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#11 | |
diyAudio Member
Join Date: Apr 2002
Location: Prague
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Quote:
![]() Error correction also considerably improves behavior of the output stage at higher frequencies - again - less work for NFB. |
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#12 |
diyAudio Member
Join Date: Feb 2002
Location: Germany
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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 |
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#13 |
diyAudio Member
Join Date: Sep 2004
Location: EU
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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) ? |
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#14 | |
diyAudio Member
Join Date: Apr 2002
Location: Prague
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![]() Quote:
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 |
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#15 | |
diyAudio Member
Join Date: Jun 2004
Location: Knoxville
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#16 | |
diyAudio Member
Join Date: Apr 2002
Location: Prague
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#17 |
diyAudio Member
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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 |
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#18 |
diyAudio Member
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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 :
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Detail of PNP to NPN switch spike :
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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? |
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#19 |
diyAudio Member
Join Date: Apr 2002
Location: Prague
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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 |
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#20 | |
diyAudio Member
Join Date: Apr 2002
Location: Prague
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Thread | Thread Starter | Forum | Replies | Last Post |
Re: New error correction amp | mefistofelez | Solid State | 12 | 7th October 2010 06:55 AM |
error correction et al' | sq225917 | Digital Source | 4 | 7th August 2007 10:16 AM |
Will this work as Error Correction? | lumanauw | Solid State | 4 | 8th October 2004 10:52 AM |
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