Forums

Class B distortion

Started by Unknown December 20, 2014
<meow2222@care2.com> wrote in message 
news:32096492-16c1-4832-95f4-677075a6d070@googlegroups.com...
>Like my other recent projects, getting a job done at the lowest possible >parts cost with labour as unskilled and ill equipped as possible, and using >only scrap parts with no data. I've got a simple class A amp, now want a B >for battery apps. The point of the exercise is to provide designs that some >people in poverty might learn to build to sell. >
So why throw more parts at it? They'll be more than happy with a class C complementary emitter follower type circuit. Tim -- Seven Transistor Labs Electrical Engineering Consultation Website: http://seventransistorlabs.com
On Tuesday, December 23, 2014 12:39:49 PM UTC-5, meow...@care2.com wrote:

> Like my other recent projects, getting a job done at the lowest possible =
parts cost with labour as unskilled and ill equipped as possible, and using= only scrap parts with no data. I've got a simple class A amp, now want a B= for battery apps. The point of the exercise is to provide designs that som= e people in poverty might learn to build to sell. What kind of nutty requirement is that?
>=20 >=20 > NT
On Saturday, December 20, 2014 4:02:32 PM UTC-8, meow...@care2.com wrote:
> On Saturday, December 20, 2014 12:11:02 PM UTC, Phil Hobbs wrote: > > Yup. And to get anywhere near it, they have to run a huge input error voltage--as much as a volt
> Does that mean if I ran the opamp at gain=1 it would at least reduce the problem to some degree? The bad news is it will be any old opamp, eg LM358, LM324 etc.
No, not really; in particular, LM358 and LM324 are anomalously bad because their class B output stages are intended for low quiescent power. It was a deliberate decision to insert a dead band in the output drive. You really want to find a class AB op amp instead, like TL072 or TL074, if you care about distortion (and there's a bit of a quiescent current penalty). In a pinch, you can put a pullup resistor onto an LM324, and make it operate in class A.
On Mon, 22 Dec 2014 07:50:00 +1000, <meow2222@care2.com> wrote:

> On Sunday, December 21, 2014 10:08:00 AM UTC, David Eather wrote: >> On Sun, 21 Dec 2014 10:34:21 +1000, <meow2222@care2.com> wrote: >> >> > On Saturday, December 20, 2014 8:57:22 PM UTC, Jan Panteltje wrote: >> > >> >> + >> >> | >> >> |/ c >> >> -------| NPN >> >> | |\ e >> >> in ---- +|\ | R | >> >> | >------===----| >> >> -- - |/ | |-------- out >> >> | | |/ e | >> >> | --------| PNP | >> >> | |\ c | >> >> | | | >> >> | - | >> >> -------------------------- >> >> >> >> For some value of R the cross over is eliminated as the opamp drives >> >> the load for small signals. >> > >> > Yes, I tried that in spice. It seems the R has to be very low for an >> > opamp though. 0.65v on 8ohm load = 75mA. 75mA 0.6v = 8ohms for the >> R... >> > plus 8R load makes a 16R load. Can a random opamp drive that? >> > >> > >> > NT >> >> Jan's circuit works. R is actually there for stability not to drive the >> speakers (check it out 20ma output current into 8 ohms - a whopping >> 3.2mW, >> into a standard speaker is below the noise floor in most indoor >> locations). An R about 4.7k +/- 50% should be fine. >> >> If you already tried this and crossover was really bad there must have >> been something very wrong in your circuit. Even with a 741 the cross >> over >> distortion is above 20k. > > I did sim it. Putting that R in made little difference, still heavy > crossover distortion. Maybe I need to go much lower with R. How well > your average opamp can cope with very low R I'm not so sure, lets hope > Spice models it ok. > > > NT
OK here is my results: http://i356.photobucket.com/albums/oo1/davideather/opampampcircuit_zpscb9ffc2c.png http://i356.photobucket.com/albums/oo1/davideather/opampampcircuit_zpscb9ffc2c.png distortion at 1k was under 0.5% and at 10k under 5% - I didn't filter out the output above 20kHz so the figures are better than that. Output was about 3 watts, too much for the output transistors for a continuous signal but OK for music and voice.
meow2222@care2.com wrote:
> > Standard plastic/metal trannies. Real life Tj will vary a good bit between > samples, since the specs of the parts used are either unknown or ignored. > Whether its 175 or 125, Vbe still shifts a long way. > >> This would go far better if you just stated what you are trying to >> achieve rather than discuss op amp design. > > Like my other recent projects, getting a job done at the lowest possible > parts cost with labour as unskilled and ill equipped as possible, and > using only scrap parts with no data. I've got a simple class A amp, now > want a B for battery apps. The point of the exercise is to provide designs > that some people in poverty might learn to build to sell. > > > NT
The VBE shift will be present in the swing of the amp. How you are seeing a VBE type error in the output doesn't make sense. I suspect the problem is your circuit and not the part. You do know that 175 is beyond mil spec. I had figured this was a well rig project, but now I don't know what to think.
On Wednesday, December 24, 2014 2:24:52 AM UTC, David Eather wrote:
> On Mon, 22 Dec 2014 07:50:00 +1000, <meow2222@care2.com> wrote: > > > On Sunday, December 21, 2014 10:08:00 AM UTC, David Eather wrote: > >> On Sun, 21 Dec 2014 10:34:21 +1000, <meow2222@care2.com> wrote: > >> > >> > On Saturday, December 20, 2014 8:57:22 PM UTC, Jan Panteltje wrote: > >> > > >> >> + > >> >> | > >> >> |/ c > >> >> -------| NPN > >> >> | |\ e > >> >> in ---- +|\ | R | > >> >> | >------===----| > >> >> -- - |/ | |-------- out > >> >> | | |/ e | > >> >> | --------| PNP | > >> >> | |\ c | > >> >> | | | > >> >> | - | > >> >> -------------------------- > >> >> > >> >> For some value of R the cross over is eliminated as the opamp drives > >> >> the load for small signals. > >> > > >> > Yes, I tried that in spice. It seems the R has to be very low for an > >> > opamp though. 0.65v on 8ohm load = 75mA. 75mA 0.6v = 8ohms for the > >> R... > >> > plus 8R load makes a 16R load. Can a random opamp drive that? > >> > > >> > > >> > NT > >> > >> Jan's circuit works. R is actually there for stability not to drive the > >> speakers (check it out 20ma output current into 8 ohms - a whopping > >> 3.2mW, > >> into a standard speaker is below the noise floor in most indoor > >> locations). An R about 4.7k +/- 50% should be fine. > >> > >> If you already tried this and crossover was really bad there must have > >> been something very wrong in your circuit. Even with a 741 the cross > >> over > >> distortion is above 20k. > > > > I did sim it. Putting that R in made little difference, still heavy > > crossover distortion. Maybe I need to go much lower with R. How well > > your average opamp can cope with very low R I'm not so sure, lets hope > > Spice models it ok. > > > > > > NT > > > OK here is my results: > > http://i356.photobucket.com/albums/oo1/davideather/opampampcircuit_zpscb9ffc2c.png > http://i356.photobucket.com/albums/oo1/davideather/opampampcircuit_zpscb9ffc2c.png > > distortion at 1k was under 0.5% and at 10k under 5% - I didn't filter out > the output above 20kHz so the figures are better than that. Output was > about 3 watts, too much for the output transistors for a continuous signal > but OK for music and voice.
That's almost identical to what I simmed, but distorted. When I get time, which probably won't be this week, I'll try & see what change fixes it. Thank you :) I had it running on lower voltage, 30v is a big mismatch for an 8 ohm 0.5-3w amp. NT
On Tuesday, December 23, 2014 6:06:50 PM UTC, Tim Williams wrote:
> <meow2222@care2.com> wrote in message > news:32096492-16c1-4832-95f4-677075a6d070@googlegroups.com... > >Like my other recent projects, getting a job done at the lowest possible > >parts cost with labour as unskilled and ill equipped as possible, and using > >only scrap parts with no data. I've got a simple class A amp, now want a B > >for battery apps. The point of the exercise is to provide designs that some > >people in poverty might learn to build to sell. > > > > So why throw more parts at it? They'll be more than happy with a class C > complementary emitter follower type circuit. > > Tim
Most purchasers won't want raw class B. I'll include the option though for the few that do. NT
On Wednesday, December 24, 2014 2:29:09 AM UTC, miso wrote:
> meow2222@care2.com wrote: > >=20 > > Standard plastic/metal trannies. Real life Tj will vary a good bit betw=
een
> > samples, since the specs of the parts used are either unknown or ignore=
d.
> > Whether its 175 or 125, Vbe still shifts a long way. > >=20 > >> This would go far better if you just stated what you are trying to > >> achieve rather than discuss op amp design. > >=20 > > Like my other recent projects, getting a job done at the lowest possibl=
e
> > parts cost with labour as unskilled and ill equipped as possible, and > > using only scrap parts with no data. I've got a simple class A amp, now > > want a B for battery apps. The point of the exercise is to provide desi=
gns
> > that some people in poverty might learn to build to sell. > >=20 > >=20 > > NT >=20 > The VBE shift will be present in the swing of the amp. How you are seeing=
a=20
> VBE type error in the output doesn't make sense. I suspect the problem is=
=20
> your circuit and not the part.
I assume so
> You do know that 175 is beyond mil spec. I had figured this was a well ri=
g=20
> project, but now I don't know what to think.
Maybe I didnt explain well enough upthread. Its not designed to hit 175, bu= t if I can pick an option that's bias stable over the full range then its o= ne less cause of failure in the real world, where what people do with the d= esign is outside of my control. The helper tr circuit, with trs biased by t= he opamp's quiescent current, falls down on this point, otherwise it would = be a very promising option. A circuit that works reliably for end users is going to sell far better tha= n something that gains a bad rep for frying itself. NT