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No-global-loop amplification
No-global-loop amplification
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Old 29th September 2017, 03:56 PM   #1
vzaichenko is offline vzaichenko  Russian Federation
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No-global-loop amplification
Arrow No-global-loop amplification

Hi All,

A Simpelstark amplifier has been prototyped, tested, measured, auditioned.
See the links below:

1) Version 1.1 schematic and PCB views;
2) Spectrum measurements;
3) Assembled board view.

I will most likely try to update the layout, designed by Alex MM, and test it later on.

Cheers,
Valery
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Original message - Einbahnstark, etc.

As many times discussed, feedback is inevitable, being present everywhere around us. We also use it every time we design an audio amplifier.
Important point the way we use it.

No doubt, its possible to design a good (or not so good) amplifier with both global NFB loop in place or no global loop at all in this case, several local NFBs are utilized for controlling important parameters.

After testing / auditioning different designs throughout the recent years, I come to conclusion I like the way the good no-global-loop amplifiers sound. Those amplifiers are well-known in the world of vacuum tubes for decades. However, my focus are solid-state amplifiers and hybrids, designed with series of local feedback (preferably covering a single stage) loops in mind. Damir and other members presented a number of excellent works in this area.

In this thread, Id like to share my experience in designing an important part of any amplifier the front-end, providing all the voltage gain utilizing only local feedback loops.

Picture 1 illustrates possible arrangement in principle.
Input stage (transconductance, Gm = 0.5 mA/V) is a heavily degenerated CE circuit thats where the most of distortion is born, however at the volume of degeneration used, its low enough for the purpose.
Emitter degeneration provides a series-series feedback, providing the desired transconductance ratio and linearizing the transfer function.
2-nd stage is a high-precision cascaded current mirror with current gain of around 2x highly linear mirroring with a little bit of gain.
Finally passive transimpedance conversion resistor load also very linear, assuming both the output impedance of the current mirror and input impedance of the next stage are high enough.
To satisfy the latter assumption, we have to use a high-quality unity-gain buffer between this resistor and the OPS, as OPSs input impedance, although it can be rather high, is normally more (especially in case of MOSFET drivers) or less capacitive and rather modulated (especially in case of BJT arrangement).
Small value capacitor provides minimal correction, eliminating possible frequency response artifacts above 1MHz.
It's easy to arrange a balanced input or a fully balanced architecture here - no global loops make it beautiful (just servo needs a bit of redesign).

Picture 2 illustrates practical implementation of the above ideas.
The circuit demonstrates pretty good performance although, Im presenting simulated results here, I have measured and listened a rather close (a bit less sophisticated) design, showing similar results and sounding just great.

Pictures 3, 4, 5 and 6 illustrate:
- Spectrum at 1 KHz (20V RMS0;
- Spectrum at 20 KHz (20V RMS);
- Square wave response at 20KHz (60Vpp);
- Frequency response.
Harmonics 2, 3 and very little bit of 5. Looks like a spectrum profile from the vacuum pentode 😊

There may be many different implementations of the stages, including jFETs at the input, folded cascode instead of current mirror, simpler unity gain buffer, etc.

In terms of OPS selection it must be a low-distortion one. Class A / AB (well-designed CFP, properly biased EF, MOSFET outputs, some error-correction options, non-switching arrangements, etc.).
I used NS-OPS, showing THD < 0.05% open loop throughout the whole audio bandwidth.

What do you think?

Cheers,
Valery

P.S. A simpler no-global-loop headphone amp design is presented here:
Aureaux high-quality no-global-loop headphone amp
Attached Images
File Type: jpg @No-Global-Loop-Amplification.JPG (108.3 KB, 1546 views)
File Type: jpg Einbahnstark_2.0-Sch.JPG (263.9 KB, 1567 views)
File Type: jpg Spectrum_1KHz_20V_RMS.JPG (109.1 KB, 1530 views)
File Type: jpg Spectrum_20KHz_20V_RMS.JPG (109.4 KB, 1456 views)
File Type: jpg Square-wave_20KHz_57Vpp.JPG (89.7 KB, 1438 views)
File Type: jpg xFrequency-response.JPG (90.9 KB, 263 views)
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Last edited by vzaichenko; 20th February 2018 at 09:55 AM.
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Old 29th September 2017, 05:26 PM   #2
BesPav is offline BesPav  Russian Federation
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Hi, Valery!

Nice approach and good linearity.

It also could be done slightly simpler, without highly degenerated diferential stages, which in turn adds noise.

Something like this:
IMG_6487.JPG

R4 define load of the input diamond buffer and also current consumed from rails.
Full Wilson mirrors reflects consumed current to I/V load.

Last edited by BesPav; 29th September 2017 at 05:55 PM.
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Old 29th September 2017, 05:35 PM   #3
mlloyd1 is offline mlloyd1  United States
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as is usually the case from you - this one looks interesting, too!

could designing the layout to facilitate isothermal pairing of certain devices eliminate the need for the DC servo?

mlloyd1
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Old 29th September 2017, 06:25 PM   #4
vzaichenko is offline vzaichenko  Russian Federation
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No-global-loop amplification
Quote:
Originally Posted by BesPav View Post
Hi, Valery!

Nice approach and good linearity.

It also could be done slightly simpler, without highly degenerated diferential stages, which in case adds noise.

Something like this:
Attachment 637873

R4 define load of the input diamond buffer and also current consumed from rails.
Full Wilson mirrors reflects consumed current to I/V load.
Yep - diamond is a good one, I'm going to play with it
Using dfferential pairs makes it easy to arrange a balanced input.
On the other hand, we can try a dual-diamond configuration, for example.

Thanks for inspiration
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Old 29th September 2017, 06:30 PM   #5
vzaichenko is offline vzaichenko  Russian Federation
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No-global-loop amplification
Quote:
Originally Posted by mlloyd1 View Post
as is usually the case from you - this one looks interesting, too!

could designing the layout to facilitate isothermal pairing of certain devices eliminate the need for the DC servo?

mlloyd1
I think, the right isothermal pairing plus a trimmer for zeroing-out some initial offset may work. Requires some live testing though
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Old 29th September 2017, 06:49 PM   #6
BesPav is offline BesPav  Russian Federation
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Quote:
Originally Posted by vzaichenko View Post
Using dfferential pairs makes it easy to arrange a balanced input.

Yup.
But the most valueable property of the balanced input is a good CMRR. It must reach at least 60 dB at 20 kHz.
So there are next thoughts.
1. Source output impedances must be aligned to 0,1% or better.
2. Balanced receiver must be placed before volume regulator and better to before input selector
3. Balanced receiver must be builded around 0,1% or better resistances, may be matched matrix.

Quote:
Originally Posted by vzaichenko View Post
On the other hand, we can try a dual-diamond configuration, for example.

Yup. And load them to each other.
With proper layout technique you could achieve low parasitic capacitance to ground and improve CMRR accordingly.
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Old 30th September 2017, 08:24 AM   #7
vzaichenko is offline vzaichenko  Russian Federation
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No-global-loop amplification
Agree - "external" balanced receiver will give better result in terms of CMRR.
We use THAT-1206 ones in our preamp - those are fantastic.

Exactly, dual-diamond configuration with the diamonds, loaded to each other. Rather simple and efficient. Also, a good starting point for a fully balanced setup (including two OPS modules).
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Old 30th September 2017, 08:42 AM   #8
Ultima Thule is offline Ultima Thule  Europe
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Quote:
Originally Posted by vzaichenko View Post
What do you think?

Cheers,
Valery
Hi Valery, I think it's interesting to read your philosophical ideas, not many lay out their philosophies regarding audio amplifier designing as you have done here, but rather plant out tiny tidbits "here and there" so one have to spend quite some time here on DiyA in order to get some idea what each ones philosophy is, even if I don't have so much to add I can only say that I concur with many of your sound philosophies.

Regards Michael
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Old 30th September 2017, 10:19 AM   #9
dadod is offline dadod  Croatia
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Valery, as always you have very good ideas how to design different amplifiers. You use diamond OPS here, how you do thermal compensation here, I never built any diamond power OPS yet (only for preamp)? How you get THD 0.002% if OPS alone has 0.05%?
I designed no GNFB but never built yet, to many project, have you built that one, if yes, how you like the sound?
This is my NGNFB amp and it uses current mirror with current gain of 2.12 GainWire-NGNFB-classB-PowerAmp
Those are my preamps with no GNFB, also uses current mirror. Current conveyor as a voltage amplifier and No NFB line amp (GainWire mk2)
Maybe you can get some ideas from it, like to use supper pairs.

P.S. pity you don't use LTspice
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Old 30th September 2017, 11:09 AM   #10
jwilhelm is offline jwilhelm  Canada
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Input boards for the NS-OPS series amplifiers have arrived for this design. I hope to do some live testing this coming week. I've designed the boards to accept RCA or XLR input connectors for either single ended or differential designs.
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