Amp and cabinet question
Moderators: rickenbrother, ajish4
1/2 each, and the total impedance is in parallel - meaning if you put two 8 ohm cabs you are putting a 4 ohm load on the amp...
1/Zeff = 1/Z1 + 1/Z2
Thus, a 4 ohm cabinet and an 8 ohm cabinet in parallel gives an effective impedance of 2.67 ohms. Over the long term this will not be very good for most amps...
1/Zeff = 1/Z1 + 1/Z2
Thus, a 4 ohm cabinet and an 8 ohm cabinet in parallel gives an effective impedance of 2.67 ohms. Over the long term this will not be very good for most amps...
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Some amps will work OK with different impedances. I have an older Peavey head that says on the back 210 watts 4ohms and 300 watts 2 ohms. Tubes are much more forgiving than transistors though.
'73 4001 MG '88 4003S JG '89 4003S FG '91 4003S MG
If it's a SS head you will get more power with two cabs of say 8 ohms each than one 8 ohm cab if your head is capable of 4 ohms which almost all SS heads are nowadays. All SS heads can work with different impedances, but they all have a lower limit, tube heads should have the correct impedance. The power does split up in half with equal impedances. If a SS head puts out 210 watts at 8 ohms it would probably be close to 300 at 4 ohms (two 8's paralled) with each cab getting 150. A tube head will put out the same or less power with a mismatch as they have taps for different impedences and should be matched to the correct impedance. You will shorten power tube life with a mismatch, probably lose power and get a krappy sound.
No, cabinets do not need to be the same impedence, as long as the amp can handle it. For example:
Your amp is able to handle a 2 ohm load (assume it says so on the back panel)...you usually play through a 4 ohm 2x10 like my Carvin cab. If you put an 8 ohm 15" in you will get an effective load of 2.67 ohms - so your amp will be OK (nominally).
What happens power-wise is that 2/3 of the power the amp puts out goes to the 2x10 (the lower load), and the other 1/3 to the 8 ohm 1x15". Which in this case is probably what we all would want.
Where the problem comes in with mismatched loads is if the 8 ohm cabinet is a dual speaker cab, or you have a PA system split through a cross-over to power subs and another set of mains...once the idiots in my band wanted to run an extra set of high ends which would have put the load down to 2 ohm on one side of the power amp, and the other side would have still been 4 ohm.
Now, I think our power amp can handle the load, but from a sound re-inforcement perspective the power of the amp would have been going 2/3 to the highs, when if you could control it you'd rather have more power to your subs. Bass frequencies need MORE POWER.
And Bob Young is correct about the power difference - halve the impedence you increase the output power by about 40%...
Your amp is able to handle a 2 ohm load (assume it says so on the back panel)...you usually play through a 4 ohm 2x10 like my Carvin cab. If you put an 8 ohm 15" in you will get an effective load of 2.67 ohms - so your amp will be OK (nominally).
What happens power-wise is that 2/3 of the power the amp puts out goes to the 2x10 (the lower load), and the other 1/3 to the 8 ohm 1x15". Which in this case is probably what we all would want.
Where the problem comes in with mismatched loads is if the 8 ohm cabinet is a dual speaker cab, or you have a PA system split through a cross-over to power subs and another set of mains...once the idiots in my band wanted to run an extra set of high ends which would have put the load down to 2 ohm on one side of the power amp, and the other side would have still been 4 ohm.
Now, I think our power amp can handle the load, but from a sound re-inforcement perspective the power of the amp would have been going 2/3 to the highs, when if you could control it you'd rather have more power to your subs. Bass frequencies need MORE POWER.
And Bob Young is correct about the power difference - halve the impedence you increase the output power by about 40%...
Above e-mail is inactive. try ed_ardzinski@**** where **** is Hotmail.com or Yahoo.com. I tend to see things inthe hotmail box quicker...
"halve the impedence you increase the output power by about 40%..."
That depends on the power amp in question. My Aragon 2004 will double (100%) it's output power from 100 watts rms @ 8 Ohms to 200 watts rms @ 4 Ohms per channel. However, going to a 2 Ohm load, the amp will only put out about 300 watts rms per channel. That is with both channels driven simultaneously. Many quality amps can double it's output when halving the impedance; much of this depends on the design of the power supply of the amp. Most PA and MI amps do not have adequate power supplies, therefore, most only increase their output by approximately 50% when going from an 8 Ohm load to a 4 Ohm load.
That depends on the power amp in question. My Aragon 2004 will double (100%) it's output power from 100 watts rms @ 8 Ohms to 200 watts rms @ 4 Ohms per channel. However, going to a 2 Ohm load, the amp will only put out about 300 watts rms per channel. That is with both channels driven simultaneously. Many quality amps can double it's output when halving the impedance; much of this depends on the design of the power supply of the amp. Most PA and MI amps do not have adequate power supplies, therefore, most only increase their output by approximately 50% when going from an 8 Ohm load to a 4 Ohm load.
Hahaha....I'm 6'7" and that went over my head...
I know this stuff is simple with you guys...but I know I'm not alone in my "duh-ness" when it comes to ohms ,watts,impedence, parallel etc...
I should have paid more attention in shop class...
I know this stuff is simple with you guys...but I know I'm not alone in my "duh-ness" when it comes to ohms ,watts,impedence, parallel etc...
I should have paid more attention in shop class...
1973 4001 MG cb fwi
1986 4003 Shadow
2012 4004Cii FG w/gold trim
1986 4003 Shadow
2012 4004Cii FG w/gold trim
As Jeff said, really good amps will double power to the load when the impedance of the cabinets connected to it is halved. That is dependant on two main things. As Jeff said, the power supply and I add the output devices.
The power supply needs a large enough mains transformer to be able to deliver current to the power supply and output circuitry. The power supply also needs massive amounts of capacitance to hold a large reserve charge to put out current as needed when you dig into that low E with furvor.
And of course the output devices (since we are talking about halving impedance/doubling power we are talking solid state devices - tubes have transformers and always deliver rated output to the loads - usually 2 and 4 ohms or 4 and 8 ohms). MOSFETs are the usual output devices now-a-days and you need enough devices to deliver the current to the load for that doubling of power. If both exist, the amp will double its output power when you are using an 8 ohm cab and add the second 8 ohm cab. Or using an 4 ohm cab and add a second 4 ohmer, if the amp is rated to operate into a 2 ohm load.
As for dividing power to two different impedance cabinets, look at it as the amp wanting to deliver all the current it is capable of on demand, and the speakers trying to resist that current delivery. An 8 ohm cabinet will resist accepting the current from the amp twice as much as a 4 ohm cabinet would. That is why the 4 ohm cab will get 2/3 of the power and the 8 ohm one only 1/3 of the power, when those two cabs are in parallel. The total impedance seen by the output section of the amp would be the 2.67 ohms as reported by Ed.
In this example, using a 600W amp, the 4 ohm cab will need to be rated capable of at least 400W operation, and the 8 ohm cab would need to be rated at least 200W.
As to matching cabs to amps power rating wise, it is better to have a cab that is rated higher than the amp's capability than less or even the same. Using an 800W cab with a 100W amp works but the probably outcome there would be a blown amp or more likely blown speaker driver - the amp will run out of steam and put a flattopped signal to the cab, and speaker voicecoils do not like that. When you exceed the heat dissapating capability of a voicecoil, it will burn open. So be sane in matching amp power and speaker power ratings.
I hope this is clear and not too long-winded.
The power supply needs a large enough mains transformer to be able to deliver current to the power supply and output circuitry. The power supply also needs massive amounts of capacitance to hold a large reserve charge to put out current as needed when you dig into that low E with furvor.
And of course the output devices (since we are talking about halving impedance/doubling power we are talking solid state devices - tubes have transformers and always deliver rated output to the loads - usually 2 and 4 ohms or 4 and 8 ohms). MOSFETs are the usual output devices now-a-days and you need enough devices to deliver the current to the load for that doubling of power. If both exist, the amp will double its output power when you are using an 8 ohm cab and add the second 8 ohm cab. Or using an 4 ohm cab and add a second 4 ohmer, if the amp is rated to operate into a 2 ohm load.
As for dividing power to two different impedance cabinets, look at it as the amp wanting to deliver all the current it is capable of on demand, and the speakers trying to resist that current delivery. An 8 ohm cabinet will resist accepting the current from the amp twice as much as a 4 ohm cabinet would. That is why the 4 ohm cab will get 2/3 of the power and the 8 ohm one only 1/3 of the power, when those two cabs are in parallel. The total impedance seen by the output section of the amp would be the 2.67 ohms as reported by Ed.
In this example, using a 600W amp, the 4 ohm cab will need to be rated capable of at least 400W operation, and the 8 ohm cab would need to be rated at least 200W.
As to matching cabs to amps power rating wise, it is better to have a cab that is rated higher than the amp's capability than less or even the same. Using an 800W cab with a 100W amp works but the probably outcome there would be a blown amp or more likely blown speaker driver - the amp will run out of steam and put a flattopped signal to the cab, and speaker voicecoils do not like that. When you exceed the heat dissapating capability of a voicecoil, it will burn open. So be sane in matching amp power and speaker power ratings.
I hope this is clear and not too long-winded.
Amp and cabinet question
Agreed on the details about the impedance/watts relation. I'm strictly a "book physicist" when it comes to electronics. I can do some simple wiring.
I makes a lot of sense that both the power supply and the output circuitry can change the the end behaviour of a circuit. I was merely considering an output stage with a "perfect" power supply. Essentially the frictionless pulley attached tot he massless rope.
The important (maybe practical is a better term) thing to remember is that the amp will put out more juice if you have more speakers attached. Speaker cabs are almost always wired in parallel (again, I'd love to be corrected if I'm wrong!) so the reciprocal formula applies. But the actual values are the impedances, which are complicated values that depend on EVERYTHING in the circuit.
So yes, my 40% comment is an idealization. IIRC correctly he equation in physics class had a power of two somewhere, leading to a factor of 1.4. My guess is the more modern amps react differently to the speaker load. Reactance is the 'imaginary' part of impedance, which can be a complex number (yukk! heavy math!). As I remember it, impedance is essentially effective resistance of a circuit.
Here's a little light breakfast reading for those interested in the physics.
http://en.wikipedia.org/wiki/Impedance
I makes a lot of sense that both the power supply and the output circuitry can change the the end behaviour of a circuit. I was merely considering an output stage with a "perfect" power supply.
Essentially the frictionless pulley attached tot he massless rope. The important (maybe practical is a better term) thing to remember is that the amp will put out more juice if you have more speakers attached. Speaker cabs are almost always wired in parallel (again, I'd love to be corrected if I'm wrong!) so the reciprocal formula applies. But the actual values are the impedances, which are complicated values that depend on EVERYTHING in the circuit.
So yes, my 40% comment is an idealization. IIRC correctly he equation in physics class had a power of two somewhere, leading to a factor of 1.4. My guess is the more modern amps react differently to the speaker load. Reactance is the 'imaginary' part of impedance, which can be a complex number (yukk! heavy math!). As I remember it, impedance is essentially effective resistance of a circuit.
Here's a little light breakfast reading for those interested in the physics.
http://en.wikipedia.org/wiki/Impedance
Above e-mail is inactive. try ed_ardzinski@**** where **** is Hotmail.com or Yahoo.com. I tend to see things inthe hotmail box quicker...
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throw_this_away
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"As to matching cabs to amps power rating wise, it is better to have a cab that is rated higher than the amp's capability than less or even the same. Using an 800W cab with a 100W amp works but the probably outcome there would be a blown amp or more likely blown speaker driver - the amp will run out of steam and put a flattopped signal to the cab, and speaker voicecoils do not like that. When you exceed the heat dissapating capability of a voicecoil, it will burn open. So be sane in matching amp power and speaker power ratings. "
There are people who will disagree... with some of your logic, but I don't want to get into it because I don't knoe if I believe it myself.
I think it is more preference than anything. Some of the hardcore SS guys at Talk Bass think you need a 1500W rack rig going through a cab rated at half the wattage for clean and clear headroom at low notes (can you say overkill?). Trick is not to crack the volume much past 2 or 3. ;P
My observation with tube amps is that you have far more usable headroom, and thus more wattage bang for the buck. Manufacturers tend to pair tube heads with cabs rated around double the Wattage of the amp (300W svt and 800W 8x10, 200W YBA200 and 400W matching cab).
As I see it, SS sound better when you don't push them to clipping, so you can do with a lower powered cab. Tube sounds better when pushed hard so a higher wattage cab is ideal.
There are people who will disagree... with some of your logic, but I don't want to get into it because I don't knoe if I believe it myself.
I think it is more preference than anything. Some of the hardcore SS guys at Talk Bass think you need a 1500W rack rig going through a cab rated at half the wattage for clean and clear headroom at low notes (can you say overkill?). Trick is not to crack the volume much past 2 or 3. ;P
My observation with tube amps is that you have far more usable headroom, and thus more wattage bang for the buck. Manufacturers tend to pair tube heads with cabs rated around double the Wattage of the amp (300W svt and 800W 8x10, 200W YBA200 and 400W matching cab).
As I see it, SS sound better when you don't push them to clipping, so you can do with a lower powered cab. Tube sounds better when pushed hard so a higher wattage cab is ideal.
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jwr2
I do a lot of seat of the pants engineering ... I build my own basses and my own speaker cabinets ... right now all of my cabinets are 8 ohm cabinets ... I have 3 ampeg bass heads ... they like to run with a 4 or 8 ohm load ... the high the number the more the resistance ... the higher the resistance the weaker the sound is coming out of the cabinet ... the lower the resistance the stronger the signal ... I never run my ampeg heads with less than 4 ohms ... too low of resistance will pull too much power out of the heads and can damage the bass head ... many bass amps are designed to operate in a 4 to 8 ohm environment ... stay in that range and you will be safe ...
2 8 ohm cabs = 4 ohms resistance ...
2 8 ohm cabs = 4 ohms resistance ...
The conventional wisdom is to use higher rated cabs than amps with tube amps and equal or lower rated cabs with SS heads. In my experience as long as you have a good clean sound you don't have to worry. I've used single 180 watt 8 x 10 cabs for years with no problem with a tube SVT although if I hear the cab distorting (muddy sound) I play less hard and turn down. And I do occasionally push the head.
SS heads are more likely to burn out voice coils with their distortion and tube heads usually don't but mechanically push the voice coils beyond their limits with the resulting rubbing sound many of us are familiar with.
SS heads are more likely to burn out voice coils with their distortion and tube heads usually don't but mechanically push the voice coils beyond their limits with the resulting rubbing sound many of us are familiar with.