[denjo]

Hi
My Cary 300 SEI integrated amplifier (300B SET) has a 4/8 ohm impedance selector to match the speaker's impedance. The Harbeth Super HL-5 is rated at 8 ohms but dips below 6 ohms a couple of times throughout the frequency spectrum. Although a safe choice would be to leave the amplifier selector at 8 ohms, I have been tempted to toggle it at "4 ohms". That way, it seems, I should be getting more power which might help in giving the speakers more power to control the bass and LF. However, I need to know whether this might potentially damage my amplifier or speakers or both.

Any advice or shared experience would be much appreciated.

Best Regards
Dennis

[doug s.]

you won't damage anything, but theoretically, you will be halving the amp's watts output into 8 ohm speakers if run from the 4 ohm tap.  if the speakers are efficient enough & the amp has enough watts at half-power, you could certainly try it...

doug s.

[denjo]

Thanks for your advice Doug! In that case I should just stick with the selector at 8 ohms since it will give me more power here.

Best regards
Dennis

[mitch stl]

>> I have been tempted to toggle it at "4 ohms". That way, it seems, I should be getting more power...

That's not the way tubes work. The point of the output impedance selector switch is to better match the amp's output to the speaker.  Output tubes are a high voltage device. Depending on the amp, you could be looking at 400 volts or more output at the tube. That is too much voltage (and not enough current) for your speaker so you run it through an output transformer to lower the voltage and increase the current. The 8 ohm tap comes off a different point in the transformer's winding than the 4 ohm tap. Neither tap increases the power from the output tube. Using the correct tap allows a more efficient transfer of the power you do have.

All speakers have dips and peaks in their impedance curve. That's why they figure a "nominal" impedance. That's the number you use to match the speaker to the amp.

As an example, if you've ever driven a car with a clutch and a manual transmission, you don't start from a dead stop in 4th gear. You use the appropriate gear for the task at hand. Same thing with a tube amp's output impedance. Use the one that is the best match for your speaker.

[denjo]

Hi mitch stl
Thanks for your explanation. The car analogy is a good one, now I understand this subject a little better! Since the Harbeths are specified as 8 ohms nominal, I think your views confirm that I should really leave the selector at 8 ohms and leave well alone!

Many thanks!

Best Regards
Dennis

[Duke]

Not to take anything away from mitch stl's explanation, but my parents have a JoLida 302b and back when they had 16 ohm speakers in my opinion they sounded best driven via the 4-ohm tap (8-ohm is the only other choice).  Now they have 8-ohm speakers, and once again the 4-ohm tap sounds best (though in both cases the 8-ohm tap plays louder).  I can't explain it, and I'm not trying to say anything other than you might try it both ways just in case.   

Duke

[doug s.]

Not to take anything away from mitch stl's explanation, but my parents have a JoLida 302b and back when they had 16 ohm speakers in my opinion they sounded best driven via the 4-ohm tap (8-ohm is the only other choice).  Now they have 8-ohm speakers, and once again the 4-ohm tap sounds best (though in both cases the 8-ohm tap plays louder).  I can't explain it, and I'm not trying to say anything other than you might try it both ways just in case.   

Duke

that's why i said dennis could try it.  my experience w/my (since sold) coincident victory's, which are 14 ohms nominal, warying from 10-18 ohms, was that the did fine on 8 or even 4 ohm taps, if the amp's power was sufficient, but on low wattage amps, (ie: 4-5wpc), they preferred 16 ohm taps.  they were quite efficient, at 97db/1w...

doug s.

[Steve]

Yep, damping increases, total distortion and power output both decreases. Should be no problem with the output tube running 8 ohm speak on 4 ohm tap.

A general principle I hope will allow you to relax is that with class A1 operation, maximum plate dissapation occurs with no output power, no input signal. As the power output rises from zero, the output power is subtracted from the plate dissapation. So if the plate dissapation at idle, no output power, is 40 watts, and the volume is increased to 10 watts output, the plate dissapation is now approx 30 watts.

Class A2 is a little different, but you should have no problems.

Hope this helps.
Steve

[jeffreybehr]

FWIW, I agree that:
1. No harm will be caused by using either output tap;
2. Changing taps does NOT change the power available, but the 8-Ohm selection will increase the amp's Voltage gain and will require a slightly lower volume-control setting for the same output level;
3. Something mentioned but only briefly is that changing taps changes the way the amp damps the speaker's back-EMF and hence bass performance.  The 4-Ohm setting will damp more--that is, the amp has a higher damping factor--and the bass will sound tighter, less 'bloomy', while the higher setting will damp less and allow the bass to sound 'fuller'.

Try the other setting for more than a few minutes; you may prefer the sounds.

[mitch stl]

I think the additional commentary is helpful and interesting to Dennis, but keep in mind that his original question was wondering if switching his tube amp to the 4 ohm setting would give him "more power."

It won't.

Of course, unlike the car analogy, there is no clutch to burn out by trying 8 ohm speakers on the 4 ohm tap.

Dennis, I'm working on a slightly longer reply that may help you better understand how tube amps deliver power to a speaker. However, generally speaking, if you have a 50 watt tube amp, that is the most it is going to deliver before clipping. Doesn't matter if you have an 8 ohm speaker or a 4 ohm one. Nor does it matter if you use the 8 ohm tap or the 4 ohm one. If you are on a quest for "more power" then your solution is to buy a more powerful amp.

This is different from a solid state amp that will often increase in power as the speaker impedance drops (assuming the amp design is capable of delivering the extra current. Many amps aren't.) Of course, even in this case, the power issue is driven by the impedance of the speaker and not a setting or switch on the amp.  (Think of a wall outlet. A 15 amp wall socket can deliver about 1800 watts of power. If you plug in a clock radio, it draws maybe 10 watts no matter what the wall outlet is capable of providing.)

Here's a bit of additional information that I've added to this post in an edit.

Let's say your output tube is at 400 volts and needs to see an impedance of 4400 ohms. Obviously your speaker isn't going to be of any help handling things straight off the tube - the voltage is too high for the speaker and the speaker's impedance is too low for the tube. The transformer handles the conversion from a higher voltage to an appropriate lower one for the speaker but still allows the tube itself to see the correct impedance.

The transformer has two sets of windings in it. One set of windings has a lot of turns that is connected to the output tube. This gives the higher impedance needed by the tube. The other set of windings has a much lower turn count and is connected to the speaker. The ratio of the high turn count to the lower turn count reduces the voltage at the output. The tube gets its high impedance and the speaker gets a lower voltage.

The selection between an 8 ohm and a 4 ohm speaker connection is accomplished tapping into the speaker side of the 8 ohm transformer windings before they are finished. This gives a still lower voltage at 4 ohms than at 8 ohms, so the total power output stays the same. (Assumes an 8 ohm speaker connect to the 8 ohm tap and a 4 ohm speaker connected to the 4 ohm tap on a tube amp.)

However, what some have talked about is the fewer number of turns on the 4 ohm winding has a lower DC resistance. That means it absorbs any back-EMF generated by the bass speaker driver better than the 8 ohm tap which has a higher resistance. Whether this extra dampening factor is of any use in a particular set-up depends on the speaker involved and a host of other factors, not the least of which is the personal preference of the listener. Just give it a whirl and if you like the 4 ohm tap better, great! Just keep in mind that using an 8 ohm speaker on a 4 ohm tube amp tap will effectively result in a bit less power, not more. So there is a trade off and you have to choose which is more important to you.

[denjo]

Duke, Steve, Mitch and Doug S

Thanks very much for your replies and insights - very helpful indeed!

Well, fortified by the views tht switching the taps would not damage amp and/or speakers, I tried the 4 ohm tap briefly but somehow felt that the sound was more "together" via the 8 ohm tap. My Harbeth dealer says that from his experience with other tube owners, he feels that the 4 ohm tap usually gives a better sound. At the end of the day, I have to trust my own ears and what may apply to a system might not apply to mine.

Many thanks,

Best Regards
Dennis

[TRM]

very happy 4ohm tap with 8ohm speaker user here