Amplifier Specs


Can someone explain what slew rate and damping factor are and how they affect an amplifiers sound/performance. Thanks in advance
robk
Simplistically the easy explanation is: "slew rate" is how fast the amp reacts to signal and delivers the output power and is related to amp design and output device specification; "damping factor" is the ability of the amp to "damp" the motion of the speaker cone and is related to the output impedence of the amp and the impedence of the speaker. Damping factor is also effected by the amount of feedback used in the amp circuit. Hope this helps? regards, Richard at www.vantageaudio.com
Hi, Richard. Good to see you here. Thanks for the clarification. Are those numbers bigger the better in general? How about "negative feedback"?
Hello Ken. There are many factors involved in determing "damping factor" and it depends on circuit design, so sometimes bigger is not always better. Damping factor is roughly the ratio of amp output impedence v speaker impedence. So if the amp is, say, 0.1 ohm and the speaker is 8 ohms the factor is approx 80. Negative feedback is used in circuit to control devices, for example transistors, and ensure they give a reasonable performance under repeat conditions. It basically tries to overcome the inherent shortcomings of the device in question and give better performance figures. This is a very rough and ready explanation which I hope helps. Regards, Richard.
How much damping factor you need depends on the speaker you intend to use. Some speakers are specifically designed for use with tube amps that generally deliver low damping. Coupled with an amp with high damping factor will produce an over damped bass response, dry and muddled. I think it has to do with the Q factor of the woofer. Maybe someone out thier in Audiogon land has a better grasp of the tech aspect.Suffice to say, matching the right amp to a speaker can make a huge difference.
OK I know, I mean out there. Oops.
You have to consider that speaker cones have a true damping that is determined by lots of factors, including the damping effect of the voice coil resistance. The amp impedence does not add much to the voice coil total resistance and in reality has little effect on the total damping. Speaker damping is usually engineered to achieve the desired low frequency rolloff. The aim here is to get a low damping figure for the speaker system as a whole to get the best bass response. But it is true that getting the right amp to match the speaker system still makes a huge difference.
You have to consider that speaker cones have a true damping that is determined by lots of factors, including the damping effect of the voice coil resistance. The amp impedence does not add much to the voice coil total resistance and in reality has little effect on the total damping. Speaker damping is usually engineered to achieve the desired low frequency rolloff. The aim here is to get a low damping figure for the speaker system as a whole to get the best bass response. But it is true that getting the right amp to match the speaker system still makes a huge difference. Richard at www.vantageaudio.com
Sorry, the above post was duplicated! Well, we are talking about "stereo" speakers here, yes? Richard.
Keep in mind that slew rate and damping factor are not like the world land speed record, where engineering goes into pushing the envelope higher and higher. Prudent amplifier designers design for performance, not specmanship. Is more better? Only to a point. The necessary slew rate is directly proportional to the highest frequency the amplifier needs to produce and to its maximum output voltage. Thus, the slew rate for a full-range or high-frequency amplifier should always be high enough to permit full output voltage swing beyond 20 kHz. But an amp can have an excessively high slew rate, too, allowing it to pick up RFI on the inputs and either pass some of it on to the speakers or demodulate it and produce unwanted noise and jabber in the audio. Similarly, the output impedance of an amp is usually due to the output network, which decouples RF that comes in from the speaker cables. In the real world, though, your speaker cable resistances are usually going to totally overwhelm the effect of the output impedance on damping the speaker cone. Any effective damping factor (that's taking into account both the amp's output Z and the speaker cable resistance) of, say, 100 or higher will provide you with optimum bass control--as long as you have good speakers. There are good ways and there are bad ways to get amazingly high slew rate and damping factor in an amplifier design. Take out the ultrasonic filtering and you can get a super-high slew rate spec, but the amp would be vulnerable to the RF effects I mentioned above. Take out the output network and you could have a damping factor of maybe 20,000, but you'd let RF into the circuitry and you might make the amp unstable with certain reactive speaker loads. Beware specmanship.