Audio amp...Phase splitter...Why?

[Xtrchessreal]

I have been racking my brain to learn about audio amlpifiers. My EE
courses never covered them. I can't find any books in the library.

Audio is apparently either difficult to understand or intentionally
cryptic to make it seem so.

I have a question for anyone that can explain it.

In a Class AB Tube push pull amplifier the OT output Transformer is fed
a signal and an inverted signal. WHY?

What is the purpose of this two signal input to the OT?

Here is my guess: Because the inverted AC signal causes a larger
potential difference across the OT gaining as much power as possible
for the output.

How is the split signal 0 degree phase and 180 degree phase then
reconstructed on the secondary into one signal across the speakers.

 

[John Popelish]

Xtrchessreal wrote:
(snip)

In a Class AB Tube push pull amplifier the OT output Transformer is fed
a signal and an inverted signal. WHY?

The output transformer is a sort of electrical see saw. Tubes pull
down, only (draw current from positive supplies) so to get an audio
that swings both positive and negative, one way is to rock the "see
saw" one way with one tube and the other way with the other.

What is the purpose of this two signal input to the OT?

It is a way to combine the power from two tubes into a single signal
with low distortion, since some of the distortion since the positive
and negative swings are produced symmetrically.

Here is my guess: Because the inverted AC signal causes a larger
potential difference across the OT gaining as much power as possible
for the output.

How is the split signal 0 degree phase and 180 degree phase then
reconstructed on the secondary into one signal across the speakers.

The two signals are combined as a single magnetic field in the core of
the transformer (with one tube magnetizing it one way, and the other
tube magnetizing it the other way) and the secondary extracts power
from this alternating field.

Here is an example of a simple push pull (actually pull pull, but on
opposite sides of the see saw) amplifier.
http://members.aol.com/sbench/norcpp.html
and the page with it and more examples:
http://members.aol.com/sbench101/#PowerAmps

 

[John Popelish]

I have been racking my brain to learn about audio amlpifiers. My EE
courses never covered them. I can't find any books in the library.

Audio is apparently either difficult to understand or intentionally
cryptic to make it seem so.

I have a question for anyone that can explain it.

In a Class AB Tube push pull amplifier the OT output Transformer is fed
a signal and an inverted signal. WHY?

What is the purpose of this two signal input to the OT?

Here is my guess: Because the inverted AC signal causes a larger
potential difference across the OT gaining as much power as possible
for the output.

How is the split signal 0 degree phase and 180 degree phase then
reconstructed on the secondary into one signal across the speakers.

Here is another basic tutorial on audio tube amplifiers:
http://www.hans-egebo.dk/Tutorial/amplifiers.htm

 

[Phil Allison]

"Xtrchessreal"

I have been racking my brain to learn about audio amplifiers. My EE
courses never covered them. I can't find any books in the library.

Audio is apparently either difficult to understand or intentionally
cryptic to make it seem so.

** Audio is far from simple.

But it looks so simple.

That is why there are soooo many arguments.

I have a question for anyone that can explain it.

In a Class AB Tube push pull amplifier the OT output Transformer is fed
a signal and an inverted signal. WHY?

** If you take the *whole* primary winding, there is really no such
inversion.

What is the purpose of this two signal input to the OT?

Here is my guess: Because the inverted AC signal causes a larger
potential difference across the OT gaining as much power as possible
for the output.

** The use of two tubes with a *centre tapped transformer* means DC bias
current in the tubes flows in opposite directions in each half of the
primary winding. Hence the DC magnetic field cancels out and the iron core
is NOT magnetised as it would be otherwise.

With a single tube design this cannot happen, the iron core has to be much
larger and fitted with a small air gap to prevent magnetic saturation. When
a single tube amp is at idle - the iron core is always half fully
magnetised.

Using two tubes in so called "push- pull" mode also means tube non linearity
tends to cancel as well.

The two tubes are driven with identical but antiphase signals so that AC
current will flow in the primary in one direction at any moment - hence
generating a voltage at the secondary.

Googling with "push pull amp"' get lots of good hits.




......... Phil

 

[Pooh Bear]

I have been racking my brain to learn about audio amlpifiers. My EE
courses never covered them. I can't find any books in the library.

Audio is apparently either difficult to understand or intentionally
cryptic to make it seem so.

Only if taught badly. Audio is gneric 'low frequency' electronics really.

I have a question for anyone that can explain it.

In a Class AB Tube push pull amplifier the OT output Transformer is fed
a signal and an inverted signal. WHY?

What is the purpose of this two signal input to the OT?

So that one tube handles positive going outputs whilst another handles the
negative.

Here is my guess: Because the inverted AC signal causes a larger
potential difference across the OT gaining as much power as possible
for the output.
No.

How is the split signal 0 degree phase and 180 degree phase then
reconstructed on the secondary into one signal across the speakers.

That's what a centre tapped transformer does by its nature !

Graham

 

[John Larkin]

"Xtrchessreal"

Not in the last, say, 40 years.

Actually, neither.

** Audio is far from simple.

But it looks so simple.

That is why there are soooo many arguments.

DSP is complex. Differential equations are complex. Control theory and
Laplace transforms are complex. Filter design is complex. But not much
argument is provoked by them.

So something else must be going on.

John

 

[Xtrchessreal]

Thanks JP,PA, and PB for your time to answer my questions. I spent a
few hours going over the Tube in general getting a feel for the actions
taking place inside.

So, now I feel like I know why there is a phase splitter in a Push Pull
type output stage. Basically the tube has to operate in a positive
current flow from cathode to plate using a grid to control the
flow/current inside the tube. The grid is biased with a less positive
voltage in order to control the current from cathode to plate. That
control bias can be designed in several ways. The main idea is that in
a push pull type amplifier the two ouput tubes must run in a positive
current from cathode to plate. As the AC signal is flowing in the
positive to negative cycle the tubes (naturally to the circuit) take
the positive portion of the signal and amplify it to the output
transformer. Hence the transformer is center tapped and the signal is
fed from each output tube 180 degrees at a time. There may be some
signal loss during the crossover from each output tube or at least some
noise or distortion introduced to the output.

I am guessing that the transformer is inverse wound from the center tap
to the bottom such the the negative portion of the signal causes flux
to flow in the same direction as the top half with the positive portion
of the signal. Thus the entire cycle is reconstructed on the
secondary. Also, that each tube power output is additive to the total
output less any losses from noise etc in the transformer. So If my amp
is running at 29 watts per tube plate dissapation then the total output
of the amp is near 58 watts RMS depending on the efficiency of the
transformer.

I was initially confused with a few different things. 1st that the
inverted signal was adding in a delayed signal 180 degrees behind the
initial signal. 2nd that the inverted signal was adding to the
potential difference or subtracting depending on the phase you are
looking at. 3rd that both tubes were in operation during the whole
cycle of the signal.

I am now wondering about the phase splitter circuit.

In my amp Marshall 2204 a single tube is used to split the signal and
there is a feedback loop fed to the same phase splitter circuit.

Without the feedback the tube still produces two signals inverted from
each other - that is right yes or no?

The Feedback loop I know is canceling some portion of the input signal
but I don't know why it is there. If that could be explained that
would be very nice.

I appreciate your input and your time. Appropriate books are hard to
find in stores and libraries so please accept my apologies for asking
too much.

Thanks for your help.

X

 

[[email protected]]

Push pull is used because it is more efficient, you get a lot more
power out for the same sized components. The transformer is normally
wound it just has a tap halfway down the winding. Crossover distortion
is avoided by ensuring that both tubes conduct near the crossover
point. Negative feedback is used to reduce distortion, you'll find lots
of full explainations about it on the web or in text books.

 

[Phil Allison]

Push pull is used because it is more efficient, you get a lot more
power out for the same sized components.

** Efficiency is simply the ratio of power input to power output.

Not the "size" of the bits used.

BTW The number of components in a PP stage is way higher.

The transformer is normally
wound it just has a tap halfway down the winding. Crossover distortion
is avoided by ensuring that both tubes conduct near the crossover
point.

** Really ? Been Googling have we ??

Push pull *tube* amps generally operate in class A up to half power or
more.

Crossover problems are impossible.

Negative feedback is used to reduce distortion,

** Improvements in frequency response and damping are irrelevant then ?

you'll find lots
of full explanations about it on the web or in text books.

** You must take you on advice some day.




......... Phil

 

[John Popelish]

Thanks JP,PA, and PB for your time to answer my questions. I spent a
few hours going over the Tube in general getting a feel for the actions
taking place inside.

(snip)

It looks like you have made considerable progress.

There may be some
signal loss during the crossover from each output tube or at least some
noise or distortion introduced to the output.

(snip)

Or there may be extra gain at the cross over, when both tubes are
contributing to the output at the same time. The exact case depends
on how the tube gain varies near cut off.

Also, that each tube power output is additive to the total
output less any losses from noise etc in the transformer. So If my amp
is running at 29 watts per tube plate dissapation then the total output
of the amp is near 58 watts RMS depending on the efficiency of the
transformer.

Almost right. Tube plate dissipation is a loss. So the output is the
supply power minus plate dissipation and all other losses.

I was initially confused with a few different things. 1st that the
inverted signal was adding in a delayed signal 180 degrees behind the
initial signal.

An inversion is not actually a time delay.

2nd that the inverted signal was adding to the
potential difference or subtracting depending on the phase you are
looking at.

Now you have the idea. Inversion of input and output allows the
possibility of subtraction, so that the plate current can be rising on
one tube or the other as the signal approaches either positive or
negative peaks.

3rd that both tubes were in operation during the whole
cycle of the signal.

In some push pull designs, they are. This is a push pull class A
amplifier. They have high losses, but cancel the even harmonic
distortion of a single ended class A design, and also the DC net
current in the transformer primary.

I am now wondering about the phase splitter circuit.

In my amp Marshall 2204 a single tube is used to split the signal and
there is a feedback loop fed to the same phase splitter circuit.

So this tube is in between a plate resistor and a cathode resistor of
equal values. As it varies its current, the drop across those two
resistors rise and fall similarly, except that one resistor has
voltage varying on its most positive end, and one at its most negative
end. So the voltages at those ends varies in the opposite direction
from each other.

Without the feedback the tube still produces two signals inverted from
each other - that is right yes or no?

Yes. But both those signal outputs have more distortion.

The Feedback loop I know is canceling some portion of the input signal
but I don't know why it is there. If that could be explained that
would be very nice.

(snip)

The more gain, the more cancellation. So throwing away some of the
gain with negative feedback makes the lower net gain more stable
throughout the signal swing.

 

[Rich Grise]

I have been racking my brain to learn about audio amlpifiers. My EE
courses never covered them. I can't find any books in the library.

Audio is apparently either difficult to understand or intentionally
cryptic to make it seem so.

I have a question for anyone that can explain it.

In a Class AB Tube push pull amplifier the OT output Transformer is fed
a signal and an inverted signal. WHY?

What is the purpose of this two signal input to the OT?

No average DC on the transformer primary, ergo, it (theoretically) doesn't
saturate. (It might still on the peaks, but that will just distort the
signal, not let the smoke out.)

Here is my guess: Because the inverted AC signal causes a larger
potential difference across the OT gaining as much power as possible
for the output.

That, too, and the fact that you can run class AB which is much more
efficient than class A.

How is the split signal 0 degree phase and 180 degree phase then
reconstructed on the secondary into one signal across the speakers.

PLEASE!! PLEASE!!!! It is _NOT_ "180 degrees out of phase", it is
INVERTED!!!!!!!

One common way I've seen it is to use a triode, that's half ordinary
amplifier, and half cathode-follower, with equal cathode and plate
resistors - the signals will be mirror images of each other. (i.e., one
goes up when the other goes down.) NOT 180 degrees out of phase!!!!!

Admittedly, at any given frequency, a sine wave shifted by 180 deg.
is indistinguishable from its counterpart that's only inverted, but
if the frequency changes, the phase will probably change because
there's something in the circuit that delays the signal, like an
inductance or capacitance. But merely inverting it is not the same
thing - it's not frequency-dependent.

Hope This Helps!
Rich

 

[Phil Allison]

"Rich Grise"

No average DC on the transformer primary, ergo, it (theoretically) doesn't
saturate. (It might still on the peaks, but that will just distort the
signal, not let the smoke out.)

** If transformer core saturation occurs at all - it is invariably at the
lowest frequencies an amp is capable of AND squashes the waveform at or
near ZERO crossings.

On a scope, it looks like LF crossover distortion.

On a Marshall, the first signs appear at about 45Hz and full output.



......... Phil

 

[Phil Allison]

"John Popelish"

So this tube is in between a plate resistor and a cathode resistor of
equal values.

** Nope.

That "single tube" is a 12AX7 twin triode.

Marshalls use a differential ( cathode coupled) phase splitter.



.......... Phil

 

[John Popelish]

"John Popelish"


** Nope.

That "single tube" is a 12AX7 twin triode.

Marshalls use a differential ( cathode coupled) phase splitter.

Phil, do you know where I can see a schematic, so I don't have to keep
guessing for this poster.

 

[Xtrchessreal]

You guys are very helpful here. Thank you for your input.

I will be more thorough of circuit descriptions in the future. The
Marshall does have a 12xa7 in the so called phase splitter circuit
though in some schematics it is written as ECC83. I do not have enough
tube knowledge to know if that is the same thing or if one is a
specific tube name while the other is a general tube function name or
they are completely different triodes.

I do know that schematics do not always have the actual components
listed or circuit diagram as the device was released out the door.
Sometimes components change due to inventory the day it was assembled
as opposed to what it was supposed to have according to schematic or
BOM.

The term "Phase splitter circuit" in this amplifier is not a well
chosen name because it does lead a person, at least a new person in
electronics, to believe that the signal is 180 degrees out of phase.
This I was struggling with initially believing the signal was delayed.
You are taught this idea of lagging and leading Eli when learning about
phase and phase diagrams etc. In this Marshall the circuit is actually
producing a normal and an inverted signal so I see why using the phase
term at all is not a good idea. Perhaps it should simply be called
"Inverted signal splitter circuit" though the schematic does call it as
I said above.

Is this a difference of European vs American terminology perhaps?
Whatever the case may be this is another example of why I feel that so
much of Audio Electronics and Electrical Engineering seems so cryptic
almost as though to make it harder than it really is. The real issue
for me in that regard is I can't find any books on audio electronics
that explain this stuff independently of the rest of Electronics. All
of my text books completely miss Tubes and the only mention of audio is
in the form of a radio amplifier and receiver which gets one half page
out of 890.

Why has the electrical engineering curriculum so completely stopped
teaching audio? Last I looked everyone has a stereo, mp3 player, 5.1
audio card for the PC, a Home theater, TVs, and what about musicians,
guitar players, keyboardists, Disc Jockeys and rappers, singers, etc.
why am I not taught audio in all of these areas? I will ask my Dean.

If anyone out there has enough knowledge to write a book on audio you
may make some money on it. For me understanding the signal the
electrical/physical signal as it propagates through the audio amplifier
would be very helpful to me. Seeing the various voltage dividers and
tube electrodes in motion would give me something real I can picture in
my head. Math by itself just doesn't do anything for me.

Thanks for all of your help

 

[Phil Allison]

"John Popelish"

Phil, do you know where I can see a schematic, so I don't have to keep
guessing for this poster.

See:

http://www.drtube.com/schematics/marshall/jcm800pr.gif



.......... Phil

 

[Pooh Bear]

You guys are very helpful here. Thank you for your input.

I will be more thorough of circuit descriptions in the future. The
Marshall does have a 12xa7 in the so called phase splitter circuit
though in some schematics it is written as ECC83. I do not have enough
tube knowledge to know if that is the same thing

It is the same thing. ECC83 is the European name for an American 12AX7.

Graham

 

[John Popelish]

Xtrchessreal wrote:
(snip)

... Perhaps it should simply be called
"Inverted signal splitter circuit" though the schematic does call it as
I said above.

Is this a difference of European vs American terminology perhaps?
Whatever the case may be this is another example of why I feel that so
much of Audio Electronics and Electrical Engineering seems so cryptic
almost as though to make it harder than it really is.

(snip)

I think, since the earliest active devices were tubes, anything having
to do with them has a lot of historical baggage.

If anyone out there has enough knowledge to write a book on audio you
may make some money on it.

I wouldn't bet on it. At least not unless it is written in Chinese.

For me understanding the signal the
electrical/physical signal as it propagates through the audio amplifier
would be very helpful to me. Seeing the various voltage dividers and
tube electrodes in motion would give me something real I can picture in
my head. Math by itself just doesn't do anything for me.

I agree. Until I can wave my arms around and talk about an electronic
effect in conversational terms with almost no math, and make sense to
myself, I don't feel like I have any real understanding of a mechanism
or process.

By the way, in case you haven't heard, when doing Google searches for
understanding (as opposed to sales or patents or whatever), a very
fine word to add is "tutorial".
E.g. ["tube amplifier" tutorial], ["audio amplifier" tutorial],
"single ended amplifier" tutorial], etc.

 

[Jasen Betts]

I think, since the earliest active devices were tubes, anything having
to do with them has a lot of historical baggage.

not true, the earlest active devices were spark gaps.

Bye.
Jasen

 

[Rich Grise]

You guys are very helpful here. Thank you for your input.

I will be more thorough of circuit descriptions in the future. The
Marshall does have a 12xa7 in the so called phase splitter circuit
though in some schematics it is written as ECC83. I do not have enough
tube knowledge to know if that is the same thing or if one is a
specific tube name while the other is a general tube function name or
they are completely different triodes.

They're the same tube, but numbered 12AX7 in the US and ECC83 in Europe.

Cheers!
Rich