Microphone Onboard Amplifier Idea

This may be a cheap alternative to onboard mic amps eventually.
Why am I building this? I could just build the u87 or Gus' circuit (which i really like). However, the 2N3819 usually does not have enough gain to hook up to a 10:1 transformer and get enough volume at the output. It needs to be sorted to stringent specs that give you a few dozen out of a thousand tranistors.

The idea here: be able to use about half out of a dozen FETs in a circuit, and use a lower cost output transformer.
http://mysite.verizon.net/res75okq/sitebuildercontent/sitebuilderfiles/micamp.pdf

The first stage is biased so that it has a 40 gain, pretty consistently (32dB). The second stage is just a source follower and the transistor need not be sorted. Now we can us a low turns transformer like the OEP to balance the output. The stepdown of the transformer is 12dB in theory, but in reality it is 13 to 14dB. The resulting net gain from capsule to the output is 18 to 20dB depending on the loading applied.

I have prototyped it up the voltage probe where it says "Built to this point". The left side with the 1n capacitor and capsule has not been built yet.

The charge pump was posted here a few months back. At some point the two shall be paired up and stuffed into a mic. Comments and suggestions are welcomed about the circuit.

Cheers,
Tamas

Why not an NPN for the 2nd transistor? BJTs can make better followers than fets.

I was vacillating between FET or BJT. As far as sound goes simple followers are probably much alike. I will test it with a garden variety emitter follower and see how it responds to loading.

Thanks,
Tamas

I have never seen 32dB of gain from a single JFET stage. 20dB is more like it. But hey, I'll take your word for it.

As mentioned above, an emitter follower would make a better output stage. Then again, the base current could load the drain of the amplifying JFET. Perhaps a good approach would be a compound JFET/BJT follower output stage--or at least a bootstrapped emitter follower, to keep the input impedance fairly high.

Dave,

Neumann U87 single FET preamp with 47 k drain resistor (with transformer load) and capacitor bypass/grounded source, have approx. 34 dB gain.

--Bo

The original reason for going with the second stage FET was to present nearly no load to the first stage. That way all the voltage gain is preserved.

The gain has been confirmed by measurements using the prototype and several FETs out of a 100 piece bag.

Tamas

"I have never seen 32dB of gain from a single JFET stage. 20dB is more like it. But hey, I'll take your word for it."

The Hamptone gain block with one fet has a gain between 26 and 30dB depending on the resistors used.

chrissugar

Dave;
High transconductance fets rock.