Comparison of JFETs for mic applications

[MicUlli]

???
"J-Fet as noise source in Mic's is tertiary or lower order"
"Putting a lower noise J-Fet into the circuit usually makes things worse"
???
Contradiction in one post

 

[ricardo]

I go into some detail on 'electronic' noise of a LDC mike in Zephyr.doc You have to join.

You can clearly see the effect of EVIL resistors even in a Schoeps type circuit.

I need to pontificate at length on da various aspects of Condensor Microphone noise and especially the audible significance & annoyance of each noise type.

But don't hold your breathe for dis beach bum to tear himself away from da Great Barrier Reef

 

[jp8]

I would be interested to know of any other manufacturers, it seems NEC (Renesas) and OnSemi are out of that business. I would not be surprised if all of the traditional Japanese, American, and European device vendors have completely ceded that market to Chinese vendors now.

Toshiba still has a nice variety of very usable JFETs. Not only the well known 2SK209, but also the 2SK879 and 2SK880 (see my post #128), which Toshiba promotes for use in condenser mics and audio circuits. See attached datasheets. Active parts and available from Mouser and Digikey.

Jan

 

[Mr. Moscode]

1f=1femtoFarad or 10E–15 Farad

Thanks. That's in the territory of stray capacitance. FWIW a femtofarad is abbreviated or written as fF when I looked it up.

 

[ccaudle]

Toshiba still has a nice variety of very usable JFETs

I did not see any indication that the Toshiba devices include an on-die low leakage diode for biasing like 2sk660, 2sk596, etc.

 

[ccaudle]

a femtofarad is abbreviated or written as fF

SPICE schematics just use the metric prefix, the units are implicit to the device type.

 

[k brown]

I go into some detail on 'electronic' noise of a LDC mike in Zephyr.doc You have to join.

You can clearly see the effect of EVIL resistors even in a Schoeps type circuit.

I need to pontificate at length on da various aspects of Condensor Microphone noise and especially the audible significance & annoyance of each noise type.

But don't hold your breathe for dis beach bum to tear himself away from da Great Barrier Reef

Why do you hold so much info hostage to people having to join MicBuilders?

 

[abbey road d enfer]

Thanks. That's in the territory of stray capacitance. FWIW a femtofarad is abbreviated or written as fF when I looked it up.

In Spice simulation software, units are unnecessary. A resistor marked 100 is a 100 ohms resistor, a capacitor marked 1n is a 1nF capacitor, and so on, thus a capacitor marked 1f (note the lower-case f) is a 1 femtofarad capacitor.
Though it has (almost) no relevance to the circuit's operation, it's useful because it allows copying the circuit without needing to delete an element that is not actually in the circuit.
The very same reason for 1u resistors or zero-volt voltage sources that one may see in some schematics.
Schematics in simulation are noticeably different than standard schematics; there are generally no connectors, no switches, but there may be additional elements destined to provide measurement points or offering the capability of making them inactive.

 

[abbey road d enfer]

Why do you hold so much info hostage to people having to join MicBuilders?

I find it rather surprizing that you're not already a member there.
Most members there are rather tinkerers than scientists, with some rather stellar exceptions.
I contribute there quite often when I can offer help in pure electronics.

 

[k brown]

I find it rather surprizing that you're not already a member there.
Most members there are rather tinkerers than scientists, with some rather stellar exceptions.
I contribute there quite often when I can offer help in pure electronics.

Beside the point though - if he has something relevent to a thread, why not just post it, instead of "You have to join"?

 

[abbey road d enfer]

Probably worth to mention the concept of "equivalent input noise". Whatever circuit you use, EIN is the one and only figure that counts for noise performance.
If we want to isolate the JFET as primary noise source we can use a model of a source follower (see pic sfollower).
Input referred noise density is

nd = (1 + Cin/Ccapsule) * SQRT (8kT/(3 gm)

and describes EIN perfectly. gm is the transconductance at operating conditions.

There is no way to reduce nd whatever circuit you design (at equal JFET operating conditions)...

Noise formulae are to be taken with a large spoon of salt, since the actual perceived noise is obtained by convoluting with the audition curve.
A higher noise with its spectrum shifted towards frequencies lower than 20Hz may well result in smaller perceived noise than one with lower value but happening at higher fequencies.
That's the reason for the 1G resistor, which results in more noise than a 100k, but in conjunction with the capsule capacitance, shift the noise spectrum one decade below.

 

[abbey road d enfer]

Just a remark, "Charge Amp with EXTREMELY LOW loop gain".

That's what they are quite often.

Try bootstrapping the load, a cascode on the FET.

THEN we will have something that deserves the moniker "Charge Amp".

Each one his own lexicon.

And the conclusion that extra capacititance in the charge amp deteriorates gain and SNR similarly is correct, the degree of deterioration is directly inverse to loop gain.

A "charge amp" with minimal loop gain, will behave almost like the common source circuit, one with infinite loop gain will tolerate infinite parasitic capacitance and show no degradation in Gain or SNR.

I guess that's what my simulations showed. Seems we agree.

 

[abbey road d enfer]

Beside the point though - if he has something relevent to a thread, why not just post it, instead of "You have to join"?

When making his point involves several pages, I understand he's not willing to serve it on a plate, when it's just afew clicks away.
Server space is at a premium, why encumber it with data that are already in the web?
What's more, it's like offering someone to open a dictionary rather than explaining, it offers infinite possibilities to learn new things.

 

[k brown]

When making his point involves several pages, I understand he's not willing to serve it on a plate, when it's just afew clicks away.
Server space is at a premium, why encumber it with data that are already in the web?
What's more, it's like offering someone to open a dictionary rather than explaining, it offers infinite possibilities to learn new things.

Fair point; just seems like I see it often regarding rather short-explanation details that would take up little space in a post (not long tech dissertations) - just copy/paste the info from there to here.

 

[thor.zmt]

That's what they are quite often.

Then they are really a "mixed mode" amplifier, with some limited feedback though the feedback capacitor.

A charge amp in the literal sense needs very high loop gain.

Each one his own lexicon.

True, it's always what the actual outcome desired is. Presuming one actually understands what a desired outcome idles in the first place.

Instead falling into the logical phallacy of engaging in practice without a solid theory to support it.

I guess that's what my simulations showed..

Yes.

Seems we agree.

If you are trying to say that the effect of parasitic capacitance at the input node, parallel to the capsule, is reduced by the loop gain or feedback factor, then yes, we agree.

Hence the suggestion of shooting for 60dB loop gain / feedback factor, with all signal path capacitors enclosed in the feedback loop, to gain a dramatic improvement in terms of noise, distortion and resilience to component quality.

Seeing it is trivial to achieve in a single stage and we can use 2SK170/LSK170 gainfully (pun intended) into the bargain, what is not too like?

Thor

 

[thor.zmt]

???
"J-Fet as noise source in Mic's is tertiary or lower order"

It means the noise contributed by the J-Fet is small next to other noise sources.

"Putting a lower noise J-Fet into the circuit usually makes things worse"
???
Contradiction in one post

No. It states that in circuit, replacing a correctly selected J-Fet (say 2SK660) that appears to have high noise on paper, with a much lower noise alternative (say 2SK170) actually degrades noise and distortion performance of the circuit, because of the usually much higher capacitance in circuit.

This is BECAUSE the J-Fet self noise is of tertiary importance in actual complete microphone circuits.

QED in the various measurements posted.

Thor

 

[thor.zmt]

There was an article in EE Times IIRC showing actual test results in microphone circuits.

Found it:
https://www.edaboard.com/attachments/3-15-design-ideas-pdf.132665/

Thor

 

[ricardo]

Why do you hold so much info hostage to people having to join MicBuilders?

cos as Abbey says, when I've written several pages of detailed info, just pasting it here would upset my careful formating & diagrams

In this case, the audibility of various condensor mike noise sources, I should really pontificate at much greater length than the couple of pages in Zephyr.doc

The alternative is to just shut up ... or as a concession to your sensibilities, I'll refrain from saying "you have to join" so people can discover this fun fact for themselves.

If the info is of importance to them, people will join. If it's not, they won't.

I find MicBuilders a useful repository for stuff I've done careful work on .. certainly better than some of the zillion page threads here ... where searching for pearls of wisdom (even my own humble offerings), involves sloshing through a HUGE quagmire of noise ....

There are other gurus (and pre10 gurus like me ) who found it useful too .. including the late Guru Scott Wurcer who is sorely missed.

 

[ricardo]

On the topic of info behind pay/join walls ...

Anyone know what's happened to the AES library search engine? I can't even find my own poor contributions to that august body. Not being able to link to them is affecting my pre10 guru status.

In case it isn't obvious, dis beach bum is no longer an AES member

 

[abbey road d enfer]

Found it:
https://www.edaboard.com/attachments/3-15-design-ideas-pdf.132665/

Thor

I was aware of this circuit when it was first published.
It actually does what it claims, dispensing with coupling capacitors, but many other circuits do, too.
It also solves one (non) issue, zeroing offset between outputs. I qualify that as a non-issue because many circuits rely on reasonably matched resistors to derive power from phantom.
EDIT: OTOH, it does bootstrap Cgd, but not Cgs.
IMO it's good food for thought, but it has not been developped fully.
And it certainly does not lend itself to capacitive NFB (short for "Charge Amp with EXTREMELY LOW loop gain").