What makes a amp realy fast?

[Chrabban]

hello i am just wondering, what makes a amp realy fast and have a nice tight grip over the speaker? is it the transformer or is it realy good and big capacitor? or both?

[janneman]

Lets try to reason it.

The transformer is disconnected from the power supply and from the amp about 80% of the time (that's when the diodes are not conducting), so to me it seems that the transformer has only a very limited impact on speed, if at all.

Then the caps. You can have all the power in the caps you want, but if the amp cannot get it to the output, that doesn't help either.
Also, an amp has what is called a 'power supply rejection ratio'. This says how insensitive the amp output is to any power supply variation. So an amp with a reasonable PSRR doesn't see much of what happens at the supply cap (as long as the voltage remains high enough for the required output signal).

So it appears that 'amp speed' depends on other factors.

jan didden

[Mooly]

It's not that straightforward...

Damping factor (how low an impedance the amp output is) is perhaps the "conventional" meaning of an amp having a tight grip on the speaker. In theory In practice it's somewhat different.

A fast amp is thought of as one with high slew rate and extended frequency response.

The transformer and big capacitors can go some way to making a good amplifier but it's not the whole story by any means.
Different amplifier topologies sound different, some prefer the "blameless" and technically perfect amps and other the less perfect but perhaps better sounding ones.

[Chrabban]

Janneman: so you mean that the layout and the parts on the PCB is what makes it fast and have a good hand with the speakers ? =)

Mooly: so what makes an amp to have high slew rate and extended frequency response?
and what is slew rate ?
hehe i think that i like the amps that sound a bit better then the "blameless". but to get that there has to be more experementation right ? =)

[a.wayne]

Originally Posted by Chrabban

What makes a amp really fast :

Usain Bolt ..........

[nigelwright7557]

Its getting a happy medium.
You dont want a low amp becasue it cant cope with higher frequencies.
You dont want too fast an amp because it could oscillate.

On my designs the last part of designing is getting the amp damped correctly at the VAS.
Or if a quasi also getting the lower driver damped enough to stop oscillation.

[Mooly]

Originally Posted by Chrabban

Janneman: so you mean that the layout and the parts on the PCB is what makes it fast and have a good hand with the speakers ? =)

Mooly: so what makes an amp to have high slew rate and extended frequency response?
and what is slew rate ?
hehe i think that i like the amps that sound a bit better then the "blameless". but to get that there has to be more experementation right ? =)

Slew rate is how fast the output of the amplifier can change with respect to time. It's usually measured in "volts per micosecond".

To design an amp for high slew rate means paying attention to the issues that normally limit it. For example having sufficient current available in the various stages within the power amp to charge and discharge any stray or internal component capacitance.

It's worth you getting hold of some of the books written by Doug Self such as the "Audio Power Amplifier Design Handbook"
The Amplifier Institute.

[ashok]

[QUOTE=Chrabban;2648873]Janneman: so you mean that the layout and the parts on the PCB is what makes it fast and have a good hand with the speakers ? =)

Assuming that the supplies ( trafo and supply caps) can supply the required current with reasonable voltage sag, the statement given above is probably correct.
While circuit configuration also has a part to play, the layout and critical components are just as important if not , sometimes, more important !

Slew rate just by itself doesn't come into play. Most amps can slew fast enough to cover all audio frequencies thrown at it. Doesn't necessarily make them have a 'tight' grip on the loudspeaker drivers.
Bass frequencies ( where we need the tight grip!) are FAR slower than the high frequencies we speak of when talking about slew rate.

[DF96]

Originally Posted by Chrabban

what makes a amp realy fast and have a nice tight grip over the speaker?

Fast means high slew rate - affects high treble. Tight grip means low output impedance - affects low bass. The two are unrelated. To a first approximation, high slew rate means adequate current drive within the amp so that capacitances can be charged/discharged sufficiently quickly. Low output impedance means having sufficient negative feedback.

[EWorkshop1708]

What makes an amp have good control over the speaker?

Multiple output transistor pairs
Strong VAS to drive the output stage