building an isolation cabinet, need help with ohms

[Chaotic]

Im building an isolation cabinet and wanted to know what ohm speaker to put in it. What is the most common ohm-age that most amp heads put out? If I put a 16ohm head thru an 8 ohm speaker, or vice versa, what would happen? some one please explain.

[jaxvon]

Originally Posted by Chaotic

Im building an isolation cabinet and wanted to know what ohm speaker to put in it. What is the most common ohm-age that most amp heads put out? If I put a 16ohm head thru an 8 ohm speaker, or vice versa, what would happen? some one please explain.

Do you mean what is the minimum impedance of most amp heads? Amps don't output impedance, they output current and voltage. You can connect them to any speaker that is greater than or equal to its stated minimum impedance. As for what most of them are, I don't know.

[matt thomas]

I'd use a 16 ohm speaker, that way all amps that I know could handle it, and also the output will be less, so there will be less noise to dampen in the box

narco

[Chaotic]

Other then overall volume, what are the cons if any, of using an 8 ohm head with a 16 ohm speaker? Does it effect the sound in any way?

[matt thomas]

An "8 ohm head" will just be an amp that needs a speaker with a MINIMUM of 8 ohms.

All this means is that if you attach a speaker of less than 8 ohms to it your amp may (will) overheat and die

An "8 ohm head" will run just fine with any speakers MORE than 8 ohms just fine, no problems

the sound will be defined by the character of the amp and speaker. The same speaker in 8 and 16 ohms should sound virtually the same.

narco

[Chaotic]

So then a 16 ohm speaker seems to be the best bet. I was just curious since demeter and randall make their iso cabs in either 16 or 8.

[nosebleedaudio]

What has not been mentioned is the fact that an 8 ohm speaker will draw twice as much current compared to a 16 ohm, producing a 3db increase in level, all other parameters being equal such as efficiency of the speakers.
But I would say the speaker itself will have more influence on the sound, the cabinet being the 2nd. open back or closed effects the sound, ect...

[aussie_techie]

Originally Posted by narco

the sound will be defined by the character of the amp and speaker. The same speaker in 8 and 16 ohms should sound virtually the same.

narco

while the drivers may sound the same do you find the amp sounds different under different loads?
i dont know enough about guitar amplifiers, all im going off is that many power amps dont perform or sound as good under low loads, ie at 2ohms.

[matt thomas]

I'm also guessing, but I would think higher ohms would be a tighter sound (based on logic not experience), lower resistence would get flabby, although guitar amps are made to drive typically 4 through 16 so it shouldn't make a difference. I'm sure the difference would be minor or not apparent at all.

and I did cover the 3dB level difference!


narco

[jwl]

I'd go 8 ohms, if you are installing a single speaker in the cabinet. That's what most guitar amps expect to see (if not a 4ohm load).

The 16 ohm cabinets are designed for 4x12 stacks, to get the total impedance down to 4ohms when the speakers are wired in parallel.

Just to clarify, amps don't "output" ohms, as someone said they output voltage and current. The impedance load (measured in ohms) will determine how hard the amps are working... if you go too low (like say 2 ohms) then it will overload the amp circuitry and likely cause problems (like blow the output transistors, say).

[Yago]

Hmm , not sure about some things here ...
Whilst it is not correct to say "an amp outputs impedance" , it is true to say "an amp has an output impedance" .
The impedance of the speaker should match the amp for maximum power transfere , or at least be in the specified range of the amplifier to prevent possible damage/sound degredation .
Also I am unsure about the 8 ohm speaker having 3db higher output with respect to a 16 ohm speaker , this too is dependant on the output impedance of the amplifier .
If the amplifier has a 16 ohm output impedance and you use an 8 ohm speaker the power will decrease as a larger proportion of the output voltage will be dropped across the output impedance of the amp , and not the speaker .

In short , I would find out the specs of the amplifier , and get a speaker to match .

[Chaotic]

Originally Posted by Yago

In short , I would find out the specs of the amplifier , and get a speaker to match .

Thats the issue though, I want to build a speaker cabinet that can accomodate a number of different amps/heads.

[Yago]

As (probably) already said , most amps have an acceptable range for speaker impedance . Do a little research into the amps you wish to accomodate , you will probably find that most are in the same region .

[ScottTunes]

The assumption here is that you are referring to building an isolation cabinet for a tube guitar amp...

So, to oversimplify an answer, install an 8 ohm load (speaker). That is the most common impedance required by the output section of an amp.

Tube amps *can* go up or down "one" level of impedance... Meaning an 8 ohm amp can power a 4 or 16 ohm load (speaker and/or speaker cabinet), a 4 ohm amp can power a 2 or 8 ohm, etc., with without damaging the amp. Solid state should not be mismatched! Besides, solid state distortion isn't pretty. Abusing a tube amp can be...

You wouldn't want to abuse, say an audiophile's tube stereo system because the mismatch would introduce too much unacceptable distortion. However, with guitar amps, this maybe a good thing in small doses. Though as with many things, you can't say, "if this much is good, more would be better!" More is not always better. In fact, "Less Is More" applies here too. The less mismatched the amp and speaker are, the more it sounds as it was designed to sound.

FWIW, YMMV...

[dewey decibel]

Originally Posted by jwl

I'd go 8 ohms, if you are installing a single speaker in the cabinet. That's what most guitar amps expect to see (if not a 4ohm load).

The 16 ohm cabinets are designed for 4x12 stacks, to get the total impedance down to 4ohms when the speakers are wired in parallel.

Just to clarify, amps don't "output" ohms, as someone said they output voltage and current. The impedance load (measured in ohms) will determine how hard the amps are working... if you go too low (like say 2 ohms) then it will overload the amp circuitry and likely cause problems (like blow the output transistors, say).

Originally Posted by tifftunes

The assumption here is that you are referring to building an isolation cabinet for a tube guitar amp...

So, to oversimplify an answer, install an 8 ohm load (speaker). That is the most common impedance required by the output section of an amp.

Tube amps *can* go up or down "one" level of impedance... Meaning an 8 ohm amp can power a 4 or 16 ohm load (speaker and/or speaker cabinet), a 4 ohm amp can power a 2 or 8 ohm, etc., with without damaging the amp. Solid state should not be mismatched! Besides, solid state distortion isn't pretty. Abusing a tube amp can be...

You wouldn't want to abuse, say an audiophile's tube stereo system because the mismatch would introduce too much unacceptable distortion. However, with guitar amps, this maybe a good thing in small doses. Though as with many things, you can't say, "if this much is good, more would be better!" More is not always better. In fact, "Less Is More" applies here too. The less mismatched the amp and speaker are, the more it sounds as it was designed to sound.

FWIW, YMMV...

+ 1. Most guitar amps are 4 or 8 ohms. If I was building a cab to handle multiple amps I'd also consider something like the Weber Z-Matcher...

https://taweber.powweb.com/store/zmatch.htm

[ultrasonic]

One idea is to build a box with two 8 ohm speakers. That way you could run them in series for 16 ohms, use a single speaker for 8 ohms, or both speakers in parallel giving a 4 ohm load. This would also allow you to run two different amps: one amp into each 8 ohm speaker. While you're at it, use two different types of 8 ohm speakers for a variety of tone.

General guitar amp/speaker info:

Some amps have an output impedance selector, which will allow you to properly connect different loads to an amp. Other amps have separate jacks for different impedance loads.

My experience suggests that most solid state amps specify the minimum speaker load. In most cases you can safely use a higher load, i.e. bigger number ohms. The problem with using a smaller load is that it starts to look like a short circuit to the power amp. So if your solid state amp states that it wants to see a 4 ohm load, you are probably safe to go 8 ohms, but 2 ohms is not advisable.

Tube amps, on the other hand, typically work best into a fairly exact load. The output tubes are loaded by the primary of the output transformer, which matches them to the speaker load. In this sense the output transformer can be thought of as a matching transformer.

In the real world I think most amp guys will say that the classic Fender amps can handle a 100% mismatch. That means your Deluxe Reverb (normally driving a single 8 ohm speaker) is probably ok running into a 16 ohm load, or a 4 ohm load. Vintage Marshall amps are significantly less forgiving about such mismatches, from what I've been told by amp techs.

It's hard to predict just how any random amp may react to an output load mismatch, so the best bet is to try to match. Some tube amp designers specifically mismatch the output to create a certain sound, so making assumptions about how much of a mismatch a particular amp can tolerate may have disasterously smokey results. Or at least very silent results. Especially with all the obscure boutique amps around these days.

The Weber device recommended by another poster may be a good idea, but I've never used it.

Rules of thumb:

- Always try to exactly match the load to the amp.
- Tube amps will have different tone and dynamics when operated into mismatched impedance loads. Change the load, change the sound.
- The classic Fender tube amps are always based upon 8 ohm speakers. If the amp has one speaker (i.e. Deluxe Reverb) it is an 8 ohm output, if it has 2 speakers (i.e. Twin Reverb) it is a 4 ohm output, 4 speakers (i.e. Super Reverb) is designed as a 2 ohm output. Leo only bought 8 ohm speakers. Except for the Champ, which was a 4 ohm speaker.
- Running an amp into a mismatched load is more likely to cause a problem if you operate the amp at a high volume.
- If a combo amp has an extension speaker jack it is probably in parallel with the internal speaker.
- Never run an amp with no speaker connected.
- Vintage tube amp output transformers can be very expensive.
- If an amp has separate jacks that are labelled for different impedance loads (Mesa, Sovtek, etc.), only use one jack at a time.