Near ZERO IMD buffer amp

[wenye]

Originally posted by alaskanaudio


In a true linear amplifier the measured distortion should be the same at each point in signal path. With the application of any kind of feedback, error correction, pre-distortion or whatever you want to call it have you will have higher distortion levels in the intermediate stages. This intermediate stage distortion can be very high even though the distortion at the output may look truly outstanding. This is likely to be detrimental to the reproduction of the music.

Johannes,

I agree with your point, but not at all.
Ideally, low distortion at anytime slot and anywhere the signal goes through is the best. Unfortunately, for voltage mode amplify technology, it nearly can not be achieved. Usually voltage mode amplifier (call it VMA since here) have very high open-loop gain and depend on deep NFB to achieve stabilization, low distortion and high close-loop bandwidth. When this amplifier works at open-loop status, high distortion will occur at any point of the amp. Thanks for NFB we can see a good looking on periodicity signal. When a high speed signal feed to this kind of amplifier, and signal bandwidth over the open-loop bandwidth of this VMA, then NFB can not correct the distortion immediately, transitory distortion occur. This kind of distortion someone call this thing as "TIM". In VMA, if TIM occur, both signal current and signal voltage of each stage has high distortion. Whether NFB deep or not! The problem is real! Because every natural sound is not periodicity, they consist of very high-speed components. So that almost all of VMA have "TIM" problem. Certainly, limit bandwidth or extend open-loop bandwidth to cover signal BW are compromised solution. Above is theoretically speaking of TIM problem. Have a glance at most solid-state audio amplifier design, VMA and NFB are most common technology which was employ before. And it seems nobody complain about that ¡°they sound bad¡± or ¡°this product employ NFB, so that it sound bad¡± even if NFB employ in hi-end stuffs.
Now, let¡¯s talk about another kind of amplifying technology. It called ¡°current mode amplifying¡±(CMA). In this kind of amplifier design, signal be transfers as current, and convert to voltage at the last node of gain stage. The most important characteristic good for audio or any distortion sense project of CMA is: ¡°even if voltage signal was distorted at I-V conversion stage due to not correct gain setting or any other reasons like TIM, if V-I convert stage work normally signal current still linearity! So now, how to define this stage has distortion or not? And CMA provide better better open-loop bandwidth then VMA at same of open-loop gain. As we talk above that low open-loop bandwidth will cause TIM directly! And as I said before if open-loop bandwidth over input signal bandwidth, that it will not cause TIM or just a very little TIM we can ignore it.
So, now return to your point, you said any NFB or NFB like distortion-canceling technology will cause additional distortion? I don¡¯t think so¡_.. Especially in CMA design¡_¡_

[wenye]

Hi Terry,



Well, it will surely measure worse with the 1k load however
the nice thing will be you can see the harmonic spectrum more
easily.

OK, sounds interesting and novel, that's good. Another good
reason to try various and herder loads to see how well the
pre distortion works over a variety of conditions. It will also
be interesting to see if the spectrum changes.




Agree, so I need to test it again......




Yes. If you know the conditions it will be used in then no need for
overdesign. Unfortunately we need to cater for worst case
conditions, we never know when someone is going to hang
100's of metres of cable and a few signal splits off a mic pre.
But it's a good challenge and I find high drive capability with
healthy class A always sounds better even into light loads.

Ultimately the sound is what's most important, and everyone
has their own design criteria. However as stated make sure you
check stability into various cap loads representative of typical
consumer interconnects. Even some open loop buffers can
oscillate however they usually require less series resistance
to stabilise than a typical opamp.



Congratulate! You found a good way to meet this challenge. That is why I would like to say "real condition" have to drive a high capacitance load. Also need to try...




I think this is important especially WRT it's loading effects on
previous stages. I have found that even tube based followers
had a considerable effect on the previous stages linearity. WRT your CD player OP application, it is likely in this case that
the previous stage will have a lower impedance than a typical
pre amp stage so obviously you would use a source R for
measurements that is representative of your particular
application.



For condition of the test I do before, the input source impedance is around 1.02K...... Happy to try any other source impedance and find what's different.

Originally posted by Terry_Demol
Hi Wenye,

Did you try 1k load on buffer? I'm interested to see spectrum
results.

Cheers,

Terry

After 3 days turn it in, it sound better....

Today is not a good luck day. Our AP-2322 was defective by ultra high input ac line voltage. I think the power supply of 2322 was defectived! I measure the incoming ac line voltage after 2322 smoking, actually it as high as 260V!!! It's crazy!!! ..... It seemly have to return it to AP factory....

Could you help me to accomplish other measurements what we (me and you) are concern? If it can be, I will send you 2 or more pcs of this buffer modular for measurement...... Could you?.....


Thanks
Wenye

[wenye]

Originally posted by kello

the Sheet show you buffer the THD is 0.0025%(about -92dB),the distortion just so so,same level like good OPAMP.

in you test sheet



show the residure distortion in >5KHz is better but can see more distortion in <2KHz band.meaning in this band the quality must take care( this band more importan for music or voice listen).

in the test we perfer focus in audio quality for listen,so evaluation this module is good or bad no only in one band.

so I hope can have a look all this distortion in 20-2KHz band's distributing.

BTW, can you paste the THD vs RL on 50--100K ohm the perfermance of you idea design?

1. THD of open-loop design is not easy to over an op-amp with NFB.

2. The products under 2khz usually cause by measure filter of measurement hardware and software....

3. Just listen to this evaluation board, it sound better after 3days turn in.
Well come to limit number of volunteer to evaluate the modular and give a fair appraisement (not too much contort )...

4. Please find the attach file above THD+N vs frequency.jpg for more detail what you want.

5. THD+N vs load. AP is now not available, could you help me to do the testing?

Rdgs,
Wenye

[kello]

He he....

Tony! :P


My HP audio analyzer is working fine.....But I think must take free time using the R&S have a look for calibration and confirm again ....Thanks for you big help...


No problem,Send me you board if you finished ....I'll Check and analysis all performance using the UPD analyzer and soon reply back the result to you.

[kello]

Originally posted by wenye


1. THD of open-loop design is not easy to over an op-amp with NFB.

2. The products under 2khz usually cause by measure filter of measurement hardware and software....

3. Just listen to this evaluation board, it sound better after 3days turn in.
Well come to limit number of volunteer to evaluate the modular and give a fair appraisement (not too much contort )...

4. Please find the attach file above THD+N vs frequency.jpg for more detail what you want.

5. THD+N vs load. AP is now not available, could you help me to do the testing?

Rdgs,
Wenye

I don't think so the <2KHz's distortion is cause by AP2's HW or SW.

still now,I can 't find the THD vs Fr in 20-20KHz@0dB,-20dB test result.


for the THD vs RL,you can appoint the RL load in diffiren choice in 50-100K ohm,normally we using 10K or 100Kohm for normal test.

in test result graph sheet, you can cascade result with difirence RL load.

normally, we perfer the buffer can dirver low impedance lower <1K,e.g 75ohm.if not ,we don't need the buffer.....



--- I can help you for the test,send me the modules,I keep the word about you secret , call me by moblie phone.

[X.G.]

http://www.cndiyclub.com/bbs/dispbbs...&star=1&skin=0

[bbp]

I¡®m glad to hear this

[analog_sa]

Originally posted by bbp
I¡®m glad to hear this

Apparently sound travels even slower than i thought.