Hello All.
I am trying to construct a simple preamp for a dynamic mic (to allow me to connect to the mic-in port of a PC) and have run into some noise problems. I ought to point out that I'm, at best, an electronics amateur, and generally clueless at circuit design. I'm also working somewhat blind as I'm relying on some PC based scope software that gets its data from the sound card. I'm not sure how reliable it is.
Here are a few facts:
1. The mic I'm working with is a Behringer XM8500 with an impedance of 250R. It produces a signal of about 10 mV. It has a differential o/p. (I think audio guys call this "balanced").
2. I have a very simple design: I'm using (half of) a NE5534 as a difference amp (i.e one of these), with a voltage gain of about 50, 35dB. This gives an acceptable signal to record at the PC end.
3. The CMRR of a difference amp is apparently pretty sensitive to the ratios of R3/R1 and R4/R2 so I've selected these carefully so that they're better than 1%. R1 = 3351R, R2 = 3347R and R3 = R4 = 178K3
4. The h.f. 3dB point is about 18KHz. (For l.f. rolloff see below ..)
5. The circuit is battery-powered (two 9v batteries) so I think I don't have to worry about ground loops (I think ..)
The problems:
P1. The dynamic mic seems to be astoundingly susceptible to 50Hz hum. I was hoping that it would all be common mode noise, and the difference amp would eliminate almost all of it, but I'm wrong: with the mic connected to the amp, I see a minimum of 5mV p-p hum. Given that my o/p signal is about 500-600 mV typical, this is a poor SNR.
Can anyone suggest why I'm getting so much hum in the amplified signal.
P2. The self-noise of the amp. (with no mic attached) seems to be about 1 mV p-p. This seems to be far higher than I'd expect from the spec of a 5532. This is broad-band noise, with the same power density all the way up to 20KHz or so. This manifiests itself as background hiss when I record.
What could be causing this noise ? Interference due to poor shielding ? I can't believe that it's all generated in the circuit itself. The noise from a 5532 + any Johnson noise wouldn't seem to be enough, given such a simple circuit.
P3. I am currently coupling the 5532 directly to the mic o/p. I have tried adding a couple of caps to give low freq. rolloff (3dB at 100Hz) to reduce the 50Hz hum, but this had no noticeable effect. This confuses me - surely if I roll of the gain from 100Hz I should substantially reduce 50Hz noise, no ?
P4. In addition to this, I also see 120 Hz hum - now I'm in the UK, not the States, so this can't be a mains harmonic. Cany anyone suggest where I'd be picking up 120Hz hum from in a domestic environment ?
And more generally - I see mic preamps advertised with say 70-80dB gain, but very good noise figures - however do professional designers achieve these ? Does a lot of this come down to good shielding ?
I am trying to construct a simple preamp for a dynamic mic (to allow me to connect to the mic-in port of a PC) and have run into some noise problems. I ought to point out that I'm, at best, an electronics amateur, and generally clueless at circuit design. I'm also working somewhat blind as I'm relying on some PC based scope software that gets its data from the sound card. I'm not sure how reliable it is.
Here are a few facts:
1. The mic I'm working with is a Behringer XM8500 with an impedance of 250R. It produces a signal of about 10 mV. It has a differential o/p. (I think audio guys call this "balanced").
2. I have a very simple design: I'm using (half of) a NE5534 as a difference amp (i.e one of these), with a voltage gain of about 50, 35dB. This gives an acceptable signal to record at the PC end.
3. The CMRR of a difference amp is apparently pretty sensitive to the ratios of R3/R1 and R4/R2 so I've selected these carefully so that they're better than 1%. R1 = 3351R, R2 = 3347R and R3 = R4 = 178K3
4. The h.f. 3dB point is about 18KHz. (For l.f. rolloff see below ..)
5. The circuit is battery-powered (two 9v batteries) so I think I don't have to worry about ground loops (I think ..)
The problems:
P1. The dynamic mic seems to be astoundingly susceptible to 50Hz hum. I was hoping that it would all be common mode noise, and the difference amp would eliminate almost all of it, but I'm wrong: with the mic connected to the amp, I see a minimum of 5mV p-p hum. Given that my o/p signal is about 500-600 mV typical, this is a poor SNR.
Can anyone suggest why I'm getting so much hum in the amplified signal.
P2. The self-noise of the amp. (with no mic attached) seems to be about 1 mV p-p. This seems to be far higher than I'd expect from the spec of a 5532. This is broad-band noise, with the same power density all the way up to 20KHz or so. This manifiests itself as background hiss when I record.
What could be causing this noise ? Interference due to poor shielding ? I can't believe that it's all generated in the circuit itself. The noise from a 5532 + any Johnson noise wouldn't seem to be enough, given such a simple circuit.
P3. I am currently coupling the 5532 directly to the mic o/p. I have tried adding a couple of caps to give low freq. rolloff (3dB at 100Hz) to reduce the 50Hz hum, but this had no noticeable effect. This confuses me - surely if I roll of the gain from 100Hz I should substantially reduce 50Hz noise, no ?
P4. In addition to this, I also see 120 Hz hum - now I'm in the UK, not the States, so this can't be a mains harmonic. Cany anyone suggest where I'd be picking up 120Hz hum from in a domestic environment ?
And more generally - I see mic preamps advertised with say 70-80dB gain, but very good noise figures - however do professional designers achieve these ? Does a lot of this come down to good shielding ?