Tuesday, 6 April 2010

Passive Crossovers and the Impact of Amplifier Output Impedance

In previous blogs I've referred to the enormous amount of distortion introduced by passive crossovers and in this one I show how a power amplifier can react with a loudspeaker to change the way it performs. A good quality, competently engineered loudspeaker can have its performance sabotaged by a power amplifier.

If you model second order passive crossover as there may be in a typical two way loudspeaker assuming a typical amplifier output impedance of 0.01 Ohms and then use an amplifier with a higher output impedance, say 0.3 Ohms, you'll find that the bass driver crossover frequency moves up and the tweeter crossover frequency moves down, producing an overlap in the crossover region. The higher the amplifier output impedance the more pronounced the effect.

As an example, using speaker with a nominal impedance of 4 Ohms and a classic 2nd order crossover at 2.3 kHz, increasing the amplifier's output impedance to 0.3 Ohms gave a bass crossover point of 2.4 kHz and a tweeter one of 2.2 kHz (being defined as the frequency at which 90 degrees phase shift occurs). Therefore it is likely that higher output impedance amplifiers will sound noticeably harsher, because of the extra energy produced in this region compared to a conventional amplifier.

In a recent review of amplifier with an output impedance of 0.27 Ohms, the reviewer also noted significant frequency response variations due to the interaction of the amplifier output impedance complex impedance of the speaker load.

We've measured a good quality 15 Watt valve power amplifier with slightly better than 0.1% distortion and it exhibits an output impedance, via the 4 Ohm tap, at 100 Hz  of 0.28 Ohms, at 3 kHz 0.4 ohms and at 10 kHz 0.9 Ohms. Not only will there be even worse overlap at  the crossover frequency, but also a loss of HF output (nearly 2 dB down at 10 kHz).

Single Ended Triodes would be much worse.

Saturday, 3 April 2010

How To Assess Hi Fi or Studio Monitors?

For the last few years hi fi fanatics, especially those that might be described as flat earthers, have been in denial. They really don't like Apple and iPods, despite the fact that they are probably the only reason that legacy separates systems haven't disappeared altogether. Many commentators are of the opinion that the Apple thing has re-awakened interest in hi fi.

However, so much vitriol has been directed towards poor old iPods and the AAC downloads of which 10 Billion have now sold, that many don't realise that with good headphones, or even the standard ones forced into your ears so that you get some bass but are uncomfortable, are probably the best sound quality most of us will ever hear. iPods have extremely low levels of distortion and can provide a level of clarity and neutrality not possible, even with the best hi fi system. Therefore they make a very good portable reference that you can use to help you judge the quality of any system you may be considering. They will not have the stereo image or the three dimensionality of speakers, but in other respects they are very useful indeed.

Which iPod
I don't think it matters, the later ones certainly sound better than previous generations and other, better quality PMPs are just as good. I use a Touch, which is an extremely useful computer as well as having an excellent remote control application in it for my Apple TV, but I recently heard a Nano which was at least as good. I'm sure a Shuffle will be too. My Macbook Pro is excellent as well, but whenever I've tried PCs they've sounded poor by comparison.

Which Headphones
Differences between good headphones are far smaller than between speakers which vary enormously and even more so if sub 100 Watt amplifiers are used, especially with 4 Ohm speakers, so choice is far easier and it's not necessary to spend a fortune. I use Sennheiser HD25-2 Professional Monitoring ones, they are absolutely superb and have a 70 Ohm impedance that suits PMPs. However others may prefer more discrete in-ears ones from Shure or Etymotic. The main thing is that they couple well to your head so that you don't lose bass as you do with standard ear buds.

Choosing Appropriate Recordings
The purpose of Hi Fi or high fidelity equipment is to more nearly or, as accurately as technically possible, replay what has been recorded. A Piano or voice should sound like the real thing, a group of instruments should spread out in front of you, remain in position and you should be aware of the acoustic characteristics of the venue where the recording was made. To someone who loves Classical Music, Folk, Jazz, World Music, even live Rock Bands, the comparisons are easy to make and people do successfully all the time.

However modern music is a free for all, sound quality is not an issue, but selling it fast is and certain characteristics are often exaggerated to make it sound as loud as possible so that it stands out from other tracks being played on the radio. Adverts between TV programs get similar treatment and it's called compression; the quiet bits are made as loud as the loud bits. Because it then appears louder but flat sounding, more treble is applied and more bass too, not to mention various other electronic tricks to make it "exciting". The net result is that a lot of it doesn't sound much better on a good system than an ordinary one and often it appears bass light because the treble is so brutal. None of this means you shouldn't enjoy it and collect it, but you do need to bear this in mind when assessing hi fi for possible purchase. You need also to use music with a real sound and proper musical instruments as well. If something live has been recorded to sound as it did on the day and the system does a very good job with it, it is a good one, but if you are impressed by the sound of purely electronic music without real references in and then find the real stuff sounds wrong, be wary because it probably indicates fatiguing distortion that will ultimately spoil all enjoyment.  An old Ella Fitzgerald recording is very much more useful as a reference than something modern and mostly electronically generated.

In previous blogs I've explained the benefit of better sound quality to minimising fatigue, but another is that is with an accurate and neutral system of the highest quality, you can enjoy a wider more diverse collection of music and movies without discovering irritating distortions that flatter some material while ruining everything else. I remember once a sound engineer friend explaining that his particular monitors did wonders for Saxophones. I asked if they made Pianos sound like Saxophones as well and he laughed and agreed they did!

I do hope this helps.

Ashley James

Thursday, 1 April 2010

How We Hear Stuff

The above Fletcher-Munson Curves  show that, unlike speakers, our ears do not have a flat amplitude response, in fact they are not very sensitive at low frequencies and don't become particularly so until 200 Hz or just below Middle C and from then on it increases until about 5 kHz and then decreases again. The best explanation for this is that we're designed to understand speech and do so by filtering off the high frequencies to discern clarity and intelligibility. Voice has a range of approximately 80 Hz - 3 kHz, but we listen mostly from 250 Hz-3 k Hz. We hear what people are saying by using our ears to locate the sound source, focussing them on it and then singling out the sizzly bits to understand what's being said.

In my opinion this shows clearly that we expect clarity and intelligibility from whatever we are listening to and amplitude is just one of several factors that enable us to achieve this. However as we age, so our sensitivity to higher frequencies deteriorates and by thirty five an average male's hearing has lost approximately 10 db (reduced by a factor of ten!) at 3 kHz. Although he may not realise it, he will be finding it more difficult to hear what people are saying or to discern clarity from his hi fi.

Our ear canal produces distortion, tests indicate this to slowly rise to about 8% at 105 dB and we use this to tell us how loud something is, but as we age and our hearing deteriorates and it too is distorting, so we tend to perceive sound as louder than it is and find it more difficult to tolerate. We need clarity at low levels if sound/music is to be relaxing to listen to.

We also have a sort of automatic volume control that can turn down the ear's sensitivity if sound is too loud for us, which is why young people are warned not to listen to iPods at high levels. The process is called temporary threshold shift and it can become permanent if we are exposed to high SPLs for prolonged periods. It probably accounts in part for thin, bass light recordings of some older rock music of which Led Zeppelin might be an example. The producer may have been playing much too loud and so added treble or removed bass to  compensate for threshold shift.

Anyone can test their hearing on the net these days and it might be a useful exercise to compare yours with a notional average and to get an idea of how well or poorly you hear. Women tend to be about 6 dB more sensitive than men and a 10dB drop is colossal, so it's not surprising that young people are more likely to tolerate distorted sound than their elders or want more bass to lessen much stronger (to them) treble.

The Brain
The ears are merely a means of gathering information, which they pass to the brain for processing, so the better the information they are able to gather, the less the work the brain has to do for it to be intelligible. It's hard work listening to indistinct sound.

Sine Wave Speech tests show how the brain is able to process quite distorted sound to get what it needs from it. If you do the test, play the sine wave speech first, then the proper recording and finally sine wave speech again. The first time you hear it, it will just be an incomprehensible noise, but once you know what has been said, it becomes intelligible. I'd guess this is the process of "running in" hi fi that the subjective brigade sells.

Sine Wave Speech probably explains more than most realise. For instance it could explain why most people are convinced they have the best hi fi and that nothing else is as good, or that reviewers who describe a system in very negative terms on first acquaintance, but having improved after a period of "running in", or why it is that often when people are offered significant improvements, they don't always recognise them. They have subconsciously "programmed" themselves to hear music through their system and need reprogramming before they see benefits of another.  Good headphones and an ipod are a tremendous help in providing an accurate reference, which is why professionals use them to listen out for detail speakers miss.

The ears, first and foremost need clarity, but the brain can process a poor information from them to achieve the same result. Clarity becomes more of an issue as we age because of lost HF sensitivity, which means we have to concentrate harder to hear whatever we are listening to.

Therefore hi fi, which is probably part of the process of relaxation, needs to be clear and probably at lower levels than in real life for maximum enjoyment.

It follows that adding treble by tone controls or through distortion in the replay system may also add clarity, but often it just makes things harsher and less bearable, which may explain why some hi fi enthusiasts think they don't like clarity.