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Old 17th March 2014, 03:34 PM   #1
dadod is offline dadod  Croatia
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Default Unique CFA 120/230W amp

This is going to be my next project of the CFA. Why it is Unique? This is only amp, I know, using OIC(output inclusive compensation, or pure Cherry) and thus lowers crossover output stage distortion. I did simulate similar CFA amp with lateral MOSFET output, but not yet built( I hope Chris will do real world amp of it soon). This one use all BJTs and triple output. This kind of a compensation was discussed a lot(Cherry wrote an article) and D. Self said in his book that is not possible to make practical amp with this as it's to unstable. I think opposite, it is very possible and it should be done. Here I use OIC in combination of the shunt compensation(actually two shunt compensation) and in simulation the triple does not need additional internal compensation, at list in the simulation.
As I think that most people will satisfy with 120 W/8 ohm, 230 W/4 ohm I used only three output pairs. This amp has THD at all frequencies and all powers below 7 ppm, and THD1k below 2 ppm.
I did try different IPS, and going always back to the Baxandall pairs at the input, with diamond input I gain nothing, simple diamond or bootstrapped, and here I use CCM in combination with the Hawksford cascade for the VAS. It is possible to simplify the VAS and not loose in distortion level, and I could show the simplified version too.
Distortion harmonic distribution is quite benign and this is a way to excellent sound. It is DC from the input to the output using DC servo to kip output DC offset below 1 mV. I was trying different ways to connect DC servo with as less as possible influence on the signal path, and thanks to OS I choose this way I used here.
This is just a study as I intend to add all needed protection. First idea is to use capacitance multipliers for the output stage with a protections incorporated on the same PCB and this I've use in my TT amp and it works flawlessly. Other idea is to use output transistors as protection units(not standard current limitation, sound influence is to bad) and not yet sure how to do that.
BR Damir.
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Old 17th March 2014, 03:39 PM   #2
dadod is offline dadod  Croatia
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Default Unique CFA 120/230W amp

Here are 1 kHz FFTs at different output power, and excellent clipping behavior.
THD at 10 Hz is on the same level, it should produce excellent low distortion bass.
BR Damir
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Old 17th March 2014, 04:58 PM   #3
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Very nice .... I see some very familiar techniques here.
I guess I might "borrow" some of yours .. too.

We will keep it going , despite the CFA thread off-topic stuff

OS
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Old 17th March 2014, 08:02 PM   #4
Struth is offline Struth  Canada
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Hi Guys

Can you provide a larger schematic, or make the above one expandable? Some of the values are hard to read.

It seems like there are two forms or two definitions for OIC. The Cherry cap is one form. The other is as used on Self's Compact Blameless amp (not that he invented, OIC has been around since the 1970s with other names), where the standard miller cap is split in two with both new caps at twice the value, then a 1k tied from their junction to the output node. There is a transition from OIC to straight miller, so maybe this form needs a name other than OIC (which Doug called it) - transitional OIC...?

I've used the above compensation on both VFA and CFA designs and it seems to work well. I can't read the THD on my Radford analyser.

Great design as usual. What needs to be determined are the real-world impediments or considerations for attaining the simulated performance. Do0 either of you have an APsys2?

Have fun
Kevin O'Connor
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Old 17th March 2014, 08:06 PM   #5
RNMarsh is offline RNMarsh  United States
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Quote:
Originally Posted by ostripper View Post

We will keep it going , despite the CFA thread off-topic stuff

OS
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Old 17th March 2014, 08:11 PM   #6
RNMarsh is offline RNMarsh  United States
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Quote:
Originally Posted by Struth View Post
Hi Guys

Can you provide a larger schematic, or make the above one expandable? Some of the values are hard to read.

Great design as usual. What needs to be determined are the real-world impediments or considerations for attaining the simulated performance. Do0 either of you have an APsys2?

Have fun
Kevin O'Connor
Some Monte-Carlo runs need to be run to determine spreads of parameters and their affect which could impact actual build performance.

I will test any CFA amps (only) with AP2722 and ShibaSoku 725D and others..... BUT, they cant be just pcb for me to load. Stuffed, working boards only.

Personally, still looking for the higher output CFA (approx 250W/8).

THx-RNMarsh

Last edited by RNMarsh; 17th March 2014 at 08:20 PM.
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Old 17th March 2014, 09:57 PM   #7
dadod is offline dadod  Croatia
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Quote:
Originally Posted by ostripper View Post
Very nice .... I see some very familiar techniques here.
I guess I might "borrow" some of yours .. too.

We will keep it going , despite the CFA thread off-topic stuff

OS
Thanks OS, I hope you can find something interesting to use in your designs.
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Old 17th March 2014, 10:00 PM   #8
dadod is offline dadod  Croatia
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Quote:
Originally Posted by Struth View Post
Hi Guys

Can you provide a larger schematic, or make the above one expandable? Some of the values are hard to read.

It seems like there are two forms or two definitions for OIC. The Cherry cap is one form. The other is as used on Self's Compact Blameless amp (not that he invented, OIC has been around since the 1970s with other names), where the standard miller cap is split in two with both new caps at twice the value, then a 1k tied from their junction to the output node. There is a transition from OIC to straight miller, so maybe this form needs a name other than OIC (which Doug called it) - transitional OIC...?

I've used the above compensation on both VFA and CFA designs and it seems to work well. I can't read the THD on my Radford analyser.

Great design as usual. What needs to be determined are the real-world impediments or considerations for attaining the simulated performance. Do0 either of you have an APsys2?

Have fun
Kevin O'Connor
Above schematic is expandable, look left lower corner.
Damir
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Old 17th March 2014, 10:03 PM   #9
dadod is offline dadod  Croatia
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Quote:
Originally Posted by RNMarsh View Post
Some Monte-Carlo runs need to be run to determine spreads of parameters and their affect which could impact actual build performance.

I will test any CFA amps (only) with AP2722 and ShibaSoku 725D and others..... BUT, they cant be just pcb for me to load. Stuffed, working boards only.

Personally, still looking for the higher output CFA (approx 250W/8).

THx-RNMarsh
I can't get an answer from you, what did you do with the GainWire PCB I've sent to you??
Way you did not say that you want assembled board only?
BR Damir
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Old 18th March 2014, 03:50 AM   #10
Struth is offline Struth  Canada
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Hi Guys

It does not expand and the usual little expander icon is not there. Could you reload it?

Have fun
Kevin O'Connor
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