ASM-1 parts question

[John Speth]

An alert for you "mistake prone" ASM1 builders like me.  The noise circuit has a pair of 22 ohm resistors that are part of an RC power supply filter for a high gain amplifier.  I remember that I installed the op-amp backwards which resulted in nice aromatic roasted resistors when I turned on the power.  Had these been much larger in ohms, they would have acted more like resistors than slow acting fuses.  Anyway, this underscores Tony Clark's analysis of power vs. resistor size.  I'm not saying that the 22 ohm resistors should be a higher wattage, only that they are a good candidate if for some reason the op-amp should ever fail (or jump into the board backwards [:)  ).

John Speth
johns at oei.com

-----Original Message-----
From:	Tony Clark [SMTP:clark at andrews.edu]
Sent:	Tuesday, September 23, 1997 8:51 AM
To:	Synth-DIY
Subject:	Re: ASM-1 parts question

> Gene Stopp recommends using only metal film precision resistors on the
> ASM-1 board, which makes perfect sense to me, but my question is this:  Are
> 1/8 W components acceptable?  It seems to me that because of their smaller
> size, they will be easier to fit on the board, but I don't want to make a
> mistake in accumulating parts, or CERTAINLY not in "stuffing" the circuit
> board.

   In most all circumstances, yes.  What you have to be careful of is 
making sure that the power rating is not exceeded.  A good way to do this 
is to use the formula (V^2/R = W).
   Using this formula, W = .125W (1/8W)
   We also can assume that the max voltage we can place across a given 
resistor is Vcc-(-Vcc).  Assume 30V for this.
   Then rewrite your formula (V^2/W = Rmin) or 30V^2/.125W = Rmin.
   Your smallest sized 1/8W resistor should then be 7.2Kohms.  Of course 
this is a very cautious approach.  Typically the maximum voltage across 
any given resistor will most likely be only 15V, so that would then give 
you a Rmin of 1.8Kohms.
   Consequently, 900ohms is the smallest 1/4W resistor that should be 
used at 15V!  However, although there are some values smaller than this 
(i.e. 220ohms or even 22ohms!) the voltages across them normally can't 
reach 15V.  But I'd definately not use 1/8W resistors in their place!  
Definately not anything less than 1Kohms.