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Old 9th July 2010, 09:03 AM   #1
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Default Tube for high input Z

I'm building a mic with a condenser capsule. I am planning on using a bootstrapped 6922 as a buffer. The capsule needs to see an extremely high impedance, way more than 10Mohms for full range response.
The 6922 will probably work fine, but are there other tubes with very low grid current? (The 6922 wasn't exactly made for this use).
What other tricks than low heater voltage and high anode voltage should I use to acheive zero (close to) grid current?
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Old 9th July 2010, 01:21 PM   #2
SY is offline SY  United States
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Just out of curiosity (I'm looking at doing the same thing), what capsule are you using?

Electrometer tubes may be worth a look. Low microphonics are quite important, not just grid current.
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Old 9th July 2010, 03:41 PM   #3
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It's a Chinese made capsule I got from a friend of mine. It looks well made, but I don't know which brand it is.

The mic will be built into a 2" dia aluminum tube with 1/4" walls, and the electronics will be rubber mounted. Hopefully microphonics will be nill in this microphone
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Old 9th July 2010, 05:06 PM   #4
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Negative bias is one. Be sure the grid is several volts below the cathode. Otherwise, electrons from the space charge will fall onto the grid, drawing current. The space charge potential starts at about -1V for regular currents (microamps), but since you're concerned about fractional uA, keeping it under -3V would probably be a good idea.

A small cathode, low current density (towards the cutoff region) and moderate grid-cathode spacing (low transconductance) combine to produce a low leakage tube. I would guess regular old 12AX7 and 12AU7 are just fine in this regard; 12AU7 moreso, because it has lower Rp = more reverse bias necessary to set a low plate current. You'll start with a follower, so mu doesn't help you, anyway.

Another problem is positive grid current, due to stray cathode material deposited on the grid. As the grid heats up (from the cathode's radiant heat, from the plate acting like an oven, or even from load current), this material can emit electrons. This is a problem in power tubes, where the grid is likely to run hot, and harsh conditions (that can cause cathode stripping) deposit cathode stuff on the grid. Preamp tubes that haven't been abused shouldn't have any problems with this.

RF, high gain, frame grid, etc. types are probably less suitable because they have tightly spaced grids. Older types with poorer performance are probably better (and 12AU7 is pretty old). But maybe not too old, I'm sure there's a point where things get worse (avoid a poor vacuum).

What you'll do for the circuit is, cathode follower with a constant current sink. This will keep the grid-cathode voltage fairly stable throughout the signal, so you won't have any effect from the low gain or high distortion of the follower. For a 12AU7, you might go with 300V plate, 2mA sink, and with the grid biased at 88V, the cathode should sit around 100V. That's plenty of reverse bias, and enough current to drive another grid (but keep it DC coupled, otherwise you'll disturb the constant current with resistors).

If you need a *really* low leakage input, just skip tubes. There's no shame in that. JFETs alone attain picoamp leakage easily (at least when cold). Small signal MOSFETs will even do femtoamps. There are low-leakage opamps available which have so little leakage, it's not actually possible to measure: mere background radiation makes the air conductive enough to disturb the measurement!

Tim
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Old 9th July 2010, 05:19 PM   #5
kevinkr is offline kevinkr  United States
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Look at bootstrapping the grid bias resistor to the CF output in order to raise the effective input impedance.
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Old 9th July 2010, 06:35 PM   #6
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Quote:
Originally Posted by kevinkr View Post
Look at bootstrapping the grid bias resistor to the CF output in order to raise the effective input impedance.
Right. It is what I am doing in my condenser mics with tube inputs. Tubes are 6N2P (one triode for each capsule in a stereo mike). Bias resistors I use are 18 Megaohm only. Capsules are Chinese copies of Neumann ones.
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Old 9th July 2010, 07:19 PM   #7
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Where did you source those? I'd love to get some 1/4" jobs at a price less than the bend-over-and-tell-me-you-love-it Bruel & Kjaers.
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Old 9th July 2010, 07:29 PM   #8
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Where did you source those? I'd love to get some 1/4" jobs at a price less than the bend-over-and-tell-me-you-love-it Bruel & Kjaers.
I don't have a source yet. For my prototypes I buy MXL on ePay. Pretty cheap because of horrible electronics, but capsules are good enough for prototyping.
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Old 9th July 2010, 07:52 PM   #9
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How well have they worked for recording?
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Old 9th July 2010, 08:01 PM   #10
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How about an EF86? I believe this is what most commercial tube mic folks are using these days. Three of them, triode connected in parallel are nearly identical to a VF14 which was used in the Telefunken U47. You could, of course, try to get a NOS VF14 which costs about as much as a new kidney.

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