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Inside op-amps
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Fatmangolf View Drop Down
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Points: 8998 		Post Options Post Options   Thanks (1) Thanks(1)   Quote Fatmangolf Quote  Post ReplyReply Direct Link To This Post Posted: 27 Jul 2022 at 6:39am
I agree, a good education brings knowledge and understanding like this. I hope your VAT inputs and outputs go smoothly so you can keep educating us on those of the op-amps.
Jon

Open mind and ears whilst owning GSP Genera, Accession M, Accession MC, Elevator EXP, Solo ULDE, Proprius amps, Cusat50 cables, Lautus digital cable, Spatia cables and links, and a Majestic DAC.
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Points: 16298 		Post Options Post Options   Thanks (1) Thanks(1)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 27 Jul 2022 at 7:06am
In Adobe Acrobat you should be able to zoom right in to the open-loop frequency response graph, and select print current view to get a printed copy that looks like either the first or second image.

open loop frequency response graph

open loop zoomed in

If you are familiar with semi-log graph paper you'll be able to approximate the frequencies between those marked at each decade (they're exponential).

If you prefer, you may wish to draw it on 7 cycle semi-log paper, instead of printing it out.

Now, in the tables of a.c. performance we're told it has a GBW or gain bandwidth product of 4MHz (typical, that is).

In maths, product means the result of a sum, so at 0dB it is 1 * 4MHz, and at 20dB it is 10 * 400kHz, and at 40dB it is 100 * 40kHz, and all the answers are 4MHz, which is the product.

The reason for the slope is its dominant pole capacitor. Look at the graph and you see a corner frequency (Fc) at just above 10Hz. Here I am going to approximate that it happens between 20Hz and 30Hz.

So that's the frequency response of an average op-amp, DC to 30Hz (-3dB), and this is quite typical.

The frequency response is improved by applying negative feedback around the op-amp, such that if you make Rfb be 99 times larger than Rin, its "noise gain" becomes 40dB. At 40dB you can read off 40kHz on the graph's x-axis, or you can divide its GBW by 100 and get exactly the same answer.

Are you with me so far?
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Graham Slee View Drop Down
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Points: 16298 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 27 Jul 2022 at 8:16am
LF411 schematic

The detailed schematic shows the voltage amplifier is the "Darlington" Q4 and Q5, with dominant pole capacitor Cc 10pf between Q5 collector and Q4 base.

You can also see the differential input stage collector impedances R1 and R2, which sum to make 4k if unbalanced.

(note: this is a Wilson current mirror which overcomes the Rin load of Q4)

Still working open-loop, we find by 1/2pi RC, that this bounds the GBW, by

1/2pi 4k * 10pf = 3.979 MHz which rounded is 4MHz.

Now, the graph doesn't make it plain what the DC gain is. We could guess that it's somewhere between 105 and 110 dB, but we can look in the DC Electrical Characteristics and find Avol which is Gain (Av) open loop (ol).

Typically, it's 200V/mV, which is 200,000 and if we take the log to base 10 of that, we get 5.3, and then we multiply by 20 to get dB's (that's voltage dB's), and we find it's 106dB.

So now we can obtain the true Fc, which is GBW / 200,000 = 4Mhz / 200,000 = 20Hz.

And now we can find that "imaginary resistor" from 1/2pi fC,

1/2pi 20Hz * 10pf = 795.8 Meg

Now, let's prove that to be true and find the noise gain, 1 + (Rfb / Rin), so

1 + (795,800k / 4k) = 198,951, which was obviously rounded to the "typical" 200V/mV

So, the DC gain expressed in dB is actually 105.9749225dB, and you'd never represent decibels to that accuracy, and so the nearest is 106dB, which is 200,000.

And in that case, the imaginary Rfb can be thought of as 800 Megohms!
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Points: 16298 		Post Options Post Options   Thanks (1) Thanks(1)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 27 Jul 2022 at 8:43am

This topic assumes the reader already understands negative feedback around an op-amp - in other words, closed-loop, but if not, the above image from Ray Marston over at Nuts and Volts might help.

(courtesy link: https://www.nutsvolts.com/magazine/article/op-amp-cookbook)
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Points: 16298 		Post Options Post Options   Thanks (1) Thanks(1)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 27 Jul 2022 at 12:20pm
Valve emulation pt.1

The usual method for making an op-amp supposedly perform like a valve is to offset a load on the output to one rail or another.

All that does is make the op-amp distort more than it would just driving its normal load.

It's supposed to increase the second harmonic distortion, but with all the NFB the open-loop gain affords, there isn't any noticeable quadratic shape indicated by it's output, so the rising distortion must be higher order.

To cap it off, the load is usually arranged as a current source, to make the distortion the same at all output levels. However, valve amp distortion falls at lower levels!

Most then kid themselves that the op-amp now sounds like a valve. It doesn't! It either sounds the same as an op-amp, or bloody awful.
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Fatmangolf View Drop Down
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Points: 8998 		Post Options Post Options   Thanks (1) Thanks(1)   Quote Fatmangolf Quote  Post ReplyReply Direct Link To This Post Posted: 27 Jul 2022 at 7:49pm
Great explanations, thank you.
Jon

Open mind and ears whilst owning GSP Genera, Accession M, Accession MC, Elevator EXP, Solo ULDE, Proprius amps, Cusat50 cables, Lautus digital cable, Spatia cables and links, and a Majestic DAC.
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Points: 1525 		Post Options Post Options   Thanks (0) Thanks(0)   Quote patientot Quote  Post ReplyReply Direct Link To This Post Posted: 27 Jul 2022 at 8:20pm
From a quick online search, it looks like "Michael Fidler" on ASR is selling his own phono stage:


I'm guessing he wants to bash GS product and "stealth direct" people to his own. 

I saw this recently on a forum with another product - a special tonearm. The seller created accounts at several forums and was suddenly chiming in on all sorts of turntable related posts while subtly directing various users to their product. 

In both cases, virtually no one had ever heard of the product being touted or the maker. I've seen this kind of thing in other hobbies as well. 

In some of those hobbies, the hobbyist forum requires vendors pay for a vendor account that marks their avatar and they are prohibited from posting outside of certain parts of the forum. Of course there are folks that don't want to do this, so they try "fly under the radar" as an ordinary hobbyist. 

It's all very sneaky and very distasteful behavior IMHO. 
SL-1200 MK7 (modified) + Reflex M + PSU-1 used with AT150-40ML, AT VM95ML, Stanton 680mkII + Ogura, and Shure M35X cartridges.
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