Pentodes; plate resistance; damping...

[mr mojo]

I was doing some reading this weekend, and ran across a section about feedback, pentodes and speaker damping-specificially that the high plate resistance of a pentode can cause oscillations with the voice-coil of a speaker whose impedance varies with changing loads. The feedback "damps" the interaction between plate and speaker by reducing plate resistance.

Text also said the lower plate resistance of a triode is one of the reasons no negative feedback is needed.

So that got me to thinking...seems that pentodes will be more influenced by speakers whose impedance fluctuates. So how would I go about measuring the impedance of a speaker to see how far it fluctuates between stated and actual impedance?

Also, can anyone tell me of any speakers whose impedance stays relatively stable under a load?

Best,
mr. mojo

[SY]

Speaker impedances vary all over the place for 99.9% of speakers. They vary with level for 100.0% of speakers. For a typical 2 way mini, you might see impedance max of 25-30 ohms, impedance mins of 3-4 ohms, and of course, varying phase angle...

There's a lot of good hardware/software available for doing speaker impedance measurement. Speaker Workshop is very popular at the moment and it works well (if a bit fiddly about setup).

[PRR]

> how would I go about measuring the impedance of a speaker

Use a signal generator. Put the speaker and a 300 ohm resistor in series across the output. Sweep frequency while watching the voltage across the speaker.

Say the signal generator puts out 10V. And say we are not looking for 10% accuracy (we aren't). If the speaker is 3 ohms, the output voltage will be 10V*(3/300)= 0.1V. If the speaker is 30 ohms, the output voltage will be 10V*(30/300)= 1.0V. So you can very quickly sweep frequency and see the general trend and the outstanding peaks (and dips if any).

You can calibrate with good resistors and some math. But mostly there are two impedances that matter: the lowest midband impedance (which will suck the most power and heat in the amp) and the highest bass and treble peaks (because their ratio to the lowest impedance affects frequency response).

A typical 4-inch speaker in box will be 6Ω at DC and maybe at 20Hz, about 50Ω at 150Hz, about 8Ω at 500Hz, rising above 16Ω above 5KHz.

If the alignment is tuned flat for a zero-Ω source, and you use an 8Ω source (DF=1), then around 500Hz the response will be 8Ω/(8Ω+8Ω)= 1/2 = -6dB relative to the zero-Ω source, at 150Hz it will be 50Ω/(50Ω+8Ω)= 0.86 = -1.3dB relative to the zero-Ω source. In effect the low damping gives a 6dB-1.3dB= 4.7dB bass-bump relative to the zero-Ω source.

DF=1 is kinda like a triode. Most triode amps give DF=2 or 3, so the error is less, like 1dB-2dB. Naked Pentodes give DF=5 or 10 on paper, in practice sometimes limited to 3-5 by transformer losses. A naked pentode can show a BIG bass-boom on a speaker tuned for zero-Ω source. You can tune a speaker different for high-Ω source and get flat response; zero-Ω source design generally gives deeper bass in less box size.

> speakers whose impedance stays relatively stable under a load?

The "load" we care about is the air load. For a 4-inch speaker, this will reflect-back as about 10Ω above 2KHz, falling very fast at lower frequencies. We can hardly see that, because the bigger load is the cone+coil mass, which is infinite at DC but falls past 8Ω at 200Hz toward 0.8Ω at 2KHz. So the inside of a speaker is all slanty impedances BUT for low-efficiency (<5%) speakers, the "inside" is a low impedance over the whole working range except its bottom octave. So what we see at the terminals is mostly coil resistance and inductance (and inductance is typically selected for the speaker function).

The main impedance "flaw", then, is the bass resonance. If we could use infinitely limp suspensions in infinite boxes, there would be no bass resonance: impedance would rise smoothly below about 100Hz. In fact we have lots of stiffness, usually selected to tune-up the soft corner of a no-stiffness speaker's response. If the suspension and box had zero losses, the impedance rise would be proportional to midband efficiency, roughly 100:1 or 8Ω for typical 1% efficient speakers. Zero losses never happens, and losses help muffle surround-flap and other incidental flaws, so the impedance peak tends to be 30Ω to 100Ω for 8Ω speakers. Adding loss at bass resonance reduces the impedance peak but also reduces bass output: impedance rise is a necessary part of an optimized speaker.

That's for a single driver. The sins of crossovers are complex. It is possible, with flat drivers, to design a 6dB/8ve crossover with dead-flat impedance through the crossover frequency. If the impedance rose at crossover, total response would dip because less power is being drawn at the higher impedance. In fact we are usually fighting driver droop, which suggests a crossover that dips in impedance to suck more power and compensate the driver droop. But we could go on for 9,999 pages about the complexity of crossovers.

[mr mojo]

Yowza! Thanks guys-great info as always. This is THE best place on-line for DIY audio by far.

Sy,
Thanks for the tip on Speaker Workshop and the general info on impedance. Part of the reason for my curiosity is when my DIY amp is done I'm just gonna HAVE to get new speaks to go with it!
I've listened to MG1s on a 40wpc Scott 296 and got great sound and good volume so the MMGs are now on my short list, but when I read about their impedance dipping to @ 1.5 ohm I became a little hesitant.


PRR, I gotta say I always appreciate your incredibly detailed responses. I also gotta admit, as a graphic artist by trade I've got to read it over a few times before some of it starts to
sink in.

But by all means, if you continue to be so inclined, please keep it up-I surely do appreciate it.


Best,
mr. mojo

[SY]

MG-1s have a much flatter impedance curve than normal, so that's a big help right there. I used a pair with some Dyna MkIII-based amps for many years (ultralinear connection) and was quite happy with the sound. I haven't tested MMGs. If they really do dip that low and it's in any part of the audio spectrum where you expect to need power, that suggests (I hate to say this) solid state might work better.

Or you could think about speakers which have a long and honorable reputation of being happy with tubes, like LS3/5a or BC-1.

[mr mojo]

Sy,

That would explain a lot about the MG1s-the folks at the shop dropped their chins to their chests when they heard them sing on the 296-that, of course after they said I'd need at least 200wpc to really make them shine.

Before I paint myself into a corner, I should say I read somewhere that Magnepans can dip as low as 1.5ohm-no specific models listed, and with all things on-line it may be best taken with a large grain of salt.

I've been curious to try some of those LS3/5As. I've read so many conflicting opinions about them that if I have a chance I'll have to let my ears decide for myself.

Best,
mr. mojo

[SY]

The LS3/5as are not accurate, but they image like a champ, have pretty good detail, and can almost fool you into thinking they've got bass. All that assumes an amp that doesn't mind the highish impedance with a few funny phase angles through crossover. A good p-p 35 watter will do wonders with them.

I should throw one of the newer single-driver HE designs into the mix, too, but my experience there is pretty limited.

[PRR]

> their impedance dipping to @ 1.5 ohm

From what?

If it wobbles from, say, 1.5 to 5, with an odd peak to 20, just tap your transformer to 2 ohms. If you roll your own, you can wind ANY low impedance you like. Just do the math. It may come out a little awkward for arbitrarily low loads: you can't have less than one (maybe one-half) turn which works out about 0.001Ω which suggests strap-copper.... but speakers with cable-terminals would never be wound to a milliOhm. If the load works on the other end of a few feet of wire, you can wind an audio transformer to suit.

If you don't have an iron-spinner: You could try tricks. 10K:4Ω used at 5K gives about 2Ω output. But the DCR is still around 0.4Ω, which starts to be a lot for 2Ω or 1.5V loads.

What are we talking about? Flat-diaphragm "loud"speakers? Get real. A JBL D-130 is "only" 5Ω DC but won't even touch 9Ω over the audio band, and a good 5% of that is actual air. It's a little shy above 4KHz; that's what the JBL bullet tweeter is for. Accurate? Maybe not, but more than many things I've heard. And you don't need any 200 watts to shine-up a D-130 in a living room. 2 watts will break your lease.

[pedroskova]

I'm pretty sure that the MMG is just a resistive 4 ohms like the other Magnepans.

[EC8010]

Originally posted by PRR
DF=1 is kinda like a triode. Most triode amps give DF=2 or 3, so the error is less, like 1dB-2dB. Naked Pentodes give DF=5 or 10 on paper, in practice sometimes limited to 3-5 by transformer losses. A naked pentode can show a BIG bass-boom on a speaker tuned for zero-© source.

I feel sure you meant to type "Naked Pentodes DF = 0.2 or 0.1 on paper"...