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Contact Microphone DIY - Quality

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Contact Microphone DIY - Quality

Postby EffeX » Wed Jan 27, 2016 1:26 pm

Hello everyone,

I want to build myself a nice contact (piezo) microphone, and after some research I have some questions that I hope people here might be able to answer.

These questions are mostly about the quality of the sound, seeing that the process in of actually making one doesn't seem to be very complicated:
Solder a piezo transducer to a 6,3mm jack cable, then solder the cable to a 6,3mm mono plug and done. Perhaps put it in some plasti-dip for protection. But that's where the first questions already arise...

1) Is it a good idea to put the piezo transducer in plasti-dip? If yes, what is the influence on the quality of sound?

2) Are there big differences in quality for the standard piezo transducers? What I see on the internet all seems to be the standard $0,50- transducer disc, apart from for example the Barcus Berry or the Trance Audio mic's (2.1 - Why could these be better than the standard transducer?). I wonder what the frequency response is for the standard transducers like this one. Are there significant differences for one that is 35mm or 15mm?

3) I read a lot about the high impedance that contact microphones require. It is often recommended that you use a preamp or impedance transformer before plugging it in your interface or field recorder. Would something like this work if I plug it into my Zoom H5?

No idea if there's people here who have the right knowledge, but it's worth a try I think. Thanks in advance!

For people who want to read up on this kind of mic here are two good articles to get you started:
- http://hydrophones.blogspot.co.uk/2010/10/tips-guidance-note...
- http://www.musicofsound.co.nz/blog/the-first-rule-of-contact...
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Re: Contact Microphone DIY - Quality

Postby Martin Walker » Thu Jan 28, 2016 8:14 pm

Hi EffeX, and welcome to the SOS Forums! 8-)

1. The element needs to pick up vibrations from the outside world, so encasing it in plastic isn't a good idea unless the latter is very thin (typically for instance, to record an acoustic instrument you would use double-sided sticky tape or elastic bands to attach it) However, if you want to put it into a protective enclosure, then exploring hydrophone ideas is probably sensible, such as this one:

http://maaheli.ee/main/d-i-y-hydrophones/

2. Buying a piezo element can always be a little hit and miss, although they are typically only a few pounds at most, so if you're not happy with the performance of the first one, try a different model. However, most people seem happy with even the cheapest elements, so give them a try first.

3. Yes, plugging your piezo mic into a fairly typical 50K ohm line input impedance will probably end up with the bass rolling off below a couple of hundred Hz, so you'll ideally need more like 1M ohm. That impedance transformer you link to is perfect for connecting a line level device such as a keyboard/synth to a mic input, but is not at all suitable for piezo.

If you're into a little circuit DIY, here's a basic preamp that's small enough to house with the piezo transducer:

www.megalithia.com/sounds/tech/piezo/fetamp.html

Here's a more upmarket DIY version with better performance:

www.megalithia.com/sounds/tech/piezo/opamp.html

Hope this helps!


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Re: Contact Microphone DIY - Quality

Postby Wonks » Thu Jan 28, 2016 11:01 pm

1) The piezo element needs to be compressed and released to generate its output, so as it's already likely to have some form of protective coating, anything extra is probably going to reduce the sound quality. It really needs to be squashed between the source and something to hold it in place. Piezo transducers for guitar bridges rely on a nice flat saddle slot and a nice flat saddle bottom to work properly.

2) No idea about no. 2.

3) Normal piezo recommendations are to run into an input impedance of 10 megohms for maximum output matching, (though in a fairly recent thread, Folderol or ef37a mentioned a very low impedance gave a very flat frequency response). Very high is the norm. That transformer you highlighted would not be the solution in this case.

You'd best be looking at outboard acoustic guitar pre-amps for 10 meg impedance inputs. Or you could make one yourself. Here's one I found http://www.scotthelmke.com/Mint-box-buffer.html
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Re: Contact Microphone DIY - Quality

Postby Folderol » Fri Jan 29, 2016 10:09 am

Wonks wrote:
3) Normal piezo recommendations are to run into an input impedance of 10 megohms for maximum output matching, (though in a fairly recent thread, Folderol or ef37a mentioned a very low impedance gave a very flat frequency response).
'twas me :)
Only works if the feedback is almost entirely capacitive, so becomes a like for like balance.
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Re: Contact Microphone DIY - Quality

Postby EffeX » Fri Jan 29, 2016 11:00 am

These are fantastic answers, thank you very much!

Now regarding 3), this is what Jez Riley has to say about it on his website:
" . 1/4 inch / XLR combination socket - the 1/4 inch input is mostly intended for ‘line level‘
inputs, such as those coming from a microphone with its own power supply, a mixer or a
separate pre-amp. Connecting 1/4 inch jack fitted contact mics or hydrophones to this
will only give you around 80% of the possible signal. To get the optimum results use a
1/4 inch to XLR impedance transforming adaptor (see the accessories page) - this gives
the best results: a higher signal level and warmer mid / low range frequency
response. "


Seeing that we have two different answers now I went ahead an emailed him this question:
"You've probably received tons of questions about this subject already but after thorough research I was only able to find conflicting information about the subject of wether or not a impedance transformer (as mentioned on your site) is enough for a basic contact microphone (into for example a Zoom H5). On forums people say to get a DI or preamp with the proper impedance, but on your site you state that a transformer is enough. I'm curious as to how you came to this conclusion, and I would be very grateful if you could answer me this question. "

Here's his answer:
"the impedance matching adaptors do exactly that - they match the impedance & this helps freq. response + using an xlr connects to the H5 via the 'mic' section of the combo sockets, as opposed to the 'line in' section. Using a separate pre-amp (depending on the model) would also match the impedance but would, in addition, improve upon the Zoom's internal pre-amps. So, folks on forums are confusing the two different functions of impedance matching & pre-amp improvement."

This of course still doesn't necessarily mean that he's right, but I was wondering what you guys think of this.

Once again thank you for the great answers!
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Re: Contact Microphone DIY - Quality

Postby peterdeltablues » Fri Jan 29, 2016 11:19 am

I'm suspicious of advice from anyone who seems to imply that 1/4 inch jacks and XLR have different impedances. They're just connectors - of course, they are often used to connect to different types of input circuitry, but it's sloppy writing at best.

I'd recommend building yourself a cheap high impedance pre-amp, with an output level control. At simnplest it can just be a jfet input op-amp, or you can use discrete jfets. Used as a contact mic, a piezo won't put out much voltage, but hit it (as in a drum pad) and you can generate quite a bit - I'd prefer to buffer the signal with an external pre-amp than risk overloading the inputs on your recorder.

Dipping in plasti-dip will do two things (i) provide some protection and reinforcement for the solder connections onto the disc (the solder onto the ceramic element is not that robust) and (ii) roll off some high frequencies, which might not be a bad thing.

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Re: Contact Microphone DIY - Quality

Postby Alba » Fri Jan 29, 2016 12:02 pm

I'll just throw in a link to this recent thread. It doesn't answer all the questions but its worth a look maybe. Link
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Re: Contact Microphone DIY - Quality

Postby Alba » Fri Jan 29, 2016 12:10 pm

Here's another linky to a more advanced version of that cable. This one is balanced but there's lots of good info on that page and links to more resources. Linky II
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Re: Contact Microphone DIY - Quality

Postby Alba » Fri Jan 29, 2016 12:12 pm

These look handy Linky Resurrection but i havent had a chance to check the data sheet to see if they can be dropped into a circuit using a J201 or similar.
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Re: Contact Microphone DIY - Quality

Postby Alba » Fri Jan 29, 2016 12:16 pm

You could always buy one of these type of endpin preamps and save yourself the trouble. I haven't tried one mind you. Linky Rising
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Re: Contact Microphone DIY - Quality

Postby Alba » Fri Jan 29, 2016 12:34 pm

The sound you get will depend on the instrument and the placement of the pickups. The best thing i've found regarding mounting is to get some double sided tape (the spongy mounting tape type) and sandwich the disc between two layers. The bottom layer sticks the thing into the soundboard and the top layer protects the wiring but also allows you to put a layer of aluminium foil on top which seems to cut down on noise.

I use the Maplins piezo discs All are shown

I've also found that wiring them in parallel seems to work better than series.

Placement and piezo size is a tricky one. It depends on the instrument and so a fair bit of experimentation is required. The cheap under saddle piezos sound crap imo. I prefer a disc. Though i have recently stuck an old undersaddle jobby actually under the bridge plate with thin double sided tape along with a pair of 50mm piezos salvaged from a couple of old smoke alarms and it sounds fantastic.

I have a 12 string that has two discs in it. One 27mm about 2" from the edge of the brige on the treble side and one 50mm disc about 3" from the bass side. It sounds great.

However, i would say that you aren't going to get Tommy Emmanuel's sound without spending some money and chopping the guitar about a bit and also including an internal movable mic inside the instrument and blending the piezo and mic. Blend is the keyword.

I've found the diy approach handy for some situations but tbh, you will get a better sound if you just sit down and stick a 57 or 58 in front of the guitar.

Its a lot of experimentation to get a good sound.

Funnily enough i have just been testing a guitar that i piezo'd up yesterday using my preamp cable phantom powered into a mic pre on the interface. It sounds quite good.

ymmv.
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Re: Contact Microphone DIY - Quality

Postby Martin Walker » Fri Jan 29, 2016 9:03 pm

EffeX wrote:
Here's his answer:
"the impedance matching adaptors do exactly that - they match the impedance & this helps freq. response + using an xlr connects to the H5 via the 'mic' section of the combo sockets, as opposed to the 'line in' section. Using a separate pre-amp (depending on the model) would also match the impedance but would, in addition, improve upon the Zoom's internal pre-amps. So, folks on forums are confusing the two different functions of impedance matching & pre-amp improvement."

This of course still doesn't necessarily mean that he's right, but I was wondering what you guys think of this.


The whole point of that impedance transformer is that it transforms the input impedance of your mic input from 200 ohms to 50k ohms, while simultaneously dropping the line level of the piezo element down to mic level. That's what it's designed to do (as stated in the product details - "This adaptor is designed to match 50K output impedance to 200R input impedance. It is ideal for connecting a hi-Z instrument to a microphone input").

This makes it perfect for plugging in keyboards/synths to mic inputs, and it would no doubt give rather more bass end if you plugged in a piezo element through it rather than directly.

However, it won't transform the input impedance up to the 1M ohm or preferably greater that's needed to restore the full low end of the piezo output - that would need a differently designed transformer with a different turns ratio, and would IMO probably be somewhat impractical.

More details here on piezo requirements in this SOS review:

www.soundonsound.com/sos/apr06/articles ... h_0406.htm


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Re: Contact Microphone DIY - Quality

Postby Wonks » Fri Jan 29, 2016 10:32 pm

I remember ef37a saying that the input impedance of a transformer was dependent on the impedance the output side was connected to. If I remembered correctly (and this is the case), then if that transformer was connected via an XLR to TRS lead into a line input, then the input impedance would then be (at the 250:1 ratio of that transformer) in the order of 11 to 12.5 megohms dependent on the actual line input's impedance - so ideal for a piezo.

So maybe it's just that you have to misapply the transformer to make it work?

I'm either right or wrong here, so hopefully someone will either confirm or correct me.
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Re: Contact Microphone DIY - Quality

Postby Folderol » Fri Jan 29, 2016 10:56 pm

That's only sort-of true over a limited design range. Outside of that, the transformer gets very lossy and internal capacitances, inductance and resistance all combine to make its response drop off pretty quickly.
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Re: Contact Microphone DIY - Quality

Postby Wonks » Fri Jan 29, 2016 11:00 pm

So really, you just need a pre-amp with the correct impedance designed in from the start.
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Re: Contact Microphone DIY - Quality

Postby Martin Walker » Sat Jan 30, 2016 8:00 pm

Wonks wrote:So really, you just need a pre-amp with the correct impedance designed in from the start.

I'd say so yes.

When I Googled this impedance transformer topic out of interest, quite a few people seem to be using this particular impedance transformer with piezo mics, but I suspect this is because they are marketed as 'impedance transformers', so everyone assumes they match all requirements, even though their manufacturers clearly state that they are designed to couple 50 Kilohms to 200 ohms, and nothing about their suitability with sources that benefit from AT LEAST 1M ohm :beamup:


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Re: Contact Microphone DIY - Quality

Postby Wonks » Sat Jan 30, 2016 11:37 pm

My latest acoustic guitar acquisition has an old 'bug' type piezo pickup mounted inside it that I've yet to try. If I haven't thrown it away, I should have one of those impedance converters in a bag somewhere, so I'll try running into mic and line inputs and the impedance converter into a line input and see what the results are.
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Re: Contact Microphone DIY - Quality

Postby Folderol » Sun Jan 31, 2016 9:00 am

Another point is that you never know what perfromance to expect, so the sound from one of these might seem 'acceptable'... until you hear it properly matched!

If you fancy a bit of D.I.Y. try feeding it into an 071 wired as a unity gain buffer, with a 10M input resistor to ground, and send the output of that into a line input.
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Re: Contact Microphone DIY - Quality

Postby Martin Walker » Sun Jan 31, 2016 3:46 pm

Folderol wrote:If you fancy a bit of D.I.Y. try feeding it into an 071 wired as a unity gain buffer, with a 10M input resistor to ground, and send the output of that into a line input.

Perfect! 8-)


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Re: Contact Microphone DIY - Quality

Postby EffeX » Mon Feb 01, 2016 12:57 pm

This is great guys, and let me know what the results for your experiment were Wonks.
I think i'll just try it out as well. A friend of mine has a piezo DI that I compare to that impendence transformer. I'll post the results once I've done that.
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