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Duck-Twacy 14th May 2003 10:10 AM

Resistors & doubts ;(

For the resistors (except the 0.47 and 1 ohm) of my Aleph5 I bought 0.4 watt BCC metalfilm resistors.

But they are so small (less then 4 mm)!! Will they stand the heat??? I probably will want to increase the bias a bit.


(clickable image)
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My other question. For the 0.47 and 1 ohm power resistors I have these Vishay RTO-20 resistors. But the are 5%. Will this impact the sound of the amp in a negative way?????

I first ordered Caddock MP816 (1%) but the shop couldn't deliver them (had to buy 50 pieces). The exchange between the Caddocks an Vishays was done by the shop themself, only sending me a small mail (not even stating the type of Vishays). Although I like the owner of the shop, I'm not really happy with this. Of course the difference in price was refunded (and put into other parts).

I just wish I had discovered Welborn labs sooner, I would have bought everything there.

Since I was planning to make my Aleph very good, this worries me a bit.
I have invested a lot af money in all the other parts so far (more than a 1000 euro)
For instance 4 of these babies
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(I don't mean Rickie :) )

Hope you can tell me if my worries are justified of if its only in the mind ;)


Mad_K 14th May 2003 10:33 AM

The 0,4W is good for the circuit, even with increased bias.

For the source resistors, 5% is not good :eek:
Not to worry though. What you need to do is to match the 5% the same way as you matched the output mosfets.
-> Send a known amount of current (0,5A-1A or thereabout), and note the voltage drop across the resistor. This I would do even with 1% resistors.. It is essential that the resistors match as much as possible, so that the outputfets share the load equally. This has more to do with reliability than sonics.

-You can easily get 1% tolerance from 5% parts this way;)

Duck-Twacy 14th May 2003 10:45 AM

I'm afraid I didn't match the mosfets myself :goodbad:

Can't I just measure the Vishays with my Fluke 111 multimeter?

Oeps checked the specs. resolution 0.1 ohm, so guess not :bawling:

Mad_K 14th May 2003 10:58 AM

This is not difficult!

I use an old psu (wall-wart or what you call it) from my pc-sub (2A, 15VDC) and two 47R 17W resistors in paralell. Then you put your mosfet or resistor in series and measure VGS or V (=U) across the part. I take the measurment after 5 seconds. Measure the voltage across the two 47R, and you know the current, I. Then it's a simple R=U/I for your source resistors;)

Duck-Twacy 14th May 2003 11:15 AM

Still If I decided to replace them with the 1% Caddocks MP816, would it be sufficient to only replace the twelve 1 ohm resistors and leave the eight 0.47 ohms 5% Vishays, or are these just as important?

Mad_K 14th May 2003 11:49 AM

You could use the 5% for the 0,47R, and replace only the 1R with 1%. This will be good enough:)

Duck-Twacy 14th May 2003 07:36 PM

So if I first try to match the 1 ohm resistors, how do I group them? In 2 sets of 6 resistors for left and right channel, or maybe also within each channel two sets of 3??
What if one channel has about 0.95 ohm and the other channel around 1.05 ohm??

Nelson Pass 14th May 2003 09:50 PM

I wouldn't worry about the 5% on the Vishays, even if they
were really 5%. More likely they are 1% or better.

As to the .4 watt BCCs, they look awfully tiny....


Duck-Twacy 15th May 2003 07:06 AM

Thank you mr Pass. Still, I will try to measure them.

As for the little BCC ants ;) . Try reading the colour code on these thingies (I have -5 lenses! :bigeyes: ).

But now I'm worried again. What is the worst that can happen if one of the liitle resistors on the PCB gets fried? :flame:
Can that result in a unduly death of my mosfets?? :scratch: :dunno: :dead:

Duck-Twacy 15th May 2003 07:41 AM

Edit. Btw according to the datasheet (see PDF abopve) the 0.4 watt is for 40 degrees celcius ambiant temperature. At 60 degrees C (more typical for an Aleph) it is only 0.3 watt. Is this bad?
(My PCBs will be attached perdendicular to the heatsinks, at 2 or 3 cm distance minimum, so for most resistors the distance wil be greater)

(couldn't edit the above message anymore)

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