Loud mains hum with Jaycar short circuits Vol 3 #12

[Gillie]

The title says it all. Any ideas, especially re this particular kit? I am feeding a computer sound card. Hum is not present until the pre-amp is powered (ie there is no hum pickup from the output wiring or lead). The hum is present either with the input microphone connected or disconnected (ie the hum does not emanate from the input lead or the mic itself). I have strapped the input R & L hot pins as the mic is mono.

 

[(*steve*)]

Can you post the circuit, tell us how it is powered, and how it is connected to the PC.

If you short the input of the preamp, is there any change to the hum?

 

[Gillie]

Thanks for the speedy response Steve. The circuit is not here but I was hoping for a response from someone who knows this actual kit. It uses an op amp. I will try the shorting. Perhaps I need to shield it. At present it is an open board.

 

[Gillie]

Further to that. Yes, the hum disappears when the input is shorted. There is no difference whether the input mic is plugged in or not, although its impedance is 250 Ohms so I would expect a damping of the sound.

 

[(*steve*)]

Shielding may help.

Is the preamp powered from batteries? If not, try powering it from batteries to see if things improve. If things improve you may have some hum being injected from your power supply or via an earth loop.

 

[Gillie]

Unfortunately it is powered by batteries, Steve. So that one is out!

 

[(*steve*)]

Ok, that's fine. It eliminates several possible issues.

I would recommend you place the preamp in a shielded enclosure and be sure that shielded cables are used to connect the inputs and outputs.

 

[Gillie]

OK Steve. Will try first with some kling wrap over insulation before splashing out on a diecast box! Closing down for today and will report back later

 

[(*steve*)]

Kling wrap? As in the clear plastic wrap?

Aluminium foil would work better as long as you don't short anything out.

 

[Gillie]

No, not plastic of course! I meant ali foil! I'm fairly silly but not completely stupid!

 

[Audioguru]

The cable from the microphone to the preamp input is supposed to be shielded audio cable so that the shield blocks mains hum pickup. If your cable is not shielded then it is simply an antenna for mains hum to be picked up. The shield wire should connect to the ground of the preamp.

 

[Gillie]

Hi Steve and others

I completely take back my protestation that while I may be silly I was not stupid! I was! I had connected the input leads to the wrong jack socket tags. I was operating the pre-amp with nothing whatsoever across the input! Fixed that. Put all into diecast box. No hum. Gain a bit too high though.

The pre-amp is built around an LM833. Pins 2 and 6 (inverting inputs?) are fed back from the output (pins 1 and 7) via 200 k resistors. I take it I should vary that to reduce the gain? Maybe a (dual) pot?

 

[Gillie]

Now looked it all up. With a 200k feedback resistor gain is over 500 times. Seems a dual 100k pot from output to inverting input would enable gain reduction from 257 downwards. Am I on the right track?

 

[(*steve*)]

I would recommend that you simply reduce the value of the 200k feedback resistor(s). We're still operating without schematics...

 

[Audioguru]

Usually an amplifier has more than enough gain so it can be used with input levels that are lower than normal.
Then it has volume controls (not gain controls) that yours might not have.

 

[(*steve*)]

Having said that, if the preamp is clipping, then the gain must be reduced. For low noise, it's better not to attenuate a small signal before amplifying it (and thus the volume control normally follows the preamp).

 

[Gillie]

Thanks for your suggestions everyone. The design was based around a dual op-amp LM833. It called for negative feedback resistors of 200k between the op-amp output and the non-inverting input (on each of the two channels (stereo)). That gave a theoretical gain of over 500 times, much too great for my application.I'd like to report that my idea of changing the op-amp feedback resistors to 100 k ganged pots has worked very well. It enables me to set sensitivity on the fly. I'm using Audacity software and the soundcard on my(homebuilt) W7 machine, and the original purpose of the preamp was to increase the high quality low output mic level to a level suitable for the (bog standard) soundcard. Having got past my original blunder of the mis-wired input jack socket, all has been plain sailing. Totally noise free in diecast box with battery supply. Sorry no circuit diagram. Trying to learn how to extract that from my paper and input to this forum! However, typical op-amp circuit.

 

[Gillie]

I am now going to try to post the circuit.