Extreme slew rates like 200V/us or 300v/us no use in general but can it be achieved? - Page 2 - diyAudio
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Old 10th February 2013, 07:00 PM   #11
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by rhythmsandy View Post
so how to achieve very low output imp? anytips?
Huge NFB levels?
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Old 10th February 2013, 07:11 PM   #12
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wouldnt that rob the bandwidth?... and microdetails..
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Old 10th February 2013, 08:06 PM   #13
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Originally Posted by rhythmsandy View Post
wouldnt that rob the bandwidth?... and microdetails..
You say so. Please explain why it would be the case (not the bandwidth anyway, it will be increased in the same proportions of the FB ratio, not robbed)
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Old 11th February 2013, 12:06 AM   #14
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With amplifiers that use global feedback, a high open loop gain at low frequencies will produce a very low impedance once the loop is closed. For example, if your amp has an open loop output impedance of 1 Ohm, and an open loop gain of 100dB at 50Hz, if you close the loop with 20dB of total gain, you're left with 80dB reduction in output impedance. That turns the 1 Ohm into .0001 Ohm at 50Hz (one decimal place for each 20dB). Of course, that's theoretical, and you still have wire resistance from the amp output to the speaker that adds to the total.

So, lots of feedback and large gauge wire will improve the chances for tight, smooth, effortless bass. That and some decent speakers of course.

Last edited by pete_schumacher; 11th February 2013 at 12:09 AM.
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Old 11th February 2013, 01:18 AM   #15
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Sandy is asking better questions and he has touched upon one of the big marketing hypes in hi fi.

The best (subjectively) amplifier I ever built had a slow slew rate of 1.25 v/uS. Tube amps (with their output transformers) are notoriously slow and faster speeds were easily available with ordinary parts and topologies once transistors became practical.

A slower amplifier will not produce full power bandwith. Is full power bandwidth necessary for high fidelity reproduction of musical programs? No, it is not.

As far as "slamming" bass, damping factor was mentioned and is an important consideration. Damping factor is however highly overrated as well and anything over 50 or so is more than adequate. But the power supply has a great effect on bass performance. Adequate transformers to reduce voltage droop and generous filter capacitors have a tremendous effect on percieved bass response. High slew rates have no effect on bass response.

There are different philosophies and the present mindset is high speed and very high loop gain. Many excellent amplifiers have only had 20-30 dB of loop gain though; if your circuit is linear enough this can work quite well. Tube amps were like that; it would be nigh on impossible to design a stable tube amp with higher loop gain.
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Old 11th February 2013, 02:19 AM   #16
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Quote:
Originally Posted by rhythmsandy View Post
what factors affect the slew rates? is it possible to achieve slew rates of order of 200V/us or 300V/us? Ive read the book frm Douglas slef where he states that there is no use of getting higher slew rates and doesnt make difference...
He's not listening to how amps sound, he's saying that based on his technical understanding.

I reckon there's a correlation between higher slew rates and better sound, but I very much doubt that the higher slew rate is directly responsible. Rather its a marker for better HF linearity and its that linearity which is what gives rise to better sound.
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Old 11th February 2013, 02:22 AM   #17
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Originally Posted by abraxalito View Post
He's not listening to how amps sound, he's saying that based on his technical understanding.

I reckon there's a correlation between higher slew rates and better sound, but I very much doubt that the higher slew rate is directly responsible. Rather its a marker for better HF linearity and its that linearity which is what gives rise to better sound.
true linearity is the key but here we do not want amp to stint at any instance no matter what kind of current demands happens...
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Old 11th February 2013, 02:24 AM   #18
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Yeah - but people do tend to over-estimate the current demands of your typical domestic loudspeaker. Just like high slew rates, high current capabilities tend to shift product.
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Old 11th February 2013, 02:24 AM   #19
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And possibly a marker for competent layout in that the amplifier has this slew rate capability with stability (supposedly) in the first place..
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Old 11th February 2013, 06:30 AM   #20
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Originally Posted by Bonsai View Post
Marcel, the LTP current can be set independently of the ULG frequency. You set the ULG based on the closed loop amplifier gain, LTP degen resistors and Cdom. In my e-Amp design, the slew rate is 155 V/us and uses conventional Miller comp (you can set it for TMC also using jumpers).
Hi Bonsai,

The point I was trying to make is that you need some degree of local feedback (such as long tailed pair degeneration resistors) to be able to increase the bias current without (substantially) increasing the unity loop gain frequency. If I understand you correctly, that is the same point you are making, in which case we agree.

By the way, I hate the use of the term current feedback for an amplifier with series feedback at the input, shunt feedback at the output and a low open-loop input impedance at the negative input, because the term current feedback originally refers to feedback configurations with series rather than shunt feedback at the output, which has nothing to do with whatever the open-loop input impedance at the negative input may be. Anyway, that's a different discussion.

Best regards,
Marcel
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