FET opamp for condensor microphone?

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Svart

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Jun 4, 2004
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Instead of having a FET and some other bits in a condensor microphone, why don't we use a low noise FET input opamp and be done with it? Some of the new video opamps are superb.

Discuss.
 
no you are right in the technical aspect, who really needs fast slew, low power consumption and extrememly low noise without matching parts and such? Besides, who is actively designing a super low noise audio opamps these days? All the work is going into video and RF opamps, audio has gone by the wayside.

Sometimes GOOD ENOUGH just isn't good enough and sometimes I have strange ideas and would like to discuss them.

But something bugged me just now, I asked a question and hoped that forum members would discuss this and why it would or would not work, not offer up reasons why my question was "wrong" without discussing the actual theory.
 
Svart, couldn't you tell I was pullin' your chain? Loosen up and let that stick fall out, willya? :razz:

All I can say is that with great speed comes great responsibility. And I've worked with video and RF for years, so I'm not some hobbyist dilettante who has never worked with this stuff.

No developments in audio ICs, huh? I guess you don't get the trade rags. The semi houses are pushing new ones every month, it seems. Sure, the areas of improvement are mostly in terms of higher integration, lower voltage/power and smaller size... but it's not an accurate statement, to say that audio IC development has ceased.
 
> why don't we use a low noise FET input opamp and be done with it?

Done already. Some of the low-buck condenser mikes use a TL072 chip as head-amp.

Between the high electrical impedance, and acoustic noise, the TL072 may not be limiting the noise factor enough to fret about. My AKG 414 has total output noise of 1.25uV, TL072 has self-noise around 2uV: not a big difference. Maybe 4dB more noise for 28dB less dollars. You can fiddle diaphragm size and bias, select TL072, and shave the difference to nearly-nothing.

The alternative, a triode worked at fairly low current (so you don't burn your hand) has self-noise in the 1uV-3uV range.
 
And neither am I.

Sorry NYD, with all the attitudes floating around here these days it's hard to tell when someone is busting your nuts or actually letting their opinions unfairly guide their view of others.

Most newer high speed opamps are very stable at low gains. Many of them are designed for use with low voltage, single railed, low current supplies. This is exactly what we need for a microphone, something that can work in an environment of low current availabillity, with extremely low self noise off of some single rail.

I'm not talking color here, I am talking pure fidelity.
 
output noise

What about input noise? A lot of the older opamps did have a lot of input device noise, at least 5-6 times more than some of the new parts have, unfortunately the FET parts are a little more noisy on the voltage side.

I would bet that the discrete method is actually noisier right?
 
Why not to put the whole preamp into mic head? For an example the FETbloak DOA would be fine (you'll need the 1G resistor still). All you need is a stiffer power source. Obviously you couldn't use this kind of mics without the "phantom booster", which would be just a regular 48V supply but instead of the 6.8k resistors had somewhat smaller resistors (or active circuit) and maybe some kind of short circuit protection in case a usual mic cable fails.
 
I kind of like the CAD schematic I saw here. A TL072 type working push pull.

I was thinking a TL071 etc with a pull down to a NPN PNP output to a nice transformer

Kind of like a cheap fet47

I have been thinking about this I have not built so I don't know if it would work with the charge amp setup feedback network the Fet 47 has.

I wonder if it will have some of the sound some people post about with a FET 47 on Kick

Cheap china capsule in front of a drum don't have to worry as much for the capsule replacement cost.
 
I've been trying to lay off the use of emoticons because I considered them an easy-out for people who couldn't convey their tone and intentions in writing (and that's definitely me sometimes!). But maybe I'd better go back to using them. :roll:

Believe me, I'm not in favor of the current trends in audio IC design; as you say, it's all geared toward cellphones, I-Pods and other miniaturized consumer products, not professional audio applications. On the other hand, I do think there's kind of a knee-jerk, faster-amp-as-panacea-for-all-ills reaction among a segment of the audio community (and I'm not accusing Svart of being among them). Having designed some video circuits myself, I know those ICs can be finicky critters and I'm not inclined to use them where I don't need a 6MHz bandwidth. Then again, I build audio circuits all the time that use tubes or transistors that have quite usable amplification up into VHF, so the problem of high frequency stability is not confined just to video ICs.
 
it's all geared toward cellphones, I-Pods and other miniaturized consumer products, not professional audio applications

Exactly. I'm not saying to embrace them and accept them for normal audio use, but I am saying that they aren't going to go away. They ARE the future so we might as well try to introduce them into our designs and figure out intelligent ways to use them to get the outcome that we desire.

As this is my philosophy, I am not against trying it to see if it works.. sort of the ANTI kneejerk reaction that NYD talked about. We know what FETs work best as impedence converters in mics but that doesn't stop others from trying all other models and brands right? Same thing here, why use 12 discrete parts if there is one part that can do a better job with less hassle? I know a lot of folks love that discrete sound and a lot of folks love the tube sound.. that's all anyone talks about around here.. But I have to ask, WHY? is it because we are used to hearing the shitty MXL mics all day and we subconciously link that to the parts used? Lets face it, most parts are plenty good enough, it's the usage that kills them.

So lets get back to the discussion of the use of a yet unspec'd opamp in a mic and get away from the political discussion that seems to plague every other thread in the forum.
 
i'm ready to give this a shot.. I have some opa627 and opa637 to start with..

What is the typical gain needed for the overall circuit?
 
>> Quote: output noise

> What about input noise?


AKG 414 is unity gain. Output noise IS input noise. (Yes, it is 2-stage and the second stage could add noise.... but the 2nd stage works at much lower impedance with BJT inputs so I doubt it adds enough to matter.)

> the FET parts are a little more noisy on the voltage side.

Not for source impedances over 1 Meg.

> I would bet that the discrete method is actually noisier right?

That's a VERY GENERAL assertion. Sure, I have a totally discrete Radio Shed phono preamp that is noisier than a 741 chip. And a discrete-input mike amp that is lower noise than any chip made 10 years ago.

In general: the parts can be the same. But they usually are not. Multi-mask chip processing is costly, so input devices are small-area, and you don't want to reject entire chips for a little excess noise. Each chip has high development costs, a limited market, high per-unit cost. Single devices are cheaper per acre, are often produced by the millions, price may be so low you can buy a bucket-full and sort-out the best ones.

Then again, the intrinsic matching on a chip may allow some clever tricks......

And there are a few high-volume/profit applications where a low-low-noise chip is worth the development and marketing costs.

For large-diaphragm audio-band condensers, head-amp noise is not a Big Problem. TL072 may be at the upper end of what we should accept (yet I have great recordings made with preamps of 6dB higher noise level), and strains bad in low impedance.... but golly, it is 30 years old and costs 30 cents each. Yeah, you can probably find something with lower input noise and a more serious output stage. Maybe even something which will drive term-75 loads (150 ohms) at -86dB corruption. And maybe for under $4, which is nothing compared to the mechanical costs of capsule and a nice housing.
 
Page 41 in the PDF microphones at the neumann site

looks inverting to me
 

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