New to bass-Amp and ohms ?

[David Wells]

Guitar player for years. New to bass. I am used to tube guitars amps. Going to be playing bass just jamming with my brother in law on drums for fun.

Bugera Veyron amp 1000w 8 ohms 2000w 4 ohms(really 250 at 8 ohms and 500 at 4 ohms)

Bugera BN115 cab 8 ohm 1600w cab(really 400w)

My confusion with bass coming from guitar and reading about bass amps and cabs here is about different amount of power and volume with ohm settings.

Seems that I would get more volume out of cab if it were 4 ohms to match the amp which peaks power 4 ohms.

So assuming all this is real that bass amps give out more power at the lower ohm load……would I really notice or perceive a volume or clean headroom difference if all things being the same if the speaker in the cab was 4 ohms instead of 8 ohms?

I know two 8 ohm cabs for a 4 ohm total would be louder but thats a big part due to more speaker cone surface.

I am just trying to get my mind around the amp giving more power like double at a four ohm load as opposed to an eight ohm load and wondering if I should use another eight ohm cab or just a different four ohm cab instead if it really makes a big difference?

Any help to clear my confusion appreciated? Thank you!

 

[Wasnex]

With solid state, the biggest thing you need to know is the minimum impedance the amp can handle. In general, solid state amps make more power as the impedance decreases, but this is not always true. Some amps double the power when impedance drops by one factor, some increase power by about 60-70%, and some hold the power constant. Check the specs to see what your amp does.

Tubes amp make the same power as long as the see the expected load. So if a tube amp has three output taps such as 2, 4, and 8 ohms, it will make the same power at each impedance. If you mismatch the impedance, the power goes down and it is harder on the amp and tubes. In some instances, it can be dangerous. Before running an impedance mismatch, make sure it is safe.

Don't be confused by the difference between max power and max SPL. If your solid state amp is rated for 4 ohms minimum, the amp will most likely make its max power at 4 ohms. But 4 ohm speakers are often less efficient than 8 ohm speakers.

Check the sensitivity rating of you cab. You need to the 1W/1m rating rather than the 2.83V/1m rating. 2.83V at 8 ohms is 1W. 2.83V at 4 ohms is 2W. To change the 4 ohm 2.83V rating to 1W/1m, simply subtract 3dB.
For example if you have a cab rated as 98dB 2.83V/1m at 4 ohms, subtract 3db: 98-3=95db 1W/1m. This seems to be used to hide the fact that 4 ohm speakers are often less efficient.

Now to determine how loud your rig will play:
Start with the 1W/1m sensitivity rating and then add three dB for every doubling of power you can use. Consider the amount power the amp can produce and also how much power the speaker can handle. Use the smaller of the two ratings.

So if you have a cab with 97dB 1W/1m sensitivity rating, it will make 100dB with 2W, 103dB with 4W, 106dB with 8W, etc. If the max useable power is between doubling, then you estimate a number between 0-3dB depending on how close you are to the next doubling of power.

If you add a second identical cab, the sensitivity rating goes up by 3dB. So two cabs with a 97dB 1W/1m sensitivity rating, will make 100dB with 1W. Using two cabs also doubles the power handling. So if the amp can provide enough power, two cabs cab play 6dB louder than one cab. The come from +3dB in higher sensitivity and 3dB from doubling the power handling. It's often better to get an 8 ohm cabs, because then later if you need to play louder you can get a second cab instead of having to replace your rig.

 

[David Wells]

I appreciate the time and effort to explain all of this! I slowed down and re-read several parts to make sure I followed it all. It makes sense and clear now. Cool! I think I will look up the manual and get the numbers online as that company seems to list a lot of these numbers in their manuals. Along with overestimated power ratings to get our attention.

Since my thread/post I was wondering about the use of an attenuator hooked up but not used for attenuation would work to possibly achieve max power at 4 ohms into the attenuator and then from the attenuator to the 8 ohm cab.

My understanding is that you just have to match your amps output to the attenuator or in my case one that has 4, 8 and 16 ohm options. I have read it does not matter after the attenuator what ohm rating the speaker is. I have read it on the google multiple times over the years. That does make it truth but it seems to be the popular opinion.

So my theory would be that hooking up to an attenuator 4 ohm tap but not using any attenuation would make the amp see 4 ohms and be able to deliver max power as it would see a 4 ohm load and the 8 ohm speaker load does not register to the amp.

Maybe there are other complicated factors, maybe there would be tone suck issues even with no attenuation as it may not be fully bypassed even with the dial set at no attenuation. No idea. Just a thought as I did recently pick up a Bugera PS1 attenuator to try with a guitar amp.

If this idea makes no sense in real world application or if it is simply wrong or I missing factors please anyone chime in so I can learn from feedback. Thank you!

 

[Wasnex]

I appreciate the time and effort to explain all of this! I slowed down and re-read several parts to make sure I followed it all. It makes sense and clear now. Cool! I think I will look up the manual and get the numbers online as that company seems to list a lot of these numbers in their manuals. Along with overestimated power ratings to get our attention.

Since my thread/post I was wondering about the use of an attenuator hooked up but not used for attenuation would work to possibly achieve max power at 4 ohms into the attenuator and then from the attenuator to the 8 ohm cab.

My understanding is that you just have to match your amps output to the attenuator or in my case one that has 4, 8 and 16 ohm options. I have read it does not matter after the attenuator what ohm rating the speaker is. I have read it on the google multiple times over the years. That does make it truth but it seems to be the popular opinion.

So my theory would be that hooking up to an attenuator 4 ohm tap but not using any attenuation would make the amp see 4 ohms and be able to deliver max power as it would see a 4 ohm load and the 8 ohm speaker load does not register to the amp.

Maybe there are other complicated factors, maybe there would be tone suck issues even with no attenuation as it may not be fully bypassed even with the dial set at no attenuation. No idea. Just a thought as I did recently pick up a Bugera PS1 attenuator to try with a guitar amp.

If this idea makes no sense in real world application or if it is simply wrong or I missing factors please anyone chime in so I can learn from feedback. Thank you!

Most attenuators are built for one specific impedance. If you have a 4 ohm amp and cab, you buy a 4 ohm power attenuator.


The Bugera PS1 has three different inputs: 4, 8, or 16 ohms.

The Quick Start guide says:

Failure to observe the following points may result in damage to the amplifier or loudspeakers. Damage caused in such a way are not covered by warranty.

• Observe the correct impedance for your loudspeaker and use the appropriate output socket
• Always use loudspeaker cables for connection between the amplifier and the loudspeaker. Never use other types of cable such as instrument cable and microphone cable

This makes me think that the speaker impedance should match the output impedance of the amp. Also it suggests that is absolutely necessary to connect a speaker for the safe operation of a tube amp.

Keep in mind the PS1 is only rated for 100W.

I have a fairly high-end attenuator: Aracom PRX150-DAG. This unit uses a transformer with multiple taps that allows you to select impedance for both the amp's output and the speaker load:

So this one really can be used for impedance matching.


Anytime the Aracom is in the circuit, it applies at least a 3dB cut. This is equivalent to cutting the available power in half.

This model does have a setting where it can be used as both an attenuator and a loadbox. In this particular setting the impedance of the speaker does not matter. In fact, it's not even necessary to connect a speaker (see position F below):

In position F the manual says the minimum attenuation is 16dB.

The PRX150 is rated for 150W. I bought it to tame some of my vintage guitar amps. Not really suitable for most of my tube bass amps as the power handling is too low.

 

[agedhorse]

Do NOT use an attenuator, it will not “convert” a speaker cabinet to 4 ohms, not is it safe for you amp.

 

[David Wells]

Perfect! This is what I needed to hear!

The Bugera PS1 attenuator at least cannot be used with the idea I had in mind. No problem. I am glad I asked. Thank you both!

 

[David Wells]

If I need more volume I will just add another 8 ohm cab. Just keep dumping more cabs at my brother in laws house. Lol

 

[David Wells]

Looking back I bet my thought that you just need to get the ohms of the amp and attenuator to match and that the speaker/cab does not matter are likely from thinking about most of the Weber brand attenuators that use an actual speaker motor in their attenuators. Maybe in that case it is that way. Probably where my train of thought was based on. Anyways, I am not messing with an attenuator now with bass!

 

[Killing Floor]

Haha, so true. Guitar players don’t have Ohms. Just plug any head to any cab and it works.
Only half kidding. But almost any amp can damage almost any speaker if used improperly.

 

[Bassamatic]

You can hardly hear any difference between 8 and 4 ohms, with equivalent speakers. I have a 2x15 with a switch that lets me change it from 4 to 16 Ohms so I can use it with other speakers. There is only a slight audible difference when I change it from 4 to 16 Ohms.

There is very little difference to your ear with double the amp power. +3dB power is barely detectable. However, The 4 Ohm output is useful for running 2 x 8 Ohm cabs. When you double the cone area, it DOES sound twice as loud to your ear. So get another cab if you want more SPL.

 

[David Wells]

Thank you! That info really helps!

 

[GreyBeard]

Do NOT use an attenuator, it will not “convert” a speaker cabinet to 4 ohms, not is it safe for you amp.

WHAT HE WROTE,,

 

[GreyBeard]

Trying to converting an 8 ohm speaker to 4 ohms is a waste of time. Match your amp to your speakers, crank it up and rock on. If it isn't loud enough, get a bigger rig.

 

[David Wells]

Thank you! I actually just picked up another cab off Craigslist for $150. Another 8 ohm cab. A 1990’s Ampeg SVT-410HE. The older 200w version. I posted some questions about it in another thread. So now I can try out some 10’s and mix the 10’s and the 15 for a combined 4 ohm load for more volume if desired and my amp is peak power at 4 ohms. So this all should work fine for me until I become a better bass player and then some. For now its just me and my BIL on drums and I will play guitar then switch over to bass so I will not even open up a question about mixing speakers and mixed speakers in a full band situation. We are just looking to get better and have fun. Lol

I am new here but one very common theme I noticed here looking around is two things…. If you want more volume add more speakers and strong feelings regarding mixing speaker size. I will not poke the bear on that topic. Lol

 

[George Dennis]

Thank you! I actually just picked up another cab off Craigslist for $150. Another 8 ohm cab. A 1990’s Ampeg SVT-410HE. The older 200w version. I posted some questions about it in another thread. So now I can try out some 10’s and mix the 10’s and the 15 for a combined 4 ohm load for more volume if desired and my amp is peak power at 4 ohms. So this all should work fine for me until I become a better bass player and then some. For now its just me and my BIL on drums and I will play guitar then switch over to bass so I will not even open up a question about mixing speakers and mixed speakers in a full band situation. We are just looking to get better and have fun. Lol

I am new here but one very common theme I noticed here looking around is two things…. If you want more volume add more speakers and strong feelings regarding mixing speaker size. I will not poke the bear on that topic. Lol

First thread: impedance, attenuation and mixing cabs!
Welcome to TB!

 

[agedhorse]

Thank you! I actually just picked up another cab off Craigslist for $150. Another 8 ohm cab. A 1990’s Ampeg SVT-410HE. The older 200w version. I posted some questions about it in another thread. So now I can try out some 10’s and mix the 10’s and the 15 for a combined 4 ohm load for more volume if desired and my amp is peak power at 4 ohms. So this all should work fine for me until I become a better bass player and then some. For now its just me and my BIL on drums and I will play guitar then switch over to bass so I will not even open up a question about mixing speakers and mixed speakers in a full band situation. We are just looking to get better and have fun. Lol

I am new here but one very common theme I noticed here looking around is two things…. If you want more volume add more speakers and strong feelings regarding mixing speaker size. I will not poke the bear on that topic. Lol

Mixing speakers isn't a problem when the speakers are designed from the beginning to be mixed. This means that the real engineering has been done and the measured performance matches the predictions.