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
Reply by ●December 24, 20142014-12-24
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
Reply by ●December 24, 20142014-12-24
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
Reply by miso●December 23, 20142014-12-23
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.
Reply by David Eather●December 23, 20142014-12-23
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
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.
Reply by ●December 23, 20142014-12-23
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
Reply by Tim Williams●December 23, 20142014-12-23
<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
Reply by ●December 23, 20142014-12-23
On Tuesday, December 23, 2014 8:45:40 AM UTC, miso wrote:
> meow2222@care2.com wrote:
> =20
> > One of the issues with some circuits is that output tr Vbe drops to abo=
ut
> > 0.35v at 175C (Tj). Diodes glued onto output trs would only rise to Tca=
se,
> > resulting, in a basic implemetation, in runaway bias. Everything can be
> > fixed with more bits of course, but I'm trying to do this one as rock
> > bottom as possible.
> >=20
> > Enjoy your xmas!
> Um 175 dec C? You have ceramic packages? You have characterized a diode o=
n=20
> the chip to determine Tj? Diodes on the output? Glue?
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. W=
hether its 175 or 125, Vbe still shifts a long way.
> This would go far better if you just stated what you are trying to achiev=
e=20
> rather than discuss op amp design.
Like my other recent projects, getting a job done at the lowest possible pa=
rts cost with labour as unskilled and ill equipped as possible, and using o=
nly scrap parts with no data. I've got a simple class A amp, now want a B f=
or battery apps. The point of the exercise is to provide designs that some =
people in poverty might learn to build to sell.
NT
Reply by miso●December 23, 20142014-12-23
meow2222@care2.com wrote:
> One of the issues with some circuits is that output tr Vbe drops to about
> 0.35v at 175C (Tj). Diodes glued onto output trs would only rise to Tcase,
> resulting, in a basic implemetation, in runaway bias. Everything can be
> fixed with more bits of course, but I'm trying to do this one as rock
> bottom as possible.
>
> Enjoy your xmas!
>
>
> NT
Um 175 dec C? You have ceramic packages? You have characterized a diode on
the chip to determine Tj? Diodes on the output? Glue?
This would go far better if you just stated what you are trying to achieve
rather than discuss op amp design.