Make sure you are looking at the correct graph -- AFAIK the
frequency responses are about equivalent in terms of flatness.
Maybe you are confusing the low frequency section with the whole
spectrum? The graphs to compare are on page 6 (lower graph) and on
page 4 (middle graph).
The "modded 3 wires" in the Simonelli document is AKA the "Jerry
Lee Marcel" mod, JLM mod, or "Marcel mod". The SP48
doc here mentions it as well, on page 7.
The main reason for the mod is to reduce sensitivity (i.e. to
handle loud sounds better.)
To modify a 2-wire capsule, you do the "linkwitz" mod to it --
exactly how this is done depends on the capsule you are using, but
if you google for that you should be able to dig up some info. Let
me know if not. However note that you can also use a 3-wire
capsule and you don't need to mod it.
In case you still want to build the JLM variant, here are my
notes, gathered from many wise people here (sorry my notes don't
credit authors here):
The third version introduces Jerry¡¯s 2k2 between the FET
source and the capsule case. To do this, a 2 wire capsule like
WM61a must be modified to 3 wire or a 3 wire capsule like Primo
EM182 used.
This does the following :
? Gain & Sensitivity reduced by 9.3dB ie 9.3dB current
feedback
? Max. output (2.7% THD) increased by 10.6dB
? Together this increases the max. spl by 20dB for 2.7% THD to
135dB spl .. a very good spec
It also
? increases the Output Z but as this is already probably 50k, it
is of no consequence
? increases the inherent electrical noise (assuming FET,
capsule, preamp are noiseless) by about 3dB (The other noise
contribution in this supa circuit is the 2k Input Z of the
preamp).
...
JLM mod increases max SPL about 21dB, lowers sensitivity, and
with primo capsule might increase noise by about 1dB.
...
2k2 & 120k as suggested by Jerry & tested by Stefano
is certainly a good starting point for their low THD mod.?
Adjust the 120k for Vds between 5-10V
...
Will just point out that though the 2k won't degrade the noise
of WM61a (tested), the Primo EM184/172s will probably see about
1dB more noise (theoretical).
"If you do the "Marcel mod", the 2k2 MUST be AT the capsule ..
even if R1 & C1 are at the XLR.? After modifying the capsule
to insert the 2k2, you treat the assembly (which is now a 3
terminal device) like a 2 terminal capsule by ignoring the
source connection."
-c
