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-   -   Op-amp open loop bandwidth. (http://www.diyaudio.com/forums/solid-state/51366-op-amp-open-loop-bandwidth.html)

beppe61 11th February 2005 08:44 AM

Op-amp open loop bandwidth.
 
Dear Friends,

In the data sheet of the op-amps I can easily find the Open Loop Gain (usually huge).
To find the open loop bandwidth is a mistery to me.

Is someone willing to help me ?

Thank you very much indeed.

Regards,

beppe61

diy_audio_fo 11th February 2005 09:33 AM

Re: Op-amp open loop bandwidth.
 
Quote:

Originally posted by beppe61
Dear Friends,

In the data sheet of the op-amps I can easily find the Open Loop Gain (usually huge).
To find the open loop bandwidth is a mistery to me.

Is someone willing to help me ?

Thank you very much indeed.

Regards,

beppe61

In a data sheet you find two sections:

- a table with numeric data
- a series of graphs

In the table section you can find

- the numeric value of the open loop gain (large signal voltage gain AOL usually expressed in V/V)

- the numeric value of the Gain Bandwidth Product (GBW usually expressed in MHz)

The open loop bandwith is = GWB/AOL (if the AOL is expressed in V/mV you have to divide the result by 1000)

In the graphs section you can find a graph titled Open Loop Frequency Response. The Open loop bandwidth is the frequency where the line of the graph starts to tilt down.

The numbers you find using the two ways I suggest have to be equal (more or less)

Regards
Marco

beppe61 11th February 2005 09:44 AM

Dear Marco,


Thank you very much fro your kind and valubale reply.
Am I wrong or the open loop bandwidth for the majority of op-amps is very small ?
What can this imply ?
I am referring to all those discussions about TIM distortion and so on.
I am very intersted in knowing your point of view.
As you can see from my posts here I am looking for simple line preamp schematics.
I posted one.
Do you have any suggestion?

Thank you very much indeed.

Regards,

beppe61

jcx 11th February 2005 03:53 PM

Low open loop bandwidth in reasonable (single pole compensated) op amps implies high gain at low frequencies

What this means for audio is subject to some debate here

Feedback theory and distortion measurement (IMD distortion as well as harmonic) of feedback amplifiers shows that more loop gain lowers distortion

Some people claim that high feedback amplifiers donít sound as good as similar technology amplifiers with less loop gain, or flat loop gain over the audio frequency range

TIM, PIM distortion mechanisms that Otala discussed in JAES in the Ď70ís are often cited as technical reasons to explain the perceived differences

I donít find PIM to be a convincing argument for preferring low gain-high bandwidth circuits Ė the missing link has to be demonstrating that PIM is audibly 100x more objectionable than ďAMĒ IMD products since both are reduced by high negative feedback whereas wide bandwidth-low loop gain designs accept higher AM distortion over the whole audio band to avoid the AM to PM conversion that comes from using a integrating loop gain characteristic over the audio band

beppe61 12th February 2005 12:41 PM

Dear Mr. Jcx,

Thank you for your kind and thorough reply.
But I understand that Mr. Otala became famous not on the basis of his theory but in relation with the sonic quality of his well received projetcs, or I am mistaking ?
Someone became audio classics (Harman Kardon Citation XX for instance. A every audio lover dream).
What I wanted to imply is that a low open loop bandwidth is a situation that worries me on principle.

May be I am too banal.

Kind regards,

beppe61

stevebeccue 12th February 2005 05:28 PM

Op amps often have open loop bandwidths near 100 Hz.
They are intended to be used closed loop, which has all
sorts of consequences, including wider bandwidth.

If you desire a "linear" amp, i.e. low distortion, you must
use negative feedback to linearize the horribly non-linear
gain devices. If not "loop" feedback, as in an opamp, it
is "local" feedback. I can't comment on whether anyone
can hear the difference.

Best Regards,

beppe61 12th February 2005 05:55 PM

Dear Mr. Stevebeccue,


Thank you very much for your kind reply.
If understand well (please be sure that I don't want to be ironic at all, I am just trying to understand) you mean that no difference between two preamps with similar measurements?
I read the opinions of lots of people that much prefer, sonically speaking, a preamp without feedback (i.e Nelson Pass Aleph P) to an op-amp based line preamp (i.e. Morrison Elad) even if the latter has much better measurements.
I am posting here to try to understand if this is a myth or a reality.

Thank you very much for your post that I think useful to the discussion.

Kind regards,

beppe61

lumanauw 13th February 2005 01:14 AM

Mr. Otala is the base of Electrocompaniet power amplifier company.

There's one man that can comment about High OL, Low OL, TIM, PIM, AM-PM, etc impact to sound (how audibly they are). He is called John Curl (JC) here.

I'm sure he is able to comment, but wheter he is willing or not is another question, since he is a professional designer, still making commercial amps now. These things are "secrets" for designers like him.

beppe61 13th February 2005 11:01 AM

Regarding the mentioned great and famous John Curl I seem to have read that in his last and absolutely wonderful Blowtorch preamp he has adopted a topology with no feedback.
Food for thought.

Regards,

beppe61

jan.didden 13th February 2005 11:13 AM

Quote:

Originally posted by lumanauw
Mr. Otala is the base of Electrocompaniet power amplifier company.

There's one man that can comment about High OL, Low OL, TIM, PIM, AM-PM, etc impact to sound (how audibly they are). He is called John Curl (JC) here.

I'm sure he is able to comment, but wheter he is willing or not is another question, since he is a professional designer, still making commercial amps now. These things are "secrets" for designers like him.

He has commented on that in many posts here. If you read them, read the whole thread. There are many knowledgeable people here who don't agree whith with many of his opinions, and back that up with facts. Very educational discussions.

Jan Didden


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