Hi everybody! I am very new to electronics and am trying to build a preamp to fulfill the following goals

1. Amplify microphone input signal (~10-50mV ptp) to roughly 1V ptp to power earbuds
2. Attenuate low ( < 5Hz) and high (> 20,000 Hz) frequencies

I am using a J310 Jfet and NE5532p OP AMP.
VGS(off) = -2 -> -6.5 V
IDSS = 24 - 60mA

JLI 2555



I have kind of ignored the source-follower circuit and load resistance, as I am not sure how to implement it with my J310 JFET. My current circuit is as follows.

I took inspiration from this circuit. Single-Supply, Electret Microphone Pre-Amplifier Reference Design (ti.com)
My problem is when I plug in my circuit all I hear is noise (and can sometimes see voltage clipping from my JFET), If I hum into my mic I can hear it through the noise, however I can't hear regular talking. My current output (noise) looks like this.

Any help would be much appreciated.

 

Hi gabe,
Welcome to AAC.
The NE5532 OPA is not suitable for that circuit, it is a dual power supply type.

E

Update: please post your LTSpice asc file.

 

Hi Eric!
I really appreciate the fast response, I changed to an LM 358 and am getting a very similar output, I believe there may be more than one problem with my circuit. I have the following op amps available if you think a different one would be better

I also have long wires and alligator clips connecting things (I wouldn't think this would generate an un-usable output if it was the problem)

I would really appreciate any other advice you might have, I believe may issue may stem from jfet biasing, but I'm not sure. I may need to buy a different jfet.

 

Hi gabe,
please post your LTSpice asc file.
E

 

Here is the asc file, I appreciate you taking a look!

 

hi, g.
There is no Drain load,???
E

 

Hi Eric, I accidentally forgot to update my file, this is what it is like currently, however I do not have a source load. Another problem I may be having is my power supply is not connected to ground, only hot and neutral.

 

Hi Eric, this is what's coming straight out of my JFET by itself, I don't know if this is a power supply issue or if I'm creating some oscillations in my circuit (perhaps not having a ground from the wall?), but this is clearly the noise I'm hearing and it is suspiciously 60 HZ (wall power???).


This is now what the noise is narrowed down to.

I think I need to order a power supply cable with ground, instead of just hot and neutral.

 

ALL of the electret microphone capsules that I have seen include that FET transistor inside, and if you are providing an additional gain stage with an external FET there might be way too much gain.
Biasing the non-inverting input to half the supply is valid, but then I see it is also tied to the common, which is incorrect.

 

hi gabe,
Unzip this file, it will help with the electret mic simulation part, replace your FET.
E

 

I went back to post #1, and one obvious possible source is mentioned: "When I plug in my circuit"!!
Plug in to WHAT?? With what sort of connection?? With what sort of cable to the connection??An unshielded cable can provide several volts of assorted noise. Likewise an incorrect connection to a plug. So now a more complete description is in order.

 

"When I plug in my circuit"!! Plug in to WHAT??

Hi,
The TS clearly indicates that it is his version of the electret, is what he is plugging in, that causes the problem.
He is using the preamp OPA circuit as shown in the data link he posted.

E

 

I did not think that was clearly indicated. In addition, it is not clear what it is plugged into, or how the connection is actually made. And certainly the connection at the low level end of an input circuit matters a great deal. It has not been made clear that the TS is working with an electret element that does not include an internal FET. so there are a few items that have not been adequately clarified, as I see things.

 

Hello,

Try this:

Also R3 may be removed from the schematic.

Bertus

 

Any help would be much appreciated.

The JLI-2555 has a FET inside the housing so should be configured as below.
Generally 2.5 volts across the element is sufficient but requires decoupling through R1 and C1 to help eliminate power supply noise.

 

Hello,

The datasheet shows that there is no fet inside the module.
An external fet should be used.

Bertus

 

I did not think that was clearly indicated. In addition, it is not clear what it is plugged into, or how the connection is actually made. And certainly the connection at the low level end of an input circuit matters a great deal. It has not been made clear that the TS is working with an electret element that does not include an internal FET. so there are a few items that have not been adequately clarified, as I see things.

Hi MisterBill!
I believe your unshielded cable idea holds a lot of merit. I am using unshielded cables and alligator clips, and It seems when I move them near electronics the noise spikes. However, I think having a floating voltage for my circuit may be a larger problem, I have ordered a new power supply cable to ground my circuit properly. I don't know how exactly this 60Hz noise is getting in my circuit, however I don't think a floating voltage source is helping.

To further clarify, I do not believe my version of the JLI 2555 includes a JFET.

I really appreciate everybody's help! It means a lot as an EE student about to enter my electronics classes trying to get a head start.

 

To further clarify, I do not believe my version of the JLI 2555 includes a JFET.

Stand corrected but still believe the FET should be decoupled.
The spec sheet shows 1.5 volts on the drain of the FET.

 

The data sheet certainly does not show any FET internally. BUT the data sheet also lists the output impedance as 2k2 ohms (2200ohms). That happens to be the value of the load resistor in the provided circuit. The output impedance of just the electyret element is very large.
Thus I smell a discrepancy. And as the outline drawing shows two terminals aside from the threaded case terminal, that tells me that one terminal is the drain and the other is the source.
And I can promise you that an un-shielded high impedance, small signal, connection will certainly pick up 60 Hz noise floating in the air.

 

Hi gabe,
Can you confirm you are attempting to build this circuit from your link, both in simulation and hardware.?
If yes, for the simulation circuit, use the electret mic model in post #10 to drive the OPA amp, as shown in this circuit.

For the hardware version, use an actual electret mic module.

Also confirm which OPA type you are planning/trying to use.

E