DOA's w/ LED's!? | What-2-Build Next..

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Jul 3, 2012
stumbled over these pics on accident when i dug up an Post from '12 when i was trying to build a clone of the Sontec Eq. 


So there are (illuminated) LED's on these DOA's... now i just had my 1st expiernce w/ DOA's this month w/BA512's in the Hairball Lola kit i just finished & the (2) two DIY-990's i have extra to use at will ;D

My question is what DOA's are thee that have LED's on them, when i was going through my reasrch on discrete op-amps i did not come across ANY designs that had LED's ...and to be honest (YES! i like them STRICTLY for their eye-candy! I ADMIT IT OK!! lolz;)

I have (2) two DIY-990's from hairball left over, i got (2) BA512's and (2) DIY-990's to see which i like better in my Lola, i have the BA512's in there now but have not been able to test my Lola yet as it is a 500-series module and i have to finish the enclosure & supplies for it (as i built my own & did not purchase a 500-series enclosure or lunchbox as the Lola is most likely the only 500-series i will own for the next 6-months). 

If i were to buy another 500-series kit... it would be the MISSING-LINK-II for sure, that puppy i have read some gOoOood things about ;)

any comments, advice, ideas, Pour-it-on Baby!! :")


Pics of my builds mentioned about:
Hairball Audio Lola, 1176 Rev-A:

...the REDD47 is next ;)
using Mullard ef86 + HA100x ;D

...hope to hear some good ideas from you guys on what to use my extra DOA's for :")  also any info on the magical Christmas-tree DOA's & where to find them :chuckles:



Well-known member
Staff member
Nov 30, 2006
Hickory, MS
The LED is probably used as a roughly 2V voltage reference for an internal current source and a little eye candy.  I did something similar inside a delay line kit back in the 80s (to set bias V for a BBD).  A little odd to have an LED inside a closed chassis but I did it mostly to amuse myself.  ;D

I remember when LEDs first came out and they were very expensive novelties. By the '80s they were cheap enough to use in odd applications just for chuckles.



New member
May 27, 2014
There is a schematic in this thread as an attachment in reply #14

No values but FET and BJT parts are cited, BD139/140 output transistors.
The pictures here look good enough to read resistor bands for working out most of those.
The examples pictured in the OP look like the input FETs are an obsolete dual-matched package, rather than the 2sk170s (also out of production) from the schematic.
Also, there are two individual Rs tied together in place of a trimmer in the example pics. That might show as a single R in the schematic.

I've seen pictures of several slightly different Igor layouts for this amp or closely related variations. Maybe due to component availability? Not sure if sold as part of kit or just showed up in support thread pictures of Igor's builds.

They all seem to be FET inputs cascoded with BJTs, generally with a (dc balance?) trimmer and pairs of LEDs and caps near each rail. Should be possile to work out given the available schematic and some math.

I guess the BJTs aren't quoted but iirc, they are Pn4249 and Pn2484. The 4249 is $0.40 part 60v/40MHz but the 2484 is hard to find, I think. There is a 2n2484 available in a TO can for ca. $1+.
I would guess there are a number of suitable NPN/PNPs substitutes.

On closer inspection, this OP pics have +2 transistors compared to the schematic.
Also looks like the wrong number of diodes at the output but hard to tell.
There are 2 transistors coupled to the OTs, I'm guessing those are the extra.
Darlington paired with the BD outputs, maybe?
The two wrapped pairs of BJTs maybe the input cascodes and current mirror as shown in the schematic.
The one wrapped to the dual input FET package must be NPN on the FET sources.
I am also guessing R5/R6, explicit in the schematic are the trimmer footprint on the PCB.
R1/R2 must be the yellow banded pair - maybe R = 499R/1%?



Well-known member
Jan 30, 2010
Maine USA
A '990 has two diodes to bias-up the current-sources.

Replace the diodes with an LED. Re-figure the current sources' emitter resistors 2.3X bigger to account for the difference of voltage (some understanding and math required). The diode bias may be small to get LED eye-candy, decrease the diode bias resistor to taste/sight, then re-compute those emitter resistors for the new voltage.

You will lose 2V-3V maximum output due to excess drop in the Vas current source.


Jul 3, 2012
thank PRR, probably not the smartest change when performance comes before candy;) but thank-you I'm pretty sure i will try it out though on my next set of DOA's hehe, any advice on my next set of DOA's?  i'll do a search now, but if anyone wants to blurt out some models, I'm pen ears  :")

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