Question About Impedance Matching

[sudon't]

I know very little about electricity, (don't touch it), or electronics. But I do know that you're supposed to match your amp's output to your speaker's impedance. That is, use the eight ohm outputs for eight ohm speakers.
But, what if your speakers are rated at six ohms? Do you use the four ohm outputs, or the eight ohm outputs? And, if I'm not asking too much, could some give me a basic explanation of why?
Forgive my ignorance, but is it, as I suspect, basically a math problem? If so, is there a simple way to do the math to get a proper impedance match between other components? I think this would also be helpful for adjusting phono preamps, (when they are adjustable), to cartridges, for instance.
TIA!

 

[JoeESP9]

Usually amps or receivers that have 8 and 4 Ohm taps are transformer coupled tube amps. Try your 6 Ohm speakers on both sets of taps. Which ever sounds best is what you should stick with. Either way it's not critical.

99%+ of all solid state amps have speaker connections that have no impedance figure attached to them. With them it doesn't matter at all.

Impedance matching other components is a non issue. As long as the input impedance of a component is at least 10 times the output impedance of the component feeding it there is nothing to be concerned about.

For all practical purposes impedance matching components is a non-issue!

With phono cartridges the load they are working into effects their frequency response. Most moving magnet cartridges work fine into a 47K Ohm low capacitance (200 to 400 pf) load. The capacitive load a moving magnet cartridge will see includes the capacitance of the connecting cable and the capacitance of the phono preamp.

Low output moving coil cartridges are designed to work best into a lower load (10 to 100 Ohm) and are relatively insensitive to capacitive load.

 

[ConradH]

The term "impedance matching" gets a bit iffy when applied to audio matters. In the RF (radio) world one is usually dealing with a single frequency and impedance can have a single value. That makes the math work out in a straight forward manner. When it comes to loudspeakers the impedance varies all over the place, so the idea of a perfect match goes out the window. At best we could get an imperfect range that we might call acceptable.

The idea of impedance matching is to get maximum power into the load. That happens when the source impedance (output impedance of the amplifier) is equal to the load (input impedance of the speaker). Already we have a problem with the concept since the design goal of most power amps is to have as low an output impedance as possible, aka high damping factor. Even with tube amplifiers that might sport a modest damping factor of 15, which is an output impedance of .5 ohms on the 8 ohm tap, it’s clear that the formal concept of impedance matching can’t apply.

I had to drag you through all this to make it clear that the term “impedance matching” should be forever banished from the audio dictionary and left to the RF world. What we’re really concerned with is making sure the amplifier can deliver the necessary voltage and current to the loudspeaker from as low a source impedance as possible, without damage. With a solid state amp it’s usually a design issue you can’t do anything about. With a tube amp, you have a choice of transformer taps that trade off voltage and current so the amplifier circuitry on the other side of the transformer “sees” reasonable values of voltage and current for a given speaker. Tubes operate at high voltages and low currents. Speakers operate at low voltages and high currents. Thus the necessity of the transformer, unless your name is Futterman.

The above logic also applies to low level signals like preamp to amp. Matching is never what you want, but low impedances driving higher impedances. The rule of 10X is good, but is also a simplification that can often be violated with no penalty.

Here’s some simple math that may or may not be useful. Say we have an amplifier that’s designed to put out 30 watts rms. It has taps for 4, 8 and 16 ohms. 30 watts into 4 ohms is 10.95 volts rms and 2.738 amps. Into 8 ohms it’s 15.49 volts rms and 1.936 amps. Into 16 ohms it’s 21.91 volts rms and 1.37 amps. (voltage = SQRT(P*R) and current = SQRT(P/R))

You can see that a 4 ohm load requires more current and less voltage for a given power compared to the 16 ohm load. The maximum voltage from the amplifier will only be what’s shown above, nor will it be happy trying to deliver more current. The only way to get the maximum power output is to use the load a given tap is designed for. Let’s say you put a 16 ohm speaker on the 4 ohm tap. Current won’t be a problem but the maximum voltage will be 10.95 volts rms. 10.95 squared over 16 gives you 7.5 watts. Not very impressive. Now put a 4 ohm speaker on the 16 ohm tap. Here we have a problem because plenty of voltage is available, but not so much current. 1.37 squared times 4 gives you (surprise) 7.5 watts. This case is bad because if you turn it up the amp will try to deliver more current than it was designed for and may be damaged. In the previous case it will just clip.

The usual caveats apply- I make lots of mistakes so somebody check my math. I’ve also oversimplified various things and ignored others. The bottom line is that speaker impedance varies all over depending on frequency. It’s bad to demand more current than an amplifier was designed for, so always use taps of less than or equal to the nominal speaker impedance. Even a 16 ohm speaker on a 4 ohm tap should be stable and happy, if not optimum.

Also, without an impedance plot for a speaker you really don’t know the impedance. An ohmmeter reading isn’t definitive, but many 8 ohm speakers will read 6 ohms when you make a DC reading using an ohmmeter. Lacking any other data that makes a good spot check.

If that 6 ohm number for your speaker was obtained with an ohmmeter, attach it to the 8 ohm tap and enjoy. If it really was a published number it’s still close to 8, so I’d probably run the 8 ohm tap for normal listening levels, knowing nothing would be overly stressed.

 

[catman]

G'day all, just to add, here's almost everything that you've ever wanted to know about 'impedance' and audio: http://sound.westhost.com/impedanc.htm

http://sound.westhost.com/cable-z.htm

http://sound.westhost.com/articles/cartridge-loading.html Regards, Felix aka catman.

 

[charles 1973]

Most SS Amps and speakers will tell you all you need to know about on the back of the unit's as to the compatbility of the two. It may say for example 4 ohms minimun Impedance on the back of the amp. So basicly a speaker that has a nominal impedance of 4 ohms or higher is fine.

Tube Amps are a little different and often have a 4 ohm and an 8 ohm Tap. Just use the one that is closer to the speakers impedance rating. If the speaker say's 6 ohms, either the 4 ohm or 8 ohm tap should be fine. Try them both if you wish.

Also it's not uncommon for a speakers impeadance to range from 5 - 15+ ohms. A Speaker that is Tube friendly is one that has a relitivly level Impedance.

 

[sudon't]

I don't know if this would've made any difference in the advice everyone's given, but perhaps I should have mentioned that I'm using a SS amp - a McIntosh MC 352 - which has 2, 4, and 8 ohm taps. It has the "autoformer", whose function I'm not really clear about.

I had to drag you through all this...

And I'm glad you did. This is the kind information I was hoping for, though I have to admit, in my naiveté, I thought it would be a little more straightforward!
What I had heard, or read, was that using the lower impedance outputs would increase the damping factor. I tried both the 8 and 4 ohm taps, but couldn't hear any real difference, so I went with the 4 ohm output, based on what I'd heard - which may, or may not be nonsense.
But, if I'm understanding what everyone is saying, maybe it was the better choice. The amp, at 350 wpc, should have plenty of spare power. 3.5 watts is a comfortable listening level, (for the other people in the house), so it would look something like this:

(3.5 * 4) = 14 √ = 3.74 volts
(3.5 / 4) = 0.875 √ = 0.935 amps

(3.5 * 8) = 28 √ = 5.29 volts
(3.5 / 8) = 0.4375 √ = 0.66 amps

So, again, if I'm understanding correctly, it might be better to go for the extra amperage from the 4 ohm taps? Probably it's a piddling difference, or I'm completely wrong, but I do want to try and understand how things work.

Low output moving coil cartridges are designed to work best into a lower load (10 to 100 Ohm) and are relatively insensitive to capacitive load.

OK, I'm hearing different things about this. My old, (very old), preamp was designed before the advent of the MC cartridge, so there's no possibility of adjustment. I bought a high output MC, (2.5 mV), plugged it in and it worked.
But soon, I'm demo-ing a phono preamp, (Pro-Ject Tube Box SE II). What appealed to me was the fact that it is adjustable, so that, whatever cartridge I decide to use in the future, I will be able to adjust for it. Surely there must be some kind of guidelines for setting the preamps impedance to match the cartridges output?
For instance, normally one might use the MM section for a high output MC. But I have heard that it's better to use the MC section, if you can adjust for it. This is not so much of a problem with an adjustable unit - although they use jumpers, which is a bit of a pain. But I have this Counterpoint preamp which has a slot, (holes, really), on the circuit board of the MC section, where you can plug in a resistor. How would you decide which resistor to order? You would likely want to try a few, but where to start?

 

[sudon't]

G'day all, just to add, here's almost everything that you've ever wanted to know about 'impedance' and audio

Very helpful - thanks, and cheers!

 

[ConradH]

Wow, that amp is so rugged and so much overkill it probably doesn't matter what tap you use! I'd just go for the 8 ohm tap and be happy. See if the others sound any different, but I'd be surprised.

Cartridge loading depends on the cartridge. Some respond more to changes than others. IMO, Catman knows more about this topic than I do. My cartridges have proven rather insensitive to it, so I just go with the factory recommendations.

 

[JoeESP9]

I don't know if this would've made any difference in the advice everyone's given, but perhaps I should have mentioned that I'm using a SS amp - a McIntosh MC 352 - which has 2, 4, and 8 ohm taps. It has the "autoformer", whose function I'm not really clear about.


And I'm glad you did. This is the kind information I was hoping for, though I have to admit, in my naiveté, I thought it would be a little more straightforward!
What I had heard, or read, was that using the lower impedance outputs would increase the damping factor. I tried both the 8 and 4 ohm taps, but couldn't hear any real difference, so I went with the 4 ohm output, based on what I'd heard - which may, or may not be nonsense.
But, if I'm understanding what everyone is saying, maybe it was the better choice. The amp, at 350 wpc, should have plenty of spare power. 3.5 watts is a comfortable listening level, (for the other people in the house), so it would look something like this:

(3.5 * 4) = 14 √ = 3.74 volts
(3.5 / 4) = 0.875 √ = 0.935 amps

(3.5 * 8) = 28 √ = 5.29 volts
(3.5 / 8) = 0.4375 √ = 0.66 amps

So, again, if I'm understanding correctly, it might be better to go for the extra amperage from the 4 ohm taps? Probably it's a piddling difference, or I'm completely wrong, but I do want to try and understand how things work.


OK, I'm hearing different things about this. My old, (very old), preamp was designed before the advent of the MC cartridge, so there's no possibility of adjustment. I bought a high output MC, (2.5 mV), plugged it in and it worked.
But soon, I'm demo-ing a phono preamp, (Pro-Ject Tube Box SE II). What appealed to me was the fact that it is adjustable, so that, whatever cartridge I decide to use in the future, I will be able to adjust for it. Surely there must be some kind of guidelines for setting the preamps impedance to match the cartridges output?
For instance, normally one might use the MM section for a high output MC. But I have heard that it's better to use the MC section, if you can adjust for it. This is not so much of a problem with an adjustable unit - although they use jumpers, which is a bit of a pain. But I have this Counterpoint preamp which has a slot, (holes, really), on the circuit board of the MC section, where you can plug in a resistor. How would you decide which resistor to order? You would likely want to try a few, but where to start?

The specs for a LOMC cart will give you the manufacturers recommended loading for that cartridge. A resistor (or load) with a value the manufacturer recommends is the best place to start. I have tried running HOMC carts into a head amp and a transformer. That didn't last long. The results weren't IMO very good. I run my Sumiko Blackbird straight into my ARC SP-9. The Ortofon MC20 I sometimes use goes through my Marcoff PPA-1 head amp first.

As with many things the load you give a phono cartridge is not set in stone. With MC carts having a couple of different values for load resistors isn't expensive even if you go to something exotic like Vishay metal films. Experimenting is fun and informative. Most moving magnet phono stages are set at 47K Ohms. For them the capacitive load is much more critical. Felix (The Catman) should chime in here. I seem to remember him trying and using different load resistances for Shure MM carts.

If you're happy with your current MM phono stage you may want to consider a SUT (Step Up Transformer) or head amp instead of an entire phono stage for LOMC carts. Either would be connected to the cartridge and before your phono preamp.

 

[sudon't]

A resistor (or load) with a value the manufacturer recommends is the best place to start.

I'll have to look and see what they recommend. As for the old MM stage, I'm getting rid of that old preamp, (a fifty-year-old MX-110). I picked up a Counterpoint, which has both MM and MC settings, as well as the possibility of swapping resistors. I want to have the phono stage upgraded, but in the meantime, plan to demo that Pro-ject phono pre. A little experimentation will be fun, if nothing else.
I'm not familiar with step-up transformers for cartridges, (yet another piece of electronics to lust after? Gee, thanks). I'll have to read up on those. But I have the high output MC at the moment, so that's not an issue yet.

See if the others sound any different, but I'd be surprised.

No, I guess you're right. I certainly didn't notice any difference. I suppose this might be more of a concern with some of the class-A SETs. Nevertheless, very glad to learn something.
Thanks for your help, guys! Really appreciate it.