Amps rated for 8 and 4 ohms

[cool9]

I wanted to know what kind of selection there is for amps rated at 8- and 4-ohm loads. I made a mistake of accidentally purchasing 4-ohm B&W speakers and I have a Sansui AU-517 60wpc rated at 8-ohms. I don't know what type of Class it is but the amplification and transformers are explained (output phases, push-pull and differential amplification) in the manual and it's very complex. Two of the transformers which have current limiters to protect the transistors from overload.
I read the AK sticky (which is wrt Class AB amps only) on powering 4-ohm loads and yes my amp does get real hot when I drive these speakers at higher volume. In the sticky cooling (of the transistors, heat sinks, etc.) is discussed and it seems that it's something that's very difficult to achieve. So I think I might look into an amp that is rated for both 8- and 4-ohm loads. It's either that or try to return the 4-ohm speakers (which I'm not sure is possible).
I read this good link (Rod Elliott, Elliott Sound) which details the different amplifier Classes, what kind of output devices they have and their respective load requirements. The article does state this: "Most power amps operate in Class-A up to about 5 to 10mW, after which they become Class-B". http://sound.westhost.com/class-a.htm

 

[BinaryMike]

Your AU-517 is generally regarded as class AB, as are almost all products of this type, but they're really quite close to pure class B. The only thing that makes 'em technically class AB is a very tiny but critically important amount of idle current in the output stage.

This doesn't have much to do with handling four-ohm loads, however. That's governed by the amp's ability to deliver high load current without distortion, and by its ability to dissipate waste heat. There is a little you can do to upgrade an existing amp. You can put stronger output transistors in it, and you can reduce thermal impedance between output devices and heat sink. Forced-air cooling is very effective, but often too noisy.

 

[cool9]

So randomly taking an amp as an example to power 4 ohm speakers:

Denon PMA-900V:
120W min. RMS/channel (8 ohm load) w/less than 0.02% THD from 20-20000 Hz
1 kHz (4 ohm load) 200W + 200W (DIN, THD 1%)

As your power approaches 200W/channel you'd introduce up to 1% THD (or more)?
Because the 4 ohm rating is at 1 kHz the THD could go higher than 1% THD at other frequencies?
That's not the greatest scenario (but not the worst) you'd want to have in your audio setup, correct?
THD starts to become audible around what values? (The standard is you'd like to have less than 0.1% THD?)
And of course, you'd want the amp to have an excellent heat transfer design if you push the load to its limit?

 

[jan_stevns]

....snip....

THD starts to become audible around what values? (The standard is you'd like to have less than 0.1% THD?)

If you look at the specs for most loudspeakers, you may begin to wonder a bit why many people are so concerned wether a amp has 0.1% THD or 0.2%

:scratch2:

 

[BinaryMike]

Because the 4 ohm rating is at 1 kHz the THD could go higher than 1% THD at other frequencies?

The 4-ohm power rating for 1% THD was probably derived by deliberately pushing the amp to the onset of clipping. Back off below clipping, and distortion undoubtedly drops to a much lower value, but still well in excess of the value achieved with 8-ohm loads.

 

[cool9]

If an amp's specs don't have a rating for 4 ohms is the manufacturer implicitly (not directly expressed) telling you not to use the amp for 4 ohm loads? Using an amp according to its printed specs can seem like a game of chance for some amplifiers. Some things are explicitly stated and some things are left out.

 

[BinaryMike]

If an amp's specs don't have a rating for 4 ohms is the manufacturer implicitly (not directly expressed) telling you not to use the amp for 4 ohm loads?

Yes, but there's still grey area. Many so-called 8-ohm speakers actually deserve to be specified lower, and almost all speakers are 'reactive' to a degree, which means that impedance can dip very low at some frequencies. Amp designers know all about these issues. They also know that some customers will use 'em with 4-ohm loads regardless, and they take steps to minimize warranty returns. This can include output current limiter circuits that impair sound quality at relatively low volume settings.

 

[Retro Stereo]

Just go find yourself a good used Classe' CA-400 Amp and you'll never have to worry about having enough power for any speaker you buy.






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[Kim G]

Just go find yourself a good used Classe' CA-400 Amp and you'll never have to worry about having enough power for any speaker you buy.






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There is one available for only $2400 on Agon.
Probably a little more scratch than that nice little Sansui.

Kim

 

[Retro Stereo]

There is one available for only $2400 on Agon.
Probably a little more scratch than that nice little Sansui.

Kim

That price is actually pretty high, most of the other ones I've seen lately have been going for under $2000 on a consistant basis. They originally sold for $5500. Considering that this would be the last amp you'll ever need for "any" speaker, I consider $2000 a bargain.



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[Bob_in_OKC]

You could spend $300 on an old Adcom and run 4 Ohm speakers all day.

 

[Retro Stereo]

You could spend $300 on an old Adcom and run 4 Ohm speakers all day.

Yeah, I'd love to see what one of those Adcom's would do with my Thiel CS3.6 Speakers when they dip down to 2 ohms or less, oh wait, I already know, peter out......

I'll just keep my CA-400, thanks.




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[Bob_in_OKC]

Yeah, I'd love to see what one of those Adcom's would do with my Thiel CS3.6 Speakers when they dip down to 2 ohms or less, oh wait, I already know, peter out......

I'll just keep my CA-400, thanks.

You're welcome, Mr. Sarcastic Smarty Pants. I'm not saying an Adcom is the equal of a CA-400. I'm saying it doesn't have to cost $2000 to drive 4 Ohm speakers.

 

[whoaru99]

If you look at the specs for most loudspeakers, you may begin to wonder a bit why many people are so concerned wether a amp has 0.1% THD or 0.2%

:scratch2:

Yes, a point very few consider because loudspeaker distortion specs are not clearly/openly published by very many mfg.

 

[cool9]

Well, seeing that my income is $24K/year I figure I could drop my health insurance and cut back on my diet then I could pick up that $2000 amp. And cross my fingers that I don't need a colonoscopy or a root canal.

 

[whoaru99]

I've never much worried about using 4 ohm loads...just give the unit PLENTY of breathing room (meaning no stack, no closed cabinets, decent air space all around) and you'll most likely be fine. This would be for "normal" use. If you're going to try to use the thing for an all night rave party or a frat house party with the volume cranked to 11, then I might reconsider that opinion.

 

[Retro Stereo]

You're welcome, Mr. Sarcastic Smarty Pants. I'm not saying an Adcom is the equal of a CA-400. I'm saying it doesn't have to cost $2000 to drive 4 Ohm speakers.

I won't disagree with your assessment if the speakers in question are easy to drive 4 ohm speakers (I have a few pair of easy-to-drive 4 ohm speakers), but as you know, not all 4 ohm speakers are easy to drive. My "BIG" amps before I got the Classe' were Pioneer SPEC 2's which were benched at 430 watts per channel into a 4 ohm load, but speakers like Infinity RS 2.5s/4.5s, AR-9s, Thiel CS3.6s (especially these speakers), and even Dahlquist DQ-10s, sucked those amps dry. I'd love to have you come over sometime to hear the difference between using a big Adcom amp and the Classe' amp on my Thiels (4 ohm speakers) sometime. Night and day.




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[Bob_in_OKC]

I won't disagree with your assessment if the speakers in question are easy to drive 4 ohm speakers (I have a few pair of easy-to-drive 4 ohm speakers), but as you know, not all 4 ohm speakers are easy to drive. My "BIG" amps before I got the Classe' were Pioneer SPEC 2's which were benched at 430 watts per channel into a 4 ohm load, but speakers like Infinity RS 2.5s/4.5s, AR-9s, Thiel CS3.6s, especially these speakers, and even Dahlquist DQ-10s, sucked those amps dry. I'd love to have you come over sometime to hear the difference between using a big Adcom amp and the Classe' amp on my Thiels (4 ohm speakers) sometime. Night and day.



Retro

I'd love to have the chance to do exactly that.

Just to clarify to the OP where I'm going with the Adcom comments...Adcom amps like the GFA-555II are a quality mid-fi solution that can handle reasonably demanding loads...not a giant killer.

On the topic of 4 Ohm speakers...agreed it's certainly important to consider which ones we're talking about. My only 4 Ohm speakers (ProAc Response TriTower) are on the easy-to-drive side, so I run them with a 50-watt tube amp.

 

[Retro Stereo]

Well, seeing that my income is $24K/year I figure I could drop my health insurance and cut back on my diet then I could pick up that $2000 amp. And cross my fingers that I don't need a colonoscopy or a root canal.

Wow, that's my exact income right now as well ($2000 a month). I sold off a bunch of my other equipment/Pioneer collection to afford the Classe'. Where there's a will, there's a way. Hopefully, it "will" be the last amp I'll ever need, at least I know I won't need anything bigger.



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[Retro Stereo]

I'd love to have the chance to do exactly that.

Just to clarify to the OP where I'm going with the Adcom comments...Adcom amps like the GFA-555II are a quality mid-fi solution that can handle reasonably demanding loads...not a giant killer.

Wholeheartedly agree. If he's looking for good value driving an "easy-load" 4 ohm speaker, Adcom is his friend.




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