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Old 12th January 2013, 12:14 PM   #31
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Metal oxide resistors should have less inductance than wirewound, and they are often recommended for passive loudspeaker crossovers (L-pad) for that reason. How much less I can not say.
But it is possible that carbon resistors are also non inductive depending on fabrication method. Metal film resistors are inductive since resistive element usually have the form of spiral. In Japanese amps one can see emmiter metal film resistors in the form of cube and it is possible that these are made in special way to make them non-inductive. But it is impossible to buy them anywhere.

I think that Rod recommended carbon emmiter resitors for his P101 Mosfet amp because these are non-inductive.
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Old 12th January 2013, 01:21 PM   #32
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What is the temp coeff for 2KV low pF caps ( blue in colour) ?
I had some 100pf /2 kV which I measured at room temperature 28 deg C and dipped in hot water ( about 80 deg C). The value didn't seem to change drastically. Certainly not 50%.
Maybe I should do it again now to make sure I didn't goof the measurement !
I couldn't find any COG/NPO in the local retail market. I have a few which I bought in Singapore.
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Old 12th January 2013, 02:05 PM   #33
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Carbon film , metal film and metal oxide resistors are all fabricated similarly and spiral grooving, used in production to economically trim the values of film types, creates an inductive component to them all. It will vary somewhat, according to the length of spiral path necessary for each value in the individual manufacturer's formulation/decade range. Low value 5W box wirewounds (at least) are wound for low inductance (bifilar) below 100R. That is open to question but the specs of common brands like NOBLE, XICON seem correct, at <10% of the nominal single layer solenoid wound value, using an LCR bridge.

In any case, the original P3a design is quite stable with 30MHz Ft output and 150 MHz Ft driver transistors - as is. There are tens of thousands of these in wide application around the globe. It is probably the most tested and proven of all DIY amps! The main threat to its stability is ill-conceived PCB layout and wiring as DIYs are wont to experiment and try for themselves. In such a simple design, it's inevitable that someone will find a limit to how far it can be pushed before stability is stupidly compromised. In that case, I wish them lots of toasted parts for their trouble!

The reason for using COG/NPO ceramics is not for thermal stability of compensation but for distortion reasons, which these types don't measurably incur whist other grades can be disastrous due to their non-zeroing behaviour with AC signals. John Curl has much to say on the subject as does D. Self.

I can't find a direct reference to the age of AN485 - It could well have been the inspiration of this and many other audio designs when the audio knowledge base was still dominated by vacuum tube technology. ESP P3 design goes back to about 1970 according to Rode and I think he should know.

BTW Ashok, I can't guarantee it, but most quality sources of ceramics <100pF are actually NPO. It's not hard to test with a little air heating in any case. You only find COG in SMD parts, AFAIK.
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Old 13th January 2013, 05:50 AM   #34
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Why would anybody toil with proper board layout when original boards are affordable at (AU) Dollars 26.-? And you get Rod's assistance as bonus.
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Old 13th January 2013, 06:08 AM   #35
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Local shop has 100pF NP0 ceramic and WIMA polypropilene film/foil 100pF. Which one is better for P3A?
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Old 13th January 2013, 10:58 AM   #36
jaycee is offline jaycee  United Kingdom
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Some people see DIY as DIY... and that using a PCB means you have only assembled a kit... so they would rather get the experience of making their own layout. Also, some people are unable to buy the PCB's due to import restrictions, taxes, etc.

Polypropylene are the better parts
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Old 13th January 2013, 02:44 PM   #37
effebi is offline effebi  Italy
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Hi Sakis,
You have a good memory about metal cans!
but it was not that implementation.
It was the rather standard one that I mounted the Rod's board
Tecnics SU-Z1 to refurbish.
To stay on the DIY side I had used my mix of BJTs, in particular a Sanyo 2sA1209 as a VAS ...
Thanks also the valuable Sakis help with the Technics schematics, that build was quite a success at the end and still today it plays on a classic Dance Studio like a mini-PA system every day!
BTW, for the Cdom I used a 500V 100pf mica and the board HAS the 100 uF filters installed as standard (PCB is a"second version").

Last edited by effebi; 13th January 2013 at 02:48 PM.
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Old 13th January 2013, 04:17 PM   #38
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Quote:
Originally Posted by ashok View Post
What is the temp coeff for 2KV low pF caps ( blue in colour) ?
Guessing at the brand but since those blue 2kV are cheap and everywhere, they are Chinese and marketed in the US as XICON.
They are branded variously elsewhere. I think then, they are likely Y5P coefficient but will be unsuitable for the comp. cap by their
formulation anyway.

Is this appearance similar?http://www.mouser.com/catalog/specsheets/XC-600138.pdf
or perhaps TDK's more expensive one http://www.mouser.com/ds/2/400/e413_ck45-6483.pdf
These have a -20/+50% temperature characteristic and I think that also implies a no-go
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