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Electronic guys & gals! - I need help with a preamp/amp/cab switcher project!

Discussion in 'Amps and Cabs [BG]' started by IvanMike, May 19, 2005.

  1. IvanMike

    IvanMike Player Characters fear me... Supporting Member

    Nov 10, 2002
    Middletown CT, USA
    Hey there

    You might recall me asking about the availability of the nifty amp/cabinet switcher made by ampeg. If you've never seen one, they're really quite cool. IIRC, they're a 2 space rackmount unit, on the front of which are two 8 position rotary knob switches (kind of like the selectors on some home stereo receivers for selecting between radio/tape/cd, etc.). Anyhow, you plug the speaker output of up to eight amplifiers into the back, and run speaker cords to up to eight seperate speaker cabinets from the back as well. by turning the first knob you select which amp you want to hear (you have to plug your bass into this amp yourself), and by turning the second knob you select which cabinet you want to power with that amp. a very cool device. unfortunately ampeg only makes these available to ampeg dealers.

    when i was looking for something similar jmx gave me a link to a similar device.
    looks cool, but i think i'd like to build one myself with a bit more connectivity and options.

    essentially, i'd like to build a device for my own use to do quick comparisons of preamps, amps, cabs, basses, etc. If possible I'd liek it to be a rackmount unit, and with the type of connectivity i'm looking for, It would probably have to be 3 or 4 rack spaces high. I'd like to have 3 or four rotary switches like the ampeg has, so i want to know where to find switches like these, some of which are good for speaker level signals, and some of which are good for instrument and line level signals.

    Anyhow, the idea would be to be able to plug 8 speakers into the back and be able to select between them with one rotary control. Then I'd like to be able to plug the speaker outputs of 8 amps into it (including at least one poweramp) and select between those with a second rotary switch. This should be ok for solid state amps, (as they don't need a load), but a unit like this would need to run all of the unselected amps into a dummy load if you used amps with tube power sections with it (i wonder if ampeg's unit wasn't ok to use with the tube svts?)

    Then, to improve upon the ampeg unit, I'd like to be able to plug the output from a bunch of preamps into it, and use a third rotary control to select which preamp's signal i sent to the poweramp's input. I'm assuming i might need to use a different switch for that, or will the same type of switch be ok for the line level signal? I'm also assuming that running the remaining preamps into nothing would be ok, (even if they still had a bass signal going into them), or would that somehow damage a preamp if it had a cord running out of it that terminated into infinite resistance?

    To make the unit really cool, it would be neat to have the switch that controlled which preamp's output was being used select the same preamp's input at the same time (maybe if there were two rotary swiches mounted on the same pole?), so i could have a bass plugged into the unit itself and by turning one knob i could select agiven preamp, running the signal from my bass to its input and running its output to the poweramp all at once. If no such swich exists, then i suppose i could have a fourth rotark switch that selected what preamp's input i ran my bass into rather than having to keep unplugging and plugging my bass (or using an external a/b box).

    On that same thought, it would probably be cool to have yet another input for the bass and rotary knob that would switch between the inputs of the various amp heads plugged into the unit as well.

    OF course, if i wanted to get really crazy i could have a bunch of inputs for basses on the front too, but i think that's going a bit overboard. :p

    the whole idea is to be able to do quick comparisons of preamps, amps, and cabs. I've found that the longer you spend between listens, the less able you are to make good comparisons.

    my question? where do i get the kind of switches i'm looking for? and are there in fact multi position switches that would be able to switch two things at once (like the input and output of a preamp)?????? I suppose i should ask where a good place to get the chassis for a rackmount unit is too.
  2. Passinwind

    Passinwind I know nothing. Commercial User

    Dec 3, 2003
    Columbia River Gorge, WA.
    Owner/Designer &Toaster Tech Passinwind Electronics
    Yeah, you can get multi-ganged rotary switches, but high current ones will not be cheap unless you can find a surplus vendor. eBay's not a bad place to start looking. Beware of make-before-break ones unless you're positive you need them to work that way.

    I'd look at using high power relays for the speaker/amp side, and smaller ones for the line level switching. Then you can use any old rotary or standard double throw switches to control the relays, and have each switch control how ever many relays you need it to. The trick in doing this is to make it fail-safe, so for instance two amp power sections can't inadvertantly be shorted together. That's doable, but it takes some clever thinking to get there. Time delayed relays are often a key consideration here, in my experience.

    As far as loads for the tube amps, a simple short circuiting jumper may suffice in most cases, especially if you make sure no input signal can happen unless the amp is active in the test matrix.

    Speaking of the matrix, I personally would go for one of those spiffy computer driven relay interfaces. Then you could program all sorts of logical permutations, and you maybe wouldn't be rewiring things nearly as often as your needs change. I'll see if I can find any of my old notes from my last job, where I had to design some robotic motor switching stuff. What's your realistic budget target?

    I like the Sescom heavy duty rack cases pretty well: www.sescom.com . Also try Mouser and Digikey.

    Or you could just trade an Ampeg dealer some hookers 'n snow for one of theirs. (ducking) :cool:
  3. BillyB_from_LZ

    BillyB_from_LZ Supporting Member

    Sep 7, 2000
    The real-live Ampeg ones come up on eBay every once in a while. IIRC at least one of the amp inputs is tailored to tube amps (as the SVT-CL is connected).

    Maybe Jerrold would be willing to help you acquire some vendor name/part numbers for the switches and perhaps a schematic for the whole unit.

    I imagine that an easy way to do all of the switching would be with an industrial PLC (programmable logic controller) or maybe even a program written for your PC or laptop that controls a relay driver board and a bunch of relays.
  4. geshel


    Oct 2, 2001
    You'll also want it so that there's no "pop" when you switch preamps. You can get switches that have less switching-noise, or another option is that you somehow mute the speaker output during the switch.
  5. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    The Ampeg one is pretty simple, it's not really meant for quick switch A/B testing of anything and everything. It isn't set up to match levels, etc, etc.

    It's more intended to be a sales floor tool, demo any of these with any of those.....

    I would suggest if you want to make one, to keep it simple, 2 or 3 at most at a time.
    Any more gets confusing anyway.*

    There was another versiion, that was made for Audio Centron, which was really set up for selecting two different speakers, and then doing an A/B switch between them.

    Even it didn't do volume matching. Again, largely a floor sales tool.

    * Not only will more get confusing, but with more than a couple or three speakers they normally can't be put in similar "acoustic" positions with respect to the walls etc. If you do manage that, then they are probably in different places for the player/listener.
    So any way you slice it, you get "less good" results with too many choices at once.
  6. tombowlus

    tombowlus If it sounds good, it is good Gold Supporting Member

    Apr 3, 2003
    North central Ohio
    Editor-in-Chief, Bass Gear Magazine
    John Files has something similar set up at Bass Emporium. I don't recall seeing a brand name on it, and I think that John mentioned that he had either made it, or had someone make it for him. He had just about every amp and every cab hooked into it (at least 8 amps, and probably well over 20 cabs). I found this to be a very effective tool for quickly auditioning a variety of cabs, but I agree with Jerrold that it is more of rough comparison tool, rather than a finely tuned diagnostic device.

    That said, if you (or someone else) were to make a similar unit, being the gear freak that I am, I might be interested in buying one off of whomever made it... :p

  7. geshel


    Oct 2, 2001
    The real trick would be to make one that adjusted for different gains in the pre- and power-amp sections, on the fly.

    If you made something like this electronically you could do it - before switching to the next preamp or power amp it would send a test pulse through and compare the input and output, then normalize the gain (to whatever the current "master volume" setting is).
  8. Matt Call

    Matt Call Supporting Member

    Aug 1, 2004
    Minneapolis, MN
    I've seen one of the Ampeg ones for sale in the last two weeks. I think it was at a local music store.
  9. IvanMike

    IvanMike Player Characters fear me... Supporting Member

    Nov 10, 2002
    Middletown CT, USA
    well gee, find out for me where it is and let me know.........

    thanks for the replies so far. i'm aware of the limitations of the ampeg design, but honestly that's all i'm looking for in terms of accuracy. anything that would be quicker than plugging and unplugging for comparison would be cool. having used the ampeg units man a time back in the day i found them to be good enough for my purposes.
  10. Matt Call

    Matt Call Supporting Member

    Aug 1, 2004
    Minneapolis, MN
    I'll see what I can do. It's not really "local" per se, it was in Minneapolis. I'm probably going to be going up to play a few things, I'll keep you posted.
  11. 44me


    Jun 17, 2002
    Bedford, NH USA
    Power rotary switches would be simpler to configure, but relays will give you more flexibility for switching amp outputs and speakers, and will be much easier to find with suitable ratings. Use a double pole, double throw on the output of each amp to connect to either a ‘speaker bus’ or a dummy load (switch both output leads to keep everything isolated). Use more of the same relays to connect each speaker to the ‘speaker bus’. The dummy loads can simply be 4 ohm, 10W resistors as long as you don’t run significant output level when the dummy load is connected.

    As Passinwind’s pointed out, the tricky part is driving the relays to ensure that you don’t inadvertently connect two amp outputs together. To keep things simple you’ll probably want to use signal level rotary switches to drive the relay coils. Some relays open much slower than they close, so you’ll want to pay attention this spec. Often a clamp diode is connected across a relay coil to provide a path for the inductive energy flow when the coil is turned off (making life much easier for your signal level rotary switch), but this will greatly increase the opening time. Adding a zener diode in series with the clamp diode with a rating at least as high as the relay supply voltage will eliminate this problem.

    It’s OK to leave unused preamp outputs open, but it’s likely you are going to have problems with pops when you switch them in. A simple way to take care of this would be to disconnect all speakers before you switch a preamp output. This requirement makes computer control attractive, but things can get complicated fast.
  12. IvanMike

    IvanMike Player Characters fear me... Supporting Member

    Nov 10, 2002
    Middletown CT, USA
    so just for kicks, if i go looking for one of these on e-bay is there a model # or something i should look for????

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