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Old 25th March 2003, 01:16 PM   #21
paulb is offline paulb  Canada
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I did get an answer from Rod Elliot on this, quite some time ago. He says, and I quote:

Doug Self is a protagonist of the "Optimum Class-B" theory. In my experience, increasing bias current reduces distortion - I have *never* seen it increase again as bias is increased.

If the amp is any good to start with, you won't hear any variation in distortion at all. A sinewave is the best test, but most amps have distortion below the audible threshold.


traderbam,
Sorry for the glib reply. This is a great city. You should come visit your brother and spend a day or two hiking or skiing in the Rockies.
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Old 25th March 2003, 01:28 PM   #22
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Paul: glib? I didn't think so. I have been to Calgary and Banff several times and it is a beautiful place. You are very lucky.

I wonder what Rod Elliot means by "A sinewave is the best test, but most amps have distortion below the audible threshold". How can it be the best test if most amps cannot be distinguished by using it? Or have I missed something?
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Old 25th March 2003, 04:11 PM   #23
paulb is offline paulb  Canada
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Keep in mind that Rod's comment was a quick email reply. I'm not sure exactly what he meant by sinewave being the best test.
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Old 25th March 2003, 07:15 PM   #24
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I think Self is correct if the criterion is THD measurement. Measured distortion often increases at higher bias on BJT Class AB amps.

It might be fair to speculate that Rod Elliot's statement refers to subjectively experienced distortion, and of course the statement "often increases" might not apply to his designs.
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Old 25th March 2003, 07:48 PM   #25
Pabo is offline Pabo  Sweden
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Default Optimum bias point!

Self has measured the optimum bias level in form of emitter voltage in a BJT stage and as was mentioned earlier by MarelvdGit depends on the BJT transfer function. Self comes to the conclusion that about 23mV per emitter resistor is the optimum bias point but it is sligthly dependant on the resistance value.

The best test tone for an amplifier should be a signal in which the level varies all the time and in which the highest frequency is 20kHz. A sine wave does just that. An intermodulation test signal seems even better as it consists of multiple sine waves at different frequencies but the result when measuring IM-distorsion is allmost exclusively related to the THD-distorsion.
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Old 25th March 2003, 09:52 PM   #26
mlloyd1 is offline mlloyd1  United States
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Default Re: Re: The sweet spot!!!

mikek:
would you say more about this:

Quote:
Originally posted by mikek
.....
Self's schemes for maintaining thermal stability over the long term are not very convincing......eventually bias will drift
Does your comment include the bias circuit he uses for the class a design? That one seemed more stable than necessary to me. Maybe I missed something?

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Old 25th March 2003, 10:29 PM   #27
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Default Re: Re: Re: The sweet spot!!!

Quote:
Originally posted by mlloyd1
mikek:
would you say more about this:



Does your comment include the bias circuit he uses for the class a design? That one seemed more stable than necessary to me. Maybe I missed something?

mlloyd1
Hi mlloyd..
Self's class A design does not require thermal compensation for bias stability, as it uses negative feedback for quiescent current control.......

I was refering to his thermal compensation methods for class B amps., which evidently ameliorate the problem of thermal tracking and longterm bias-drift, but cannot completely eliminate it, as this would only be satisfactorily accomplished by the transistor manufacturers incoorparating such a thermal compensating transistor directly on the same silicon real estate as the power transistor....

P.S: Still haven'nt received those error-correction schematics you promised?
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Old 26th March 2003, 05:59 AM   #28
Pabo is offline Pabo  Sweden
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Default Bias correction circuitry

As mikek mentioned the thermal stability is poor in Selfs designs as well as in any other designs except for some Kenwood amplifier ten years ago where they actually integrated a sensing diode in the transistorchip.

Audio Amateur presented a couple of amplifiers about five years ago where they used an OPAMP to measure and correct the bias current. The problem as we all know is that the bias current can only be measured in the overlap region in which the total voltage over the emitter resistors is the lowest. The circuit was based on a OPAMP which measured the voltage over the two resistors and discharged a capacitor down to the lowest detectable voltage plus a diode drop. The capacitor was then charged through a high value resistor until the next time the voltage came down to this low level again.

When I saw the circuit I had recently read Selfs book and I therefore saw great opportunities in getting the critical bias current stable.

After having worked on the circuit for a while I tested it at lower frequencies and discovered that it didn't work. The time constant had to be so long that the voltage over the capacitor did't increase more than a few millivolts between each discharge pulse which also ment that I was better of with thermal compensation

As I see it the only way of doing this is to use a sample and hold circuit which is triggered with a zero detector. Has anyone seen such a circuit?
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Old 27th March 2003, 04:35 AM   #29
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The easiest way to measure bias in a complementary follower during operation is to measure the emitter to emitter voltage (the voltage across both emitter resistors in series) and take the minimum value. A minimum-hold circuit (opposite of a peak-hold) will put out a usable value for driving an opto-isolator used as a bias transistor.

If the circuit is not a complementary follower, this still can be done, but becomes slightly more complicated in the sensing.

I produced such a circuit for Threshold on my way out the door, and it was shown at CES, but never produced.
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Old 27th March 2003, 05:49 AM   #30
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There are at least two ways to prevent quiescent current stability
problems without sample-and-hold-like circuits and without a sensing diode on the same die as the output transistor.

One way is current dumping, where a low-current class A and a high-current class C stage are combined in a very ingenious error correction scheme, which has been used commercially in the QUAD solid-state amplifiers since 1976 or so. The patent on this technique has passed its due by date, so everyone is free to use it, also commercially.

The second way is by using a non-linear common-mode loop. As
far as I know, it was invented by Johan H. Huijsing and Frans Tol in 1976, it is frequently used in operational amplifiers, but rarely in audio power amplifiers. The Philips TDA1514A power amplifier IC is an exception; it uses a harmonic mean loop.

Build a non-linear circuit which senses the currents through both output transistors and which generates an output signal that depends mainly on the smallest of the two output device currents. That is, the non-linear circuit should be a smooth approximation to a minimum selector. Then make an additional feedback loop that increases or decreases the currents
through both output devices until the output of the non-linear network equals a reference value. This kind of loop is known as a class AB bias loop or non-linear common-mode loop.

For example, if the non-linear network calculates the harmonic mean of the output device currents (2*I1*I2/(I1+I2)), and the feedback loop makes this equal to Iref, then in the quiescent point, I1=I2=Iref. When the output current of the amplifier is much greater than the quiescent current, one output device will conduct a large current defined mainly by the output voltage and load impedance, while the non-linear common-mode loop makes
the current through the other output transistor approach 0.5*Iref.
Therefore, the output stage automatically becomes non-switching.

Making a harmonic mean circuit with discrete transistors or with
cheap transistor arrays is difficult, but there are other non-linear
functions which give similar performance. For example, you can easily approximate an e^(-I1*R*q/kT)+e^(-I2*R*q/kT) function with a couple of small resistors and two matched small-signal transistor pairs. This is actually what I use in my power amplifier.

Coming back to the conventional complementary emitter follower,
the factor ln(2) in my previous post is a mistake. It can be shown
that when the bias voltage across each emitter resistor equals kT/q in the quiescent point, and the transistors behave according to the exponential law Ic=Is*exp(Vbe*q/(kT)), then the transconductance in the quiescent point equals the transconductance when one output device is switched off and the other conducts a large (infinite, in theory) current. This should be pretty close to the optimum. Indeed, kT/q~=26mV at room temperature, which is close to Self's 23mV.
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