Apogee's and BUZZZZZZZ

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Apogee's and BUZZZZZZZ

Postby Graz on Fri Aug 06, 2010 1:48 am

BUZZ

Whats that – a buzz???? Many have heard this, at first it starts low, sometimes only on the very rare odd track. Once noticed we get tuned in to the sound, and hear it whenever it crops up. The fact is this is THE problem with old planar Apogee's, and the only solution is service. The first culprit is the damping foam degrading over time. Most of the original Apogee's have been around over 20 years now, and so can claim long service! But when it is that time – there is no getting around it. Chances are as the process is gradual you have been listening to compromised performance for a while...

A ribbon replacement may do the trick – but in some cases the magnets have been wrongly placed from the factory, this requires several or all of the magnets to be removed and re-attached in the correct position. Installers have measuring protractors for magnet placement, devices to clearly show where magnets have been attached wrongly.

What follows is a detailed explanation of buzz, first posted by myself on the old forum a few years ago;-


UNDERSTANDING THE BUZZ

For years we have fought the monster buzz and the theories for it and manifestations in sound have been numerous. I have pulled down so many Apogee’s now and it is still something that you think of when you gut a ribbon that looks reasonable in presentation, measures well and has reasonable foam. Cut down in their prime?

Well to add to the other theories try this conjoined pair. Firstly, the buzz is the combination of resonant frequencies of a randomly excited planar driver that has areas of null or partial reverse phasing. What? I will explain. The Apogee ribbons (as all ribbons) are simple flat examples of left-hand-rule in practice, stimulated by audio frequency electricity. They have a set magnetic circuit (if within tolerances – not all conform here!) and a set ribbon circuit that is supposed to integrate with the magnetic circuit in such a way as to always be in phase. A nice theory, and IMHO the reality is the source of the unwanted energy “leakage” that excites the second part of my theory - Structural resonance. All ribbons are structures, and in the case of the bass the ribbon is being asked to do some amazing things, all at once. Start/stop proportionally correctly over it’s entire field area in a linear manner while not contributing too much noise of it’s own structure ringing. A tough call when you consider the speed and fragility of a ribbon. Even tougher when you consider the construction is a bonded group of dissimilar materials that have to work well with strength, mechanical and electrical behaviors in an efficient way for a long time, every time. When you strike any suspended structure you get a form of resonance or ringing, and ribbons are no different here. The nature of the older ribbons (FR/Scintilla/Diva/Duetta/Caliper) is such that the construction is built face down using strips of non-conductive sticky kapton, for the purpose of holding the circuit together. As this is largely a random process with good intentions rather than “rigid parameters” one ribbon will not exactly resemble another, even if the template has been followed. In some areas the ribbon will be backed with no kapton (just bare unsupported foil), others one piece of kapton, areas with two pieces overlapping usually unintentionally, and in rare cases even three pieces. All cases produce local structural rigidity variations that respond differently to excitement from the audio signal, either enhancing or decreasing structural transmission of different areas of the spectrum covered by the driver, and in some cases adding noise outside the band. These are all at rather decreased levels in comparison to the main reproduced sound usually, as the edge damping foam absorbs a reasonable amount of the unwanted energy. Later production ribbons such as the SRA/SG/Stage/MG ribbons were better in this respect as they were extremely linear by comparison in regard to structural resonances having a single piece of MYLAR caught in an aluminium sandwich. The waved conductor edges made for increased panel integrity through overlap, minimising the panel flex problems on the field meridians that the circuit uses. In fact – the wave introduces as many problems as it solves in this respect, as the magnetic field is simply not this shape, promoting areas of null/conflict that are visually kept in check by the corrugation the waves traverse. Due to the general accuracy of the etched ribbons vs the old hand cut ones the buzz in one is generally cleaner, lower, and more tolerable than the hand-cut type.
When the buzz gets out of check.;-

1) The primary cause is when the foam breaks down over time, leading to the unwanted energy not being damped at all. This is extreme, and sounds like a kazoo!

2) Null areas are usually caused by problems of magnet movement usually during the adhesive setting time from new. Rotation throws a slant on the field in a local area that makes the response less or in extremes opposite to that required.

3) Field conflict (see pic 1). Exactly what it sounds like! When a ribbon that is designed to intersect a magnet or gap between two magnets is out of position across the whole width motion can actually be reversed! IMHO this was the prime cause of the old panels having “tired” areas along the circuit insulation edges. As ribbons are generally poor with vertical dispersion this effect will mainly be noticed when the inaccuracies are at ear level, but still part of the mix. The most regular one I seem to find is caused by inaccurate alignment with the end intersecting traces on dual/quad trace basses, often being as much as 3mm out of alignment. The greatest percentages of error are on such ribbons, being at their worst at the top and/or bottom, and various stages to correct in the centre areas. Original hand-cut dual trace basses such as the Caliper/Duetta/Diva/Scintilla 1-4 all have more susceptibility to this one as their circuits are smaller (particularly the Caliper Sig) and therefore percentage of error on field integration inaccuracies greater. I have seen errors off 22%+ on these ribbons in the past!! Ribbons such as the Scintilla 1 ohm, and Full Range are better here as the circuit design is much simplified and easier to cut, and field integration error percentages comparatively smaller.

4) Over braced (see pic 2). Areas where Kapton is stacked during hand production, not a problem with the etched ribbons. No visual clues on this one, just a few “local” contributions over the ribbon. My worst examples of this belong to the Full Range and 1 ohm Scintillas – as I have regularly seen 200% Kapton variations on the back of these things!

If you believe what I am talking about here then the buzz is the sum sound of the inaccuracies of the ribbons field integration to a greater degree, and variations of the ribbons structural ringing to a lesser degree, noticeable when the damping fails. And a bad ribbon will make the damping fail faster, as it has to work harder…

I often get asked by people who did not evaluate speakers before buying if I can teach them how to “tune out the buzz”. The problem is that IMHO the buzz is not a tuning issue, so while you can mess with the tensioning bolts to minimise local buzzing in a random way, your result will be two speakers tuned to minimise structural/tolerance issues rather than a nice matched stereo pair of speakers. I have never installed a panel yet that buzzed on installation – even way out of tuning. Having considered that led me to my present buzz opinions.

All doom and gloom – NO, just an explanation of the only real downside to the full planar Ap’s as I see and hear it. As we all know Apogee’s do sound great when compared to almost anything else out there, but tighter tolerances can improve the sound and lengthen the lives of our speakers.
Picture one is a Caliper Signature ribbon, one of the last made by Apogee. The owner claims these speakers had new factory ribbons done just before Apogee closed, and that before - and after they always had "a buzz". The ribbons measure exactly as they should, and the damping foam proved to be in reasonable order. Note the spacing between the lines - this should be even in spacing between all lines, but you don't need a ruller to see the differences. This ribbon's traces can NEVER properly integrate with an even magnetic field.

Image


Picture 2 shows the back of a ribbon, and if you look carefully you will see areas of foil exposed, and areas of multiple stacking of Kapton. Some of these variations do not run the same across the entire width of the ribbon, and for this reason the way sections of ribbon handle "ringing" will vary considerably. Accurate placement of adhesive Kapton is really quite hard as the adhesive used on it is really quite vicious, and where it lays is where it sticks!

Image

Ribbon production today and for the last decade has been a more accurate process, and together with the Installer's magnet positioning measurement capability the potential for better performance from an Apogee has never been so great!


Take care - Graz
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