Discussing audio amplifier design -- BJT, discrete

"Jon Kirwan is a FUCKWIT TROLL "


DO NOT FEED THE TROLL !!!!!!!!!!!!!!!

DO NOT FEED THE TROLL !!!!!!!!!!!!!!!


TROLLS are DESTROYERS of all NEWSGROUPS.
-------------------------------------------------------------



... Phil

On Tue, 26 Jan 2010 16:15:45 -0800, John Larkin
jjlarkin_at_highNOTlandTHIStechnologyPART.com> wrote:

On Tue, 26 Jan 2010 12:57:13 -0800, Jon Kirwan
jonk_at_infinitefactors.org> wrote:

I'd like to take a crack at thinking through a design of an
audio amplifier made up of discrete BJTs and other discrete
parts as an educational process.

.......<snip>.........

Back when transistors were young, and transistor manuals (GE,
RCA)
were published, there were tons of such circuits around. They
all
pretty much converged to a few forms, and haven't changed much
since.

I remember reading in popular electronics about some audio
amps that I couldn't even come close to following at the
time. The series of them with the name 'tiger' in them.

On Tue, 26 Jan 2010 12:57:13 -0800, Jon Kirwan
jonk_at_infinitefactors.org> wrote:

I'd like to take a crack at thinking through a design of an
audio amplifier made up of discrete BJTs and other discrete
parts as an educational process.

snip

When I was first getting interested in power amp
design (back in the '70s) I started collecting
schematics for all the power amps I could get my
hands on, to compare them. I noticed that the
schematics for simple bipolar op-amp ICs were
remarkably similar to those for big discrete power
amps. If you have an old National Linear Databook
(or don't mind a lot of rooting around on the Web
for individual datasheets), you might take a look.

You can build a pretty decent amp with only a
handful of transistors. The same basic circuit
can be used for a wide range of output powers,
just by changing the power supply voltages and the
output device ratings.

Best regards,

Jon Kirwan wrote:
On Tue, 26 Jan 2010 16:15:45 -0800, John Larkin
jjlarkin_at_highNOTlandTHIStechnologyPART.com> wrote:

On Tue, 26 Jan 2010 12:57:13 -0800, Jon Kirwan
jonk_at_infinitefactors.org> wrote:

I'd like to take a crack at thinking through a design of an
audio amplifier made up of discrete BJTs and other discrete
parts as an educational process.

......<snip>.........


Back when transistors were young, and transistor manuals (GE,
RCA)
were published, there were tons of such circuits around. They
all
pretty much converged to a few forms, and haven't changed much
since.

I remember reading in popular electronics about some audio
amps that I couldn't even come close to following at the
time. The series of them with the name 'tiger' in them.


That was probably an adaptation of RCA's 70-watt power amp,
published in their transistor manual of the mid-60s. A true
70-watt continuous output was pretty hefty then. Still is, in
fact, for many applications. The design was state-of-the-art,
using their 4000 series transistors which were specifically made
for audio. THD was <0.25% at 70W (pretty good for the time).

IIRC, Popular Electronics published it virtually unchanged (I
don't think they credited RCA with the design). I vaguely recall
their calling it 'tiger' something and the article title included
the word "indestructible". The latter term was because it
included current clamping for the output transistors as
short-circuit protection, a thermal fuse and a normal fuse. PE
claimed that they abused it with short-circuits and reactive
loads, and the worst they got was a blown fuse.

I remember that design fondly, if not perfectly, because I spent
a lot of time analysing it and others in the RCA manual.

Jon Kirwan wrote:
On Tue, 26 Jan 2010 16:15:45 -0800, John Larkin
jjlarkin_at_highNOTlandTHIStechnologyPART.com> wrote:

On Tue, 26 Jan 2010 12:57:13 -0800, Jon Kirwan
jonk_at_infinitefactors.org> wrote:

I'd like to take a crack at thinking through a design of an
audio amplifier made up of discrete BJTs and other discrete
parts as an educational process.

......<snip>.........


Back when transistors were young, and transistor manuals (GE,
RCA)
were published, there were tons of such circuits around. They
all
pretty much converged to a few forms, and haven't changed much
since.

I remember reading in popular electronics about some audio
amps that I couldn't even come close to following at the
time. The series of them with the name 'tiger' in them.

That was probably an adaptation of RCA's 70-watt power amp,
published in their transistor manual of the mid-60s. A true
70-watt continuous output was pretty hefty then. Still is, in
fact, for many applications. The design was state-of-the-art,
using their 4000 series transistors which were specifically made
for audio. THD was <0.25% at 70W (pretty good for the time).

IIRC, Popular Electronics published it virtually unchanged (I
don't think they credited RCA with the design). I vaguely recall
their calling it 'tiger' something and the article title included
the word "indestructible". The latter term was because it
included current clamping for the output transistors as
short-circuit protection, a thermal fuse and a normal fuse. PE
claimed that they abused it with short-circuits and reactive
loads, and the worst they got was a blown fuse.

I remember that design fondly, if not perfectly, because I spent
a lot of time analysing it and others in the RCA manual.

On Wed, 27 Jan 2010 17:31:00 +1000, David Eather
eather_at_tpg.com.au> wrote:

Jon Kirwan wrote:
On Tue, 26 Jan 2010 16:15:45 -0800, John Larkin
jjlarkin_at_highNOTlandTHIStechnologyPART.com> wrote:

On Tue, 26 Jan 2010 12:57:13 -0800, Jon Kirwan
jonk_at_infinitefactors.org> wrote:

I'd like to take a crack at thinking through a design of an
audio amplifier made up of discrete BJTs and other discrete
parts as an educational process.

I imagine this will be broken up into three sections; input
transconductance, transimpedance VAS, and output driver. But
other arrangements (such as combining the VAS and output
driver using a signal splitting BJT) would work for me, in
learning.

I said "BJT" and "discrete" but I'm also open to the idea of
using BJT pairs, such as the BCV61 and BCV62. In the case of
current mirrors, that may make sense. But not high-priced,
elite and/or hard to get, or obsolete. And no FETs. This is
to be about learning to design with BJTs.

SMT vs through-hole isn't an issue for learning about a
design, I suppose. If I need to build up some section and
test it with a signal, I'll probably want to do it quickly
and without having to buy services every step of the way. So
I may 'dead bug' SMT parts to get there. (The basic idea
here is to learn, not to make something tiny.)

Although I have some other applications, right now I'd like
the target use to be as a computer speaker system (not unlike
those dirt cheap, sadly almost all of them 10% THD, systems
sold today into this market. Except that I'd like to work
through the design on my own, from start to end.

Given what I understand right now from a very short search on
the topic, the input should be taken as a maximum of 1.0Vrms
and the input's load should appear to be something like 10k
ohms. If someone knows different from that, I'll accept the
criticism and change that spec.

I'd like to consider a tone control and a volume control to
be included.

Output is to be into a small 8 ohm speaker. With that
maximum 1.0Vrms at the input and the volume control set to
maximum the wattage into 8 ohms should be around 10 watts.
Since human hearing won't tell much difference between 8
watts and 12 watts, this is a bit of a sloppy spec and I'm
open to anything in the area of 5-20 watts... though I'm
really wanting to keep the rail voltages down to something
modest and the BJTs not having to tolerate hugish Vce.

Now that I say this, an odd idea comes to mind because the
CFL light bulbs include two TO-220 BJTs that can handle quite
a high Vce on them. I could cannibalize those. But to be
honest, I'm still not needing high watt outputs. So there's
no reason to think about scavenging such parts.

I would like to design it to work into 4 ohms as a margin
bound and not as a design goal, but even 5.6 ohms would be
acceptable.

I'm not looking for this to be done quickly, either. If it
takes months of only occasional back-and-forth, I'm fine with
that. Also, I expect to do my work and don't expect someone
else to hand-hold me from complete ignorante to complete
enlightenment. I just need someone to slap my face when
I say something terribly stupid and/or point in a truely
useful direction when I need it. Or else someone who is
wanting to explore this with me and willing to work for it.

Is anyone here willing to consider a sincere discussion?

Jon
Back when transistors were young, and transistor manuals (GE, RCA)
were published, there were tons of such circuits around. They all
pretty much converged to a few forms, and haven't changed much since.
I remember reading in popular electronics about some audio
amps that I couldn't even come close to following at the
time. The series of them with the name 'tiger' in them.

I'm not so much interested in _circuits_, per se, as I am in
learning about topologies, various ideas built upon them, and
then the specific details of designing towards a specific
implementation. For example, I enjoyed learning about
bootstrapping as a general idea _and_ as it applies to a
couple of specific areas. Having both theory _and_ specific
practice helps firm up the ideas better.

I could post some circuits from the old manuals, it that wouldn't
spoil what you want to do.
It may serve as a point of discussion. Would you be willing
to discuss their details and the broader theories as applied?

10 watts into computer speakers sounds like a lot. Most AM radios
didn't make one watt. You might experiment first to see how much power
you really need.
Oh, I figure one watt is enough, too. As a practical matter
and as a consumer using a device and not as a designer trying
to learn something. That's what... 3Vrms? Into 8 ohms? A
voltage gain of 3, given 1Vrms input? I'm wanting to learn
some things, not place one BJT (okay, not really, but it
almost seems like that) down as an emitter follower and then
calling it good. ;)

Up front, I thought I'd like to deal with perhaps something
on the order of about 10Vrms into 8 ohms. I figured that is
enough 'bad' that I'd have to cope with some interesting
corners along the way; but not enough 'bad' that I'd have to
deal with too much all at once.

For example, at around 10 watts or so, it's enough that I may
need to seriously consider avoiding class-A operation of the
output stage and move to class-B, instead. But it is low
enough that there is some room to discuss each, as well as
class-AB biasing, too. More power and I'm almost certain I'm
pushed into class-B. Less power and.. well, who cares that
much? At one watt or so, just class-A and be done with it? I
won't learn the reasoning behind trade-offs that way.

There's more. I just figured at about 10 watts I'm likely to
learn some things but not be forced to learn so much that I'm
overwhelmed.

I'm open to specific advice about all this, of course.

Jon
I like 10 watts as a starting size - at this size you have to start
doing things the way the big amps do, but it is not so big as to be
outrageously expensive, for example you still use a relatively small
power supply, heatsinks, and inexpensive transistors,

You appear to confirm my instincts.

and at the end you
can use it with your PC and really blow those 320 watt PFPO (peak
fantasy power output)speakers away.

Well, mostly I'm just trying to learn... not impress others
about the results. :)

I like your hesitation on class A. You want an amp with some power
output and class A is very inefficient, never more than 25% and often
way less. This would add greatly to the cost - a 40 watt power supply,
heatsinks capable of getting rid of the same as heat while keeping the
transistor junction temperature low, and beefier transistors. You also
get to put up with a shorter service life from all that heat. The "big
thing" with class A is there is no crossover distortion, which can lead
to better overall distortion figures, but the cost is huge - a kit for 2
x 20 watt class A sells for $600.

Egads. My instincts said class-A would add a lot to weight
and cost, but no idea a mere 20W kit could sell for $300!

My particular bias for an amp this size is to go class AB with a split
power supply. The majority of quality audio amps follow this topology
and this is, I think, I great reason to go down this design path (what
you learn is applicable in the most number of situations). I should hunt
down a schematics of what I'm seeing in the distance (which can/will
change as decisions are made) - some of the justifications will have to
wait

I'm fine with taking things as they come.

As far as the class, I guessed that at 10 watts class-A would
be too power-hungry and probably not worth its weight but
that class-AB might be okay.

I have to warn you, though, that I'm not focused upon some
20ppm THD. I'd like to learn, not design something whose
distortion (or noise, for that matter) is around a bit on a
16-bit DAC or less. I figure winding up close to class-B
operation in the end. But I'd like to take the walk along
the way, so to speak.

The first step is to think about the output. The basic equations are

(1).....Vout = sqrt(2*P*R)

With R as 8 ohms for a common speaker and 10 watts that is 12.7 volts -
actually +/- 12.7 volts with a split power supply.

If you don't mind, I'd like to discuss this more closely. Not
just have it tossed out. So, P=V*I; or P=Vrms^2/R with AC.
Using Vpeak=SQRT(2)*Vrms, I get your Vpeak=SQRT(2*P*R)
equation. Which suggests the +/-12.7V swing. Which further
suggests, taking Vce drops and any small amounts emitter
resistor drops into account, something along the lines of +/-
14-15V rails?

Or should the rails be cut a lot closer to the edge here to
improve efficiency. What bothers me is saturation as Vce on
the final output BJTs goes well below 1V each and beta goes
away, as well, rapidly soaking up remaining drive compliance.

(2).....Imax = sqrt(2*P/R)

This comes out to 1.6 amps. You should probably also consider the case
when R speaker = 4 ohms when initially selecting a transistor for the
output 2.2 amps - remember this is max output current. The power supply
voltage will have to be somewhat higher than Vout to take into account
circuit drive requirements, ripple on the power supply and transformer
regulation etc.

Okay. I missed reading this when writing the above. Rather
than correct myself, I'll leave my thinking in place.

So yes, the rails will need to be a bit higher. Agreed. On
this subject, I'm curious about the need to _isolate_, just a
little, the rails used by the input stage vs the output stage
rails. I'm thinking an RC (or LC for another pole?) for
isolation. But I honestly don't know if that's helpful, or
not.

Are you OK with connecting mains to a transformer? or would you rather
use an AC plug pack (10 watts is about the biggest amp a plugpack can be
used for)? The "cost" for using an AC plug pack is you will need larger
filter capacitors.

I'd much prefer to __avoid__ using someone else's "pack" for
the supply. All discrete parts should be on the table, so to
speak, in plain view. And I don't imagine _any_ conceptual
difficulties for this portion of the design. I'm reasonably
familiar with transformers, rectifiers, ripple calculations,
and how to consider peak charging currents vs averge load
currents as they relate to the phase angles available for
charging the caps. So on this part, I may need less help
than elsewhere. In other words, I'm somewhat comfortable
here.

I should also ask if you have a multi meter, oscilloscope (not necessary
but useful)and how is your soldering? But it would be wise to keep this
whole thing as a paper exercise before you commit to anything.

I have a 6 1/2 digit HP multimeter, a Tek DMM916 true RMS
handheld, two oscilloscopes (TEK 2245 with voltmeter option
and an HP 54645D), three triple-output power supplies with
two of them GPIB drivable, the usual not-too-expensive signal
generator, and a fair bunch of other stuff on the shelves.
Lots of probes, clips, and so on. For soldering, I'm limited
to a Weller WTCPT and some 0.4mm round, 0.8mm spade, and
somewhat wider spade tips in the 1.5mm area. I have tubs and
jars of various types of fluxes, as well, and wire wrap tools
and wire wrap wire, as well. I also have a room set aside
for this kind of stuff, when I get time to play.

Jon

Have a look at
http://en.wikipedia.org/wiki/Electronic_amplifier
The bits on class A might be interesting as it says 25% efficiency and 50%
obtainable with inductive output coupling (i.e. with a transformer)
which is what I said, not what blow hard Phil said.

"David Eather = a LIAR and a Wanker "


** Learn to trim - wanker!!

I like your hesitation on class A. You want an amp with some power
output and class A is very inefficient, never more than 25% and often
way less.
** Class A amplifiers are up to 50 % efficient.
Yes, if you use a matching transformer,

** 100%, absolute BULLSHIT !!!!!!!

YOU KNOW NOTHING MORON !!!!!!!

This would add greatly to the cost - a 40 watt power supply,
** 20 watts is all that is needed.
IBID

** Fuck you - you damn imbecile.

heatsinks capable of getting rid of the same as heat while keeping the
transistor junction temperature low,
** Junction temps can settle at 125 C with no problems.
Most transistors are spec'ed at 125 C for 1000 or 2000 hours.


** More, 100% absolute BULLSHIT !!!!!!!

This RATBAG just keeps piling on the LIES !!!!!

people would like there devices to run a bit longer than this. At least
one semiconductor manufacturer believes that for each lowering of the
junction temperature by 10 C doubles the life of transistor


** MORE 100% absolute BULLSHIT !!!!!!!

This RATBAG just keeps piling on the damn LIES !!

You also get to put up with a shorter service life from all that heat.
** Bull.
I think just about everyone will disagree with you on that point.


** What some fuckwit like you choses to "think" is of zero consequence.


heat dries out electrolytics

** ROTFLMAO !!!

Shame BJTs are not electrolytic.

You FUCKING MORON !!!!!!!!


and heat stress cycling is a major failure mode of semiconductors


** No heat cycling occurs with class A.

Huge amounts occur with all class B and AB designs.

You FUCKING KNOW NOTHING MORON !!!!!!!!

The "big thing" with class A is there is no crossover distortion, which
can lead to better overall distortion figures, but the cost is huge - a
kit for 2 x 20 watt class A sells for $600.
** Irrelevant what some unspecified kit sells for.

No, it is an example of how costly class A can become.

** Absolute fucking CRAP !!!!!!!!

YOU are an example of how STUPID people can become.

An extreme example of an damn LIAR too.

A 20 watt class A power stage ONLY needs bigger heatsinks compared to
usual low bias, class AB operation.

** No big expense there.



My particular bias for an amp this size is to go class AB with a split
power supply. The majority of quality audio amps follow this topology
** Bollocks.
Well, somewhat recently the trend has gone to mono block IC amps so I'll
take that on the chin

** Go drop dead - you pig ignorant LYING WANKER !


Only some very high powered hi-fi and pro-audio amps use additional DC
rails.

So what.


** Proves you WRONG - you stinking LIAR !!!!!!

The power supply voltage will have to be somewhat higher than Vout to
take into account circuit drive requirements, ripple on the power supply
and transformer regulation etc.

** There is no variation in the DC current draw from a PSU with class A
amplifiers - in fact, this is the very definition of class A operation
of an audio amp. So the DC rails will remain steady from no drive to
full output.
True!

** Proves you WRONG AGAIN - you stinking LIAR !!!!!!

Also, the heatsink will cool considerably when the amp is operated at
full
sine wave power.

Not relevant at the moment.

** Course it is - you DAMN LIAR !!!!!!!!!
How is the heat sink (currently) relevant when there is no spec for the

GO DROP FUCKING DEAD

- YOU BULLSHITTING IMBECILE !!!!!!!!!




..... Phil

"David Eather is a Fuckwit LIAR"

Have a look at
http://en.wikipedia.org/wiki/Electronic_amplifier
The bits on class A might be interesting as it says 25% efficiency and 50%
obtainable with inductive output coupling (i.e. with a transformer)
which is what I said, not what blow hard Phil said.


** ROTFLMAO !!!

So this Eather WANKER gets his WRONG info from bloody Wiki !!!!

Here is the actual quote:

" Class A amplifiers are the usual means of implementing small-signal
amplifiers. They are not very efficient; a theoretical maximum of 50% is
obtainable with inductive output coupling and only 25% with capacitive
coupling."

So, the para is clearly about "small signal" class A stages

- ie RC coupled pre-amp stages !!!!!!!

Not class A ** POWER AMPS ** !!!!!

Wot a fucking FUCKWIT !!

BTW

Inductive coupling also refers to the use of chokes as the collector or
plate loads for ( single ended ) class A operation.

Way over this Eather Google Monkey's pointy head.



..... Phil

Have a look at
http://en.wikipedia.org/wiki/Electronic_amplifier
The bits on class A might be interesting as it says 25% efficiency and
50% obtainable with inductive output coupling (i.e. with a transformer)
which is what I said, not what blow hard Phil said.


** ROTFLMAO !!!

So this Eather WANKER gets his WRONG info from bloody Wiki !!!!

Here is the actual quote:

" Class A amplifiers are the usual means of implementing small-signal
amplifiers. They are not very efficient; a theoretical maximum of 50% is
obtainable with inductive output coupling and only 25% with capacitive
coupling."

So, the para is clearly about "small signal" class A stages

- ie RC coupled pre-amp stages !!!!!!!

Not class A ** POWER AMPS ** !!!!!

Wot a fucking FUCKWIT !!

BTW

Inductive coupling also refers to the use of chokes as the collector or
plate loads for ( single ended ) class A operation.

Way over this Eather Google Monkey's pointy head.



So how are class A POWER AMPS more efficient?

Jon Kirwan wrote:

On Wed, 27 Jan 2010 17:31:00 +1000, David Eather
eather_at_tpg.com.au> wrote:
snip

My particular bias for an amp this size is to go class AB with a split
power supply. The majority of quality audio amps follow this topology
and this is, I think, I great reason to go down this design path (what
you learn is applicable in the most number of situations). I should hunt
down a schematics of what I'm seeing in the distance (which can/will
change as decisions are made) - some of the justifications will have to
wait

I'm fine with taking things as they come.

As far as the class, I guessed that at 10 watts class-A would
be too power-hungry and probably not worth its weight but
that class-AB might be okay.

I have to warn you, though, that I'm not focused upon some
20ppm THD. I'd like to learn, not design something whose
distortion (or noise, for that matter) is around a bit on a
16-bit DAC or less. I figure winding up close to class-B
operation in the end. But I'd like to take the walk along
the way, so to speak.

10 watts / PPM thd? Mmmm... maybe more like .1 - .05 % are realistic and
a few detours to see what would help or harm that.

The first step is to think about the output. The basic equations are

(1).....Vout = sqrt(2*P*R)

With R as 8 ohms for a common speaker and 10 watts that is 12.7 volts -
actually +/- 12.7 volts with a split power supply.

If you don't mind, I'd like to discuss this more closely. Not
just have it tossed out. So, P=V*I; or P=Vrms^2/R with AC.
Using Vpeak=SQRT(2)*Vrms, I get your Vpeak=SQRT(2*P*R)
equation. Which suggests the +/-12.7V swing. Which further
suggests, taking Vce drops and any small amounts emitter
resistor drops into account, something along the lines of +/-
14-15V rails?

Or should the rails be cut a lot closer to the edge here to
improve efficiency. What bothers me is saturation as Vce on
the final output BJTs goes well below 1V each and beta goes
away, as well, rapidly soaking up remaining drive compliance.

(2).....Imax = sqrt(2*P/R)

This comes out to 1.6 amps. You should probably also consider the case
when R speaker = 4 ohms when initially selecting a transistor for the
output 2.2 amps - remember this is max output current. The power supply
voltage will have to be somewhat higher than Vout to take into account
circuit drive requirements, ripple on the power supply and transformer
regulation etc.

Okay. I missed reading this when writing the above. Rather
than correct myself, I'll leave my thinking in place.

So yes, the rails will need to be a bit higher. Agreed. On
this subject, I'm curious about the need to _isolate_, just a
little, the rails used by the input stage vs the output stage
rails. I'm thinking an RC (or LC for another pole?) for
isolation. But I honestly don't know if that's helpful, or
not.

Mostly not needed, if you use a long tailed pair for the input / error
amplifier, but you might prefer some other arrangement so keep it in
mind if your circuit "motorboats"

Are you OK with connecting mains to a transformer? or would you rather
use an AC plug pack (10 watts is about the biggest amp a plugpack can be
used for)? The "cost" for using an AC plug pack is you will need larger
filter capacitors.

I'd much prefer to __avoid__ using someone else's "pack" for
the supply. All discrete parts should be on the table, so to
speak, in plain view. And I don't imagine _any_ conceptual
difficulties for this portion of the design. I'm reasonably
familiar with transformers, rectifiers, ripple calculations,
and how to consider peak charging currents vs averge load
currents as they relate to the phase angles available for
charging the caps. So on this part, I may need less help
than elsewhere. In other words, I'm somewhat comfortable
here.

Ah, then there are questions of what voltage and VA for a transformer.
So there are questions of usage (music, PA, PA with an emergency alert
siren tied in etc) and rectifier arrangement and capacitor size /
voltage to get your required voltage output at full load.

I should also ask if you have a multi meter, oscilloscope (not necessary
but useful)and how is your soldering? But it would be wise to keep this
whole thing as a paper exercise before you commit to anything.

I have a 6 1/2 digit HP multimeter, a Tek DMM916 true RMS
handheld, two oscilloscopes (TEK 2245 with voltmeter option
and an HP 54645D), three triple-output power supplies with
two of them GPIB drivable, the usual not-too-expensive signal
generator, and a fair bunch of other stuff on the shelves.
Lots of probes, clips, and so on. For soldering, I'm limited
to a Weller WTCPT and some 0.4mm round, 0.8mm spade, and
somewhat wider spade tips in the 1.5mm area. I have tubs and
jars of various types of fluxes, as well, and wire wrap tools
and wire wrap wire, as well. I also have a room set aside
for this kind of stuff, when I get time to play.

OK. Next serious project, I'm coming around to your place!

Your gear is
better than mine. I had to ask, rather than just assume just in case my
assumptions got you building something you didn't want to, and got you
splattered all over the place from the mains, or suggesting you choose
the miller cap by watching the phase shift of the feedback circuit - I
don't read a lot of the posts so I didn't know what you could do.

Jon

Have a look at
http://en.wikipedia.org/wiki/Electronic_amplifier

The bits on class A might be interesting as it says 25% efficiency and
50% obtainable with inductive output coupling (i.e. with a transformer)
which is what I said, not what blow hard Phil said.

The amplifier, it
seems to me, dictates the parameters. So that comes later,
doesn't it?

On Thu, 28 Jan 2010 11:17:02 +1000, David Eather
eather_at_tpg.com.au> wrote:


Have a look at
http://en.wikipedia.org/wiki/Electronic_amplifier

Done.

The bits on class A might be interesting as it says 25%
efficiency
and 50% obtainable with inductive output coupling (i.e. with a
transformer) which is what I said, not what blow hard Phil
said.

What I first see there is the amplifier sketch at the top of
the page (I don't really care too much about arguing about
efficiencies right now -- I'm more concerned about learning.)
The input stage shown is a voltage-in, current-out bog
standard diff-pair. First thing I remember about is that R4
shouldn't be there and better still both R3 and R4 should be
replaced with a current mirror. R5 should be a replaced with
a BJT, as well. I assume the input impedance of that example
is basically the parallel resistance of R1 and R2, but if we
use split supplies I'd imagine replacing the two of them with
a single resistor to the center-ground point. There's no
miller cap on Q3, I'd probably replace the two diodes with
one of those BJT and a few resistor constructions I can't
remember the name of (which allows me to adjust the drop.)
The feedback ... well, I need to think about that a little
more. There's no degen resistors in the emitters of Q4 and
Q5.

Jon Kirwan wrote:
On Thu, 28 Jan 2010 11:17:02 +1000, David Eather
eather_at_tpg.com.au> wrote:


Have a look at
http://en.wikipedia.org/wiki/Electronic_amplifier

Done.

The bits on class A might be interesting as it says 25%
efficiency
and 50% obtainable with inductive output coupling (i.e. with a
transformer) which is what I said, not what blow hard Phil
said.

What I first see there is the amplifier sketch at the top of
the page (I don't really care too much about arguing about
efficiencies right now -- I'm more concerned about learning.)
The input stage shown is a voltage-in, current-out bog
standard diff-pair. First thing I remember about is that R4
shouldn't be there and better still both R3 and R4 should be
replaced with a current mirror. R5 should be a replaced with
a BJT, as well. I assume the input impedance of that example
is basically the parallel resistance of R1 and R2, but if we
use split supplies I'd imagine replacing the two of them with
a single resistor to the center-ground point. There's no
miller cap on Q3, I'd probably replace the two diodes with
one of those BJT and a few resistor constructions I can't
remember the name of (which allows me to adjust the drop.)
The feedback ... well, I need to think about that a little
more. There's no degen resistors in the emitters of Q4 and
Q5.

Either the circuit was designed by someone with a limited
experience or it was deliberately presented this way for clarity
as an illustration of the basic topology.

In a practical design using an unregulated power supply, R1
should be split into two with a capacitor from the split point to
ground. This is to decouple the input stage for stability as well
as for hum filtering.

R6 should also be split and the split point
bootstrapped with a capacitor to the mid point of the output
stage.

Talking about RCA's 70W amp got me nostalgic about those days.
Here's a 1W amp using _germanium_ transistors:
http://img716.imageshack.us/img716/2583/1wamp.png
This is one of my early solid-state designs based, of course, on
topologies I'd learned by studying others' designs. It's no hi-fi
by any stretch of imagination, but I actually constructed a few
of these in the early 70s for myself and for friends. One of them
fed the input from an early Sony Walkman to drive an 8-inch
Philips dual-cone "Hi-Q" speaker and gushed over how good it
sounded!