# How many "amps" needed to bench test an amp?



## ryan s (Dec 19, 2006)

I have 2 power supplies converted to 12V. One does 5A, the other 8A I believe. Both were enough to fire up a HID ballast, so they are capable.

I know that amps rarely, if ever, draw their full current rating in normal use. Would an 8A power supply be enough to bench an amp for basic functionality tests? 

Nothing over 500W, and no loads under 4 Ohms. Probably hooking 2 coaxs together for an 8 Ohm load for testing bridging.

The wires are dinky though...like 20ga...but they're only about 10" long.

Oh, they both have short circuit protection.


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## t3sn4f2 (Jan 3, 2007)

ryan s said:


> I have 2 power supplies converted to 12V. One does 5A, the other 8A I believe. Both were enough to fire up a HID ballast, so they are capable.
> 
> *I know that amps rarely, if ever, draw their full current rating in normal use.* Would an 8A power supply be enough to bench an amp for basic functionality tests?
> 
> ...


That statement applies to averaged current draw only. An amp can and does easily draw max current at any point that the volume knob and the music calls for it to do so.


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## Oliver (Jun 25, 2007)

Try a 55 amp or higher supply for max testing of amplifier draw.

http://cgi.ebay.com/34-Amp-12-Volt-...emQQptZLH_DefaultDomain_0?hash=item3359f78e45



> This Power Supply Is Brand New Adjustable Voltage 10 to 13.8 VDC
> 12 (13+) volt DC 34 Amp Regulated Switching Power Supply


Mebbe something like this ^^^^


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## turbo5upra (Oct 3, 2008)

I have a 12 amp.... It to me is more than enough to power up a amp and give it a decent "run". Don't most amps idle @ 2 or under amps?


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## [email protected] (Jun 12, 2008)

I have a old computer power supply I converted for use. It works fine for anythign I have thrown at it. I think I get around 20 amps with all my wires combined. I use it to te4st amps when I get them and to run an amp on speakers to break them in. I have used it on a 5-ch amp with no problems


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## Oliver (Jun 25, 2007)

Ohm's Law: E = IR ; I = E/R ; R = E/I


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## t3sn4f2 (Jan 3, 2007)

a$$hole said:


> Ohm's Law: E = IR ; I = E/R ; R = E/I


doo da mafe


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## ErinH (Feb 14, 2007)

The fuse rating onan amp can be an indicator but nothing more. Keep in mind the fuse rating will be a worst case scenario. You won't likely won't be loading all channels down unless its a mono amp. 
Also the efficiency will work against you.


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## ryan s (Dec 19, 2006)

Ugh...math. That's why I have a BA :laugh:

For example, I have an old Phoenix Gold MS250 I need to test. It has a 25A fuse. Since I'm not going to push it, how much draw should I count on? Are there any formulas for efficiency as the volume is increased/decreased? I'm used to calculating for "max draw, full tilt" but I don't know if there are any ratios or something as a guide for "quarter tilt" or whatever.

Thanks for the replies everyone. I wish mutliquote was working...a "group thanks" will have to do :laugh:


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## ErinH (Feb 14, 2007)

Not very scientific but how about this: (total fuse rating)/(number of channels driven). That's a worst case. So, for a 4 channel rated at 80 amps total, you'd need only 20amps for powering one channel. 

Not exact but it should give you a ballpark. 

As for efficiency, I can't say for certain. I believe I've seen someone say that A/B amps are typically 60-70% efficient, while D class is 80%+ 


I'd ask chad about this. He has much more experience on the matter.


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## audiogodz1 (Jan 5, 2010)

I've been playing with amps and power supplies for 2 decades in my stores and shade tree and I measure advice based on how many times someone has had their hands in the cookie jar. One guy who has ran 2 power supplies and it worked both times at that amperage is good..... someone who has ran 20 power supplies in every situation is my kinda guy. I'm sorta the latter. I've hooked up an easy 300 to 400 amplifiers on a good assortment of power supplies so I have got the feel for when an amp is going to clip the power supply off on you. 

First, don't run ANY amp on anything smaller than 20 amps. Example.... I have a ten amp that will turn on my PPI art 600, but volume never can get above 1/5th before it kills the power supply. I know the amp works, that's about it. On a 20 amp I can get almost to halfway @ 4 ohm before it clips the power supply. 

In my old stores I ran a 60 amp and it clipped at 2 ohm stereo on a PPI a600 on it's biggest peaks wide open. So even at 60 amps under a heavy song you can clip that huge power supply. 

I've tried more efficient amplifiers on 30 amp Astron power supplies such as the Coustic 460u or the Ample A226, both putting about 250 RMS/500 pep to the speaker. I was able to run them balls out with no clipping. 

You just need to get as big a power supply as possible, you will ALWAYS need the extra juice. Nothing is more frustrating than having an amplifier just get warmed up and you roll the knob and everything cuts off. 

Nothing below 20, realistically 50 and up. You won't even get an 8 amp to produce hardly any sound before it clips off. Remember you also have to run the head unit and whatever else is on it too.


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## sqshoestring (Jun 19, 2007)

I just tested an insignia that is class AB 2x110rms iirc. I ran it for a while at 20A on a sine wave and got it warm, then was burping it to 35A and it would clip hard. But I have 40A of PS behind the 35A meter that was pegging. I could only test small amps with it so I have another 30A to add to it, if it works its an old beast. And that amp has 50A of fusing, lol. The voltage on the sub went from 9 to 24 on the DMM, I checked it today and it runs 32v+ rails idle. Now all I wanted to do was make sure it didn't blow up, but when I start power testing I can't run short on power or have the voltage dropping.

One day I clipped the subs on my kicker while running the 4ch part pretty hard on fronts and rears. The clamp meter showed a peak of 74A and rms around 30-40, it was bass music. That is 4x70rms and 1x420 class D. That was around 13.5+v as my car does not drop, but my PS sag to 12.8 or so....that would mean more than 74A to max the kicker 5ch on the bench. I'll have to see if I can adjust them when I get my load hooked up. It really depends on the size and class of the amp, but you need quite a bit to work an amp over. There is a big increase in draw the last third of volume.

On the other hand I was clipping the kicker, with a scope not clipping maybe it takes much less than 74A I don't know.


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## ryan s (Dec 19, 2006)

Erin: I recall seeing something about how efficiency drops way down when the amp is only being partially pushed...something like less than 20% at low volumes. It might be at a site like BCAE and I probably overlooked it :laugh:

Audiogodz: Thanks for the sharing your experience! Guess I'm going to do it the pain-in-the-butt way for the moment (swapping them in the car) since I need these amps gone before spending any more money :laugh:

I was planning on using one of my power supplies for the headunit and speakers. I've done it before off the 8A one...but of course I didn't push it.

sqshoestring: That's interesting to see it putting out 30-40 per channel while pulling 74A...sounds like quite a setup you've got going on :surprised:

Thanks for the help guys. Looks like I'll be doing it the fun way today :laugh:


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## Oliver (Jun 25, 2007)

ryan s,

Then you'll know for sure !


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## ryan s (Dec 19, 2006)

Good thing I got the fuse block, more 8ga, and Y cables the other day! :laugh:


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## ErinH (Feb 14, 2007)

have you considered pairing your power supply with a battery? Think about it... we run our entire systems off a battery.
I run my entire system off my battery, while having my PS assisting it when tuning or listening.

I'm sure you could find a decent battery for not too much if you look around. Then use the two together to power whatever you want.


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## TrickyRicky (Apr 5, 2009)

Am pretty sure the power supply suppost to accept a battery before adding one on. I use a AudioAuthority 2/77 50amp hybrid supply. It accepts an extra battery or even two, or pairing it up to another 2/77. Circuit City use to use them for their display models. I bought mine from egay that was used in a CC.

Anything under 10amps, is only enough to power up a HU. You can power up an amp, but dont expect to get anything over 200watts.


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## ryan s (Dec 19, 2006)

Could always steal my dad's tractor battery...it's out for the winter and it's probably 3/4 the size of my car's :laugh:


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## circa40 (Jan 20, 2008)

To test for function, 8-10a is probably more then enough. 




ryan s said:


> I know that amps rarely, if ever, draw their full current rating in normal use. Would an 8A power supply be enough to bench an amp for* basic functionality tests*?


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## sqshoestring (Jun 19, 2007)

ryan s said:


> Erin: I recall seeing something about how efficiency drops way down when the amp is only being partially pushed...something like less than 20% at low volumes. It might be at a site like BCAE and I probably overlooked it :laugh:
> 
> Audiogodz: Thanks for the sharing your experience! Guess I'm going to do it the pain-in-the-butt way for the moment (swapping them in the car) since I need these amps gone before spending any more money :laugh:
> 
> ...


Efficiency is lower at lower volumes with class AB, that is how hybrids are better. They lower the rail voltage at low output...high rail and low output makes waste. But, power use is a log function to db so it takes around 10x the power to go twice as loud. That means an amp draws way more current at 100% than it does at 90% output. In theory a 100w amp would use 10w at half volume ignoring waste and other effects. So low efficiency at low output is not that big a deal, since power use is much lower there anyway. A class D or hybrid does get rid of much of this waste.

8A should be enough for most HU, they can only push 18wrms or so.

I didn't measure the channels on the kicker, it was 74 peak draw and at same time was 30-40A RMS draw all on the 12v 5ga wire from the battery, car running. Just showing you need 74A to hit peak, but only 40A to run it though it would clip on peaks then....in theory, because that peak would then drop the voltage and could shut the amp down or cause other issues and distortion/etc.

The easy way is just try it and you will find out. Run a DMM on it checking voltage, when it exceeds the PS it will start dropping and more it will shut down. A cap can help a little with that but not that much.

Some info on amp power draw: Amplifier Installation Guide
The only real way is to test an amp, but that is close enough for most uses. Note an amp can draw more clipping, which is above what it is rated at.


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## ChrisB (Jul 3, 2008)

I've been having great luck with my IOTA DLS-55 for test bench use since I purchased it. It also doubles as a battery charger or keeps my system going when I am playing around with the RTA.

Expanding further on the old school class ab amps.... I had a LP 5002 that was measured to pull 85 to 90 amps peak current depending on what you were doing with it but required a 60 amp slow blow fuse. I am surprised it didn't shut down Jeanious2009's power supply.


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## sqshoestring (Jun 19, 2007)

I ran a marine battery on a 1A auto charger to test amps for a while. It was in a vented utility room or you need to be careful of the fumes batteries can make. I got rid of it when I got a larger PS. They will drop voltage under load so hard to test power with them, but you can test for sound/output sort of/etc. I test amps on a 10A supply all the time just to see if they work.

Another way is use a $10 DMM on amperage draw they take 10A, on a 10A supply or with a 7.5A fuse, it will show you idle current. Most amps are under 2A with nothing hooked to them, huge amps might be more. Now I have a power panel with amp meter in it so I use that.

For a while I ran the 10A and only used the battery with it to run amps hard. 13.8v will charge a 12v battery. I would recommend a small 1-2A automatic charger if you must use a battery, leave only that on it when not in use. Otherwise take the battery outside and charge it once in a while. Higher charge rates make more fumes, not good in a house or around flames/people/etc. I guess some of the gel batteries are better, I had a delco starting marine.

Large class AB can hog a lot of current. Way back I was in a store and playing with a 3002 LP. We put the PA2 line driver on it to compare with and without, running a huge full range hatch box with 12s in it, maybe a kicker (stillwater). It went louder, more clear, and the power supply started to freak out. But it was pounding, everyone in the place was looking over there lol. I don't know what supply they had, must have been 50A or better it was huge and they had three listening rooms full of stuff.


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## ryan s (Dec 19, 2006)

Was just heading outside and pulled the fuse on the PG MS250 since it's in the way of the power terminals. It's supposed to have a 30A fuse but has a 25A installed. Half of the plastic insulation on the fuse is bubbled and warped a little, but the fuse isn't blown (regular ATC).

Now I'm scared to put it in the car. I don't like melted fuses in amps sold as untested. The board and components look fine, though. No burn marks or leaky caps that I can see.


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## TrickyRicky (Apr 5, 2009)

ChrisB said:


> I've been having great luck with my IOTA DLS-55 for test bench use since I purchased it. It also doubles as a battery charger or keeps my system going when I am playing around with the RTA.
> 
> Expanding further on the old school class ab amps.... I had a LP 5002 that was measured to pull 85 to 90 amps peak current depending on what you were doing with it but required a 60 amp slow blow fuse. I am surprised it didn't shut down Jeanious2009's power supply.


Yes suprisingly it didnt. It pushed very nicely. My power supply is only 50 amps, and the amp didnt even clip the power supply. As a matter of fact I tested my modded 8002SW and it also didnt pull anywhere 50 amps (but I also couldnt turn it all the way up, because neighbors call po-po on me already).

Not to mention I liked the 5002 you sold me more than the other two 5002's I have. We both know the 5002IQ needed proper repair, but my other functional 5002IQ didnt perform quite as well as the one you sold me. Keep in mind I used 8-ohms load on my LP's.


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## ryan s (Dec 19, 2006)

Went outside and started messing around...too cold. It was 45 yesterday and I was out in just a Tshirt. It's almost 45 today but the weather people never mentioned the 30 degree wind chill 

So now I sit inside and think. What should I do with the PG amp? Throw a good 30A fuse in and hope it works?


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## TrickyRicky (Apr 5, 2009)

ryan s said:


> Went outside and started messing around...too cold. It was 45 yesterday and I was out in just a Tshirt. It's almost 45 today but the weather people never mentioned the 30 degree wind chill
> 
> So now I sit inside and think. What should I do with the PG amp? Throw a good 30A fuse in and hope it works?


If the amps blows up or even melts a fuse which the manufacture stated to use, then something is wrong with the amp. Always start small then add to the fuse size. I would start with a 20 amp fuse and if that blows up, then use a 25, if that blows up, then something is wrong. 

When I bench test my amps I use a 50 amp maxi fuse (because my amps require a bigger fuse) but I would never use a bigger fuse than what the manufactures suggest. Unless you want something to smoke.


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## ryan s (Dec 19, 2006)

It's just one half of the plastic that's a little melted so I don't know for sure if the amp caused it...the previous owner might have been a noob, blown the stock fuse by not disconnecting the battery or something, and grabbed whatever he/she could find.

I'm disconcerted by the sight of it, though :/ I'll try a 20 in it when I test it.


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## sqshoestring (Jun 19, 2007)

Those fuses can melt from a bad connection in the holder. Hard to say if in the amp, or that fuse came from someplace else since it is not correct. The amp should play on a 10A fuse at a normal volume, maybe lightly on subs. I'd start it up in a 10A, if it works at all then put the proper size in and run it.

Make sure the holder is clean and not corroded or something, push the fuse in/out a few times to clean the contacts.


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