2SK660 in ext polarized mic?

[k brown]

Pardon my ignorance, but the 2SK660 is specified for use in electret mics - is there something about it that would make it not suitable with an externally polarized capsule; input cpacitance, or...?

 

[Lampie519]

It would be indeed strange if there was as it has been marketed for that specific purpose.
My guess is that it is for PC ECM's. So a cheap solution...

 

[Khron]

Right now i can't recall if it's the 2SK660 or the 2SK596 (both having the dual diodes between gate and source), but my Shure KSM27 uses one of those, and it's not an electret.

 

[k brown]

It would be indeed strange if there was as it has been marketed for that specific purpose.
My guess is that it is for PC ECM's. So a cheap solution...

The data sheet specifies 'ECM' (Electret Condenser Microphone); hence the question why they would specify electret, not just 'condenser microphone'.

 

[k brown]

Right now i can't recall if it's the 2SK660 or the 2SK596 (both having the dual diodes between gate and source), but my Shure KSM27 uses one of those, and it's not an electret.

Thanks.

 

[Khron]

Yep, it's the 2SK660 in the KSM27:

https://groupdiy.com/threads/shure-ksm27-ksm141-circuitry.61079/

 

[kingkorg]

Pardon my ignorance, but the 2SK550 is specified for use in electret mics - is there something about it that would make it not suitable with an externally polarized capsule; input cpacitance, or...?

I wouldn't say specified, it says suitable for use in ecm. I'd interpret that as a marketing hint. No particular reason not to use it with externally polarized capsules, but large capsule externally polarized would cause the diodes to clip the signal at relatively low levels.

As electrets are usually small diameter, lower output capsules it could be the reason why they marketed it this way.

 

[k brown]

I wouldn't say specified, it says suitable for use in ecm. I'd interpret that as a marketing hint. No particular reason not to use it with externally polarized capsules, but large capsule externally polarized would cause the diodes to clip the signal at relatively low levels.

As electrets are usually small diameter, lower output capsules it could be the reason why they marketed it this way.

Interesting point I hadn't considered.

 

[analogguru]

..... is there something about it that would make it not suitable with an externally polarized capsule; input cpacitance, or...?

Not capacitance but maybe input resistance of only about 30MOhm...

 

[k brown]

Yep, it's the 2SK660 in the KSM27:

https://groupdiy.com/threads/shure-ksm27-ksm141-circuitry.61079/

Did you happen to note the value of the capsule-to-FET decoupling cap in the KSM27?

If @analogguru is correct, it should be around .033 uF?

 

[Eric B]

One point that was not noted above is that FETs designed for electret capsules are designed with particularly low input capacitance. Electret microphones are typically very small diameter and therefore have low capacitance. If a large FET with high input capacitance were to be used, a capacitor voltage divider is formed and the signal would be attenuated. That ruins the Signal to Noise Ratio.

 

[k brown]

One point that was not noted above is that FETs designed for electret capsules are designed with particularly low input capacitance. Electret microphones are typically very small diameter and therefore have low capacitance. If a large FET with high input capacitance were to be used, a capacitor voltage divider is formed and the signal would be attenuated. That ruins the Signal to Noise Ratio.

You're describing the opposite case from this inquiry, which is about using specifically the 2SK660 with externally polarized capsules (namely Oktava MK-012, Rode NT-45O, MXL 603 and Behringer B-5).

I've already built a few of these and they sound fine, but I may be missing the last octave or so by having only 820pf coupling caps (?).

 

[Eric B]

I interpreted the ensuing discussion as being about how and why FETs designed for electrets are different from other FETs which might be used in capacitor microphones. The other difference, which was discussed, is the presence of the biasing diodes. Depending on the circuit used, those could limit the amplitude of signal which could be passed. I'm sorry if my input to the discussion wasn't helpful for you.

 

[Khron]

Did you happen to note the value of the capsule-to-FET decoupling cap in the KSM27?

If @analogguru is correct, it should be around .033 uF?

There isn't one; the only thing between the front diaphragm wire and the JFET gate is a surface-mount ferrite bead.

The little capacitor next to it is "just" the pad capacitor, with the other end going to the switch.

 

[k brown]

There isn't one; the only thing between the front diaphragm wire and the JFET gate is a surface-mount ferrite bead.

The little capacitor next to it is "just" the pad capacitor, with the other end going to the switch.

Hmm - so the full polarizing voltage is going into the gate of the FET?

 

[Khron]

Hmm - so the full polarizing voltage is going into the gate of the FET?

No, why would it? It's going to the backplate. The capsule itself IS a capacitor, remember? And noone says it's mandatory to have any part of the capsule grounded or anything.

 

[k brown]

No, why would it? It's going to the backplate. The capsule itself IS a capacitor, remember? And noone says it's mandatory to have any part of the capsule grounded or anything.

Oh, I see - so, like Schoeps, this is not a grounded capsule.

So I still need to figure out if @analogguru is correct, and the impedance of the 2SK660 gate really is as low as 30M, the decoupling cap needs to be a lot bigger than 1000p, as all the capsules I'm using (post #12) are grounded.
https://groupdiy.com/threads/most-bang-for-buck-sdc-project-yet.81760/

 

[analogguru]

You don't need to figure out if I am correct, simply because I don't need to be correct.... but FIY here is the datasheet of a similar FET, the 2SK596.... maybe you want to consider if you want to have a look at the top of page 2 of the datasheet....

 

[k brown]

You don't need to figure out if I am correct, simply because I don't need to be correct.... but FIY here is the datasheet of a similar FET, the 2SK596.... maybe you want to consider if you want to have a look at the top of page 2 of the datasheet....

Yes, I was puzzled that the input resistance wasn't in the 2SK660's specs. How did you come up with the estimate of 30Meg? I really need to figure out what value of capacitor to use.

 

[analogguru]

If you don't want to believe datasheets and what can be labelled as common knowledge I can recommend you to do a search for "HP4070D", invest about $ 20,--, buy such an Multimeter and you will be able to measure resistances up to 2 GOhm.