Class B w/o crossover distortion (1975) - Page 19 - diyAudio
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Old 23rd October 2010, 03:52 PM   #181
Elvee is offline Elvee  Belgium
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Here are the voltages:
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File Type: gif VischVolt.gif (20.1 KB, 435 views)
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Old 23rd October 2010, 06:17 PM   #182
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Quote:
Originally Posted by Elvee View Post
Here are the voltages:
Now the circuit works also by me correctly (still by simulation - at least concerning the DC conditions).
Only my first estimate in my previous post causes the simulation error. My second estimate (cathode of D2 seems to be also wrong connected) wasn't right.
Attached Files
File Type: pdf Circl.Elvee Visch-II.pdf (9.2 KB, 99 views)
File Type: pdf Circl.Elvee Visch-II Frequency Response.pdf (5.0 KB, 46 views)
File Type: pdf Circl.Elvee Visch-II Clipping Behaviour.pdf (8.7 KB, 51 views)

Last edited by tiefbassuebertr; 23rd October 2010 at 06:26 PM.
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Old 2nd April 2011, 05:08 PM   #183
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Time to wake this thread up again.

Here's another modification to the Visch topology - a buffer that combines the reactive bridge from the Quad current-dumping circuit with a simplified Visch darlington output stage, driven by an ultra-low distortion opamp. It's biased in Class-AB with about 25 mA quiescent current in the output stage. There's still a lot of optimization to be done, but the initial results look very promising. At 10V output amplitude into 8 ohms, THD20 is below -120 dB, with H2 dominating.

It's probably most suitable for driving low-impedance headphones, but it could also be used as a line-level buffer. All the devices (except the opamp) are commodity jelly-beans types - the output darlingtons could probably be substituted by TIP122s with nominal impact on sonics.
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File Type: jpg visch_quad_darlington.jpg (82.0 KB, 348 views)
File Type: jpg visch_quad_darlington_6w_fft.jpg (148.7 KB, 335 views)

Last edited by linuxguru; 2nd April 2011 at 05:09 PM. Reason: fix line-break
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Old 2nd April 2011, 07:49 PM   #184
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very interesting idea. Unfortunately I don't understand, from where flows the output current through RL.
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Old 3rd April 2011, 01:00 AM   #185
artu is offline artu  Chile
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This is a Feed Forward error correction amp isn't it?
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Old 3rd April 2011, 12:01 PM   #186
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It's missing the junctions where the wires meet at 90 degrees - an LTSpice .wmf problem. Just assume that all wire-crossings have a junction, except where there is a U-shaped dog-leg at the crossing.

The load is driven by U2, U3 (darlingtons) through L1 (part of the reactive bridge C1-R3-L1-R4). There's a small feed-forward correction (a few mA) from the opamp through R4. It's possible to decrease distortion a bit by decreasing R4 (and rebalancing the bridge), but at some point the odd-harmonic character of the LME49860 output stage will start dominating.
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Old 3rd April 2011, 06:58 PM   #187
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Quote:
Originally Posted by linuxguru View Post
It's missing the junctions where the wires meet at 90 degrees - an LTSpice .wmf problem. Just assume that all wire-crossings have a junction, except where there is a U-shaped dog-leg at the crossing.

The load is driven by U2, U3 (darlingtons) through L1 (part of the reactive bridge C1-R3-L1-R4). There's a small feed-forward correction (a few mA) from the opamp through R4. It's possible to decrease distortion a bit by decreasing R4 (and rebalancing the bridge), but at some point the odd-harmonic character of the LME49860 output stage will start dominating.
Thank you for your explanations.
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Old 4th April 2011, 06:12 AM   #188
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Elvee, I don't mean to nitpick but your VischB crosses above 1A.
And thats just the collectors of the 3055's. Haven't even added
driver currents into that figure yet...
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Old 4th April 2011, 06:31 AM   #189
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Now, thats more like B... 100mA crossing for the big transistors.

Late discovery: With Schottkys as shown, R1 R2 can be Muntz'd.
Attached Files
File Type: txt VischDist1.asc.txt (4.1 KB, 30 views)

Last edited by kenpeter; 4th April 2011 at 06:39 AM.
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Old 4th April 2011, 07:50 AM   #190
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by kenpeter View Post
Elvee, I don't mean to nitpick but your VischB crosses above 1A.
It does, and it doesn't:
It is one of the difficulties with this circuit, the dynamic stability of the quiescent current.
The first pic shows the circuit operating at a very low power.
The Iq is a nice ~30mA. Note however that the positive and negative crossings occur at a different level. This is caused by the time-constant of the Visch "memory cap". Yet, the frequency is not very low.

The second picture is made at 25V^ output; now the crossing occurs at 1.8A.
This could be seen as a "feature": dynamic biasing following the music level.
But in addition to the increased dissipation, it also has a major drawback: it takes time to settle to the new condition: the third pic is taken under the same conditions, but at the beginning, when the circuit didn't yet have time to adapt.
There is a severe clipping for a number of ms.


When the circuit is near idle condition, it spends most of the time servoing the Iq.
But at higher current levels, the waveform on D1 becomes trapezoidal, and the servo action occurs only in the linear mode, during the transitions.
But if, on the other hand the circuit is not perfectly balanced, the duty cycle of the trapeze (almost a square wave) will dominate the average voltage and cause a shift in the bias.
I have made a number of real prototypes, and the problem seems almost impossible to solve satisfactorily.
Attached Images
File Type: gif VischLoPo.GIF (60.3 KB, 220 views)
File Type: gif VischHiPo.GIF (55.4 KB, 203 views)
File Type: gif VischStartUp.GIF (60.6 KB, 89 views)
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File Type: txt VischP.asc.txt (3.4 KB, 5 views)
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