Tube Amps / Music Electronics
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|7/3/1998 2:35 PM|
|anonymous||Are two speakers better than one?|
Here's a silly one. When connecting a second speaker to the "Ext. Speaker" (Fender type)jack, what happens, does the power get split between the two speakers, volume staying about the same, or does the volume double approximately? You get my drift. I don't have the seperate speaker yet, so that's why I'm asking.
|7/3/1998 10:31 PM|
Not a silly question, but the answer's not simple.
A tube power amp can only put out its maximum power when it drives a reasonably-well matched speaker. If you mis-match it up or down, the available power goes down. All the Fenders I see in my schematics book just connect the external speaker in parallel, so this would mismatch the speakers lower than perfect for the power amp; the overall power the amp can put into the speakers will go down. The power splits between the speakers in the inverse ratio of their impedances if they're not the same impedance.
However, that does not mean that the amp is half as loud. The human ear responds logarithmically, which means that half (or double) power sounds only a little quieter (or louder) if everything else is equal. To double the volume you need ten times the power in the same speakers.
Speakers vary a great deal in how efficiently they convert electricity to sound, so if you install a very efficient speaker in parallel, the overal sound may go down, stay the same, or even conceivably go up (although this is not the most likely thing to happen). A more efficient speaker may be much more economical than another bigger amp. The devil is in the details.
You MIGHT also get better coverage of a venue by placing two speakers separated by some distance, and so have more real loudness over a greater area with two speakers at lesser total power than one higher power one at a single place. This is, by the way, also true with nuclear weapons. No, I'm not kidding - that's why the US doesn't have huge nuclear bombs, just lots of little (20 kiloton to 5 megaton)ones.
With solid state amplifiers, the amp approximates a voltage source, and does not need (and can't use) speakers that are matched to its internal impedance. Instead, there is most often a nominal or rated impedance and a lowest safe impedance. If you have a solid state rated at 50W into 8 ohms, and connect another 8 in parallel, it will probably put out something like 100W into the new 4 ohm combined load if it can do that without burning up or activating the protection circuits. Some solid state amps are rated down to 2 ohms combined speaker impedance loading.
So - you have to know your amp and your speakers, and your stage layout to make a sensible projection of what happens.
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|7/4/1998 10:33 AM|
What you've said makes a lot of sense, but it seems to me that back in my "playing out" days (when the only joints I did up certainly weren't SOLDER joints!) I'd always get more apparent volume if I plugged my combo amp into an extension cabinet. It wouldn't always sound better (depending on the quality and impedance of the ext cab), but it always sounded LOUDER. As far as I know I don't think I ran into any serious impedance mismatches which would have drastically reduced the efficiency of the amp. Of course if the speakers were wired up out-of-phase with each other it wouldn't work right at all...
Getting back to modern stone-free times, rather than just plugging into the ext spkr jack, wouldn't it be better to first check out the impedances involved and then figure out a way to hook up all of the speakers in series and/or parallel so that it would add up to the desired impedance, making up a custom speaker wiring harness if necessary? If the speakers aren't matched in impedance or efficiency, they may never sound good together, but you could always try different linkages with test cables and jumper clips just to see if one of the combinations does work well.
In dealing with speakers of mismatched efficiency (not impedance), in a parallel configuration the more efficient speaker will sound much louder. Is that also true if they were hooked up in series?
|7/4/1998 1:15 PM|
Could well have been the distrbuted source thing. I always liked the sound better from multiple sources.
Tube amps do go down in output power no matter whether you mismatch up or down. Hifi tube amps sometimes have output transformers that match at the minimum distortion point instead of the max power point for the output tubes (which are most often different) and so sometimes the power will go up slightly for a hifi amp mismatched from 6600 ohms on the plates (for 6L6 or EL34) to 4K or so, but mostly guitar amps are already designed for the power maximum point; besides, half of 6600 on the plates is 3300, past the 4K max.
The drop in power is small in loudness terms. half power is a just-noticeable drop in volume. The room acoustics can easily have 12db peaks and valleys.
Yep. That's a far cry from just plugging in an extension speaker, though. The easy ways aren't good and the good ways aren't always easy. Depending on how many speakers and what kinds, you may not be able to come up with the right impedance.
Dinking with the speakers to get the best way is a REALLY GOOD IDEA, but not many guitarists have the patience or skills to do that.
Hmm... hadn't thought about that before. Let's see - equal impedances, so each speaker gets the same voltage and currents; the simplistic answer is yes, the more efficient one sounds louder, but I suspect that the greater amount of energy that the more efficient one expends into the air gets reflected into a bigger load on the electrical terminals, so the real impedances (as opposed to the nominal ones) can't really be equal.
I think "probably" is the right answer.
Speakers in series are, by the way, less damped and therefore have a different tone from speakers in parallel. Each speaker sees an extra impedance (the other speaker) in series between it and the amp, so the amp damping is lower. This may not be significant to tube amps with low damping already.
|7/5/1998 12:37 PM|
||RG, can you explain damping?|
I've not seen that explained anywhere before
|7/6/1998 5:37 AM|
Unplug your speaker cabinet. Leave its the speaker terminals OPEN. Now gentlty tap the speaker cone. The cone is "soft" and it will make a small "boom" sound that will die quickly. The frequency of this "boom" sound is the resonant frequency of that speaker inside that cabinet.
Now short the speaker terminals and repeat the experiment. The cone will be "harder" and the boom will die much quicker. How much quicker will depend on your speaker. That piece of wire is damping the speaker.
If the speaker was perfect, the boom would totally disappear and it would just "click". This would mean that it was perfectly damped and would follow the input transients perfectly. Because of internal wire resistance, friction of the mechanical parts, etc. it will always retain a little "boominess" even with its terminals shorted.
The "ideal" amp (from a hi-fi point of view) would look to the speaker like that piece of wire and kill its natural "boominess". The greater the damping factor of an amp, the closer it is to a piece of wire (zero output impedance). As output impedance gets higher, more of the speaker "boominess" survives, giving a less tight sound that is undesirable in hi-fi but can be good in a guitar amp for certain playing styles.
|7/6/1998 7:13 AM|
GFR's example is a good one to illustrate the effects of damping.
To the techie, damping is the ability to control ringing in a resonant circuit. The electro mechanical setup in a speaker either by itself or in a cabinet is resonant at one or more frequencies, so on any transient - like almost any musical signal - the speaker/cabinet wants to ring at these frequencies. The most prominent one is the speaker/box resonance(s).
Just like a speaker has an impedance, so does the amplifier. A speaker can (and does) "drive" signals back into the amplifier anytime it is moving even slightly out of synch with the applied signal, acting like a big, powerful microphone. The ringing on a transient will do nicely to cause this. If the amplifier has a very low impedance, it acts like a big load and damps out any motion that is not exactly like the signal.
The term damping factor is just the impedance of the speaker divided by the impedance the speaker "sees" looking back into the amplifier speaker terminals. In stereo/hifi stores, they talk about more damping factor being better, and discuss how DF of 400 is 'way better than only 300, but the fact is that after a DF of about ten or so, the speaker is about as damped as it's going to get and other secondary effects cut in. Tube power amps with feed back can get to here, but the non-feedback ones are generally in the units.
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