Using diodes LEDs to bias tube in mics ? & capacitors in mics & other mic ????

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gary o

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Dec 28, 2004
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Hi firstly can I just say Im more a singer than a mathmatision so dont understand all the theory so please forgive me... but I know what I like the sound of & Im just trying to build my perfect mic I have almost come to the conclusion that its gonna be a tube mic nothing solid state has come close to sounding better YET..... also come to the conclusion that capsule is to me about 95% the sound of a mic tho am intrigued & willing to experiment to find different flavours, I have 4 capsules to play with favorite is Violet Vin67 closely followed by the great Tim Cambell CT12 then a Oktava 219/319 then a Lomo k47 type... so far the capsule always wins

It worries me as I see soooo many different circuits being made lately & people going to great lengths to try n find a certain tube to match an old obsolete tube but then they stick in a cheaply made capsule that in my case dont know if it even sounds close to the classic capsule it claims to sound like (for $90) .....tho it is fun to try the circuits are we chasing our tails....Im not having go I just wanna spend my money in the right place if u know what I mean

Im wondering if I can make a mic that makes say my least or 2nd to least fav capsule sound as good as my best capsule and so on.....so far I havent messed with de emphasis circuits ( dont have right capsule yet)

So on to the question about the cathode bias .....I have often read on the net that electroytic caps & other caps are bad things to have in audio ? is that so, I see some mics avoid them, been reading about using diodes or LEDs to set the cathode bias ? am I talkin crap (dont answer that) did I read about using a infa red LED ? did I also read you could just use another PSU to send a small varable cathode voltage to tune a mic ?

I see some circuit use a cap just before the grid ...... so the cap effects the sound..... Is the cap less design better ......

 
Diode - nonlinear element. The nature of its work does not look like resistors.
I tried to do bias by the LED. If you do not use a capacitor connected in parallel, the sound gets a lot of distortion. In addition, decreases the level of the signal (feedback). Basically, sound cool, but weird. Best of all, negative voltage is applied directly to the grid , cathode attached to the ground. There are several possible solutions:
1. Submit the negative voltage for heating  and make the divider that  forms a negative offset from the heat. Most common solution for fixed bias.
2. Make a separate negative voltage controlled from the power supply (best).
3. Used batteries (can be a miniature).
 
o3misha said:
Diode - nonlinear element. The nature of its work does not look like resistors.
I tried to do bias by the LED. If you do not use a capacitor connected in parallel, the sound gets a lot of distortion. In addition, decreases the level of the signal (feedback). Basically, sound cool, but weird. Best of all, negative voltage is applied directly to the grid , cathode attached to the ground. There are several possible solutions:
1. Submit the negative voltage for heating  and make the divider that  forms a negative offset from the heat. Most common solution for fixed bias.
2. Make a separate negative voltage controlled from the power supply (best).
3. Used batteries (can be a miniature).

Very interested in making the separte negative voltage, are there mics that do this .....would something like LM317 work ......
 
One of my tube microphones uses LED biasing.  You need the right LED and you need to think about how to bias it correctly.
 
Interesting weird yet cool sound to be had o3misha ...... dont know hoe to bias correctly that is why I ask  :)..... I have been reading folks here been finding differents circuit schemes changing the sound of there mics .... I cant seem to make my experimental circuits sound much different when same capsule is used, difference in output level but not much in sound .....
 
Thanks Gus didnt find anything here found schematic via goohle looks interesting & like something I should make ....
 
So why is external neg voltage best for mics & why arnt we all using this .......

Thanks
 
Separate negative bias voltage can adjust without soldering inside the microphone. In addition, when the cathode is soldered to the ground, there is no influence of the cathode capacitor that is present with autobias. It is always better in terms of sound. There is only one disadvantage: fixed bias on the grid you have to adjust the operating point of the  tube whenever you decide to change the tube.
 
THanks o3misha cool so in mostmic circuits I can lose cathode bypass cap & achieve better sound most important with the remote neg supply brilliant why isnt thismore common .... I need to construct a neg supply now .....

thanks
 
o3misha said:
Separate negative bias voltage can adjust without soldering inside the microphone. In addition, when the cathode is soldered to the ground, there is no influence of the cathode capacitor that is present with autobias. It is always better in terms of sound. There is only one disadvantage: fixed bias on the grid you have to adjust the operating point of the  tube whenever you decide to change the tube.

Or look at C12, where you get the best of both worlds.  ;)

 
So the C12 has 4u7 bypass cap & adjustable bias ?....

Im putting together MY ultimate mic so far .....Its basically the MGM church mic with the high B+ thus higher capsule polarization voltage 70V a Tim cambel capsule & orig T14/1 ( or maybe maxes MK47 BV8 tran ) havent tried that yet

The MGM design sounds best to my ears with the parts Im using when compared to other circuits I have tried so far with the parts I have not sure if its the higher capsule tension or the amp running at higher voltage that seems to make it sound nice or the design...

This is why I wonder about best cathode bias arrangment....I dont mind making anothet PSU and have extra cables coming from the mic

Im thinking of adding  swich for 33M 100M 200M 1G grid res..... & switch for 33u 47u 1u 2u 3Uu output cap, maybe switchable feedback too also my PSU can provide 120V 150V 275V for the B+

I would like to learn more about the MGM church circuit to find out what it is I seem to like about it.

thanks
 
Better stick with 60V capsule polarization, especially with a edge terminated capsule like Tim's. Some capsules can do a bit more, but you're quickly approaching a point of diminishing returns. All you can possibly gain is a couple more dB capsule output; the sound doesn't change a whole lot within the sensible voltage range.

Also keep in mind that you have to be very careful with switches in the high impedance area. You quickly get parasitic capacitance. Also many switches don't have enough isolation resistance.
 
gary o said:
So the C12 has 4u7 bypass cap & adjustable bias ?....


No, the C12 has grounded cathode and is self-biased in the PSU.

We discussed it before here and here.

I know it looks complicated but basically the plate current causes voltage drop across a resistor (R8) in the power supply, and this (negative) voltage is applied to the grid. Thus, the grid bias voltage is dependent on the plate current, and the tube is effectively biasing itself - just as in a cathode-bias arrangement.

One advantage is that there is no need for a cathode bypass cap, another is that the bias does not need to be adjusted manually when changing tubes. I see it as a clever way to kill two birds with one stone and achieve self-bias without the bypass cap.

 
Hi fellas thanks for replies

Rossi I didnt know that about edge terminated capsules thanks,I got my 75V numbers from other DIYers ,Im running my church mic at 260V so I guess may work out a little less capsule voltage .... I read of how higher voltage therefore diaphram tension raises the resonant frequency & wondered if it is this I like the sound of or just the amp sounding cleaner or maybe the feedback .....can I adjust capsule voltage by cahnging R6 & R7 in this circuit....

http://prorecordingworkshop.lefora.com/composition/attachment/3b8aafb94c605cc1db0400f0ad524599/1199745/ChurchMicSchematic.jpg

I only had one high res value resistor 1Gig to start with & as I understand a higher grid res combined with capsule capacitance sets corner frequency is that right ? & 1G with capsule will & does sound kinda full bassl,which I like , but want to try other values so thought might try the switch.... or maybe just find fav value ......

Hi Dan thanks for your help again I do remember we went over this , my trouble is I never fully understand whats going on but its sinking in the C12 seems to have as you say best of both, I wonder could I implement this in my church mic..... As I say nothing wrong with church mic sound as I have it &Im trying to see what makes it sound nice to me .... I made circuits with exact tube tran & capsule but this has a much nicer sound.....I see it has a more unusual design & also the higher B+.......sooo interesting

Cheers
 
I did measurements with center terminated capsules, and there was almost no difference in frequency response within the sensible voltage range. And I suppose, when you do get a noticeable sound change due to higher tension, then you already approach or exceed the limits of the capsule. Keep in mind that it's not enough for the diaphragm to be just below the suck-in voltage. You really need to be well below that point. Also keep in mind that you have to double the voltage for 6 dB more. Going from 60 to 75V barely gives you 2 dB more output. 60V is a safe value for nearly all classic capsule designs; it's what they were designed for.

Yes a lower grid resistor affects LF response. Theoretically it also affects the noise performance, but in my experience a 60 or 100 Meg grid or FET gate resistor is not noticeably noisier than 1G. 60 meg, as in the U47, is an interesting value, because the LF cutoff is around 30-40 Hz. That's just below the usable range of musical instruments; you lose a lot of rumble without sacrificing actual signal. However not every capsule and circuit sounds good at 60-100 meg, so you gotta try values and listen.
 
I read somewhere some one saying mr Church designed the circuit to pick up less rumble by tightening the M7 & K7 diaphram with the higher 75V but I guess other ways to do that like the grid res...

Also read this circuit maybe slightly worse for noise..... I havent found that.... I have no real classic mics to compare sound or noise too ( have a MK012 MK101 head which I think is great for the money) but I have one of Maxes MK47 kits in a eq47 body that I copare my experiments too & the church mic is quieter than that & sounds nicer to me with same capsules I try.....it seems to have a cleaner sound hard to discribe of coarse,,,,Im going to run it at lower voltage and see how it sounds.... will also try grid res values OP caps & borrow Mk47 transformer , I love the sound of this thing so just need to fine tune it & decide which capsule I like better I might put it in the EQ47 body

thanks for great info
 
I read somewhere some one saying mr Church designed the circuit to pick up less rumble by tightening the M7 & K7 diaphram with the higher 75V but I guess other ways to do that like the grid res...

Also read this circuit maybe slightly worse for noise..... I havent found that.... I have no real classic mics to compare sound or noise too ( have a MK012 MK101 head which I think is great for the money) but I have one of Maxes MK47 kits in a eq47 body that I copare my experiments too & the church mic is quieter than that & sounds nicer to me with same capsules I try.....it seems to have a cleaner sound hard to discribe of coarse,,,,Im going to run it at lower voltage and see how it sounds.... will also try grid res values OP caps & borrow Mk47 transformer , I love the sound of this thing so just need to fine tune it & decide which capsule I like better I might put it in the EQ47 body

thanks for great info
I'm building the Church circuit currently. Would r6 be the best way to drop capsule down to 60v?

I'd really like to understand this circuit but I'm probably not there yet.

Gary O, saw you were still active...if you see this, would be really cool to know your findings. I've heard your vid clip of a mic in a 47 body through your ba6a and it was killer! Was that the Church?

church_schematic.jpg
 
yes, r6/r7 is voltage divider for capsule bias
Thx shabtek. I've got my voltages ok now and passing audio but can't get rid of the massive heater hum. 6.3vac no ct into FWB 2 caps and dropping resistor grounding negative side.
Could someone shed some light on what configuration of 6.3vdc supply would work here?
 
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