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 ?
I have a pair of Zu Druids MK IV 08 and added a 51 ohm Mills resistor across the terminals of the tweeter. The overall impedance (technically just resistance as measured by my ohm-meter) was 7.9 ohms on each side.

The harshness was definitely removed and sound level was altered given the same volume position on the control amp. The things is that I also replaced the Kimber Kap with a Mundorf silver oil cap, which would have helped soften some of the high end bite. Overall though, it's a more pleasant sound to my ears than before the mods. It's also a pretty easy mod to reverse.
Impedance matching between amp outputs and speaker can have strong effects. Resistors are cheap and easy, but if you settle on the advantage, here's an even better way to do it:


The AntiCables autoformers for load matching do a superb job and improve dynamics over going the resistor route.

Funny you should mention the anticable site as a friend talked to me about it as well. Really happy with the sound of the speakers now and will probably leave the resistor in place for awhile. Speakers seem more coherent- bass response/detail is improved and sound stage width and depth greater. Noticed that the Zu Union and new Soul II both utilize an 8 ohm driver. I wish that Zu would offer an 8 ohm driver version of the Superfly driver as there seems to be few amps that are optimized for or have a 16 ohm tap.
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.
Thank you for the information . That explains why the speakers sound cleaner/clearer now as the distortion has been reduced. Checked the resistors for heat and did not seem even warm to the touch after 1.5 hours of listening. I am happy with the amplifier,although with a fixed output it may not be as versatile as i like. Agree that the resistor is not an ideal solution and hope that an 8 ohm driver will be added to the Zu lineup
I'm interested in this topic, but I have to admit, as a newbie it's a bit over my head. Phil, I looked at the anticable site, but it seemed like the autoformers were made for dividing the ohms to a smaller number...8 ohm tap on an amp to 4 ohm speakers...the issue with the super fly's is they are designed for 16 ohm taps and my amp has 8 ohm taps, in this scenario is the autoformer still an option? Sorry if I am totally misunderstanding the concept here...

Dcr...for the resister route, do they come with instructions or is there a resource that shows how to do it...sounds simple based on your description, but still would like to see how it's supposed to work...
Purchased the Mills 15 ohm resisitors from parts express . they are rated @ 12 watts so may not be suitable if your amp is high powered. consider that the output from the amp will be halved by the reisistor meaning half will go to the speaker and half to the resisitor. my amp is 15 watts so half will be about 7.5 watts . the resistor is not noticeably warm or hot after playing the amp.
there are higher rated vishay resisitors (50 watts ) available from mouser.com but the mills resistors are working fine.

the resistor has a wire lead at either end - there is no positive or negative . with needle nose pliers gently bend the wire to form 90 degree angle @ the barrel of the resistor. at the far end of the wire - bend in to circular shape to fit around the terminal posts on the speaker. remove the cardas clamp and fit the circled ends around the posts and place the spade lugs over them before tightening down the clamps.
if you like email me, and i would be happy to forward pics
Thanks...sounds super easy, I'm going to give it a try.
>>I looked at the anticable site, but it seemed like the autoformers were made for dividing the ohms to a smaller number...8 ohm tap on an amp to 4 ohm speakers...the issue with the super fly's is they are designed for 16 ohm taps and my amp has 8 ohm taps, in this scenario is the autoformer still an option?<<


You're right that the autoformers are intended to multiply the speaker impedance that an amp sees. But if you wire them "backwards," your amp will see 1/2 the Superfly's rated impedance. From Paul Speltz' web site FAQ:

"10) What happens if I run them backwards?

Most everybody uses the ZEROs to MULTIPLY the speaker's impedance by connect the BLACK and WHITE leads to the speaker, and connecting two of the other four leads to the amplifier. In contrast, you can use the ZEROs to DIVIDE the speaker's impedance. This can be used to optimize your speaker's impedance to the rare amplifier that prefers a low impedance speaker, like the Decware Zen tube amplifier that has only a 2 ohm output tap. This can be done by connecting the BLACK(-) and WHITE(+) leads to the AMPLIFIER, and connecting two of the other four leads to the SPEAKER.

-Divide speaker's impedance by 2 = YELLOW (+) & BLUE (-) to Speaker (ie. 8 ohm speaker becomes 4 ohms)

-Divide speaker's impedance by 3 = YELLOW (+) & BROWN (-) to Speaker

-Divide speaker's impedance by 4 = GRAY (+) & BROWN (-) to Speaker

You will notice the only thing that changed is that the connections to the speaker and to the amp are FLIP-FLOPPED...thus making it an impedance DIVIDING autoformer, instead of an impedance MULTIPLYING autoformer.

Please experiment for best sound. As your speaker's impedance is reduced, the Zen amplifier will be able to source more and more power into the speaker down to the maximum power point of 2 ohms (Zen amp). Best sound will be achieved when a balance between most power and "critically" damping the speaker is achieved. Reducing the speaker's impedance to low will under damp the drivers causing an overly blooming bass. Trust your ears, and when you achieve the best sound possible, you are done. An interesting article on critically damping your speakers can be found here: www.otlamp.com/articles/tomcik/index.html"