I'm looking for simple schematic for condenser microphone preamp (NOT electret)

• I'm looking for simple schematic for condenser microphone preamp (NOT electret)
  Posted by dusan on 24 Nov, 2022 10:34
• I want to build preamp for condenser microphone capsule. It is not electret, I disassembled one and it is condenser mic. When I google for "condenser mic preamp" 99% of schematics are for electret which is not what I want.
  
  I'm looking for simple schematic I can build with parts I have. I don't have many parts but I have:
  
  - various NPN/PNP (30 different types)
  - jfets: 2SK30A, BF245A, 2SK246, 2N5457, J201
  - bs170 (and few other mosfets in TO220)
  - largest resistor I have is 10M (100pcs)
  - opamps (TL071, OP27G, LT1028, NE5532, LM258, JRC4558, TL064IN)
  - other (NE555P, ICM7556IPD)
  - various diodes (30 different types)
  - 1/4w E12 resistors (1R..10M)
  - 9V battery
  
  I assume I will need charge pump to make higher voltage so I have these caps:
  - 100V polyester caps (1n..470n)
  - 50V ceramic caps (5p..100n)
  - most of my electrolytic caps ends at 16V, I have few 35V and few random oddbals in higher voltages
  
  Is there a schematic I can built with these parts?
  If not what should I buy to build something simple?
• #1 Reply
  Posted by jonpaul on 24 Nov, 2022 10:44
• Great question! |
  
  See classic Neumann and other German valve condenser mics.
  
  https://en-de.neumann.com/v-402
  
  https://gyraf.dk/schematics/schematics.html
  
  Some have JFET or other FET input.
  
  The power is separately derived usually a 48V phantom on studio mics.
  
  
  Jon
  
  
• #2 Reply
  Posted by wasedadoc on 24 Nov, 2022 11:39
• Simpler than the above but perhaps still not as simple as the OP would like is the Mackie input schematic shown in the first post at https://www.eevblog.com/forum/repair/mackie-mixer-with-noise-on-two-channels/
  
  Edit to make correction: No, that circuit is not applicable for use on the raw output of a capsule.  It assumes it is preceded by a capsule pre-amp.
• #3 Reply
  Posted by magic on 24 Nov, 2022 18:25
• Start with measuring capacitance of the capsule. AFAIK typical values are single or two digit pF and a quick calculation demonstrates that gigaohm biasing resistance is needed to maintain frequency response down to 20Hz. Resistors of such values are available, but a little exotic. They can nevertheless be obtained from big distributors like Mouser and sometimes from auction sites.
  
  Maybe you could experiment with biasing by leakage of reversed PN junctions or stuff like that, but a quick glance at the mic schematics linked by jonpaul shows that they all used resistors.
• #4 Reply
  Posted by TimFox on 24 Nov, 2022 18:35
• A bootstrapped gate resistor can achieve AC impedances of that magnitude with reasonable DC resistance.
  ("Bootstrapping" involves positive feedback less than unity to the "bottom" of that resistor, easily obtained from a tube cathode or JFET source.)
  The noise contribution of the lower resistance will still be that of the resistance, but the high-pass filter from the capsule capacitance will be determined by the bootstrapped impedance.
• #5 Reply
  Posted by dusan on 08 Jul, 2023 15:01
• Eventually I purchased two 1G resistors and after few trials and errors I came up with this schematic. It works fine, the volume is loud, I'd say it's about 2x cleaner than best electret mic I have. Consumption is 1mA at 9V. I thought I would need charge pump and use 50V or so but it works at 9V too.

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• #6 Reply
  Posted by dusan on 08 Jul, 2023 15:09
• Update: Wav sample is first electret microphone (different circuit), then beep, then this condenser microphone. electret_vs_condenser.wav (975.36 kB - downloaded 34 times.)

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  electret_vs_condenser.wav