Need help constructing amplifier S-Sub

[Fedess]

Hi!,

I constructed this Class AB amplifier called "S-Sub" which the author says it's designed specially for woofers. I want to use it in a biamped system. I recently made it but I cannot get it working, cause I have a problem with output DC.

I have -25V at the output (obviously it's wrong), where the author says that this should be between -0,1V and 0,1V and then adjust it to 0V (or as close as possible) with the pot labelled R5b on the schematic.

Power supply used for the testing is +/-30VDC.

I have basics knowledgement on electronics and following the schematics I found that between D2 and R9 I should have -25,3V (-30VDC from the supply plus 4,7V from the regulating zener, it gives abaout -25,3V). I measured it with the tester and that's the value I got.

At that point we see it's linked to Q9 transistor's base, and I also have -25V at its collector.

The same value is measured at the base and emitter of Q13, base and emitter of Q14 and the base and emitter of the output transistor Q15...

I am missing lot's of things from the schematic because I don't understand it at all, so I couldn't diagnose where the problem is...

I atteched the schematic indicating the points where I measured with the tester.

I checked the circuit few times, found some others mistakes and corrected them, but now I don't know what I'm doing wrong.

Note: Transistors Q12, Q13 y Q14 are KSE350 (replacement for MJE350); and transistors Q6, Q7, y Q9 are KSE340 (replacement for MJE340) .

I also connected the circuit with two 22 ohm 5W series resistors (one at +V and the other at -V) for the testing...just in case...(I don't think they are causing this problem!)



Here is the link to the schematic:
http://www.pcpaudio.com/pcpfiles/pro...psub/etapa.pdf

Hope someone can help me!

[jaycee]

R17 does not look right to me. It would appear to defeat the working of Q9!

Q9 itself could be damaged. You may also need to adjust R6 for your use of an MJE350. I'd stick to BD139/140 unless you are able to work out and adjust part values appropriately. In addition, MJE340/350 are not good drivers (Q7, Q14) and may compromise performance.

What power transistors for the outputs are you using?

[AndrewT]

Do not connect a load to the output. This would very likely blow all your output devices.

Have you powered up through a light bulb tester?
Does the bulb stay lit?

more test voltages would help.
Vr3, Vr?, Vr4, Vr5, Vr6,

All the Zeners are dissipating very low Watts.
They will not regulate at all well if the Zener current<<10% of max rating.

[jaycee]

To be honest, for a low power amp, it's not a fabulous design. For a subwoofer amp... I'm not impressed. How much power do you want in your sub?

[dangus]

I'm just going to suggest making sure that any transistor substitutes are in fact equivalent devices. And check that all transistors are installed with the correct lead orientation. Those are a couple of things that gave me trouble.

[sregor]

Just a wild guess - did you use 8R2 or 8K2 for R4? Correct would be 8R2 ( 8.2) More voltages would be helpful. Good luck.

[Fedess]

hi! thank you all for the replies!

jaycee: at the author's web he says R17 is there to be "dominant" against other parassits resistances, also to reduce the high ("infinit") impedance and to not allow it (the impedance) to be variable...

I want this amplifier to be rated at about 100W at 8 ohm. So higher voltage is required, and the author suggest using MJE340 and MJE350 when using more than +/-40VAC (56VDC) because they support that higher voltage. He notifies that they are worse compared to BD139 and BD140...

I think the power supply would be something between +/-56V and +/-63V (this depends on the transformer I will be able to get), so I can get the decired power.

The output power transistor I'm using right now are MJ15024 and MJ15025.

I changed Q9 and Q12, now using BD139 and BD140 respectively, but still have the same problem...

Supposing that I switch back to MJE transistors, what value do you suggest for R6? (or explain me a bit, if you want, why should I change it and how to calculate it)


AndrewT: I haven't connected any load because of the high DC voltage at the output, I want to fix that first...

I didn't use a bulb, just 47ohm in series (for the first test) and now 22ohm (still testing...) and they don't get hot.

I will measure voltages across the resistors and zeners and post them in a few minutes!

dangus: I have already done that, anyway I will check everything again

sregor: That's a very good observation! R4 is 8.2 ohm. Unfortunatly I don't have a higher voltage supply right now


PD: sorry for my 'not very good' use of english!

[Fedess]

Originally Posted by jaycee

To be honest, for a low power amp, it's not a fabulous design. For a subwoofer amp... I'm not impressed. How much power do you want in your sub?

It's not for a subwoofer but for an 8" woofer in a bi-amped system. As I said in the previous post I want about 100W @ 8 ohm (per channel)

Well, I didn't choose it because I wanted something special. The design is not complex, the author offers the PCB masks for free, I was sure I could get all the components needed, and I made the "Public" amp from the same author (which is the only DIY amp I have right now), and the author recommends it for the purpose I need... so at the time of making decisions I think there was no reason for not choosing it.

I consider I'm a beginner yet, so I didn't want to go for something more complex but very accurate (not for now!); I want to experiment a little more before!

[Fedess]

I changed Q6 and Q9, they are now BD139. And BD140 for Q13 and Q12. I still have -25,3V at the output...

Measurements (using those new transistors):

VR3= 2,49V
VR?=0,23mV (yes, less than 1 mV)
VR4=0V (I put the tester at the 200mV scale and measured 0,00mV)
VR5=1,82V
VR6=1,72V

D1=4,13V
D2=2,41V (D1 and D2 are the 4,7V zeners)
D3=8,85V (it's the 9,1V zener)

(I measured voltage across each component, not taking GND or another point as reference)

Note: Input is shorted to GND

I think there is something wrong with D2 and have doubts about those 0V across R4...

[sregor]

VR3 = 2.49 and VR4 = 0 would indicate Q11 open. VR4 should be about 0.6 volts less than VR3. Also, since D2 has insufficient drop - would guess there may be a problem with Q5 or Q9 (base currents should be much less than the 2.5 mA going through R9. Also, by more voltages I only meant more voltage measurements, not higher voltages. Good luck.
Edit: Reread last post and saw you changed Q9. Maybe D2 is bad, but doesn't explain negative voltage at output.