Instrument microphone

Hi there, this one took a while to reach my desk so apologies for the slow reply.

First thing's first, you really wouldn't put a microphone in front of a speaker that way. The sound pressure it picks up is not reflective of what you hear. It's a weird combination of the size of the speaker, the distances and what not.

The way to do this correctly is to put a "tap" into the instrument line in. I expect such things exist but they shouldn't be difficult to design as you really only need a high-impedance pickup to duplicate the output from your electric plank to another amplifier or the mixing desk.

When I'm recording for groups (on the occasions I do) and even speaking events I always take a tap from the mixing desk where possible because those guys have the best sound.

The basic circuit is just a "follower" ideally using a FET op amp like TL072 which is cheap and easy to find. You don't need super-high quality for this purpose as we're often distorting the sound with overdrive and valve simulations.

I've got the parts to make a pre-amp that will add a "valve-like" behaviour when it starts to tip into overdrive but it's a case of time to make it.

The circuit to quietly siphon off your playing to another amp is pretty simple - there are loads of designs on the web (many explain it in silly detail) but what you need is called a "follower" because the output "follows" the input. Put 1v on the input you get 1v out - but the circuit that's supplying that 1v just won't notice (electrically speaking) and the output from the design will easily drive a guitar amp.

The TL072 is a dual amp but if you're more traditional, the classic 741 (which may cost a little more as they aren't used much now) will also likely work. As a single in an 8-pin DIP it might be easier to handle.

You will need to power it somehow and the usual way is a couple of 9V batteries providing a total of 18v for many hours before needing replacement. A slightly better way can been seen in the schematic of the Mathew headphone amplifier (see the PDF under that post). That requires a little more voltage (12-16) from a wall-wart and you split the voltage "biasing" the input at 6-8 V. In the Matthew amp, I've used a separate amplifier to supply the biasing voltage but this is there to keep the noise to an absolute minimum. 

In practise you'll see lots of designs where a couple of high-value resistors (usually about 100K) are strung between +Ve and 0V with the centre tapped off to bias the non-inverting input. The inverting input is wired directly to the output giving a net gain of 1 but a reduction in input impedance of 1000s of ohms down to just a few ohms (less actually but this is limited by the output drive capability).

My son-in-law has a plank he's built based on the Red Special so I'll have to ask him if we can try a design when I have a few minutes and a couple of suitable sockets.

Separate note, if you want wireless headphones by far the easiest way is to get one of those pre-made Bluetooth transmitters and drive that from your monitor out (audio, not your screen).

Even a decent quality one will cost you about £20 or less (or maybe a fiver on eBay). Someone already designed those so we don't have to go to all the trouble.

Hello, first off all thank you for your complete answer. 

I am really surprised about your words since a lot of people are using microphones like Shure SM57 or Sennheiser E906/E609 for examples.

I have an line out jack output on my amplifiers also but I aways though it was better to use a microphone to pick the sound in front of the amp! 

So with your explanations, I now have some points to start researching over the web to understand everything better. 

Thanks a lot again.

Happy to help. You can try this for yourself with pretty much any microphone you might have lying around for your PC (even a Bluetooth headset although the telephone mode does cut the quality).

It's true that if you swing a Sure SM57 (dynamic, vocal) over the edge of your amplifier cab, you will pick up the sound of what you're playing but it won't pick up the sound accurately and more important it will also collect sound from the whole stage causing all manner of weird phase effects and distortions.

But if you do happen to have a microphone for your computer you can try this at home by recording the sound from your television, radio or even computer and then comparing what you record to what you actually hear.

The difference with your guitar pickup is that it's immune to other sounds (this doesn't apply to acoustic guitars which have microphone as pickups). As the strings vibrate they disturb the field around the pickup coils and that's amplified by your cab. You can see this if you remove the strings and then tap the guitar's body. No iron/steel near the pickup, no sound.

Dr. Brian May (of Queen) built his own from scratch and some old bits of furniture before he joined the iconic band and it still works today, albeit with improvements. His book and to lesser degree, the website, describe it all in detail: including the pickups. Mind you, with an IQ that's the sum of mine and Matts (and then some) it's not surprising and even then it took him and his dad two years to complete.

Yes, it's logical after all. 

I will not get the sound of the amp+speaker. But if I use a microphone straight on the cab, I will not get the exact sound of the speaker either. That's totally logical anyway, worthless. 

Thanks