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8ohm vs 16 ohm- Does speaker impedance really alter the tone?


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I'm just wondering and would like to ask & discuss, how speaker impedance affects tone.

 

 

Say for example, the Marshall 1960A 4x12 cabinet, that has four Celestion 75watt speakers. (I have this very cabinet, but unfortunately I currently don't have the chance to try this out, and also very wary of stuffing things up. Plus I want to know what other people think of it.)

 

 

You can hook it up to the amp in 8ohms stereo, 16ohms mono, and 4 ohms mono.

 

Now, say the output of your amplifier is monaural (mono), either one speaker cable, or two cables carrying the same signal (ie not a stereo signal w/left & right).

 

 

Provided that everything is connected properly, How would the speaker impedance alter the tone?

 

 

I've heard of speaker mismatching, where an 8 ohm output is connected to a 16ohm speaker, etc. This works when the resistance (impedance) of the speaker is higher than the signal going into it (if it was lower, it will become f.u.b.a.r.).

 

 

Does the higher impedance have a larger headroom, ie doesn't "crunch" so easily/break up? Does it sound smoother? Does it handle the signal better? Does it enhance certain frequencies, or reduce some? Does it keep the bass tighter?

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^^ Cool, thank you.

 

I'm wondering if I'd be better off connecting the cab up to the amp in 8ohms or 16ohms. I already have another cab connected to another amp in 8ohms mono, so I want to see if I'd be better off having the Marshall cab at 16ohms.

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It makes a little difference, but the explanation is long and technical and it doesn't make a big enough difference for you to hear it.

 

There are some amp designs that just wouldn't sound right with a given speaker impedance, and as such they don't even have that setting built into it. But whatever. Let's not get technical.

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It makes a little difference, but the explanation is long and technical and it doesn't make a big enough difference for you to hear it.


There are some amp designs that just wouldn't sound right with a given speaker impedance, and as such they don't even have that setting built into it. But whatever. Let's not get technical.

 

 

I don't mind a bit of technicality, as long as it ain't too nerdy & academic (like on the boogie board...).

 

 

But if the difference is negligible, then I suppose it wouldn't matter how it's hooked up, provided there's no bad mismatching.

 

 

I'm still wondering though... :confused:

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Think about it - At 16 ohms you'll have increased voltage and reduced current. At 8 ohms, increased current, less voltage. Current means heat, which is bad. Lower voltage means voltage drop in the cable you're using gets more important, esp. when running 100 tube watts into 16AWG wire.

 

Some OT's only come with 8 and 16 ohm secondary taps. The 4 ohm tap would be too expensive to add since you'd have to design for higher current and more heat.

 

Also the amount of negative feedback changes if any (higher output impedance = more negative feedback).

 

 

So, in theory the higher impedance you use the tighter and louder your cab/amp will be.

 

In practice? YMMV, but most people seem to prefer the 8 ohm tap on their OT, possibly due to the signal going through less wire in the OT, designing the feedback loop around the 8 ohm tap, they like some voltage drop/power amp overload, 8 ohm jacks covered with fairy dust, etc.

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Think about it - At 16 ohms you'll have increased voltage and reduced current. At 8 ohms, increased current, less voltage. Current means heat, which is bad. Lower voltage means voltage drop in the cable you're using gets more important, esp. when running 100 tube watts into 16AWG wire.


Some OT's only come with 8 and 16 ohm secondary taps. The 4 ohm tap would be too expensive to add since you'd have to design for higher current and more heat.


Also the amount of negative feedback changes if any (higher output impedance = more negative feedback).



So, in theory the higher impedance you use the tighter and louder your cab/amp will be.


In practice? YMMV, but most people seem to prefer the 8 ohm tap on their OT, possibly due to the signal going through less wire in the OT, designing the feedback loop around the 8 ohm tap, they like some voltage drop/power amp overload, 8 ohm jacks covered with fairy dust, etc.

 

Thanks for that info, Jopop :thu: . It seems to me that 16ohms is more favourable (tigher sound), despite the negative feedback.

 

But wait a minute, if there's voltage drop due to the poweramp being overloaded (8ohms), would that mean the poweramp will distort more?

 

Because as well as tightness, I want as much poweramp distortion as possible.

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Choosing a given speaker load to maximize power amplifier distortion is a little "pennywise and pound foolish" if you ask me. I think for the most part, you won't hear a difference unless you plan on modding your amp. If you're looking for more power amp distortion, get an attenuator.

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Thanks for that info, Jopop
:thu:
. It seems to me that 16ohms is more favourable (tigher sound), despite the negative feedback.


But wait a minute, if there's voltage drop due to the poweramp being overloaded (8ohms), would that mean the poweramp will distort more?


Because as well as tightness, I want as much poweramp distortion as possible.

 

I believe he's wrong about the voltage drop.

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I believe he's wrong about the voltage drop.

 

 

If i am i would be very happy to be corrected! :thu:

 

Voltage drop in a conductor is generally calculated with this formula:

 

(Delta V)=P/V*L*cos phi*rho*A

Delta V is voltage drop in volts

V is RMS voltage

L is condutcor length in meters

Cos phi is the cosinus of the amount of degrees the voltage is out of phase with the current

Rho is the cable's resitivity (0.0175ohm for copper conductors)

A is the cross-section area of the conductor (We don't have AWG in europe)

P is power in watts

 

P is calculated using (among others), this formula:

 

P=V*I*cos phi

V is voltage, I is current in amperes, cos phi is.. well you should know now.

 

I is calculated like this: V/Z - Voltage divided by impedance

 

 

 

 

So naturally, to reduce voltage drop there are a couple of solutions -

 

* Increase V and impedance (easy with guitar amps and cabs)

* Use a better conductor (silver? not a probable solution)

* Increase cable cross-section area (common solution)

* Shorten the cable used

 

 

Now, if anything is wrong, please let me know, you'll be doing me a favor :wave:

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Thanks for that info, Jopop
:thu:
. It seems to me that 16ohms is more favourable (tigher sound), despite the negative feedback.


But wait a minute, if there's voltage drop due to the poweramp being overloaded (8ohms), would that mean the poweramp will distort more?


Because as well as tightness, I want as much poweramp distortion as possible.

 

Yes in theory 16 ohms impedance should give a tighter sound due to more negative feedback in the amp, and should be a bit louder due to less voltage drop in the cable. 8 ohm impedance should be less stable due to less feedback, and distort earlier.

 

The voltage drop will be the same in volts all things being equal, but the percentage is lower. Say you have 6 volts out in 8 ohm and 0.4 volts voltage drop - Increasing to 16 ohms will give 12 volts out but still 0.4 volts voltage drop, however it will make less of a difference with higher voltage. Say you have a 2 volt voltage drop on a cable with a source voltage of 4 volts - not good. Very bad. Say you have 2 volts of voltage drop on a 11000 volt line - totally irrelevant.

 

 

Anyway as i said i mosly see people preferring the 8 ohm tap. No idea why.

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Wait i am probably way off on the voltage drop thingie..

 

Voltage drop is I * Z according to wikipedia

As I goes down, Z goes up proportionally (i think)

 

In reality then it won't matter.

 

Damnit, i was wrong :mad:

 

 

The negative feedback stuff is correct though. I'm not an amp guru though but i know bits and pieces. I wish i could give more info.

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He's right that the voltage and current change to maintain the same power (in a tube amp with the impedance selector changed to match impedance with the cab). But not the heat because it's a function not only of current, but also resistance and other factors which cancel out the changes (if we're assuming the system can handle the power).

 

He's wrong about the negative feedback though (on most amps). A few amps take the feedback off the speaker output and so that will change when the impedance selector is changed, but most amps take the feedback directly off one of the OT taps so it's unaffected by the selector. Even if it does change, it's because the circuit is being changed and providing a different amount of feedback, not because the load impedance changed (which is what I think the original question was getting at).

 

Basically, when you match the impedance with an OT, the impedance itself doesn't change the tone, but the other things that you change to get the different impedance (series vs. parallel, OT windings) might depending on the amp. And yes, the explanation is nerdy.

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He's right that the voltage and current change to maintain the same power (in a tube amp with the impedance selector changed to match impedance with the cab). But not the heat because it's a function not only of current, but also resistance and other factors which cancel out the changes (if we're assuming the system can handle the power).


He's wrong about the negative feedback though (on most amps). A few amps take the feedback off the speaker output and so that will change when the impedance selector is changed, but most amps take the feedback directly off one of the OT taps so it's unaffected by the selector. Even if it does change, it's because the circuit is being changed and providing a different amount of feedback, not because the load impedance changed (which is what I think the original question was getting at).


Basically, when you match the impedance with an OT, the impedance itself doesn't change the tone, but the other things that you change to get the different impedance (series vs. parallel, OT windings)
might
depending on the amp. And yes, the explanation is nerdy.

 

I am wrong about the negative feedback too? :confused:

 

http://aikenamps.com/GlobalNegativeFeedback.htm

 

As far as i can see the voltage fed back to the amp will vary with what tap you connect the speaker to, given that the feedback tap stays on the same tap?

 

Hmm.

 

Also, you're saying heat is dictated by power and not current? Then, why do power plants increase voltage on HV lines in order to run more current through thinner wires?

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I am wrong about the negative feedback too?
:confused:

http://aikenamps.com/GlobalNegativeFeedback.htm


As far as i can see the voltage fed back to the amp will vary with what tap you connect the speaker to, given that the feedback tap stays on the same tap?

You get a different voltage on each tap, but as long as you have the correct load on one of the taps, those voltages on each tap won't change.

 

Also, you're saying heat is dictated by power and not current?

Um, no, I didn't say that, I don't think :confused:

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Actually, let me take back the last part of my last post regarding heat and power.

 

I didn't say it was dictated by power, but I was implying a relationship. What you have to realize there's a difference between dissipated power and transmitted power.

 

The power plant HV lines you mentioned are transmission lines and the object is to minimize the power dissipated by them, but speakers are dissipating all the power they receive.

 

The heat produced is directly related to the amount of power a circuit dissipates (not transmits).

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