The topic of measurements is a touchy subject in the audiophile world. I once had a technical conversation with the late A.J. Conti from Basis Audio at one of the old Newport shows. At one point he asked me why audiophiles seemingly ignore how component engineering and measurements impact the sound. I simply responded it is easier for the audiophile to believe the mysticism that exists within their hobby, as well as look at their hobby more from the perspective of art, where everything is subjective, than science, where objectivity comes into play. It was not hard to see how my response resonated with A.J. as he became very animated while we continued the conversation. As we parted ways, I could still see A.J. shaking his head. I cannot say I blame him for his reaction, in fact I sympathize with it.
As humans we all hear differently and what sounds good to one person might not sound good to another. In addition, how our ears perceive and interpret sound cues cannot be compared with how test equipment captures measurements. However, measurements are a significant factor in helping an audio engineer determine if their design will reproduce good sound. As my friend Jack Elliano from Electra-Print once told me: “If it measures good, then it sounds good. The test equipment does not have a dog in the hunt.” In addition, Daniel von Recklinghausen, who worked at EAD and KLH is famous for saying: “If it measures good and sounds bad, then it is bad. If it sounds good and measures bad, then you have measured the wrong thing.”
Roger Modjeski was a stickler for measurements. He owned lots of test equipment, including a Healthkit Distortion Analyzer he built in his early teens. Roger often read John Atkinson’s component review test measurements in Stereophile. He once told me about a rave review by Art Dudley (the component escapes me) only to have Atkinson measure the component and wonder if he received the wrong component to measure. The component measured so poorly that Atkinson was in disbelief over what Dudley could be hearing. Roger was also known for taking measurements and publishing specs of his equipment that other designers ignored or did not want to publish. His efforts were highly focused on reducing noise levels as much as possible, and ensuring any distortion in his designs was benign making his components sound extremely transparent. An aspect of the sound that unfortunately was lost on most audiophiles.
For today’s Roger’s Corner let us see what Roger has to say on what measurements are considered when designing an audio amplifier. There is one measurement in particular that is very interesting. The question for you is which measurement do you think it is?
Question: I would like to know what all the measurable variables are that one considers when manufacturing an amplifier. I am guessing this list is more exhaustive than the spec sheet typically posted. If not correct me.
Answer: Good Question. One thing to do is follow John Atkinson's measurements of amplifiers in Stereophile. I find most amplifier specs to be missing something that should be there. Perhaps because that spec is not so good. So here is the list.
- Distortion from 0.25 watts to full power over 20-20,000 cycles. This spec came from the FTC.
- Damping factor can be a most important spec if it is below 8-10. However, much more than 10 does not make much difference. In other words, some solid-state amplifiers have damping over 100. That is okay, but at what sonic cost. Damping factor will have a great effect on frequency response if your speaker has a widely varying impedance curve. Do not be impressed by super high damping. It is not going to matter.
- -3dB power bandwidth. Jadis did not publish this for the JA200 because the spec would be horrible. Here is a 4 chassis amplifier that can do 200 watts but not to 20K Hz. The spec at 20-20K Hz is more like 5 watts. I have measured it.
- Noise is often specified in dB below full output. Not a good way to spec this. I would prefer a simple statement of noise in mV both wideband and A weighted*. Good luck on seeing that one on a spec sheet.
- Power consumption will give you some idea of how much the amplifier is going to heat your room.
That pretty much covers the common specs. However, it is worth mentioning MTBF (mean time between failures) which is spec’ed on parts in your computer, industrial power supplies, and many other things. Unfortunately, I have never seen this spec on an amplifier. The number for most amplifiers would be very low. MTBF for things like hard drives is 300,000 to 1,200,000 hours. See the following link: http://knowledge.seagate.com/articles/en_US/FAQ/174791en. The MTBF of amplifiers varies widely and is only known by those of us who fix them. I am very proud of the MTBF for my amplifiers. I can only state it in years, and it is at least 30 years**.
Additional Notes:
*Roger routinely listed the noise spec in mV for his products.
**In reality, some of Roger’s designs have been in continuous service for 40 plus years.