Advantage or disadvantage of using 8 ohm tap verses 4 ohm tap

I have speakers rated at 8 ohms but it dips in one spot to 3.5 ohms. My tube amplifier offers an 8 ohm or 4 ohm option. Due to the dip I have chosen to use the 4 ohm tap. My question is will this work the power tubes harder and therefore will they need replacing more frequently or will there be no additional strain on the amp or tubes by using the 4 ohm tap even though the speaker is basically rated as 8 ohm?

 

On a tube amp, the choice of tap makes no difference to the amp. You are free to choose whichever sounds best.

 

whichever sounds the best. I have the same option, my speakers are nominal 6 ohms and the 8 ohm tap clearly sounds better. hopefully it will be obvious to you which one sounds the best.

 

My amps have a triode/pentode switch (33/75 watts) and in the triode setting 4ohms sounds best. I am experimenting with the pentode setting in the 4ohm tap and will try the 8ohm later. Prior to going too far I just wanted to be reassured no issues would occur. I had been told it was OK, just wanted a little more reassuring.

Thanks.

 

Not seeing the specs, I have to guess that the impedance dip is in the low frequency range, which is typical of 3-way speakers. It is probably fine to use either tap of the amplifier. It does matter though. The amplifier can only deliver its rated output at the specified impedance via the output taps. So the speaker must match the tap as closely as possible to achieve this. Where it becomes complicated is that the speaker never has a constant impedance, rather it varies with frequency of output. So, the lower impedance at lower frequencies being mated with the lower impedance tap should result in deeper and tighter bass response of the system. The opposite is true with the mid and top end if the impedance rises a bit with frequency, usually with output peaking at mid frequencies. Then, the 8 ohm tap will deliver more power into those frequencies. So by listening to your system, you can determine which tap sounds like it offers the most correct, ie flat response. I would try this with the mid and HF controls flat, as well as speaker placement to make them sound flat in-room where possible. If you find yourself having to turn down or turn up the level controls of the mid and high frequency drivers, you need to experiment a bit more with room placement as there will be response deviations due to those settings. The long and the short of it is that you want to use the tap that provides the flattest response when the speakers controls are set to the nearest to flat position and the cabinets are placed in a position that provides the most flat bass response.
-Bill

 

Most amps will be most dynamic and have the tightest bass and most extended range in the pentode setting. You need to choose the right tap and speaker placement to get there though. It may be more pleasant listen on many recordings in triode mode, but you'll probably be listening to a more colored sound overall. It depends on your room and listening distance. If you are in a small room, near field, then triode may be more clear and sweet throughout. It tends to lose some top and bottom end extension and control as well as dynamics as the speakers become more difficult to drive and the room size increases IME.
-Bill

 

For those who have more expertise than me, the impedance curve for the OP's speakers can be found here. SoundStageNetwork.com | SoundStage.com - NRC Measurements: KEF Reference 1 Loudspeakers .

I kinda think they should be rated at 4 ohm anyway.

 

I don't see where those are his speakers, but if so, I agree with you.
-Bill

 

You are correct, I just bought a pair of KEF Reference 1s and am using a tube amplifier 33/77 (Triode/Pentode). Room is 11 x 15, so on the small side. I have noticed exactly what every one has commented on, you all are very smart folks. Haven't tried the Pentode mode in the 8 ohm tap but will give it a chance. All the oomph goes out in Triode 8 ohm tap so the 4 ohm is my choice but was worried if this could have a bad outcome for the amp. From what I am reading the answer is no. I have been just recently experimenting with the Pentode setting and definitely more muscle behind the music, maybe slightly less sweet, but this is in the 4 ohm tap. I will try the 8 ohm tap in Pentode mode and listen to what happens. I am tending to agree that the speaker might best be served as a 4 ohm speaker. Testing will tell, thanks all. Great information and reassurance that I am not harming my amps.

Michael

 

The amplifier will work harder when the tap selected is higher than the impedance seen;

easier on the amplifier if a lower impedance tap is selected than actually seen however maximum available output power will be reduced.

Best to match as closely the tap with the nominal speaker impedance (the predominant value as shown in the speaker's impedance curve) for the maximum distortion free power, as KT88 says.

 

I would use the 4 ohm tap with those speakers in either mode. Since you can just flip a switch to change modes, it's a no brainer. Just select the 4 ohm taps and then use whichever mode of operation best suits the music. Personally, I find more potential in the Pentode modes and so would use that for speaker set-up for best bass response. You can always flip to triode if you play a particular acoustic piece that is better served that way.
-Bill

 

You just answered my question, which I hadn't even asked. My new PrimaLuna has 4 and 8 Ohm taps, my Wharfedale's are 6 Ohm rated. Will try both to see which sounds better.

 

The main difference between different taps is that the maximum amount of available power will be altered. Otherwise, sonic quality is pretty much unaffected. Generally speaking, lower taps provide the greatest current and the least voltage; visa versa for the higher taps.

Different speakers draw varying combinations of voltage and current; the idea is to match these demands as closely as possible with the output stages of the equipment.

 

Thanks for sharing this.

Unless I am misunderstanding something, in my case, the nominal speaker impedance assigned by the manufacturer seems to be incorrect. From Stereophile Bowers & Wilkins 683 S2 loudspeaker Measurements

So, based on that info, does it seem the speaker is nominal 8 ohm? Or more like 4 ohm?

 

Probably closer to 6, but it is an average as there is never a constant load at a specific impedance for any speaker.
-Bill

 

Thanks, Bill. Would there be any harm in using the 4 ohm tap for the above speakers?

 

No. It should sound a bit warmer if that helps.
-Bill

 

Thanks.

Can you explain a bit about why that is? I am trying to understand how the choice of output on an amp affects tonality.

 

If the output matches the load exactly (not gonna happen, this is theoretical), the power is most efficiently transferred across the bandwidth of the amplifier. If the impedance is mismatched, more or less power is lost at the extremes of impedance and frequency. Usually, an amp benefits from a lower tap as all loads are lower than the tube output impedance before the transformer, and the lower the tap, the more closely it matches the lower frequencies, which typically draw more power anyway.
-Bill

 

Many thanks for explaining, Bill.

I just did some comparisons between my 6 ohm and 4 ohm taps. I like the sound better on the 6 ohm, a bit more open on top and more even tonality.

I didn't mention, but the speakers are less than a week old and still breaking in. I was getting a persistent thumpy sound from the woofers for the first few days using the 6 ohm, so I switched to the 4 ohm and the bass tightened up a lot. But the upper frequencies dulled so now I am back to the 6 ohm. I am just hoping with more time the woofers smooth out. I am at 43 hours now. These are the speakers: 683 S2

 

You are using a tube amp with Kef speakers? Wow. And in triode mode? Wow x 2. That amp must be working hard in either mode.

 

Some contend that Direct Coupled (output transformerless) amplifiers produce better results with speakers having wide impedance swings.

 

Actually they are not working very hard. I have settled in on Ultra-Linear, 4 ohm tap and they have a great sound. No harshness, extended both in the highs and lows. Obviously it isn't going as low as if it were a bigger box with more woofers but there is a feeling of missing low end. Plays louder than I would ever want. I am using the longer tubes in the rear ports. Large transformers I believe is what is helping all this out.

 

OK, just to update, I have new speakers (B&W 704 S2) that have over 200 hours of play time on them. While breaking them in, I have often thought they were a bit dull on top and heavy on the bass. (They are more than 2 feet off the rear wall and that's as far as they can get.) While breaking them in, I had them on the 6 ohm tap on my Yamaha amp listed in my profile. The only other option is a 4 ohm tap. The speakers are rated 8 ohm.

So today I was listening to a James Taylor CD and it sounded a bit bassy and like JT was behind a veil. For the heck of it, I switched to the 4 ohm tap and sure enough, the bass was tamed and JT was much clearer! I tried some other recordings and found that the 4 ohm tap gave me less bass (or perhaps tighter bass) and a more open midrange and treble. I finally can hear the famous B&W midrange!