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Old 5th October 2005, 06:22 PM   #1
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Question FirstWatt F1 Gathering materials

Guys,

I've decided to try building the FirstWatt F1. I've put together a BOM based on the schematics in the service manual. This is my first attempt to build an amplifier so I had a few questions which may seem rather naive to the experts

PowerSupply capacitors : Do I need to buy any particular brand/quality what should I look for?

Resistor/Capacitors in the input path (r1-r4, c1,c2) I'm assuming that you need to put high quality resistors capacitors here? Again any recommendations on brand/quality/source or what should I be looking for?

MOSFETS: I've been reading some of the posts here about the importance of matching FETs. How many do I need to purchase in order to be able to get a matched set? for example to match the 2 IRFP240s do I need to buy 4, 6, 8?

Thanks in advance,

--Chris
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Old 5th October 2005, 08:16 PM   #2
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I used the Panasonics from Digikey.

The resistors are RN55D type 1% except for the 3 watt
resistors, which are also Panasonic from Digikey.

I matched the devices to .01V, and the two P channel are
matched to the two N channel. If you match to .1V for Vgs
you can trim the Sources resistors to get the offset down to
the 50 mV spec without much difficulty. If necessary, I usually
trim the P channel Source resistors, as that has little effect on
the gain figure.

I would probably buy 2X to ensure matching, and the single
current source N channel can be the odd device out of match.

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Old 5th October 2005, 10:04 PM   #3
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Crack open a Mouser and Digikey catalog stud.
The resistors Nelson uses are in there, they're Vishays.
Good signal caps are WIMA.
Get the stuff you can from Digikey cause there's no sales tax.
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Old 6th October 2005, 06:59 AM   #4
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Mr. Pass,

Thank you for responding so quickly. I've went ahead and put together a 95% BOM for the project. I have a couple more naive questions

The PS Toroid : will a 250VA Avel 18V+18V work? I believe on another F1 thread the person was using a 300VA toroid.

The Line capacitor in the PS: The schematic shows ".0033 LINE", I'm assuming its 0.0033 uF, so a BC Components 0.0033UF 275VAC X2 should work?

C3 and C4 : Are there any particular ratings required for these capacitors?

I'm planning to use a 3RU rack mount chassis with heatsinks on both sides and 1/2 of the back panel. I should be able to get aproximetly 5.25"x40" of area for heatsinks.

I have a complete BOM except for the mentioned capacitors and heatsinks, with your permision I'd like to post it here for anyone else interested in the project.

Thanks again for all the help

--Chris
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Old 6th October 2005, 07:10 AM   #5
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DIY_newbie

What do you expect the total cost to be?

Thanks

Richard
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Old 6th October 2005, 12:57 PM   #6
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DIY_newbie,

>> The Line capacitor in the PS: The schematic shows ".0033 LINE", I'm assuming
>> its 0.0033 uF, so a BC Components 0.0033UF 275VAC X2 should work?

A 3.3 nanoF at 400 V AC {Style Orange drop} check for the noise level
and the position of the transfo ...

>> C3 .. C4 : Are there any particular ratings required for these capacitors?

For these 15 000 microF about 35 volts service, 85 or 105 degreeC ; 3000 hour
mini ; computer type with low ESR ; good panasonic's TSUP or TSHA series
Nichicon VR or PW series

>> The PS Toroid : will be a 250VA Avel 18V+18V work?
Attention the F1 draws 100 watts by channel and with a coeff of 2/3
that goes to 250 to 350 VA for a stereo amplifier...
Look for a "audio type" for better quality ...

Good luck.
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Old 6th October 2005, 01:53 PM   #7
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Quote:
I matched the devices to .01V, and the two P channel are matched to N channel.
Mr. Pass

At what current are you matching them, and how do you match P channel pair to N channel pair?

Bartek
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Old 6th October 2005, 02:05 PM   #8
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Quote:
Originally posted by Alain Dupont
[B]DIY_newbie,

>> The Line capacitor in the PS: The schematic shows ".0033 LINE", I'm assuming
>> its 0.0033 uF, so a BC Components 0.0033UF 275VAC X2 should work?

A 3.3 nanoF at 400 V AC {Style Orange drop} check for the noise level
and the position of the transfo ...
I'll look into these.

Quote:
Originally posted by Alain Dupont
>> C3 .. C4 : Are there any particular ratings required for these capacitors?

For these 15 000 microF about 35 volts service, 85 or 105 degreeC ; 3000 hour
mini ; computer type with low ESR ; good panasonic's TSUP or TSHA series
Nichicon VR or PW series
I apologize for not being clear, I meant C4 and C5 on the channel schematic. The 220uf and 1000 uf capacitors near the current sources.

Quote:
Originally posted by Alain Dupont
>> The PS Toroid : will be a 250VA Avel 18V+18V work?
Attention the F1 draws 100 watts by channel and with a coeff of 2/3
that goes to 250 to 350 VA for a stereo amplifier...
Look for a "audio type" for better quality ...

Good luck.
I just looked up the specs on the Avel here :
http://www.partsexpress.com/pdf/avelspecs.pdf

The specifications say that the 250VA is 93.2% effecient. Doesn't this imply that I should be able to pull (250VA / 18V) * 93.2% = 12.94 Amps? How do I convert this to a Wattage figure? Isn't this simply 18V*12.95A = 233 Watts? 15% overhead?

--Chris
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Old 6th October 2005, 02:46 PM   #9
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Newb,

Forget those fancy shmancy calcs for the tansformer. They don't
apply to nuthin you're doing. You're pulling huge amounts of
current for tiny bits of the 60 Hz cycle. The 93% efficient means that at rated
power, the transformer is heating itself using 7% of the power
it's pulling at the primary, and 93% of the power being pulled at
the primary is making it to the secondary. So your calc is correct.
BUT, like I said, it doesn't apply to your situation, that calc is for
a RESISTIVE load, and your load is about as far from resistive as it could possibly be.

Our man Nelson has
a rule of thumb to provide a VA rating that is 4x the idle power.
So if the idle power of your F1 is for example 100W, you should
use a 400 VA if they come like that, or a 500VA. Forgive me
Master if I've messed up. If it's a question of 250 VA or 300 VA
go with the 300. It's gonna hum less, and it'll give more oomph to the supply. When in doubt, go bigger on the supply as long as it will fit in your box.

I used Nichicon Super Through (type KG), 22,000 uF, 2 per rail in my Aleph 2s. I'm sure you could be happy with one of those per rail for a small amp like an F1. Do you need 2 per amp or one per amp ? They're rare and you may have trouble getting them.

Also, on the FET matching, you need 2 pairs, no ? If so you'll get a better result if you match out 16 transistors rather than 8. But 8 might be OK. What are they, like $1 each or less ???

W.
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Old 6th October 2005, 02:47 PM   #10
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DIY_newbie,

>> I meant C4 and C5 on the channel schematic. The 220uf and 1000 uf
>> capacitors near the current sources.

35 volts good quality Panasonic M series or Nichicon VR, PW series

>> Doesn't this imply that I should be able to pull (250VA / 18V) * 93.2% =
>> 12.94 Amps?

max is 6.94 amps for each secondary winding
But you are working on a class A ; and in this case we do not use full capacity of
the transfo ; we use a 2 or 3 coeff otherwise the heat noise, saturation etc will be present and so it's not a good idea to load the transfo more than 50 %

so for 50% load or even better 33 % load ... look at the Aleph-X excel datasheet
you will be able to draw 3.47 Amp from each secondary
Thats 3.47 Amp * 24 Volts = 83 VA * 2 channel = 166 VA max for 50 %
and these 166 VA are considered on full resistive load

The 6% lost in efficiency is somehow negligible here...
it's lost in magnetic filed, heat, copper loss, coupling between primary and secondary

PS: look at other examples of Nelson's design, Aleph series... for suggested VA

regards.

Alain.
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