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Old 26th March 2007, 11:57 AM   #1
gsdali is offline gsdali  United Kingdom
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Default Preamps for acoustic instruments

I have an acoustic octave mandolin with pickup and I need to build a preamp. I alreadfy have the parts on the way to build a Till FET amp, but I have a spare t-amp, (from one of the cheep $35 amps) and was wondering if anyone could think of a way of converting that for use as a high impedance per-amp (5 to 10 Mohms input impedance), with an output at line level or electric guitar level.

Or am I being crazy and showing a lack of understanding of electronics?

any other designs anyone would like to suggest?

Thanks for any help and advice.
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Old 26th March 2007, 03:28 PM   #2
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What actually is in the mandolin at the moment?, is it just a microphone (and what type), or does it take a battery (like acoustic guitars do).

Is it for recording or live performance? - I hate acoustic guitars for live performances, too much feedback from the internal mikes!
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Old 26th March 2007, 04:01 PM   #3
gsdali is offline gsdali  United Kingdom
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It has a piezo soundboard pickup, with an internal preamp, but this does not provide enough boost and the output impedance is still very high. This is for live use, and yes feedback is the bane of my existence.
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Old 26th March 2007, 04:05 PM   #4
teemuk is offline teemuk  Finland
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With reasonable volume levels acoustic feedback becomes an issue with most hollow-bodied instruments - usually regardless of mic configuration. I don't know about mandolins, as they have a pretty small body, but I guess it's still the same thing with them. Stuffing the instrument with some damping material helps but basically makes the tone very different so what an acoustic amplifier really need is a circuit that can eliminate feedback. I also guess some sort of tone controls would not do harm either.

Question is, are you looking for to build a gain stage or a preamplifier? It's the extra features like tone controls, feedback killer, limiter, DI etc. what really make a preamp any useful. If the T-amp (or any other circuit) does not have them then it is IMO good as nothing - unless you are just looking for to boost up the signal a bit.

I don't think it's a good idea to convert a T-amp to preamp - if I remember correctly that's a digital power amp, right? You can pretty much build a basic gain stage setup around a simple opamp stage with a lot less trouble.

With some simple modifications you can even convert that Till-gain stage to a setup you need ("high impedance per-amp (5 to 10 Mohms input impedance), with an output at line level or electric guitar level"). Naturally, just increase the input impedance. It still has a very low gain (about 2 if I remember right) but if you bypass the JFET's source with a capacitor you can increase it. Basic circuit theory... Now it's up to your pickup how much gain you need to reach a line level (and which one of them ) or pickup level (whatever that is). ...However, IMO that's still just a basic gain circuit and not very useful when compared to a preamplifier with some other features.
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Old 27th March 2007, 09:43 AM   #5
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Quote:
Originally posted by gsdali
It has a piezo soundboard pickup, with an internal preamp, but this does not provide enough boost and the output impedance is still very high.



Is there something wrong with the preamp?, pointless including one if it doesn't give enough gain and has a high output impedance. Usually they have an opamp and run off a 9V battery.

Quote:

This is for live use, and yes feedback is the bane of my existence.
Yes! - I told my daughter I won't do the PA for any more acoustic gigs - use their normal electic bass and guitar instead!.
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Old 27th March 2007, 10:22 AM   #6
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Soundboard pickups are useless for live sound. Completely and utterly useless. No pre or feedback eliminator will solve this.

If you're working with real PA, then you need decent under bridge transducers from Fishman, Ashley, or LR Biggs. With them, the sound engineer at least has a fighting chance of getting some level out of your instrument.
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Old 29th March 2007, 06:15 PM   #7
Minion is offline Minion  Canada
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You might also consider building a Small electret Mic that you can mount close to or inside the sound hole of your Mandolin and then connect the Mic to the Live PA system and get the Phantom powered required for the Mic and any Preamp from the Sound board or use Battery power.....

You can build a very small and stealthy good quality Mic with a $5 Capsule (WM61 or maybe TSB-165) and maybe another $5 in parts that would sound much better than your Peizeo and have a Mich higher Output and Lower output impedance......

I have a simular Mic mounted inside my Clasical Guitar and it really sounds nice, I actually mounted a XLR Jack to the Guitar and have the Mic suspended inside the Body of the Guitar and all of the Electronics fit in a 1/2 inch x 1 in Area and I just plug the Guitar into a Mic Preamp when I need to Play it amplified or record....

Just a suggestion as it works great for me....


Cheers
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Old 1st April 2007, 09:49 AM   #8
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Acoustic instruments fitted with a (suitable) pickup - are best interfaced via a DI Box into the PA system.

If you wish to use a guitar amp - find a DI Box with a jack socket output (buffered, low impedance) - not the usual parallel linked to the input type.

Plenty of information on the following Web site:

Orchid electronics
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