Hi,

Again I mean this in good intentions,

Am I missing something then,

The R27 is in series with the base of Q10, it drops the voltage, and thus the current
by [(VCC - Vbe) / (R27+R34)]. Not including HFE parameter which would raise the input impedance higher.
This is with collector opened.

The 2 diodes keeps a good base bias voltage on Q10 to help put the amp into Class AB mode.

Needs to have some base current to keep from cross over distortion.

For fun I breadboarded the circuit last night using different resistor values to design it for an output current of 1mA, with no load, and got a 16vP-P with no crossover distortion.

I'm not sure you're missing anything to be fair. More likely i'm missing something! Don't know if you saw my thread introducing myself but i pointed out i'm a bit on the rusty side of things - like about 27 years rusty!

However, without digging through to find my course notes (which i've got a horrible feeling disappeared somewhere) i would have said the output is Class B - i could be wrong on this of course but i agree that you need to bias the transistor on to reduce crossover distortion. Also i was always taught to look for the worst case scenario, in which case you'd have 24V trying to get through a single 15Ω resistor, hence my power calculations. The fact that the resistor is frying suggests the worst case scenario in his circuit exists. If his power supply is one of the kind that self regulates the current O/P then it could be it's momentarily shutting down as it's delivering in excess of 1.5A, hence the fluctuation he describes.

I'm sort of guessing here that if he soldered everything on stripboard, he's either not made a cut where he should have done or got a minute blob of solder linking 2 tracks or something along those lines. It could also be that he's not got the correct value resistors in somewhere.

I wasn't questioning if the circuit worked as per the design shown, more the fact that he may have altered the design as in the very first post it's a little ambiguous whether the circuit shown is the original or the one he modified.

 

There is nothing inherently wrong with the circuit design.

 

I'm afraid this is getting to be moot if the OP has not returned by now.

While I don't think that people check on the internet to be told to replace random components (they can do that without advice) I did Sim that the builder had miscounted the holes in the protoboard and connected R34 directly from ground to +24V and that explained everything. (insert smiley face here)

Properly used, a simulator can solve an amazing amount of problems and be very educational, but properly using them is an art all of itself. When it comes to assembly errors (I call them factory f***ups when I'm working on machines) they are so numerous that it boggles the mind. I don't think anybody could Sim all the assembly errors that are possible, even in a 3 transistor amplifier. That is why there is no substitute for measuring and thinking.

 

I'm beginning to wonder if the OP has disappeared in the puff of smoke that was his resistor too!

The assembly errors are too numerous to mention - there's 27 different possibles just on the transistors alone! That doesn't include bridged tracks from solder between the pins!

As for manufacturers assembly errors, i refer to them as "finger trouble" - the fingers have put things in the wrong place for example.

I've suggested several possible things to look at, last but not least was measuring the voltages and temporarily lifting components out of circuit to enable tests to be carried out.

However, it would seem we are going to be left as much in the dark as the OP was when starting the thread unless he comes back!

 

I have worked several jobs as "QC" (quality control) and saw many amazing things that could go wrong on the assembly line. Imagine a machine with the case grounded to what electricians call "bond", one secondary of the power transformer grounded to the sensing circuit board, and another secondary of the power transformer grounded to the output board which is opto-isolated from the first board.

Now, rotate the ground wires so the bond is on the sensor board, the wrong secondary ground is on the output board, and the ground of the other wrong secondary is on the 120 volt bond. Who would Sim that?! You could tell me to replace random parts for a week and it wouldn't fix anything.

I also meet the strangest things when working on machines that have been sold and put into service. If it hasn't been working properly until it broke, you have to include every possible error in your mind. Only when it has been working properly and then broke can you even consider Simming for the answer.

 

The comments about possible assembly mistakes may be useful to a lot of people, even if the OP never shows up again. Perhaps this should have been given more emphasis early on, as in my experience, they are very likely in anything put together by hand, especially when someone is new to the job.

Both stripboard and solderless breadboards are particularly fertile grounds for such problems. Apart from the mistakes others have mentioned, it is easy to connect a component or link wire into the wrong strip.

Another area for confusion is the top view/ bottom view issue, where one may not be clear about the component pinouts given in manufacturers' data. Typically these are described from a top view, but a minority of parts (notably electronic valves/tubes) are viewed from the bottom.

I would advise anyone building anything from scratch to check out the circuit as thoroughly as possible BEFORE switching it on for the first time: this can save a lot of frustration, and of course expense on ruined components.

 

@ #12
and
@ LAIRD SCOOBY,

Sir's,
I apologize if I came on as being beligerant in my posts,


The only reason for this apology, is because, I did not realize your technical background (#12),in this field, until your last post, when you wrote about being a (QC),

as well as Laird Scooby, I did not know your technical background, until I read what yoyu wrote in your personal background in the part of the forum where you sign up at. (whatever area that is called).

You both are well advanced in this field of electronics, compared to me, where this is only a hobby for me, not a carrer.

So I HUMBLY back away, and I do deserve whatever sarcasm you had coming to me, seeing that I am the one that's putting my 2cents in, when you guys are the ones who make a living in this field, and its only my hobby,

In other words I have no room to argue, when you guys, disprove of a method I may suggest.

And to all the other people who work in this field, I apologize for any technical retro arguments that I come up with ,seeing that you guys are above me in this field.

So again please accept my apologies, to all of you who are professionals in this field.

Thankyou.

 

Ok, hobbist. You made a beginners error. I've done that too.

Even with the decades of work I've put in, I learn something almost every day on this site. There are so many specialties in electronics that nobody can know it all. We are lucky to have so may brilliant people here. I can't even understand most of what papabravo says!

No matter how good you get, there will always be somebody that knows more. Every once in a while, somebody will pop up with, "I worked there and this is what happened". Everybody has to bow down that that kind of experience!

I've learned to never be too sure of myself, whether turning off a 440V 3 phase disconnect (and finding it didn't disconnect!) or trying to ascertain the skill level of the person I'm trying to answer (I fail regularly at that).

One of the things I learn over and over is humility.