simple mono guitar preamp

Discussion in 'General Electronics Chat' started by maggie, Aug 15, 2011.

  1. maggie

    Thread Starter New Member

    Aug 15, 2011
    13
    0
    I would appreciate any helpful comments, i'm trying to teach myself enough electronics to build or hack a small amplifier for my guitar and acoustic instruments. I made one of the internet project 386 opamps. and I hacked in to an old portable radio to make another, but i need a preamp for my quiet instruments. I tried the combo op-power amp from forrest Mimms' Radio Shack book, but couldn't make it work, I tried the 741 project that I saw on the internet,and it worked on the breadboard but not after I attacked it with a soldering iron. i saw many comments about the crappiness of the 741, and some alternate suggestions, like theTDA2822, but that is stereo, so would that also work for a mono preamp? Someone said the LM386 was a battery eater, and sounded like a clock radio. For some applications, I like that crude funky sound, but not for everything. I guess I'm looking for suggestions alternatives to the 741, and the 386 opamps, but must be equally simple and easy to assemble, that would sound good in a large room or maybe a coffee house. I would appreciate any comments, thanks.
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
    Hello,

    Do not expect much from battery powered amplifiers.

    Here is a page from the EDUCYPEDIA with some links to audio amplifier circuits:
    Audio amplifiers

    Bertus
     
    Last edited: Mar 15, 2014
  3. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The amount of amplifier power into a fairly large speaker (8") that is in an enclosure designed for it determines loudness.
    Small speakers sound like headphones playing while laying on a table, small squeaks.
    An LM386 power amplifier has an output of only 0.4W when driving a fairly large 8 ohm, speaker which is about the same loudness as a cheap clock radio.

    Years ago there were many car radio power amplifier ICs that produced 14W into 4 ohm speaker when the battery was charging at 13.8V but many of those ICs are not made anymore.

    A guitar pickup sounds best when it drives a preamp that has a very high input impedance like this one: (see my attachment). The preamp drives the power amp.
     
    Last edited: Aug 15, 2011
  4. maggie

    Thread Starter New Member

    Aug 15, 2011
    13
    0
    thanks, guys for responding. Bertus, that is an awesome link, I will look it over. AudioGuru, I didn't get the attatchment that you referred to. Thanks for the input. I guess at my level of inexperience, I'm not ready to tackle tubes and high voltage complicated stuff. My general idea was to use piezo rod for pickups and a 3 or 4 inch speaker. The one I finished, is audible in my living room, and for playing the down home bluesy things, it has a retro, home- made raspy sound I kind of like. But for celtic and bluegrass and other things, not so much. Thanks again, you guys are great.
     
  5. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The preamp circuit I posted is eaxtremely simple. It uses a field-effect transistor, not a vacuum tube but they sound similar. It is a preamp for a normal magnetic guitar pickup but might work well with a piezo pickup.

    A 3" speaker is used in a cheap clock radio and sounds awful. The 3" speaker drivers in my pc speakers have rubber surrounds, big heavy magnets and sound pretty good. I have heard a few good-sounding 4" speakers.
     
  6. maggie

    Thread Starter New Member

    Aug 15, 2011
    13
    0
    Audioguru
    Do you know where you posted the preamp or how I can search for it?

    And instead of the 386 for power amp, you suggest a replacement with way more watts ( 2.5w-7w big enough?) and a 4" to 8" speaker for what I'm trying to do?
    thx
     
  7. MrChips

    Moderator

    Oct 2, 2009
    12,446
    3,362
    If you want an acoustic guitar to sound great in a coffee house you need a lot more than an LM386. To bring out a deep mellow tone you need good bass response which means a decent sized speaker and a powerful amp. I would go with at least a 10" or 12" speaker and about 100W. I use an automobile stereo amp. The left and right channels are bridged to drive the one speaker for more power. For preamp I use a TL071. My whole amp is powered by a 12V sealed lead acid battery so the amp is portable. Nice to take to camp or cottage country and I sit around the camp fire and entertain the whole crowd. I play classical, jazz and bass. I'll post some pictures of my amp later.
     
    maggie likes this.
  8. #12

    Expert

    Nov 30, 2010
    16,310
    6,817
    Here's a simple preamp that I made for a Rhodes mechanical piano in the year 2000.
    Excellent impedance match for an electric guitar, undetectable noise, lasts hundreds of hours on a rectangular 9V battery. The power switching is up to you to design.
     
    maggie likes this.
  9. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    I am sorry. Now it is attached to post #3.
     
  10. maggie

    Thread Starter New Member

    Aug 15, 2011
    13
    0
    Aha!--thank you.
     
  11. MrChips

    Moderator

    Oct 2, 2009
    12,446
    3,362
    Here is my homemade battery operated portable guitar amp and bass to show scale.
    The speaker is 12" 450W.

    [​IMG]


    Top of the amp showing controls.
    [​IMG]


    Inside the amp showing the 12V battery and car stereo amp:
    [​IMG]
     
  12. maggie

    Thread Starter New Member

    Aug 15, 2011
    13
    0
    Mr. Chips-I am just learning, hence the questions about old op amps. I am curious, what level of expertise is needed to make this from a car radio. Is it a matter of finding the input/output and hooking up jacks/speakers? Is it simple to match the amp power to the speaker? Thus far I have only built projects that provided a parts list, so nothing to calculate.
     
  13. #12

    Expert

    Nov 30, 2010
    16,310
    6,817
    From an old guys point of view, it's childs play to use a car amplifier to run a guitar speaker. I whipped one up in about 30 minutes when I needed a practice amp for a new band. I used an old piece of one by six (wood) for the base.

    Car amps are often designed to drive 4 ohm speakers, so the usual 8 ohm speaker will give it no problems. The amp I found in my junque box used "line level" input which is about 1 volt, and a guitar can do that, no problem. I used a Radio Shack 13.8 volt, 3 amp power supply and was plugged in after about 30 minutes of making connections.
     
  14. MrChips

    Moderator

    Oct 2, 2009
    12,446
    3,362
    You have to begin somewhere. How handy are you with tools, a hammer, wrench, screwdriver? When I was 19 years I built a vinyl turntable, quadraphonic amp and four speakers partly from kits. I didn't have access to a power saw so I got the lumber yard to cut all my wood panels for the speakers exactly to size. Everything else just required careful planning.

    [​IMG]

    Connoisseur turntable built from a kit.


    [​IMG]

    The speaker uses a 3-way cross-over to drive three speakers, tweeter, mid-range and 12-inch woofer.

    [​IMG]

    The quad amplifier uses preassembled Sinclair modules. I had to design and build the power supply and a time delay relay to switch in the speaker outputs.

    Most of the hard work is in planning the physical layout and construction.

    (How did I finance my hobbies at 19? I was a student from overseas. It was summer between my 1st and 2nd year at university. I worked on the farms and saved my pocket money to pay for my projects.)

    For the guitar amp, most the components were bought at a surplus store - 12" grill, corner protectors, cabinet handle and felt covering. Speaker, auto amp, battery are standard off the shelf. Matching the power rating of the amp and speaker is not important. It just means that if one is much more than the other you would be wasting some money. Each channel of the stereo amp is capable of driving 4-ohm speakers. I made sure that the speaker was 8-ohm and that the amp was capable of being bridged, that is, one channel output goes to one connection on the speaker and the other channel goes to the other connection. There must be a switch on the amp that puts one channel 180-degrees out of phase with the other.

    Note in the photo that the signal from the preamp/controls is just an RCA audio cable into the amp. The only thing I had to do is open the amp to tap off +15V and -15V to power the preamp. You can use the FET preamp powered by a 9V battery as suggested by others.

    If you are looking at building your own circuits there are lots to be found at Elliot Sound Products:

    http://sound.westhost.com/
     
    Last edited: Aug 16, 2011
  15. maggie

    Thread Starter New Member

    Aug 15, 2011
    13
    0
    Thanks#12and Mr chips- this car stereo thing might be do-able for me, if the pre-amp will handle my piezo pickups. it seems fairly simple. Not sure about the bridging 2 channels but I'll figure it out.
    For input can i just solder a jack to the volume pot tabs? I already have a cabinet with an 8 ohm speaker in it, waiting for me to re-purpose it into an amplifier.
     
  16. MrChips

    Moderator

    Oct 2, 2009
    12,446
    3,362
    Here is what is meant by bridged outputs:

    [​IMG]

    In the upper diagram, the two channels would produce the same signal at the output and there would be no sound from the loudspeaker.

    In the lower diagram, one of the channels is inverted, i.e. out of phase with the other channel. This doubles the output to the loudspeaker. If the car stereo amp can be bridged there will be a switch on the amp to allow you to achieve this.

    The other benefit of bridging the outputs is that the large value output capacitor to the speaker is eliminated, assuming that the amps are properly designed to give zero DC outputs, i.e. they must be perfectly balanced.
     
  17. #12

    Expert

    Nov 30, 2010
    16,310
    6,817
    I have to say you're using your head. If you don't have an input jack on the car amp, sneaking in at the volume control is a pretty good way to do it. Just be attentive to grounds/shields so you don't pick up too much hum.
     
  18. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    When the output voltage swing is doubled then the output power is almost quadrupled (about 3.5 times) because then the output current is also almost doubled. Then an amplifier that produces 4W into 4 ohms can produce 14W when two of them are bridged.
     
  19. maggie

    Thread Starter New Member

    Aug 15, 2011
    13
    0
    I stole the volume pot idea from a youtube video.
    I think I understand bridging, not too sure if I can do it. If it means soldering 2 leads into one, for each channel, then hooking those 2 up to the speaker, I think I can do that once I figure out which where the channel outs are.
    This is the greatest website ever. The videos and worksheets here are amazing, and you all are so nice.
     
  20. maggie

    Thread Starter New Member

    Aug 15, 2011
    13
    0
    Wondering if I can substitute another FET for the J201, and if so, still use exact same schematic for the preamp??
     
Loading...