Can RF bias be used to cancel crossover distortion?

[Jan Panteltje]

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

Ah, what I tried to say is that any distortion is reduced,
also that caused by hysteresis.
This because audio tape recordings made with bias are pretty much clean.
The required bias amplitude depends on the tape characteristics
(BH curve among other things).
So I would expect these parameters to be present in your math, if
it represented a compete detailed model.
Maybe this site can be of some help:
http://gcwebtw.com/Knowhow/MagneticRecording/MagneticRecording.htm
Look under 'V', where its says:
'Setting the peak bias field approximately equal to the coercivity creates
a curve which is more linear (at least for low values of H!)'

So he we have coercivity as a new parameter.....

 

[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

Indeed yes.

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, althoughit 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?

It is wrong. As you reduce the bias current in a bipolar, the little
"re" decreases. At some point, the "gm" of the output section falls
below 1/2 its normal value. Decreasing below that point leads to an
increase in distortion. This means that the AB case is lower
distortion. You can push the idle current number down but never quite
to zero.

 

[Arie]

"Jan Panteltje" ...

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.
...

Reminds me of the 'Mule' amplifiers I used to build in the 70's, output
stage was a Ge type PNP (forgot which) + Si type NPN (2N3055). Biasing was
just a 1N4004, thermally coupled to the output stage. No extra R in series,
no trimming. Idle current about 50...100mA. Sounded very good for its
purpose: guitar amplifiers. Did a whopping 20W 'rms' (O.K., with a bit of
clipping) with 4 transistors.

Arie

 

[[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....

?

In real life the load is reactive. I think that while in the dead zone
of the class B amp, it will not be well behaved, i.e. damped.

 

[Jan Panteltje]

In real life the load is reactive. I think that while in the dead zone
of the class B amp, it will not be well behaved, i.e. damped.

You are right, much of the benefit is lost if I add an inductor in
series with the load:
ftp://panteltje.com/pub/l_without_rf.gif
ftp://panteltje.com/pub/l_with_rf.gif

So the dead zone should still be a small as possible.

 

[Rich Grise]

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 ?

It sounds like you're approximating class D here. ;-)

Cheers!
Rich

 

[Kevin Aylward]

Ah, what I tried to say is that any distortion is reduced,
also that caused by hysteresis.
This because audio tape recordings made with bias are pretty much
clean. The required bias amplitude depends on the tape characteristics
(BH curve among other things).
So I would expect these parameters to be present in your math, if
it represented a compete detailed model.
Maybe this site can be of some help:
http://gcwebtw.com/Knowhow/MagneticRecording/MagneticRecording.htm
Look under 'V', where its says:
'Setting the peak bias field approximately equal to the coercivity
creates a curve which is more linear (at least for low values of H!)'

One could argue here that the HF bias forces the curve (samples) for low
frequencies to average the +Hc to -Hc curves, forming a net (average) curve
that always goes through the zero point, so may be in this way that
hysteresis effects are reduced. Seems very plausible, now that I have
thought about it. The sums are probably a bit messy though.

The author does make a statement that is obviously incorrect though, to wit,
"The frequency of the bias is not critical so long as it is greater than the
maximum signal frequency ". Essentially, *any* non-linear process will
result in sum and difference terms showing up, so if the bias were say,
20khz, and the signal were 19khz, then a 1khz signal would be produced. So,
on this basis alone, one needs 2F. Typically, the bias is always > 60kz and
that is for a machine with a 15khz signal BW.

Kevin Aylward

www.kevinaylward.co.uk

 

[Martin Griffith]

On Mon, 13 Oct 2008 20:46:42 GMT, in sci.electronics.design Jan

snip

Also the bias symmetry (the waveform) needed to be very precise to reduce noise, some tape
machines had a special adjustment for that (Studer?).

Not on Studer, up to the A80 range, maybe Ampex.
The A80 IIRC had a master LC oscillator at 240KHz for bias, but this
was divided (SN7492?)down to 80KHz for the erase then through a LC
filter for each signal, no funny distortion tweaks, just simple


martin