Headroom and Amp Class for Dummies

[agedhorse]

Mind you if its a 180W amplifier being used in a typical domestic situation it probably is only used at lower power unless he has some very insensitive speakers.

Yes, but it’s a class AB amp that operates in class A at low power. This is in fact the A part of the definition of class AB.

 

[undercutter]

There are different classes of 212’s, some are better than others by a wide margin.

You are overthinking the headroom part.

okay, after plugging a ton of spec info into chatgpt, it has determined that my best course of action would be to look for a single 800w 4ohm Mesa Boogie Subway 2x12 Ultra-Lite Cabinet to run on the A channel with the option of getting a second identical (or complimentary) cabinet to run on the B channel in the future as necessary. That way i can use my current power amp to its fullest potential while setting myself up with the option of upgrading my janky separate preamp/power amp setup to a new 800D in the future.

Now I just need to find a single 800w 4ohm Mesa Boogie Subway 2x12 Ultra-Lite Cabinet for under $1000! Wish me luck I guess.

Thanks for the info.

 

[Wasnex]

I must be misreading something. Why is 500w into an 8 ohm Acme B2 ok, but 500w into a 4 ohm Acme B2 not ok?

500W into an Acme Low B2 is not okay. The expectation with a 500W amp is you will never use all of the power.

350W RMS is assumed to be the thermal rating of the voice coils. This is sort of a long term average. The voice coils can take short transients that exceed 350W as long as the long term average does not go above 350W.

Music is not constant power. It has dynamics and the power level varies. AFAIK bass typically has about a 12dB crest factor. Crest factor is the ratio between the peak power and dynamic power. 500W RMS is 1000W peak. So we assume if the Peak power is 1000W the long term RMS power will be 12dB lower. Let's do the math figure out the long term RMS power.

Power Crest Factor of 12dB = 10Log(1000/X)

• Solve for X
• The formula can be rewritten as 12 = 10Log1000 - 10LogX
  • 10Log1000 = 30 We sub this back into the formula
• 12 = 30 - 10LogX
• 12 - 30 = -10LogX
• 10LogX = 18
• LogX = 18/10 = 1.8
• X = 10^1.8
• X = ~63.1W

Math Proof (insert ~63.1 back into the original formula from above)

• Crest Factor = 10Log(1000/63.1) = 12dB

So with 1000W peak and a 12dB crest factor, it is assumed the long term RMS power is about 63W RMS. Since the woofer voice coils are rated for 350W RMS we don't expect them to overheat. However, with bass you need to be concerned about mechanical power handling. Notice the 300W limit at 30.87hz. If you exceed 300W RMS at 30.87 hz even for a short period of time, the woofers will most likely receive cumulative damage and eventually fail. In other words, the woofers can't handle anywhere close to 500W at 30.87hz.

 

[james condino]

They can't handle the full 500 watts blasting away at max capacity, but they definitely sound much better with more available power.

I use a 900 watt class D head for 99% of my gigs; it reacts completely different that one of my old Ampeg tube heads at a much lower rating.

Depending upon the day's needs. I'll bring a pair of ACME B1s, a single B2, a pair of B2s, or a B2 + a B4; all of the cabs are 8 ohms. They all work fine, I just have to be aware of the limitations when I'm on the gentle side of things and not push past a reasonable limit. (Sometimes I'll bring them all and run through a 2000 watt head for festival type big stages.) All of those combinations react noticeably better with the 900 watt head vs when I use a smaller 600 watt head by the same company. Almost every ACME owner that I know who uses them a lot occasionally blows a driver, but so do the rest of the cab owners I know. They are $100 and easy to replace.

 

[agedhorse]

I disagree, most players do NOT blow drivers at all when they power their rigs appropriately.

 

[james condino]

No way I would want to argue with you or think I might have a chance, but almost every regular gigging musician I know blows cab, breaks necks, blows fuses, & burns up amps all while having the time of their life giving 200% to the audience while jumping off stage into the crowds and regularly bleeding on stage.

I'd rather live that life 1000x over than cautiously never live up to my full potential or save a $100 speaker.

The side of me that spent the better part of a decade in engineering school agrees 100% that there are more responsible way to manage my equipment.

 

[Wasnex]

No way I would want to argue with you or think I might have a chance, but almost every regular gigging musician I know blows cab, breaks necks, blows fuses, & burns up amps all while having the time of their life giving 200% to the audience while jumping off stage into the crowds and regularly bleeding on stage.

I'd rather live that life 1000x over than cautiously never live up to my full potential or save a $100 speaker.

The side of me that spent the better part of a decade in engineering school agrees 100% that there are more responsible way to manage my equipment.

You don't have to blow speakers to live life to the fullest.

With Acme cabs in particular, if you want to get rowdy, you need a bunch of cabs.

One low B2 at it's mechanical limit (300W) can produce about 117.8dB

Two Low B2 gets you about ~123.8dB. 124dB is my personal minimum, so a pair would get very close.

With a pair of Low B4s, you are up at about ~129.8dB with 1200W. Now we're talking.

 

[agedhorse]

You don't have to blow speakers to live life to the fullest.

With Acme cabs in particular, if you want to get rowdy, you need a bunch of cabs.

One low B2 at it's mechanical limit (300W) can produce about 117.8dB

Two Low B2 gets you about ~123.8dB. 124dB is my personal minimum, so a pair would get very close.

With a pair of Low B4s, you are up at about ~129.8dB with 1200W. Now we're talking.

Agreed, enough rig for the gig is paramount to gear survivability.

 

[agedhorse]

No way I would want to argue with you or think I might have a chance, but almost every regular gigging musician I know blows cab, breaks necks, blows fuses, & burns up amps all while having the time of their life giving 200% to the audience while jumping off stage into the crowds and regularly bleeding on stage.

I'd rather live that life 1000x over than cautiously never live up to my full potential or save a $100 speaker.

The side of me that spent the better part of a decade in engineering school agrees 100% that there are more responsible way to manage my equipment.

We will have to disagree.

To live up to my full potential, I make sure that I have adequate tools and the experience to use them correctly. It's called setting up for success, not failure. It's what makes folks pros in the touring sound industry, the right gear, enough of it, and no excuses.

 

[el murdoque]

Nonexistent in the bass amp world.

This is the Reußenzehn Bass Studio Amplifier.
It runs a single KT-88 in Class A, getting something bewteen 15 and 20 Watts from it.

Yes, these are incredibly rare, since they were hand made by a single guy, but they DO exist!

 

[agedhorse]

This is the Reußenzehn Bass Studio Amplifier.
It runs a single KT-88 in Class A, getting something bewteen 15 and 20 Watts from it.

Yes, these are incredibly rare, since they were hand made by a single guy, but they DO exist!

Naaah, this is quite the stretch in the literal sense.

That’s not a proper usable bass amp for anything but very low volume use. Not practical for 99.999% of players. It’s a super specialty piece more than a bass amp imo.

Sure a one-off type amp (or anything for that matter) can be built, I have built some very weird things myself, but that doesn’t make them even remotely practical, viable amps.

How many here could even practice with a 15-20 watt amp?

 

[el murdoque]

Naaah, this is quite the stretch in the literal sense.

That’s not a proper usable bass amp for anything but very low volume use. Not practical for 99.999% of players. It’s a super specialty piece more than a bass amp imo.

Sure a one-off type amp (or anything for that matter) can be built, I have built some very weird things myself, but that doesn’t make them even remotely practical, viable amps.

How many here could even practice with a 15-20 watt amp?

Careful, your argument is carrying you on a slippery slope, and that ice looks thin!

I would like to point your attention to the Ampeg PF-20T. This amp also has but 20 watts, and the last time I checked, it was considered a proper amp by the vast majority of users.
It furthermore has a feature set in the rear end that is very similar to the Reußenzehn. Both run without a load attached, both have, apart from the speaker output, two balanced line outs, one from the preamp and one that's coupled to the output transformer. Which might make them specialized gear for recording sessions and home play equipment for the connoisseur, but also a viable tool to send to FOH.

And both are, volume wise, in the ballpark of the B-15 and that's a bass amp nobody can dispute.

 

[agedhorse]

Careful, your argument is carrying you on a slippery slope, and that ice looks thin!

I would like to point your attention to the Ampeg PF-20T. This amp also has but 20 watts, and the last time I checked, it was considered a proper amp by the vast majority of users.
It furthermore has a feature set in the rear end that is very similar to the Reußenzehn. Both run without a load attached, both have, apart from the speaker output, two balanced line outs, one from the preamp and one that's coupled to the output transformer. Which might make them specialized gear for recording sessions and home play equipment for the connoisseur, but also a viable tool to send to FOH.

And both are, volume wise, in the ballpark of the B-15 and that's a bass amp nobody can dispute.

Not sure why this is such an issue, but let me clarify what you have already misquoted...

1. I never said "proper amp", I said "That’s not a proper usable bass amp for anything but very low volume use". This makes it a challenge as a gigging amp.

2. Yes, both can be used as a preamp, without a speaker load. This is excactly why I said that it was "a super specialty piece more than a bass amp imo". A bass amp presumes that the power amp is a fundamental part of the amp's operation.

3. Unless you are playing really small venues or always have real PA available, I just don't see this being a usable for most gigging users unless they are looking for a studio or specialty piece.