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Peizo+Condenser preamp
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Uncle Krunkus
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PostPosted: Thu Mar 22, 2012 12:22 am    Post subject: Peizo+Condenser preamp
Subject description: Degeneration cap brings sudden WOW factor!
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Hey everybodies,

I've got a friend (supreme bass player) who is building harps from really nice solid and veneer maple.
They cover 2 octaves, nylon string, with the prospect of incorporating a pickup and pre-amp in the sounding box.
We'd like to have a piezo flat against one of the sounding boards, plus a small condenser mic hanging directly off the preamp board.

Does anyone know of a good pre-amp schem, optimised for a flat piezo disc pickup, or a good condenser mic insert pre-amp, or maybe both in one circuit? I could combine them if need be, as there will be a "blend" pot plus a volume pot.

Any good ideas, cautions, and/or links would be greatly appreciated.

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Last edited by Uncle Krunkus on Sat May 19, 2012 7:14 am; edited 5 times in total
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PostPosted: Thu Mar 22, 2012 2:54 am    Post subject: Reply with quote  Mark this post and the followings unread

Yeah, I've got a good idea. Ditch that pickup scheme and use the one that was invented here at electro-music by Dr.Justice and myself. It works great and does a good job of capturing the true string vibrations in the signal. If you want to test it out you can do an easy one-string test on an existing guitar with just a few wires, two alligator clips, a small super magnet, and a 1/4" jack. I can talk you through it and if you like the sound of the test then you will know what to do to build it into the harp. Recommended!

Les

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Uncle Krunkus
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PostPosted: Thu Mar 22, 2012 3:24 am    Post subject: Reply with quote  Mark this post and the followings unread

I think I know what you're talking about Les, but,.....
Nylon strings.
Thanks for the interest though.

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PostPosted: Thu Mar 22, 2012 3:33 am    Post subject: Reply with quote  Mark this post and the followings unread

Doh! Yeah, the invention requires metal strings! Oh well...

See Make magazine's web site here:

http://blog.makezine.com/video/

which is their video page and look at the second video, collin's lab, for a funny and entertaining tutorial on how to make a contact mic preamp to electrifiy an acoustic guitar (or your head (see video) ). Worth a watch if you're doing this sort of thing even if you don't use this exact circuit.

Les

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Uncle Krunkus
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PostPosted: Thu Mar 22, 2012 1:56 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks Les,
that piezo pre-amp is super simple and just what I needed to incorporate into the final idea.

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PostPosted: Thu Mar 22, 2012 2:11 pm    Post subject: Reply with quote  Mark this post and the followings unread

Oh, good, glad i could help out this time Smile

Les

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Uncle Krunkus
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PostPosted: Sat Mar 31, 2012 2:34 am    Post subject: Reply with quote  Mark this post and the followings unread

So,....

I'd like to combine this piezo pre-amp :-
http://blog.makezine.com/tag/collinslab/

with this condenser insert pre-amp :-
http://www.zen22142.zen.co.uk/Circuits/Audio/ecmmic.html

It seems that the piezo pre-amp has an output impedance of 220K, whereas the condenser pre-amp has an output impedance of 2K.

What can I do to bring these into line with each other?
Can I sum them passively across a "blend" pot of say 10K?
What theoretical stuff am I missing here?
BTW,... It will be running off a 9V battery, so simplicity, and a lack of bi-polar designs would be best.
Any help would be greatly appreciated.

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PostPosted: Sat Mar 31, 2012 3:02 am    Post subject: Reply with quote  Mark this post and the followings unread

Uncle Krunkus, this is a question of Thevenin equivalent circuits, which you can google to bone up on the theory. Some possible solutions include:

1. just put a 220k resistor in series with the low impedance amp, and make sure they drive a load impedance of at least 220k. This way the signals will be about the same in amplitude.

2. Use a difference amplifier with voltage follower inputs (there opamp circuit).

3. Design an opamp circuit that balances out the two impedances.

4. Buffer each output with an emitter follower.

I would simply choose option 1.

Les

p.s. these answers assume the output impedance is primarily resistive, which is typically the case.

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PostPosted: Sat Mar 31, 2012 3:10 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks Les,
1 or 4 sound best to me.
Nice and simple.
Just like me! Laughing

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DES



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PostPosted: Sat Mar 31, 2012 8:15 am    Post subject: Reply with quote  Mark this post and the followings unread

Something else to consider....companies like Fishman and Barcus Berry make (made?) dual preamps for acoustic-electric guitars that had condenser and piezo inputs along with volume and tone controls. Taylor guitars is also using a brand (forgot who) that does this and sounds great. Potential drawback is that their housings can be bit ugly...but you can always move the parts into something better. Also price could be a consideration. Could also look into service schematics for the above as well....
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Oskar



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PostPosted: Mon Apr 02, 2012 7:53 am    Post subject: Reply with quote  Mark this post and the followings unread

Krunky, that's a really interesting question! There are many variables to consider, including the following:
- Budget?
- How good do you need it to sound?
- If you go down the DIY road, how many man hours are you willing to invest in getting the components right, the installation, placement, yadayada...
- Is doing it youreslf imperative, or can you live with a ready-made solution that does the job straight out of the box?

If you want to look at ready-made solutions, you can go for the expensive brands like Barcus Berry, Schertler and others. There are so many options from makes such as Fishman, LR Baggs, Seymour Duncan and others that I'd probably classify as mid- to moderately high price. In the more cheap and cheerful category I would mention Shadow, probably others as well.

Point is, I suppose, What are your goals, budget-wise, sound-wise, manhour-wise? And so on and so forth...

If you go for the DIY thingy, then Les and others on here have excellent know-how to get you in the ballpark, as it were, then it's a matter of tweaking it until you feel you've got it right.

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PostPosted: Mon Apr 02, 2012 2:32 pm    Post subject: Reply with quote  Mark this post and the followings unread

He's building a few of them, so the idea is to get one happening, and make the next a bit better, etc. It's all about value adding to what are already very beautiful hand made harps. For the pre-amp to be hand made as well will only add to charm.
I've almost finished the basic stripboard layout, which I'll knock up, test, modify, update layout, knock up V2, etc. etc.
If I put them together myself, with parts I already have, then it will all be cash positive, (which we desperately need these days! Laughing ). So I won't be turning to anything I can get off the shelf.
There are also fairly strict dimension restrictions, so I'm designing with these in mind from the start.
I'll take a photo when I get the first one together.

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Oskar



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PostPosted: Mon Apr 02, 2012 4:34 pm    Post subject: Reply with quote  Mark this post and the followings unread

One thing I remember from my lovely Yamaha steel string that got stolen: It had both a piezo undersaddle pickup, as well as a goose-neck condenser inside. It was always necessary to play around with the phase switch to remove that horrible, boxy squawk you're liable to get in that kind of setting. SO, I'd recommend a sort of phase switch or variable phase control or something. Also, I remember being slightly frustrated with my not being able to process the two signals separately, both for EQ, compressing/limiting, modulation FX and delay/reverb. Being a complete nincompoop when it comes to actual elctronics I'm not gunna tell you how to build this, but some way of controlling the phase alignment, as well as the possibilty of having two separate signal paths for piezo and condenser/dynamic mic is something I PERSONALLY would love to have in such an instrument.
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PostPosted: Mon Apr 02, 2012 11:18 pm    Post subject: Reply with quote  Mark this post and the followings unread

Based on Oskar's sage advice above, I'd recommend buffering the output of the two amps with voltage followers and running them out separate jacks. Phase adjustment can then be done by later systems be they analog, digital, or computer based.

Les

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Uncle Krunkus
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PostPosted: Tue Apr 03, 2012 1:30 am    Post subject: Reply with quote  Mark this post and the followings unread

Unfortunately, (or?) we can only have one output jack due to the space constraints.
Just a Volume pot, Blend pot, and output jack.
But that's okay, because I think it will be largely a case of piezo or condenser, rather than piezo and condenser.
I understand completely the potential for phase problems, and that is something we will need to optimise for, as I improve the design, but it's not just a case of optimising the electronics, and I'm yet to see how much signal either pre-amp can acquire.

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Uncle Krunkus
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PostPosted: Tue Apr 03, 2012 3:53 am    Post subject: Reply with quote  Mark this post and the followings unread

So to increase the output impedance of the piezo pre-amp to around 220K do I just add a 200K from the output to ground? (After the cap) Or do I put it in series with the output?
I'm gonna start by just hanging the two outputs across a 10K(?) pot, and take the blend output off the wiper. Is this right?
Thanks dudes.

I'm gonna start knocking up the first stripboard prototype tonight. It will probably get a few changes yet, but I need something to find the right physical layout, and start testing.

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Oskar



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PostPosted: Tue Apr 03, 2012 12:14 pm    Post subject: Reply with quote  Mark this post and the followings unread

Would it be possible to use a stereo output jack? That way, you can route the signal sources to different paths.
Sorry to be harping on about this, but I speak from experience.
Btw, sounds like a VERY exciting project!

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PostPosted: Tue Apr 03, 2012 8:30 pm    Post subject: Reply with quote  Mark this post and the followings unread

Uncle Krunkus wrote:
So to increase the output impedance of the piezo pre-amp to around 220K do I just add a 200K from the output to ground? (After the cap) Or do I put it in series with the output?
I'm gonna start by just hanging the two outputs across a 10K(?) pot, and take the blend output off the wiper. Is this right?
Thanks dudes.


Put the 200k in series with the low impedance output. Use a 500k or 1MEG pot to ground. Use 250k pot if necessary. In a tight pinch, just to try it, can use 100k pot if you don't have bigger. 10k will attenuate the signal too much.

Les

p.s. if it's an either-or situation, use a switch instead of a phase adjust?

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PostPosted: Tue Apr 03, 2012 9:20 pm    Post subject: Reply with quote  Mark this post and the followings unread

I meant for the two outputs to go to either side of the 10K pot as a blend control. Then take it's wiper to the top of a 500K (bottom to ground) (volume) with it's wiper as the final output.
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PostPosted: Wed Apr 04, 2012 1:40 am    Post subject: Reply with quote  Mark this post and the followings unread

Uncle Krunkus wrote:
I meant for the two outputs to go to either side of the 10K pot as a blend control. Then take it's wiper to the top of a 500K (bottom to ground) (volume) with it's wiper as the final output.


That configuration will work, but again I'd use a large blend pot and here's why. You will have two Thevenin equivalent sources (voltage source in series with Thevenin resistance) with a source impedance of 220k. If you blend into a high impedance load with a 10k, you will only get about 5% blend range because the blend pot is so small compared to your sources. If you use a 500k blend pot you'll get about a 50% blend range because the blend pot is as big as the two Thevenin resistances.

Note that you're not going to get 100% blend range unless you buffer the outputs of the two sources with voltage followers (which have close to zero Thevenin equivalent resistance). Also note that the gain pot in your passive arrangement should certainly be 1MEG because you want a high impedance load so your signal doesn't get attenuated too much. Another note: if you buffer with voltage followers you can use a 10k pot no problem and then your gain pot can be as low as 100k also.

I recommend using the voltage followers, in which case adding the 200k resistor in series with the low impedance load is unnecessary and you can use the pots you want to use. Also with the followers your source impedance will be lower so you will get less low pass filtering from the capacitance of the cable.

Thinking further, if you are going to buffer with a voltage follower then your gain pot can be an active arrangement with a third opamp, and that way you can all but completely eliminate the cable's effect on the sound. Have you ever noticed some guitarists saying they prefer the tone of one brand or length of cable over another? That's because of the cable capacitance robbing them of those sweet high tones. On a harp, I'd imagine this factor is even more important.

Thanks for offering me the opportunity to advise you, Uncle, it has helped me rethink this category of circuits. Good luck, and may the electromotive force be with you!

Les

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PostPosted: Wed Apr 04, 2012 1:47 am    Post subject: Reply with quote  Mark this post and the followings unread

Inventor wrote:
Good luck, and may the electromotive force be with you! Les


I'm waiting for Les to invent a new dance: "Come on baby, do the electro-motion!" Cool

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PostPosted: Wed Apr 04, 2012 5:00 am    Post subject: Reply with quote  Mark this post and the followings unread

Oskar wrote:
Inventor wrote:
Good luck, and may the electromotive force be with you! Les


I'm waiting for Les to invent a new dance: "Come on baby, do the electro-motion!" Cool


Hahhahaha!

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PostPosted: Wed Apr 04, 2012 5:08 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks heaps Les,
I will read that over a number of times, and I think I'll get it eventually. (keep the gods at bay!) Laughing
I'm such a brain fart with circuit theories.
You guys make the world go round!!!

Das Krunkenoid.

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PostPosted: Wed Apr 04, 2012 9:14 am    Post subject: Reply with quote  Mark this post and the followings unread

Doktor Krunkenstein, he hass ze makings of an electroharp Gott. Ziss vill be ver, ver gut!
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PostPosted: Fri Apr 06, 2012 4:13 am    Post subject: Trying a jpg Reply with quote  Mark this post and the followings unread

Here's the layout so far,
It's not working quite right yet, but this is where it'll get interesting.
The piezo is coming through quite well, regardless of blend point. (found a 50KLin for the blend pot (same profile as the Vol Pot,)...is there a problem with that?)
The condenser section is basically dead (I think, but it's hard to know how to test for the bad spot)
Volume pot, (500KLog) seems to work well, with usuable levels below the half way point.


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Electret/Piezo pickup and pre-amp stripboard V1.0 by Uncle Krunkus
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