tk@halmi
Well-known member
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 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