SS Amp speaker replacement 8ohm vs 4 ohm... bad idea?

[Five Aces]

I'd appreciate any opinions or thoughts on this...

I have an old Fender Deluxe90 DSP SS amp with the original 4Ω, 12” Celestion G12T-100 speaker. I was looking to swap a Eminence Legend 121-8 12" 150W 8Ω speaker in it's place... initially to try the resulting tone change.
Besides the fact that I like and already had the Eminence Legend from a previous amp swap, my thought was as follows...

1) The SS amp should be fine operating an 8 ohm load, which would reduce the output from 90W to aprox. 65W? Plenty of power still for my use, plus, thinking it would unload the amp a bit and give me additional headroom?

2) I'd like to add an Ext. Spkr Jack (wired in parallel) on the back of the amp, allowing me the option to occasionally connect an additional 8 ohm external speaker cab, and the resulting load would be 4 ohms, which is what the amp was originally designed.

Thoughts? ...bad idea? I'm open for comments!

 

[printer2]

Do not think you will get much in the way of additional headroom, there would be no problem with your changing speaker(s) otherwise.

 

[Lynxtrap]

You will probably loose some treble, the sound will be a bit dull IME.

 

[TequilaCaster]

OP, your plan sounds 'aces' to me.

 

[Five Aces]

You will probably loose some treble, the sound will be a bit dull IME.

Hmmm... is it because of using an 8 ohm speaker? Or are you saying the Eminence Legend is dull in comparison to the OEM Celestion that is in it now? ...Difference in freq. range perhaps?

 

[Lynxtrap]

Hmmm... is it because of using an 8 ohm speaker? Or are you saying the Eminence Legend is dull in comparison to the OEM Celestion that is in it now? ...Difference in freq. range perhaps?

Because of the 8 ohm speaker. Running a SS amp through a higher impedance speaker (than the amp is indended for) results in lower output and less treble/presence.

 

[Beebe]

Because of the 8 ohm speaker. Running a SS amp through a higher impedance speaker (than the amp is indended for) results in lower output and less treble/presence.

I worked as a design engineer in commercial AV for ten years and never heard of such a thing. Doesn't mean there isn't something different about guitar amps that makes this true.

 

[Leonardocoate]

Increasing the resistance will not hurt the amp (SS only) but the sound will be a bit darker. You might like the end result. I have a cab with 4 4ohm speakers that where wired in series/parallel to maintain the 4 ohm and they where very bright and sounded great with a guitar. I then experimented by wiring them in series to get 16ohms and they sounded bassy or dark. It works great as my bass cab, but playing guitar through it is not so good. I have conducted other experiments that get similar results, not always as noticeable. I added a cliff jack to a Micro Cube (4ohm) and played it through a 10" 8ohm Lil Buddy and it sounds spectacular. I guess my point is to go for it. You won't hurt anything

 

[robt57]

Safe direction, but when I have done it the tonal change was not made up for with the speaker.

Side note; Just find the right speaker. I have a $40 Peavey Studio Pro 65 that went from 55% accetible sounding to 90% when I stuck a Princeton 65 Fender Speaker in it.

There is a lot to be said for speaker hording.

 

[schmee]

No problem

 

[printer2]

There is a lot to be said for speaker hording.

I don't understand.

On the change from 4 ohm to 8 ohm, there is no reason the amp will deliver a darker tone.

 

[Lynxtrap]

I worked as a design engineer in commercial AV for ten years and never heard of such a thing. Doesn't mean there isn't something different about guitar amps that makes this true.

I have experienced this in several amps, and this is my understanding of the physics behind it (I am ready to be proven wrong):

Impedance rises with frequency. An 8 ohm speaker has twice the impedance of a 4 ohm speaker already from the outset.

If the power amp is designed for a 4 ohm speaker, it is not capable to deliver the voltage to drive the higher load (8 ohm speaker) to full power, and this gets more noticeable with high frequencies where the impedances might be, say, 20 ohms for the 4 ohm speaker vs 40 ohms for the 8 ohm speaker.

It may or may not be a problem, in this case we'll have to see what the OP thinks after trying it.

 

[Five Aces]

....It may or may not be a problem, in this case we'll have to see what the OP thinks after trying it.

Because of holiday prep and family coming to town, this will probably not happen until after Christmas.
This is all fascinating info and insights. Curious as to the results since it's all subjective to individual interpretation of the resulting sound! I've heard demos that shows a simple difference of increased volume makes some minds equate that as brighter.
In any case, as long as electrically there is no worry of harm, it should be interesting.
I'll post followups.....

 

[printer2]

I have experienced this in several amps, and this is my understanding of the physics behind it (I am ready to be proven wrong):

Impedance rises with frequency. An 8 ohm speaker has twice the impedance of a 4 ohm speaker already from the outset.

If the power amp is designed for a 4 ohm speaker, it is not capable to deliver the voltage to drive the higher load (8 ohm speaker) to full power, and this gets more noticeable with high frequencies where the impedances might be, say, 20 ohms for the 4 ohm speaker vs 40 ohms for the 8 ohm speaker.

It may or may not be a problem, in this case we'll have to see what the OP thinks after trying it.

A SS power amp acts as a voltage source. So changing from 4 to 8 ohms will produce less power, but that is only because the speaker is only asking for less current, not because the amp can not put out more power. The amp does not care if it is feeding 4 ohms, 8 ohms, 40 ohms. If the amp is given a 1V signal and that produces 10V at the output it will put out 10V at 4,8,40 ohms. Some amps have current NFB and rather than operating as a voltage source it will operate (to a certain extent) as a current source. So with rising impedance it will put out more power at that impedance that it would as compared to operating as a voltage source.

 

[archetype]

I don't understand.

On the change from 4 ohm to 8 ohm, there is no reason the amp will deliver a darker tone.

Looks like you're horn hoarding, too. Hornding?

 

[Lynxtrap]

A SS power amp acts as a voltage source. So changing from 4 to 8 ohms will produce less power, but that is only because the speaker is only asking for less current, not because the amp can not put out more power. The amp does not care if it is feeding 4 ohms, 8 ohms, 40 ohms. If the amp is given a 1V signal and that produces 10V at the output it will put out 10V at 4,8,40 ohms. Some amps have current NFB and rather than operating as a voltage source it will operate (to a certain extent) as a current source. So with rising impedance it will put out more power at that impedance that it would as compared to operating as a voltage source.

OK, thanks for the explanation. This is going above my level of knowledge so I won't and can't go into an argument about it.

I might have been wrong in talking about voltages. Perhaps it is current and power that should be looked at.

What effect does the current demand have on the sound when the impedance approaches something like 40 ohms (as opposed to 20 ohms) at those high frequencies, while most of the signal consists of information in the 150 - 2k HZ range?

Sorry if I mislead someone. I have always heard of this phenomenon, and I have experienced it myself in several cases. Perhaps I was imagining it all.

 

[printer2]

OK, thanks for the explanation. This is going above my level of knowledge so I won't and can't go into an argument about it.

I might have been wrong in talking about voltages. Perhaps it is current and power that should be looked at.

What effect does the current demand have on the sound when the impedance approaches something like 40 ohms (as opposed to 20 ohms) at those high frequencies, while most of the signal consists of information in the 150 - 2k HZ range?

Sorry if I mislead someone. I have always heard of this phenomenon, and I have experienced it myself in several cases. Perhaps I was imagining it all.

With tube amps you may be right. The transformer has a limited frequency bandwidth and a change in the load impedance can shift the response. With a SS amp the transistor or mosfet (generally) does not care if the output is open, the NFB around the amp keeps things in check.

 

[printer2]

Looks like you're horn hoarding, too. Hornding?

So many speakers, so little time. I have four sets of an 8" and a horn to be used as small monitors or for mains to be used in house concert situations. Have a pair of Electrovoice horns that I could pair up with a 12" each, need to find a tweeter for them though.

 

[Lynxtrap]

With tube amps you may be right. The transformer has a limited frequency bandwidth and a change in the load impedance can shift the response. With a SS amp the transistor or mosfet (generally) does not care if the output is open, the NFB around the amp keeps things in check.

Maybe so. I will try to actually test it and do an A/B comparison.

 

[printer2]

Maybe so. I will try to actually test it and do an A/B comparison.

I was going to mention that a change to the voice coil of a speaker may change the speaker's response, the mass and inductance may be different with the different impedance. Whether it is theoretical or there is a audible effect?