CFA Topology Audio Amplifiers

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#3,841
Again -on the thermal issues ... 😕 ?

At least on a NAD/NX type amp , 1st/2'nd stage cancel with temp. My
hawksford's cascode cancels out the active devices. LED CCS cancels out -
(LED forward V drops w/temp).
I seen the NAD372 (Junie) , only the OPS diamond/drivers were coupled (different OPS vbe).
All else was "sloppy" , no input diamond pair coupling ... VAS was all separate ,
as well -- BUT , it works !

BTW - Junie's VAS blew up , luckily it was driving that High-Z input
diamond compensated OPS.

OS
 
#3,843
john curl said:
Well I was getting somewhat 'high handed' Richard and it was not the only time that you hung up on me. I haven't called you since, you might notice. It's OK.
Click to expand...

You do that a lot, BTW... but thats OK, too. Its just you being you. Call anytime you feel like it.

So, what about that thermal problem you had with your CFA? Thats a lot more interesting.

Better yet... why not revisit the CFA and see what you come up with today. Maybe do it here.

Thx-RNMarsh
 
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#3,844
Esperado (3833)
I have, personally, no experience on "no GFB" with power amps. Neither on negative impedance. No instability ? No added distortion to the speaker coil, due to Eddy currents ?
Thank for sharing.
Small negative output impedance within minus 0.2 ... 0.3 Ohm perfectly suppress back EMF from the speaker and no (some, any) instability is not.
Instability begins to manifest itself in the form of increasing returns (hum) loudspeaker at 150 ... 200 Hz at lower output impedance lower than minus 0.4 ohms.
best regards
Petr
 
#3,845
Petr1951 said:
Esperado (3833)
I have, personally, no experience on "no GFB" with power amps. Neither on negative impedance. No instability ? No added distortion to the speaker coil, due to Eddy currents ?
Thank for sharing.
Small negative output impedance within minus 0.2 ... 0.3 Ohm perfectly suppress back EMF from the speaker and no (some, any) instability is not.
Instability begins to manifest itself in the form of increasing returns (hum) loudspeaker at 150 ... 200 Hz at lower output impedance lower than minus 0.4 ohms.
best regards
Petr
Click to expand...

How did you do it -- circuitry?


Thx-RNMarsh
 
#3,849
RNMarsh (3845)
How did you do it -- circuitry?


It's enough to take in a properly configured output stage distortion corrector Hawksford (ECH). And after setting the insert between the generator and the output stage resistor 20 ... 30 kohm. More accurate adjustment of the negative output resistance can be done using one of the balancing (ECH) schemes corrector described in the book Bob Cordell.


best regards
Petr
 
#3,850
Waly said:
Well, I would think that, in audio, the snake oil has to be branded and subjectively evaluated as well. It seems like every audio expert has his own snake oil, and then any other brand is snake oil. Your snake oil makes no exception.
Click to expand...
The big problem with you is you believe electronic for audio is a science. I'm very sorry to inform you that you're wrong, it is technology.
An imperfect assembly of imperfect components, witch react with a lot of evils to a huge number of various environment you can't put on paper.
I'm sorry to inform you that simulator is a nice tool, but just a caricature.
I'm sorry to inform-you that we know very little about the way our brain+ears works and can be fooled.
I'm very sorry to inform you we know very little about how our beloved and imperfect musical signals are destroyed in our imperfect electronic assemblies.
I'm sorry to inform you that, as long you will be the 'believer' you are, yes this is what you are, you will never be a good designer for audio.

Because all the great designers i had met in my life, with no exceptions, were modest, doubtful, curious, open minded, experienced, with respect for others. Everything that you are not. They lie on what they can see, listen, reproduce. And they had done this a lot during years, endlessly. (And, with no doubt, J.C. is one of them).
You ? You are here in front of a lot of great audio designers. They all know what you just have learned at school or in books: A theoretical knowledge, just information, not knowledge, a very little part of the game.
They have experience.
Do-you have respect for what they share ? No. Do-you ask yourself the question: Why are they so exited with CFAs ? No.
A lot of people reports similar experience, did this question your certitudes ? NO.
Did-you decided to listen to some samples of CFAs, in order to see what is behind or about ? Never.
You just pontificate against a topology that you have never experienced. An endless troll.

That is stupid, that is not scientific, that helps nobody (neither yourself), that is boring.

I'm sorry, but you are the one who don't believe UFOs exists. Think twice: the only think you can say is "i don't know, never seen one of them".
 
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#3,851
Petr1951 said:
Esperado (3833)
I have, personally, no experience on "no GFB" with power amps. Neither on negative impedance. No instability ? No added distortion to the speaker coil, due to Eddy currents ?
Thank for sharing.
Small negative output impedance within minus 0.2 ... 0.3 Ohm perfectly suppress back EMF from the speaker and no (some, any) instability is not.
Instability begins to manifest itself in the form of increasing returns (hum) loudspeaker at 150 ... 200 Hz at lower output impedance lower than minus 0.4 ohms.
best regards
Petr
Click to expand...

Having a negative output impedance from a power amplifier can sometimes be a useful tool for altering the speaker damping. However, bear in mind that both closed-box and vented systems are designed and voiced assuming a normal amplifier with fairly high damping factor. The Qts of the woofer will be decreased by a negative amplifier output impedance and alter the bass alignment. There can also sometimes be some interactions with crossovers if a negative output impedance is put at the amplifier output.

BTW, there is an important exception to my comment about the way speakers are voiced. If the speaker designer is a lover of tube amps, he will probably voice them with a tube amp, which typically may have a DF of 20 or less, which means some output impedance. This can result in the speaker sounding different when driven from a solid-state amp with very low output impedance, and not necessarily the way the speaker designer intended.

Cheers,
Bob
 
#3,852
Bob Cordell (3851)
Having a negative output impedance from a power amplifier can sometimes be a useful tool for altering the speaker damping. However, bear in mind that both closed-box and vented systems are designed and voiced assuming a normal amplifier with fairly high damping factor. The Qts of the woofer will be decreased by a negative amplifier output impedance and alter the bass alignment. There can also sometimes be some interactions with crossovers if a negative output impedance is put at the amplifier output.


Bob!
Negative resistance is intended to compensate for the resistance of the speaker cable, see Patent: Patent US4441085, N03F 1/34, publ. 03.04.1984
then cable simply will not exist!
Here is another example of a negative output resistance
618e97a84b0f.png
[/URL][/IMG]
http://radikal.ru/fp/f86f43e5fbb04b07a76a558e6e2d044c][/URL]
best regards
Petr
 
#3,853
ostripper said:
But , with many being pin compatible with the TL .. one still has a choice.
The dual package is the way , as SO many NE5532's exist in the waste stream.
Fixed the Caps !! (closer Vcc-gnd's + /+vcc to -Vcc 3 cap)

OS
Click to expand...

Yes, with the dual 8 pin DIP you will have lots of choice.

I think offsets at the amp output of +- 5 mV worst case untrimmed, which is what you will get with a general purpose OP amp, should be ok. If you want something considerably better than this, then indeed, go for higher precision devices - there's a big selection out there. But, they will cost ya!
 
#3,854
MiiB said:
I have several posts ago shoved a CFA front-end where the current is thermally compensated by a CCS, which has opposite tempco of the input pair. (The four transistor glued together) If currents then are mirrored into the VAS. Then there's no drift at all. I have built several and seen it by practical implementation. The first versions I built was terrible as I had to run everything very low in order to have room for the drift. The latter cures it.
Click to expand...

My findings on offset stability are similar. Practical, DC coupled diamond input CFA's don't need a servo. You do need to trim the offset out. So, if using a pot, two resistors and a cap are a concern then I guess use a servo.
 
#3,855
Bonsai said:
My findings on offset stability are similar. Practical, DC coupled diamond input CFA's don't need a servo. You do need to trim the offset out. So, if using a pot, two resistors and a cap are a concern then I guess use a servo.
Click to expand...

You state your findings , I agree that- as is- your NX (at least simulated)
does well for a "IC-less" design (offset + thermal).

But, Why -o -why.. do the OEM's ALWAYS use that dang servo. I'm up
to about 38 commercial CFA amp schematics and ALL use it (the servo).

Could it be a long term offset/thermal stability issue.. or is it that a
TL/NE IC +cap is just a dollar and solves the problem (better) ?

PS - after simulating and studying all the 38 , 70% offset the diamond
w/ inverting servo's fed to the diamond. Properly implemented ...
just nV's of AC are fed back to the diamond. The lifted ground ,
and the Vreg for the diamond are far more "polluted" (100's of uV's).

Perhaps the misconception is that the IC will inject noise or other "badness"
into the diamond. With just DC and a couple nV of AC being
fed back through a 470K resistor - it truly amounts to nothing.


OS
 
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#3,856
I think the reasons are

1. They don't like pots - you need a good quality pot and that's expensive. In high volume production qty's a TLO72 is probably about 15c and the cap and R's another 20c
2. You have an additional production step to measure the offset and dial it out.
3. If the amplifier has thermal drift problems, you either need to capacitively couple, or you have to use a servo. Even if you capacitively couple, you will often still need to dial the offset out

These are all good reasons to use a servo.

However, I prefer a pot since if fits in well with my zener regulated front end approach, or in the case of the e-Amp, the cascode bias voltage.

In the e-Amp I used a 1000uF coupling cap and dialed the offset out with a pot. In the nx and sx the DC stability is very good, so no need for decoupling cap or servo - only a pot to dial out the initial offset.
 
#3,859
MiiB said:
The servo also protects you from sources with dc offset, my servo is capable of dealing with up to ,25V. With a servo you could also loose the feedback capacitor
Click to expand...

Without a input cap , .25 v is TOO much. the diamond is just too sensitiive.

Variations in the lifted ground , or the filters's (lifted ground"s) natural offset , no issue.

It takes less than 2 mV (diamond) to correct the whole amps offset.
The diamond is nearly as sensitive as a quality op-amp's input. 😱

OS
 
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#3,860
ostripper said:
But, Why -o -why.. do the OEM's ALWAYS use that dang servo. I'm up to about 38 commercial CFA amp schematics and ALL use it (the servo).
Click to expand...
Just a funny story. I had to repair a Revox A77. All the trim pots where dead. The cursors where deep black, not touching the carbon ring any more, and falling down apart as soon as you touched them. All of them, and they where many in this machine.Replaced, +two special caps, and the machine was working like the first day.

For my own usage, i always replace trim pots, after tuning, by sets of resistances of appropriate values and i highly recommend this.
 
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