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Speakers for tube amps

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Do you have experinces with this. is it better to have a 8 ohm impedance for tube amps in general, than a 4 ohm with a bit higher sensitivity?
I guess the 8 ohm needs less current and will have a better damping factor.

I guess to really know, you need the same speaker in both impedances!
 
yup, the speaker impedance will affect the selection of the output transformer, as the impedance of the speaker will have an effect on the reflected impedance the output stage sees. Many output transformers have multiple secondary taps for different impedance speakers. Sensativity of the speaker is the important number for a tube amp.
 
if you only have an amp output of 20watts you will not be able to drive most modern 8 or 4ohm speakers.

you will need to match the impedance of the output transformer to that of the speakers if it is not already matched

this may mean working on the transformer which you will need to discharge before any work on it
 
Some things to keep in mind.

There is no such thing as a truly flat impedance curve or at least I've never seen one. They will always have a resonance and some climb with frequency and some do even weird things. Magnepans climb and have built in Fletcher Munsen loudness curve. If the amp is Class A power is delivered to the load by the load so it's totally impedance dependent. Keep cable length to a minimum. Mono blocks are best to sit right at the speaker with only a few inches of cable. Long cables do weird things because of capacitance/inductance and loss. With tube amps you want a speaker with at least 96 db measured 1 meter with 1 watt input signal. Most speaker manufacturers don't rate their speakers this way but they should. They used to but don't anymore. Look for Klipsch LaScalla, Belle, Heresy, Cornwall, K-horn and in Altec Model 19, Valencia and the studio monitors A7-500, 844A, A8,604E and in EV Sentry IVB or Gillum G3. Ray
 
pointy said:
if you only have an amp output of 20watts you will not be able to drive most modern 8 or 4ohm speakers.

Yup, not really correct. In fact, I'll go as far aas to say that statement is just plain wrong. My tube amp can drive most modern speakers and it only puts out 5 watts. I even took it to a speaker store and tried it out with most of the speakers in the store. It worked well with all the speakers they had and drove all of them to a suitable volume level, and that was in a listening room that was much bigger than my listening room at home.

Originally posted by pointy you will need to match the impedance of the output transformer to that of the speakers if it is not already matched
Actually, if you already have the output transformer, your choice is only to match the impedance of the speaker to that of the amplifier. Regardless, an unmatched set of speakers and output transformers will lead to higher distortion and less power transferred to the speakers. Whether or not this is actually a problem depends on the particular setup, but a few ohms in either direction isn't usually a problem.


Originally posted by pointy this may mean working on the transformer which you will need to discharge before any work on it

Um, unless you want to disassemble the transformer and start removing or adding windings (good luck on that) there is nothing to work on. also, transformers don't store energy, so there isn't anything to discharge with respect to the transformer. Maybe you meant to discharge the capacitors in the amplifier, of course, that is something completely different that discharging a transformer (whatever that is).
 
Power rating

The power rating should be root means square (RMS) not music power or any other rating. With these small box, infinite baffle acoustic suspension, low sensitivity 85db/1watt/1meter or less, yes probably the amp power does matter. The problem there is the amp rums out of steam or winds up clipping on deep bass, bottoming out the woofer excurssion and causing distortion out the wazoo. You never want the amp to even approach clipping, that's why you want a very efficient speaker. If the amp is clipping then it is internally hemoraging distortion through the roof. If the speaker has a decent enough magnet structure and 10,000 gauss or more in the air gap it will have a high sensitivity. The problem is that manufacturers want to skimp and sell cheap speakers.
 
I will agree with that. Acoustic suspension is more of challenge to a low-power tube amps, but that said I am constantly amazed what a low power tube amp can do.

Another factor not mentioned yet is the space. In our old apartment, the little 8WPC PP Thomas amp I have could drive my old insensitive (87dB IIRC) speakers as loud as I could stand and sound good doing it. Our home has a larger rec room in the basement and the little amp in the same setup just runs out of steam. It works fine for casual listening levels, but is no fun when you want to crank it. That amp now serves our office and it is perfect for the job. The 35WPC Dynaco that took its place can rock those speakers to the point where I am fearful of damaging them (looking at the woofer excursion). These old speakers are "rated" to "100W", whatever that means.

My Boston Acoustic A70s are acoustic suspension and are rated 90dB. They can be driven ear bleeding volumes with an 8W amp, but the bass is lacking for sure. The Dynaco has no trouble with them and the result is literally wall shaking.

If you are on a tight budget, look for vintage speakers. I love the tight bass of the A70s, but they are not as deep as some others. I got them for $40 *with* new foams on the woofers. If you have more to spend, look at something a bit more modern. Klipsch has made some really nice ones over the years that are quite sensitive, but some folks are turned off by the horns.

I now have a pair of KLF-10s and they are amazing. I have never heard bass that deep without a subwoofer and hearing it through a Tubelab SE with 45s is even more amazing (2WPC). When hooked up to the Dynaco...well it's just loud...like Metalica concert-meets-boxing-match loud.
 
Critical Damping Factor

Hylle said:
... Is it better to have a 8 ohm impedance for tube amps in general than a 4 ohm with a bit higher sensitivity?
Hylle,

To my experience small power (<5W) amps can drive sensitive speakers to satisfying levels, but the main issue here is one of amplifier damping factor, not output power. For a given speaker there is one value of internal resistance and consequently one value of damping factor which results in critical damping.
 
pointy said:
most speakers designed to work up at 100watts will sound s**t at only 10 to 20 watts

they are made for use with tranny amps and are of low quality

If by shitt, you mean less than brain splittingly loud, then I would agree with you. Otherwise, I'd have to agree with everyone else that has contributed to this thread, and in particular, with planet10 who said "That is a huge -- and inaccurate -- generalization.".

These speaker ratings are at peak power and not continuous power. I seriously doubt you are running your 100w rated speakers (or 100 watt semiconductor amps) at an output power of 100 watts for anything longer than a few milliseconds at a time. If you did, you'd likely set your speakers on fire and blow the output stage of your amp.

Lets be reasonable here and refer to the power of an amp as the quiscient power (which is an average power), not the peak instantaneous power that the manufacturers have fabricated in an attempt to market underpowered amps to uneducated consumers.
 
Take a look at Eminence speaker specs. They are guitar speakers and so they aren't the hi-fi stuff you guys are talking about. But check out the specs on the link, and take a look at the pdf spec sheet for some interesting graphs of impedance vs frequency. The linked speaker is the one I've retrofitted into a 15W guitar amp that sounds really super in an open back cabinet.

The speaker puts out 100 decibels with one watt at one meter. The grey line on the graph is the impedance. Note the big spike at 84 hz, a minimum at 350 hz, and then the gradual rise above that.

For the skeptics, this speaker sounds just as good at low bedroom volumes as it does cranked so loud it hurts. I would venture a guess that hi-fi speakers in sealed cabinets might have problems at very low volumes due to the acoustic properties of the sealed enclosure, not the characteristics of a free mounted speaker.

http://eminence.com/guitar_speaker_detail.asp?model=RAGINCAJUN&speaker_size=10&SUB_CAT_ID=4

http://eminence.com/pdf/ragincajun.pdf
 
blaaberg said:
Hey,

What if the impedance is very low about 0.5 ohm. Is there a tranny out there, that could do the job?

The input to output impedance ratio for a transformer is constant. That is to say, a 8000:8 ohm transformer will act like a 1000:1 ohm transformer if the secondary is connected to a 1 ohm load and a 4000:4 ohm if connected to a 4 ohm load.

So, look for a transformer with the proper impedance ratio and use it.

pointy said:
ps.......i'm still right so there
:bigeyes:
 
frugal-phile™
Joined 2001
Paid Member
Re: Some things to keep in mind.

grhughes said:
If the amp is Class A power is delivered to the load by the load so it's totally impedance dependent... With tube amps you want a speaker with at least 96 db measured 1 meter with 1 watt input signal.

So much misinformation!!

I'm happily using a 3.8 W amp with 90 dB speakers.

Class A has nothing to do with it. If an amplifier has a high output impedance then its power output will tend to follow the impedance curve -- the design of my loudspeakers is done specifically to take advantage of this with my amps.

dave
 
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