Extreme slew rates like 200V/us or 300v/us no use in general but can it be achieved?

[rhythmsandy]

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...

but when I see krell amps they have that super fast slam and iron fist bass. checked the slew rates its 120V/us but hows that value achieved? what factors really make that slew rate.. compensation capacitors?

reducing it to very low value might incur stabilities isnt it?

making a 10pf comp capacitor gives 64v/us and 5pf can give 110v/us

but are there any other parameters to be considered?

[Elvee]

Originally Posted by rhythmsandy

what factors affect the slew rates? is it possible to achieve slew rates of order of 200V/us or 300V/us?

That can be achieved relatively easily, even using a relatively simple circuit and crappy components.
This buffer example exceeds 500V/µs.
Alternative buffer topologies
The paramount ingredient is the topology: by simply decreasing the cap values in a conventional circuit, you hit the limit very quickly

[Jsixis]

a lot of PA power amps have extreme slew rates, I feel it is more marketing hype then actual audio improvement.

[jxdking]

any greater than 10v/us is enough for audio PA

[MarcelvdG]

Basically, for amplifiers with one dominant pole that is determined by a compensation capacitor:

Slew rate = (maximum output current of the stage before the compensation capacitor)*(voltage gain of all stages after the compensation capacitor)/(capacitance of the compensation capacitor)

When Miller compensation is used, the actual stage having the Miller capacitor is neither before nor after the compensation capacitor.

A well-known trick to improve slew rate is increasing the bias current of the input stage and applying local series feedback to it (assuming that the input stage is the one and only stage before the compensation capacitor). Only increasing the bias current usually means that you will need a bigger compensation capacitor, so you don't win anything. Another trick is to use a class AB-biased input stage that can deliver large peak currents when needed, like the examples that Elvee showed.

[Walkalone]

Originally Posted by Jsixis

a lot of PA power amps have extreme slew rates, I feel it is more marketing hype then actual audio improvement.

PA audio don't have high slew rate. About <80V/uS

High-end amp will do it. They have very large BW and Extremely high slew rate.
Spectral DMA50: 1000V/uS BW: 0 - 1.2MHz
Magtech amp: 500V/uS
Goldmund Telos: 300V/uS
....

[Bonsai]

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).

However, it's quite possible get 300 or 400 V/us using Miller inclusive comp using this same design, although I did not pursue this option.

CFA topologies offer the possibility of even higher SR's because they do away with the input transconductance stage (i.e voltage to current LTP), with the feedback current effectively driving the compensation capacitor directly via the trans impedance stage. Opamp CFA's thus configured have beef designed that offer in excess of 1000 V/us.

So, how much SR do you really need? Some practitioners say 1 V/us per peak output voltage is a good guide, which I would think is ok.

[rhythmsandy]

thats great replies is there any perceived difference? especially in bass? does it gives more firm bass?

[pete_schumacher]

Originally Posted by rhythmsandy

thats great replies is there any perceived difference? especially in bass? does it gives more firm bass?

At bass frequencies, slew rates don't matter as much as damping factor. High slew rates are important for keeping IMD under control at higher signal frequencies. But for tight, controlled bass, you want low output impedance.

[rhythmsandy]

so how to achieve very low output imp? anytips?