Topic: SP-CMC-4U and other SP mics with new "low-sensitivity-mod" for very high SPL

[sjoe]

I would have to strongly disagree that the context of the applied method has no relevance..

I'm talking about the FET circuit itself. It doesn't matter that the FET happens to be inside a microphone.  The FET circuit design works the same way regardless of what it's attached to.

This mod had come about not because of my knowledge of the technique you describe but purely because of experimentation.

OK, but it's not magic or voodoo. The links I provided show exactly what is happening with the 4.7k mod.   The addition of that resistor configures the FET into a textbook example of a source-follower circuit, and the standard math and engineering principles that apply to that circuit also apply when the FET is inside a microphone.

There's lots of information out there about what the source-follower circuit does and how it works.  If you're not interested, that's fine but I've seen several technical people discussing this here and maybe one of them can make good use of it.  Consider it my contribution to the research.

none of the schematics you show actually show my method

Look at the very first diagram on the rason.org site. That's exactly what the 4.7k mod does, it would be R2 in that schematic.

If it helps you to visualize things, pretend that .1uf cap on the input is the microphone's diaphragm.  R1 and the 4.7uf output cap are the standard battery box components.

Joe

[Church-Audio]

I would have to strongly disagree that the context of the applied method has no relevance..

I'm talking about the FET circuit itself. It doesn't matter that the FET happens to be inside a microphone.  The FET circuit design works the same way regardless of what it's attached to.

This mod had come about not because of my knowledge of the technique you describe but purely because of experimentation.

OK, but it's not magic or voodoo. The links I provided show exactly what is happening with the 4.7k mod.   The addition of that resistor configures the FET into a textbook example of a source-follower circuit, and the standard math and engineering principles that apply to that circuit also apply when the FET is inside a microphone.

There's lots of information out there about what the source-follower circuit does and how it works.  If you're not interested, that's fine but I've seen several technical people discussing this here and maybe one of them can make good use of it.  Consider it my contribution to the research.

none of the schematics you show actually show my method

Look at the very first diagram on the rason.org site. That's exactly what the 4.7k mod does, it would be R2 in that schematic.

If it helps you to visualize things, pretend that .1uf cap on the input is the microphone's diaphragm.  R1 and the 4.7uf output cap are the standard battery box components.

Joe

I dont think I ever used the word "voodoo" to describe my mod actually just the opposite. Like I said before there is no such thing as a simple electronic circuit that has not been done before in one way or another. I simply was the first one to use it to convert 3 wire mics into high performance 2 wire mics. Many companies were selling these 3 wire mics with out doing very basic distortion measurements to see if the mics there were selling were actually in spec. I was one of the first to measure some of the more popular stealth mics and determined that they did overload when used in 2 wire and I was the first to find a solution to that problem. I did not want to put a product out that distorted. So I experimented and came up with my 4.7k mod.

[SparkE!]

What the 4.7k mod Chris does is electrically identical to the 2-wire Linkwitz mod, except that the drain and source connections are reversed.  The type of JFETs that are used in these mics are sufficiently symmetrical to allow to swap the source and drain connections (which is what the 2-wire Linkwitz mod does) and still get substantially the same performance.  So yes, Chris's mod produces substantially the same results as making the 2-wire Linkwitz mod.  It's just an easier way of accomplishing nearly the same thing.  Personally, I prefer the 3-wire Linkwitz mod because it uses the JFET in the polarity that its manufacturer specified, it attenuates the signal less than Chris's mod and I sometimes use the mics going straight to a NJB3 where I can't afford the additional loss in signal amplitude.

The thing that people aren't talking about here is that most battery boxes have their own pull-up resistor.  If you want to be compatible with a conventional battery box, then you can't use a 2 wire Linkwitz modded capsule because the battery would be in the wrong polarity.  And of course you can't use the 3 wire Linkwitz mod with a conventional battery box because a conventional battery box is wired for 2 wire connections, not 3.  That's where Chris's mod shines.  It uses a conventional battery box, it maintains the correct polarity of current flow through the transistor, but it incurs an additional 6 dB of signal attenuation with respect to the 3 wire Linkwitz mod.  To me, that's a small price to pay for compatibility with your conventional battery box.  Not only that, but if your conventional battery box uses a 4.7k pullup (and most do), then Chris's mod minimizes the harmonic distortion.  In fact, it should have harmonic distortion that is a little bit better than the 3 wire Linkwitz mod.  The price for the lower distortion is a very slight increase in noise floor.  (It's pretty common in electronic circuits to be able to trade noise performance for harmonic distortion or vice versa.)  In the case of loud source material like you have at most live music venues, the increase in noise floor is imperceptible and therefore, of no consequence.  For reference, here are the 2 and 3 wire Linkwitz mods as shown on the page referenced by Roving Sign previously in this thread:

[sjoe]

if your conventional battery box uses a 4.7k pullup (and most do), then Chris's mod minimizes the harmonic distortion.

I'm fairly certain the boxes sold by SP use 10K resistors.   (The one I bought many years ago did, at least.)  If that's the case, is 4.7k still the optimum value for the source resistor?

How about running the mics straight into an R-09 without a battery box?   I've done this so I know it works, but the same question applies.    Can we measure the open circuit voltage and short circuit current at the R-09 mic jack to determine the effective value of the internal pullup?   Or does the R-09 use some kind of current source circuit?   I don't have an R-09 service manual yet, so I can't tell.   If it turns out that the R-09 uses a 2.2k pullup (for example), what happens to the source resistor value?   Do we still want 4.7k there?

Last but not least, does anyone know the types or part numbers of the FETs used in these microphones?  It seems to me that the circuit design would be a lot better/easier if we knew the actual characteristics of the devices.

Joe

[SparkE!]

if your conventional battery box uses a 4.7k pullup (and most do), then Chris's mod minimizes the harmonic distortion.

Last but not least, does anyone know the types or part numbers of the FETs used in these microphones?  It seems to me that the circuit design would be a lot better/easier if we knew the actual characteristics of the devices.

Joe

For Panasonic WM 61 capsules, it's reputed to be 2SK3372 (a Panasonic part number - surprise, surprise)

As far as what is the best resistor, it varies by mic element, by bias voltage and by battery box resistor, but in most cases it's going to be 4.7k. Of the common values, Chris really did pick the right one.