I thought I could get by under similar circumstances with a fairly high power stereo tube amp driving a pair of speakers with 81 db efficiency and a 3 ohm load. It was great if the music was quiet or solo acoustic but it didn't work worth a darn on anything requiring more "ooomph" such as symphonic. I didn't want to get rid of my tubes and was able to drive the mids/tweets with tubes and SS on the woofer... This happened to work great in my case and I got the best of both worlds but it is often difficult to get tubes and SS to mate well.
While I don't have any experience with your particular amp, I can state with confidence that the Wolcott Presence will meet the match - for whatever that is worth. -aj
Tube amps (except OTL types) use an output matching transformer. This transformer presents (relatively) fixed impedance (usually settable to 4, 8 or 16 ohms) to the speakers. The Matching Theorem (see any good EE circuits textbook) says that a source (i.e. amp) with fixed output impedance supplies maximum power when the load (i.e. speaker) has the same impedance.
Thus tube amps normally don't do well with demanding loads with impedances that vary across the audio spectrum. For example, when the speaker is attached to the 4 ohm tap on the amp, it gets maximum power when its impedance is also 4 ohms. Most speaker impedances vary across the audio spectrum, so power delivered to the speaker also varies with frequency. This effect is partly responsible for some of the tube sound.
Another way to look at it is that the inherent impedance of the transformer limits the current.
We can turn the matching theorum around, and ask what source (amp) output impedance delivers the most power to a given load (speaker). In this case, the lower the source impedance, the more power you get into a given load.
Transistor amps normaly don't have output transformers (there are a few exceptions). They tend to have very low output impedances. Many, (especially those built with massive power supplies and running class A) have output impedance much less than an ohm and can deliver large amounts of current into low impedance loads.
So if you need lots of current, solid state is the way to go. You can get there with tubes, but you need a lot of them, and massive (expensive) output transformers to go with them. They still supply maximum power only at the selected output impedance, though.
I don't think a 6 Ohm load will require that much current. 84 db's will require power though. A high power tube amp should fit the bill. Though a high power tube amp usually comes with a high bill. I'm not familiar with your speakers but a high power VTL should fit the numbers. Of course fitting the numbers doesn't mean it will float your boat sonicaly.
There are a number of tube amps that will be able to handle that sensitivity. I can't comment on specific matches, but I can tell you that there are several (VTL and smaller) that might work. Feel free to email me if you want any specifics on my own experiences with other speakers. I have been on a real quest to match my current and planned inefficient speakers with "tube-like" SS. I can tell you that it will be much easier and cheaper to find high current tube amps than tube-like SS...
BTW nice answer Ghostrider!
Best of luck,
I second Angela100's suggestion of Wolcott amps (disclaimer - I'm a Wolcott owner & dealer). No, they won't double their power output going from 8 ohms to 4 ohms, and again into 2 ohms, like a beefy high-current solid state amp will. But, they will deliver their rated power into a 1.7 ohm load, which is pretty darn good for a tube amp. Especially for a tube amp that uses EL34 tubes, which are traditionally noted for their midrange performance, rather than their stability into difficult loads. But Wolcotts are the amplifier of choice of many Sound Lab owners, and Sound Labs are a difficult speaker to drive.
A feature of the Wolcotts that is quite welcome with demanding loudspeakers is the adjustable output impedance control, which allows the user to fine-tune the amplifier/speaker interface.
I'd be happy to answer any specific questions you might have.
We represent the Wyetech Labs SET Tube amps and found them to drive very difficult speaker loads better than much higher powered tube amps. the Topaz stereo amp which is rated at 25 watts can out drive my 175 watt Airtight monos. a very well designed power supply is the key!
I agree with artistic audio...power supply takes the cake. I've heard any number of well made lower powered tube amps, drive the hell out of something that the numbers indicate would be a mis-match. Consider the size of your room-the type of dynamics that are important to you and audition. Wright sound company makes amps that drive things they shouldn't....I've heard their 2a3's drive and 86db 8 ohm speaker at 3.5 watts, its not a combo I would advocate but the fact that it makes good music makes a point...their 300b amp drove the speakers as loud as I could stand-check them out at www.thewright-sound.com
The idea of a 'high current' amplifier is a bit mythological. If your speaker is a 6 ohm load (not particularly difficult for tubes, BTW), then it is easy to show, using Ohm's law, what is really happening.
Let's say that you've settled on a tube amp that produces 115 watts. Let's further say that you have a transistor amp that will produce the same power into 6 ohms. By using a little algebra (Ohm's law is Resistance = voltage/current), we can derive how much current will be flowing in each case.
Power = (current)squared X Resistance, so:
115 = (4.38) squared X 6.
This is the same regardless of whether it is tube or transistor. IOW, 'high current' has little or nothing to do with it.
You'll probably want to use the 4 ohm tap to keep the load on the output tubes at a good operating point for low distortion.
Since the amp specs are rated from its 8 ohm tap, it likely that you will have a little less power on the four ohm tap as the transformer is less efficient on the four ohm tap. The loss will be minor.
My guess is the real issue is whether or not 110-115 watts is enough power. Unless you are in a really small room (and depending on your listening habits) there's a good chance you'll be running the amp out of power if you try to play it loud.
It seems then, if I'm understanding all of this correctly, that high current is what's needed to push the power into highly resistive or low impedence loads? So if my speakers stay somewhere close to 6ohms w/o much in the way of wicked impedence curves, I should be fine with just some high wattage, correct?
The ARC amp that I'm in the process of buying weighs close to 100lbs so I'm presuming that it has the kind of HUGE output transformers that will be required.
Here's another question...If my speakers are 6 ohms, should I use the 4 ohm or 8 ohm taps? Should I be conservative and go with 4?
Thanks again for all of the insightful advice. I think I'm starting to get it.
I have a 6ohm speaker(87dB) and use a 60 watt per channel tube amp with out any issue, and from time to time I like to let it rip. btw I use the 4 ohm taps on the amp- to me it sounds better. ~Tim
As long as your nominally 6 ohm speakers don't have any dramatic impedance dips or peaks, I think the ARC's will do fine. Use the 4 ohm tap - it's better to mismatch the load on the high side of the amp output impedance rather than the low side. You'll effectively lose about 20% of potential output power.
Given the nominal nature of speaker impedance and output impedance, I doubt you'll notice any difference.
BTW Ralph - The equivalent circuit for the amp looks like am ideal voltage source in series with an impedance. This impedance represents the output impedance of the amp. It can limit current into low impedance loads since it is in series with the speaker.
Also, for those whose math is rusty, when dealing with alternating current (i.e. audio signals) the voltage, current, and impedance are complex quantities (i.e. each is in the form a+bi, where i = sqrt(-1) ). For impedances, the imaginary part is the reactive component, and the real part is the resistive component. Complex arithmetic models the energy storage behavior of capacitive and inductive elements in the circuit.
Yes, Ghostrider, the output impedance of course is a current limiting factor as more and more of the output voltage is dropped across the output section as opposed to the load as the load impedance is reduced.
However in the case of the ARC this is not a serious issue with a 6 ohm load. So the issues of a 'high current' amplifier are rendered moot, for the most part.
Were it a 3 ohm load, I think a different solution (amplifier) might be advised. There is also a guy on the ASOG (otlamp.com) named Paul Speltz who makes a torroidal autoformer (a variation of one we used to make) for this sort of application. It has taps for 4,3,2 and 1 ohm loads and sets the amplifier load at 16 ohms. Works quite nicely on those otherwise hard to drive loads that tubes don't normally like.
Allows one to use tubes with, say, Appogee Scintillas, if you could find a set these days. For the most part though 3-4 ohms seems to be the most challenging most speaker manufacturers are doing these days.
High Current amps are not "myth". The fact is that there are two good reasons for this so-called "myth" existing.
The first is the case of speakers that actually have low impedance dips and/or are simply low impedance. In such cases amps that are not designed to handle the higher current AT the power supply's voltage will have problems of several sorts. Including blowing up or blowing fuses.
The second case is where the current available from the power supply is insufficient to supply enough current to a low - Z load at peak output - and the rail sags in voltage. Often the rail sag can be significant. Old tube amps are notorious for this, as are "big" solid state receivers as well as all too many otherwise decently made solid state power amps.
As for tube amps, it is perfectly possible to design a tube amp to drive almost any load, low-Z or not. You simply have to have the proper output transformer to match the plates of the tubes. I know of one person who uses 2 x 211s in parallel for a SE amp that runs some ML CLSII speakers. It works just fine, not surprisingly.
In the case of a tube amp, one often finds "soft clipping" so that tube watts *seem* to be more since the advent of clipping can be excused by the ear much more easily than the often hard clipping of a bipolar transistor amp.
The issue of amp power vs speaker sensitivity is often more related to (as someone mentioned) your room size/listening distance, the sort of music you listen to and the level at which you listen to it. How absorptive your room is also plays a role in the perceived volume.
With that 100+ watt tube amp you should be able to listen to peaks up around 100dB with no strain - this implies an average "medium" listening level of about 88-90dB and source material that is not super dynamic (where the average level is >20 dB below the peaks...
Of course, try it and see what happens!
Bear, thank you again for sharing your insight.
Thank you all so much for your thoughtful and very insightful (if not, at times, a bit too technical - for me anyway) responses.
I should receive the amp late next week, if all goes well, and will report back on my experience. Upon reading through the literature on my speakers, I found that the impedence dips to 3ohms at its' lowest point, but typically operates in the 6ohm range. Connected to the 4ohm taps on the ARC amp, I would guess this should still be relatively safe...no?
My listening room is not very big but does have some HIGH ceilings. (The room is approximately 15' X 18' but the ceilings vault to about 20') I'll be anxious to see how it goes.
Thanks again all!
That is an important difference. It is good to think in terms of % when dealing with impedance. It will probably work out.
Dan - I think you'll be OK. The speaker seems reasonably well behaved and the 4 ohm taps should do fine.
I will recommend to try other taps & listen.I am currently driving ML Aerius i from the 8 O. on my ARC VT-100 with better result than the 4 O..The ASerius go down to -2 O. in the upper range!I also have a ss amp (odissey stratos) which is VERY powerful but the VT-100 is a way better for music & voices.Yves