noise on That1512 preamp

Hello,

I've just finished a preamp based on the That1512 chip.





The preamp functions, but there seems to be some mains hum, I'm not sure if its because of some wiring fault or because the preamp and power supply are squashed into an old computor power supply box and are to close together. The following graph shows the interference. the top trace is the preamp on maximum gain with no mic connected, and the bottom trace is the soundcard with nothing connected.


Has anyone got any suggestions as to how I could reduce the noise? Different case? I tried to shield the power supply from the preamp by using a piece of biscuit tin screwed to the case.

Many thanks

Albert

Measuring without defined source impedance makes no sense. Use a 150 ohm resistor between pin 2 and 3 and check the mic preamp META, there's one or two threads about that IIRC.

Samuel

Your input caps are (physically) large and very close to the power transformer, so they will pick up a lot of hum, naturally.

Sorry about that, here is a new graph with 150 Ohm load. Top trace is preamp, bottom trace unconnected soundcard.



Am I right that it is mains induced hum?

You'll need to show an exact schematic (including PSU) for further help. In addition to this it will be helpful if you post a sound-file of the noise, about 30 s long.

I have no idea about the absolute level of the noise. Is it noticeable when used with a microphone?

You'll need to use a source resistor for the AD converter measurement as well.

Samuel

[quote author="mkoijn"]Am I right that it is mains induced hum?[/quote]

Looks like magnetic fields from the transformer, given 50Hz and 150Hz prominent. Were it 100Hz we would suspect full-wave-rectified ripple.

Attention to loop areas in low level/high gain portions of the circuit is needed, if you can't move the transformer away. Sometimes changing the transformer orientation can help a lot.

Thanks, The point about it being magnetic interference is useful. I will look into it.

You may benefit by using a piece of that same case metal or mu-metal to shield your input section and the power supply. Another thing to check is your grouding scheme. Do a search of "ground loops" and "star-grounding" on this forum and others.

Good luck
gb

You can prove whether the hum is being induced from the transformer - by extending the wiring and moving it outside the case.

If this does dramatically reduce (or eliminate) the hum, then it might be possible to improve magnetic screening around the device in the case - but it might be easier to try another sort of transformer.

A toroidal type has the lowest hum field by far - and very small models (from a few watts upwards) are readily available - so will certainly fit in the space.

JG

Thank you all for the suggestions. I will try extending the cables of the transformer and moving it away from the case. Still not solved the noise. I tried making a complete shield for the transformer out of a biscuit tin. It didn't seem to make any difference at all.

Regards

Albert

Thank you all for the suggestions. I will try extending the cables of the transformer and moving it away from the case. Still not solved the noise. I tried making a complete shield for the transformer out of a biscuit tin. It didn't seem to make any difference at all.

Regards

Albert

So, I moved the transformer out of the case and extended the cables. This reduced the noise considerably. There is still a peak at 50Hz of aprox 15db above the rest of the trace. It seems the transformer does radiate a lot of noise.

Try different orientations, as well as adding distance, while observing the spectrum.

If the system is that sensitive you may want to think about identifying where you have significant loop areas in your circuit and minimizing them, for your system is going to see other mains-related fields in normal use, besides the transformer.