Tube amps, metal cone speakers, DIY in general - do they mix?

[fjhuerta]

Hi all,

After listening for some months to my latest two designs, I've reached a conclusion... I don't know if its wrong, but it certainly seems to match what I've observed.

I've noticed I'm always trying to make speakers as sensitive as possible. Due to this fact, my designs always hover around 4 Ohm. Whenever working with metal cone drivers, though, due to the sharp filters required, impedance sometimes drops below that mark. Also, when using two 8 Ohm woofers in parallel (or one 4 Ohm), impedance drops to 3.5 or so Ohm. Meanwhile, the tweeters hover at around 6 Ohm or a bit more (depending on the model).

Anyway, this results in a bright, forward sounding speaker when used with my tube amps (MC-30's, MC-40, etc). Even when using them through the 4 Ohm tap - it doesn't help.

IIRC, a tube amp behaves like a CC source, so low impedances bother it quite a bit. I'd assume that, with DIY designs, this is a sometimes overlooked fact -since SS amps can handle almost any load gracefully.

Am I correct in my assumptions?

[JonMarsh]

The impedance sensitivity of a tube amp (acting more like a current source) will certainly interact with the speaker load line, and be colored by it- that is why some of the earlier Avalon designs had additional zobels to make the load line as flat as possible.

The only way to deal with this if you want to stick with your tube amps is to design your speakers accordingly- that means getting sensitivity not by low impedance, but by driver selection and larger boxes (Hoffman's Iron Law).

The RS270 is a better choice for getting good sensitivity; consider perhaps a three way design with it and the RS52, and my inexpensive favorite to combine with it, the D26. Due to driver overlap you'll still wind up with a lowish impedance in the mid to high frequencies, but bass impedance should be higher, and closer in general values starting with a nominal 8 ohm, 90 dB/watt driver. However, due the afore mentioned Iron Law, the T/S parameters of the RS270 will dictate a large box if you go ported- 75 liters is a good point to consider.

~Jon

[fjhuerta]

Jon, so I'd be correct in assuming that tube amps really need flat impedances to work their best?

I'm looking at my latest design's impedance curve. From 100 to 400 Hz, it dips below 4 Ohm, right to 3 Ohm. The mids and highs go from 4 to 12 Ohm (yeah, I know - I was trying to design for the least amount of components!). Would this impedance curve really affect FR so much?

If so, then I'll return to SS. Tube amps are too much of a hassle for me. Personally, I'm starting to believe their sonic "character" is the result of distortion and the interaction between load and output impedance.

[Hdale85]

Well If you like the tube sound you could always get a tube preamp and run a SS amp. I've heard a system like this and sounded very good. You'll still get that harmonic bloom that tube amps get but you'll also have the low end authority that SS gives you.

[fjhuerta]

Well If you like the tube sound you could always get a tube preamp and run a SS amp. I've heard a system like this and sounded very good. You'll still get that harmonic bloom that tube amps get but you'll also have the low end authority that SS gives you.

Yeah, that's what I've used in the past - a Foreplay III or a Mac C-22 with a Rotel 1070. And I do like it a bit more than other SS preamps I own.

[eb15]

There are several factors that affect the sound of a "tube amp", and I've noticed low-impedance speakers affecting the sound of solid state amps as well.

You might want to try an alternative brand/design before you give up on the genre.

[Kevin Haskins]

I like tube flavor too. I use a tube preamp rather than an amp because tubes at high power are costly and will heat your house in North Dakota.

I'm using a Foreplay 3. Its a simple circuit and Dan & Paul have really optimized it over the years. I've considered doing a PCB for it and stuffing it in a box (don't like the shoe box thing) but I just don't have the time. Too many projects already.

[Dennis H]

Jon, so I'd be correct in assuming that tube amps really need flat impedances to work their best?

I'm looking at my latest design's impedance curve. From 100 to 400 Hz, it dips below 4 Ohm, right to 3 Ohm. The mids and highs go from 4 to 12 Ohm (yeah, I know - I was trying to design for the least amount of components!). Would this impedance curve really affect FR so much?

If so, then I'll return to SS. Tube amps are too much of a hassle for me. Personally, I'm starting to believe their sonic "character" is the result of distortion and the interaction between load and output impedance.

Javier, if you have it modelled in LspCAD, just insert a 4 (or 8 ) ohm resistor right after the generator to simulate the transformer's impedance and you can see how much the frequency response changes.

[Dennis H]

PS -- were you using the tube amp when you had trouble getting some of the forum designs to sound right?

[ThomasW]

PS -- were you using the tube amp when you had trouble getting some of the forum designs to sound right?

Everythings starting to fall into focus now.....

[JonMarsh]

Javier, if you have it modelled in LspCAD, just insert a 4 (or 8 ) ohm resistor right after the generator to simulate the transformer's impedance and you can see how much the frequency response changes.

Yup, the way to get a clear view! :T

I'm not dissing tubes, and that's why I'm I'm into zero loop feedback solid state (lower output impedance, no output device replacement/bias issues, similar freedom from edge and glare). But designing a speaker to work in a neutral manner with tube amps is a very specific sort of task.

Now, conventional solid state amps may have a given small signal output impedance (due in large measure to feedback) but how they sound working at significant volume into a real speaker is more dependent on open loop output stage impedance and power supply capability. Many SS amps can strain when a somewhat reactive 4 ohm load is present (like the first test version of the Modula NeoD CC running on an Adcom stereo power amp, compared to the Aragon we switched it to).

I really liked the big Sonic Frontiers vacuum tube monoblocks, but they were very dear, and the company is no longer around.

These days I'd covet a pair of Ayre MX-R monoblocks...

~Jon

[fjhuerta]

Well... yeah... I may have used the tube amps while testing the Nats... since I didn't really know there was so much of an issue. ops: Did I do wrong? :W

For the record, the difference between SS and tube amps (at least with my designs) is huge - and it really messes up FR. I think a hybrid approach (tubes on the pre, SS on the amp) makes a lot of sense to me now (well, it only took me like, 2 years to learn about this!).

Thank you so much for your input. I thought I was imagining things, but now that I simulated my speakers with a 4 Ohm resistor after the source, I can see how much FR is affected. And yes, the speakers definitely are brighter! I'll post some images... for reference.

[fjhuerta]

Grey is the reference, without the resistor installed.



A -5.3 dB difference in the bass area. Wow.

[Dennis H]

Javier, if you love your tube amps, there's still hope. Just do your modelling with the resistor inline. You're building speakers for yourself so they don't need to work for everyone. I'd still measure the drivers with an SS amp though so you're working from "real" driver SPL curves and only adding the 4 ohms once to the circuit simulation.

[JonMarsh]

Grey is the reference, without the resistor installed.



A -5.3 dB difference in the bass area. Wow.

Just what I'd expect. I wondered what was happening when you were so unhappy with your Natalie P build, but that was a time when I was extremely busy at work and didn't have the bandwidth to investigate...

Dennis's follow up suggestion is good, to, but I'd consider doing a "normal" design, then using conjugate networks (zobels) to flatten the impedance curve. Even one pair of my Arvo Part designs used that to smooth the midrange impedance.

~Jon

[Doug Lockwood]

Javier, if you have it modelled in LspCAD, just insert a 4 (or 8 ) ohm resistor right after the generator to simulate the transformer's impedance and you can see how much the frequency response changes.

Dennis H,

You have the right Idea, but I believe the resistor will be more on the order of 1 to 2 ohms, since typical damping factors of 4 are common, and the Zout is the reflected Zout of the tubes * Turns ratio squared.

Doug

[fjhuerta]

Thanks for all your comments - I feel as if I had been "in the dark" (so to speak) about the real behaviour of a tube amp into a load.

I returned my SS Rotel to my main system, and now my speakers sound great!

I'll now have to think long and hard about keeping my tube amps....

[fvoelling]

I have to admit that most of this stuff is way over my head, but now I'm wondering how the Modula MT (RS28 version) would behave with a tube amp vs solid state?

I was contemplating using my Jolida integrated tube amp with the MT that I'm building and using the Emotiva RPA-1 solid state in my HT for the L/R front channels instead. Is that a good idea?

[cjd]

If you want tubes in your system, I believe the best place for them is in the pre-amp stage.

C

[Doug Lockwood]

have to admit that most of this stuff is way over my head, but now I'm wondering how the Modula MT (RS28 version) would behave with a tube amp vs solid state?

My Brother built the 27TCFD Version of the Modula MT for one of his friends. We listened to it with an ARC VT50 (for hours) both with and without a sub and it sounded great. It really depends on the amp.


If I was designing / redesigning a speaker, I would test the drivers with the amp I was going to use. This is DIY, after all.

Doug