Lads, we're in deep now. Below is the list I've assembled for the full mod suite. This is no pussywillow hack; we're going to strip the mic down to the PCBs, take off a load of components, stuff the naked boards with new bits, clean it up, then put it back together. Then hopefully we can use it!
This is a work in progress; the BOM and process below are still unconfirmed until I've completed a full mod myself using them, so these will be subject to edits and changes. Please don't hesitate to contribute! Most component values are drawn from Jim Williams' comments in other threads, where he generously shares info on his mods, as does Jim J. Additional info comes from the Dutch page linked to earlier in the thread, which also provided the SP-1 schematic. Thank you to Jim Williams, Jim Jacobson, and nvrecording.dl!
There's still a puzzle piece or two to fit in here. Besides simply replacing the original components, we can get better results if we do some fiddling with the electrical balance of the circuit.
Remember the OP? (Maybe he'll pop back in someday…)
Look at 3 & 4 on Cooney's original mod list;
"3) Replace the 2 1n4148 diodes with UF4001's
4) Replace the 6.2v zener diode with either a 7.5v or 8.2v zener diode- check DC converter voltage at the UF4001's and try to achieve ~60vs" Cooney, OP
And then digest this acorn from Jim W.;
"…I up the polarization voltage from 40 to 57 volts which increases output by around 5 db..." Jim Williams.
Ok, we're trying to get more current to the mic capsule here. Why? This will increase the output, according to Jim. And here's another clue; look at the old Schoeps schematic… see the two pots in the circuit? (P1, P2). The symbol is like a rectangular resistor with a diagonal line through it. There's one pot by what could be an output transistor, and another by the resistor off the FET, R1
. WTF! Why do we need a pot there? It's to change the way the current flows! The pot is just a variable resistor, right? So we can change the values of resistance at those two points in the circuit, changing the way the circuit behaves, and balancing the response of the mic. Holy shit!
Now, this feature doesn't seem to have been incorporated in the modern mic designs that copy this circuit. Our Dutch connection suggests finding the ideal balance point using a pot, measuring the value, then replacing the pot with a fixed resistor, or set of resistors of exact equal value. The process is described here: http://www.nvrecording.nl/nhc/AudioT...ncemike_en.htm
Mod-masters like Jim seem to be using fixed resistor values which they have tested and found to work on this circuit. Changing the zener diodes is probably a part of that as well. I'm still in the process of working out that element of this mod. Anyone with more knowledge, please share! I think it's more valuable to understand why we are making changes, rather than blindly following a list.
Remember, this mod, and the BOM, have not been tested; I'm documenting this so others can learn from my mistakes, as I make them.
Now, let's mod… SP-1 Slutz Mod BOM: BETA
THIS LIST IS UNCONFIRMED
Confirmed items will have an asterisk added *
Mouser / Digikey links pending.
PLEASE CONTRIBUTE! ... and point out my errors...
>1 x polystyrene or polypropylene .001uf --- use MIT MultiCap
>1 x 100 uf Panasonic FR cap
>2 x 330 uf Panasonic FR caps
Electrolytic Capacitors: --- use Nichicon or Panny
>2 x 330n cap
>4 x 22n cap
>2 x 1n cap
FET / JFET:
>2 x JFET bipolar transistors Hitachi 2sa1083's, matched
>1 x FET fast J305 Siliconix
>7.5v or 8.2v zener diode
>2 x UF4001 diodes
>2 x 1 gig ohm resistors --Mini Mox 200 metal oxides
Resistors, Vishay/ Dale CMF55:
>2 x 2k2
>2 x 150k
>2 x 150 ohm matched pair
>1k5 SP-1 Slutz Mod, Process:
Replace capsule to FET cap:
>Replace C13 capsule to FET cap
with polystyrene or polypropylene .001uf --best- MIT MultiCap
Replace JFET bipolar transistors:
>Replace he bipolars T1,T2
with Hitachi 2sa1083's, matched pair
Replace the FET:
>Replace T3, the 2sk170 FET
with a J305 from Siliconix
, R9 1 gig ohm
with Mini Mox 200 metal oxides
>All other resistors to be Dale CMF55's
>R1, R2 replacements MUST be matched pair 150 ohm .5%
Replace ceramic caps with nice film caps:
>Change 47 uf 50v cap to a Panasonic FR series, 100 uf.
>Change 220 uf caps to 330 uf Panasonic FR's.
>The ouput caps (C3 and C4
? Or is it C1
& C2?) are enlarged to allow 20 hz to pass and are bypassed with more MIT caps.
(From the Dutch Mod) - Replace C3, C4
each by, or connect 4,7u/63V electrolytic (NO tantalum) capacitors in parallel (the + wires connected to the bases of T1 en T2). NEED CLARIFICATION ON ABOVE ITEM
Replace electrolytic caps:
>Replace with Panny FM electrolytics or Nichicons
>Change the 2 1n4148 diodes to UF4001's
>Change the 6.2v zener diode to either a 7.5v or 8.2v zener diode
>Check DC converter voltage at the UF4001's, look for ~60vs"
Ok, still a lot of shit to figure out here... the current balancing process, and which output caps to replace. There are also a few items I haven't put on the mod list process, which I've seen mentioned in other threads, such as removing certain resistors, and other things.
I'm assembling all the above components, so I can document the process of the Slutz Mod as I perform it. I mostly buy from a local supplier, but I may have to order certain bits they don't stock; I'll add purchase links as I find them, or as others contribute.