Help me troubleshoot this design, please

[teemuk]

Hello everybody. I'm in the middle of assembling this amplifier but it seems to have some stability problems. When I first tested it, it oscillated on a low frequency for few seconds (not very loudly though) and then short circuited. I had the amp hooked up on a light bulp current limiter so the output devices did not die. No component seems overheated or blown and the amp ran very cool... Of course, I shut it down immediately when I noted the short. This is where the things go beyond my comprehension...

If I remove the wire link marked in the schematic the output devices indicate no short circuit: When powering on, the light bulp burns on a dim glow for a second and then fades out - during short it is very bright. Now, this would indicate that the short is on the bias/class-A driver side. However the DMM shows no shorted tracks or components there. As far as I understand, the only thing to cause such drastic current draw would be a falsely operating VBE multiplier - more specificly a short between it's base and emitter or something similar. However, it's resistor values test ok, so does the transistor - I even deliberately ran the amp underbiased when I powered on the first time. The vbe mult transistor is mounted on the same heatsink as the power transistors and drivers.

The circuit works fine on spice simulation but sometimes this has little meaning in reality - perhaps I have something wrong in the design in the first place? This has gone beyond my comprehension: My test results indicate that everything should work but still something is wrong. Perhaps someone can offer a fresh idea for what to troubleshoot and help me out on this...

I should add that during the first test the zobel was omitted and the output plus vbe mult transistors have a quite long wiring. The driver and VBE multiplier transistors (except the class-A driver) are BD179 and BD180 instead of ones shown in the schematic and the power transistors are TIP141.

[mrshow4u]

My guess is the bias setting is too high. Try shorting C to E on BD139. Restore the link and then see if the current limit lamp comes on. It might be just a misadjusted bias pot.

[ashok]

Lf oscillations could be due to ( ground) wiring. What appears to be LF oscillations could also be really RF oscillations! Are you looking at a waveform on a scope ? A picture of the set up might give us a better perspective to your problem.
In addition the BD140 driver generally had a resistor and diode on its emitter in more recent circuits. Supposedly to duplicate the emitter load on the upper driver. I might be wrong ...check D Self.
Cheers.

[mrshow4u]

pardon my ignorance, but what is that 200k, 47uF stuff yanking on the output line??


....and how is the input transistor biased??

[teemuk]

Originally posted by mrshow4u
pardon my ignorance, but what is that 200k, 47uF stuff yanking on the output line??


....and how is the input transistor biased??

That trimmer is used to set the DC level to half of Vcc at the output (for symmetrical drive). Together with the input transistor's base resistor it also handles input transistor biasing since the 47uF capacitor bypasses AC signals. The following resistor/cap sets the negative feedback.

As requested here's a picture of the set - I haven't taken a newer picture, which would show the circuit left from wire link in the schem. It's built on a small veroboard mounted on the top left bolt holding together the heatsinks. The two blue + yellow wires come from the vbe transistor and they were shortened to about 1/3 of the length shown in the picture. The grey wire (on left)goes to speaker terminal (where the output cap - and later the zobel - are mounted) and the red and black wires go to power supply. The "lower current" circuit board gets the supply and ground connections by other wires not shown in the picture. All black ground wires go to star ground point. Granted, the layout is messy and the wiring too long but currently I have no possibility to make a proper PCB. Unfortunately, I do not own a scope but it will probably be a future investment.

[mrshow4u]

...yes, sorry. I was looking at it wrong. Sorry about that.

[teemuk]

Originally posted by ashok
In addition the BD140 driver generally had a resistor and diode on its emitter in more recent circuits. Supposedly to duplicate the emitter load on the upper driver. I might be wrong ...check D Self.
Cheers.

Ah, you are meaning a circuit that I know as "Baxandall-diode". (Took a while to comprehend that - at first I thought you were speaking about the internal structure of the transistor itself). I am well aware of this circuit but I decided to omit it for simplicity's sake, nice that you mention it though. Leaving it out will increase distortion a bit but as far as I know it should not cause issues with stability.

[davidallancole]

Have you measured your voltage across the Vbe multiplier. From what you have there, would the bias voltage not be 4V (1900/470=4)?

[teemuk]

Originally posted by davidallancole
Have you measured your voltage across the Vbe multiplier. From what you have there, would the bias voltage not be 4V (1900/470=4)?

Hmm... The spice simulation shows 1,7 V across it rather than four when the trimmer is set at 1900 ohms. I haven't tested this since I'm not too keen on keeping the amplifier on during a short circuit for a too long time. The light bulp limiter is a crowbar safety measure allowing me to flip the power on and notice a problem without blowing things up but I fear a long term short circuit (such as making the voltage measurement over vbe mult) would be pushing it too much.

I know little about vbe mult circuits; only that the bias voltage requirement is about 0,7 times the number of BE-junctions. If the transistor has a high gain the ratio between R2 (collector to base) and R1 (base to emitter) should be around (Vrequired / 0,7) - 1. In this circuit the result would be four resulting, for example, into 4 x 470 ohms = 1880 ohms. In spice simulation this value was proven insufficient for BD139. A higher beta transistor model (2N222A) indeed shows a figure around 4 V @ 1900 ohms. Perhaps the spice model I have has an error (it's from ON Semi)and the bias current is indeed set too high. But I gather that the Hfe figure counts pretty much and BD139 has a pretty low one. However, I do not suspect an error in the amount of bias voltage because....

I mounted the Zobel network plus did the very simple test mrshow4u adviced and shorted the vbe mult base to collector (before that I changed the BD179 to brand new BD139) - still a short circuit.

To me all this indicates that the problem can not be in the bias circuit but I just can't believe it. I can not understand any other way the circuit would make the power transistors draw such as high current as in a short. The Vcc supply for the "low current" circuit goes through very high resistance - if there is a short instantly from wire link to ground then at least my DMM does not show it. Also, I do not have a load attached, which should be a reason good enough to rule out the theory of oscillation in the input that would drive the amp at maximum level. Still, even without the load attached an error in the vbe mult circuit would push the output hard.

[latala]

first please check supply polarity the tip 142 have a diode as part ofthere structur I think ?
Second If thats in order check each and every device for a short !
the circuit is so simple it should work !
regards trev