Project : Mono Preamp

Discussion in 'The Completed Projects Collection' started by Wendy, Nov 24, 2013.

  1. Wendy

    Thread Starter Moderator

    Mar 24, 2008
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    [​IMG]

    I've been tagged (volunteered actually) to assist a group with their sound system. Boom boxes with removable speakers are good for this, but sometimes you need a bit of amplification between the wireless mic receiver and the main amplifier.

    This design has a variable gain between 1 to just over 20. I measured a max gain of 21.7. I made the lid using a 3D printer, which was a luxury. You can adapt the dimensions given to a conventional lid. For those lucky souls who have access to a 3D printer I included the .stl file.

    The frequency response was flat up to 100Khz, and I saw low role off around 20 Hz (theoretical is 16Hz).

    Here is the breadboard and schematic:

    [​IMG]

    The parts not on the parts list were:

    Project Box - BG Micro Part Number ACS1157 ($2.01)
    Wall Wart DC Power supply - 9VDC to 18VDC
    Matching Power Input Jack for wall wart
    U1 - NJM4565 (I suspect almost any decent dual op amp will work, but this one is designed for low noise).

    This project is very low power, almost any DC wall wart in the voltage range will work. Do make sure the wall wart is completely isolated from the AC line. I have made a prototype before this one shown which will be recycled, and will make 2 more before I am done. I was aiming for maximum simplicity as well as performance.

    .
     
    Last edited: Dec 2, 2013
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  2. Dr.killjoy

    Well-Known Member

    Apr 28, 2013
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    Looks all good over there....



    Really good job.
     
  3. Wendy

    Thread Starter Moderator

    Mar 24, 2008
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    OK, I built S/N 002, shown below:

    [​IMG]

    For the halibut I put in a 1458, which is a dual op amp only slightly better than a 741. The amazing thing to me is it worked at all, but to give you a comparison I took 2 pictures, both at 10Khz and just shy of clipping. I used a square wave because it is loaded with harmonics and is a good test waveform.

    1st, the 1458
    [​IMG]

    Next, the JCR4565
    [​IMG]

    The high frequency roll off is immediately obvious. To anyone who knows both op amps this comes as no surprise, but I thought the new folks might find it interesting.

    .
     
  4. cumesoftware

    Senior Member

    Apr 27, 2007
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    Hi Bill,

    Nice amp! May I suggest the LM4562 (aka LME49720)? It has very low noise and THD (0.00003%), so it fits well into your application.

    Regards, Samuel Lourenço
     
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  5. bertus

    Administrator

    Apr 5, 2008
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    Hello, Bill,

    Also the LM833 or NE5532 might be used in your preamp.

    Bertus
     
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  6. Wendy

    Thread Starter Moderator

    Mar 24, 2008
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    Funny thing, I got the JCR4565 as samples from Tanner's electronics. I've bought more since then. I have some other projects I'm going to audition them in.
     
  7. Markd77

    Senior Member

    Sep 7, 2009
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    Have you got some of the capacitors numbered differently on the schematic to the parts list?
     
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  8. Wendy

    Thread Starter Moderator

    Mar 24, 2008
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    Yes, I do. I'll fix it tonight when I have more time. Thanks.

    _________________________________________________

    Done.
     
    Last edited: Dec 2, 2013
  9. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Maybe a metal box would be a good idea too Bill?

    With a large signal input the EMI noise etc will be low compared to the signal, but it's possible someone might want to use this with a low output guitar or mic, where a shielded box would be of benefit.
     
  10. Dr.killjoy

    Well-Known Member

    Apr 28, 2013
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    In my opinion the white face looks better off .. Also I would move the power plug to the sides or top and bottom in order to keep the face less cluttered and make it easier to adjust if needed to ..

    But keep up the Great work
     
  11. Wendy

    Thread Starter Moderator

    Mar 24, 2008
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    Whoever needs that application could definitely use a metal box, I was after a decent look and good performance. If you want to pick up AC hum just park your stuff too close to the Oscope though.

    I told my friend at the time I wasn't too worried about the plastic color. I tried very hard to make the labels as part of the lid, but the software (not the printer) wasn't up to it. No big deal though, these suckers are usually parked in back of the amplifier.

    Did I mention one of my near future projects is a 100W class D amp?
     
  12. Dr.killjoy

    Well-Known Member

    Apr 28, 2013
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    Can't wait for that ...:D:D
     
  13. arenwi

    Member

    Dec 29, 2014
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    Hi Bill,

    I had a post asking for solution for an amplifier and it cames to heare, many tahnks

    About your schematic. I found the virtual ground circuit in the midle of the schematics.
    The left circuit is the amplifier and you only use the virtual ground to reference for the imput and output?? nothing more in the circuit?? The rest of the circuit are with the normal grouns??

    Ok I´ve never test the ground of the microphone and output conected to the virtual ground. It sound a solution of mi problem¡¡¡¡

    I will tell something
     
  14. Wendy

    Thread Starter Moderator

    Mar 24, 2008
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    A virtual ground created by the op amp as shown is very low impedance. It can not take a lot of current, but to a signal both its AC and DC characteristics are much less than 1Ω. This makes it ideal to use as a ground in a AC amplifier.

    Note that R3 is going to virtual ground, and will allow the op amp to use the configurations we all know and love, and can predict with great accuracy. You want 600Ω input? Make R3 600Ω and that is what it is.
     
  15. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    Great circuit.... question, shouldn't R6 be 470 Ω instead of 4.7K ?
    Won't the LED burn too dimly with R6 being that high?
    EDIT: never mind... I just noticed the 9-20V allowed power input range... I'm too used to working at 5V TTL ...
     
  16. Wendy

    Thread Starter Moderator

    Mar 24, 2008
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    Actually, for a lot of these circuits dim is good. Modern LEDs can be bright, and overwhelm everything around them, when all you really want is notice that there is power to the gadget.
     
  17. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    Right... now I recall that most musical instruments amps have LEDs that burn dimmer than circuits used for other purposes... wonder if it's aesthetics, since bands play mostly in the dark at night clubs and the like...
     
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