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Not an amp, but building my own A/B/Y switch to switch between 2 amps. Couple ?'s

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  • Not an amp, but building my own A/B/Y switch to switch between 2 amps. Couple ?'s

    First, sorry if this belongs in "Theory and Design" or somewhere else.

    I built a buffered AB switch so that I can use two small amps for practicing and still have 2 channels like my main amp. It works well and is very transparent sounding but as expected the cable run between Amp A, the Device, and Amp B acts as an antenna because everything shares the same ground inside the pedal. The problem can be corrected well enough to enjoy the device by laying out the cables judiciously but it is still annoying. A couple of my friends have expressed interest in having one of their own but I want to work this out before I release one into the wild. Here's the schematic of what I have built so far:
    Click image for larger version

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    I am looking for an isolation transformer that I can use on one of the outputs to avoid this. Would something like this do the job?
    Also, would placing the switches after the output buffers rather than before them have any impact on the radio reception or noise? My thinking was that connecting the signal lead to ground for the output that isn't selected would prevent that amp from picking up radio.

  • #2
    The isolation transformer is a good idea.
    I Googled 'A B switch ground isolation' and came up with this site.
    Link: http://www.geofex.com/FX_images/humfree2.gif

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    • #3
      I just posted on making a version of the Jensen 4-way guitar splitter. In its original form (at Jensen Transformers) it uses 3 $70 isolation transformers and a $35 op amp
      http://www.jensen-transformers.com/as/as014.pdf
      BUT you can bring it in under $100 by using Edcor 10K:10K matching transformers (EDCOR - PCW10K/10K) and a cheaper LF356N op amp

      RG's design is great and inexpensive, while the Jensen design contains a high cut control and should have better frequency response.

      Note the Jensen is a Y technically but with switched output its a A B C D etc

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      • #4
        I haven't noticed any hiss or hum from mine, just the radio problem. The $2 TI LM324 in there does a pretty decent job.

        Both of these examples use 10K:10K transformers on the outputs, so I think I might shop around for ones with the best frequency response. Fitting them inside the stomp box enclosure I used might be tricky, I might just end up building a separate passive box for them.

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        • #5
          RG's transformer is way cheap, but the Edcor PC10K/10K ($6.37) has a better freq response, essentially 20-20kHz, and is quite small at 1.312" x 1.625" x 0.635"

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          • #6
            There's no way you should get radio interference. Have you scoped the signals - oscillaton is more likely, due to capacitive loading on the op amps, from the cables. Small series resistor (~1k - 2k2) on each output acts to isolate the op amp from the cable capacitence. See if that helps.
            Also the 10uF output coupling caps need a resistor (~10k) to ground after, to prevent any leakage current affecting the bias of the amp they're plugged into.
            However, an AB box doesn't need buffers, unless isolating transformers or super long cable runs are used; it can just be passive changeover switch.
            Pete.
            My band:- http://www.youtube.com/user/RedwingBand

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            • #7
              Have you seen this? ampselector
              Looking through the pictures in the instructions, the part number is visible. Looks like it's an Xicon 42TM018-RC, about 2.50 from Mouser.
              -Mike

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              • #8
                cool kit,

                general guitar gadgets makes a kit for RGs design,
                Amp A/B/Y Switcher Box
                maybe some of the layout there would help noise

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