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Getting the Right Feedback: The Alpha Hi-Fi

Jane Morecroft


We all know the electrifying thrill of listening to a favourite band in a crowded, sweaty stadium, or the sublime sensation of a violinist playing Vivaldi's Primavera. But rarely do we feel those same emotions when we put a CD on. Part of that missing link is undoubtedly ambience, but is there something else lacking? My father, a hi-fi designer who has won awards for design ingenuity, certainly seems to think so. I track him down in his hi-fi listening room; to get some answers.


Dad, surely a good hi-fi system is all about bass and volume, so what's all the fuss about modern-day technology not making the grade?


My father, a quiet spoken man with amazingly white hair, shakes his head:


High volume and ample bass-load are not the secret to triggering a strong emotional response in listeners. Very often it is the complexities and delicate tones of say a musical track, rather than the volume, that would trigger that emotion, but all too often these are hidden by hi-fi distortion.


What kind of hi-fi distortion?


The main problem lies in the process within an amplifier known as low distortion feedback; this creates the illusion of perfection by covering up flaws in the system, so sound problems are nearly always blamed on another link in the audio chain rather than the amplifier itself.


So what's the solution then?


All engineers in the hi-fi field have their own theories and believe in them passionately. But I am convinced that if we are to get close to live sound we need to address two key areas: the incorrect use of materials and inaccurate amplifier electronics; specifically the use of negative feedback within an amplifier.


Correct materials? I latch onto the less technical-sounding problem - in what sense?


Non-metallic materials in amplifiers; my Dad answers. It is unimaginable that in 2006 we still make all our amplifiers out of metal. We make most other things like say cameras, that process light, out of plastic. But amplifiers that process electrical currents are made from metal, a material that interacts with the currents in the circuit! How crazy can you get?


That does sound crazy, I reply, although I must admit this relatively obvious problem had never occurred to me before.


What about the electronics?


The feedback issue is an important one because when everything is aligned perfectly an amplifier can perform to its maximum capability: whether it's a hi-fi system amp or an integrated amp such as the ones used in studio mixing desks - it will affect everything.


So, I hazard, the real culprits of poor sound fidelity are the wrong materials and the electronics within an amplifier.


My father sits down and pauses for a moment.


Of course there are other factors, such as the quality of modern-day recordings.


He puts an old Ella Fitzgerald CD on the hi-fi. A melodious, slightly crackling voice flows out of the speakers, taking me to a place I just can't reach with say, a radiohead album. And I don't think it's the style that's the problem.


My father nods, enthusiastic now.


Poor hi-fi sound is in part caused by recordings which are so overworked in the studio that a good amplifier would struggle to filter and sort through all the faults. If one listens to an old track, which would have been produced more simply, it is much easier to get closer to the live version: that delicate, punchy sound that should arrive from an inky black background. That is what identifies an excellent amp.


My father uses the example of two singers harmonising: in real life the result would be captivating, he says, but in the studio they would record the voices separately and then mix them together electronically.


Human hearing cannot unscramble the timing distortion in such a signal, so we just hear a clear but hard sound. But if the studio used only one microphone to record both singers' voices, already mixed; in the air, the result would be much more exciting, as with live sound.


He adds:


Of course a good amp would be more able to handle and manage a bad recording.


Now I understand that an amplifier 'amplifies' sound. But I'm not a hundred percent sure how it works.


My father uses a mirror analogy to explain:

Imagine you are standing between two parallel mirrors and that from them you can see your own reflection going away into the distance, becoming smaller and smaller. Move to one side and these repeated images become clearer as the view is no longer obstructed by you, but you see fewer images as they veer off to one side. To get the maximum number of reflections you need to be exactly between both mirrors.


Yes Dad, but what have mirrors got to do with an amp?


My father smiles, a patient smile:


An amp is like the person seeing his reflection, only in its case the reflections are inside the feedback loop. For the amp, the images are all the same size but they go back in time, not space. The amplifier also goes one step further as repeated images are generated inside the negative feedback loop - reflections generated by reflections, as the amplifier tries repeatedly to correct errors. When everything is perfectly aligned the amplifier sees only one image, the original; everything else disappears.


Whilst I struggle with the mirror concept my father expands on the feedback problem.


There is no doubt that one of the keys to the problem is the way negative feedback is used. The 1927 invention reduces distortion by exchanging excess gain (the amount of amplification) for distortion. It's really a very clever system - errors are actively cancelled out by the feedback action.


But, he explains, greater precision is needed to correctly control feedback, and therefore sound.


Poorly aligned feedback can create a sound which is either too harsh or soft; instead feedback must be identified as a fixed control frequency in its own right, and not just as a product of the audio frequency.


So is the hi-fi industry working on this?


Not really! Instead music designers and enthusiasts are resorting to past technology. This explains the revival of valve amplifiers and transistor designs that were leading-edge technology 35 years ago! They are thought to give better sound.


And do they?


My father is thumping his foot and shaking his head, is that in response to the music or my question?


Sometimes, but that is mostly for unintentional reasons. Valve amplifiers, for example, perform better than many modern-day amps because they have simpler circuits (so there is less margin for error). But really we have the technology today to move forward much more effectively in all design areas than the valve and transistor designs of the 60's and 70's.


So basically what you're saying is that to get closer to live sound, amplifiers need to be designed better and made of non-metallic materials?


He smiles that patient smile again.


Something like that.


Closing the door to the listening room I hear my father put another track on his system. As thoughts of amplifier circuits and feedback whizz through my head I feel a greater respect for my father's drive to create that 'inky black sound'.



Denis Morecroft began designing hi-fi in 1978 and since then has won numerous awards for design ingenuity including most recently a 2002 WHAT HI-FI Magazine award for his hi-fi cables.