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dataplex 5th January 2013 05:43 PM

LM1876 as part of an electromagnetic actuator

i want to make an array of super ebows for an instrument i'm making.

in the past ive been using LM386s, but they have proven to be too weak and/or distort too much.

the LM1876 seems like a good choice to go for the new instrument, and i've been doing experiments with gainclone schematics (actually haven't been working properly for me yet... doh!!)

however, i was wondering if these Hi-Fi amp schematics might be a little too complicated for this kind of application.

what i want to do is amplify a signal from a piezo - or magnetic pickup - and drive a solenoid actuator positioned under a string - causing it to vibrate/feedback.

would you guys be able to help me with sorting out a more simple schematic for this kind of application? or at least throw some ideas at me?

i have a 100va 2x 18v transformer, and some LM1876s. what do you think?

all the best,

DUG 5th January 2013 06:29 PM

Solenoid loading vs peak current from the amplifier is one thing to consider.

Solenoid impedance would be another.

There were some ap notes somewhere in the APEX Micro site that addressed reactive loading and SOA for amplifiers...Might want to take a look at that:

Apex Application Note Library - Apex Microtechnology

dataplex 5th January 2013 06:40 PM

hey, thanks!

Electromagnet Catalog - Magnetic Sensor Systems - MSS Electromagnets

The model I will be using is E-77-82, 28 AWG wire. i know andrew macpherson uses these coils with the LM1876 for his magnetic resonator piano - Magnetic Resonator Piano | Music . Entertainment . Technology

dataplex 5th January 2013 08:32 PM

any other thoughts on the schematic?

dataplex 7th January 2013 09:21 AM


JMFahey 7th January 2013 12:52 PM

I think you are overstressing everything, both amp and solenoid.
Your 2x18V transformer will provide 2x25V rails, way too much considering you will be driving what practically amounts to a pure inductor.
Your LM1876 will not like it very much.

And that solenoid is factory rated for 1.25W (using DC current).
Don't know what frequencies you will use, (nor the inductance), so can't calculate current through that coil, but the potential exists to fry that solenoid.
Yours is an inusual application, so I guess you won't get much input easily from others, but I suggest you build a normal, by the book, 1875 circuit, load it with a solenoid, and sweep it at different voltage levels (to know what's *really* necessary) and watch for overheating, measure current (insert a 0.1 ohm resistor in series and scope it), etc.
As a side note: even if the driving frequency clips, it's not bad, the string will pick the fundamental anyway .
Note 2: place the solenoid close to the string and check how much power is really needed to make it vibrate.

dataplex 7th January 2013 01:31 PM

hey jim, thanks for the reply! your insight is very valuable!

driving my strings with clear harmonics, full frequency range is optimal. however, i could probably do fine with something like 2000 - 14.000hz. im not sure what inductance means (sorry, im looking it up right now!) but the coil resistance of the solenoids i bought will be a little under 8 ohms.

i need significally more power than the LM386 i used prior to this design... today i made some progress - i actually got the LM1876 chip working with a custom handwound 8 ohm solenoid. the output using my iphone music jack is perfect, but the coil was heating up A LOT. might have to heatsink the manufactured coils when they arrive at my studio.

one question that i need help with -

is there a preamp schematic you would suggest i could build to pre-amping a signal from a piezo, /and or magnetic pickups for the LM1876?

thank you!

tauro0221 7th January 2013 02:17 PM

Why you do not use the LM388? It have a lot of power to drive your 8 ohm coil.

tauro0221 7th January 2013 02:18 PM

Sorry I should said LM3886 from National.

JMFahey 7th January 2013 04:06 PM


however, i could probably do fine with something like 2000 - 14.000hz.
Are you making musical instruments for bats?:eek:
Are you sure about those frequencies?
As an example, guitar strings vibrate (fundamentals) between 80 and 1200/1500Hz


but the coil was heating up A LOT.
Not surprising, I think you must re-evaluate what you are doing.
The very successful EBow which I think you are trying to emulate, is a hand held device powered by a 9V battery, must use a few milliwatts at most.:
The Amazing EBow :: Home


might have to heatsink the manufactured coils when they arrive at my studio.
I think you must re-evaluate what you are doing.[2]

Yes, I might suggest a preamp, please be a little more specific about your instrument so I can guess what the signal levels will be.
And please check the frequency range. :)

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