Can RF bias be used to cancel crossover distortion?

[Jan Panteltje]

In audio tap[e recorderas RF bias is used to work around the magnetic hysteresis of the tape.

But what if we use RF bias in an audio amp with crossover distortion?
I tried this without bias:
ftp://panteltje.com/pub/rf_bias_against_xover_distortion_no_bias.gif
and then this with bias:
ftp://panteltje.com/pub/rf_bias_against_xover_distortion_with_bias.gif
The blue line is the filtered output at R2/C1

The signal is 1000Hz 8V.
The bias, if present, is 2V.
The top voltage source is 100kHz 2 V.

Does this make sense ?

The reason I tried this is that I once had a mixer with a switchmode supply,
that did feed through to the main amp,
everybody liked that sound, but later I found that there was
RF from that mixer in the speaker leads....

?

 

[MooseFET]

In audio tap[e recorderas RF bias is used to work around the magnetic hysteresis of the tape.

But what if we use RF bias in an audio amp with crossover distortion?
I tried this without bias:
 ftp://panteltje.com/pub/rf_bias_against_xover_distortion_no_bias.gif
and then this with bias:
 ftp://panteltje.com/pub/rf_bias_against_xover_distortion_with_bias.gif
The blue line is the filtered output at R2/C1

The signal is 1000Hz 8V.
The bias, if present, is 2V.
The top voltage source is 100kHz 2 V.

Does this make sense ?

The reason I tried this is that I once had a mixer with a switchmode supply,
that did feed through to the main amp,
everybody liked that sound, but later I found that there was
RF from that mixer in the speaker leads....

?

All sorts of distortions can be blurred out using dithering in ADCs so
the answer has to be a qualified yes. The down side is that the
amplifier is doing work to amplify this RF. This makes it sound
unlikely as a way to save power.

 

[Jan Panteltje]

In audio tap[e recorderas RF bias is used to work around the magnetic hys= teresis of the tape.

But what if we use RF bias in an audio amp with crossover distortion?
I tried this without bias:
 ftp://panteltje.com/pub/rf_bias_against_xover_distortion_no_bias.gif
and then this with bias:
 ftp://panteltje.com/pub/rf_bias_against_xover_distortion_with_bias.gif
The blue line is the filtered output at R2/C1

The signal is 1000Hz 8V.
The bias, if present, is 2V.
The top voltage source is 100kHz 2 V.

Does this make sense ?

The reason I tried this is that I once had a mixer with a switchmode supp= ly,
that did feed through to the main amp,
everybody liked that sound, but later I found that there was
RF from that mixer in the speaker leads....

?

All sorts of distortions can be blurred out using dithering in ADCs so
the answer has to be a qualified yes. The down side is that the
amplifier is doing work to amplify this RF. This makes it sound
unlikely as a way to save power.

True, but it gets rid of a _whole_ lot of bias schemes

 

[Kevin Aylward]

In audio tap[e recorderas RF bias is used to work around the
magnetic hys= teresis of the tape.

But what if we use RF bias in an audio amp with crossover
distortion? I tried this without bias:
ftp://panteltje.com/pub/rf_bias_against_xover_distortion_no_bias.gif
and then this with bias:
ftp://panteltje.com/pub/rf_bias_against_xover_distortion_with_bias.gif
The blue line is the filtered output at R2/C1

The signal is 1000Hz 8V.
The bias, if present, is 2V.
The top voltage source is 100kHz 2 V.

Does this make sense ?

The reason I tried this is that I once had a mixer with a
switchmode supp= ly, that did feed through to the main amp,
everybody liked that sound, but later I found that there was
RF from that mixer in the speaker leads....

?

All sorts of distortions can be blurred out using dithering in ADCs
so the answer has to be a qualified yes. The down side is that the
amplifier is doing work to amplify this RF. This makes it sound
unlikely as a way to save power.

True, but it gets rid of a _whole_ lot of bias schemes

Yes.

Kevin Aylward
[email protected]
www.kevinaylward.co.uk

 

[Kevin Aylward]

In audio tape recorderas RF bias is used to work around the magnetic
hysteresis of the tape.

Oh... Thats a new one on me. I have a calculation on this showiing that the
the basic reason for distortion reduction is parametric amplification. To
wit:

http://www.kevinaylward.co.uk/ee/tapebias/tapebias.html

If you would care to point out any errors, I might be inclined to change my
view. Do you have any papers analysing why you believe HF bias reduces
distortion due to hysteresis, and not just the inherent non-linear behaviour
of tape?

But what if we use RF bias in an audio amp with crossover distortion?
I tried this without bias:
ftp://panteltje.com/pub/rf_bias_against_xover_distortion_no_bias.gif
and then this with bias:
ftp://panteltje.com/pub/rf_bias_against_xover_distortion_with_bias.gif
The blue line is the filtered output at R2/C1

The signal is 1000Hz 8V.
The bias, if present, is 2V.
The top voltage source is 100kHz 2 V.

Does this make sense ?

Probably. I have never really thought about this particular application of
HF bias, but the theory is quite general, so it would, on the face of it,
appear to explain your pictures.

The reason I tried this is that I once had a mixer with a switchmode
supply, that did feed through to the main amp,
everybody liked that sound, but later I found that there was
RF from that mixer in the speaker leads....

Can't say if that was related or not, so I wont.


Kevin Aylward
www.kevinaylward.co.uk

 

[Eeyore]

In audio tap[e recorderas RF bias is used to work around the magnetic hysteresis of the tape.

But what if we use RF bias in an audio amp with crossover distortion?
I tried this without bias:
ftp://panteltje.com/pub/rf_bias_against_xover_distortion_no_bias.gif
and then this with bias:
ftp://panteltje.com/pub/rf_bias_against_xover_distortion_with_bias.gif
The blue line is the filtered output at R2/C1

The signal is 1000Hz 8V.
The bias, if present, is 2V.
The top voltage source is 100kHz 2 V.

Does this make sense ?

The reason I tried this is that I once had a mixer with a switchmode supply,
that did feed through to the main amp,
everybody liked that sound, but later I found that there was
RF from that mixer in the speaker leads....
?

All sorts of distortions can be blurred out using dithering in ADCs

Indeed yes.

so the answer has to be a qualified yes. The down side is that the
amplifier is doing work to amplify this RF. This makes it sound
unlikely as a way to save power.

Besides, it's easier to design a good AF amplifier.

I've managed 0.0008% @ 1 kHz and 350W and immeasurable crossover distortion (not visible on an
analyser output on the scope either) and no it wasn't Class A, although it did have a fairly hefty
quiescent bias current. See the thread on using mosfets for audio.

Graham

 

[Jan Panteltje]

Oh... Thats a new one on me. I have a calculation on this showiing that the
the basic reason for distortion reduction is parametric amplification. To
wit:

http://www.kevinaylward.co.uk/ee/tapebias/tapebias.html

Now that is nice, I first though about writing it out, but then,
as I was doing other stuff at the same time, spice was easier to quickly check
if the idea worked.
In my younger days, before we were let lose in the studios, we got a 9 month in the
school benches audio video training from the network (they do not do that anymore I think).
One old person, from the radio section, explained the RF bias in tape recorders
thing, he explained it very clearly, graphically, in the time domain, with the
tape (IIRC) leaving the record head and the magnetisation following the tape's BH curve
getting weaker and weaker, and finally settling for one value.
Perhaps you can see a similar thing in the spice simulation I presented,
if you enlarge the area around the cross-over.
Anyways, I will not check your math, you likely are better then me at that,
but it seems you were there first

If you would care to point out any errors, I might be inclined to change my
view. Do you have any papers analysing why you believe HF bias reduces
distortion due to hysteresis, and not just the inherent non-linear behaviour
of tape?

The tape's BH curve _is_ hysteresis, maybe I did not make myself clear?

Wanted to draw ASCII, but this is much better:
http://en.wikipedia.org/wiki/Hysteresis
Look at 'magnetic hysteresis', this is also what you see if you magnetise -
demagnetise the tape (iron).

Probably. I have never really thought about this particular application of
HF bias, but the theory is quite general, so it would, on the face of it,
appear to explain your pictures.

I can think of practical applications, a small RF, and it must be small as it
does limit the drive on top and bottom (it should be a pure sine wave not clip),
maybe modulated so it is weaker for strong signals?, and a setpoint of the power
amp just below cutoff.
The extra RF would use little power, perhaps less then a little bit of bias would,
as it would see a high impedance anyways (speaker) or even a wide LC filter?

 

[Eeyore]

Oh... Thats a new one on me. I have a calculation on this showiing that the
the basic reason for distortion reduction is parametric amplification. To
wit:

http://www.kevinaylward.co.uk/ee/tapebias/tapebias.html

If you would care to point out any errors, I might be inclined to change my
view. Do you have any papers analysing why you believe HF bias reduces
distortion due to hysteresis, and not just the inherent non-linear behaviour
of tape?

You are correct Kevin. It's to get round the non-linear behaviour. I should have
spotted that myself but was only skimming. There is doubtless some hysteritic
behaviour but that's not the primary purpose of bias.

Graham

 

[Ancient_Hacker]

Yes, and you can fly to the moon by pulling up on your bootstraps.

What you're doing is dithering the signal around zero, hoping to get
at least one transistor into a bit of its active region.
Which is not unlike trying to push-start your car by throwing rocks at
it.

In the real world you're going to have problems:

(1) The transistors are not going to be 2N3904's, but power
transistors or FETs, usually not designed for RF. You didn't simulate
this.

(2) You may improve the crossover region, but you're going to mess up
the signal peaks. You didn't simulate this.

(3) Unless your customers have a FCC license, you'll need to add a
passive low-pass filter to the output, and maybe some shielding.

(4) When your cost accountant sees the extra cost of the HF
transistors, the filter, and shielding, they'll fill their pants.

 

[Martin Griffith]

In audio tap[e recorderas RF bias is used to work around the
magnetic hys= teresis of the tape.

But what if we use RF bias in an audio amp with crossover
distortion? I tried this without bias:
ftp://panteltje.com/pub/rf_bias_against_xover_distortion_no_bias.gif
and then this with bias:
ftp://panteltje.com/pub/rf_bias_against_xover_distortion_with_bias.gif
The blue line is the filtered output at R2/C1

The signal is 1000Hz 8V.
The bias, if present, is 2V.
The top voltage source is 100kHz 2 V.

Does this make sense ?

The reason I tried this is that I once had a mixer with a
switchmode supp= ly, that did feed through to the main amp,
everybody liked that sound, but later I found that there was
RF from that mixer in the speaker leads....

?

All sorts of distortions can be blurred out using dithering in ADCs
so the answer has to be a qualified yes. The down side is that the
amplifier is doing work to amplify this RF. This makes it sound
unlikely as a way to save power.

True, but it gets rid of a _whole_ lot of bias schemes

Yes.

Kevin Aylward
[email protected]
www.kevinaylward.co.uk

So, if I understand it correctly, if you add RF wiggles to a nasty O/P
stage you reduce the distortion.

I have a feeling that that you only need to add RF when the signal is
approaching the crossover zone, and not when the input signal is
significant.
Is this a fair assumption?

martin

 

[Jan Panteltje]

On a sunny day (Sat, 11 Oct 2008 22:59:47 +0200) it happened Martin Griffith

stage you reduce the distortion.

I have a feeling that that you only need to add RF when the signal is
approaching the crossover zone, and not when the input signal is
significant.
Is this a fair assumption?

martin

Yes, but in doing so you would [have to] modulate the RF signal in a special way.
Because the signal would be minimum for both the pos and neg audio peaks,
while maximum for zero....

One can ask how much you can hear crossover if the amplitude is maximum....

 

[Jan Panteltje]

Yes, and you can fly to the moon by pulling up on your bootstraps.

What you're doing is dithering the signal around zero, hoping to get
at least one transistor into a bit of its active region.
Which is not unlike trying to push-start your car by throwing rocks at
it.

In the real world you're going to have problems:

(1) The transistors are not going to be 2N3904's, but power
transistors or FETs, usually not designed for RF. You didn't simulate
this.

(2) You may improve the crossover region, but you're going to mess up
the signal peaks. You didn't simulate this.

(3) Unless your customers have a FCC license, you'll need to add a
passive low-pass filter to the output, and maybe some shielding.

(4) When your cost accountant sees the extra cost of the HF
transistors, the filter, and shielding, they'll fill their pants.

huh, ;-) it would be green, and the perfect audio amp, so you
can ask whatever you want for it.

 

[Martin Griffith]

On a sunny day (Sat, 11 Oct 2008 22:59:47 +0200) it happened Martin Griffith

stage you reduce the distortion.

I have a feeling that that you only need to add RF when the signal is
approaching the crossover zone, and not when the input signal is
significant.
Is this a fair assumption?

martin

Yes, but in doing so you would [have to] modulate the RF signal in a special way.
Because the signal would be minimum for both the pos and neg audio peaks,
while maximum for zero....

One can ask how much you can hear crossover if the amplitude is maximum....

A silly question, since I haven't stuck a scope across an audio
amplifier for at least 10 years, and I really suck at high power audio
(it really,really makes me fall asleep)

How large is the crossover zone, a couple of diodes worth of V, 1400mV
or so, into 4 ohms?

martin

 

[Jan Panteltje]

A silly question, since I haven't stuck a scope across an audio
amplifier for at least 10 years, and I really suck at high power audio
(it really,really makes me fall asleep)

How large is the crossover zone, a couple of diodes worth of V, 1400mV
or so, into 4 ohms?

martin

I think, from a distortion POV, say that area where a change in input voltage
does not result in a [similar] change of output voltage.
That is why I used the NPN-PNP pair without bias, so 1.4V say.
But of course, if we were to do this for thermal stabilisation,
you could bias it so it was only .2V say.
If we were to do this to improve further on Yeore's .000xxx % distortion
numbers, then maybe we should refer to a 'non-linear' zone, and
that could be any size, but likely small.
The smaller that size the less RF we need, so I have been thinking about making a separate
box that just adds some millivolts of RF to the input, and a separate loudspeaker filter.
The both could be sold for 10000$ to audiophiles, of course their amp would have
to have the required bandwidth, say 200kHz, but I think most real audiophiles have that sort of amp.
....

 

[Martin Griffith]

A silly question, since I haven't stuck a scope across an audio
amplifier for at least 10 years, and I really suck at high power audio
(it really,really makes me fall asleep)

How large is the crossover zone, a couple of diodes worth of V, 1400mV
or so, into 4 ohms?

martin

I think, from a distortion POV, say that area where a change in input voltage
does not result in a [similar] change of output voltage.
That is why I used the NPN-PNP pair without bias, so 1.4V say.
But of course, if we were to do this for thermal stabilisation,
you could bias it so it was only .2V say.
If we were to do this to improve further on Yeore's .000xxx % distortion
numbers, then maybe we should refer to a 'non-linear' zone, and
that could be any size, but likely small.
The smaller that size the less RF we need, so I have been thinking about making a separate
box that just adds some millivolts of RF to the input, and a separate loudspeaker filter.
The both could be sold for 10000$ to audiophiles, of course their amp would have
to have the required bandwidth, say 200kHz, but I think most real audiophiles have that sort of amp.
...

That 31.4159kHz will be quite expensive, won't it , or you could do a
2 pi version fot repbulicans

martin

 

[[email protected]]

In audio tap[e recorderas RF bias is used to work around the magnetic hysteresis of the tape.
But what if we use RF bias in an audio amp with crossover distortion?
I tried this without bias:
ftp://panteltje.com/pub/rf_bias_against_xover_distortion_no_bias.gif
and then this with bias:
ftp://panteltje.com/pub/rf_bias_against_xover_distortion_with_bias.gif
The blue line is the filtered output at R2/C1
The signal is 1000Hz 8V.
The bias, if present, is 2V.
The top voltage source is 100kHz 2 V.
Does this make sense ?
The reason I tried this is that I once had a mixer with a switchmode supply,
that did feed through to the main amp,
everybody liked that sound, but later I found that there was
RF from that mixer in the speaker leads....
?

All sorts of distortions can be blurred out using dithering in ADCs

Indeed yes.

so the answer has to be a qualified yes. The down side is that the
amplifier is doing work to amplify this RF. This makes it sound
unlikely as a way to save power.

Besides, it's easier to design a good AF amplifier.

I've managed 0.0008% @ 1 kHz and 350W and immeasurable crossover distortion (not visible on an
analyser output on the scope either) and no it wasn't Class A, although it did have a fairly hefty
quiescent bias current. See the thread on using mosfets for audio.

Graham

I think I remember reading something by Douglas Self about proberly
biased class-b actually being better
than class-AB, so could it have been even better with less bias? or
maybe it only applies to bipolars?

-Lasse

 

[Eeyore]

In audio tap[e recorderas RF bias is used to work around the magnetic hysteresis of > > > the

tape.

But what if we use RF bias in an audio amp with crossover distortion?
I tried this without bias:
ftp://panteltje.com/pub/rf_bias_against_xover_distortion_no_bias.gif
and then this with bias:
ftp://panteltje.com/pub/rf_bias_against_xover_distortion_with_bias.gif
The blue line is the filtered output at R2/C1

The signal is 1000Hz 8V.
The bias, if present, is 2V.
The top voltage source is 100kHz 2 V.

Does this make sense ?

The reason I tried this is that I once had a mixer with a switchmode supply,
that did feed through to the main amp,
everybody liked that sound, but later I found that there was
RF from that mixer in the speaker leads....
?

All sorts of distortions can be blurred out using dithering in ADCs

Indeed yes.

so the answer has to be a qualified yes. The down side is that the
amplifier is doing work to amplify this RF. This makes it sound
unlikely as a way to save power.

Besides, it's easier to design a good AF amplifier.

I've managed 0.0008% @ 1 kHz and 350W and immeasurable crossover distortion (not > visible on an

analyser output on the scope either) and no it wasn't Class A, although it did > have a fairly hefty
quiescent bias current. See the thread on using mosfets for audio.


I think I remember reading something by Douglas Self about proberly
biased class-b actually being better
than class-AB, so could it have been even better with less bias? or
maybe it only applies to bipolars?

I could tell you a few things about him. His designs don't even come close to mine.

Graham

 

[Kevin Aylward]

Now that is nice, I first though about writing it out, but then,
as I was doing other stuff at the same time, spice was easier to
quickly check if the idea worked.
In my younger days, before we were let lose in the studios, we got a
9 month in the school benches audio video training from the network
(they do not do that anymore I think). One old person, from the radio
section, explained the RF bias in tape recorders thing, he explained
it very clearly, graphically, in the time domain, with the tape
(IIRC) leaving the record head and the magnetisation following the
tape's BH curve getting weaker and weaker, and finally settling for
one value. Perhaps you can see a similar thing in the spice
simulation I presented, if you enlarge the area around the
cross-over. Anyways, I will not check your math, you likely are
better then me at that, but it seems you were there first




The tape's BH curve _is_ hysteresis, maybe I did not make myself
clear?

The BH curve shows hysteresis, but that aspect is not important as far as
the basic1st order distortion is concerned. For the analysis it is assumed
that the loop area of the B-H curve is zero. I don't see how HF bias could
reduce the *additional* distortion of hysteresis.


Kevin Aylward
www.kevinaylward.co.uk

 

[Jan Panteltje]

The BH curve shows hysteresis, but that aspect is not important as far as
the basic1st order distortion is concerned. For the analysis it is assumed
that the loop area of the B-H curve is zero. I don't see how HF bias could
reduce the *additional* distortion of hysteresis.

But it does, I think your model needs some refinement perhaps?
Although your model porves a general case for reducing distrotion by using
RF addition, it is perhaps a bit too simplified for the magnetic tape case.

I could be wrong, I have forgotten most of that tape stuff (40 years ago huh),
what do you think?

 

[Kevin Aylward]

But it does,

How do you know that? Have you done, or know of actual tests that
distinguish between non-linearity and hysteresis effects?

I think your model needs some refinement perhaps?

Its a more complicated to include memory (phase) effects, one needs a
voltear seriers, or such like, but even this I think is a problem for
hysteresis.

Although your model porves a general case for reducing distrotion by
using RF addition, it is perhaps a bit too simplified for the
magnetic tape case.

I don't think so, as far as basic features.

I could be wrong, I have forgotten most of that tape stuff (40 years
ago huh), what do you think?

It accounts for some main, known factors. For example, the nyquist limit is
2 X fmax, this analysis shows that you need at least 3 times fmax for the
bias signal, and this is exactly what is done. Typically bias frequencies
are 60khz-80khz.


Kevin Aylward
[email protected]
www.kevinaylward.co.uk