AudioKinesis Cabs Part VI...

[adam montminy]

Thanks a lot for taking the time to explain all of that, Duke! That actually corresponds exactly to some of my experience with tube amps and cabs. In general, I found that all of my conventional modern ported cabs with tweeters and/or mids sounded uncontrollable in the low end and had a certain harshness in the upper mids. I didn't get any of those qualities with any of my solid state amps with those same cabs. I only half-understand exactly what all of your information means but it's helpful to know that I'm not crazy when I'm noticing these things in my gear experiments.

 

[steve_rolfeca]

That's interesting but I have actually found the exact opposite in my experience. ...I've found that tube amps are very picky about cabs whereas solid state amps don't seem to care what cab they're plugged into for the most part.

...As it turns out, there are quite a lot of characteristics inherent in tube amps that effect their interactions with speaker systems.

And vice-versa, in some (admittedly rare) circumstances.

I built a pair of simplified Klipsch corner horns when I was a teenager. I used a fairly random woofer, intended for small ported boxes.

When I tried to drive them with my Japanese SS integrated amp, something about the crazy impedance curve drove the amp into low frequency oscillation. It just motorboated, making a funny chugging noise.

Not knowing what was going on, I plugged them into my dad's tube setup, made by Quad. Only 15 watts out of each monoblock power amp, using pairs of KT66's...

...But the damping factor was so high, that the speakers played perfectly, and sounded pretty damned good!

 

[Interceptor]

Duke, thank you.

That explained so much of what I'm hearing as I have shifted to tube amplification after 40+ years of solid state bass rigs. Designing around the impedance bumps is a dance I had wondered about, but hadn't gone and experimented with.

 

[adam montminy]

I just have to laugh sometimes. This is a topic that is well-understood by everyone who works in the field (designers, engineers, etc). People have written books about it apparently, there is a ton of data and measurements and here we have an award-winning speaker designer presenting some of this great info and there will always be someone who claims that it's all hoo-ha, internet legend, and has no basis in fact. Haha! Anecdotal evidence is good and valuable but without actual knowledge of facts and understanding how things work, we enter the world of mysticism and end up trying to re-invent the wheel. Sure, you can pair any amp with any cab and it will sound like a bass guitar but if you're interested in making some musical combination, some combos will work much better than others and it might help some people to know why.

 

[JimmyM]

I just have to laugh sometimes. This is a topic that is well-understood by everyone who works in the field (designers, engineers, etc). People have written books about it apparently, there is a ton of data and measurements and here we have an award-winning speaker designer presenting some of this great info and there will always be someone who claims that it's all hoo-ha, internet legend, and has no basis in fact. Haha! Anecdotal evidence is good and valuable but with actual knowledge of facts and understanding how things work, we enter the world of mysticism and end up trying to re-invent the wheel. Sure, you can pair any amp with any cab and it will sound like a bass guitar but if you're interested in making some musical combination, some combos will work much better than others and it might help some people to know why.

Well if you know so much about it, why did you ask?

Also, I made very clear that my opinion was based on my (nearly 40 years) experience with tube amps and different cabs sealed and ported and not based on technical info. But my experience is what it is, and that includes plugging an SVT into a ported cab that Duke made that sounded way excellent when powered with an SVT. But if ported cabs don't work well with tube amps, please explain why the most legendary recording amp of all time is the B-15, a tube amp plugged into a ported cab.

 

[tombowlus]

Personally, I think all this hoo-ha about only certain cabs working with tube amps is the stuff of internet legend and has no basis in fact. Pair up a good sounding amp with a good sounding cab, and it matters not the format of the amp powering it.

I have played through some cabs where the speaker cones would dance like crazy (making bad, farty tones in the process) when I pushed them with tube amps, but which behaved in a much different (and more controlled) fashion when pushed with solid state amps (of much higher output power rating).

 

[tombowlus]

Well if you know so much about it, why did you ask?

Also, I made very clear that my opinion was based on my (nearly 40 years) experience with tube amps and different cabs sealed and ported and not based on technical info. But my experience is what it is, and that includes plugging an SVT into a ported cab that Duke made that sounded way excellent when powered with an SVT. But if ported cabs don't work well with tube amps, please explain why the most legendary recording amp of all time is the B-15, a tube amp plugged into a ported cab.

AND, I have also heard plenty of ported cabs that did sound/perform great with tube amps. So, IME/IMHO, while porting is certainly a factor which influences how drivers perform in a given enclosure, there are numerous other factors at play.

 

[tombowlus]

Well the most obvious is, the cab needs to be wide enough for the tube amp's feet.

Beyond that, tube amps in general behave differently into a speaker's impedance curve than solid state amps do. A solid state amp approximates a constant voltage source, so it tries to deliver the same voltage regardless of the speaker's impedance curve. So if a solid state amp delivers say 200 watts into 8 ohms, it will deliver 100 watts into 16 ohms, and it will try to deliver 400 watts into 4 ohms and 800 watts into 2 ohms... but usually it hits some limiting factor first (usually it lacks the ability to deliver enough current to continue doubling its wattage each time the impedance is cut in half).

A tube amp usually behaves more like a constant power source, particularly if it has low amounts of global negative feedback (which normally results in a high output impedance, or low damping factor, and a really nice-sounding distortion profile). So this type of amp tries to deliver approximately the same power regardless of the speaker's impedance. So if a tube amp delivers 200 watts into 8 ohms, it will probably deliver close to 200 watts into 16 ohms, and close to 200 watts into 4 ohms.

So now let's consider a hypothetical speaker impedance curve, and look at how these different types of amps behave into it. This is a two-way bass-reflex speaker tuned to about 50 Hz, so it's in the ballpark for what we might encounter in a bass cab:

Let's assume this speaker was "voiced" for a solid state amp... most are. So if we use a tube amp, we'll be getting more wattage into the impedance peaks than the designer expected. So we will have a more upper-mid emphasis because more wattage than the designer planned on will be going into that 1.5 kHz peak; the upper bass will be louder (which may or may not be a good thing) because of the impedance peak at 70 Hz; and since the peak at 30 Hz is below the tuning frequency, it will tend to make the cab a bit more likely to fartout on a low-B than if we used a solid state amp. This speaker may very well sound better with a solid state amp than with a tube amp, unless it just so happens that the changes are improvements to our ears.

Let's look at another impedance curve, this time just a woofer (no mid or tweet) in a sealed box, like an "old school" bass cab:

Relative to a solid state amp, our tube amp will put more wattage into the speaker at the 45 Hz impedance peak, and it will also put out increasing wattage as we go up in frequency. Now more than likely a sealed-box bass cab would have it sbass peak in the 60-to-80 Hz region, so that is where the low-end "boost" would happen. So this speaker will have more bottom end, as well as more top end,when driven by a tube amp - and since both are usually desirable in a bass cab, it's not surprising that "old school" bass cabs tend to sound better with tube amps.

In the real world the differences may or may not be as dramatic as I have described here, depending on the specifics of the amplifiers and speakers involved. But the principles described are correct to the best of my knowledge and according to measurements I've made.

For a more thorough look at the subject of different amplifier paradigms, you might take a look at this paper: Paradigms in Amplifier Design

That was probably a longer answer than you were looking for. Sometimes my inner geek just takes over.

Or, I could have just said, "What Duke said."

 

[JimmyM]

I have played through some cabs where the speaker cones would dance like crazy (making bad, farty tones in the process) when I pushed them with tube amps, but which behaved in a much different (and more controlled) fashion when pushed with solid state amps (of much higher output power rating).

AND, I have also heard plenty of ported cabs that did sound/perform great with tube amps. So, IME/IMHO, while porting is certainly a factor which influences how drivers perform in a given enclosure, there are numerous other factors at play.

Thank you. There are, indeed, numerous other factors at play, and to narrow it down to ported vs sealed only is a gross over-simplification.

 

[JimmyM]

Or, I could have just said, "What Duke said."

Duke's the long version, you're the short version

 

[adam montminy]

Well if you know so much about it, why did you ask?

Also, I made very clear that my opinion was based on my (nearly 40 years) experience with tube amps and different cabs sealed and ported and not based on technical info. But my experience is what it is, and that includes plugging an SVT into a ported cab that Duke made that sounded way excellent when powered with an SVT. But if ported cabs don't work well with tube amps, please explain why the most legendary recording amp of all time is the B-15, a tube amp plugged into a ported cab.

I don't know much about it at all. That's why I asked {Duke.} I don't work in that field. I repair saxophones for a living. I'm sorry I wasn't clear. I can see how one could have misinterpreted my wording. I was just looking for a little scientific explanation for what I have experienced with my ears and I knew that Duke would be just the right person to provide such a thing.
Thanks

 

[adam montminy]

Well if you know so much about it, why did you ask?

Also, I made very clear that my opinion was based on my (nearly 40 years) experience with tube amps and different cabs sealed and ported and not based on technical info. But my experience is what it is, and that includes plugging an SVT into a ported cab that Duke made that sounded way excellent when powered with an SVT. But if ported cabs don't work well with tube amps, please explain why the most legendary recording amp of all time is the B-15, a tube amp plugged into a ported cab.

Also, I wasn't saying that ported cabs don't work well with tube amps. I said the ported cabs I've tried didn't work well with the tube amps I've tried so I was inquiring to Duke whether or not ported cabs can work well with tube amps. I was simply looking for information on the subject. Thanks for providing your experiential input.

 

[adam montminy]

I have played through some cabs where the speaker cones would dance like crazy (making bad, farty tones in the process) when I pushed them with tube amps, but which behaved in a much different (and more controlled) fashion when pushed with solid state amps (of much higher output power rating).

That's exactly the sort of thing I was talking about. My cabs would almost start feeding back on certain low notes, just booming out of control, not even at super loud volumes either. I'm glad to hear that I'm not the only person who has experienced that. It's also good to know that not all ported cabs behave this way with tube power. Thanks for that.

 

[Coolhandjjl]

Duke, so regarding the ss vs tube comparisons, the cab impedance curve is almost moot for a tube head as the tube head attempts to deliver the same wattage throughout the range, whereas the ss head will deliver less at the cabs impedance spikes. Do I got that right?

 

[DukeLeJeune]

Wow, I kinda stirred things up there!

I should have emphasized that I don't know enough about the specifics of tube and solid state bass amps to say how much of a difference there is in practice. Amps can be along a continuum ranging from "approximately constant voltage" to "approximately constant wattage (within a limited range of impedances)", with solid state amps generally towards one end and tube amps generally towards the other end, but I don't know how spread out along the continuum tube amps for bass cabs are. The spec that comes into play is "output impedance" or "damping factor", but I don't think it's given very often.

And it's not nearly so cut-and-dried as "ported for solid state, sealed for tubes", which probably seems like what I was saying with the two examples I used. Sorry I gave that impression! You see, the absolutely vital piece of the puzzle that was missing from the examples in my last post, and which matters more than the impedance curve, is of course the frequency response curve. We'd need the cab's frequency response curve (presumably measured using a voltage-source amp), the amplifier's output impedance, and the cab's impedance curve to make a good prediction of what the tone of that cab will be when driven by that tube amp.

I'll write more later - gotta get back to making sawdust for now. Just don't ask me why I think tube amps sound better. That gets really geeky and really controversial. And it's probably out of my league, as I'm not an amplifier guy.

 

[DukeLeJeune]

Duke, so regarding the ss vs tube comparisons, the cab impedance curve is almost moot for a tube head as the tube head attempts to deliver the same wattage throughout the range, whereas the ss head will deliver less at the cabs impedance spikes. Do I got that right?

I don't think we can safely say that, because our world is totally calibrated to solid state amps. Our modeling programs make their predictions assuming solid state amps. The woofer manufacturers' published curves are made using solid state amps. I use a modeling program that lets me change the output impedance of the amplifier, but it's not totally user-friendly because it treats it like a series resistance and lowers the speaker's efficiency accordingly, so comparing the two frequency response curves is not as straightforward as it should be.

And I'm not certain at what damping factor values the amp's behavior crosses the line from "more like a constant-voltage source" to "more like a constant-wattage source". There is probably a fairly large range of output impedances among push-pull transformer-coupled tube amps for bass cabs, and there may likewise be a fairly large range among solid state amps - I just don't know.

 

[staccatogrowl]

I always admire the honesty of "I don't know. ." Wish more used the phrase.

 

[McFarlin]

But someone has to mop up all that green slime.

 

[slagheap]

Well the most obvious is, the cab needs to be wide enough for the tube amp's feet.

Beyond that, tube amps in general behave differently into a speaker's impedance curve than solid state amps do. A solid state amp approximates a constant voltage source, so it tries to deliver the same voltage regardless of the speaker's impedance curve. So if a solid state amp delivers say 200 watts into 8 ohms, it will deliver 100 watts into 16 ohms, and it will try to deliver 400 watts into 4 ohms and 800 watts into 2 ohms... but usually it hits some limiting factor first (usually it lacks the ability to deliver enough current to continue doubling its wattage each time the impedance is cut in half).

A tube amp usually behaves more like a constant power source, particularly if it has low amounts of global negative feedback (which normally results in a high output impedance, or low damping factor, and a really nice-sounding distortion profile). So this type of amp tries to deliver approximately the same power regardless of the speaker's impedance. So if a tube amp delivers 200 watts into 8 ohms, it will probably deliver close to 200 watts into 16 ohms, and close to 200 watts into 4 ohms.

So now let's consider a hypothetical speaker impedance curve, and look at how these different types of amps behave into it. This is a two-way bass-reflex speaker tuned to about 50 Hz, so it's in the ballpark for what we might encounter in a bass cab:

Let's assume this speaker was "voiced" for a solid state amp... most are. So if we use a tube amp, we'll be getting more wattage into the impedance peaks than the designer expected. So we will have a more upper-mid emphasis because more wattage than the designer planned on will be going into that 1.5 kHz peak; the upper bass will be louder (which may or may not be a good thing) because of the impedance peak at 70 Hz; and since the peak at 30 Hz is below the tuning frequency, it will tend to make the cab a bit more likely to fartout on a low-B than if we used a solid state amp. This speaker may very well sound better with a solid state amp than with a tube amp, unless it just so happens that the changes are improvements to our ears.

Let's look at another impedance curve, this time just a woofer (no mid or tweet) in a sealed box, like an "old school" bass cab:

Relative to a solid state amp, our tube amp will put more wattage into the speaker at the 45 Hz impedance peak, and it will also put out increasing wattage as we go up in frequency. Now more than likely a sealed-box bass cab would have it sbass peak in the 60-to-80 Hz region, so that is where the low-end "boost" would happen. So this speaker will have more bottom end, as well as more top end,when driven by a tube amp - and since both are usually desirable in a bass cab, it's not surprising that "old school" bass cabs tend to sound better with tube amps.

In the real world the differences may or may not be as dramatic as I have described here, depending on the specifics of the amplifiers and speakers involved. But the principles described are correct to the best of my knowledge and according to measurements I've made.

For a more thorough look at the subject of different amplifier paradigms, you might take a look at this paper: Paradigms in Amplifier Design

That was probably a longer answer than you were looking for. Sometimes my inner geek just takes over.

excellent lesson. reckon could be why my Super Twin conversions sounded better with sealed cabs than with my ported LDS Faital 2x15. the LDS sounds killer with the D800 - but i can't pick it up with one hand and run up a stairwell like i'm able with my H1203s!

 

[slagheap]

That's interesting but I have actually found the exact opposite in my experience. Most of my ported cabs had uncontrollable low end with my tube amps for example. I've found that tube amps are very picky about cabs whereas solid state amps don't seem to care what cab they're plugged into for the most part.
I asked Duke because anecdotal evidence is not as useful to me as actual information from someone who knows the "how" and "why" about the topic. As it turns out, there are quite a lot of characteristics inherent in tube amps that effect their interactions with speaker systems.

as a matter of fact that was my experience ( over the top bass in a ported cab, ) with a big tube amp. the same cab is great with a good microhead. very small sample size, plus a host of other undefined variables, though.