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Zu Soul Superfly 8 ohm experiment

Have been Zu owner for many years starting with Tone progressing to Druid -then Essence and now Soul with nano tech drivers. Drive the speakers with Unison Research Preludio which is a 15 watt single ended class a amp which utilizes 6 ohm output taps. After reading a review of the speakers which suggested the speakers sounded best when paired with an amp that utilizes 16 ohm output taps i posed the question to Sean. he suggested i could experiment altering the impedance of the speakers by utilizing a 16 ohm resistor to lower the impedance to 8 ohms. I have generally been happy with the sound of my system except for a bit of hardness/glare in the higher frequencies that shows up on some recordings . i attributed this to the recording technique as ell as less than optimal listening environment (10x12 room ) also wondered if perhaps the preludio was not happy with the 16 ohm impedance. Decided to give Seans' suggestion a try and purchased a pair of 15 ohm Mills resistors from parts Express. Wrapped the leads around each of the speaker terminals -placed the spade lugs of the cables over the leads and tightened down the connector. have to admit I did not expect much, but was pleasantly surprised. Thought the speakers sounded cleaner - vocals clearer and a definite decrease in the high end glare/hardness . After changing speaker cables I had been bothered by what seemed to be an exaggeration of sibilance . but noticed that it now sounded much more natural and the hardness associated with it improved. wonder if any other Zu owners have had a similar experience ?
dcr

Showing 1 response by atmasphere

atmasphere's avatar
atmasphere
12,098 posts
 
Its really important with a transformer-coupled amplifier to load the tap of the output transformer at the impedance it is designed for.

If you put a 16 ohm load on the 8 ohm tap, the transformer can ring (add distortion) which will make it brighter and harsher. Conversely loading the 8 ohm tap with 4 ohms will load the output tubes too hard and will also cause additional distortion, in addition to a loss of power.

However putting a 16 ohm resistor in parallel with a 16 ohm loudspeaker means that although the result is 8 ohms, half of the amplifier power will be turned into heat in the resistor. If you have plenty of power maybe that is not a problem, although the resistor might get quite warm.

Obviously a more elegant solution is to have an amplifier with a 16 ohm tap. FWIW there are a few amps that are optimal when driving 16 ohms. That would be- any amp with a 16 ohm tap and most OTLs. Transistors, although they make less power into 16 ohms, also make less odd ordered harmonic distortion (IOW they sound smoother). It strikes me as a bit odd that a tube amp, if transformer coupled, would not have a 16 ohm tap.
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