Question on Pro Amps and current draw

Discussion in 'Archived Threads 2001-2004' started by Steve_D, Aug 10, 2001.

  1. Steve_D

    Steve_D Second Unit

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    I was considering buying a pro amp to power 2 passive subs. I had my eye on the Crest Audio CA4, rated at 435 watts into 4 ohms, with minimal fan noise based on user reports.
    I checked the specs and the current draw maxes at 15 amps! Excuse me, but my entire HT is powered by 1 15 amp circuit! Even the 1/3 power rating was like 9 amps.
    If I put one of these on without running another circuit....how will there be anything left over for the rest of the components like the RPTV, 2 channel amp, receiver, etc...
    In comparison, my B&K Ref 4420 is capable of delivering 300 watts into 4 ohms and 75 amps peak current but only has a max draw of 6.8 amps. I guess the B&K stores power in the caps to use during peak flows.
    Is this partly why pro amps tend to be less expensive watt for watt? Are they designed for area with multiple 20+ amp circuits?
    Now, I don't pretend to be an electrical engineer...so could someone tell me if I'm wrong or right about these comments?
    TIA
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  2. brucek

    brucek Second Unit

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    Steve,
    Remember that power is the "current squared multiplied by the resistance". So, an increase in power from 300 to 435 watts has a large affect on the current drawn.
    I think if you look at the max current rating of that Crown amp it says that 15 amps is the circuit breaker size required. That amp idles at 1.4 amps. Instantaneously, it can draw a lot more. Continuously, 400 watts dissipated will draw 10 amps through a 4 ohm load. That's your max current for that amp. Believe me, you will not be dissipating 400 watts with your amp continuously.
    When we are talking about AC power, a normal 120 volt AC 15amp line can supply about 1800 watts of continuous power . If it draws more than that, the panel breaker trips.
    Amplifiers don't draw their full spec'd AC current on a continuous basis - their instantaneous power comes from their DC power supplies from appropriately sized filter capacitors (this is that peak current of 75 amps you mention for the B&K). The AC supply only keeps the "well" full, so the DC power supply can keep up with it's demand from the load.
    That said, once you start getting into large amplifiers such as this combined with many other HT pieces of equipment, it's time to install a dedicated 20 amp circuit.
    brucek
     
  3. Wayne A. Pflughaupt

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    Steve,
    As you can see, Bruce knows his stuff. To emphasize, you could probably get away with using one amp or the other on a single circuit with the rest of your gear, but it’s really pushing it if you try to use both.
    Nice to see you on this forum, Bruce!
    Regards,
    Wayne A. Pflughaupt

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  4. Steve_D

    Steve_D Second Unit

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    Thanks Bruce, your answer was exactly what I was looking for...
    Exactly confirmed my suspicions that I need to start pricing electricians....I am definitely going to look at upping the line to 20 amp at least and hopefully running a second.
    I was trying to decide between another B&K or maybe a pro style amp for passive twin subs. If I'm understanding everything correctly, given my current "limited" power situation, I would do better with amps that store more power in the caps, and don't draw as much at max. In other words, the Crest can deliver more clean power than the B&K...but the B&K has bigger "wells" so it doesnt have to draw as much at any one time.
    I also understand that 99.8% of the time I'll be no where close to full wattage/amp draw...it's just that why buy 8 300 watt amps if they are really restricted to more like 8 x 150.
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  5. brucek

    brucek Second Unit

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    Steve,
    You're making a couple of assumptions that aren't quite correct. Sometimes it's difficult to explain these things.
    Firstly, I think it is a very good idea to install a dedicated 20 amp circuit to your stereo/HT equipment when you start to use high power amplifiers like that Crest. If you install a second circuit, it's also wise to ensure it's on the same phase as the first.
    quote:
    I also understand that 99.8% of the time I'll be no where close to full wattage/amp draw...it's just that why buy 8 300 watt amps if they are really restricted to more like 8 x 150.
    [/quote]
    No, they are not restricted. They just don't use that much power all the time, but it's available as headroom.
    When it comes to high power amplifiers or any other type of Class AB amplifier (your Crest and B&K are AB), they certainly don't draw AC current and thus power from the AC wall outlet equalling their maximum specification on any continuous basis. A 400 watt amplifier will have a specific idle current (the Crest is 1.4 AC amps - that's 168 watts from the AC line). Its' 400 Watt specification says that it has the ability to dissipate 400Watts continuously into a 4 ohm load given a AC signal input from 20Hz to 20Khz with a resultant THD+N of 0.1%. This is pretty good for that bandwidth. At 1KHz, where most amps limit their specifications to, the THD+N is 0.01%. This is a good amplifier.
    To produce this kind of power is has a fairly large +/- DC power supply rail of 82 volts. There is lots of headroom in this amp. It has massive power supply capacitors. This means it can respond to quick transients in the music and supply the DC power needed without taxing its power supply too much.
    So your question of why you need the 300 watt amp instead of the 150 watt amp is simply because it's there when you need it. It provides you with a large dynamic range and power when required.
    quote:
    If I'm understanding everything correctly, given my current "limited" power situation, I would do better with amps that store more power in the caps, and don't draw as much at max. In other words, the Crest can deliver more clean power than the B&K...but the B&K has bigger "wells" so it doesnt have to draw as much at any one time.
    [/quote]
    No, power supply capacitors are there to respond to the quick demands for power. All amplifiers are the same in this regard. Perhaps the B&K claims to be a so called "high current design" meaning they claim to have larger than normal capacitor storage that can instantaneously supply 75 DC amps of current, but don't get too excited about that. It's more of a marketing thing than anything else. The Crest has ample storage in its capacitors as well.
    Here's the point I was trying to make in my original post. Don't confuse AC and DC current. When you read a specification that says an amp can supply 75 amps and you wonder how this is possible since the AC wall circuit you are using is a 15 amp circuit. Well, the 15 amps is the total AC amps available from the wall at 120 volts AC, while the 75 amps is a DC current available instantaneously from the DC power supply.
    I took the liberty in my previous post to estimate and use a rough rule of thumb to tell you that it wasn't likely that you'd draw more than 10 amps of current from the wall with the Crest. You see, in high power amps, to produce this power they require large +/- DC supply voltages. In fact, they can get close to the line value of 120 volts. Using this fact, you can estimate the absolute max AC current draw knowing the max continuous current through the load at full power and assuming a low "turns ratio" in the power supply transformer. My point was that the Crest doesn't draw 15 amps from the wall.
    The other point I was trying to get across is that very high power amps such as the Crest you are interested in at 400 watts sometimes require an upgrade to your AC circuits that you're using to supply them. You'll likely be fine with your "limited" AC circuit, particularily at normal listening levels, but again, it's probably a good idea to upgrade to a 20 amp or maybe even x2 20 amp circuits while you're at it, because it doesn't cost a lot more to put in two circuits while one is being installed.
    brucek
    [Edited last by brucek on August 11, 2001 at 03:49 PM]
     

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