Aug 282011

4-ohm or 8-ohm Speaker Taps ? That Is The Question

This is one of the most common questions clients ask me – “My tube amplifier has 4-ohm and 8-ohm taps, which one should I use ?”.
I would quote user kt88 from the forum, as he gave a good explanation.
“It’s not a good idea to mismatch impedances. Why do you think tube amps have multiple sets of taps? Obviously, it’s important. By keeping the speakers connected to the taps that most closely approximate your speakers load, you will get the most efficient transfer of power at full bandwidth.”
4-ohm and 8-ohm speaker output taps on the rear of an amplifier.
rear - speaker output taps
Connecting an 8 ohm speaker (or any higher impedance) to a 4 ohm tap …
“It’s a moving target because speakers have complex impedance curves, not a simple and static impedance. So speaker makers will rate their speakers nominal impedance which will provide you with the closest impedance for choosing a tap. The real fun part comes when you buy a 6 ohm speaker ;). What happens when you connect an 8 ohm speaker (or any higher impedance) to the 4 ohm tap is that the output transformers do more of the work and they get a bit hotter, so the better the transformer quality, the better transfer and lower distortion you have. The high frequencies will begin to roll off slightly in this example as well.”
Connecting an 8 ohm speaker to an 8 ohm tap …
“Connecting the same 8 ohm speaker to the 8 ohm tap will result in the most linear frequency response and greatest efficiency of power delivery across the band. Some amplifiers can deliver more power into lower frequencies at lower impedances due to the interface with and the design of their transformers but it is always advisable to keep the load matched with the tap on the transformer.”
Connecting a 12 ohm speaker to a 4 ohm tap …
“The situation can be a bit more costly if you take it further by connecting say a 12 ohm speaker to the 4 ohm tap. Under those conditions, the transformer will receive a lot more voltage and begin to get very hot as it will not be able to dissipate all of the energy into the load. So you could blow a transformer, which essentially destroys all but the more expensive amplifiers as replacement is very expensive. The surest way to destroy an output transformer is to operate a tube amp without a load connected.”
Connecting a 4 ohm speaker to an 8 ohm tap …
A bit of a reverse situation is formed inside the amp when you connect a 4 ohm speaker (or any lower impedance) to the 8 ohm tap. The output tubes work harder in this arrangement and may not last as long. Again, you will not get the most efficient power transfer to the speakers but this time, the lower frequencies will be limited in power“.
Connecting a 2 ohm speaker to an 8 ohm tap …
“So you can use taps as a sort of mild tone control in this way as long as the output transformers and output tubes and surrounding circuitry of the amplifier is of good quality and as long as you don’t get too far away from the target impedance. In other words, a 2 ohm speaker on the 8 ohm tap is stretching it too far here as well.”
Rule of thumb …
“A good rule of thumb is to never be off by more than 100% of the value, so nothing below 4 ohms or above 16 ohms should be connected to the 8 ohm taps. Look at it as a factor of two and anything more or less than that will be destructive to the amplifier. The speakers are not really affected here other than the limiting of their power bandwidth. If you are going to mismatch impedances, to avoid any amplifier damage, you really need to be careful of input levels as well as that is what drives the voltage swing inside the amplifier. If you crank up the preamp and get a lot of power output, you are going to see higher voltages inside of the amp. Nothing wrong with that as long as the amp is designed to handle it and you are operating it within its specifications but if you are mismatching its expected load and output, you can get into some problem areas.”
“There are multiple taps on a tube amp for these reasons. You use each for specific loads and if you use the wrong one, in some situations you could have some problems.”
I hope this helps some readers here about different speaker taps found in their tube amps. I’ve always had good amps with excellent transformers, so I did break the rule of thumb mentioned above once in a while, as I like the 4 ohm tap for its more weighty bass, at the expense of a bit of highs rolling off, which is still acceptable in my system. This varies of course depending on one’s system. On one occasion with a 6-ohm speaker mated to an integrated tube amp which had both 4-ohm and 6-ohm taps, I found minimal sonic difference between these so I just hooked the 6-ohm speakers to the 6-ohm taps.
* If you want to share more on this topic, or if you have other hifi tips to share, just reply using the comment form below this post, thanks.

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