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PMA 7th September 2004 07:37 PM

Quote:

Originally posted by Eva

So using a standard LTP and VAS would do exactly the same job I think, but in a less fashionable way

Any amplifier with global feedback is just an 'error correction' amplifier

Again - not exactly. The major drawback of your approach lies in very high order harmonics (strongly Iq dependent - hard to maintain optimum Iq for whole temperature and signal range), produced by standard a la Self class B/AB output stage. Feedback factor reducing with increasing frequency is unable to correct them and what is worse, it creates new compounds of harmonic distortion, not contained in a signal before closing feedback. And - low THD is only a very small part of audio design ;) . Necessary, but not sufficient.

Error correction also considerably improves behavior of the output stage at higher frequencies - again - less work for NFB.

capslock 8th September 2004 12:00 PM

changes for bipolar output transistors?
 
Hi Pavel,

how would you change R7-R11 for bipolar output transistors? Would you place a complementary emitter follower before or after this stage?

Regards,

Eric

Tailor 8th September 2004 12:38 PM

:((
 
Hi!

Is that only me, that don't understand how that circuit works ??

This type of Hawksford correction:
http://www.diyaudio.com/forums/attac...&postid=330446
is for me enigma too :(

And yes, I have already read Hawksford papers, twice.

Could someone be so kind and explain it to me (us maybe) ?

PMA 8th September 2004 01:28 PM

Re: changes for bipolar output transistors?
 
Quote:

Originally posted by capslock
Hi Pavel,

how would you change R7-R11 for bipolar output transistors? Would you place a complementary emitter follower before or after this stage?

Regards,

Eric

Eric,

the circuit's behavior depends on components values considerably. I have simulated it for diferent MOSFET output pairs and I have to tell that resistor values matter if you change for example to 540/9540 from the 413/118 used. Please see different resistors for N-MOS and P-MOS, this results from different characteristics of both types. Simulation result worked well for 418/113 with 1 exception - R7 had to be decreased from 10k (simulation result) to 2k (experimental result). Decreasing R7 increases Vbias and Iq.

I assume that the circuit could be used for BJT's. Then we should omitt D1-D4 diodes and change values of all resistors, or decrease current of both CCS's (high CCS current helps to drive MOSFET's capacitance). I do not have enough courage to recommend the actual values of resistors for BJT's. I would suggest to simulate the circuit first and then to build it. It should be easier to use double EF as the output stage, to have more "space" for error voltage correction function.

Regards, Pavel

cunningham 9th September 2004 12:57 AM

Quote:

Originally posted by PMA


Again - not exactly. The major drawback of your approach lies in very high order harmonics (strongly Iq dependent - hard to maintain optimum Iq for whole temperature and signal range), produced by standard a la Self class B/AB output stage.

Error correction also considerably improves behavior of the output stage at higher frequencies - again - less work for NFB.

You plan to recreate very high order harmonics(to cancel those generated by amplifier) with an OP-AMP? How's the slew rate? Hope there isn't too many transistors in that OP-AMP chip...miller will sqander any very high frequencies.

PMA 9th September 2004 06:58 AM

Quote:

Originally posted by cunningham


You plan to recreate very high order harmonics(to cancel those generated by amplifier) with an OP-AMP? How's the slew rate? Hope there isn't too many transistors in that OP-AMP chip...miller will sqander any very high frequencies.

I did not catch your point well. The opamp, as mentioned, is the AD844, current feedback opamp with slew rate of 2000V/us. I do not plan to recreate very high order harmonics, but trying to prevent their creation, that's why the error correction is done. The high order harmonics are produced by operation of standard class B and AB output stages.

Eva 9th September 2004 07:55 AM

PMA is right in this point :

That 'four-transistor-exotic-op-amp' acting as a gate driver has potentially very high bandwidth due to its 'single-transistor-in-the-signal-path' nature and thus linearizes crossover distortion to some extent up to higher frequencies that any global feedback system

However, it provides relatively poor linearization of output devices in its linear region at audio frequencies, but this job is accomplished by global feedback

Anyway, the switching spikes I've seen in VAS current waveforms of real circuits [not simulation] when driving AB output stages with slow Ft=3Mhz bipolar output transistors had less than 1uS width so we are talking about harmonics above 500Khz

Never trust simulations of the switching or saturation behavior of bipolar or MOS devices. Models doesn't reflect at all actual device behavior in these circumstances

Eva 9th September 2004 09:00 AM

This is a test amplifier I designed and built last week :
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And these are some actual measurements. Supply rails are +-40V and load is 3.4ohms resistive. Signal is 10Khz sine. There is actually a single set of drivers and oputput devices with 50mA bias. Blue trace is the output voltage, and red trace is the voltage drop across R32 so it clearly shows the input signal plus the error-correcting signal coming from the LTP

The circuit has global feedback up to 2.5Mhz and uses pole-zero compensation to mantain high open loop gain up to 10Khz [102dB@1Khz, 97dB@10Khz] with stability. Note that even such a small 68pF C.dom capacitor strongly dominates the VAS input impedance over the output stage base drive current. Note also that PSPICE simulation is worthless here since it shows absolutely different waveforms

Overall waveforms :
An externally hosted image should be here but it no longer works. Please upload images instead of linking to them to prevent this.


Detail of PNP to NPN switch spike :
An externally hosted image should be here but it no longer works. Please upload images instead of linking to them to prevent this.


Detail of NPN to PNP transition, no clear spike :
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Note also the width of the spikes that clearly reveal harmonics starting at 1Mhz but no distortion below 1Mhz

Finally, the questions are :

- Do you think that these >1Mhz non-linearities may be ever audible?
- Do you think that Hawksford's 'output stage linearizing op-amps' are worth trying for such a circuit?

PMA 9th September 2004 05:15 PM

Dear Eva,

I am sure that you have no doubts about the fact that I do also measure, simulation is just a tool to find component values in quite effective way.

My question is different - are you willing to e-mail me and write a little bit about your professional background?

Regards,
Pavel

PMA 9th September 2004 05:25 PM

Quote:

Originally posted by Eva


Do you think that these >1Mhz non-linearities may be ever audible?
- Do you think that Hawksford's 'output stage linearizing op-amps' are worth trying for such a circuit?

According to a large number of our listening tests there is an audible difference between the same stage with and without error correction. There is also an audible difference between well HF filtered and non-filtered signal from a CD player. There is a considerable improvement when using audio link signal transmission with buffered output and properly terminated signal cable at both ends. These phenomenae should be inaudible. I am an EE with 25 years experience in analog measuring instruments design. I have no exact answer. The only think I know is that our traditional measurements have very very low correlation with audible results. Our biggest fault is that we mostly measure in repetitive, periodical and selective manner.


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