Power amp question

Discussion in 'Amps and Cabs [BG]' started by Troglodyte1911, Sep 27, 2022.

  1. Troglodyte1911

    Troglodyte1911 Supporting Member

    Oct 20, 2020
    I salvaged an Extron XPA 2001-70V 200W power amp from a conference room on the off chance I could use it with a pre-amp to drive a cabinet. Do any of you with more knowledge know if this could be used for that? I don’t think they make an instrument cable that would connect to the green output, but maybe something custom could be made. Before I recycle it I thought I would ask.

    Attached Files:

  2. bholder

    bholder Affable Sociopath Gold Supporting Member Supporting Member

    Sep 2, 2001
    Vestal, NY
    Received a gift from Sire* (see sig)
    Power amp is power amp, ought to be workable. Have you tried to look up the specs? The important ones are power and accepted output impedance range.
    Killing Floor likes this.
  3. bholder

    bholder Affable Sociopath Gold Supporting Member Supporting Member

    Sep 2, 2001
    Vestal, NY
    Received a gift from Sire* (see sig)
    Killing Floor likes this.
  4. Troglodyte1911

    Troglodyte1911 Supporting Member

    Oct 20, 2020
    Thanks for looking in to it. That was one of the things I don’t know. Checked an ohms calculator and it looked like it would be around 25 ohms, but don’t know if it would work or damage a cab.
  5. bholder

    bholder Affable Sociopath Gold Supporting Member Supporting Member

    Sep 2, 2001
    Vestal, NY
    Received a gift from Sire* (see sig)
    Well I'm sure someone here knows the implications, it's early yet, give it a bit of time. ;) Just beyond my depth, sound-tech-wise...
  6. basscooker

    basscooker Commercial User

    Apr 11, 2010
    Northern KY
    Cab fan, hobbyist
    Low quality audio, never really designed or intended for much more than like background music and announcements. Probably sound meh at best. Agreed it will amplify, but at what bandwidth?

    I didn't check the manual, but I do know 70v amps aren't used in MI or consumer audio for a reason.
  7. What ^ said.

    Do you want to hook up your bass to the Airport's Announcements or Grocery store's PA?
  8. Troglodyte1911

    Troglodyte1911 Supporting Member

    Oct 20, 2020
    Lol, off to be recycled!
  9. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    No, it can’t be used because the 70 volt line is designed to drive much higher impedances (on the order of ~200 ohms for 200 watts output). An 8 ohm load would look like a short circuit to the amp.
  10. This is not the amp you're looking for.
    70 volt outputs is a speaker distribution scheme used mainly for PA announcement or paging systems. Think offices, schools, hospitals, other large areas. "Cleanup in aisle 6."
    They require a special transformer at each speaker in the system to transform to the speaker's nominal impedance. These systems require careful planning, and setting the right taps on transformers to work properly. Audio quality may not be that great since the primary use is for voice. Low end will be lacking.
  11. bholder

    bholder Affable Sociopath Gold Supporting Member Supporting Member

    Sep 2, 2001
    Vestal, NY
    Received a gift from Sire* (see sig)
    Might make for an interesting "Clean Up On Aisle 9" or "Attention S-Mart Shoppers" effect. But nah hahaha.
  12. Killing Floor

    Killing Floor Supporting Member

    Feb 7, 2020
    Austin, TX
    I mean, kinda! The red zone is for …boink dink dink bum bum bum…
    I would be down.
    S-Bigbottom likes this.
  13. Distributed public address systems, like in school where there is one speaker in each classroom.

    I am not sure how to best use amps like these for live music amplification. My suspicion is it will be under-powered as a bass amp.
  14. I once had access to a college football stadium sound system. It was a modern system that included a mixer. The project was to play the studio radio station on the stadium PA for a special event on campus. My job was to do the audio interface to the PA. I was so temped to grab a D.I. and my bass and see how it did.
    Bad enough that teachers in classrooms near the stadium complained about the sound level of the student station.
    Can't imagine the pushback given my level of bassmanship (or poor resemblance thereof).
    S-Bigbottom and Killing Floor like this.
  15. 1958Bassman


    Oct 20, 2007
    There's no reason to assume that a 70V/100V power amp isn't capable of decent sound, but you absolutely don't want to connect it to speakers that don't have a transformer to decrease the voltage. This type of amp is for 'constant voltage systems', which are designed to overcome long speaker wire distances and because the voltage loss is overcome in this way, smaller wire can be used. The concept is similar to power transmission- the lines on high towers is extremely high, which is to help overcome losses over distance, then the voltage is decreased through the local transformers that distribute the power to the facilities that need it- the voltage on high tension lines is often more than 26,000VAC.

    The spec sheets show that this amplifier is only rated for 70V/100V systems, not 4 Ohm or 8 Ohm loads.

    If you look at the details of their equipment, you'll see speaker frequency response rated 58Hz-22KHz- that's not what a typical paging speaker is going to do, it's for auditorium and large venue use. While the speakers may have a position on the power tap control for 8 Ohms, that's only to make it usable for a low impedance system, it doesn't mean the amplifier you bought can use typical speakers.
  16. 1958Bassman


    Oct 20, 2007
    To a great extent, 200W is 200W, but the speakers used in the systems that have a 70V/100V amplifier use autoformers.wjicj are transformers with one pair of input terminals/wires and several pairs for the output to the speaker. The setting is chosen, based on the power handling of the speakers and the SPL needed at the listening positions, determined by the speaker's dispersion, sensitivity and distance to the listening area. If a 50W amplifier can be sufficient, and mane of the great ones over time have been low powered, 200W should be just fine. However, the power of this type of amp is shared by the speakers and if the speaker power taps are set incorrectly, the load will be too much for the amp to handle because, just like a bunch of speakers can be wired in parallel to achieve impedance that's too low for a given amplifier, current is the reason for the amplifier's failure.

    Here's a scenario- the place where sound is needed will be covered by 7 speakers (yeah, this works with these systems), each speaker is at a different height and a different sound level is needed for each. The speakers have power tap settings, often 1W, 2W, 4W, 8W, 15W (for smaller applications) or higher power, like the one in the photo. The recommended setting for the power taps chosen should be around 80% of the amplifier's output or the amp will overheat and fail prematurely.

    HolmeBass likes this.
  17. iiipopes

    iiipopes Supporting Member

    May 4, 2009
    This ^. For the OP: a 70v system is designed to run, among other things, elevator-music-style systems in department stores and office buildings, an array of old-fashioned PA horns at a stadium, etc. You can't just plug a regular cab into a 70v system. See the longer explanation here, including the short paragraph about needing an impedance transformer necessary if you do want to run an 8- or 4-ohm speaker: Constant Voltage (70-Volt) Audio Systems For Beginners

    Too bad it wasn't one of the classic vintage Bogen amps, as those amps had different taps on the transformer and an array of screw terminals in the back so you could wire a conventional cabinet to one, and the back panel was labeled as to which screws you attached the speaker cable depending on which cabinet and which impedance rating.
    Last edited: Sep 27, 2022
    Old Garage-Bander likes this.
  18. And to add to all of the above, the transformer required to use it with a typical bass guitar cabinet would be huge, heavy and expensive.
    agedhorse and iiipopes like this.
  19. Gizmot

    Gizmot Supporting Member

    Mar 22, 2009
    Pittsburgh area
    No one has mentioned the mechanical differences between amps designed for fixed installations and those designed to be able to handle the rigors of being transported, bounced around, hooked and unhooked, etc. To be fair, some amps could handle it, but many won't. It's like buying a sports car when you need a 4-wheel drive truck.
    agedhorse likes this.
  20. Engle

    Engle Supporting Member

    Damn, something I know about and it’s already been well answered!

    Many power amps that are intended for 70v will also have an 8/4 ohm output. If this one doesn’t, you can’t use it.

    I have used fixed installation amps (I have one for monitors) and they can sound good, but if they have a transformer for 70v, that’s a lot of extra weight, and you often have to adapt from screw terminals and Phoenix connectors to connect your gear.