What is the optimum -3db bandwidth for amplifiers with feedback ? - diyAudio
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Old 31st December 2006, 01:56 PM   #1
mikelm is offline mikelm  England
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Default What is the optimum -3db bandwidth for amplifiers with feedback ?

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
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Old 31st December 2006, 02:17 PM   #2
AndrewT is offline AndrewT  Scotland
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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.
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Old 31st December 2006, 02:18 PM   #3
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Old 31st December 2006, 02:29 PM   #4
mikelm is offline mikelm  England
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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
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Old 31st December 2006, 03:21 PM   #5
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Default Re: What is the optimum -3db bandwidth for amplifiers with feedback ?

Quote:
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

==============================================
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Old 31st December 2006, 03:40 PM   #6
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Default Re: Re: What is the optimum -3db bandwidth for amplifiers with feedback ?

Quote:
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

Quote:
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
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Old 31st December 2006, 03:52 PM   #7
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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
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Old 31st December 2006, 04:08 PM   #8
mikelm is offline mikelm  England
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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
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Old 31st December 2006, 04:18 PM   #9
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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.
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Old 31st December 2006, 04:52 PM   #10
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I am beginning to wonder if there is confusion between open loop bandwidth, closed loop bandwidth and gain bandwidth product.
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Old 31st December 2006, 05:49 PM   #11
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The distiction between the three is reasonably clear. ( though I'm a bit foggy of the presise definition of gain bandwidth product - can it be used to define closed and open loop situations ? )

my original question refers to HF -3db point closed loop gain - there is no confusion here.

The open loop unity gain of such an amplifier would be very high, 50 - 100meghz perhaps.

I guess I will have to try for myself and find out.

I agree that the lower HF distortion may be difficult to distingush but I have always found in the past the lowering noise at all frequencies reaps sonic rewards so I think if it is do-able the extra PSNRR will be an asset.

mike
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Old 31st December 2006, 05:52 PM   #12
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I was hoping to get some input from those who had explored this region.

Perhaps I will have to find out for myself ...
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Old 31st December 2006, 05:57 PM   #13
AndrewT is offline AndrewT  Scotland
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How about posing your question in the Cordel interviews -Mosfet maybe.

The thread seems to have attracted the attention of well known experts in the field.
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Old 31st December 2006, 06:16 PM   #14
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Mike,

why not go have a listen to a Spectral power amp and see how you like it.
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Old 31st December 2006, 07:08 PM   #15
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Other considerations:
Is this the small signal bandwidth or Full power BW?
FET, BJT or other?
What is the RF like at your location?
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Old 1st January 2007, 03:20 AM   #16
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Default Bandwidth

When determining the bandwidth of both amps and preamps, I use the rule of 10: For the upper limit, I go 10X above and for the bottom end, I go 10X below. So, I shoot for -3dB at 2Hz and 200KHz. This gives a nice, flat (~ -0.1dB) response from 20-20K.

Following this rule, first mentioned by an electronics professor in college, I have always had great preamp and power amp performance and no HF oscillation or RF problems.

This is used, of course, in conjunction with good designs, good PCB layouts, sufficient open-loop gain, HF compensation, etc.
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Old 1st January 2007, 03:55 AM   #17
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i have an amp design in the works, with a GBW of about 2Mhz, open loop. one must also take certain phase shifts (due to propagation delay in the components) into account when feedback is applied to set the gain at a useable level (such as about 30 for a 100w amp), because some of this propagation delay is going to show up at the - input of your diff amp and make your amp susceptible to oscillation. this happened with my design, and it took a bit of capacitive modification of the frequency response curve (to a -3db rollover of 30khz) to keep the amp stable when clipping is reached. when you run an ac transfer sim of your design, if you have any peaking of your response curve, you've found an instability. it usually occurs at the rollover freq or higher on the down slope of the frequency response curve. your rollover should be smooth. because of junction capacitances (or interelectrode capacitance in tubes) it is possible in ALL amp designs.

in my design, the peak was at 1Mhz, and the bugger would oscillate at 1Mhz at clipping. after adding some capacitance in the right place, the amp settled down, and is ready for prototyping.
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Old 1st January 2007, 05:32 AM   #18
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i don't think there is such a thing
as a general optimum -3dB upper bandwidth

it depends on the actual circuitry of this amplifier
it also depends on chosen transistors
very much, I think it depends on how fast the output stage transistors are

Ft, as have got it, is the frequency, in MHz,
where a transistor does not provide any gain no longer

Most used power transistors have Ft like 3 or 4 MHz
there are some faster Bipolar with 6, 10 and even 40-50 MHz
But for 90% of our usual diy designs it would be <=6 Mhz

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

There are 3 different levels, where we would add some capacitance
to tune our power amp = restrict upper frequency:

1. input filter
it is good to remove such noise and things that are not actual audio information,
BEFORE it enters amp.
Because once inside, they would make feedback create leftover from the correction job.

2. within and between stages
here we can adjust and slow done some fast transistors
so they do not feed frequencies into slower next stage
such signals can not be handled very good by slow devices
and will also only give extra correction work to global feedback
if they even can get through ...
To this category we count also gate stoppers for MOSFET
which reduces the speed of those power FET, a bit.

3. in the global feedback network itself
this sets the overall bandwidth of the amplifier
defines the band where any any gain will be performed

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


There is no such general -3dB optimum, that would suit every, or even most, power amplifiers.
However for each amp + load setup, there is a maximal upper frequency, that is possible.
As many amplifiers use similar transistors ( monkey see - monkey do )
and the usual topology ( Op-Amp style )
we end up in somewhat same area, for -3dB overall bandwidth:
.... something like 50-200 kHz.


The General Rule is to try to trim and make the amplifier
as fast as possible in conditions at hand
and still do a performance we can be satisfied with.
A too fast amplifier will have more distortion,
than if it is tamed a bit.

There is no good reason to slow down an amplifier,
just for the sake of making it slow
.



lineup
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Old 1st January 2007, 05:47 AM   #19
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Thank for your answer lineup - and happy new year, I'm still 2006 here

your points are thorough and all make sense to me.

but my question is this

If we have an amplifier that can comfortably handle a very high bandwidth, what is the highest bandwidth above which we do not notice any benefit.

I just wonder if someone out there has done this stuff and come to some conclusions.

Anyway I guess I will have to build a few amps and find out for myself ...
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Old 1st January 2007, 11:43 AM   #20
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Quote:
Originally posted by mikelm
...
but my question is this
If we have an amplifier that can comfortably handle a very high bandwidth, what is the highest bandwidth above which we do not notice any benefit.
...
What do you mean by "notice any benefit"?
Sonically or in measurements?
Theoretically what one wants to achieve with an amp is a perfectly flat response over the range 20Hz to 20kHz. Since perfection is not of this world, we are happy with some roll-off at both ends. If you want -0.1dB at 20kHz you need -3dB at 200kHz or so. If you are willing to compromise and accept -1dB at 20kHz you just need -3dB at 30kHz or so.
Can you hear the difference between the same amp, if you just change this single parameter, between -0.1dB at 20kHz and -1dB at 20kHz? I can't, and I doubt anybody can reliably ABX the difference in sound, even when playing pure tones; so the difference in sound would be rethorical.

In practical terms, the rule-of-thumb that DCPreamp wrote above is a good one, actually it's even high-end. It was quite impossible to achieve with BJT output stages just a few years ago but nowadays you can aim that high with FETs. But it's definitely not a requirement for or a guarantee of amplifier sound quality.

There is a very simple experiment you can build on a protoboard in less than an hour and check for yourself the benefits - or lack of - of very high bandwidth amps.
Just build yourself one of the CMoy variants with a modern op-amp and FET output buffer. These little amps have > 1 MHz closed-loop bandwidth usually. Use a good Sennheiser headphone and just add a switch for a -3dB low pass filter at 30kHz and at 200kHz. Just a warning: these little things tend to oscillate...

And HAPPY NEW YEAR!
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Old 1st January 2007, 04:21 PM   #21
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My recent goal is to have flat distortion 20 - 20khz ...

agreed - large bandwidth alone does not = good sound

I think it has to be combined with very low noise ( good PSRR ), good stabilty, reasonably low distortion and good dynamics ( micro and macro )

( ....and also, most important, a good front end ! - a transparent amp is is not much of an asset if the front end is sounding horrible. )

Normal bandwidth amplifiers have OLG that begins to roll off at quite low frequency. This then has to be rolled off even more so that in closed loop mode they are stable. This reduces the amount of feedback db in the high midrange and HF area

With wide band amp the roll off begins at higher frequency and the compensation for stability can operate at a frequency way above audible sound.

All of this means that there is are more db's of feedback available at high audible frequencies.

In my experience this gives a richer fine texture to the sound which I enjoy.

Also in feed back amps, PSRR, to some extent, relies on the amps feedback.

In my experience one very major factor in getting better sound from audio gear is reducing noise in general, but particularly the HF sonic & ultra sonic power supply noise.

So, if one has a broadband amp with a low pass filter at the input this can be a powerful means of preventing broadband PS noise from reaching the output.

My question was ( and still is ... ) to what extremes can one apply this principal and still notice an improvement both in reproduced HF texture, and the sweet smooth sound that comes with very low noise throughout the system.

I will continue to explore this area and let u now what I find

Happy New Year

cheers

mike
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Old 1st January 2007, 07:17 PM   #22
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make your bandwidth as wide as your design will tolerate. if your amp bandwidth is 1Mhz with no instability (and doesn't act as an AM receiver). then use it. you will never have a full power output at 20khz from most music sources anyway (that would also eat a lot of tweeters). but your distortion measurements at 20khz would probably be close to your measurements at 1khz. that's assuming of course your output devices are fast. bipolars begin going into common mode conduction around 100khz with most designs, which will eat fuses as well as output devices.
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Old 1st January 2007, 07:17 PM   #23
jcx is online now jcx  United States
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Quote:
Originally posted by mikelm
My recent goal is to have flat distortion 20 - 20khz ...
Not possible, different distortion mechanisms have different frequency dependence - Vbe modulation is flat over audio freqs, nonlinear Ccb causes increasing distortion with frequency, thermal modulation falls off quickly with frequency...


Quote:

Normal bandwidth amplifiers have OLG that begins to roll off at quite low frequency. This then has to be rolled off even more so that in closed loop mode they are stable. This reduces the amount of feedback db in the high midrange and HF area

With wide band amp the roll off begins at higher frequency and the compensation for stability can operate at a frequency way above audible sound.

All of this means that there is are more db's of feedback available at high audible frequencies.
Not a useful way to look at it, most amps use single pole compensation, therefore larger GBW product gives larger open loop gain at all frequencies where the gain has a 1st order slope and is a quantity we should want to maximize - the limit is determined by the speed of the output devices (usually the slowest transistors in a amp), slow output devices add extra phase shift and cannot operate stably in single pole compensated amps at as high a overall amplifier GBW (even if we add more front end gain to make up for the low output Q GBW) as faster devices http://www.diyaudio.com/forums/showt...995#post647995

The low/high frequency open loop gain terminology is misleading, for single pole open loop response there is a low frequency corner determined by the open loop DC gain intercept with the single pole slope, for the same GBW a high DC gain gives a low open loop corner frequency and low DC gain gain gives a high open loop corner frequency - but with less gain than the "low open loop corner frequency" at every frequency below the "high corner frequency"
http://www.diyaudio.com/forums/showt...927#post489927

Quote:

Also in feed back amps, PSRR, to some extent, relies on the amps feedback.

In my experience one very major factor in getting better sound from audio gear is reducing noise in general, but particularly the HF sonic & ultra sonic power supply noise.

So, if one has a broadband amp with a low pass filter at the input this can be a powerful means of preventing broadband PS noise from reaching the output.

"A General Relationship Between Amplifier Parameters, And Its Application to PSRR Improvement"

http://citeseer.ist.psu.edu/cache/pa...ip-between.pdf

This paper shows that high open loop gain is a necessary condition for high psrr, it also shows some topological ways around this limitation

filtering the input to the amp should have nothing to do with ps noise at the output, psrr is measured with zero input signal
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Old 1st January 2007, 08:46 PM   #24
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Hi unclejed613,

Quote:
make your bandwidth as wide as your design will tolerate.

Thanks for your comment - this is what I will now experiment with.


Hi jcx,

Thanks for your comment aswell - I think I will have to organise my ideas and choose my words a little more precisely in future - but despite my unprecise expression I hope u managed to get the general thrust of my reasoning.

I mention that I have i/p low pass filter because without it one might just be amplifying HF noise present at the i/p which is not that clever if the objective is to reduce HF noise at the o/p !

I will not comment much further on this now because I feel it's time to make a few more broadband amplifier designs, test what I have been talking about and find the answers to the questions I have been asking.

I have seen some designs in spice that do indeed have distortion charactoristics that are flat up to 20khz and now I will see if I can realise this in real life.

Spice, I know, does not show thermal distortions very easily but I usually try to design thermal distortions out - at least on the i/p stage one way or another.

cheers

mike
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Old 1st January 2007, 11:29 PM   #25
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Quote:
Originally posted by lineup
i don't think there is such a thing
as a general optimum -3dB upper bandwidth

it depends on the actual circuitry of this amplifier
it also depends on chosen transistors
very much, I think it depends on how fast the output stage transistors are


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


There is no such general -3dB optimum, that would suit every, or even most, power amplifiers.
However for each amp + load setup, there is a maximal upper frequency, that is possible.
As many amplifiers use similar transistors ( monkey see - monkey do )
and the usual topology ( Op-Amp style )
we end up in somewhat same area, for -3dB overall bandwidth:
.... something like 50-200 kHz.


The General Rule is to try to trim and make the amplifier
as fast as possible in conditions at hand
and still do a performance we can be satisfied with.
A too fast amplifier will have more distortion,
than if it is tamed a bit.

There is no good reason to slow down an amplifier,
just for the sake of making it slow
.



lineup

Quote:
Originally posted by unclejed613
make your bandwidth as wide as your design will tolerate. if your amp bandwidth is 1Mhz with no instability (and doesn't act as an AM receiver). then use it. you will never have a full power output at 20khz from most music sources anyway (that would also eat a lot of tweeters). but your distortion measurements at 20khz would probably be close to your measurements at 1khz. that's assuming of course your output devices are fast. bipolars begin going into common mode conduction around 100khz with most designs, which will eat fuses as well as output devices.
lineup
The General Rule is to try to trim and make the amplifier
as fast as possible in conditions at hand
and still do a performance we can be satisfied with.
A too fast amplifier will have more distortion,
than if it is tamed a bit.
There is no good reason to slow down an amplifier,
just for the sake of making it slow
.


unclejed613
make your bandwidth as wide as your design will tolerate.

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


Here is your answer, to the 'optimal -3dB'
for every amp / load ... there is a max upper limit
to get best performance within the audioband - your chosen criteria for 'good'

there are 5 factors that will effect what high freq you can use:

1. amplifier circuit type and topology
2. transistor type and individual devices data
3. PCB layout / practical set up of amplifier
4. what load or different loads you will use
5. what kind of music you will be playing/amplifying - the sound source material



regards
lineup
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