Which Power Amp should I use for my Wharfedale Diamond 9 Speakers

Dicky B

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Hi All,

Please help, I have a pair of Wharfedale Diamond 9.0 Speakers, 75 Watts with 6 Ohms impedance and I have a couple of oldish Stereo Power Amps and am not sure which is best suited for the speakers.

The first and oldest Power Amp is a Hitachi HA-3800 and according to the info on the back, it's 300 watts with connections for up to 4 speakers; A or B - 4 Ohms to 16 Ohms per speaker or A and B - 8 Ohms to 16 Ohms per speaker.

The second Power Amp is a Kenwood KA-660D, 200 watts also with connections for up to 4 speakers; A or B - 4 Ohms to 16 Ohms per speaker or A and B - 8 Ohms to 16 Ohms per speaker (exactly the same as above).

I've been trying to get my head round this as there is a lot of conflicting information on the net regarding the mysteries of correct power output and impedance matching from your power amp to your speakers. I'm sure I've got something wrong here but from what I've read the power amp should provide about twice as much wattage per channel (at the speaker's nominal impedance) as the continuous power rating of the speaker. Therefore as my speakers are 75 watts with a nominal impedance of 6 Ohms I will need a power amp that can produce 150 watts per channel into 6 Ohms. Does this therefore mean that I need a 300 watt stereo power amp? Presumably I would also need to connect my speakers to the A or B connections as the 6 Ohms of the speakers falls within the 4 to 16 Ohms impedance provided by the A or B connections. However I have also read that you should try to match as closely as possible the Power Amp impedance with that of the combined impedance of the speakers. Well according to the formula for calculating combined speaker impedance (I have two 6 Ohm speakers therefore 6 (Ohms) divide by 2 (number of speakers) = 3 Ohms, the lowest impedance either of my power amps will deliver is 4 to 16 Ohms which means that since the combined speaker impedance is lower than the output impedance rating of the power amp I could damage my power amp.

Any suggestions gratefully received.
 
It is not a matter of matching the power and impedance of a speaker precisely to an amp. You need the power ratings and the impedance to be within a range.

For example, both your amps appear 4 ohms stable. When you connect two 8 ohm speaker, the result is 4 ohms. So that implies the working range on both amps is 4 ohms per channel to 16 ohms per channel, and that is somewhat standard on all amps.

As to the power rating, the ratings are up to a given wattage, but there is some flex there. With some speakers you will see power ratings like 25w to 100w. This implies that the manufacturer thinks you need at least 25w per speaker to make the speakers work properly, and they are recommending that you not drive them with over 100w.

So, much like impedance, we have a working range - 25w to 100w.

Now the key is the impedance per amp channel, not the overall load on the amp as a whole. Each internal amp is a stand alone entity, and we are concerned about how many speaker are placed on that one internal amp channel.

To get the value of the PER CHANNEL load, you take the impedance of one speaker and divide it by the number of speaker ON THAT ONE AMP CHANNEL.

So, if you only have one speaker on each channel, then the impedance per channel is 6 ohms and within the working range. However, if you attempt to use two pair of speaker, meaning two speaker on each amp channel, we have 6/2 = 3 ohms, and that falls outside the working range of 4 ohms to 16 ohms.

So, if you have stereo, then you have left and right amp channels. If you connect one speaker to the right channel and one speaker to the left channel, everything is fine.

If you connect two speakers to the left channel and two speakers to the right channel, then not so fine.

The left and right speaker terminals are connected internally, so both left speaker connect to the same amp.

Back to power. The 75watt rating on the speakers is a guideline, not an absolute. These speaker should work fines on an amp that is UP TO 75 watts per channel.

The proper specifications on the Hitachi HA-3800 is 35w per channel.

Two sources indicated that the Kenwood KA-660D is 70w per channel.

So, both amps are in the working power range of your speakers.

However, it is not power as such that blows speakers, it is volume. Any amp large or small when turned up too loud has the potential to damage speakers.

Even at pretty high volume levels, the average power is only a few watts. So, there are some people who use amps well above the power ratings of their speaker, on the assumption that they are never going to drive them with full power. But, they have to be cautions and use common sense, and not drive the speakers at excessive volume.

Whether an overpowered or underpowered amp, common sense is your biggest source of safety.

In short, your speaker will work with either of your amps.

By the way, the 300w and 200w you are seeing on the back of the amps, that is the power supply power, not the amp channel output power.

Does that help your understanding? I know I can ramble on too long sometimes, but hopefully I've explained it well enough for you too understand.

Steve/bluewizard
 
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Many many thanks BlueWizard, you've answered my query perfectly, it took a few re-reads for it to sink in but I've got it now, much more detailed explanation than anything else I've read elsewhere. Not only was I misinterpreting the power supply power on the back of the amp with the output power (easily done though since I have no paperwork for either amp, why the hell don't the manufacturers state the output power on the back of the amp?) but I was also getting mixed up with what constitutes a 'channel'.

One final query though, if the power output of either of my amps falls within range of the speakers (75w) and is what I need to achieve then why have I read in several places that the power amp should provide about twice as much wattage per channel (at the speaker's nominal impedance) as the continuous power rating of the speaker?

Thanks again.
 
I touched on that. Some people operate on the theory that you are never really going to use all the power available in an amp, so you can get an amp bigger than your speakers are rated at.

And that is true, if you can trust yourself to use the amps wisely and conservatively. I always say - It is not overpowered or underpowered amps that blow speakers, it is the (pardon the expression) Idiot running the volume control.

Again, you can damage speaker with any amp. In your case, you could destroy your 75w speakers with a 25w amp poorly used. Though it would take some extreme volume to do so.

In most cases, there is little point on playing above 50% volume (12 o'clock on the dial). At the most, you can go up to about 60% volume (1 or 2 o'clock). Above that is nothing but distortion and danger. I typically play in the 10 o'clock to 11 o'clock range even for a party. And that is very loud, but still within the safe operating range.

So, this idea that your amp needs to be twice the power rating as your speakers, that's just somebody's opinion, and a slightly ill-informed opinion at that.

If we follow that advise, in your case, you would need a 150w amp. But off the top of my head, the only 120w to 150w consumer amps I'm aware of are in the £800 to £1000 price range. If you can afford amps like that, you are not going to be using Diamond 9.0 speakers.

- - - - - - - - - - - - - - - - - - - -

One last point regarding power and volume, though somewhat on the technical side.

Power and Volume are not linear. That is, they don't have a one-to-one correspondence. Meaning, that if you turn your amp up twice as loud, you are NOT using twice as much power, you are using TEN TIMES MORE power.

With each slight turn upward of your volume control, the volume increase by 3db, that represent TWICE as much power; a slight change in volume DOUBLE the average power consumed.

So, let's start with 1 watt, which is pretty loud by the way, and move the volume control up in tiny little nudges, with each uniform nudge, we get an equal and slight increase in volume. But this is what happens to the power -

1w, 2w, 4w, 8w, 16w, 32w, 64w, 128w

Now, when you are on the high end of the volume dial, you can see that each nudge upward of the volume control, even though the change in volume is slight, the actual power is leaping upward massively; 16w becomes 32w which in turn jumps to 64w, which in turn jumps to a massive 164w. Yet you perceive small uniform increase in volume with each step.

This is what blows speaker, the massive surge in power for what is perceived as a slight increase in volume. And this is precisely why it is VERY WISE to stay away from the high end of the volume dial. Above 50%, the dial becomes very sensitive to power, and small turns upward equal huge jumps in power.

For what it is worth.

Steve/bluewizard
 
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Thanks again BlueWizard, informative as ever. You mention that you do not turn the volume beyond the 10 or 11 o' clock position even for a party, however I have read that if using a mixer then the master volume on the mixer should be down low but the power amp volume should be turned up to the maximum setting. What's your view on this?

Many thanks

Dicky B
 
Dicky B,

In a situation like that I wouldn't turn the power amp to full volume. If you have an amp you can try it, with no input signal at full volume you start to hear a lot of amplified hiss, which is circuit noise being amplified. In this case I would turn the volume up to about 3 o'clock, and let the pre-amp to most of the controlling.

Steve/bluewizard
 
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