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PostPosted: 06 May 2018, 18:15 
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Joined: 26 Dec 2016, 03:46
Posts: 194
Location: Bayarea
I just did a rough drawing, no guaranty it will work, it's easy to try.

1) I add a 75p cap from output of the opamp to the -ve input, this is to start rolling of the gain at about 100KHz C=1/(6.28 X 20Kohm X 100KHz).

2) I change 47K feedback resistor to 20K, don't want to use too high a value to avoid extra pole formed with the input capacitance of the -ve input.

3) I eliminate the trim pot, you don't need to trim the gain. Gain is set by the feedback resistor G = 1+ 20K/1K = 21. Don't hang stuffs on the -ve input if it can be avoided, it's a sensitive junction that can pick up noise.

4) I increase the 10uF to 100uF to give better low frequency response.

5) Don't use MC1458. Try OPA604, it's +/- 22V supply, good drive current, THD of 0.001% or better. It's a single opamp, you only need one. If you insist in dual opamp, try LM4562.

Attachment:
Opamp amp.JPG


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PostPosted: 06 May 2018, 21:17 
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Location: Bayarea
I looked back to the schematic. Q1 has to thermal coupled to either Q2 or Q3. This is not just a simple voltage multiplier, it's the temperature tracking that is important. Vbe is about -2mV/deg C, you need thermal tracking to keep the idle current in check. Or else, you will have a run away condition.

What that means is Q1 has to be either mounted on top of Q2 or Q3 to keep the temperature tracking each other. OR mount on the heatsink right next to Q2 and Q3 so the temperature closely tracking each other. Just the fact you use 2N3904, this is not the kind of package that can be bolted onto Q2 or Q3 or onto the heatsink. You need something like a TO-126 type transistor. I choose a low Vbe type transistor so you get better voltage divider spread. Big power transistor has lower Vbe like 0.64V, if you use a transistor that has 0.72V Vbe for Q1, you will NOT get the temperature compensation correct. FYI, I bought over 15 transistor to test the Vbe, I use KSD1691, this has the lowest Vbe that comes close to the big power transistors. You might want to check this out.

You want to design power amp, you really need to get the book of Audio Power Amplifier by Bob Cordell. the devils are ALL in the detail. Things are very subtle, but it's very important.


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PostPosted: 07 May 2018, 02:01 
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Joined: 23 Feb 2017, 02:02
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My amp uses a cfp output stage, without anny compensation added to the output stage, zero oscillations
The op amp is a properly compensated stable high gain made out of descrete components.

darington shows reduced frequency response and higher distortion compared to CFP when loaded up with eight. However no load results are fairly similar.
Further more CFP is much more tollerant to capacitive loadings and oscillates less.

If I were to compensate this output, having a few pf arround 33pf or so accross base to collector of all Szlkai connected transitors may or may not work.
I've tried arround 33pf in my output before, did not reduce distortion or have any postive effects, infact it increased my distortion.

What yungman drawn up is very good, It should take away the oscillations. If not then you've probally done your output stage wrongly, then revert back to standard output stage.
After you got a standard stage with emitter resistors working, go back to your emitter resistorless


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PostPosted: 07 May 2018, 05:09 
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Location: Bayarea
Bob Cordell talked about potential oscillation problem of CFP, also, CFP has gain, frequency response is much lower than simple emitter follower. ( in CFP, it's fT/gain of the common emitter transistor). So it will introduce a pole much earlier.

Doug Self has write up on crossover distortion of CFP, it's much sharper on the wing spread plot.

BTW It would be better to put about a 1ohm resistor at the emitter of those big common emitter transistors like Q4 and Q6. Without emitter resistor, current sharing is going to be bad and gain is going to be higher and more prone to oscillation.


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PostPosted: 07 May 2018, 05:23 
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In my testings built in real life, CFP showed superior charteristics in all area.
In class ab. cross over was also much much lower than darington.


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PostPosted: 07 May 2018, 11:47 
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Location: Bayarea
KochiyaYamato wrote:
In my testings built in real life, CFP showed superior charteristics in all area.
In class ab. cross over was also much much lower than darington.


Can you provide more info, I am going by the book and avoid designing with CFP. My designs are all complementary 3EF( eg. PNP NPN NPN EF). Also, Doug Self said you cannot use more than two output pairs with CFP to improve crossover distortion. You can literally half the crossover distortion by doubling the number of output pairs in simple EF OPS. In my designs, I use 9 output pairs.

Nelson Pass use many pairs of CFP in his Threshold Stasis designs. I own the Nakamichi PA-7 that he designed. I am really not impressed either with the sound, THD or stability. THD is 0.04% across frequency range which is not low.


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PostPosted: 07 May 2018, 15:30 
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Joined: 23 Feb 2017, 02:02
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I've used multiple parallel transistors in CFP without anny plorbem.
Many guys on the book do circuit simulator and belive it 100%, circuit simiulator has been very inaccurate for me many times so I just skipped it.

Darington driving 8 ohms had shown allot of distortion similar to cross over for my amps.
Even at class a like 2.5AMP bias once I try to output more than arround 1.25Amps into 8 ohms cross over starts apperaing.
This is arround 18v pk-pk. voltage swing is 9v postivie or negative so 9/8 is current

This is three pairs of transistors.

With CFP and same three parallel outputs the amp was able to output sine wave perfect at class ab and class a up untill clipping

http://www.tonepublications.com/old-sch ... amplifier/
Online it says PA-7 is single endded, the output is not directly connected to speaker but through capacitor for dc blocking, that is mainly why I think your not impressed with the sound.
There also isn't anny global feedback loop so distiortion is quite high another reason for poor sound. All my testing showed more feedback better sound given that amp is stable.


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PostPosted: 07 May 2018, 19:17 
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Joined: 26 Dec 2016, 03:46
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Location: Bayarea
The website is wrong about PA-7, it's not single ended. It's is push pull 8 stage CFP. Attached is the schematic with values I put in as I bought it in non working condition and I had to fix it.

Attachment:
PA-7 with value LL.jpg


Now you got me interested in experimenting with CFP. I so wish they have big transistor with fT over 30MHz.

I use LTSpice, the models have been modified by Bob Cordell, I got this from his website. I think it all depends on how fast you push the design and how you test. I designed the amp to have -3dB at about 300KHz at large signal with slew better than 30V/uS. I think this has advantage of better transients and much lower THD at 20KHz. I achieved 0.003% @20KHz with 46V peak to peak driving 4ohm with +/-33V rails ( to keep the amp cool as I have 1A bias). If I use bigger chassis with bigger heat sink, I can go much higher power over 100W at this THD. When you push for high frequency response, compensation gets tricky and LTSpice is not accurate. I talked to LTSpice group and they just said you can only take it as guide line.

BTW, when I said stable, I don't mean stable with the 4ohm load resistor and see no oscillation, it's too easy to get it stable like that. Even Marantz is wrong in their test procedure of testing stability. They put a 2uF on a test jig with 8ohm load and connect to the amp by a cable. That cover up everything. I test with with no resistor load( as speaker is like open circuit at 300KHz). Then capacitor right AT the banana output posts of the amplifier so I don't have the series impedance to separate the cap from the output of the amp. Then trigger with squarewave sweep to over 3MHz to trigger oscillation.

I can make my amp stable to 0.022uF cap at the output post, the Nakamichi start oscillate with only 1500pF without even square wave trigger. LTSpice failed flat iin stability.

It is important to have no resistor load. That's the reason so many amps on the market are unstable, this have been talked a lot in forums. With the resistor load and connect with speaker cables, you can pile up the capacitors!!! That's why taming stability is a pretty big job.


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PostPosted: 08 May 2018, 00:24 
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Joined: 23 Feb 2017, 02:02
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I forggot a very important point in my amp, with a pure resistive load

The daringron output has have crossover like distortion with sine.
CFP has zero crossover like distortion.
Both amps were excatly the same front end driver but in the end modified to run on CFP.

However with darington output, it will not crossover up untill clipping with a inductive load like speakers.

With my latest CFP output stage and high gain front end stage with lots of feedback, no output inductors but has 4.7R +0.1uF zobel on output.
The amp shows zero oscillations and less than 5% over or undershoot with a pure 0.1uF load showing extreme stablilty.
OP AMP sellers selling infinate capacitive load op amps say having plently of high frequency gain and bandwith + low impdeance ouput prevents op amp from oscillating under pure capcitive load.

My older schematic have lower gain front end, the amp oscillates with pure capacitive load.

Your amp schematic shows a front end using a mosfet input cascoded and using simple gain stage. The gain stage has current limiting viaQ107, the gain stage is then sent to a common emitter buffer connected a vbe multiplier then drives allot of outputs.
Its a very simple low gain front end.


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PostPosted: 08 May 2018, 00:39 
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Joined: 26 Dec 2016, 03:46
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Location: Bayarea
You happen to live in the Bay Area? I am interested in meeting people that are into amps in my area!!! PM me if you are and are interested in talking.

Yes, I have zobel network. I don't use output inductor. It's a long story, I found the inductor really increase THD and higher frequency if the load is non linear......like speakers.

I have to start thinking about experimenting with CFP OPS stage. I avoid it because of reading by Bob Cordell and Doug Self. BUT like everything else, they are engineers like us and they are human. I have very good luck with 3EF, so I never challenge what they say.

How do you measure crossover distortion? Since I use 9 pairs output transistors, and I have 1A bias current, I really cannot make the THD tell me any useful info when I measure with different amplitude output.


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