Help me with some community service

[Joe Hart]

I've been convicted of molesting an amp with too much volume and distortion, so I have to do some community service...

I’m trying to throw together a page similar to the famous “Technology Of…” series. Can some of you smart folk check it out and let me know what I have wrong or what I could do to improve it? Anything from technical stuff to simply rewording something. Thanks.

www.rabbathrecordings.com/ebay/DOD250Test.htm

-Joe Hart

[Fret Wire]

Hi Joe, curious..why a dual OA? For more selection? Normally, this ckt uses pins 2,3,4 (gnd), 6,7 of a single IC.

R7 will probably work better (less cramped) with a rev log.

Nice writeup, BTW

[Joe Hart]

A dual op-amp for more selection. But I guess it could whatever (single, dual, quad).

And R7 should be a reverse log pot, but those are harder to come by, but you're right, I will add that in.

Thanks.
-Joe Hart

[jrc4558]

C4 blocks the DC current from hitting the output. Since the capacitor is known to block DC and let the AC through, that's what allows us to have audio output from our pedals. I think it's called AC coupling or smth.

[Peter Snowberg]

Very cool Joe!

A couple quick notes...

R1 does have an effect on input inpedance, but the whole value is set by R1+R4+((((R2+C2)/2)+R3)/2). To distill it, R4 is the main component setting the input impedance. I think R1 is more for RF filtering than anything else.

The bias is set by R2 & R3 with C2 to filter out noise in the bias supply.

C4 is a DC blocking cap that passes only AC on to the following parts. Without this cap the opamp would be trying to eat current flowing from the gorund connection. Constantin is right. This is a.k.a. "coupling cap"

R8 adds to the output impedance but it also forms an RC filter along with C5 to color the tone.

All in all.... GREAT JOB!

[vdm]

hey cool - i never knew that making the resistors in a voltage divider bigger saved battery power!! is this just a little bit or is it significant???

other than that - awesome work with the page! and there is nothing that sticks out to my that needs some blatant work

trent

[MartyMart]

Great job Joe  

For someone's "first" build it would be very helpfull........nice !

Marty.

[Ge_Whiz]

The 1N4148 diodes are 'silicon', not 'silicone' (silicones are plastics or oils). Minor point, but may as well get it right.

The 'amount of control' produced by a pot depends on whether it is used as a variable resistor (two lugs, or middle lug connected to end lug, two wires to pot) or a true 'potentiometer' aka 'voltage divider'.  In the latter case (as with R9 here), the signal level at the output, and incremental change as the pot is rotated, depends only on the rotation angle as it is set by the ratio of the two halves of the pot split by the rotor. The point is, although the pot value also has an effect on output impedance, a wide range of pot values could be substituted for R9 (at least 20k to 1M) with little effect on the result (C5 may need tweaking).  I applaud your encouragement of newbies not to sweat over exact component types and this is another case where leeway is possible.

It may also help newbies to include diagrams of (a) pinout details of a standard dual opamp; (b) what you mean by 'asymmetric diodes', and (c) how to wire the pots so that the controls work the right way.

Nice article and a good choice, well done.

vdm: Voltage divider resistors control a current flowing from battery +ve to ground, so they lead to a constant 'leak' from the battery. Using Ohm's law, the leakage current is inversely proportional to the total resistance, so yes, the effect is significant. However, the resistor values have to be small enough to allow sufficient current to flow to meet the demand of the part of the circuit connected to the divided voltage, so the values are a fairly careful compromise.

[strungout]

Oy.

I was gonna suggest pinout diagrams, but Ge_Whiz took care of that, and also some pics (or links to) capacitor/resistor types, and markings, either as you mention them, or as a "useful links" section at the end of your article. Sending people where you know the information is simple and easily understandable helps avoid them finding a page that isn't, and getting discouraged.

Great work!

Ciao.