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Old 29th May 2008, 06:39 PM   #931
massimo is offline massimo  Italy
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Did anybody noticed that in the close up picture in post #926 at the top of this page Nelson added a 2.4 ohms resistor in parallel to the 0.47 ohm source resistor of the Nch mosfet (Q4)?
Moreover, the gate resistor seems to be 150 ohms in lieu of 47 ohms indicated in the published schematics. (Assuming that the left hand resistor close to the gate is R14)

A special tweaking or........???
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Old 29th May 2008, 06:54 PM   #932
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Quote:
Originally posted by massimo


??? ???
Why?
8 mA in each resistor squared * 10 ohms

Yeh Why???
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Old 29th May 2008, 07:03 PM   #933
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I'm sure the Master tweaks all of the production units.

R1 andR2 are also part of the 60 ohm feedback loop, try that with 20V across it.

Bill
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Old 29th May 2008, 07:10 PM   #934
juma is offline juma  Germany
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Quote:
Originally posted by stefanobilliani

I think ZTX 450 and 550 has BASE at the middle pin ;
With the printed face in front of you ,the left pin is collector .
Yes, the base is in the middle, but with the printed face in front of you , the right pin is collector (picture from datasheet attached)


Quote:
Originally posted by massimo

??? ???
Why?
8 mA in each resistor squared * 10 ohms
You forget the current that is coming from output through 100 Ohms / 3 W resistors.
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File Type: gif ztx.gif (5.1 KB, 1947 views)
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Old 29th May 2008, 07:11 PM   #935
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Default philosopy about input sensitivity vs output?

Could someone please elaborate about the current prevailing philosophy
of input sensitivity vs output wattage and specs, with any technical
justification for this position? I'm just a hobbiest. I'm not a EE. I'm
not trying to agrue any position. I am hoping to gain an understanding
of the issues.

Again, I'm not a EE, so please feel free to correct any of my mistakes.
Here is my understanding of the situation:

By my simulation, with a 1.5V input (3Vpp), and an 8 ohm load, this amp
outputs about 12V (22Vpp). 12V * 2.6A = 31.2W, what I think would be
considered peak. 31.2W * .707 = 22W RMS. So with an input of 1.5V,
we get an output of about 22W RMS. I'm not sure if discussing the
simulator's predicted distortion measurements is valuable, so let me just
say that the simulated circuit's THD measurement is in the same
ballpark as what Nelson published.

Now 12V is obviously no where near the 24V rail voltage, and the amp
is not clipping at the 1.5V input. The amp is getting close to clipping
at an input around 3.7V (7.4Vpp). At 3.7V input, we get about a 22V
(44Vpp) output, with distortion in the ballpark of .5% (again, simulated).
22V * 2.6A = 57.2W peak. 57.2W * .707 = 40W RMS.

I'm under the impression that most amps have a specified full output
rating with an input of around 1.5V. Or at least many give their specs
that way. I'm not saying that one should judge amps on their specs alone,
but I use specs to get me in the ballpark of devices I want to compare.
One viewpoint is that this amp is a lot more powerful than a 22W amp.
Should we say that this is a 22W amp with almost 3dB of headroom?

Another observation that is really interesting to me is that the 3.7V input
that led to the output being slightly shy of clipping in the F5 is exactly the
same as what was designed for the Zen 5. This makes me think that
Nelson has a philosophy about input and output ratings.

So it seems to me that there are many different ways, and each way
could be factually correct, that one could give specs or generally
classify an amp. It also seems to me that Nelson has a design
philosophy that he uses in this regard. Would anyone care to explain
this philosophy, including a technical justification for this position? It
probably wouldn't take too deep of a technical discussion to loose me.
But if I don't ask, I may never know...

thanks,
Robert
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Old 29th May 2008, 07:21 PM   #936
massimo is offline massimo  Italy
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Quote:
Originally posted by juma

You forget the current that is coming from output through 100 Ohms / 3 W resistors.
Yes, you're right.
It doesn't look Nelson has used other than Dale RN55 resistors at the input of the amp reviewed by 6moons, anyway. (the two + two resistors close to the JFets, iI mean.)
Even in the article, a part from the blue Pana 3W, he suggested to use 1/2 W resistors throughout.
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Old 29th May 2008, 07:58 PM   #937
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Quote:
Originally posted by Nelson Pass


The closest I've got is to get a 1 dB peak at about 800 KHz.
There are two peaks, and it seems to relate to stray input capacitance and the value of the input resistor.

Stray input capacitance -- my bad -- I should have remembered that it quacks like a current feedback opamp so it's some sloppiness in the layout, even a few pf's causing peaking. Viewers are strongly advised of the adult nature of the material which will appear on your screen:

Click the image to open in full size.

That's out to 2 MHz.
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Old 29th May 2008, 08:14 PM   #938
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That's really interesting. Frankly, I didn't put any thought into
the input resistance vs bandwidth - I simply defaulted to the
known "won't oscillate" values at the Gates of the JFETs.

My actual circuits don't correspond to these results though, so
I'll take the time to play with it and come back with some info.
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Old 29th May 2008, 08:35 PM   #939
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The bandwidth will also be a function of the feedback resistance.

It's not that my layout is sloppy per se, it's just that not enough consideration was given to the capacitance issues. I will try doing this "dead-bug style" with some 50 ohm coax rather than Pomona clips, maybe another using surface mount input resistors to each of the gates.
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Old 29th May 2008, 08:45 PM   #940
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That goes without saying, since the feedback resistance affects
the open loop gain.
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