[sdiy] LED Drops

[mike ruberto]

Hey Roy,

I went digging in my shed and found an old backplane pcb with some T1
red LEDs but thanks for the offer.

I knew I'd seen the JLM Audio circuit somewhere before.. it's from the
LA_4A Leveler.
http://www.waltzingbear.com/Schematics/Urei/LA_4A.htm

Regarding the circuit itself, doesn't the opamp provide the proper
bias to overcome the emitter resistors when some feedback is applied
to the circuit?

Mike

On Dec 9, 2007 5:21 PM, Roy J. Tellason <rtellason at verizon.net> wrote:
> On Sunday 09 December 2007 16:02, mike ruberto wrote:
> > Ok this is cool, the catalog has alot of good info.
> >
> > LOL "WE PUT THE "DIE" BACK IN DIODE" That website is a trip!
>
> I didn't run across that bit,  but the online rendition of the printed catalog
> is really quite impressive!  Or was,  until my browser crashed for some
> reason.
>
> > I want to build this circuit...
> > http://www.jlmaudio.com/JLM%20Hybrid%20Opamp%20circuit.pdf
> >
> > but all I have is a green LED.
>
> Oh my.  In that particular case I really wouldn't substitute...
>
> > It seems the green have a higher voltage drop than the red.
>
> Yes,  somewhat.
>
> > So it looks like I'll have to play around with resistors or a combo of
> > regular diodes.
>
> I acquired a whole bunch of (mostly red) LEDs by buying a couple of assortment
> packs.  Years ago now, and I haven't come anywhere near using them all up.  I
> also take LEDs out of stuff being scrapped,  and snagged a bunch from a guy
> who was disposing of a lot of old computer cases,  which is handy in that
> they have the leads with connectors attached.  Surely you can pick some up
> somewhere...?  I don't suppose it would be that big of a deal to drop one or
> two into an envelope?
>
> Most similar circuits I've seen had _three_ diodes in there,  not two.  Two of
> them just compensate for the base-emitter drop of the transistors,  and that
> leaves the drop across the emitter resistors to overcome -- if you don't have
> a bit more bias than two diodes would give you,  you'll end up with somewhat
> more crossover distortion than what you probably want.
>
> --
> Member of the toughest, meanest, deadliest, most unrelenting -- and
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> -
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