Speaker impedance questions (SE KT88)

[porkchop61]

Hi all-
I've just finished building a pair of TQWP speakers to use with Mikael's SE KT88 amplifier.
BD TQWP speakers
I've built the speakers with 8 ohm speakers (2 used per speaker), and I've wired them in parallel so it's a 4 ohm load to the amp.

My question is, is there any benefit to wiring the speakers for series (16 ohm load) over the 4 ohm load? I'm using Hammond OT's, and have all the taps wired to a rotary switch so switching between impedance isn't an issue. Or can I try running the OT's at taps other than what the speakers are wired for? What does this do to the sound or power output, and can it damage anything?
Thanks in advance.
Glenn

[Tom Bavis]

With the Hammond transformers, you use all the secondary for 4 or 16 Ohms, so there really isn't an advantage to either. If there were only one tapped secondary, then it would be best to use 16 Ohms, since it would use the entire secondary winding. Improvement would be mostly in high frequencies - due to improved coupling.

[porkchop61]

Thanks Tom.
So no real advantage to change my speaker wiring then.
I'd actually like to add a tweeter to my speakers, but it will make for a weird impedance due to using three 8 ohm speakers.
Glenn

[Tom Bavis]

Speakers aren't resistors. The impedance varies with frequency, and typically the the crossover is limiting signal to one speaker or the other depending on frequency. So generally an 8 Ohm woofer and 8 Ohm tweeter makes an 8 Ohm speaker. There are complications of course, especially around the crossover frequency.

[porkchop61]

Yes, I know the impedance varies with the frequency. These speakers have no crossover, so that isn't a consideration in this case. I only wanted to try adding a ribbon tweeter with a capacitor to extend the upper frequency range. I just didn't want to add another nominal 8 ohm speaker to the existing two drivers. This gives a weird impedance (nominally again). Maybe a nominal 12 ohm impedance isn't a big deal to the OT's, but do I use the 8 ohm or 16 ohm tap? That's what is really puzzling me.
Glenn

[porkchop61]

Bump?

I'm still wondering how to safely add a tweeter to my BD pipes speakers. How does adding a mis-matched load on the OT affect the OT? Is it damaging, or just not efficiently transferring the power to the speakers?

Glenn

[Sherman]

Originally posted by porkchop61
Bump?

I'm still wondering how to safely add a tweeter to my BD pipes speakers. How does adding a mis-matched load on the OT affect the OT? Is it damaging, or just not efficiently transferring the power to the speakers?

Glenn

If the impedance the amp is feeding into is a little low then power output goes up a bit and so does distortion. If the impedance is too high power output goes down and so does distortion.

If impedance is way too low the amp has to work too hard and could possibly be damaged when trying to play at a high sound level. Also when the impedance gets to low you may well notice the sound is pretty crappy. Bass goes away and there isn't any "sparkle" in the high end. What sounded like a nice unit with proper impedance sounds like a table radio with too low impedance.

Tubes have a pretty high tolerance for mismatching impedance but I wouldn't put a 4 ohm speaker on a 16 ohm tap.

As for impedance varying with frequency response- it happens even with a single driver speaker. It isn't only an issue with multiple-driver speaker systems.

Check the impedance/frequency response curves for the driver you've used. You'll probably notice that at the low end impedance is lower than the nominal 8 ohms. Above maybe 5 KHz the impedance probably climbs rather steeply. Peak impedance could well be 16 or 20 ohms or more. (You'll have to look at the curves for the driver to know for sure.

Adding a tweeter or supertweeter to a single driver speaker can be as simple as using a cap. Putting the high-pass frequency high enough that impedance on the main driver is high and the combination with the tweeter is acceptable. In fact the frequency/impedance curve of the of the combination may be flatter than the single driver alone. (That doesn't necessarily mean it will sound better, just that it presents a more constant load to the amp.)

[Robert McLean]

I think what Tom was trying to explain a couple of posts ago is that when you install a tweeter in the simplest way, ie a capacitor and the tweeter just connected across the main driver, you dont really changed the nominal impedance. There is a low frequencey at which the driver has the nominal impedance, and then as frequency rises the impedance rises. When you put the tweeter and capacitor across it, that frequency rise gets flattened out. Its like a zobel network used to flatten speaker impedance curve with the added bonus that you get high frequency sound out of it.

Any way the point is the impedance curve is flattened out, and you can consider it to have the same nominal impedance as the main drivers. so add the capacitor and tweeter across the woofers, and hook it all up to the 4 ohm tap if you kept the woofers in parallel, or to the 16 ohm tap if you changed your mind and put the woofers in series.

As for your question about damaging the OPT, no, not unless the mismatched load is a complete open circuit or complete short circuit and the volume is turned up. An impedance less than nominal will result in less power transfer, an impedance more than nominal will result in more distortion, especially with pentodes if the load line goes past the "knee" of the plate curves. The range of impedances that work well is fairly broad.

Short answer is just go ahead and do it.

[porkchop61]

Thanks Sherman and Robert.
I do have a capacitor in series with the + terminal (2.2uF) just to keep the bass out of the driver. This gives a crossover of around 9k for the 8 ohm tweeter. BTW: These are vintage Radio Shack "ribbon tweeters" from the 80's.

I remember Radio Shack selling "super tweeters" in a separate housing with binding posts for adding to an existing speaker system. I guess I'm thinking something like that.

I actually have been running the tweeters in parallel with the speakers in an external cabinet. They really do add a nice sparkle to the top end of the Tang Band drivers. I just didn't want to run this setup long term if it would cause damage.

Thanks again.
Glenn