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Bob Cordell Interview: Error Correction
Bob Cordell Interview: Error Correction
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Old 11th November 2006, 02:06 AM   #421
Wavebourn is offline Wavebourn  United States
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Bob Cordell Interview: Error Correction
Quote:
Originally posted by lumanauw
The input differential pair is pair of Q201-Q202.
There are 2 sets of back to back diodes, one set is between the bases and another set is between the collectors.
Are these 2 sets = "soft clipping"?
It decreases gain of the differential stage affecting feedback, when overload protection starts working. Far from soft clipping.
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Old 11th November 2006, 03:52 AM   #422
lumanauw is offline lumanauw  Indonesia
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Quote:
Far from soft clipping.
Then what is soft clipping?
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Old 11th November 2006, 04:12 AM   #423
lumanauw is offline lumanauw  Indonesia
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is this soft clipping? If the voltage is 28V (4ohm). Switch S801 is on (connected). Anode of D802 will be sitting at about -0V9 and katode of D804 sitting at +0V9.
If the dioda D802-D804 conduction is 0V6, then there is limit of -1V5 to +1V5
If the input signal at base of Q402 more positive than +1V5 or more negative than -1V5, then it will be "absorbed" by C410, signal bigger than +1V5 and smaller than -1V5 will not enter the Q402's base.

Is this soft clipping?
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Old 11th November 2006, 05:08 AM   #424
Wavebourn is offline Wavebourn  United States
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Bob Cordell Interview: Error Correction
It is kind of compressor, it adds distortions after output of the amp shows some voltage close to the certain level.


Quote:
Originally posted by lumanauw
is this soft clipping? If the voltage is 28V (4ohm). Switch S801 is on (connected). Anode of D802 will be sitting at about -0V9 and katode of D804 sitting at +0V9.
If the dioda D802-D804 conduction is 0V6, then there is limit of -1V5 to +1V5
If the input signal at base of Q402 more positive than +1V5 or more negative than -1V5, then it will be "absorbed" by C410, signal bigger than +1V5 and smaller than -1V5 will not enter the Q402's base.

Is this soft clipping?
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Old 11th November 2006, 03:08 PM   #425
Bob Cordell is offline Bob Cordell  United States
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Default Re: Re: Folded emitter followers

Quote:
Originally posted by darkfenriz


I am not familiar with it in practice.
What is surprising or unexpected in HF behaviuor of this circuit?

regards
Adam

Adam,

One just has to be careful because the output is fed back to the collectors of the folded emitter followers with unity gain. At high frequencies the impedance of Ccb gets smaller. If that, in combination with some other reactive or peaking effects gets you close to positive feedback at high frequencies, you may have ringing or oscillation. It is a good exercise to SPICE this circuit. A very slight amount of LFP between the output node and the tied collectors of the folded EFs, done properly, can help. One also needs to be mindful of inductance in the base circuits of the folded EFs.

Bob
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Old 11th November 2006, 04:47 PM   #426
Bob Cordell is offline Bob Cordell  United States
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Default Importance of Damping Factor

Quote:
Originally posted by gerhard



I think this damping factor is pretty much overrated. The voice coil is accelerated and stopped by current (hopefuly, and as far as the amplifier is concerned), and an average 8 Ohm speaker has abt. 4-6 Ohm DC resistance. Now let your amplifier have 1 mOhm or 10 or 100 mOhm source impedance, -- it makes no difference. A value as bad as 100 mOhm would count as confession of incompetence.

regards, Gerhard

As a factoid, my B&W 804 S features 4.4 Ohms at DC (just checked), and although not measured myself, it won't drop below that over frequency IIR the data sheet correctly.

Sometimes it is unfortunate that it is called damping factor, because that terminology tends to focus the issue too much on that one aspect you have mentioned here.

The real issue is that low damping factor implies relatively high output impedance. A classic vacuum tube amplifier, or even a solid state amplifier with no negative feedback, may only have a damping factor of 10. This corresponds to an output impedance of 0.8 ohms.

Now recognize that most loudspeakers have widely varying impedance as a function of frequency - not just in the bass region. It is not uncommon for a so-called 8-ohm speaker system to have a minimum impedance of 4 ohms and a maximum impedance of 40 ohms. Think about what this does to the final voltage frequency response as seen at the speaker terminals.

At places where the speaker impedance is actually the rated impedance of 8 ohms, we are down by about 1 dB as a result of the amplifier's 0.8 ohm output impedance against the 8 ohm load (relative to the amplifier's no-load output). At places where the speaker impedance is very high, we are down very little, so relatively speaking we are up 1 dB relative to the nominal. Where the speaker impedance dips to 4 0hms, we are down about a total of 2 dB, so we are down by 1 dB relative to the nominal. We thus have a frequency response swing of +/- 1 dB. This is definitely non-trivial.

Often, these large speaker impedance variations occur in the crossover region.

Cheers,
Bob
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Old 11th November 2006, 05:12 PM   #427
PMA is offline PMA  Europe
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Bob,

for this class A push-pull output stage with strong local feedback, the output impedance was 0.1 ohm.

Cheers,
Pavel
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Old 11th November 2006, 05:13 PM   #428
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Default Re: Importance of Damping Factor

Quote:
Originally posted by Bob Cordell



... We thus have a frequency response swing of +/- 1 dB. This is definitely non-trivial.

Often, these large speaker impedance variations occur in the crossover region.

Cheers,
Bob
Isn't the room influence much larger than that? I suppose you could hear the same difference in sound between your MOSFET and KT88 amps in every room you listened to them and probably even with different loudspeakers?
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Old 11th November 2006, 05:24 PM   #429
Wavebourn is offline Wavebourn  United States
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Bob Cordell Interview: Error Correction
Quote:
Originally posted by PMA
Bob,

for this class A push-pull output stage with strong local feedback, the output impedance was 0.1 ohm.

Cheers,
Pavel
Do you mean 0.05 Ohm?
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Old 11th November 2006, 05:27 PM   #430
PMA is offline PMA  Europe
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Quote:
Originally posted by Wavebourn


Do you mean 0.05 Ohm?
In theory, almost yes. Simulation showed 0.07 ohm. But amp was built and had been operated for 2 years. Measurements showed damping factor 80 at 8 ohm load.
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