Discussing audio amplifier design -- BJT, discrete

[Jon Kirwan]

I've seen R3 used in that position too, but never gave it much
thought until you brought it up. Offhand I still can't see a
reason for it either. Maybe for stability against a local
oscillation? Perhaps taking some time to think about it will
bring some revelation. Or someone else can save us the trouble
and enlighten us.
<snip>

I had earlier said I thought you might be right about this R3
value. Now, I don't. I think it deals with something else
-- unregulated rail voltage variations.

In this thread, you've posted circuits with a resistor on one
side and the VAS on the other of this structure. The VAS
yanks one end around while the other side mostly follows it
around. However, the current through the resistor varies, of
course. Even if one of those BJTs+2 diodes thingies is used
for a current source instead of the resistor, which does
improve things, it still isn't very constant. Using the 2
BJT structure doesn't change that fact, though it does impact
variations. No matter how you arrange it, resistor or
current source, the fact is that the current into the Vbe
multiplier device changes around as the VAS yanks around one
side of it.

This variation means that Q1's Ic varies. To accomodate that
variation, Vbe varies. Since Vbe varies, so does the
multiplied value. And for no _other_ reason than variations
in signal. That changes the bias. Changing the bias changes
the quiescent current. Etc.

(Also, I suppose, the Early effect will add yet another
slight modification, since the Vce is slightly changed so is
the Ic for the same Vbe. The higher required Ic (assuming
the current source or resistor is supplying more current,
instead of less) requires a higher Vbe, as stated. So the
multiplied voltage at Vce is higher. But that multipled
voltage also slides over on the Vce axis for whatever Vbe
that has become and that suggests still more Ic due to Early
effect, so it is a positive feedback contributing to the
already existing problem, I think. I haven't tried to work
out just what percent it contributes, though.)

So a cludge fix for this is to insert a resistor in the
collector, which will act in the opposite direction to some
degreee. I'd imagine this would create a second degree poly
curve, with a maximum somewhere but gentle 'arms' outward,
which means less variation of the Vbe-multiplied value with a
tweakable peak point based upon a nominal Ic.

I imagine this is NOT nearly as important for class-A
operation, though, since it is already "biased up" and
variation at that point of operation probably isn't so
important. It _would_ matter, I think, in other classes of
operation.

Thinking as I am that I don't want to go with class-A, I am
trying to think of still better ways of replacing R3 with (or
adding) an active device to further improve it. Anyone have
a suggestion there?

Jon

 

[Jon Kirwan]

I think it deals with something else
-- unregulated rail voltage variations.

__and__ variations due to the VAS driving the output stage.

Jon

 

[Jon Kirwan]

<snip>
(Also, I suppose, the Early effect will add yet another
slight modification, since the Vce is slightly changed so is
the Ic for the same Vbe. The higher required Ic (assuming
the current source or resistor is supplying more current,
instead of less) requires a higher Vbe, as stated. So the
multiplied voltage at Vce is higher. But that multipled
voltage also slides over on the Vce axis for whatever Vbe
that has become and that suggests still more Ic due to Early
effect, so it is a positive feedback contributing to the
already existing problem, I think. I haven't tried to work
out just what percent it contributes, though.)
<snip>

I'm rethinking this. I think the Early effect counters the
effect, slightly. The higher Ic does increase Vbe. The
increased Vbe does increase Vce. So Vce is higher and we are
also on a different Vbe curve in the Vce vs Ic graph where
the Early effect shows up clearly. But Ic is 'given' because
it is being divided by the structure, so I should have
rotated the chart in my mind and using Ic as the independent
variable. Had I done that, I would have 'seen' that a given
rise in Ic would have suggested (holding Vbe constant for a
moment) a certain change in Vce. But that a now slightly
higher Vbe would have chosen a new Vbe curve that (before
rotating the chart) is _above_ the earlier Vbe curve, which
will cut the new Vbe curve where that same Ic intersects it a
little sooner on the Vce axis. Thus, it acts against.

My gut was telling me that nature works to oppose change and
I should have gone with that instinct, I think.

The effect is small. On the order of about 0.1%, roughly.

Jon

 

[Jon Kirwan]

I had earlier said I thought you might be right about this R3
value. Now, I don't. I think it deals with something else
-- unregulated rail voltage variations.

In this thread, you've posted circuits with a resistor on one
side and the VAS on the other of this structure. The VAS
yanks one end around while the other side mostly follows it
around. However, the current through the resistor varies, of
course. Even if one of those BJTs+2 diodes thingies is used
for a current source instead of the resistor, which does
improve things, it still isn't very constant. Using the 2
BJT structure doesn't change that fact, though it does impact
variations. No matter how you arrange it, resistor or
current source, the fact is that the current into the Vbe
multiplier device changes around as the VAS yanks around one
side of it.

This variation means that Q1's Ic varies. To accomodate that
variation, Vbe varies. Since Vbe varies, so does the
multiplied value. And for no _other_ reason than variations
in signal. That changes the bias. Changing the bias changes
the quiescent current. Etc.

(Also, I suppose, the Early effect will add yet another
slight modification, since the Vce is slightly changed so is
the Ic for the same Vbe. The higher required Ic (assuming
the current source or resistor is supplying more current,
instead of less) requires a higher Vbe, as stated. So the
multiplied voltage at Vce is higher. But that multipled
voltage also slides over on the Vce axis for whatever Vbe
that has become and that suggests still more Ic due to Early
effect, so it is a positive feedback contributing to the
already existing problem, I think. I haven't tried to work
out just what percent it contributes, though.)

So a cludge fix for this is to insert a resistor in the
collector, which will act in the opposite direction to some
degreee. I'd imagine this would create a second degree poly
curve, with a maximum somewhere but gentle 'arms' outward,
which means less variation of the Vbe-multiplied value with a
tweakable peak point based upon a nominal Ic.

I imagine this is NOT nearly as important for class-A
operation, though, since it is already "biased up" and
variation at that point of operation probably isn't so
important. It _would_ matter, I think, in other classes of
operation.

Thinking as I am that I don't want to go with class-A, I am
trying to think of still better ways of replacing R3 with (or
adding) an active device to further improve it. Anyone have
a suggestion there?

Jon

Side bar:

The small signal analysis of the Vbe multiplier, if I got it
right, is based squarely upon the small-re of the BJT which
is, itself: (kT/q)/Ic. There is, of course, also the value
of R2, but since its effect is only affected by the change in
base current, it's contribution is divided by beta. So the
actual equation is something like:

R = (1/Ic)*(kT/q)*(1+R2/R1) + R2/beta

For a 2X multiplier where R2=R1, this is:

R = (2/Ic)*(kT/q) + R2/beta

Note the variation on Ic! (We _want_ the variation on T but
do NOT want the variation on Ic.) With R2=1k, for example,
that part contributes 5 ohms. But with Ic=5ma, for example,
the other part of it is sitting at 10.4 ohms -- a total of
15.4 ohms. So a variation of half an mA in Ic suggests a
7.7mV change in the bias point, ignoring any residual Early
effect on it.

I mentioned the Early effect being on the order of 0.1%. It
took me a moment to think about it, but the figure works out
to something like this:

R_early = dV/dI = -Ic/VA*R^2

It's a negative resistance that adds to R. If R is 15.4 ohms
and Ic is around 5mA and VA=100V, for example, you get about
-10mOhms. Which is roughly a factor of 1000 less than 15.4
Ohms. Which is where I get the 0.1% as a ballpark estimate.

The fuller equation would be:

R = (2/Ic)*(kT/q) + R2/beta - Ic/Va*R^2

Which requires a quadratic solution to solve for R. I'm
comfortable, for now, that I can ignore the Early effect and
just focus on the broader R figure for the Vbe multiplier,
applying changes in Ic to it to see how the voltage shifts
around.

So, in the diagram, R3 now reduces the effect by that change
in Ic*R3. If R3 is on the order of the above computed R,
that generally sets things so that right at that Ic used to
compute R the effect of R3 will be just in the right amount
to compensate for nearby changes in Ic.

I think.

Jon

 

[pimpom]

In your "unbiased" circuit, try adding a 1K resistor from the
bases to
the output. Now the flat spots in the output waveform become
slopes.
But the transfer function is now continuous, so negative
feedback can
reduce distortion without ugly slewing problems. And with zero
bias,
there's no idle power dissipation and no possibility of thermal
runaway.

The second circuit, with the bias diodes, is a likely firebomb.

John

I thought it would be obvious that those were simplified
arrangements purely for illustration. No one in his right mind
would think of using them in a practical audio amp.

 

[pimpom]

Mag tapes had inherently gross crossover distortion. The fix
was to
add a pretty high-level "bias" oscillator to the record path to
smear
it out. The bias voltage might be 20 volts at 60 KHz, way
bigger than
the record signal going into the head.

DC bias of the recording head was also used in many cheap
portables. The erase head was a permanent magnet that swiveled
out of the way on playback. On recording, the erase head
magnetised the tape to saturation in one direction. The dc bias
is polarised in the opposite direction, with enough strength to
place the operating point in the linear region. The noise level
is higher than with AC erase and bias, but it works.

 

[Paul E. Schoen]

DC bias of the recording head was also used in many cheap portables. The
erase head was a permanent magnet that swiveled out of the way on
playback. On recording, the erase head magnetised the tape to saturation
in one direction. The dc bias is polarised in the opposite direction,
with enough strength to place the operating point in the linear region.
The noise level is higher than with AC erase and bias, but it works.

I had one of those cheap cassette recorders, and it worked OK. But I had an
8-track tape player in my car and I wanted to be able to record, so I built
an AC bias circuit (I think mine was 40 kHz), using a circuit that I found
in an old databook. It incorporated the RIAA non-linear amplitude curve as
well. I also made a device which used a cheap turntable and crystal pickup,
with two J-FET (2N3819) linear amplifiers and VU meters.

http://en.wikipedia.org/wiki/RIAA_equalization
http://freecircuitdiagram.com/2008/...amp-a-pre-amplifier-with-riaa-response-curve/

This was in 1970, and the only decent piece of test equipment I had was a
refurbished HP140A scope, with 100 kHz bandwidth and a fast blue phosphor
CRT made for photography. I still have everything except the 8-track tape
player for the car. It was stolen, along with most of my tapes, when I
foolishly neglected to put them in the trunk when I parked in a marginally
bad neighborhood in Washington, DC in 1972.

Paul

 

[Paul E. Schoen]

RIAA is for records. Tape used NAB equalization.

Thanks for clearing that up. I distinctly recall the RIAA curve, but it was
probably because I was recording from records. I may have also included an
NAB curve, but IIRC it was a rather simple circuit. I remember
special-ordering a choke or a transformer, but that may have been for the
bias. This was forty years ago! I don't feel like trudging through the snow
to my other house (which is unheated) just to open up the old equipment or
possibly find the databook which has the circuit. I was just happy to get
the thing to work and I wasn't too critical about the sound quality. I
found an interesting discussion of NAB equalization:
http://home.comcast.net/~mrltapes/equaliz.html

That was for 15 in/sec, which was popular for high quality reel-to-reel. I
think 8 track tapes were 7.5 or 3.75 in/sec. And cassettes were (and
probably still are) 1-7/8 in/sec.

Paul

 

[Paul E. Schoen]

Didn't all that stuff make the car hard to drive?

Not at all. My car was a 1965 Chevy Malibu that was good on the curves.
Most importantly, it had a Class A driver

Paul

 

[pimpom]

Not at all. My car was a 1965 Chevy Malibu that was good on the
curves. Most importantly, it had a Class A driver

Was the driver good on curves _inside_ the car?

 

[Jon Kirwan]

<snip>
It is not a big stress. You can always use the junk-box transformer and
if it really isn't suitable replace it latter. For your consideration -
the RMS power of even compressed samples of music is only about 20% of
the peak.

There are a few variations on that figure. RCA did a lot of research in
the area and found that Radio broadcasts of compressed FM signals of
"Rock Music" - an undefined term, was the most demanding at 15%. Some
companies are trying to redefine that. IRF who call the same figure 1/8
of max power (12.5%) - which just happens to make their newest audio
mosfets look really good. It might be the other way around. They may
really believe it, and designed the mosfets to match. I forget where but
some group stated the 20% figure with respect to new modern music
styles. IIRC they were regarded as technically competent in the area and
had no axe to grind or wheelbarrow to push - so I filed the info away.
In any case an overestimate leads to a more conservative design and 5%
is not much. I'd be wary of definition of "modern music" too - badly
played organ music can be a stream of full amplitude waveforms that only
change in frequency at random intervals.

I'd use the junk box transformer and forget about allowing for the
electricity company slackness and just choose good sized caps that are a
reasonable price. I think a learning experience allows for a little
compromise.

Okay. I'm back to the power supply, again. (I'm convinced
that my junkbox unit will work fine -- I think it can hold
maybe 18V minimum under load on each rail. Which seems more
than enough headroom for 12.7V, plus output stage overhead.

I take a little issue with your use of terms in this phrase,
"RMS power of even compressed samples of music is only about
20% of the peak." Power is average and I don't think RMS
applies to power. Volts-to-power is a squared-phenomenon. So
are amps-to-power. RMS makes sense for those two. But power
is an average (integrated Joules divided by time.)

So I believe I have to interpret your meaning as suggesting
that the short-term power required (also an average of some
ill-defined kind, I suppose) when playing music can be a
factor of 5 times more than its long-term average power. You
also mentioned a figure as low as 12.5%, which would suggest
a factor of 8 used as a margin instead of 5.

But a requirement to support short-term power levels is
really just a compliance requirement on the power supply
rails, isn't it?

So put another way, if I wanted a long-term average of 10W
output and I wanted the extra margins required to support the
worst case estimate of a factor of 8 for short-term power
bursts, then I'd need to design rails that support a voltage
compliance level substantially higher. The parts would need
to withstand it, too. And because of the much higher rail
voltages that need to be dropped most of the time, the output
BJTs would need to have just that much more capacity to
dissipate.

Or put still another way, assuming that my output swing at
the output stage emitters cannot exceed a magnitude of 15V
and that everything is sized for dissipating 10W, does this
mean the amplifier is a 10W amplifier that can support a peak
of 14W=(15^2/(2*8))? (Which isn't so good, considering your
comments above regarding "music?")

What is meant when one says, '10 watts?'

This gets worse when I consider the class of operation,
doesn't it? I mean, class-B might be specified as 10W into 8
ohms, but wouldn't that be 20W into 4 ohms? But if class-A,
it's pretty much 10W no matter what?

I'm beginning to imagine amplifiers should be specified as to
their peak output voltage compliance into 8, 6, and 4 ohms;
instantaneous and sustained without damage to the unit. For
example, 35V into 8 ohms instantaneous, 15V sustained. Or
80W instantaneous, 15W sustained. That way, someone might
have some knowledge about how well it might handle _their_
music at, say, 15W average power. And could compare that
against another unit specified as 20V into 8 ohms, 15V
sustained.

How does one know what they are buying? What a headache.

Jon

 

[Jon Kirwan]

20V into 8 ohms, 15V sustained.

I mean "20V instantaneous into 8 ohms, 15V sustained."

Jon

 

[Paul E. Schoen]

Okay. I'm back to the power supply, again. (I'm convinced
that my junkbox unit will work fine -- I think it can hold
maybe 18V minimum under load on each rail. Which seems more
than enough headroom for 12.7V, plus output stage overhead.

I take a little issue with your use of terms in this phrase,
"RMS power of even compressed samples of music is only about
20% of the peak." Power is average and I don't think RMS
applies to power. Volts-to-power is a squared-phenomenon. So
are amps-to-power. RMS makes sense for those two. But power
is an average (integrated Joules divided by time.)

So I believe I have to interpret your meaning as suggesting
that the short-term power required (also an average of some
ill-defined kind, I suppose) when playing music can be a
factor of 5 times more than its long-term average power. You
also mentioned a figure as low as 12.5%, which would suggest
a factor of 8 used as a margin instead of 5.

But a requirement to support short-term power levels is
really just a compliance requirement on the power supply
rails, isn't it?

So put another way, if I wanted a long-term average of 10W
output and I wanted the extra margins required to support the
worst case estimate of a factor of 8 for short-term power
bursts, then I'd need to design rails that support a voltage
compliance level substantially higher. The parts would need
to withstand it, too. And because of the much higher rail
voltages that need to be dropped most of the time, the output
BJTs would need to have just that much more capacity to
dissipate.

Or put still another way, assuming that my output swing at
the output stage emitters cannot exceed a magnitude of 15V
and that everything is sized for dissipating 10W, does this
mean the amplifier is a 10W amplifier that can support a peak
of 14W=(15^2/(2*8))? (Which isn't so good, considering your
comments above regarding "music?")

What is meant when one says, '10 watts?'

This gets worse when I consider the class of operation,
doesn't it? I mean, class-B might be specified as 10W into 8
ohms, but wouldn't that be 20W into 4 ohms? But if class-A,
it's pretty much 10W no matter what?

I'm beginning to imagine amplifiers should be specified as to
their peak output voltage compliance into 8, 6, and 4 ohms;
instantaneous and sustained without damage to the unit. For
example, 35V into 8 ohms instantaneous, 15V sustained. Or
80W instantaneous, 15W sustained. That way, someone might
have some knowledge about how well it might handle _their_
music at, say, 15W average power. And could compare that
against another unit specified as 20V into 8 ohms, 15V
sustained.

How does one know what they are buying? What a headache.

Yes, as an extension of what (I think) Mark Twain said, there are lies,
damn lies, statistics, and specifications. Then there is the matter of
testing. An amplifier is a complex entity and its performance depends on
the power supply, the load, its components, environmental conditions, and
the nature of the signal being applied. So it may seem fair to level the
playing field by testing with a pure sine wave at certain frequencies and
determining that it maintains a certain level of maximum distortion without
overheating or shutting down over an extended period of time in a
controlled environment.

But in real life there are many more factors involved, and the actual
performance in an individual situation may vary widely. Power is indeed an
average function, but the ability to provide power involves efficiency and
a duty-cycle rated function of maximum temperature of components, and also
the ability of the power supply to maintain a certain voltage level for
long enough to "ride out" brief peaks in the signal of typical music.

The power that can be supplied to various loads depends largely on
impedance matching. But most solid state amplifiers are capable of
supplying a certain amount of current, so if it is optimized for eight
ohms, it may be able to provide even less continuous power at 4 ohms, but
possibly more peak power.

You have brought up some good points. But for most purposes, an amplifier
rated conservatively at 10W continuous power should be plenty for home
music listening. When pushed beyond its normal limits, much depends on how
the amplifier handles overloads, and your personal threshold of annoyance
when the inevitable distortion occurs.

Paul

 

[Jon Kirwan]

Yes, as an extension of what (I think) Mark Twain said, there are lies,
damn lies, statistics, and specifications. Then there is the matter of
testing. An amplifier is a complex entity and its performance depends on
the power supply, the load, its components, environmental conditions, and
the nature of the signal being applied. So it may seem fair to level the
playing field by testing with a pure sine wave at certain frequencies and
determining that it maintains a certain level of maximum distortion without
overheating or shutting down over an extended period of time in a
controlled environment.

But in real life there are many more factors involved, and the actual
performance in an individual situation may vary widely. Power is indeed an
average function, but the ability to provide power involves efficiency and
a duty-cycle rated function of maximum temperature of components, and also
the ability of the power supply to maintain a certain voltage level for
long enough to "ride out" brief peaks in the signal of typical music.

The power that can be supplied to various loads depends largely on
impedance matching. But most solid state amplifiers are capable of
supplying a certain amount of current, so if it is optimized for eight
ohms, it may be able to provide even less continuous power at 4 ohms, but
possibly more peak power.

You have brought up some good points. But for most purposes, an amplifier
rated conservatively at 10W continuous power should be plenty for home
music listening. When pushed beyond its normal limits, much depends on how
the amplifier handles overloads, and your personal threshold of annoyance
when the inevitable distortion occurs.

Paul

It sure has been an education, so far. Now I am beginning to
understand the technical motivation for LOTS of rails and the
ability to select between them (perhaps automatically) in
those fancy-pants amplifier designs; dropping in (or out)
stacked BJTs as needed. Though I am loathe to even attempt
thinking more about them.

.....

Now, I want axial leaded diodes for the bridge. From
simulating a load of 8 ohms, 1kHz, average power of 10W, and
my secondary winding resistance of 2.6 ohms, I'm finding that
each diode suffers under a quarter watt of dissipation. So,
any recommendations about diodes? Obviously, for a one-off,
cost is not really an issue. How important is 'fast
recovery'? (Outside of its impact on dissipation.) Seems
that anything with 100V or better for reverse voltage
standoff, 1/4 watt or better, should work. Leakage probably
isn't that important (except against as it may add to
dissipation.)

I'm expecting to use caps on the order of perhaps 2.2mF 50V,
to be secure about the rails. But I expect to want to play
with that, once everything is working, to see just how bad I
can make it while seeing what that means for the output. And
then see if I can calculate a prediction that isn't too far
from those results, on paper.

Jon

 

[pimpom]

....

Now, I want axial leaded diodes for the bridge. From
simulating a load of 8 ohms, 1kHz, average power of 10W, and
my secondary winding resistance of 2.6 ohms, I'm finding that
each diode suffers under a quarter watt of dissipation. So,
any recommendations about diodes? Obviously, for a one-off,
cost is not really an issue. How important is 'fast
recovery'? (Outside of its impact on dissipation.) Seems
that anything with 100V or better for reverse voltage
standoff, 1/4 watt or better, should work. Leakage probably
isn't that important (except against as it may add to
dissipation.)

Fast recovery is not important here since the diodes work at
mains frequency.

10W sinusoidal into 8 ohms = 1.58A peak = 0.503A dc average for
Class B. Add some mAs for the driver stages. That's slightly more
than 0.25A each for diodes in full-wave rectification. The
ubiquitous 1N4002 to 1N4007 rated for 1 Amp diodes will do fine.
They differ only in the maximum reverse voltage ratings and cost
almost the same. As a matter of convenience, I stock only the
1000-volt 1N4007. At less than 2 cents US each retail, I buy them
in batches of hundreds at a time.

I'm expecting to use caps on the order of perhaps 2.2mF 50V,
to be secure about the rails. But I expect to want to play
with that, once everything is working, to see just how bad I
can make it while seeing what that means for the output. And
then see if I can calculate a prediction that isn't too far
from those results, on paper.

I have my own rule of thumb here for acceptable levels of ripple
and load regulation. I divide the full supply dc voltage with the
current at maximum output. This gives the equivalent dc load as
seen by the power supply. In the sample design under
consideration, that's roughly 30 ohms on each side of the split
supply. Calculate the reactance of the filter capacitor at the
pulsating dc frequency which is twice the mains frequency for
full-wave. My rule of thumb is to get an R/Xc ratio of the order
of 50 for a medium quality amp. Your choice of 2200uF agrees well
with this.

 

[Jon Kirwan]

Fast recovery is not important here since the diodes work at
mains frequency.

Thanks. That had certainly crossed my mind as I was writing.
I just wanted to be sure I hadn't missed something important.

10W sinusoidal into 8 ohms = 1.58A peak = 0.503A dc average for
Class B. Add some mAs for the driver stages. That's slightly more
than 0.25A each for diodes in full-wave rectification. The
ubiquitous 1N4002 to 1N4007 rated for 1 Amp diodes will do fine.
They differ only in the maximum reverse voltage ratings and cost
almost the same. As a matter of convenience, I stock only the
1000-volt 1N4007. At less than 2 cents US each retail, I buy them
in batches of hundreds at a time.

I pull them out of CFL lamps before disposal. So they are
"free" to me. I've quite a few, now. 1200V PIV, I think.
Way overkill. But free. I'll use them.

I have my own rule of thumb here for acceptable levels of ripple
and load regulation. I divide the full supply dc voltage with the
current at maximum output. This gives the equivalent dc load as
seen by the power supply. In the sample design under
consideration, that's roughly 30 ohms on each side of the split
supply. Calculate the reactance of the filter capacitor at the
pulsating dc frequency which is twice the mains frequency for
full-wave. My rule of thumb is to get an R/Xc ratio of the order
of 50 for a medium quality amp. Your choice of 2200uF agrees well
with this.

Thanks for your thinking on this. I used more mathy stuff to
get there, but I like your practical slice through all that.
It is easy to follow.

So I'm settled on those particulars, now. The only thing I
don't have, right now, are the caps. Well, maybe. I just
found a 2.2mF, 35V cap. So that gives me one. I've got all
kinds of 200V caps, up to about 470uF. But still looking for
one more 'something close' on the order of 35-50V. I'll keep
looking through the junk box some more. Might turn up
another one.

If so, I then need to figure out all the mounting stuff for
the hardware. I have the AC plugs and grommets and fuse
holders. I can also pull a transorb out of the junk box
(from those CFL lamps, again.) The transformer doesn't have
any brackets or mounting holes in the laminated steel core so
I will have to fashion one from a simple strap of metal,
drilled out. Then to wire it all up and do the smoke test
and verify the output, with and without a load on it.

There and done, I'm ready to move on.

Thanks,
Jon

 

[pimpom]

Thanks for your thinking on this. I used more mathy stuff to
get there, but I like your practical slice through all that.
It is easy to follow.

Rules of thumb are often based on previous mathematical
derivations, as it was in this case. However, after having done
umpteen calculations where absolute precision is not needed, the
novelty wears off after some time and one tends to be satisfied
with being able to intuitively predict the outcome within a per
cent or so without actually putting anything on paper. It's been
firmly etched in my mind for 40 years that 1000uF has a reactance
of 1.5815 ohms (usually taken as 1.6) at 100Hz (twice the mains
frequency here) and I quickly derive Xc for other values from
that within a second. Then I mentally divide the equivalent DC
resistance of a load (not necessarily an audio amplifier) with
that reactance and have a good idea of what to expect in terms of
ripple voltage amplitude, regulation, DC voltage, peak diode
current, rms transformer current, etc.

Heck, it's past 4 am over here. Time for bed. Bye.

 

[Paul E. Schoen]

Fast recovery is not important here since the diodes work at mains
frequency.

10W sinusoidal into 8 ohms = 1.58A peak = 0.503A dc average for Class B.
Add some mAs for the driver stages. That's slightly more than 0.25A each
for diodes in full-wave rectification. The ubiquitous 1N4002 to 1N4007
rated for 1 Amp diodes will do fine. They differ only in the maximum
reverse voltage ratings and cost almost the same. As a matter of
convenience, I stock only the 1000-volt 1N4007. At less than 2 cents US
each retail, I buy them in batches of hundreds at a time.

At one time I had two reels of 5000 each of 1N4004 that I got surplus, but
I sold most of them. I also have a bag of about 1000 pieces of 1N4003. So I
have pretty much a lifetime supply. Either one is OK for 120 VAC mains and
perfect for lower voltage applications. But now my new designs are mostly
SMT. I was going to keep the thru holes and use the "free" parts I had, but
I figured that the labor cost of inserting, soldering, and clipping leads
on 6 diodes on 40 boards might be more than the $0.06 each for the S1G SMT
diodes. Once a commitment is made to SMT it is usually cost-effective to
use as many such parts as possible. I never fully analyzed it, though. I
figure about 2 minutes for the six diodes. At $60/hr, or $1/minute, I spend
$2/board for the leaded parts. The SMT assembly is probably $0.05 per part,
so I spend a total of $0.66 per board.

I have my own rule of thumb here for acceptable levels of ripple and load
regulation. I divide the full supply dc voltage with the current at
maximum output. This gives the equivalent dc load as seen by the power
supply. In the sample design under consideration, that's roughly 30 ohms
on each side of the split supply. Calculate the reactance of the filter
capacitor at the pulsating dc frequency which is twice the mains
frequency for full-wave. My rule of thumb is to get an R/Xc ratio of the
order of 50 for a medium quality amp. Your choice of 2200uF agrees well
with this.

Some time ago I came up with a rule of thumb of 1000 uF per amp, and I
revised that to 2000 uF per amp. I used an RC time constant of 8 mSec
between peaks for a 37% discharge from peak which holds the approximate RMS
value, and for a typical 8 VDC power supply at 1 amp R=8 ohms. So C =
..008/8 = 1000 uF. But two time constants gives only 13% discharge so 2000
uF is much better. For a 16 VDC supply, 1000 uF is OK, and as the voltage
doubles the required capacitance is halved. So for most low voltage
applications, 1000 to 2000 uF per amp is reasonable, and easy to remember.

Of course, if you enjoy mathematical analysis, you can spend time working
out effects of winding resistance and capacitor ESR and acceptable ripple.
Or you can just use LTSpice. But if I need a quick and dirty junkbox power
supply, 1000 uF/amp is good enough to grab and go.

For example, using LTSpice, I find a 12.6 V transformer and I want to make
a 12 VDC power supply at 1 amp. Using a 1000 uF capacitor and a 12 ohm
load, my output is 13.3 V which has a peak of 16.1 V and drops to 10.4 V,
which is a 35% drop as predicted. With 2000 uF it drops to 12.6 VDC so my
output is high enough to provide the 12 VDC I wanted with a regulator. Of
course there are line variations and transformer regulation, but not bad
for a quick estimate.

If I wanted 24 VDC, and I had a 25.2 V transformer, a 1000 uF capacitor
gives me a minimum of 26 VDC for a regulator with a little bit of headroom.

Now I actually add a simple emitter follower voltage regulator with a
2N3055 and two 12 V zeners and a diode in series, with 220 ohms and a 100
uF cap. I get an output of 24.18 VDC which varies from 23.99 VDC to 24.30
VDC. Adding the regulator improves the minimum voltage excursion on the
1000 uF main filter capacitor to 27.6 VDC.

Since I was originally designing for just such a regulated power supply, my
"grab-and-go" estimates for main filter capacitors seems to work out quite
well. And I found it more fun to build and test the circuit using LTSpice
rather than with math. Filter capacitors of this size are typically -20% /
+80% tolerance, so chances are the results will be even better than
expected.

Paul

 

[Jon Kirwan]

Rules of thumb are often based on previous mathematical
derivations, as it was in this case.

Don't mistake me. All I meant to say is that I _am_ new and
therefore took a slower approach, not having developed the
well worn ruts from good experience as you have done. And
that I enjoyed seeing your way of cutting through it.

However, after having done
umpteen calculations where absolute precision is not needed, the
novelty wears off after some time and one tends to be satisfied
with being able to intuitively predict the outcome within a per
cent or so without actually putting anything on paper.

I think I clearly understood exactly that from your writing.

It's been
firmly etched in my mind for 40 years that 1000uF has a reactance
of 1.5815 ohms (usually taken as 1.6) at 100Hz (twice the mains
frequency here) and I quickly derive Xc for other values from
that within a second. Then I mentally divide the equivalent DC
resistance of a load (not necessarily an audio amplifier) with
that reactance and have a good idea of what to expect in terms of
ripple voltage amplitude, regulation, DC voltage, peak diode
current, rms transformer current, etc.

Heck, it's past 4 am over here. Time for bed. Bye.

I didn't expect this and it all looks as though I may have
unintentionally implied something. If so, I hope you will
re-read what I wrote and understand that I'm merely
commenting upon my own painstaking processes, which are at
this point in time important steps for me to take, and in no
way commenting about anything you are saying (except perhaps
that I agree and otherwise like the way you thought about
it.) That's all there was there.

Thanks very much again,
Jon

 

[Paul E. Schoen]

I didn't expect this and it all looks as though I may have
unintentionally implied something. If so, I hope you will
re-read what I wrote and understand that I'm merely
commenting upon my own painstaking processes, which are at
this point in time important steps for me to take, and in no
way commenting about anything you are saying (except perhaps
that I agree and otherwise like the way you thought about
it.) That's all there was there.

Thanks very much again,

I can't speak for pimpom but I don't think any offense has been taken.
There are many ways to approach any problem and sometimes "quick and dirty"
is appropriate while other times a careful mathematical approach
considering all factors is required. There are some areas of mathematics
where my eyes glaze over and it becomes gobble-de-gook, while I can design
a circuit in my head and visualize currents and voltages and waveforms
which can then be verified and improved by using a tool such as LTSpice.
Previous to that I would rely on actual breadboard circuits and using test
equipment (with a good understanding of its limitations) to see how it
performs.

Also, I think this thread has about run its course, and it may be time to
start a new one. It has now morphed into power supply design (as it applies
to audio amps), and it seems to be more suited to sci.electronics.design.
You may consider yourself a beginner but your theoretical knowledge and
mathematical analysis is beyond the range of basics. It seems that your
lack of direct experience and practical "knack" will soon pass as you build
and test a hands-on circuit.

My main criticism would be that you tend to limit yourself too much by
using scavenged parts and freebies in a junkbox. I tend to do that myself,
and often wind up with an inferior design or one that acts abnormally
because perhaps a part is damaged or is not really the best choice given
the wide range of new devices available. And, unless your budget is
severely crimped, you can order new parts with guaranteed specs that will
result in a more predictable and satisfactory outcome, and if it is a
worthwhile design, others may use the same parts and benefit from your
work.

I have an old power supply right here that I built when I was in high
school and I've been itching to rebuild it to be more useful. But it has a
pair of 2N1540 transistors and a pair of 2N554 and two 450 uF 50 V metal
can capacitors and an RT-204 "Selenium Rectifier Type" transformer and a
1N2976B stud mount 12V zener, and the meters are 0-10 VDC and 0-3 Amps. I
no longer have the schematic and what I've been able to trace does not seem
to make much sense to me now. It is nicely packaged in a Bud Portacab but
I'd really like to have at least 0-15 VDC and more like 5 amps and better
regulation and current limiting rather than the crude 3 amp fuse it has
now. So should I use these old obsolete parts (those are Germanium
transistors!), and make compromises to get it working again or should I
design from scratch and make it do what I really want or do I just put it
back in the junk pile and buy what I'd like for a hundred bucks or so? If I
could just get it working OK in a few hours I could live with the limited
output, and maybe I could add a x2 switch so I can get 0-20V with the same
meter, or I could make a new scale and change the internal resistor, or...
so I wind up with one or two days work and I talk myself out of it again...

Paul