If the nominal absolute impedance is high enough, then variations are less of a problem. I too enjoyed my 11-ohm-nominal LS3/5As with small tube amps. If the impedance variations are relatively smooth (I'm thinking of many ESLs) then the response effect will be a smooth tilt, not too terrible a situation. But speakers, like many from B&W for example, with several high-order crossovers, and many highly reactive impedance regions, will be hard on high-output Z tube amps.
The S-5 site isn't particularly specific and the rated output impedance of '8 ohms' may or may not be a classic error. However the 10GV8 is a combination hi-mu triode/pentode so it's safe to assume the K-12G doesn't have high damping factor. Speakers with complex bass alignments, especially those that already tend towards bass emphasis as many commercial seakers do, should be auditioned with your amp if at all possible before purchase. Personally I would probably stick with acoustic suspension.
High impedance due to X-over filters, is not tube-friendly either. Gives a higher mid output on speaker with tubed amps, especially when this amp has a low feedback in the circuit. This can be compensated with a calculated extra filter consisting of a L, C and R in series over speaker connection. Frequency in calculation is the X-over frequency, wanted impedance taken from impedance speaker, eg 8 ohm or whatever.
High impedance of speaker does'nt "thread" amp though.
High impedance of speaker does'nt "thread" amp though.
To quote SY, I half-agree. But higher impedance deviations from the nominal impedance will be less troublesome than lower impedance deviations. As a simple example, suppose you have an amp with 1 ohm output resistance driving an 8 ohm nominal speaker. You already have a nominal divider loss of 8/(1+8) = -1dB "across the board". If the impedance doubles at some frequency to 16 ohms (resistive, to keep it simple), the loss is reduced: 16/(1+16) = about -0.5. If the impedance drops to half, 4 ohms, then the divider loss is 4/(1+4) = about -2dB. So the impedance peak caused a RELATIVE response change of +0.5 dB, while the impedance dip caused a -1dB change in response.
I’ve always considered the impedance peak of the second-order resonance of a sealed box bass alignment as a gift, if it’s used appropriately. Often, the impedance peak covers a broad range and can be pretty high. Since this is where a lot of musical energy exists (not to mention movie soundtracks), the resonance reduces the need for current/power and makes life easier for all amps, but especially higher impedance tube amps. The impedance peak causes a response peak too of course, but in the prior example the impedance peak could cause no more than a 1dB response peak no matter how high the impedance peak, whether 20 ohms, 40 ohms, whatever. Sometimes there is a free lunch…
This impedance rise in the lower ranges can be used to your advantage when designing the amp. I have recently been experimenting with a triode connected SE EL-34 amp. It uses no global feedback, but cathode feedback is applied in the output stage.
I have tried several different operating points for this amplifier. The point that gives great sound is 420 volts, 60 mA, and a 3 Kohm load. Measuring the output power with this load yields barely 5 watts. A 5 K load with this voltage and current, or a more conventional lower voltage, higher current operating point with the 3 K load makes more power, but distorts with heavy bass.
Why is this? It seems that my speakers have a gentle impedance bump at 70 Hz. The higher supply voltage allows the amp to work into this higher reflected load impedance without clipping.
The picture below was taken with the scope directly across the speaker while a CD with a bass guitar solo was playing. The scope is set on 5 volts per division. Here is about 35 volts peak to peak across my speakers without any clipping. I can't get above 17 volts with an 8 ohm load resistor.
This is a case of tweaking the amp (and its operating point) to match the speaker. It required the use of a different output transformer than I had origionally chosen.
I have tried several different operating points for this amplifier. The point that gives great sound is 420 volts, 60 mA, and a 3 Kohm load. Measuring the output power with this load yields barely 5 watts. A 5 K load with this voltage and current, or a more conventional lower voltage, higher current operating point with the 3 K load makes more power, but distorts with heavy bass.
Why is this? It seems that my speakers have a gentle impedance bump at 70 Hz. The higher supply voltage allows the amp to work into this higher reflected load impedance without clipping.
The picture below was taken with the scope directly across the speaker while a CD with a bass guitar solo was playing. The scope is set on 5 volts per division. Here is about 35 volts peak to peak across my speakers without any clipping. I can't get above 17 volts with an 8 ohm load resistor.
This is a case of tweaking the amp (and its operating point) to match the speaker. It required the use of a different output transformer than I had origionally chosen.
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Hi,
When we talk about "tube friendly" HI-FI speakers that would perform well with a small power tube amp, they would have to exhibit two characteristics: high efficiency (higher than 90dB) and small Xmax on the bass unit (under 0.5 - 1cm). These are the primary characteristics I would look for in such speakers. Secondary characteristics would then present themselves "intuitively".
Regards,
Milan
When we talk about "tube friendly" HI-FI speakers that would perform well with a small power tube amp, they would have to exhibit two characteristics: high efficiency (higher than 90dB) and small Xmax on the bass unit (under 0.5 - 1cm). These are the primary characteristics I would look for in such speakers. Secondary characteristics would then present themselves "intuitively".
Regards,
Milan
Hi Brian:
I wasn't talking about full-range speakers but woofers. I thought it was quite clear when I said "small Xmax on the bass unit".
Anyways, I don't think the starting post in the thread (or subsequent posts for that matter) was referring to full-range speakers.
As for full range speakers, from my listening experience with a 300B-Lowther4X horn and 300B-VOT, I may say the latter sounded far better to my ears.
Regards,
Milan
I wasn't talking about full-range speakers but woofers. I thought it was quite clear when I said "small Xmax on the bass unit".
Anyways, I don't think the starting post in the thread (or subsequent posts for that matter) was referring to full-range speakers.
As for full range speakers, from my listening experience with a 300B-Lowther4X horn and 300B-VOT, I may say the latter sounded far better to my ears.
Regards,
Milan
Hi Brian:
I personally don't think that impeccably smooth speaker impedance is a critical factor for a good sound in a speaker/tube amp combo. A far more important thing to me are T/S parameters of the bass unit. While it may be a bit unorthodox to define Xmax as a critical factor, I do believe (based on my research) that, together with T/S parameters, Xmax allows us to predict speaker behavior quite accurately and easily (taking into account the bass box type, of course), as well as the speaker's interaction with any given tube amplifier.
Regards,
Milan
I personally don't think that impeccably smooth speaker impedance is a critical factor for a good sound in a speaker/tube amp combo. A far more important thing to me are T/S parameters of the bass unit. While it may be a bit unorthodox to define Xmax as a critical factor, I do believe (based on my research) that, together with T/S parameters, Xmax allows us to predict speaker behavior quite accurately and easily (taking into account the bass box type, of course), as well as the speaker's interaction with any given tube amplifier.
Regards,
Milan
Now, I'm as much of a layperson you can get. But I drove a pair 87db/4Ohm Mårten Design speakers with a 40-yr-old 25-30W tube amp without problem. These are long Xmas ceramic Accuton elements. I would say that the tube amp drove the speakers at least as easy as my +100W SS amp. In fact, I think the tube amp handled them easier. The SS amp croaked under the load when you pushed it. Mårten Design and tubes, a match in heaven for all I care. At 6moons, a reviewer says the exact opposite--that the 30W tube amp he used wasn't up to the task, but that the 100W Bel Canto was. I do not question the reviewer for a second. I think the difference is the amps. You fill in the dots.
Having that rant out of the way, and basically contradicting what I just wrote, I always thought it was fairly simple.
High impedance good for tubes--high voltage, low current.
Low impedance good for SS--low voltage, high current.
If they are sensitive enough, like these 120db speakers , impedance is probably no longer an issue.
Having that rant out of the way, and basically contradicting what I just wrote, I always thought it was fairly simple.
High impedance good for tubes--high voltage, low current.
Low impedance good for SS--low voltage, high current.
If they are sensitive enough, like these 120db speakers , impedance is probably no longer an issue.
The principal consideration in the interaction between the output resistance of a tube amp (or any amp) and the bass system’s alignment is how Re and Rout effect Qes, which is a factor in Qts, the system’s bass Q. Any output resistance in the power amp or cables will effectively add to Re, increasing Qes, which in turn increases Qts. Xmax is indirectly involved in the T/S parameters. To achieve a low Qes, Bl must be high, meaning that there needs to be more coil in the gap to increase braking forces. So this is one factor working against high Xmax, and this is probably where you have rightly seen a correlation. But here are many other factors influencing bass alignment. Generally speaking a slightly over-damped bass alignment (with a zero ohm amp) will work better with a high Z tube amp than will an under-damped speaker. That’s a pretty big generalization, but I think it’s usually true. Nelson Pass has done some interesting work with driving heavily damped speakers with high output resistance amps.
Few loudspeaker SYSTEM manufacturers will publish the T/S parameters or the Xmax of their individual drivers, and so we are stuck with whatever test data we’re given – hopefully at least an impedance curve and a sensitivity rating (dB SPL at 2.83Vrms at 1M at 1KHz). I think that advising a buyer of a speaker system to shop for low Xmax is like telling a sport car shopper to look for short piston stroke, when horsepower, or more directly, acceleration data are more relevant. And even then we’re only then addressing the bass portion of the spectrum, where, granted, the biggest impedance interactions usually take place.
Few loudspeaker SYSTEM manufacturers will publish the T/S parameters or the Xmax of their individual drivers, and so we are stuck with whatever test data we’re given – hopefully at least an impedance curve and a sensitivity rating (dB SPL at 2.83Vrms at 1M at 1KHz). I think that advising a buyer of a speaker system to shop for low Xmax is like telling a sport car shopper to look for short piston stroke, when horsepower, or more directly, acceleration data are more relevant. And even then we’re only then addressing the bass portion of the spectrum, where, granted, the biggest impedance interactions usually take place.
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Brian Beck said:
I'm glad you see the correlation. Of course, no one said there were no other factors. It's just that, as I said before, I think Xmax is one of the two critical factors in defining "tube-friendly" speakers. Correct me if I'm wrong, but I think Nelson's experiments were, again, with full range speakers.
As for sports cars, I wouldn't know anything about them but I believe they do tend to be a bit more complex than your average speaker. When shopping for speakers, for example, you can simply push the cone (unless it's ceramic, naturally) and see how far it goes to find out Xmax. I don't think it's possible to gain much valuable insight from poking your fingers against any part of a sports car (but I might be wrong).
Phn: The biggest Accuton, as far as I know, has 1.05cm Xmax, which is the top limit I specified earlier. No problem there.
Regards,
Milan
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