Symmetrical 400W amplifier

[peranders]

Originally posted by traderbam
What slew rate do you consider acceptable?

The slew rate is determined by max output voltage and max frequency. 1000 W amp needs much more slew rate than a 15 W amp.

But if we talk numbers, 50 V/µs for a 200-300 W amp is normal.

[traderbam]

I believe that the formula for slew rate for a sinewave is:

SL= 2.pi.f.Vpeak in volts/sec

Where f is the sinewave frequency in Hz and Vpeak is the peak amplitude of the sinewave.
Eg: for a 40V, 20kHz sinewave a SL of about 5V/us is required. I'd say this is a typical requirement for home amps.

The bandwidth needs to be at least 20kHz. Why does it need to be any higher? After all, the bandwidth of your CD player is somewhere between 20 and 22kHz. Why should an amp sound better when it can amplify out beyond 100kHz? (except perhaps to dogs and bats). Wouldn't it be better if it rolled off at 20kHz so as not to amplify anything it doesn't need to - wouldn't this put less strain on the amp and less distortion?

[peranders]

You forget that you get nasty distortion due to slew rate limiting. If you have low feedback you won't get so much slew rate limiting but you will instead get higher harmonic distortion.

Considering what you say, why do we need 96 kSp or even 192 kSp?

OK, I talking about my taste. I don't settle for an amp that barely can make 20-20000 Hz.

You are totally right in questioning about the minimum performance.

[Diogo]

What about my question guys?

And I agree with peranders, a fast amp sounds mutch better,
try to amplify eletronic music like trance ,with a faster amp and listen to it, I don't have to use the audio analyser to be sure of that!

[Christer]

I think we can add one more reason to use cascodes, namely
that it will keep the Vce of the diff pair transistors almost
constant, thus minimizing the Early effect. Or is this considered
a minor reason, not worth the trouble?

[peranders]

Originally posted by Christer
I think we can add one more reason to use cascodes, namely
that it will keep the Vce of the diff pair transistors almost
constant, thus minimizing the Early effect. Or is this considered
a minor reason, not worth the trouble?

This is why distortion gets reduced and the speed increased.... this isn't minor reason, this is the basics of the whole stage.

[Christer]

I missed that you mentioned lower distorsion in a follow-up post,
but you didn't mention the cause for it. It seems we agree that
the cause is increased linearity by minimizing the Early effect.
I just wanted to check that I have the correct understanding of
cascoding, and it seems I have (at least, until somebody else
comes around having a different opinion from yours ).

[carlmart]

We are getting into uneasy ground here, as some respected designers like Nelson Pass or Hugh Dean do not use them, favouring a simpler resistor replacement there.

As I couldn't yet carry on my own tests there, replacing cascodes and constant current sources with resistors and see how they sound, that looks like an interrogation area for me.

My present amplifier uses both cascodes and ccs, so I think I'm familiar with their sound. Now I want to listen to the simpler designs and see if there's an issue there.

Can't say anything on current mirrors compared to cascodes though, whether one or th eother should be preferred.

Other opinions, based on actual listenings, should be welcome.


Carlos

[peranders]

Originally posted by carlmart
We are getting into uneasy ground here, as some respected designers like Nelson Pass or Hugh Dean do not use them, favouring a simpler resistor replacement there.

If I don't remember wrong Nelson Pass uses cascodes if they are put in the right places. Hadn't he some patent also?

Current mirrors and cascodes are two different things. Cascodes haven't any effect on gain which current mirrors have if they are collector loads.

[carlmart]

If I don't remember wrong Nelson Pass uses cascodes if they are put in the right places. Hadn't he some patent also?

Current mirrors and cascodes are two different things. Cascodes haven't any effect on gain which current mirrors have if they are collector loads. [/B][/QUOTE]


First of all, what I'm referring to is how certain arrangements may influence the sound quality. Whether they are current mirrors, cascodes or ellaborated constant current sources.

Second, if I'm not wrong Nelson Pass does not use cascodes on any of the Alephs that many DIYers are building, and Pass did comment on the advantages of simpler circuits.

The only Pass design I've seen that does use cascodes is the A75, though there are certainly others I haven't seen.




Carlos