What enables an amplifer to drive 4 ohm speakers?

[Dolby]

What enables an amplifer to drive 4 ohm speakers?

Is purely having hundreds of watts available? Large power supply? More current ... like H/K for example?

I've got a Onkyo TX-DS595 amplifer (75 watts @ 8 ohms) and was advised to run nothing less than 6 ohm speakers all round. However, I'm running 2x 4 ohm, 1x 6 ohm and 2x 8 ohm. I''ve been doing this since I got the reciever 4 years back with no hassles.

I now want to upgrade to a preowned Marantz SR6300 (100 watts @ 8 ohms), but to use with the same speak config. Is it better because of the 30% more watts - or doesn't it work like that ... ?

[kexodusc]

What enables an amplifer to drive 4 ohm speakers?

Is purely having hundreds of watts available? Large power supply? More current ... like H/K for example?

I've got a Onkyo TX-DS595 amplifer (75 watts @ 8 ohms) and was advised to run nothing less than 6 ohm speakers all round. However, I'm running 2x 4 ohm, 1x 6 ohm and 2x 8 ohm. I''ve been doing this since I got the reciever 4 years back with no hassles.

I now want to upgrade to a preowned Marantz SR6300 (100 watts @ 8 ohms), but to use with the same speak config. Is it better because of the 30% more watts - or doesn't it work like that ... ?

Usually it's current, and the power supply's ability to keep up with the large swings in demand. At modest levels with 4 ohm flat speakers, a lot of 8 ohm receivers can deliver no problem. Sometimes the magnitude of the impedance swings on these speakers is more meaningful than the nominal rating itself.

[nightflier]

I have a HK3480 receiver (stereo only) and while the manual does list a watt rating for 4 ohms, the tech support staff advised against it. I didn't listen and hooked up a new pair of 4 ohm speakers anyhow, because as everyone says about HK, they are rated conservatively.

Well, the amp was able to drive the speakers, but dropped most of the bass. For a while I thought I had been suckered into crappy speakers, but when I hooked up a 350W amp into the mix and used the receiver as just a preamp, the speakers sounded fantastic. After reading and talking to a lot of people, I found some anwers:

- Bass requires the most power, so the HK dropped that first before clipping.

- The rated ohms are less important than the actual swing (which with my speakers can go down to 3 ohms, apparently.

- Another factor to consider is sensitivity. Ironically my speakers were rated at 87dB, a pretty hard number to drive.

- Putting the speakers in the corners of the room (before I had the amp hooked up) helped a bit. I also added a sub and that helped a lot more.

So my question is: how does your 5-speaker system sound with the sub turned off?

[sam9]

As was said current is the key, but it can be a bit more complex than that. It is common for most amplifiers to to have a circuit that, called a VI limiter, that limits the combinations of voltage and current that the output devices are allowed to attemp. Generally the more voltage the less current is permitted and visa versa. The is a reflection of the devices capabilities. FETs, MOSFETS, bipolar transistors etc all have limits that if exceeded will damage or destroy the device. Some really esxpensive high-end amps (Krell?) get around this by using a very large number of output devoces in parrallel. Not only is this an expensive solution, but just can't fit in the limited space of a reciever.

The power supply may be a limiting factor as well. However, I think this may be becomming less of an issue as more recievers start using PWM power supplies or other technologies like BASH. Carver is another example. There may or may not be disadvantages to the kind of supplies as comparred to the more traditional but I don't think current limits are one of them.

A contributing factor with respect to design decisions is that manufactures expect most customers who really want bid-time bass to use sat-sub systems. Hence there is not such a expectation that the reciever will be called upon to drive a large inefficient woofer.

[kfalls]

As indicated above in a number of posts, current is the culprit when it comes to designing for low impedances. This means the power supply has to be able to supply the necessary current to the outputs, this means a larger transformer, larger regulators, larger (higher wattage) current limiting resistors, and more capacitance to supply the needed power during peaks. The outputs also have to be capable of higher-current.

Higher current means more heat, so the outputs have to be capable of higher temperatures. Normally this is done by mounting them to heat-sinks which are "finned" aluminum with a lot of surface area for dissipation. They may or may not have a fan blowing across them. The more heat-sink area you have the more heat you can dissipate. This is where discrete outputs have the advantage over IC outputs used on a lot of lower-end components. On high-end, high-current amplifiers the heat-sinks can be part of the outside case, but on most receivers the heat-sinks are internal. Newer or more exotic designs like switching power supplies or Carver's Magnetic Field amplifier reduce the need for larger components some, but have their compromises as well. These factors are all taken into consideration when designing for a certain size/performance/drive capability at a specific price point.

[Monstrous Mike]

What enables an amplifer to drive 4 ohm speakers?

Is purely having hundreds of watts available? Large power supply? More current ... like H/K for example?

I've got a Onkyo TX-DS595 amplifer (75 watts @ 8 ohms) and was advised to run nothing less than 6 ohm speakers all round. However, I'm running 2x 4 ohm, 1x 6 ohm and 2x 8 ohm. I''ve been doing this since I got the reciever 4 years back with no hassles.

I now want to upgrade to a preowned Marantz SR6300 (100 watts @ 8 ohms), but to use with the same speak config. Is it better because of the 30% more watts - or doesn't it work like that ... ?
If an engineer was allowed to produce amplifier specification, he would state the power capability in amps. Apparently this is not sexy enough for the marketing people so we see power rated by watts.

All the above is true, simply put 4 ohms speakers require twice as much current as 8 ohm speakers (on average). And further, since 4 ohms is a nominal impedence (i.e. an average, not flat over the entire frequency range), most 4 ohms speakers have a dip in impedence in the bass range. This is where you can get into trouble because now the the bass region (which requires the most power in the first place) will be demanding even more current. The problem is the extra heat generated and it's also a problem when reaching or surpassing the current capability of the amp/reciever.

Now you can still hook this up but I would personally not be turning up my reciever to full power ever. In the low to mid volume range, there should be no problem whatsoever.

And by the way, as far as getting more watts is concerned, keep in mind that you need to 10 times the number of watts to double the volume. So 30% more watts is probably not even going to be noticable. Given your 4 ohm speakers, I would look for a high current amp and forget about the watts.

[JoeE SP9]

Everything that has already been said about power supplies is valid. Most receivers and integrated amps are built with rather wimpy power supplies. This is why they are usually limited to 6 ohm or higher speaker impedences. If you want an amp that produces lots of current (the ability to drive 4 ohm or lower loads) you need to look at seperate power amps. All of them that I am aware of have no limitations as to the load they will drive. Even the best of them do not like sustained use into very low impedences but (Ask Florian about very low impedences) they seem to have no problem driving 4 ohm loads for any amount of time. Of course you could look into true Class A amps such as Krell's but they are rather expensive. My Acoustats are a 4 ohm load and the Hafler, Dynaco and Adcom amps I use have never complained.http://forums.audioreview.com/images/icons/icon6.gif

[Feanor]

Usually it's current, and the power supply's ability to keep up with the large swings in demand. At modest levels with 4 ohm flat speakers, a lot of 8 ohm receivers can deliver no problem. Sometimes the magnitude of the impedance swings on these speakers is more meaningful than the nominal rating itself.
Assuming it can deliver the current, an amp will have to dissipate the addtional heat that goes along with it. So an amp, (or receiver), must also have an adequate heat sinks otherwise it will overheat, possibly dripping protection devices that way.

Of course heat dissipation is a lesser problem with "digital" amps that are inherently much more efficient, i.e. waste less energy in the form of heat.