Class B w/o crossover distortion (1975) - Page 18 - diyAudio
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Old 5th March 2010, 01:21 PM   #171
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Ideas:

1) Current mirror with the LTP? Probably not worthwhile - it will improve THD20, but will also kill H2 and the desirable sonics. It will require 2 additional matched active devices.

2) Replace one of the two MUR460 silicons with a Schottky rectifier - the combined drop is a bit lower, and closer to that of a Darlington. During cross-over commutation, there should be less high-frequency 'hash'.

3) Snubber the rectifier chain with a 47nF/100nF foil cap (already done).

4) Flip the entire modified Visch output stage over to use NPN darlingtons instead of the PNPs shown. They're better spec'ed and less expensive than equivalent P-darlingtons.

5) Switch from bootstrapped VAS to current-source load (probably not really useful except for deep bass response).

(The CFP outputs are more tunable, since the upper half can be tuned independently of the lower half. But that comes at some cost in circuit complexity. When I eventually build this, I'll probably go with integrated Darlingtons, despite poor SOA compared to discretes.)
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Old 5th March 2010, 01:47 PM   #172
hahfran is offline hahfran  Germany
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According to my results a Schottky increases the odd harmonics. I have no idea why but it is significant. A fast silicon with a 100nf parallel on theo other side reduces higher odd harmonics.
5 is important the source impedance of VAS is a fine tune of the harmonics spectrum.
Normally I avoid current sources. They make some sense in a jFET LTP BJT cascode.
The preamp I have posted appears very suitable as its harmonics are the same as of the amp. As its output impedance is about 50 ohms, a series resistor does a simple job of fine tuning.
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Old 5th March 2010, 06:36 PM   #173
Telstar is offline Telstar  Italy
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Quote:
Originally Posted by hahfran View Post
According to my results a Schottky increases the odd harmonics. I have no idea why but it is significant. A fast silicon with a 100nf parallel on theo other side reduces higher odd harmonics.
I agree, considering that now there are ultrafast silicon diodes with TRR of around 20ns.
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"The total harmonic distortion is not a measure of the degree of distastefulness to the listener and it is recommended that its use should be discontinued." D. Masa, 1938
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Old 5th March 2010, 06:48 PM   #174
hahfran is offline hahfran  Germany
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Here is the complete schematics with VAS. The Q5 Q7 2SD718 are not suitable as SOA is too small but their fT is right and also hfe(f) . Further BD390 are no longer made but again
fT and hfe are right. The amp achieves at 28 Vp into the simulated speaker load a thd <0.02% with odd harmonics all 30dB below even. Even at full output swing the respective non conducting transistors ( in class B) are conducting the min current
is about 10 mA.

An integrator will be added that keeps the dc out component in the mV range, it feeds the non inverting input.
Attached Images
File Type: gif VishVAS.gif (23.5 KB, 535 views)

Last edited by hahfran; 5th March 2010 at 06:54 PM.
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Old 1st July 2010, 04:57 PM   #175
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Default Vish Class-B: Commercial Amplifier Devices ??

This topology looks very interesting. I am therefore looking to comercial amps with such power amplifier circuit.
Are there commercial audio amplifier products on the marked uses this topology?

Last edited by tiefbassuebertr; 1st July 2010 at 05:00 PM.
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Old 19th October 2010, 11:00 AM   #176
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Default Vish Class-B vs. pure Class A (Hard Running)

What about the audible sonic character?
Thank you for comments.
Any news concerning commercial products with Vish Class-B topology?
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Old 19th October 2010, 08:43 PM   #177
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Quote:
Originally Posted by wahab View Post
since you want a current feedback, why not this one ?...
with about 600 mA quiescent current, it display
exceptionnal performances...
yes, this actually very common topology sounds indeed really good -
author and developer of the "Black Devil" was Gerhard Haas, Owner of
EXPERIENCE electronics - Home -
especially by enhanced idle current of power buffer stage.
Additional enhance of sonic quality you will get either through enhancing the quiescent current of the input transistor (in the presently geniue topology <<1mA) or by replace the input transistor through a jFET (better characteristic by lower currents than small signal BjT's).
Quote:
Originally Posted by Elvee View Post
LTspice can provide tone bursts in a number of ways:
Here is one.

Picture 2 shows the detail, and picture 3 shows also the voltage across the capacitor.


It is, because the level of charge of the capacitor (which sets the quiescent current) relies on the splitting factor of the switching modulator formed by D1, Q2 and Q7.
When the average level of conduction of Q2 increases, the duty cycle of D1's conduction also increases, diverting the 15mA bias away from Q7 more of the time.
This decreases the charge going into C1, and corrects the Iq through Q2 (and Q1).
By the try to get simulation results of the Visch topology I haven't success, because the long tailed input pair don't conduct.
Is the paper with the complete article here posted (Wireless World, April 1975, p166)? Schematic and shortform desribtion is to see about post #7.
What have I overlooked (have a look to the attachement) ?
Unfortunately I don't see DC values by all simulation results here.
Attached Files
File Type: pdf Circl.Elvee Vish.ckt.pdf (7.3 KB, 103 views)

Last edited by tiefbassuebertr; 19th October 2010 at 09:02 PM.
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Old 19th October 2010, 09:07 PM   #178
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Quote:
Originally Posted by hahfran View Post
Here is another non switching circuit
Please let me know the Electronic World edition and launched year, where this circuit is inside - thank you.
I recall, the developers of this circuit comes from
University of Twente - University of Twente
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Old 20th October 2010, 07:58 AM   #179
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by tiefbassuebertr View Post
By the try to get simulation results of the Visch topology I haven't success, because the long tailed input pair don't conduct.
Is the paper with the complete article here posted (Wireless World, April 1975, p166)? Schematic and shortform desribtion is to see about post #7.
What have I overlooked (have a look to the attachement) ?
Unfortunately I don't see DC values by all simulation results here.
I see a number of problems in your sim: first there is an error in the schematic: the emitter of Q7 must be connected to the anode of D1.

Then, I see that Q4 has -38.5V on its base and -88nV on its emitter. That's impossible, there must be something wrong with your model of the BCP53.

And I found that the Visch topology is prone to latch-up, both in simulation and in reality.

To make it work in sim, I had to force an initial condition on the collector of Q7 (Vn009): -1.69V.
Attached Images
File Type: gif Visch.GIF (52.2 KB, 324 views)
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Old 23rd October 2010, 03:04 PM   #180
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Quote:
Originally Posted by Elvee View Post
I see a number of problems in your sim: first there is an error in the schematic: the emitter of Q7 must be connected to the anode of D1.

Then, I see that Q4 has -38.5V on its base and -88nV on its emitter. That's impossible, there must be something wrong with your model of the BCP53.

And I found that the Visch topology is prone to latch-up, both in simulation and in reality.

To make it work in sim, I had to force an initial condition on the collector of Q7 (Vn009): -1.69V.
Thank you for this advices. The model of BCP53 is approx. the same than BD140 and in all other circuits it works without trouble.
The not present connection between the emitter of Q7 and the anode of D1 was definitely an error from me. But after correction that, no change in results and behavior was to observe.
The input stage about
4QD-TEC: Low distortion Audio amplifier
looks familiar with the Visch amp input. Perhaps this will help to find the actually reason for not correct simulation results.
One error now I have found: the voltage at the base of Q6/5 cannot be the same. That must differ at least 1,3 volts.
The cathode of D2 seems to be also wrong connected - there must be 2 diodes in serial and connected to -15V by R8. Otherwise flows the ref current about the B-E direction instead aboput the diode(s)
Is it possible to show the dc conditions in your simulation?

Last edited by tiefbassuebertr; 23rd October 2010 at 03:28 PM.
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