Nice article, Sean there is only something I wanted to add that SS amps in all logical sence do require deep negative feedback that increases an output impedance and thus creates an addtional voltage drop while in tube case the negative feedback is minimal. So there we have even more requirements for transistor(s) to pass higher current and certainly heat. |
Here's something else to think about. It was presented to me in a way that made it very easy to understand by Bob Carver. I read this in the "white papers" for the Sunfire amps and it makes good sense.
A transistor is a WATTAGE device, not just a voltage or current device. To make things as simple as possible, wattage is the voltage multiplied by the current ( amps ) in the circuit. In other words, a transistor may be capable of 200 watts of output. If the circuit is running at 100 volts, that transistor can only pass 2 amps of current to achieve rated output at spec. After all, wattage is volts x amps and 100 volts x 2 amps would give us the 200 watt rating. We'll consider this the operation within an 8 ohm circuit.
While running more transistors in parallel will increase the amount of current that we can safely pass due to sharing the load, we are still limited by how much voltage we can safely deal with. That is, if we want to enjoy an extended lifespan from the output device and keep it operating within the designed specs. There are drawbacks to running multiple devices to share the load, as you know have to worry about them all working together and doing so at the exact same time and rate. This is where "matched parts" come into play. Only question is, how well does one have to match the parts before "perfection" is achieved in such a situation ?
Now, If we took that same example circuit from above and went from an 8 ohm load to a 4 ohm load, the same device would now be trying to pass 4 amps of current. Since resistance is halved, current is doubled. The problem with this is that we are still limited to that same 200 watts of output with good linearity from the transistor, so something has to give. That "something" is voltage capacity. Since 200 watts divided by the 4 amps that we are trying to pass leaves us at 50 volts, we've now lost a LOT of headroom ( 50% reduction or -3 dB's ) in the amplifier. It is at this point that compression ( slight squashing of the peaks ) or clipping ( complete "smashing" of the peaks ) can set in depending on how hard we are driving the amp. Not only that, we've also increased thermal losses which are attributed to reduced amplifier efficiency. This loss of efficiency is evidenced by the increased amount of heat dissipated by the amp. You pass more power, you generate more heat. More heat causes the outputs to operate out of their optimum range, causing them to perform poorer and efficiency to drop even more. Kind of a vicious circle but that is what we are dealing with at this point in time with the limited technology that we have. Superconductivity tries to deal with this by "freezing" the parts so that thermal losses don't come into play. That is a whole 'nother ball of wax though.
While some of this is "speculation", my experience is that a transistor works and sounds MUCH better when kept relatively cool or at one would call "normal operating temperature". Going above that point typically introduces distortion, smearing and "grain". Since the laws of physics dictate that a lower impedance will pull more current no matter what, we're already fighting an up-hill battle. While some semiconductors are better / worse than others in this area, they will all run into the same problem sooner or later. Increased heat and increased amplitude almost always go hand in hand with reduced linearity.
In order to keep this from happening, you either need REALLY big heatsinks or some type of forced cooling system. The cooling system itself can become another source of noise and interference, so most designers try to avoid them if possible. Since heat rises, one of the simplest things that you can do is allow a LOT of room above and around the amp to achieve maximum air flow and natural cooling to take place. This will never hurt and can only help. People using amps "shoved" into a enclosed rack are at an instant disadvantage compared to those that have open sided racks with more room between shelves. Running an amp on a completely open "amp stand" and elevating the amp via cones, spikes, small wood blocks, etc... so that you can get airflow on all 6 sides of the chassis' is about the best that anyone can do.
The bottom line is that a lower impedance speaker presents an entirely different type of situation to the amp than a speaker that is just a few ohms higher in impedance ( all other things being equal ). How it deals with the increased need for current and if it has enough voltage capacity so as not to compress or clip under dynamic conditions would become a very system and listening style specific situation.
Keep in mind that Damping factor is the difference in impedance of the amplifer output section itself and the load that it is presented with from the speaker / speaker cable. The lower the output impedance of the amp and the higher the load impedance of the speaker and cable, the higher the damping factor. A lower speaker impedance and / or a higher output impedance on the amp instantly lowers your damping factor. While some say that damping factor doesn't mean squat, theoretically speaking, a lower impedance speaker will always be less "controlled" than a higher impedance speaker. That is why it takes "SHEER MUSCLE" to handle low impedance, high reactance loads. There is no getting around the need for both high voltage and high current levels in such a situation. Since this requires BIG power supplies and a lot of ouptut devices, price goes up accordingly. As such, a "good" system based on low impedance speakers will effectively cost more than a good system based on higher impedance speakers.
Sorry for the rambling, but i had so many ideas that i wanted to convey, i got sidetracked. I hope that you can understand at least part of what was roaming through my head. If i've really put my foot in my mouth on this one, i would appreciate it if someone ( Hi "Bear" !!! ) would take the time to please clarify and correct my mistakes. Sean
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Ahhh, now that makes sense. Thank you Marakanetz. |
Rockead, it all depends on where larger current is drown. larger current is drown through the 4 Ohm and smaller is through the 8 Ohm. More resistance creates smaller current and thus creates "easier load" The heaviest load thus is a short circuit(0 Ohm in ideal case) when you connect with the uninsulated wire two binding posts per each channel of the amplifier(Don't do it at home 'cause you might damage your amp) |
Dumb question
If ohms are a measure of resistance and 8 ohms is more resistant than 4 ohms, why are 8 ohms generally easier for the amp? Wouldn't more resistance create a bigger load to the amp?
Life ain't easy being a rockhead, I tell ya |
Of course your right Marakanetz. I don't want to belabor the point, but the key words here are "gross generalization" and "usually find". I must admit that I was unsure as to whether or not I should have made the previous post. In retrospect, perhaps I shouldn't have. |
Its way toooo general, Unsound.
You can grab an example on JM Lab Electra 926 where the nominal impedance is 6Ohm but it could be dropped down to 2.5.
My Totem Forests amps have a huge and resolving bass despite their tiny size and small 6" woofer. Their nominal impedance is 8Ohms and it doesn't go bellow 6. I was driving them with Pathos TwinTowers 35W/ch with enough loudness and clarity.
I might also generalize that speakers with lower efficiency have better bass while highly-efficient horns are only good for speach or voice...? |
I know that this is a gross generalization but I usually find that speakers with lower impedances have better bass. |
Bill, Mac uses output transformers on their SS amps. As such, the amp sees a relatively consistent load regardless of the speaker it is connected to. That is, so long as the "most correct" tap is selected for that individual speaker. There are problems associated with doing something like this ( other than high cost ) though and that is why most other manufacturers avoid it. Even the best sounding tube amps ( like Atmasphere's and probably Tenor's ) don't use output transformers.
Using an output transformer is kind of like running hundreds of feet of zip cord speaker cable after working so hard to preserve the signal through-out the entire chain. It does lend a specific sonic characteristic to the signal though, and if that is your preference, so be it. I know what i like in a system and i set them up accordingly. I wouldn't expect anyone else to do any differently.
Nrchy, it sounds like you picked a speaker that was of low impedance and had a somewhat reactive phase angle. I'm sure that your 30 wpc amp could have dealt with the speaker IF the protection circuitry was not set quite as sensitive to triggering. This might have involved occasional clipping from the amp though, which happens more commonly than many folks would like to think. Whether or not the amp was a suitable match for the speakers without the protection circuitry would still remain to be seen if that approach had been taken. The fact that the manufacturer selected the trigger point on the amp that he did and the tech verified proper operation tells me that it was probably not up to driving the speakers in the manner that you wanted to use them. Sean
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In practical terms it means you are going to have major problems if you use an amp designed for 8 ohm loads with a 4 ohm speaker.
I had a relatively low powered amp (30 wpc class A, dual mono) which sounded real good. I decided to upgrade my speakers and bought a pair of Focus .7s. I was constantly kicking out the protection curcuitry in the amp. I brought the amp into the shop and had it checked out. They said there was nothing wrong with it but as soon as I began listening it did it again. Next I brought in the amp, pre-amp, and speakers along with their respective cables. The guy checked each piece out and didn't find anything wrong. I don't think he ever set everything up and listened to the system as a whole. After years of this I ran into a friend in the HiFi business in Milwaukee and explained it to him. He immediately said "it sounds like you're driving a four ohm speaker with an eight ohm amp."
I bought an amp capable of driving four ohms and never had another problem with the speakers. |
Sneaky follow on--what does it mean that McIntosh has always given its' ratings (even back in the 60's)at 2, 4, and 8 ohms? I assume its' a sign (guarantee?) of linearity. I run a 7270 and 7300. |
In almost every aspect, performance of an amplifier is "better" at 8 ohms that it will be at 4 ohms. Distortion is typically lower and more linear across the frequency range, bass will be more controlled i.e. "damped", less heat to build up or dissipate, etc...
Only problem is that higher impedances make it harder to transfer large amounts of power and current. While tubes can get around the power transfer problems as they tend to like higher impedances better, they still won't deliver the high current necessary to keep an "iron grip" on a woofer. If one can keep a reasonable impedance ( 6 - 10 ohms ) without a lot of reactive swings as frequency changes and do it with high sensitivity, you would pretty much have the perfect load for an amp. Whether or not the frequency response, transient response and dispersion characteristics of that speaker would work for you may be a whole 'nother ball of wax though : )
Unfortunately, many of the speakers that i personally like and own are both low impedance and low sensitivity. This makes for an up-hill battle all the way. There are only two cures for that i.e. either throw them out and buy different speakers that are a lot less finicky or run amps that are built like tanks. Guess which path i took ??? : ) Sean
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I absolutely agree with the above responses from artistic Audio and from Rives, The Manufacturers have focused on 4 ohm speakers to market them more effectively. They sound louder in the demo rooms than would an 8 ohm speaker at the same electronics setting. I suggest taking a sound level meter with whem auditioning speakers. Also bring your own music and don't rush the demo. Take your time and stop back a second time if necessary. Better yet, take the speakers home because the room is a big factor in the evaluation process. Good Luck |
also keep in mind that Tube amplifiers like higher impedences such as 8 ohms or higher, were as Solid State amps can drive more power into lower impedences such as 4 ohms. although even some of the best Solid State amps can still have trouble driving very low impedences such as 2 ohms. speakers that drop down this low I would stay away from because they will limit the choice of amplifiers you can use on them. |
It's a good question and typically misunderstood by many--even some designers. You are correct in thinking that the higher load would be more "accurate". It has less demands on the amplifier and is easier to drive. However, the trade off is that it has less output per watt that it's 4 ohm counterpart. For this reason many manufacturers reduce (or trade off) the load for higher output. In addition the load (4 ohm vs 8 ohm) that you are referring to is the "nominal" impedance of the speaker. Many speakers rated at 4 ohms can go down to 2 ohms at certain frequencies or even lower. This can create very difficult loads to drive and they can be very demanding on amplifiers. So, as someone once said to me after a lecture, "what's the take home pay?" These nominal impedances have little meaning. You need to see what the impedance curves are throughout the range and once you know these you can narrow down the amplifiers that might be appropriate for that speaker. A speaker with relatively flat impedance curves even if it's rated as a 4 ohm speaker will be easier to drive than a speaker that has wild swings through the frequency range. |
