Replacing GOhm resistors with diodes, the theory behind it

[abbey road d enfer]

Another abstract of the Hiller M49 technical description.
"The grid leakage resistance has a size of about 7000Mohm. This resistance has no effect on the favorable noise characteristics, but it does prevent the tubes from being blocked by grid charging in the event of equal pressure surges (e.g. gun blast)."
A bit obscure coz translation seems odd.

 

[thor.zmt]

Another abstract of the Hiller M49 technical description.
"The grid leakage resistance has a size of about 7000Mohm. This resistance has no effect on the favorable noise characteristics, but it does prevent the tubes from being blocked by grid charging in the event of equal pressure surges (e.g. gun blast)."
A bit obscure coz translation seems odd.

Yes, though this blocking distortion applies only to common cathode circuits. Common Anode Circuits (e.g. cathode followers) will not experience this problems.

Mind you, I doubt too many GDIY Members will be lugging their microphones to the nearest range to record the legendary Musique Concrete piece "Concerto for five Kalashnikovs and one Dushka" or will attempt to drag them to the front in Ukraine where I hear this piece is popular and performed frequently.

Thor

 

[kingkorg]

Now back to my own line of reasoning.
As remarked, what interested me overall was to make a bunch of decent, cheap and easy recording Mic's both large and small diaphragm using the usual cheap chinese Mic's as Body donor and the readily available capsules, as alternative to buying the overpriced China made "recording microphones" available generally. And yes, easy obtainability and low cost of all parts is paramount to me.
The emphasis is on decent, cheap & easy. P48 only use was a given as well as the option (not shown) to use non-standard P68 to power the circuit giving 60V Capsule Bias.

Here something I would try out with SMD parts on veroboard:

View attachment 105702

Fairchild J201 remains in production as MMBFJ201 in SOT-23 SMT. Cascoding lowers HD. Using the J201 for the reverse biased diode should give some of the beast results.

As we need to buy reasonable quantities to get sensible pricing, just use it, rather than buying extra diodes or 1GOhm resistors.

The rest is pretty much the old Schoeps design, except optimised to using P48 as bias source.

For J201 with a very abnormal Idss R1 & R2 may need adjusting. They are meant to have ~ 4V across them.

Using the Phantom Power source as bias, the Capsule gets 40V, not 60V which reduces output levels from the capsule by around 3.5dB. Yes, it will also change the sound, but we are not using genuine AKG or Neuman capsules and we are not trying to make accurate copies of 2,500 USD Mic's, just something cheap, easy & functional.

With a 1G resistor we get 64dB unweighted SNR @ ~94dB SPL while with J201 as diode the sim predicts 74dB. Note that is unweighted. A-Weighting will give a lot lower noise. So using the Diode gives us more SNR back than the lower bias loses and we have a VERY SIMPLE microphone.

No oscillators etc. to make 60V (and track noise into the Audio circuit).

For ~134dB SPL (without the pad enabled) THD will be < 0.5% almost pure H2. With the pad enabled HD is reduced by ~ 12dB and of course, so is SNR.

The point here is that if want soft mic overload with loud instruments or vocals, leave the Pad off, otherwise turn loud down a trifle with the Pad to get a relatively clean signal.

Now I need to get all my boxes shipped here.

Thor

This schematic could be problematic with sdc. I don't know which capsules, or bodies you had in mind, but many have one side of capsule grounded, screwed on to the body. Just pointing out in case you weren't aware.

True. A while back I proposed a microphone design that was universally rejected. It would have been based on massive numbers of "digital" mems microphones (ones that have a basically flat response), suitable DSP that allows almost any pickup pattern to be created and wireless 32Bit/192kHz digital Output.

Seems very interesting to me, do you have a link?

 

[thor.zmt]

This schematic could be problematic with sdc. I don't know which capsules, or bodies you had in mind, but many have one side of capsule grounded, screwed on to the body. Just pointing out in case you weren't aware.

I'm aware. The circuit is for complete rebuilds, not keeping existing capsules, which in most do it's are smallish electrets anyway.

Thor

 

[thor.zmt]

Seems very interesting to me, do you have a link?

I should be precise, proposed to microphone manufacturers (with striking both mid century modern and steampunk industrial designs).

It is not public domain.

They were all happy continuing to make the bad ripoffs of classic Designs instead.

Thor

 

[abbey road d enfer]

They were all happy continuing to make the bad ripoffs of classic Designs instead.

Which, on a strictly monetary point of view, makes the more sense, unfortunately...

 

[thor.zmt]

Which, on a strictly monetary point of view, makes the more sense, unfortunately...

I am not sure it does.

If you have a superior product with a very "down to earth" price, you have an outsized market advantage. The cheapest LDC capsule costs as much as 128 MEMS microphones, I was not planning on using that many (32 - 64 max - I have limited I/O lines on the processor).

There is no microphone pre, no ADC etc., no cable (except Power / USB nd RJ45 if you want to run cabled), a lot of stuff gets nixed.

And there are options to have a foldback Headphone/Line out on the microphone and to make additional interfaces targeted at instruments with foldback.

Then make/partner with a suitable Software and make a Hardware "box" and you are starting to sell a new standard recording and eventually wireless live sound standard.

So one needs to think big and act small.

And seeing how much buzz the Sphere gets, I think my Mic would be "more buzzy".

Thor

 

[abbey road d enfer]

I believe you forget one thing.
The cost of marketing.
Launching a new product takes a lot of energy, money and convincing.
I know, I've done that for about 30 years.
Launching a clone is much easier, since there is a spontaneous demand.

 

[rock soderstrom]

I believe you forget one thing.
The cost of marketing.
Launching a new product takes a lot of energy, money and convincing.
I know, I've done that for about 30 years.
Launching a clone is much easier, since there is a spontaneous demand.

True. I think he overlooked another fact, the incredible conservatism of the audio industry. It really takes a lot of energy and stamina to establish something truly new. I am happy about every new idea! That is progress.

 

[thor.zmt]

I believe you forget one thing.
The cost of marketing.
Launching a new product takes a lot of energy, money and convincing.

No, I did this many times and quite successfully.

Try:

I know, I've done that for about 30 years.
Launching a clone is much easier, since there is a spontaneous demand.

And here is the fun part, in addition to all the home and small studios there is another huge market for microphones where there would have been instant huge demand.

The issue that everyone wants to buy "ready made" and few companies pay or want to pay for any R & D, to make new things.

Almost everything is actually made by huge OEM/ODM factories in (mostly) south China and then sold though a number of "brands".

The "brands" just want to keep shifting boxes (Microphones or toilet paper makes no real difference to them) at maximum margins, the OEM/ODM factories just want to crank out boxes of minor variations of the same old stuff.

BUT, that is precisely the opportunity for disruptive products and quality products. Where everything is the same old shi...tuff with minor variations and all is poor grade (and anything really superior is boutique made) there is a HUGE opportunity.

Apple (V2.0 after Jobs returned) is great example.

End of sermon.

Thor

 

[Mr. Moscode]

This works because the grid of the Tube also includes an implicit diode. In the golden age of tubes, variations of grid leak biasing abounded. These days people go on endlessly about how it cannot work.

The understanding of how tubes work certainly has declined a lot.

Thor

To be fair the use of tubes by the every day DIYer has declined a lot. Not using a grid R in the right circumstance is/was not uncommon.