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Old 9th June 2006, 06:28 PM   #1
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Default building firstwatt f1 - questions on parts

hi,

i'm trying to build an F1 amp for my k1000s and have been studying the specs and coming up with a BOM.

I am a total newb at this though, and i have some questions about the parts:

1. some resistors are labeled as 1 3W. does this mean
1 ohm 3W rated? also, what kind of resistor should be
used, metal oxide, wirewound etc? does it matter?

2. There's a 20V zener diode. i could only find one
kind of these, is there any other rating aside from
20V that i need to look for? like resistance or current?

3. there's also two 5K potentiometers. in the picture,
these look like blue squares with a round button. what
kind of potentiometer is this? the ones i found look
like volume knobs, can this be used too? there's also
some cermet pots, are these ok to use? what ratings other than 5k do i look for?

4. there are two thermistors in the PSU specs (labeled
as TH and TH located beside C2 adn C3). would these
be the same spec as TH1 and TH2?

thank you very much guys!
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Old 9th June 2006, 07:02 PM   #2
Netlist is offline Netlist  Belgium
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1: 1ohm/3Watt is correct. From the picture I believe Nelson uses wirewound.
2: You can’t go wrong with 1.5Watt Zener.
3: http://be.farnell.com/jsp/endecaSear...U=108553&N=401
Has some more data, This would be a good pot to use.
4: Nelson made the CL60 famous; they will all be the same type.

/Hugo
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Old 9th June 2006, 08:31 PM   #3
gl is offline gl  United States
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Hi choariwap,

I have built a similar amp. There is a thread somewhere with details. Search for SOZ Transconductance. The resistors are Panasonic 3W metal oxide types. Any 3W resistors would do fine. The pots are 1/2W Bourns trimmers IIRC. At least that's what I used. Again this is not critical. And the zeners can be iN52xx or iN47xx series parts.

Cheers,
Graeme
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Old 9th June 2006, 11:00 PM   #4
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Quote:
Originally posted by gl
The resistors are Panasonic 3W metal oxide types.
Digikey: P1W-3BK-ND
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BOXES for crying out loud
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Old 10th June 2006, 07:44 AM   #5
Netlist is offline Netlist  Belgium
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It must be me but I can't find the Digikey part#. At least not with the Parts Search engine.

/Hugo
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Old 10th June 2006, 08:21 AM   #6
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Sorry Hugo,

i punched the number from memory. It should be P1.0W-3BK-ND
(i've got a photo memory for numbers, doesn't mean i'm accurate)
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Old 10th June 2006, 09:03 AM   #7
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Hi Choaiwap,

I think you got all answers.
I just want to add some mud.

I have tried both metal oxide and wirewound resistors,
and found no real difference in sound while I prefer
wirewound power resistors.

I understand the protective 20V zener is there to limit maximum
Gate-to-Source voltage to 20V, and in normal operation, no
current flow through it. In my opinion, the probability of Vgs
exceeding 20V is very low. I just use small size (0.5W) two 9.1V
zeners in series. So far, so good.

Any NTC type of 10R/5A or similar is acceptable themistor.

F1 is the amplifier we should have. Enjoy!!!
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Old 11th June 2006, 05:52 AM   #8
awpagan is offline awpagan  Australia
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Quote:
Originally posted by jacco vermeulen
Sorry Hugo,

i punched the number from memory. It should be P1.0W-3BK-ND
(i've got a photo memory for numbers, doesn't mean i'm accurate)

I have a photo memory for numbers too

remember the numbers exactly but not necessary in the right order

play's havoc with my maths sometimes


allan
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Old 11th June 2006, 06:37 AM   #9
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thanks for all the replies that takes care of the components.

now, i have somemore questions

1. what heatsink specs should i look for? i've been shopping on rs and farnell and the most reasonable ones i found are 300x300x40. what measurements would be enough for the f1?

2. on thermal pads: i have been tinkering with PC parts before and the general consensus for pc parts is that thermal grease is better than pads (something like arctic silever5). but in audio circles silpads are recommended over thermal grease. why is this so? i would imagine that conductivity issues would also be critical for sensistive PC parts like the processor.

thanks to everyone again!
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Old 11th June 2006, 07:38 AM   #10
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You can calculate the heat sink pretty accurate by multiplying the thermal resistance with the power to dissipate. E.g.: the mentioned heat sink has a thermal resistance of 0.28°C/W, the power to dissipate is 100W per channel. 0.28 * 100 = 28. Add the ambient temperature to that number and you’re done.

If you can get your hands on good, (read thin) mica, you might try them.
Silpad is easier to work with, as you don’t need the grease.
Have a look at the different datasheets provide by Farnell. Search for SIL-PAD and KAPTON.

/Hugo
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