Subbing NPN output transistors for PNP?

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abechap024

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Aug 8, 2009
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Hello
I'm building a power amp for my monitors and have a bunch of nice PNP transistors laying around for the output. The circuit calls for NPN output transistors...

Is there a way I could use the PNPs? I am not as knowledgeable as I would like to be on the theory behind transistorized circuits.

The transistors in question are TR9 and TR10

Thanks!

File



 
:D :D

You would clearly have a one of kind amplifier.

If you inspect that schematic you will notice that they are already cheating a little and using a NPN for the negative power device. This output topology is called quasi-complimentary as in "it's not complementary". This was commonly done in the not so good old days, because PNP devices were less rugged and generally inferior to NPN power devices.

To answer your question, yes, you could make a quasi-comp using all PNPs, you just have to do a mirror image of the opposite polarity driver transistor to flip the sex of it... i.s. use two PNPs on the pull down, and a NPN driver for the pull up.

If you want to mess with the design, I would consider making it full complementary and perhaps better. I have never seen a qusai-comp using PNPs, so you can be the first.  ;D

Note: You will probably need to check for oscillation and stability with a scope, when using other that designed for parts.

JR
 
Okay...I'm trying to wrap my head around this. I've modeled the circuit in a simulator and if I just switch out the PNPs for NPNs pin for pin, it says it works fine...though I don't think that can be right...

Heres what I've come up with based on Johns recommendations...don't know if I did it correct though. The simulator likes it good enough....I will hook it up, but I don't want to blow (more) stuff up! :)


naim140_mod_3.jpg

 
OK from a quick glance the top pair looks ok... NPN driver, PNP power device. The bottom pair not so much.. the emitter of the first PNP driver, needs to connect to the base of the power PNP.. It may look Ok to a simulator with no load but put a 4 ohm load on the output and the PNP will not get er dun... Look at how the two NPNs were connected in the original schematic.

JR

 
> ...don't know if I did it correct though. The simulator likes it good enough....

1) There's no load.... it may bias-up correctly no-load but be unable to do real work.

2) What is the idle current in your T3? Looks to me like 19 AMPS, rather more than desired.

WHY build THIS amp? I see the 2001 date, but it was old in 1981. No protection and just two power devices for nearly 100 Watts? Selecting TR1/TR2 "HFE" ratio to compensate offset bias resistors? There are better plans. For 2db less output, you can buy a $7 chip that is simpler, cleaner, and quite blowout-proof.
 
> I have never seen a qusai-comp using PNPs, so you can be the first.

It was pretty routine when Germanium was king.   

 

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Thanks John and PRR. I see why people turn to chips now-a-days it is quite simpler. But maybe not as much fun ;) I got one channel working and now to get the other channel. I found some NPN power transistors I had laying around so I think I'm going to make it a Complimentary output stage.

Ruckus - Its called TINA, and there is a version for free from TI. But Im using the demo of their industrial suite, you can only open it 25 times or so, but I just leave my computer on anyway. Works well...Saves on breadboards!

Abe
 
I like to joke that power amps are one of the most complicated "simple" products to design (consoles are the other). Simple in concept, but the details can bite you, especially when you go for higher power levels. That design doesn't even have simple current limiting so could get seriously stressed to the point of releasing it's smoke from a momentary short circuit.

Very basic current limiting can be established by using the voltage drop across the .22 ohm emitter resistors to feed the base of a transistor that turns on and starves the power driver's base input.  Feeding the full voltage from the .22 will current limit at about 2.5A which is lower than you want so a simple divider between the resistor and base will scale up that current limit threshold to something more appropriate.

Selecting the drivers and output devices will be a good object lesson in understanding transistor hfe (beta or current gain). Figure how much current the device needs to make full voltage into 8 ohms or 4 ohms, then divide that by hfe to see how much current the driver needs to feed the base, and so on...

I guess circuit sim can do all that if it has the right device models and accurate load/PS conditions set up.

JR
 
duu.uuuh! How come none of you gurus are asking questions like

"What are these PNP transistors Abe has lying around?  What power supply voltage?"

The unmodified circuit has serious problems and looks like someone just put in all that was fashionable in the 70's electronic comic books without understanding. If he builds it exactly as is, at least he has some comeback when the Holy Smoke escapes.

Abe, if you modify this circuit, my advice is not to use any speakers you value on the output.
 
I'm going to make one more comment on the UNMODIFIED circuit then I resolve never to look at this thread again cos the circuit sends shivers up my spine.

The '10u BC128' input capacitor and 'C6 - 47u See Text' feedback capacitor are the wrong way round.  I only mention this cos it's a Holy Smoke issue.  And DON'T USE TANTS!

There are circuits where the Holy Smoke from your speakers won't join that from the amplifier.  This is NOT one of them.

Of course if you do my mod and the Holy Smoke happens, serves you right for listening to pseudo gurus on the web ...  ;D
 
ricardo said:
duu.uuuh! How come none of you gurus are asking questions like

"What are these PNP transistors Abe has lying around?  What power supply voltage?"
His SIM said it was all hunky dory....  ;D ;D ;D
The unmodified circuit has serious problems and looks like someone just put in all that was fashionable in the 70's electronic comic books without understanding. If he builds it exactly as is, at least he has some comeback when the Holy Smoke escapes.
With whom?
Abe, if you modify this circuit, my advice is not to use any speakers you value on the output.

Low power amps like this, are a little more forgiving but sure, don't hook up speakers until you confirm it isn't oscillating, and isn't putting out DC...  I doubt he has the proper paraphernalia to do serious amplifier bench work: varaics, light bulb current limiters, non-inductive bench load resistors, yadda yadda...

Lets not ruin his fun... Hopefully he can come up with something better than that generic schematic.. perhaps you could share the obvious serious problems. The electrolytic caps being reverse biased a few mV is sloppy design, but not likely to be a smoke releasing fault.

Intentional beta mismatch of the input LTP to get nominal DC balance is not a common design practice.  :eek: but I notice the schematic isn't signed by a design engineer.  You get what you pay for, and this is all free.....

JR

 
I have not seen these guys calling themselves gurus but I think they are a lot more experienced than an average contributor here such as myself.

However, I have never designed a power amp before. Would you care to tell me why those caps are wrong way round?
 
sahib said:
I have not seen these guys calling themselves gurus but I think they are a lot more experienced than an average contributor here such as myself.

However, I have never designed a power amp before. Would you care to tell me why those caps are wrong way round?

The NPN input long tail pair, will have a small base current flowing into the base. This current drawn through the local resistor to ground will create a very small - DCV at that base. Note that the input side DCR to ground is equal the the DCR of the feedback network so these voltage drops should cancel out and make 0V at the power amp output. The nominal orientation of the input polar cap is + to the outside world which should be 0V. Same for the blocking cap in the feedback network. This small (mVs) reverse bias voltage is not likely to release smoke, it is just sloppy design. 

Note: this is not peculiar to power amps... discrete circuitry and even bipolar IC opamps have input bias current too, while some modern uber-opamps do a good job applying a first order cancellation of this bias current leaving only a tiny error current (bifet opamps have insignificant input bias current). 

JR
 
Damn! I really didn't want to be sucked into redesigning dodgy circuits ..

Input/Feedback caps.

The bias for TR1/2 come from earth (and from an O/P which is supposed to be at earth).  So earth (and O/P) is +ve wrt to their bases.

I like high I/P current OPA like 5532 for this very reason cos I can ensure a few 10mV across feedback/input Al. electrolytic caps.  Good performance and reliability.  I HATE i/p bias cancelling OPAs for this and other reasons.

Here, the wrong polarity means electrolytics will be leaky and unreliable.  Tants can stand a few mV reverse with only small leakage which is probably why he's specified one.  But if you have larger reverses, eg when the amp is switched on/off, after a few goes, Tants fail catastrophically and go SC.  So the already wonky offset on the amp is made more wonky and amplified by the full gain of the amplifier.  Maybe no Holy Smoke from the amp but from your Speakers instead.

I'm really a speaker man and only a pseudo amp, mike, DSP & LN guru.

Abe, are you building from an Avondale Audio kit?  If you are using your own PCB, be warned that the designer has a poor grasp of stability issues (eg his comments about small caps, 47p etc) and your PCB and layout may not be stable if it differs.  I'm assuming he's tested his extensively (ha!).

First I gotta say in da old days, I wrote my own Circuit Analysis programme and hence don't use SPICE etc.  I think I can do response / stability stuff as good as any SPICE but can't do non-linear stuff.  But can full SPICE do Safe Operating Area + thermal stuff?  That's what you need for Holy Smoke propensity.  Can the free versions do this?

Do Golden Pinnae amps sound different?  Of course they do!  Some (all?) if driving a real speaker load (a big guitar speaker is good) and sine swept at various frequencies & levels, you will see small bursts of oscillation on parts of the cycle.  All dependent on thermal & time history too.  On overload, some will take several milliseconds to come out.  None of this is Golden Pinnae subtle stuff.

A marginally stable amp may go into full Holy Smoke oscillation depending on thermal, time history & load.  Changing the O/P stage changes this.

Don't ask how I know all this (especially the Holy Smoke) BS.

I'm going to bow out now but I'd like an answer about SPICE.

Don't get me wrong.  It's good to play.  That's how we learn.  But Holy Smoke is no fun especially from your prized speakers.

http://douglas-self.com/ampins/ampins.htm
Unfortunately, he's taken a lot of good stuff & put it into his books.  Definitely worth getting.  Don't agree with everything he says but you can rely on his measurements.

 
ricardo said:
Damn! I really didn't want to be sucked into redesigning dodgy circuits ..

Input/Feedback caps.

The bias for TR1/2 come from earth (and from an O/P which is supposed to be at earth).  So earth (and O/P) is +ve wrt to their bases.

I like high I/P current OPA like 5532 for this very reason cos I can ensure a few 10mV across feedback/input Al. electrolytic caps.  Good performance and reliability.  I HATE i/p bias cancelling OPAs for this and other reasons.

Here, the wrong polarity means electrolytics will be leaky and unreliable.  Tants can stand a few mV reverse with only small leakage which is probably why he's specified one.  But if you have larger reverses, eg when the amp is switched on/off, after a few goes, Tants fail catastrophically and go SC.  So the already wonky offset on the amp is made more wonky and amplified by the full gain of the amplifier.  Maybe no Holy Smoke from the amp but from your Speakers instead.
Indeed good practice...
I'm really a speaker man and only a pseudo amp, mike, DSP & LN guru.

Abe, are you building from an Avondale Audio kit?  If you are using your own PCB, be warned that the designer has a poor grasp of stability issues (eg his comments about small caps, 47p etc) and your PCB and layout may not be stable if it differs.  I'm assuming he's tested his extensively (ha!).
Yes, layout matters a bunch with power amps.. recall my "difficult-simple product" conundrum. As I constantly remind folks, think of PCB traces as resistors or inductors. With power amps you also have the shared mutual current in traces, IIRC Cherry even wrote an AES paper about the implications of PCB layout for THD in output power stages.
First I gotta say in da old days, I wrote my own Circuit Analysis programme and hence don't use SPICE etc.  I think I can do response / stability stuff as good as any SPICE but can't do non-linear stuff.  But can full SPICE do Safe Operating Area + thermal stuff?  That's what you need for Holy Smoke propensity.  Can the free versions do this?
+1.. I wrote my own filter design software back in the 70s.. I never trusted spice because you had to know what you didn't know to properly load all the models. I suspect the modern stuff is better and easier than a few decades ago (especially if other people do the heavy lifting).
Do Golden Pinnae amps sound different?  Of course they do!  Some (all?) if driving a real speaker load (a big guitar speaker is good) and sine swept at various frequencies & levels, you will see small bursts of oscillation on parts of the cycle.  All dependent on thermal & time history too.  On overload, some will take several milliseconds to come out.  None of this is Golden Pinnae subtle stuff.
Add a tone burst generator to the list of amp design gear.. steady state testing may not reveal transient issues.
A marginally stable amp may go into full Holy Smoke oscillation depending on thermal, time history & load.  Changing the O/P stage changes this.
Changing anything can change everything... Even production variances with correct part numbers and values can cause problems for marginal designs.  It is good practice when designing power amps to load the output with an extra dose of capacitance to test for a much worse than real world worst case load. Then look at the square wave response to get a feel for stability margin. Perhaps shunt the output choke for stability testing, or apply the capacitlive load before the coil.
Don't ask how I know all this (especially the Holy Smoke) BS.

I'm going to bow out now but I'd like an answer about SPICE.

Don't get me wrong.  It's good to play.  That's how we learn.  But Holy Smoke is no fun especially from your prized speakers.

http://douglas-self.com/ampins/ampins.htm
Unfortunately, he's taken a lot of good stuff & put it into his books.  Definitely worth getting.  Don't agree with everything he says but you can rely on his measurements.

Unfortunately the best way to learn is to make the mistakes yourself... We've made our share, his turn to learn.

JR
 
> His SIM said it was all hunky dory.... 

> can full SPICE do Safe Operating Area + thermal stuff?

My sim "liked" running a 12AX7 (330V 1.5W max) at 1,500V 250 Watts.

It was an easy way to model a grounded-grid hi-Mu transmitter tube mis-applied as a class A2 SE audio amplifier (so I could estimate how brutal the driver had to be). The sim does not care about parts running 100X rating (except, oddly enuff, its 1N4001 breaks-down at 100.1V).

For a chuckle, I asked the idiot to plot V(pk)*I(k) to get instant Watts over the cycle (though in audio tubes you really don't need this time-resolution) to compare to 12AX7 rating (and know that I was ballparking the transmitter-tube rating).

Thermal bias stability seems to be a lifelong study. If you ask SPICE the question at two (or a sweep) temperatures you can know the static trend. This must be multiplied by cooling to know if it will run-away. If you put the bias diode ON an output device (not shown in that plan) and your sink is not woefully small, IME it probably won't runaway. (Undersized sinks are necessary for profit but are "unprofitable" in DIY; be generous!) But the best fix for runaway tends to be bad for small sounds after large sounds (the new on-die devices help).

Current meter in-line with the amp power is a brave way to bias. Little slip on the trim, 50mA meter smokes (most DMMs do not have huge overload rating on current). And the higher goal is >30mV on the little emitter resistors..... volt-ranges overload gracefully. The "34-38mA" target seems low for 0.22r resistors, although I know my ear would hardly notice.

Second-Breakdown is a more complicated question. I suppose one of SPICE's boxes can accept an equation which will burp when SOA is violated.

Back when a big transistor was two tanks of gasoline, such questions mattered. But at today's jellybean prices, forget the question, the answer is LOTs of output devices. Two TO3 per 50W was my old safety rule, and that's cheap. If not about profit/loss, eight per 200W (25W/pair) isn't excessive; also beats heavy-metal heatsinks.

> Intentional beta mismatch of the input LTP to get nominal DC balance is not a common design practice

There's 1K up top and 620 below. Both have 0.7V drop. For a BALanced pair, the bottom should pass twice the current of the top resistor. This suggests, not 1K:620, but 1K2:620 or 1K:470. Basic cocktail-napkin proportioning. With a low-pay helper, or on breadboard, the values won't be exact 2:1 due to TR4 base current, Early effect (and what does does the 22K top of TR2 do? Anything good?) and 1N4148 not being same drop as ZTX753.

With this change, input and output DC offset should be 5mV without selection.

I've never seen a "brass cased Tantalum". Is that some extra-good part which won't short-out in a decade? (Shorted C6 is not a disaster, just 100mV output offset which any 80W speaker will survive.)

The (anonymous?) designer knows something of amp design. I'd do it different.... I'm not sure how much of that is differences in taste and experience.

I do think it has more parts than ABsolutely needed; and if I'm adding more parts MY experience leads to Protection before some of this other elaboration. The short-path through 220 TR4 560 TR7 TR9 annoys me too. (A flat tire should not ruin the steering-wheel.)

But I've built, repaired, and designed many amps and my preferences may be unique.
 
JohnRoberts said:
The NPN input long tail pair, will have a small base current flowing into the base. This current drawn through the local resistor to ground will create a very small - DCV at that base..... it is just sloppy design. 

So, the negative value input bias is due to the NPN pair. It would be positive for PNP.

Do we know what the offset is in that circuit?


ricardo said:
Damn! I really didn't want to be sucked into redesigning dodgy circuits ..

Input/Feedback caps.

The bias for TR1/2 come from earth (and from an O/P which is supposed to be at earth).  So earth (and O/P) is +ve wrt to their bases.
........
Don't get me wrong.  It's good to play.  That's how we learn.  But Holy Smoke is no fun especially from your prized speakers.

http://douglas-self.com/ampins/ampins.htm
Unfortunately, he's taken a lot of good stuff & put it into his books.  Definitely worth getting.  Don't agree with everything he says but you can rely on his measurements.

Great to know that we admire the same designer. Do you have his book Audio Power Amplifier Design Handbook? If you do check out the input caps in the schematics on page 191 and 243 both of which are PNP input pair.

 

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