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Old 28th February 2006, 09:27 PM   #1
N-Well is offline N-Well  France
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Location: Z french Alpes
Default Get away from negative feedback ?

Hi DIYer community !

I'm a newbie in the fascinating world of Nelson Pass designs.
Since I'm a newbie, and before my question, I will tell a bit about .. my DIY life if you allow me!

I wanted to start with a very simple design with minimum feedback. So I've just built 4 original ZEN (yes!) + 2 BOZ modules for my medium (600 to 6Khz) and tweeter speakers.
Bass remained for now with my old class B amplier (dyi, "NDFL" design from the 80s).

The result is in fact astonishing, I must say, compare to my previous amps based on TDA 7294 chips.
My impressions:
- The ZEN seems limited in the higher frequency, but not really an issue (I did parallel 10F MKP caps on the ZEN outputs)
- The ZEN seems slightly less "clean", but not really an issue: it stays really easy to listen.
Now, the plus-plus:
- Under certain difficult musical conditions (forte of violin for example), the TDA7294 can be harsh, The ZEN stays much smoother.
- Overall impression of stage is higher with the ZEN: instruments have more corpus (barrier language issue may be here!)

Well, I must say that my ZEN amps supply are filtered with 0.11F per midrange channel and 0.05F per tweeter channel whereas the TDA7294 amp has only 0.04F per channel.

Of course, now ,I cannot plug back by old amp ... and I absolutely need to change my Bass class B amp!

I 've been reading various things on the web, in particular this thesis from Mr Cheever: http://w3.mit.edu/cheever/www/cheever_thesis.pdf
I'm not sure I got everything (80 pages...) but one of the conclusions is that the more feedback you have the more undesired harmonic distorsion you get...

The Aleph 3 looks great, plus I beleive I should away from output capacitors for bass, but this amp have some feedback. The early ZEN designs have not , or not much, but have output caps....

Now my question: what would be the best choice for the bass channel ?


PS: by the way this thesis makes reference to Mr Pass work on single ended amp.
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Old 28th February 2006, 10:22 PM   #2
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Cheever's paper does give much to think about, and I particularly
liked the space he devoted to Crowhurst's work.

The choice of the best amp for bass depends very much on what
the speaker and environment are, not to mention taste, but
if you want to experiment with no feedback, the F1 has a very
interesting bass characteristic. Of course it has no damping
factor.

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Old 28th February 2006, 10:41 PM   #3
flg is offline flg  United States
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87 Pages! I'm still reading the Jung capacitor paper Nelson's papers are a much more plesant read... Sorry, That was a bit of Negative feedback, wasn't it?
I'm going to start stuffing my new Zen V9 board tonight I'm sure there wont be any more Negative feedback than I need...
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Old 1st March 2006, 10:17 PM   #4
N-Well is offline N-Well  France
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Join Date: Feb 2006
Location: Z french Alpes
Thanks for the reply Mr Pass.

My knowledge in electro-acoustic is limited and mostly based on my dyi experience...
The F1 seems to drive speakers with a very different way than conventionnal amplifiers. After a brief tour on some other DIY forum, it looks like current source can interact in funny ways with the speaker impedance.
If I got it well, would the F1 "counter" the impedance rise at the resonance of the speaker ? Meaning that its response could greatly change. My bass speaker is mounted on a bass reflex, meaning I may hear a large increase where the resonator was tuned ?
mmh... I must study this a little more.
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