Determining Load Ratings When Not Specified (16 ohm)

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Kip Conner

Junior
Mar 13, 2011
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Athens, GA
Calling on those that wear smarty pants.

I need to determine the power output for an amp that doesn't list it in the 16ohm category. I have always just guessed based on the the listing for the 2, 4, or 8 listings. I guess that most people are buying amps for passive speakers that are rated for 8, so manufacturers no longer feel the need to list for 16ohms any more.

Is there a formula that you guys use?
 
Re: Determining Load Ratings When Not Specified (16 ohm)

The maximum output voltage should be the same if the load is 8 or 16 ohms, so I'd expect the power at 16 ohms to be exactly half of the 8 ohm spec.

If you want a formula: P=V^2/R but you probably know that one already.
 
Re: Determining Load Ratings When Not Specified (16 ohm)

Jeff Carter said:
The maximum output voltage should be the same if the load is 8 or 16 ohms, so I'd expect the power at 16 ohms to be exactly half of the 8 ohm spec.

If you want a formula: P=V^2/R but you probably know that one already.
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Yep. Most amps are voltage limited at 8 ohms, meaning they put out their maximum voltage at 8 ohms or any higher impedance. The same voltage into 16 ohms is half the power as that voltage into 8 ohms.
 
Re: Determining Load Ratings When Not Specified (16 ohm)

Not to put too fine a point on this but there will be less PS sag (aka regulation) at 16 ohms vs 8, so there will be slightly more than 1/2 the 8 ohm power. If you need more precision you could probably calculate what this sag is from looking at the relationship between 8/4/2 ohm power. However that much precision is not really important since 16 ohms speakers are rarely exactly 16 ohms.

JR
 
Re: Determining Load Ratings When Not Specified (16 ohm)

John Roberts said:
Not to put too fine a point on this but there will be less PS sag (aka regulation) at 16 ohms vs 8, so there will be slightly more than 1/2 the 8 ohm power. If you need more precision you could probably calculate what this sag is from looking at the relationship between 8/4/2 ohm power. However that much precision is not really important since 16 ohms speakers are rarely exactly 16 ohms.

JR
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For that matter, there's also going to be a smaller voltage drop across the output transistors with the higher load... another pretty marginal effect, though.
 
Re: Determining Load Ratings When Not Specified (16 ohm)

Kip Conner said:
Calling on those that wear smarty pants.

I need to determine the power output for an amp that doesn't list it in the 16ohm category. I have always just guessed based on the the listing for the 2, 4, or 8 listings. I guess that most people are buying amps for passive speakers that are rated for 8, so manufacturers no longer feel the need to list for 16ohms any more.

Is there a formula that you guys use?
Click to expand...

Hey Kip,

Two thoughts:
  1. You can pretty much bet on 1/2 the 8-ohm "power" (same voltage into an easier load) with "normal" amps (no tube whatever). Frankly, how far can you possibly be off? 1/4dB?
  2. I would worry much more about the impedance of your woofers, many "16 ohm" woofers are more like maybe 12 ohms, so you may be delivering more power than you think.
 
Re: Determining Load Ratings When Not Specified (16 ohm)

Thanks everyone, good info. I'm looking for a new amp to power some JBL 2445's HF Drivers. I'm going to be using PLX3602's for the Subs (bridged) and Mids (Stereo) and I am looking for a good match in the same family for the 2445's. I could use the same amp and dial it back in the processor, but I'm certain that there's a better choice financially speaking.
 
Re: Determining Load Ratings When Not Specified (16 ohm)

Kip,

You could drop down to the PLX1802, it can deliver 330 watts at 8 ohms, so still more than 150 watts peak which is as much as you would ever want to hear from a 16 ohm 2445.

Art
 
Re: Determining Load Ratings When Not Specified (16 ohm)

Art Welter said:
Kip,

You could drop down to the PLX1802, it can deliver 330 watts at 8 ohms, so still more than 150 watts peak which is as much as you would ever want to hear from a 16 ohm 2445.

Art
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"And here comes the icepick in the forehead!" - F. Zappa

I'd prefer to NOT hear a 2445 especially if an SE is trying to force anything above 8kHz from it.
 
Re: Determining Load Ratings When Not Specified (16 ohm)

No forcing involved. If it sounded less than average I wouldn't do it. For this application I'm OK with average.
 
Re: Determining Load Ratings When Not Specified (16 ohm)

Tim McCulloch said:
"And here comes the icepick in the forehead!" - F. Zappa

I'd prefer to NOT hear a 2445 especially if an SE is trying to force anything above 8kHz from it.
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I hear you on that. The old Rat Traps I have use 2440/2441s with Radian diaphragms in them. A nice 2-3k cut required to tame the icepick. Not much extension past 12k even with the newer diaphragms which is why they're a "4-way" box with an additional 1" horn operating from 8k on up. The 1" were TADs. I've swapped them to some new 18 Sound drivers. I got tired of paying $600 for TAD diaphragms and the 18 Sound drivers sound pretty much the same to my ear in that application at a fraction of the price and weight :) Running up to 4 drivers of the JBL (16 ohm) & 18 Sound (8 ohm) drivers off of each side of a Crest 4801.
 
Re: Determining Load Ratings When Not Specified (16 ohm)

Greg Cameron said:
The 1" were TADs. I've swapped them to some new 18 Sound drivers. I got tired of paying $600 for TAD diaphragms and the 18 Sound drivers sound pretty much the same to my ear in that application at a fraction of the price and weight :)
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Too late now, but Dave Rat discovered the series "protection" capacitors he used in the Rat Traps eliminated the amplifier damping from the diaphragms, TAD berillium diaphragms have a low resonance that can be excited by the mid and low frequency driver's SPL, causing the diaphragm to shatter.

After removing the caps he stopped having the alarming (expensive) failures.
 
Re: Determining Load Ratings When Not Specified (16 ohm)

Art Welter said:
Too late now, but Dave Rat discovered the series "protection" capacitors he used in the Rat Traps eliminated the amplifier damping from the diaphragms, TAD berillium diaphragms have a low resonance that can be excited by the mid and low frequency driver's SPL, causing the diaphragm to shatter.

After removing the caps he stopped having the alarming (expensive) failures.
Click to expand...

Yeah, I had no caps in my boxes. EAW had the same issue with the KF850T (TAD) as well. The last batch of failures I had were due to the limiters being set too loose for my 1"s and someone did a hot plug on a cranked, unmuted high hat condenser. The one thing about look-ahead limiters is that they actually need to be set correctly ;) Mine were set with the threshold about 3dB too high. Ooops. In any case, that was the last straw for me so I looked for an alternative driver and sold the TADs. Dave and I looked over the specs on several high quality/high output 1" drivers for VHF and the 18Sound looked to be about the best in it's category for extension/output/distortion. I did consider going to a modern 2" and eliminating the 1", but all the really good 2" drivers with good extension and lower VHF distortion are very pricey. So it was more economical to keep the JBLs and change out the 1".

P.S. the limiters on the 1" are set correctly now.
 
Re: Determining Load Ratings When Not Specified (16 ohm)

Luckily we have a new crazy invention called a speaker processor and an even wilder invention called Smaart software to smooth out and align so cabinets can be pleasing to the ear.
 
Re: Determining Load Ratings When Not Specified (16 ohm)

Kip Conner said:
Luckily we have a new crazy invention called a speaker processor and an even wilder invention called Smaart software to smooth out and align so cabinets can be pleasing to the ear.
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Kip,

Too bad neither can eliminate the harmonic distortion of a 2445, run a 7 kHz sine wave tone at 8 volts and report back how many dB down the Smaart RTA indicates the 14 kHz harmonic is ;^).

Art
 
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