Mixing high power and low power speakers

M

Meanjoegreene

Active member
While technically safe what’s y’all’s opinion of mixing 2 25 watt speakers with 2 90 watts in a 4x12? I’ll be running 100 watt heads and playing hard rock.

I think the sensitivity is 98 and 100.
 
As long as you realize that the cab is still a 100w cab; the rating is only as much as 4X the lowest wattage speaker.
 
I’ve had good luck mixing different sensitivities but never this much difference in watts. Hopefully the 25 watt GB types won’t be too overwhelmed
 
The frequency responses when mixing different speakers can and will always have adverse affects on the wattage distribution to the lower power speakers. It can also play hell on your output transformer, and here’s why.

Speakers are not resistive devices, it’s not a one cut 25W four times and done. They’re inductive by nature making their power consumption frequency dependent, meaning they can overheat depending on their efficiency or lack thereof. Likewise, a resonant peak at a location where the other speakers do not have a resonant peak means the speaker containing the resonant peak with absorb the majority of the power at that frequency location regardless of what is mixed with them.

Generally speaking to be safe, you really only have 50W maximum power in that cabinet and actually lower than that due to another detrimental effect called excursion induced inductive kickback, where a larger coil and magnet combination can induce unwanted current spikes to be distributed to smaller, lower wattage coil and magnet combination on the same phase, essentially generating more heat and phase distortion. Its a similar problem seen in relays when switched in and out of circuit, and also the same concept of a motor being a generator and vice versa, only it’s on a per-phase instance of a larger speaker coil/magnet combination in parallel with a smaller speaker coil/magnet combination. It’s even more prominent when you’ve hit a resonant location in frequency of the weaker wattage speaker. Power in is power out minus losses. There’s no free energy here, but you can visualize it as power sloshing around uncontrollably like water in a bucket.

So long story short, the 2dB efficiency difference isn’t enough of a perceived loudness problem as it is the larger wattage ratings of each pair. Keep it below 50W and you’ll be fine.
 
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So 2 90 watts and 2 25 watts might cause some issues with a 100 watt amp?

glpg80 said:
The frequency responses when mixing different speakers can and will always have adverse affects on the wattage distribution to the lower power speakers. It can also play hell on your output transformer, and here’s why. Speakers are not resistive devices, it’s not a one cut 25W four times and done. They’re inductive by nature making their power consumption frequency dependent, meaning they can overheat depending on their efficiency or lack thereof. Likewise, a resonant peak at a location where the other speakers do not have a resonant peak means the speaker containing the resonant peak with absorb the majority of the power at that frequency location regardless of what is mixed with them.

Generally speaking to be safe, you really only have 50W maximum power in that cabinet and actually lower than that due to another detrimental effect called excursion induced inductive kickback, where a larger coil and magnet combination can induce unwanted current spikes to be distributed to smaller, lower wattage coil and magnet combination on the same phase, essentially generating more heat and phase distortion. Its a similar problem seen in relays when switched in and out of circuit, and also the same concept of a motor being a generator and vice versa, only it’s on a per-phase instance of a larger speaker coil/magnet combination in parallel with a smaller speaker coil/magnet combination. It’s even more prominent when you’ve hit a resonant location in frequency of the weaker wattage speaker. Power in is power out minus losses. There’s no free energy here, but you can visualize it as power sloshing around uncontrollably like water in a bucket.

So long story short, the 2dB efficiency difference isn’t enough of a perceived loudness problem as it is the larger wattage ratings of each pair. Keep it below 50W and you’ll be fine.
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I wouldn’t mix them and expect to run the amp very loud, especially if downtuned. Pay attention to the frequency responses and make sure resonant peaks exist and even better if they are somewhat alike.
 
Meanjoegreene said:
So 2 90 watts and 2 25 watts might cause some issues with a 100 watt amp?
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Dude, you'll be fine. I've ran GB's with higher wattage speakers many times, and with cranked 100w plexi's
My buddy runs 25w GB's with the marshall v30's, and those are 70w. He cranks/gigs a custom 100w plexi I built him.

Only problem he had was he got too rowdy at a gig, and rammed his head stock though one of his GB's, I re-coned it for him, and all good.
 
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Thanks for the info. I might split the difference and go with some avatar fane m65s. Just hope they are a good fit for modern Marshall tones.
 
Meanjoegreene said:
While technically safe what’s y’all’s opinion of mixing 2 25 watt speakers with 2 90 watts in a 4x12? I’ll be running 100 watt heads and playing hard rock.

I think the sensitivity is 98 and 100.
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You will not be fine if you run those 100w heads past 4-5 on your master volume knob, or kick on a boost/fuzz/etc. Here's the logic.

1) You can't add the speaker power handling together for a total since they're not all the same wattage. So you'd have 50w per side, or 100w for all four.
2) The signal your amp sends doesn't know if it's going to a 90w or 25w speaker, it just sends the full signal (no crossover, or anything limiting the frequency or power soak).
3) Your 100w head is usually 100-110w clean (that's 4 on the volume knob), and 170-200w distorted at 7-10 on the volume knob.

So while your 25w speakers may last awhile at 4-5 on the MV plus a boost pedal or two, no one knows how long "awhile" is.

You're better off with four 50-90w speakers to cover your power at full blast plus a boost. Otherwise you are rolling the dice. I've had lots of clients send their 25w G12M's in for 65w voice coils after attempting what you're talking about. Typical start to destruction time was around 1-2 months.

You've been warned.

Jim
 
Scumback Speakers said:
You will not be fine if you run those 100w heads past 4-5 on your master volume knob, or kick on a boost/fuzz/etc. Here's the logic.

1) You can't add the speaker power handling together for a total since they're not all the same wattage. So you'd have 50w per side, or 100w for all four.
2) The signal your amp sends doesn't know if it's going to a 90w or 25w speaker, it just sends the full signal (no crossover, or anything limiting the frequency or power soak).
3) Your 100w head is usually 100-110w clean (that's 4 on the volume knob), and 170-200w distorted at 7-10 on the volume knob.

So while your 25w speakers may last awhile at 4-5 on the MV plus a boost pedal or two, no one knows how long "awhile" is.

You're better off with four 50-90w speakers to cover your power at full blast plus a boost. Otherwise you are rolling the dice. I've had lots of clients send their 25w G12M's in for 65w voice coils after attempting what you're talking about. Typical start to destruction time was around 1-2 months.

You've been warned.

Jim
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As texasslinger mentions above, does mixing 2 low wattage with 2 high wattage pose any additional risk compared to 4 low wattage (ie 4x25w greenbacks)? I've been playing my EVH 4x12 (4x25w greenbacks) with JP2C (100w) at band practice and cranking it, and this thread has me worried. Seems like EVH cranked his 100w heads through the same cab too. Even though those EVH Greenbacks say 20w, we all know that has been debunked and they are 25w a piece.
 
@Meeotch...my definition of cranked is with the amp volume (channel/master volume) past 7-10. Depending on the amp's circuit it can make 100w at 3, like my old 71 SuperLead and 69 SuperBass Marshall heads.

These days 100w heads are all over the map on specs, but generally I like to set the medium output at JCM 800 levels. Here's the JCM 800 manual for power I got years ago and scanned. Typically the 4% clean level is reached by 4. 10% distortion (70's hard rock tones) is achieved by 6-7. The knob goes to 10, but usually it's more gain and distortion past 8.

If you have a late 60's Plexi spec, you can get 120-130w clean (4) and 190-220w (7). Add on some boost pedals, and poof, your 25w G12M's are smoking in the boys room. I recone enough of them to know. I like to ask my clients what they were doing when they blew.

No one that gets a recone from me asks for the 25w spec after a voice coil melting experience.

JCM 800 manual below.

jcm800powerspecs.jpg

Sure you can mix 25w and 90w speakers, there's no problem with that, just the risk of the 25w getting cooked and leaving an open circuit that could then fry your output transformer. So there's still a risk...proceed at your own risk!
 
Scumback Speakers said:
@Meeotch...my definition of cranked is with the amp volume (channel/master volume) past 7-10. Depending on the amp's circuit it can make 100w at 3, like my old 71 SuperLead and 69 SuperBass Marshall heads.

These days 100w heads are all over the map on specs, but generally I like to set the medium output at JCM 800 levels. Here's the JCM 800 manual for power I got years ago and scanned. Typically the 4% clean level is reached by 4. 10% distortion (70's hard rock tones) is achieved by 6-7. The knob goes to 10, but usually it's more gain and distortion past 8.

If you have a late 60's Plexi spec, you can get 120-130w clean (4) and 190-220w (7). Add on some boost pedals, and poof, your 25w G12M's are smoking in the boys room. I recone enough of them to know. I like to ask my clients what they were doing when they blew.

No one that gets a recone from me asks for the 25w spec after a voice coil melting experience.

JCM 800 manual below.

jcm800powerspecs.jpg

Sure you can mix 25w and 90w speakers, there's no problem with that, just the risk of the 25w getting cooked and leaving an open circuit that could then fry your output transformer. So there's still a risk...proceed at your own risk!
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Thanks for such a helpful post Jim! As I understand it, another way of calculating wattage would be to use a decibel meter correct? Here's a quote I found:
"G12M25 generate ~97dB with 1 watt. So when closely mutually coupled, 4 of them acting in unison will generate >100dB at 1 watt.
Each time the power doubles, the dB generated increases 3dB. Hence 2 watts gives 103dB, 4 watts 106dB, 8 watts 109dB, 16 watts 112dB, 32 watts 115dB, 64 watts 118dB ..."

Do you suppose I could just use a decibel meter and regardless of where the volume knob is, if I'm around 112-115db at 1m then I should be well within the scope of a 100w cabinet?
 
Wattage calculations are done on an amp using an oscilloscope with the amp set to 8 ohms. Usually it's a 1khz signal with 1 watt of power to test a speaker's sensitivity/spl output. The type of speakers used will convert the watts to a volume. Higher decibel speakers will be louder. G12M's are 97db, G12H's are 100/101 db. The G12H's will sound 15-20% louder in a room. If there's enough watts to properly move the cones, you'll get more volume but you can't expect to get an increase of real impact going from 1 to 4 speakers unless the amp sends enough signal to move the cones since you're dividing the original signal to more speakers. Of course more speakers sound fuller than a single speaker in most cases given you're using the same type speaker.

Basically a more efficient speaker converts the signal to a higher SPL according to it's design limits than a lower SPL/DB speaker.

Summary: the more efficient the DB/SPL output pressure of the speaker, the louder it sounds vs a lower db/spl speaker. The actual amp circuit and output tranny have a lot to do with that as well, so there's that factor as well.
 
That all makes sense Jim thanks. I'll stick with the rated sensitivity for a single speaker even if using a quad. So I'm still curious about using a decibel meter to approximate wattage. As you mentioned, every amp's volume taper is different so that is not a rational way to judge an amp's power output. If I'm using speaker sensitivity as a guide, can I get in the ballpark? (ie with my example above, keeping my volume below 110-112dB should be safe)

However, it's not clear to me if this is only rational on a clean tone, as you have described distorted tones using more power. I just don't have an oscilloscope :D
 
O-scopes are the way it's measures, not with a DB meter (wasted my money on one of those thinking the same way, thank you).

Example, my old Gibson GA-8 Gibsonette from 1958 is a dual 6V6 single ended Class A amp, rated at 9w. When my old amp builder got it on the scope, it's clean tone ended at 3 on the volume, and it read 10.2w. So it's already putting out 113% of it's rating...clean. At 7 on the volume knob, it put out 18/19w with a singing distortion similar to Cream/Zep tones. It went into Neil Young mode after that with a lot of sag all the way up to 10, but basically unusable live, IMO past 7.

Ok, now get your Plexi 50 or 100w Marshall measured. My 1987 clone built to 69 specs puts out 55/60w clean around 4 on the volume knob, and 96w at 7. Yeah, it gets louder past that a little, but mostly more gain/saturation. Your basic Plexi nearly doubles from the clean rated power, which is shown in that JCM 800 power rating chart.

Speakers don't make more watts, but they do convert the wattage to more SPL and DB pressure, perceived as being louder. But since DB meters aren't calibrated to speakers, or amps, I don't see how you could use it without calibrating your DB meter to a known amp/speaker combo verified with other gear, and then run again to see what your gear measures it as.

So, variables are involved, that's why clean tones, 8 ohms, O-Scope, 1w at 3 feet (1 meter distance ) to the speaker from the mic, DB meters and all that have to be aligned to get an accurate reading.

The guy I used to have build my amps had some other amp maker named Dumble come to his shop to test stuff out when Santana or someone famous sent in their amps for service. Since Dumble trusted him...I kinda figured I could.

Anyway, the DB meter by itself will just show what the DB reading is, but you really don't know what amount of power you've sent the speakers without the rest of it.

Ok, probably reached the limit of my limited knowledge (or how much I care) on O-Scopes, testing, DB meters, etc since it's Labor Day Weekend and I'm still working. I hope this answered enough of your questions...if not...well, you need someone else to go further down this rabbit hole! LOL

I'm out.
 
Scumback Speakers said:
O-scopes are the way it's measures, not with a DB meter (wasted my money on one of those thinking the same way, thank you).

Example, my old Gibson GA-8 Gibsonette from 1958 is a dual 6V6 single ended Class A amp, rated at 9w. When my old amp builder got it on the scope, it's clean tone ended at 3 on the volume, and it read 10.2w. So it's already putting out 113% of it's rating...clean. At 7 on the volume knob, it put out 18/19w with a singing distortion similar to Cream/Zep tones. It went into Neil Young mode after that with a lot of sag all the way up to 10, but basically unusable live, IMO past 7.

Ok, now get your Plexi 50 or 100w Marshall measured. My 1987 clone built to 69 specs puts out 55/60w clean around 4 on the volume knob, and 96w at 7. Yeah, it gets louder past that a little, but mostly more gain/saturation. Your basic Plexi nearly doubles from the clean rated power, which is shown in that JCM 800 power rating chart.

Speakers don't make more watts, but they do convert the wattage to more SPL and DB pressure, perceived as being louder. But since DB meters aren't calibrated to speakers, or amps, I don't see how you could use it without calibrating your DB meter to a known amp/speaker combo verified with other gear, and then run again to see what your gear measures it as.

So, variables are involved, that's why clean tones, 8 ohms, O-Scope, 1w at 3 feet (1 meter distance ) to the speaker from the mic, DB meters and all that have to be aligned to get an accurate reading.

The guy I used to have build my amps had some other amp maker named Dumble come to his shop to test stuff out when Santana or someone famous sent in their amps for service. Since Dumble trusted him...I kinda figured I could.

Anyway, the DB meter by itself will just show what the DB reading is, but you really don't know what amount of power you've sent the speakers without the rest of it.

Ok, probably reached the limit of my limited knowledge (or how much I care) on O-Scopes, testing, DB meters, etc since it's Labor Day Weekend and I'm still working. I hope this answered enough of your questions...if not...well, you need someone else to go further down this rabbit hole! LOL

I'm out.
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Jim your wisdom is much appreciated! I also didn't intend to hijack the thread but it all seems pretty relevant. Happy holiday weekend!
 
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