Noise from preamp
I just made a preamp for myself .. based on a LF351 I had lying around.
I did not AC couple the output from the amp however.
I get a constant noise at the background (like an Airconditioner heard from inside the room). We have central heating and I am sure the noise upstairs does not come down so much for the mic to pick up.
Is it because of me not ac coupling the output?
In that case .. would a 0.33uF polycarbonate be ok?
The old LF351 opamp is not a low noise audio opamp. Of course it will produce hissss.
The output coupling capacitor has no effect at high hiss frequencies, its value affects the circuit's lowest bass frequency.
Uncle $crooge
I was thinking maybe the constant DC output from the preamp was contriibuting to the noise .. can it be the cause?
I seem to measure 2.5V with a cheap chinese coil meter even when I don't excite the electret. My Fluke measures ~1.7v but that keeps on changing. I am connecting this directly to the "mic in" of my laptop (I don't have a line level input)
If the output CD offset can never be a cause for the "hiss", should I replace the thing with a TL074? (The TL071 seems to be much more expensive - both are fronm TI - why?)
Schematic?
Curiosity was framed. Ignorance killed the cat.
The LF351/356 J-FET op amps are very good amps for LINE LEVEL audio applications where 0 dB signal levels are pretty high. For microphone or phono pre-amps, they are not good enough. The best amp used to be the Signetics 5532 type, don't know if they are still the best, but they are very good.
http://www.datasheetcatalog.org/data...nts/ne5532.pdf
The other source of hum in low level amps is magnetic flux: is there a transformer within 12" of your amplifier circuitry?
Last edited by bountyhunter; 12th August 2009 at 05:35 AM.
I don't see audioguru's problem with the LF351, it might not be as good as the TL071 but it's perfectly acceptable for most applications. If you're got plenty LF351s in your junk box then I don't think it's worth ordering new parts, especially if it's not part of a larger order.
I do not answer private messages asking for help because no one else can: benefit from advice I may give or correct me if I'm wrong.
Please ask on the open forum if you have a question and I'll be happy to help, if I know the answer.
The complaint is that the LF351 produces noise in the preamp circuit. A better opamp made for low level audio will not produce noise.
Uncle $crooge
Maybe the circuit is 'hooting' @ few 100 kHz? (due to bad layout maybe)
Last edited by marcbarker; 12th August 2009 at 01:03 PM.
"I think, therefore I toast" ( Mr. T. Toaster, artificial life form, kitchen appliance & Philosopher)
I think it's more likely to be a ground loop problem or the lack of shielded audio cable than the op-amp.
Last edited by Hero999; 13th August 2009 at 08:56 AM.
I do not answer private messages asking for help because no one else can: benefit from advice I may give or correct me if I'm wrong.
Please ask on the open forum if you have a question and I'll be happy to help, if I know the answer.
Aw, you guys made me draw a schematic. It was the usual stuff you know.
It's powered from a 9V PSU (7809).
Analog electronics is not my forte, so here is the schematic.
The input is twin wire unshielded, but the mic is 6'' away from the opamp input.
The output is a moulded stereo jack cable, the usual stuff.
Op amp is not decoupled (the input power supply is suitably bypassed) and the pins shown not connected in the schematic are not connected.
The schematic is truly the "true picture" of what I have... maybe I got some resistor values wrong from what it really is in my build, but I will have a closer look in the morning.
Should I upload a few audio samples?
Last edited by vsmGuy; 13th August 2009 at 02:08 AM.
I had just bought a lotta 351's. Got a few TL071 with me but they are reserved for some "better" experiments :-)Originally Posted by bountyhunter
There are no Transformers within 12 inches, but I connect my mic to a laptop , although it has a SMPS but no transformer to be sure.
How do I find if the noise is injected from within the opamp, picked up be leads or is ambient noise just amplified by the preamp?
Disconnect one end of C1 and see if hiss stops.
"I think, therefore I toast" ( Mr. T. Toaster, artificial life form, kitchen appliance & Philosopher)
I'd try removing the 9V power supply connection from between the input and the input cap. Seems to me that any ripple on the supply will also be amplified and, if it's full-wave rectified, that would make it 120Hz--which might sound a lot like an air conditioner.
Simple idea to test and prove or disprove one way or the other, at any rate.
Torben
Last edited by Torben; 13th August 2009 at 03:49 AM. Reason: Forgot a word.
Curiosity was framed. Ignorance killed the cat.
The opamp is inverting with an input impedance that is so low that it shorts the signal from the mic. The opamp should be non-inverting with a high input impedance.
The 1k resistor that powers the mic injects noise from the power supply that is amplified by the opamp. This resistor should be 10k and be decoupled with a 1k resistor in series from the supply and a 47uf capacitor to ground as a filter.
The mic is low level so should be connected to the preamp with shielded audio cable.
We are guessing that the noise is hiss or is hum. What is it?
Uncle $crooge
Oops--I had missed where the OP mentioned that it was a preamp for an electret mic. My bad. Audioguru's suggestion then makes much more sense than mine.
Torben
Curiosity was framed. Ignorance killed the cat.
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