N · 2019-07-10 5:41 pm

There is a possibility of making the topology entirely symmetrical.

Ketje · 2019-07-10 5:54 pm

Rallyfinnen said:
I don't understand how the input circuit can be in balance since there is a resistor on only one of them?

The drains are allmost a constant current source, the small voltage drop on R25 (<3V) make practicaly no difference.

Could some imbalance in the output stage also be 'pulling it out of balance'? Just wild guesses..

Yes.
To get the right amount of current in the output stage (IQ2=IQ6) the drop on R25 must give the right gate voltage for Q6.
That fixes the current in Q4 and, for balancing the input stage, the current trough Q5 has to be =Q4. So the current for Q1 = Q4+Q5 (=double of each)
If Q1 delivers to much or not enough current the difference go to Q5 (since IQ4 is fix).
Normaly R1 is chosen to have somewhat to much current to have a little reserve for the difference Q4-Q5.
Mona

mjona · 2019-07-10 9:29 pm

Rallyfinnen said:
Thank you for taking the time to assist!
I measured R25, and it is 290ohm.I have no other mods in place except the trimmer for the input current source.

Measurements:
R25:388ohm 5,21V 13mA
Zener: 8,91V
R1: 286ohm 13,47V 47mA

Far from 50/50..

The voltage drop across R25 compares with 4 to5V drop across the equivalent in the Aleph 3 circuit where the value is a match=390R. While you are getting 13 mA the Aleph 3 uses 10 to 12 mA. Splitting the difference let's say 11mA.

The Aleph equivalent to R1 value 220R has the same 4 to 5V across it =18 to 23 mA - split difference say about 21. So the Q4 to Q5 current ratio is about 50/50 at 11 versus 10 mA.

The IRF9610 needs a negative voltage from the gate to source where conduction starts to rise sharply as shown in the screen shot - so you should check and note the gate to source voltages for all three devices to give some clues on where the bias might need attention (or if any of these is faulty in some way).

Having looked at the image attached to post 16 I could not make out how the hardware earth connections go to a common point which would include that of a split rail power supply.

Rallyfinnen · 2019-07-11 8:39 am

Good morning (here)!
If I measured correctly Vgs is 3,74 and 3,94V on the input pair, and 4,17V on the current source.

If R1 is higher in resistance, the current will go down, right? Should I just keep increasing R1 until the offset balances out? Where is the limit (minimum current)?

Rallyfinnen · 2019-07-11 8:58 am

Tried 311ohm for R1, and voltage over R1 is then 15,2V, so current seems to stay basically the same, as does the offset on the speaker terminal..

Rallyfinnen · 2019-07-11 4:09 pm

OK, now the magic smoke is out of one of the amps.. Put in a potentiometer to adjust the current, but forgot to check the setting on the potentiometer before I connected power.

Suggestions on other class a kits to buy cheap, same supply voltage..? I'm pretty tired of this one now..
I built a new enclosure but have only one channel to put in it.

Rallyfinnen · 2019-07-11 4:28 pm

Quick search on ebay gets me this:
Assembled Hifi A30 Pure Class A High-current FET Amplifier Board 30W+30W | eBay
Not sure what they cloned or if it would work with +-18v though

Another one: DIY PNP A1216 JLH1969 class A power amp kit 10W+10W amplifier board kit | eBay
Single supply 25V, so not really the same supply either, and I dont like the output cap so much.. Could have put an output cap on these amps and be done with the offset..

This schematic seems familiar

but with double output fets.
HIFI PASS A3 Single-ended Class A Power Amplifier Kit 30W+30W AMP ( 2 Channel) | eBay

Well, that's about it. Worst case is to order the same kit again. Suggestions welcome!

N101N · 2019-07-11 10:05 pm

Single-ended class A? Not really. Just self-deception.

N101N · 2019-07-12 12:21 am

Single-ended push-pull? Not really. Just push-pull.

(If somebody is in doubt).

N101N · 2019-07-12 4:11 am

Mona, are you a lady if I may ask?

ubergeeknz · 2019-07-12 7:08 am

N101N said:
Mona, are you a lady if I may ask?

What difference would it make?

N101N · 2019-07-12 7:44 am

Maybe I would flirt with her...

mjona · 2019-07-12 9:27 am

Rallyfinnen said:
OK, now the magic smoke is out of one of the amps.. Put in a potentiometer to adjust the current, but forgot to check the setting on the potentiometer before I connected power.

Suggestions on other class a kits to buy cheap, same supply voltage..? I'm pretty tired of this one now..
I built a new enclosure but have only one channel to put in it.

I have done a simulation of this amplifier using similar IRF MOSFET devices and the circuit should work without extreme alterations assuming the matching is close - which they are claimed to be. A dc output of 100 mV is not good enough and the only way around this would be to buy a selection of devices as replacement and match them yourself.

Futurlec is a surplus components supplier who have IRF9610 listed for $US0.70 a piece you could buy a couple of dozen for not much outlay.

Ketje · 2019-07-12 9:53 am

mjona said:
A dc output of 100 mV is not good enough and the only way around this would be to buy a selection of devices as replacement and match them yourself.

Even with matched Q4-Q5 you will have an offset.It is because the different currents in those fet's.
And 100mV out is no big deal.
@N101N, no luck, no lady here

Mona

mjona · 2019-07-12 5:00 pm

Ketje said:
Even with matched Q4-Q5 you will have an offset.It is because the different currents in those fet's.
And 100mV out is no big deal.

Mona

The simulation gives 9 mV with R1 value changed to 220R, R11 changed to 1k, and R3 changed to 39k - all standard values.

The CCS current is 25 mA and there is a difference in the drain currents for Q4 and Q5 of 1 mA. Output stage Iq is set at 2A with +/- 18V rails.

The substitute MOSFET's in the simulation are IRF9640 and IRFP240. Q5 and Q7 are 2N5210.

Rallyfinnen · 2019-07-14 7:20 am

I actually made an order for these to see if I can get the faulty board working:
IRFP240PBF TO-247AC N-ch 200V 20A × 2
IRF9610PBF TO-220 P-ch 200V 1.8A × 2
BC550C TO-92 NPN 45V 100mA × 2

mjona · 2019-07-14 11:51 pm

Rallyfinnen said:
I actually made an order for these to see if I can get the faulty board working:
IRFP240PBF TO-247AC N-ch 200V 20A × 2
IRF9610PBF TO-220 P-ch 200V 1.8A × 2
BC550C TO-92 NPN 45V 100mA × 2

You may be lucky to get a match with just one pair of IRF9610.

Rallyfinnen · 2019-07-15 6:16 am

It seems to me that matching them is a drop in the ocean compared to the rest of the circuit, since swapping them did close to nothing to the offset, and they are not matched now. I think it's only one power fet and possibly the 550 transistor that needs replacing, just ordered some extras just in case.

N101N · 2019-07-16 4:36 am

What is the purpose / advantage of this output stage configuration?

Rallyfinnen · 2019-07-17 8:02 am

I did a half-hearted attempt to bring the burned channel to life, but now I'm tired of it.. The boards are not so great any more with all the drilling and soldering and cutting traces, and the dc-offset is a bit annoying too.
Looking to buy another class a amp to drive my 4ohm tweeters. Only a couple of watts should do it I think.
Hope for suggestions, and hopefully I can use the 18V bricks for the next ones too.

Here are a few I looked at on ebay (europe)
V18 PNP Sanken 2SA1216 JLH1969 Class A Power Amp Kit 10W+10W Amplifier Kit | eBay
DIY PNP A1216 JLH1969 class A power amp kit 10W+10W amplifier board kit | eBay
PA-05 amp PASS ACA 5W Single-ended Class A FET + MOS amplifier kit 5W+5W | eBay
Assembled Hifi A30 Pure Class A High-current FET Amplifier Board 30W+30W | eBay
HIFI PASS A3 Single-ended Class A Power Amplifier Kit 30W+30W AMP ( 2 Channel) | eBay

Hoping for some input from all the gathered experience here. Which ones are considered good sounding, reliable etc?

Zerozone class A dc offset adjustment?

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