Shure claims FET can drive 75' of cable (?)

[k brown]

The manuals for the SM90/91 and 809/819 (all use the same card and omni capsules) state that they come with 25' cables between the mic (which contains only the capsule with built-in FET), and the power/output module. It also says that the mic can drive an additional 50' of cable between the mic and the power module (which they oddly call a 'preamplifier', even though it provides no gain) with no reduction in quality.

Can a lowly little FET really drive 75' of cable?

 

[Khron]

Looks like that FET has 510 ohm resistors in its drain and source (in the "preamp" box), which seems to be low-enough impedance..?

 

[k brown]

But the 75' of cable would be between the FET and those 510 ohm resistors, so the FET connects to the cable directly.

 

[Khron]

... But the current flow would be "balanced", going into the drain and coming out the source. The ground connection is mainly for shielding, as i see it.

And what's the capacitance of 75ft of such cable then?

There's a couple of 1nF caps after those 510 ohm resistors in the preamp box, so i kinda doubt they considered they're lacking top end That works out roughly to the 47R / 22nF combo in the Schoeps circuit, doesn't it?

 

[abbey road d enfer]

With the long cable, the frequency response extends well enough beyond 20 kHz and, supposing the capsule delivers 30mV at 20kHz, which is enormous -that would be about 150dBspl - the AC current deliverd by the FET would be less than 30uA rms, which is well within the circuit's capacity of linear operation.

 

[k brown]

Hmm, very interesting - that also explains how some of Audio Technica's mics (ATM350 and choir mics) can have such long cables between mic heads and power modules. Those mic heads also have nothing in them but a FET (2SK660).

 

[Sonic]

Audio Technica's mics (ATM350 and choir mics) can have such long cables between mic heads and power modules. Those mic heads also have nothing in them but a FET

Hi! The Shure MX202 capsule also has a long cable to the XLR connector with a preamp.

 

[k brown]

I guess I was surprised to see no low-value resistors on the FET's outputs - wouldn't those isolate it from cable capacitance? But none of these mini-mics with long cables have them, so I guess not (noise issue?).

 

[abbey road d enfer]

The impedance of both nodes is governed by the 510 ohm resistors; No need for additional res.

 

[Khron]

And it's not like a measly JFET is (like) a massive-open-loop-gain GHz gain-bandwidth opamp that's on the brink of stability with a few pf on its output...

 

[k brown]

How capable would this 2SK170 circuit be driving 75' of cable (cable connected directly to FET(s); leaving out the DC blocking cap and it's associated resistors)?

Output Z at the FET source said to about 50 ohms.

 

[abbey road d enfer]

How capable would this 2SK170 circuit be driving 75' of cable (cable connected directly to FET(s); leaving out the DC blocking cap and it's associated resistors)?

I would retain a resistor there R184/R204) for stability. 1k is correct for driving 75' of cable. You may easily halve it.

Output Z at the FET source said to about 50 ohms.

That is consistent with Gm=22mS. Zout in source follower is 1/Gm.

 

[k brown]

Thanks -