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I have spent the last few days trying to prototype a TL082 electret mic preamp (based on this schematic). I swapped the TL071 in the linked example with TL082 (which I understand to be more noise-prone but otherwise similar), made the appropriate changes to the wiring (VCC moved to Pin 8, signal output moved to Pin 1, etc.) and am using 50 Ohm resistors for the Voltage Divider (as opposed to the 62 Ohm used in the example schematic) but otherwise my circuit is identical (this was laid out in EAGLE and the PCB cut via CNC, BTW). See below for my schematic.

TL082 preamp by CTA

As far as I can tell the preamp is sort of working but I am struggling to test and diagnose it. For example: an LM386 power amp will not amplify the signal from this TL082 circuit. The LM386 circuit I use for most things will not amplify the signal from the pre at all (no sound comes out). No idea why, here's a schematic of that circuit below:

enter image description here

If I plug the TL082 into a different amplifier, say a honeytone (which uses the TDA7052A IC), I can kind of hear the signal, whereas I get nothing through the LM386.

Does anyone see anything wrong with my TL082 schematic? Should I not be using LM386 to amplify this preamplified signal? Would something else be better? Any suggestions or reccs would be greatly appreciated!

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  • \$\begingroup\$ On the LM386 schematic, C4 shouldn't go directly to GND, a small-value series resistor should probably be added - something in the 20 ohm ballpark. And do ensure that those phono jacks are properly mated (audio on tip, ground on ring, shell)...it is easy to cross-wire them. \$\endgroup\$ – glen_geek Feb 19 '20 at 0:20
  • \$\begingroup\$ @glen_geek good to know! i will try adding 20 and test from there. thx! \$\endgroup\$ – caseyanderson Feb 19 '20 at 0:54
  • \$\begingroup\$ Does it improve if you change R4 and R5 from 50ohm to something closer to 50kohms? \$\endgroup\$ – pbandjazz Aug 12 '20 at 15:46
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enter image description here

Figure 1. Inverting mode amplifier with no Ri.

Also R3 would normally be connected to ground to discharge C1 and prevent pops when plugging in. As it is it will maximise pops.

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  • \$\begingroup\$ Oh interesting, thanks for this feedback. I see that I have omitted Ri, great catch. I am a little confused about what value resistor Ri should be and whether that is a pull down resistor to ground or would be in series after C1 but before the junction to Pin 2. Could you clarify which you would recommend? Re R3: I will ground that resistor and report back. Thanks! \$\endgroup\$ – caseyanderson Feb 18 '20 at 23:21
  • \$\begingroup\$ @caseyanderson R3 goes to Vcc to provide a power supply to the electret mic. You'll only be able to ground R3 if the mic has its own (battery) power supply. \$\endgroup\$ – Graham Nye Feb 18 '20 at 23:36
  • \$\begingroup\$ @GrahamNye gotcha, makes sense (and explains the EAGLE behavior I was just fiddling with). \$\endgroup\$ – caseyanderson Feb 18 '20 at 23:40
  • \$\begingroup\$ @Graham: I forgot there was an electret mic involved. Thanks. \$\endgroup\$ – Transistor Feb 18 '20 at 23:44
  • \$\begingroup\$ @caseyanderson: Your feedback potentiometer value looks low. Since \$ A = - \frac {R_f}{R_i} \$ (where A is the gain) you could set \$ R_i \$ = 1k and have a maximum gain of 10. 1k is probably to low for the electret to drive well. Try it but I'd expect 10k and 100k for the pot. \$\endgroup\$ – Transistor Feb 18 '20 at 23:46
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You don't need an Ri since that preamp is a transimpedance amplifier, which main advantage is lower noise than an inverting amplifier (input is current - output is voltage), see this application note:

https://www.ti.com/lit/ug/tidu765/tidu765.pdf?ts=1596661260110&ref_url=https%253A%252F%252Fwww.google.com%252F

I would suggest increasing the value of R1 to something closer to 50K, but that will depend on the sensitivity of your microphone, also I would not user a potentiometer there. If you wanna control volume then put a potentiometer at the output of the preamp, but not there.

You could improve a little bit your circuit following that AN.

Probably with a 10K resistor as R1 you do not have enough gain to hear something, but the LM386 circuit looks good to me.

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