What is the optimum -3db bandwidth for amplifiers with feedback ?

[mikelm]

I was wondering what the current thinking is on this.

is -3db 10 megHz better that 5 meghz closed loop bandwidth?

is -3db 20 megHz better that 10 meghz !!!

at what frequency does any further increase become irrelevant ?

should we aim at dB of feedback being constant throughout the audio bandwidth ?

anyone know about this ?

mike

[AndrewT]

I hope you are joking.

100kHz bandwidth sounds like pretty extreme for audio.

I would aim for -1db from 2Hz to 50kHz and this should allow all usable sub-bass signal and SACD and/or DVD-A signals to pass nearly uninterupted.

[lumanauw]

MikeB, where's MikeB?

[mikelm]

no jokes ..

I have Transformer volume control so my audio bandwidth is upper limited to about 100khz

broader bandwidth within in the amplifier has other benifits.

lower distortion at high ( sonic ) frequencies.

also better PSRR at high ( sonic & ultrasonic ) frequencies.

not sure if the lower distortion will be noticable but for me anything that reduces noise is good news.

I just wonder if anyone has experience in this area

mike

[lineup]

Originally posted by mikelm

is -3db 10 megHz better that 5 meghz closed loop bandwidth?
is -3db 20 megHz better that 10 meghz !!!
at what frequency does any further increase become irrelevant ?

should we aim at dB of feedback being constant throughout the audio bandwidth ?

You do not mention if it is about pre amplifiers or power amps.
And so we do not know at what voltage gains we are.

If we take a power amp with gain x20 and -3dB at 20 Mhz
it sure has got some GBW = gain x bandwidth.
And you sure need our GOOD LUCK to build that one ...

Pre amplifiers with moderate and lower gains
can have high upper freq limit.
But the point at where you get more troubles and distortion
than you get anything useful out of it
is in reality for audio much lower, than most of us like to think it is.


I agree with AndrewT here.

And I would be very pleased if I build a power amp
- with 20-50 Watt into 4/8 Ohm
- with -3dB at 50.000 hertz
- with granted stability into most normal real life loads
I probably would not feel too bad if I only managed 30-40kHz as long as distortion at 1 kHz stays low and nice.
And I do not get any ringing disturbance, when see square waves at my oscilloscope testings.

You can simulate and maybe even build power amps for 1 MHz output.
But a piece of advice:
do not use it with loudspeakers at the output!
Stick to using low capacitance low inductance pure resistive
absolute linear power resistors for load.

And enjoy the quiet, pure distortion free and very laidback sound of such resistors!
The lovely sound for extreme test data audio freak monsters.



Here is a topic where me and another guy share some of opinions and experiences
on upper band frequency limits for real audio applications.
Solid State >input filter for microphone preamp
http://www.diyaudio.com/forums/showt...72#post1091772
http://www.diyaudio.com/forums/showt...17#post1093017


Regards & New Year!
lineup have ALL HIS HEARING limited to below ~12.000 hertz


==============================================

* to test your ears upper bandwidth limit:
1. Use your CD-player & your Headphones
2. Calibrate for a normal listening volume (level) at 1.000 hertz sinus signal.
3. Without changing Volume try 10 > 11 > 12 > 13 >14 kHz
Until you can not hear just about nothing at all.

Remember that this result IS NOT -3dB .... more likely -20dB / -30 dB (my guess)
for Your Ears.
The actual -3dB frequency is considerably lower than this

==============================================

[mikelm]

Originally posted by lineup

You do not mention if it is about pre amplifiers or power amps.
And so we do not know at what voltage gains we are.

Thanks for your reply

I speak of power amps and I find 26dB gain is sufficient

Originally posted by lineup [B]
If we take a power amp with gain x20 and -3dB at 20 Mhz
it sure has got some GBW = gain x bandwidth.
And you sure need our GOOD LUCK to build that one ...

You can simulate and maybe even build power amps for 1 MHz output.
But a piece of advice:
do not use it with loudspeakers at the output!
Stick to using low capacitance low inductance pure resistive
absolute linear power resistors for load.

JLH simple class A goes to about 1meg and stability is fine if care is taken and many people love the sound of this amp. I normaly played this amp through speakers .. .

I wonder has anyone had experience with wide bandwidth power amps ???

cheers

mike

[darkfenriz]

Bandwidth should be limited by input RF filter and/or output inductor.
If there is no input filter and you ask about bandwidth BEFORE output inductor, then 200kHz is more than mandatory in my opinion (-3+0 dB).
Some power amps, including mine and I believe MikeB's or Bob Cordell's and so on... easilily exceed 1Mhz in this criterion, but still not in 5-10MegHz range. Op-amps and small signal gain stages are faster.
What for?
More feedback at 20kHz using simple compensation, lower TIM/PIM, sometimes flat feedback factor across audio...

regards

[mikelm]

thanks for your reply,

Yes my i/p bandwidth is limited by TVC < 100khz

and with this kind of amp a very small ( damped ) inductor at o/p is needed

I'm not suggesting I try to amplify 1meg + and send it to speakers !

rather I'm interested in the advantages & possible disadvantages of amps with high internal bandwidth.

I just wonder has anyone compared 1mhz with 5mhz internal bandwidth - even if input is filtered to 100khz or less

in other words, does flat, low distortion to 10 - 20khz and extra high frequency PSRR make an audible difference in others experience ?

cheers

mike

[jacco vermeulen]

Mike,

the Line Up North summs it up just right: you need good hearing to notice the difference.
Undoubtebly the reason why people like LineUp, at his age and with limited hearing, are perfectly happy with 50KHz limited bandwidth.

[AndrewT]

I am beginning to wonder if there is confusion between open loop bandwidth, closed loop bandwidth and gain bandwidth product.