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At some point I think I read a recommendation on one of the forums that someone try connecting 8 ohm speakers to the 4 ohm taps (that in some cases it might sound better) and somewhere else there was the suggestion that 8 ohm speakers could be tried on the 16 ohm taps or 4 ohm speakers on the 8 ohm taps. I was under the impression that a 4 ohm speaker on an 8 ohm tap might cause problems. What's the scoop? Does it make a difference if the amp is tube or SS? Is one more flexible than the other? If any of these mismatches is risky - how would one tell if there is a problem? Would the output transformers overheat? Or the power tranny?
Follow Ups:
Thanks for the comments guys . . . I do enjoy just playing around and trying things but it would NOT be much fun if something got damaged in the process. So you're saying that the only damage that might occur would not be to the output or power transformers or the speakers but only (possibly) to the output tubes - that it might shorten their life? I guess this question is related to an earlier post of mine about using a stack made up of Large Advents/Advent 1's with a SET 45 amp I recently built. (Thanks Jerry for your great explanation of the effects of stacking and speaker interaction) The amp uses output transformers from a single-ended console amp and does not have separate taps. According to the SAMS I have they are rated at "6-8 ohms". All of the speakers I've tried with them are nominally rated at 8 ohms when used as a single pair. I understand that the impedance varies at different frequencies and as Jerry pointed out the stacked Advents (nominally 4 ohms) would go down to about 2.5 ohms in a couple of spots - the 90-200 Hz and 1-3 Khz ranges. From the previous explanation (please correct me if I'm wrong) I surmise that in those ranges the mutual coupling effect would (to some degree at least) compensate for the difficulty the amp would have due to the dip in impedance. While I am considering getting some more efficient speakers that are better suited to a low power SET, at the moment I'm mainly concerned that I don't damage anything.
Hi Charlie,
once I asked to Joe Rosen (see "the book" pages 48-50) about speakers and impedances. He answered:"...Remember, best sonic results are obtained by "mismatching" the speaker to the NEXT HIGHEST impedance tap. I.E., an 8 ohm speaker will work best on the 16 ohm tap (unless its impedance is optimistic, as many "new" speakers are, manufacturers will often claim an 8 ohm "nominal" impedance when it is no higher than 6 and often more like 5 or even 4 ohms!), a 4 ohm speaker usually works best on the 8 ohm tap, and a 2 ohm speaker should be sold off
to some other audio-fool as quickly as possible! For the reason of this matching, I will usually select the 16 ohm tap to put my binding post, not the 4 or 8 ohm one. Also, the 16 ohm tap sounds the best on a "tapped" secondary OPT like the one in the Fisher 400... This is because when use the 16 ohm tap, it is the highest one commonly available, and therefore usually spans the entire secondary winding. When you use lower taps like the 8 ohm and particularly the 4 ohm, you are only using a fraction of the
whole secondary winding, and your transformer "coupling" to the speaker is much poorer. You can hear the degradation of performance as a dulling and thickening of the sound, and you can measure it as a loss of HF bandwidth, damping factor, and waveform fidelity"About modern "6 ohm speakers", he wrote:
"If the bass impedance is above 6 ohms, the 16 ohm tap will definitely work best. If it is 4 or 5 or just-barely 6 ohms in spots, the 8 ohm tap MAY work better"
About possible damages:
"What kills output valves prematurely is clipping, not load mismatching. As long as you are not working the amp hard all the time with a speaker load it can barely provide enough power to, the valves will last as long as the design & quality of the valves permit.
Theoretically, my suggestion of load mismatching could shorten the life of the output valves, but only because the output valves are free to draw more current because I've lowered the inhibiting impedance in their anode circuit. But as I said above, as long as you are not driving the valve hard into saturation, then the cathode will not "poison" itself (actually, you could call it "stripping") and degenerate any faster than it would if used "correctly".
In practice, I've had no problem and achieve very long output valve lives with my designs and loading.
There is no risk of damage to the speakers in any of this!"
Raver,I guess that's one piece of advice of Joe's that I disagree with. I find that generally, things sound cleaner and less strident on a lower tap, not a higher one. And the comment on modern speakers dipping below the nominal is wrong, too. All the AR's and KLH's did that and so did Advents and EPI's. It's been going on for a good long time. The other piece of misinformation is the idea that the 16 ohm tap "couples" better since it uses all of the winding is just nonsense. It's all about turns ratio. The comment on damping factor degrading is also puzzling. Damping Factor is a calculated figure of merit based on the amplifier's source impedance. Mis-matching the speaker won't change the source impedance. In fact, the 16 ohm tap (for example) will have significantly higher source impedance than the 8 ohm tap, and referenced to an 8 ohm load would calculate to a LOWER damping factor.
I accept the idea of trying different taps, but the reasons you quote from Joe don't make a good justification.
Jerry
Jerry,
thank you for the clarification. I posted Joe's advice as I know that Charlie loves Joe's posts. However I'm not so technically knowledgeable to comment on this.
So what tap would you use for a nominal "8 ohm" speaker? Would you try the best sounding one or do you have some rule?
Do you agree with Joe that mismatching taps don't damage speakers and/or amps?
A note: yesterday I opened my Spendor BC1's to rotate the woofers (as the voice coils tend to sag after 10-20 years), and I noticed a sticker on the two treble drivers, saying "16 Ohms". The woofer is not labeled. The literature coming with my pair says they are
8 ohms, but I'm not not so sure of this. Here you can find a BC1 impedance chart:
http://john.harper.free.fr/spendor_bc1.html
Any advice/opinion?
Raul
Raul,The first thing I noticed about the BC1's was that most of the area of the impedance curve lies well abobe 8 ohms, and the minimum impedance in any region with high energy on music is 10 ohms. Only at much higher frequencies where there is very little energy does it drop below 10. So, I'd start at 16 ohms for that speaker. if it sounds a little on the edgy side, then go to 8. Actually, you should try both and make a decision based on your feelings. The impedance curve is such that it won't hurt anything to use either one.
As to Joe's comment on taps. In general, he's right. But driven hard with a large mismatch could stress the output tubes with excess current. However, if the plates start to glow red, then back off and try a lower impedance tap.
Ignore the label on the tweeters. They may be rated at 16, but the impedence curve shows lower impedances in the high treble. A 16 ohm speaker can show lower impedence in a speaker system depending on the crossover network used with it, so trust the system manufacturer.
Because of the very large swings in impedence, the BC1 is a speaker that will show big differences in frequency response depending on the source impedence (Damping Factor) of the driving amplifier and the connecting cables. With a low DF, expect the speaker to show peaks at the impedance peaks, and dips in response at the impedence minimums. However, speaker designers know about this effect and compensate for it. Therefore, you need to gain some insight into what kind of amp was used by Spendor in the design process. From the era of the speaker (late 60's early 70's) it could be tubes with moderate DF's (like 12 or so) or SS amps with DF's around 30-40. It's not a good match for SET's, their DF is way too low, or for high DF SS amps like my NAD which has a DF = 150. Spendor could probably help you with that information.
Jerry
The simple fact is that some speakers are "rated" at something not even close to their minimum impedance. And the damping factor of some amps is not a good match for some speakers no matter what tap you use. What do you in situations where you have a mismatch and things sound terrible? I use the number one engineering procedure of all time . . . if it not working now then try something else. And one of the things you can easily try is another tap.Really this is a hobby and don’t you have at least a bit curiosity about how your amp and speakers are going to sound using one of the other output taps? I did. I have tried just about every combination. Sometimes by trial and error you just hit on one that sounds better to you. Really you aren’t going to loose much (except maybe for time) trying a few different settings. Give it a go . . . remember this is suppose to be fun.
--Dan
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