Adding stereo pre-amp out/power-amp in to guitar amplifier

Discussion in 'The Projects Forum' started by jsssl, Sep 28, 2015.

  1. jsssl

    Thread Starter New Member

    Sep 28, 2015
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    I'm looking to make a couple of simple modifications to a guitar amp, adding both a Stereo Preamp Out and Stereo Power Amp In, but have virtually no experience with electronics and could use some help with the wiring. I have schematics for the amplifier and a picture of the completed modifications as posted by a user on an internet forum once upon a time. The poster stated that he snipped resistors R116 and R216 and put the stereo jacks between them. The poster also said, "I split the signal with stereo switch jacks at (before) R116 and R216. At this points (stereo) u can reroute the preamp signal (send) and bypass the preamp section (return)."

    I do know how to solder, and I have the stereo jacks, but I've no clue how to wire it all together. Could anyone have a look and draw up a simple diagram showing which wires to solder to which connection? Would appreciate any help, thanks! Apologies for the file size of the schematic..
     
  2. Lestraveled

    Well-Known Member

    May 19, 2014
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    OK, I looked over your schematic. It took a while to find R116 and R216. (They are on page 8.) These points are just before LM1875 audio power amp.

    I would like to clarify what you want to do.
    - You want to disconnect the signal from the internal amplifier and send it out to an external device. (Is this correct??)
    - Do you also want to take an external signal an put it into the internal amplifier??
     
  3. jsssl

    Thread Starter New Member

    Sep 28, 2015
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    Yes on both. I want an output which, when a plug is inserted, will route the line level signal from the pre-amp out to an external source, bypassing the internal power-amp. Also, an input which, when a plug is inserted, will route the external signal to the internal power amp. With switched jacks, the amp should operate as normal when no plugs are inserted, sending the pre-amp signal through to the power-amp as normal, correct? Appreciate your help, thanks again
     
  4. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    What you describe should work (I've done the same thing in the past), if you can locate the two resistors. Ideally you do not want to cut anything; unsolder one pin of each resistor without damaging the pc board hole, stand them upright, and add wires to the free ends and to the now vacant holes. Be sure to use shielded cable. I squinted at the component locator and couldn't find the two resistors. If you do and point them out, we can figure out which end to lift.

    ak
     
  5. Lestraveled

    Well-Known Member

    May 19, 2014
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    OK, as AK said, lift one end of the resistors as indicated in the photo below. Do not just clip the resistors out. You need them.

    [​IMG]

    Then you will need to get 4, 1/4 audio jacks that have a contact on the center connector. They will have 3 connections. These are not the stereo jacks. If you are not sure about which jacks to buy, ask. Mount and wire per this schematic. PCB means the hole in the PCB. Resistor means the end of the resistor you pulled out of the hole.

    [​IMG]

    Cut the ground wires long and wrap them around the other wires as a shield. Secure the wires with wire ties.
     
    Last edited: Sep 29, 2015
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  6. jsssl

    Thread Starter New Member

    Sep 28, 2015
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    Ok, I'm confused about the 4 audio jacks.

    I'm assuming the 4 jacks are to accommodate left/right in and out, and if that's the case, why can't I just use the 2 stereo switched jacks, which suits my needs better? If there's another reason, what is it? As far as wiring, shouldn't the preamp signal be routed to the preamp out jack, and from there to the poweramp in jack, and then on to the actual poweramp? So, if there is a plug in the preamp out jack, the signal from the preamp will be routed externally. If no plug, the signal travels to the poweramp in jack, and assuming no plug (which should always be the case), will continue on. For the poweramp input, if there is a plug in the jack, the external signal will be routed directly to the poweramp.

    These two input/output jacks will never be used simultaneously. They will be used either a) to route the preamp signal externally (poweramp will not be used), or b) to route an external signal to the internal poweramp (preamp signal not used), but not both at the same time. If there is no plug in either jack, the amp should function normally, sending the preamp signal through to the poweramp.

    I've attached a photo that shows a completed modification with just the 2 stereo jacks and also a pic of the actual PCB layout with the 2 resistors highlighted to show what I believe are the proper ends to remove from the board.

    Also, I'd really like to have a better understanding of this stuff, so would I be correct in assuming that one resistor is tied to the right channel, and the other to the left channel?
     
  7. Lestraveled

    Well-Known Member

    May 19, 2014
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    Sorry, I thought you wanted them separate. I'll post another schematic later. Although it sounds like you could figure it out on your own.
     
  8. Lestraveled

    Well-Known Member

    May 19, 2014
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    OK, there you go. Both outputs on one jack, and inputs on the other. You can use both jacks at the same time with this configuration.

    [​IMG]
     
  9. jsssl

    Thread Starter New Member

    Sep 28, 2015
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    Yes, I probably could have figured it out, but the very real possibility of ruining a perfectly good piece of equipment tends to distort my certainty/confidence in these things!

    One last concern. By wrapping the ground wires, does that mean I'll be fine using unshielded wire for all connections? Would there be any benefit to twisting together each pair of wires coming from the board to provide shielding? And, should the ground wires connect to the chassis ground?

    Thanks again to the both of you for your help!
     
  10. InspectorGadget

    Active Member

    Nov 5, 2010
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    I don't think you want to wrap the 2 wires coming from the board. This would tend to couple the input and output signals and might bleed pre-effects audio into your post-effects audio output. If you want to shield them, wrap each around ground wires coming back from your plug toward the PCB but end the ground wires without terminating them on the PCB.
     
  11. Lestraveled

    Well-Known Member

    May 19, 2014
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    These signals appear to be high level so shielded cable probably is not needed. Also the impedance is 10K both in source and input, so I would not expect to get much cross coupling over a short run. Peavey makes very prudent choices in their designs. Don't make your mod too complicated. 2 or 3 turns per inch of the ground wire should be an adequate shield.

    Please post how it turns out.
     
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