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Old 12th February 2010, 08:22 AM   #111
hahfran is offline hahfran  Germany
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Yes I have been testing the simulation under various aspects...I am a retired mathematician and my first response is always I don't believe any program unless I know what is actually programmed....
Indeed the source resistance has ( and should have) an effect but I did not see any effect of the value of the cap in terms of "steady state".
I have now redesigned a bit such that emitter and collector resistors resp. are inserted from which the feedback is taken via 100 Ohms resistors each but haven't found out how to simulate iq thermal stability.
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Old 12th February 2010, 10:32 AM   #112
AKSA is offline AKSA  Australia
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Elvee,

Quote:
Q6 cannot be a NPN: this would make the positive half-side inverting, and it would conflict with the non-inverting negative side.
The topology is in fact a circlotron with PNP composites at the output, and the Rush cascode as the phase shifter.
You are right; however, the device is incorrectly drawn because it is inverted; emitter should be to the upper, 10R resistor, while collector should be to the base of the npn output and the 68R.

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Old 12th February 2010, 11:05 AM   #113
Elvee is online now Elvee  Belgium
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Quote:
Originally Posted by hahfran View Post
Yes I have been testing the simulation under various aspects...I am a retired mathematician and my first response is always I don't believe any program unless I know what is actually programmed....
Indeed the source resistance has ( and should have) an effect but I did not see any effect of the value of the cap in terms of "steady state".
I have now redesigned a bit such that emitter and collector resistors resp. are inserted from which the feedback is taken via 100 Ohms resistors each but haven't found out how to simulate iq thermal stability.
You have to use "temp" as a source for DC sweep simulation: see example
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Old 13th February 2010, 08:56 AM   #114
hahfran is offline hahfran  Germany
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Quote:
Originally Posted by Elvee View Post
I arrive at a much lower distortion figure under steady state conditions.
But you have to take care that the circuit has effectively reached its steady state before you make the measurement.
What I do is to run the circuit for a very long time (1s) with the intended signal level, and then, I look at final voltage across the cap and include .ic statements forcing that voltage at the next run.
If you rely on a .op, it wont work, because the bias level is dependent on the dynamic operating conditions.

With 15Vpk, the distortion is under 0.02%, almost pure 2nd harmonic.

But the circuit has a sweet spot regarding the source resistance: with 15K at the input, the figure is down to 0.003%.
Note that on your schematic, the cap value is 0.22 farad.
Elvee
I could not confirm this distortion level. To which schematic does it refer?
Nevertheless it appears the max output for about 1% thd is limited to about 17 volts. The QUAD 303 triple cascade goes as far 25 V and appears to show no switching, too.
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Old 13th February 2010, 11:00 AM   #115
Elvee is online now Elvee  Belgium
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Quote:
Originally Posted by hahfran View Post
Elvee
I could not confirm this distortion level. To which schematic does it refer?
Nevertheless it appears the max output for about 1% thd is limited to about 17 volts. The QUAD 303 triple cascade goes as far 25 V and appears to show no switching, too.
You can use this one without any modification: the .ic is OK for 15V.
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Old 13th February 2010, 02:04 PM   #116
hahfran is offline hahfran  Germany
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Ok. Thanx for your help. I see now why we disagree...I do pencil and paper and assume hence the amp is powered on ...LTSpice doesn't. To simulate one has to start the sine about 1 sec delayed and perform transient analysis "triggered". This doesn't seem to work except with your method of line commands. Apparently the Tdelay parameter applies to DC offset, too. Using your simulation of triggering .tran I find indeed at 20 V a wonderful harmonics spectrum. max out for thd 1% is now a matter of the values of the Baxandall clamp and to a lesser extend the value of the cap. For large signal performance analysis LTSpice doesn't care about transistor models.

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Old 13th February 2010, 03:00 PM   #117
hahfran is offline hahfran  Germany
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Ok. Up to 23 V input the harmonics spectrum remains simply nice... at 25 V thd is 0.154239% but 3rd h is higher than h2.
This looks like a wonderful candidate to qualify for active speaker project as that won't require SOAR protection and not even simple current protection as there are no passive crossovers ... the impedances are all well known thus that it is a matter of selecting a power BJT thus that for minimum impedance / max phase shift SOAR is wide enough.
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Old 13th February 2010, 04:50 PM   #118
hahfran is offline hahfran  Germany
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Ok attached the redesigned Vish R12 throu R15 being added, providing some Iq thermal
stability . It can be demonstrated in the sim by adding current sources to simulate increased Iq. Also extending R14 R15 with a parallel RC compensation is possible if necessary. Using 2SC2922 for Q5,Q7 gives a sufficient SOAR margin.
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Old 13th February 2010, 06:39 PM   #119
Elvee is online now Elvee  Belgium
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Thermal stability is not an issue: under no signal conditions, Iq is about 10mA (see #113 above), and doesn't vary much with temperature.
The current increases to over 2A under large signal conditions, but this is a "feature" of the circuit, and has nothing to do with temperature.

It is a sort of "sliding class" amplifier, but the reaction delay makes it awkward.
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Old 13th February 2010, 07:06 PM   #120
hahfran is offline hahfran  Germany
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Quote:
Originally Posted by Elvee View Post
Thermal stability is not an issue: under no signal conditions, Iq is about 10mA (see #113 above), and doesn't vary much with temperature.
The current increases to over 2A under large signal conditions, but this is a "feature" of the circuit, and has nothing to do with temperature.

It is a sort of "sliding class" amplifier, but the reaction delay makes it awkward.
There is no reaction delay I have explained that post 2084110.
Of course junction temperature is an issue. It is always an issue because of the e^kT dependency. This class B reaches max dissipation at about 2/3 of max power. Since "heat" does not flow, but diffuses it takes some time to cool the junction down. So if after a few milliseconds of max dissipation, there is a silent passage, the Iq will be pretty much higher well above 100 mA if I assume the junction temp is about 100 C . That can lead to uncontrolled thermal runaway. And very often does.
Even if not, the operating point is not constant. This may or may not be an issue in terms of sonic quality but anyway such does neither appear in class A nor in class B tube amps.
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