Can you tell just by the specs if amp and speakers are a good match?


#1

I’m fairly new to the whole audio world, so please forgive me, if my question is kinda obvious.

But I’ve been wondering if it is possible to select an amp for a set of speakers or vice versa just by the specs?

The reason I’m asking is, I heard a set of speakers with an amp producing 35W into 8Ohm and with another amp rated at 75W into 8Ohm. The more powerful amp sounded great while the less powerful one completely lacked bass. Yet the weaker one can get loud enough to cause permanent hearing damage. So why does it have the power to get very loud, but for some reason not the power to produce decent bass? And is there a way of telling just by looking at the specs?

Thanks
Simon


#2

Yes, you can tell, to a degree, if an amp is a good match for speakers… but a better approach, is simply look for an amp with “good specs”. This kind of an amp will match with many if not all major speakers.

In short, you want an amp than can produce watts into difficult loads (this may explain why some amps lack bass but have a high 8 ohm watt spec… etc.). This is known as a “high current” amp. To see if an amp is such a a thing, look at its specs… a good amp maker will show you the wattage output into different loads (known as ohms). So… a really good amp will put out double the watts for halving of the load. Example spec: 100 watts into 8 ohms, and 200 watts into 4 ohms. That is a great amp spec. Further, some really good (and expensive amps) will describe their output down to 2 ohms… 400 watts into 2 ohms. Now that is a fabulous amp (and unfortunately very expensive). Difficult loads are smaller ohm numbers and require an amp push harder (and run hotter).

Now if you have a limited budget, and can;t afford and amp that produces big current into tough loads, then you must be careful that the speakers you buy do not have a difficult load. You can see this in the makers specs… they will say some ohm number… usually 8 or 4… or something in between. 8 ohm speakers are easy to drive and do not require a beefy amp as I described above. 4 ohm speakers are tougher and require a beefier amp. The problem with speaker specs is that the loads (ohms) change with frequency… makes will say “nominal” which is a fudgy word that means “sort of” or average-like… that is why Stereophile and others graph load against frequency so you can see how low a speaker’s load really goes.

Like everything, the problems go away when you throw money at it… a lesson I learned over the years, is just buy beefy amps and life problems don’t occur. This sometimes confused with higher power amps are “good”… the real issue is their wattage into low ohm numbers… that is what to look for, not the 8 ohm number.

So… let’s look at PS Audio’s BHK 250…:
Output Power Both channels driven 120vac mains, 1kHz, 1% THD
8Ω 250W minimum
4Ω 500W minimum
2Ω Stable for musical transients

This amp is “wow”… fabulous numbers… it can “dump current” into difficult loads (4 ohm) and in the old days, we would call this a welder… seriously, it could create a welding arc to fix an auto body. I don’t know what they mean by 2 ohm there… “stable” … maybe someone from PS Audio can chime in here. But alas, this amp is expensive. But… when an amp has these great specs, it will match very well with just about any speaker out there… this is very important and as I got older, learned this lesson a few times over… (don’t ask!)

BTW, if an amp does not show its wattage output into 4 ohms (only show 8 ohm output), I simply won’t buy it. Period. They are clearly hiding something… they sacrificed current making, to show a high, cheaper-made, watts into 8 ohm. This is not good. Seriously, I would never never buy an amp that did not publish its 4 ohm spec.

Now the above commentary is about “matching” for load… for me, this priority number 1. Second, is how much volume will you get out of your speakers for a given watt of input… and for that, you need to look at a speakers “sensitivity” rating… low efficiency means you will need a higher powered amp to acheive the same volume. But here things get weird because it depends on the type of music you like, listening volume you like, your room acoustics… etc… but… if you stick to the issue of output at load, and you find an amp that will even publish their 4 ohm ratings, then you are 80% of the way there. (BTW, in my experience, any amp maker that publishes their 4 ohm ratings is most likely a maker of good amps.)… now a speaker can have high efficiency and be a difficult load… sheesh!

Oh, just just avoid speakers with “difficult loads”… Stereophile etc can help you there.

One more thing: ignore THD and other distortion numbers. Not relevant… it was in the early '70s but not today.

Peace
Bruce in Philly


#3

I just had another thought… (very dangerous)… and that is about specs vs sound. Flat out, the first is foundational, the second (sound) is consequential. Yes, amps that measure good may sound bad… but… there is no substitute for good engineering… period. The laws of physics rule… if you don’t respect them, then keep giving your money to the men in pointy hats and keep praying for good sound.

So if someone says “specs are crap, I don;t look at them, I only listen”… yea, well OK, but you are going to spend a ton of time looking for needles. Good luck. True, if you only shop in high end stores or only select reputable, established, high end brands… you probably will be fine… but… I will bet everything, that a reputable, established, high end brand has good engineering and good amp specs… or they wouldn;t be around long.

Before I go listen (or today, click… barf…) I go straight for the specs. THEN I listen.

Peace
Bruce in Philly


#4

Oh, I didn;t asnwer one of your questions about why the weaker amp played louder. This is actually a pretty cool phenomenon of crappy amps… and Jimi Hendrix.

What happened: as you turn up the volume, the amp can’t meet the current demand and the amp clips. The waveform of the music actually looks chopped off… the tops are flattened as the amp can’t swing voltage. It clips… stops going higher (or lower on the other side of the wave… sortof).

Now if you are into music… ever hear a square wave on a synthesizer? Something about the laws of physics here… but it is loaded with high frequency energy… the amp gets “louder”… actually more awful.

So here is a cool fact: did you know tweeters are burned out more by under-powered amps than high-powered amps? Yep… it is because of clipping… the “creating” of a square wave and the resulting more energy going to high frequencies.

ANother example: notice when a guitar player “overloads” his/her amp? You get fuzz… distortion… coooool… now listen closely to that distortion… loads of high frequency content in there… that is a square wave going on.

Peace
Bruce in Philly
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#5

Thanks a lot for your answers Bruce.

I just had an idea since you mentioned current.
Wouldn’t be the max power consumption of an amp a good indicator of its driving capabilities?
For instance:

  • Denon PMA-720AE 50W@8Ohms, 85W@4Ohms, max power 200W
  • Cambridge Audio CXA 60 60W@8Ohms, 90W@4Ohms, max power 600W

#6

It might if you are referring to peak wattage output, but not if the spec refers to maximum draw from the mains. In the latter case, it would depend on how efficient the amp is in converting what comes out of the wall socket into a signal that drives your speakers. This is determined in large part by the type of amplifier: Class A amps are quite inefficient (35 - 50%). Class D amps - digital switching amps like the PS Audio Stellar’s are about 98% efficient. Class AB amps (like the PS Audio BHK’s) are in between. Each design has it’s + and - and trade-off with cost.

For simplicity I’d stick with B-I-P’s advice. No chance of going wrong there.

Also, if you search the ask Paul videos, in one of them he explains the difference between the different classes of amplifiers.


#7

In the short run yes, you can tell if an amp’s able to drive a specific pair of speakers easily enough and most modern amps will drive most speakers and protect themselves from damage if they cannot. So you’re likely pretty safe if you’re not trying to go exotic. That said, in the long run you cannot tell squat from the specs as to how it sounds. You can guess but then there’s every chance you’ll be wrong. I wish there were an easy indicator. We once played with the idea of an audiophile rating system but it never went anywhere. Too much squabbling within the industry. Maybe I’ll make that the basis of tomorrow’s post. Thanks for the idea.


#8

In Germany there is an audio magazine by the name of “Audio”, which assigns speakers and amps an “audio number” by measuring the impedance of the speakers and the output of the amps. So that readers can easily match the two. But of course this number is only available for the products they reviewed and I was hoping there was a more general solution.


#9

+1 what Paul said. I hadn’t looked at the thread before, and was simply going to answer the title question with, “no” : )

If you haven’t decided on either the speakers or the amp, choose the speakers, and worry about the amp second (assuming you audition the speakers with a good amp ; )

I was just posting in the Sprout/Elac question thread (as an inexpensive example) that if you buy the small Elacs, a high-end amp might be a waste, whereas if you bought the Sprout, it will do justice to a surprising range of quality and price of speaker.

Almost any speaker will sound better with more good power, even if it seems “overkill” on paper - you get what’s called “dynamic ease”. It doesn’t have to work as hard or strain itself into distortion to give you x dB at your listening seat.


#10

There is no wrong answer here, but for fun, let me disagree with you Mr Beef. Just for entertainment.

Your comment of speakers first, then amp… this is what I disagree with. I recommend investing in a good, beefy amp that sounds good. 100 Watts into 8 ohm is more than enough if the amp puts out decent wattage into 4 ohm… just for a reference point. Then determine if the amp sounds good. If you get both, then buy the amp.

Reason for my recommendation of amp over speakers has to do with time… over the years, I have swapped out speakers more and having the foundation of a good amp makes life so much easier to do. Instead of worrying about both or what to do next, you can focus on speakers more… and… another of my opinions here: IF you invest in a good amp, the biggest affect on sound will be your speakers (all else being equal). It is more fun to dick with speakers than amps… that is for sure.

(Now toob amps are a different neurosis altogether so I am mostly referring to solid state … unless you spend a ton on a monster tube amp… although my Prima Luna integrated HP with KT150s is pretty darn good.)

Peace
Bruce in Philly


#11

No disagreement here. Good amp is a good amp, though certain speaker choices might lead you to one topology or another.


#12

I’d prefer that an amplifier output almost doubles from 8 to 4 and 4 to 2 ohms.
It is not possible for an amplifier to perfectly double-down because of parasitic losses etc.
ideally, the amplifier should also be capable of supplying the current for extended periods. The bandwidth should also be very wide.
Speakers are not necessarily 4ohm, 6ohm etc. frequency dependant impedance changes are also coupled with phase - it’s the combination of changing impedance and varying phase angle that creates potentialy big relative low impedance which the amplifiers see a d therefore have to supply a lot of current.
Stereophile mentioned this for the BHK250 - in their summary they suggested that it not be partnered with speakers that dip below 3ohms.