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Old 19th April 2012, 07:41 AM   #4471
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Quote:
Originally Posted by Wavebourn View Post
It is obvious: we amplify POWER. It can't consist of VOLTAGE or CURRENT only, it consists of BOTH.
No sure what you mean by this - in principle or just in audio power amps?
I can easily amplify voltage or current without amplifying power.

jan
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Old 19th April 2012, 09:19 AM   #4472
DF96 is offline DF96  England
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Quote:
Originally Posted by benb
unless it has a time machine
Doh! Why don't I think before hitting 'post quick reply'?

PS that was a rhetorical question!
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Old 19th April 2012, 09:55 AM   #4473
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Originally Posted by janneman View Post
Yes but those capacitor current and voltage slew rates are not a property of the cap, they are what the cap can handle without breaking.
Current slew into a cap and the resulting voltage are a function of whatever signal you impress on them (and some secondary effects like the resistance and inductance of cap connection wires and such).

jan
Completely agreed, but what's the sense in making a hell of fast amplifier, capable of say 200 V/uS as far as the electronics go, only to be slew limited to say 40 V/uS by the power supply capacitors?

This notwithstanding the accepted practice that you are done with slew rates if your slew rate is 0.5V/uS per every peak volt of output signal (SOURCE: National Semiconductor, Audio cookbook). This works out to 20V/uS at 20 kHz with nominally 100 Watts (40V) into 8 Ohms. And when do we ACTUALLY have a real world full power signal at 20 kHz?

And this is related to voltage; what about a power supply cap's capability to deliver current on demand, i.e. its current slew rate?

Don't you think all of the wonder capacitor manufacturers would just love to brag about these specs - if they could? Yet, they remain silent on technical aspects, and brag about silk and probably dairy milk they make them from.
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Old 19th April 2012, 09:58 AM   #4474
dvv is online now dvv  Serbia
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Originally Posted by janneman View Post
Yes the ringing is at the filter cutoff freq, I missed that. So, whatever the transient period (on the pic indeed a 1kHz square wave), the ringing is always at 15kHz then.
Makes it all the more surprising I heard a difference in 'sharpness'. I mean, I am pretty sure I'm rather deaf at 15kHz...

jan
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Old 19th April 2012, 09:59 AM   #4475
dvv is online now dvv  Serbia
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Quote:
Originally Posted by janneman View Post
No sure what you mean by this - in principle or just in audio power amps?
I can easily amplify voltage or current without amplifying power.

jan
We meant audio power amplifiers only.
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Old 19th April 2012, 10:00 AM   #4476
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Originally Posted by DF96 View Post
Doh! Why don't I think before hitting 'post quick reply'?

PS that was a rhetorical question!
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Old 19th April 2012, 11:09 AM   #4477
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Originally Posted by dvv View Post
Completely agreed, but what's the sense in making a hell of fast amplifier, capable of say 200 V/uS as far as the electronics go, only to be slew limited to say 40 V/uS by the power supply capacitors?[snip].
These are unrelated issues. If you have an input LTP with a 2mA tail current and a 50pF compensation cap in the Vas, your max slew rate is 4V/uS. With a 10mA tail current and 10pF compensation your slew rate increases to 100V/uS.

But that has nothing to do with anything happening in the power supply caps. Those caps are re-charged just about once every 10 (or 8.25) mS for 1 or 2 mS duration with a very high current pulse, and discharged by the amp load current.

jan
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Old 19th April 2012, 11:50 AM   #4478
Bazukaz is offline Bazukaz  Lithuania
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But why very high slew rate is needed?

One example is Quad ESL.
Doe to very capacitive loading on amplifier full power bandwidth is just few kHz depending on amplifier current rating. That would translate to max. slew rate really low compared to today's standards. And, if you try to exceed it, amplifier's short circuit protection probably kicks in. Should be horrible?

Still some people think Quad esl are one of best sounding speakers.

So why bother with square waves ? IMO it is just useful to test stability of amp/preamps.
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Old 19th April 2012, 01:06 PM   #4479
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Quote:
Originally Posted by Bazukaz View Post
But why very high slew rate is needed?

One example is Quad ESL.
Doe to very capacitive loading on amplifier full power bandwidth is just few kHz depending on amplifier current rating. That would translate to max. slew rate really low compared to today's standards. And, if you try to exceed it, amplifier's short circuit protection probably kicks in. Should be horrible?

Still some people think Quad esl are one of best sounding speakers.

So why bother with square waves ? IMO it is just useful to test stability of amp/preamps.
Fully agree.

jan
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Old 19th April 2012, 01:11 PM   #4480
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Somebody asked for the 15kHz filter topology.

jan
Attached Images
File Type: png 15_kHz_filter.PNG (118.3 KB, 74 views)
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