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Old 6th Aug 2020, 8:50 am   #1681
Craig Sawyers
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Default Re: The Audiophoolery Thread.

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I know Richard Lee well. He also designed Option 1, a dipole speaker with active crossover and multiple custom Quad current dumper amps in the bass.

Details of this exceedingly rare beast attached

Craig
There are two reasons that Option 1 was not a commercial success. First, it was expensive - about 2.5 times the price of a Quad ESL63 at the time - so around 6k to 8k in the mid-late 80s, so around 18k - 25k in today's prices. It was more than an order of magnitude more expensive than any other Wharfedale speaker.

So there was a stigma attached to having a Wharfedale speaker costing that much. There was certainly competition at that price point from Magnaplanar, Martin Logan, Apogee etc that were also dipole speakers with no brand baggage.

Therefore Wharfedale missed a trick. Creating a special brand name for Option 1, and possibly a range of speakers and amplifiers at a high price point.
And Option 1 was a truly awful name for a loudspeaker!

By the time I came on the scene it had been discontinued. They retained one pair, and it sounded superb, with spookily accurate imaging. I think they sold less than ten pairs all told. At a time when the Diamond (probably III) was selling 50,000 pairs a year.

Craig

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Old 6th Aug 2020, 10:09 am   #1682
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Default Re: The Audiophoolery Thread.

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... One of the biggest things seems to be this infatuation with an amp having to 'match' a pair of speakers ... most, even half decent amps have very straight frequency response curves so it's plain ridiculous ...
What you would call 'even half decent amps' don't cover the whole range of amps available these days. There's quite a large number of single-ended triode amps without any negative feedback out there. Their frequency response into a pure resistance may not be all that flat and, more significantly, their output impedance won't be low either. Given the wild swings in speaker impedance with frequency, there's quite a bit of scope for different combinations of amps and speakers to sound different.

Cheers,

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Old 6th Aug 2020, 10:22 am   #1683
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Default Re: The Audiophoolery Thread.

Well, a significantly non-flat frequency response and unnecessary sensitivity to speaker impedance swings immediately disqualifies an amplifier as halfway decent in my opinion.


Other people are free to like the thing, it might even be terribly prestigious and expensive, but it's in the could do better category for me.

Of course, some people may be using speakers which play up such an amplifier's foibles to flatten them, but that's a daft way of doing things.

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Old 6th Aug 2020, 11:11 am   #1684
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Default Re: The Audiophoolery Thread.

A loudspeaker drive unit is essentially current driven device - the force on a length of wire in a magnetic field is just BIL where B is in Tesla, I is in amps and L is the length of wire. Which is why the T/S parameters for a drive unit include the force factor BL.

Now of course being a resonant device, where the cone mass resonates with the compliance of the roll surround and spider (and the enclosed air of a sealed box) the current falls as the driver approaches mechanical resonance. Or double resonance in the case of a reflex enclosure. Essentially the driver produces a back emf as it approaches and goes through resonance.

Anyway, there are two choices. Either an amplifier and speaker cables with a significantly lower resistance than the loudspeaker voice coil, and allow the driver mechanics to dictate how much current is needed. Or a transconductance amplifier, where the input voltage is converted to a drive current.

The transconductance approach has been used with bass drivers (at the University of Essex, among others), but you need to apply motional feedback by sensing the cone movement and used that to control the amplifier gain. Various sensing has been used - an auxillary coil, capacitance etc. I can't recall if this has even been commercialised.

So the requirements of an amplifier feeding a practical loudspeaker are:

Enough voltage drive to meet the power demands, with a constraint arising from the Xmax of the driver(s)
Enough current capabilities to deal with short term music transients
A low output impedance so it is a flat frequency response into any loudspeaker

And all the design practicalities to make that happen.

High power valve amps with <0.1 ohm output R are likely to work just fine. But low power single ended amps are likely to produce frequency response anomalies into practical loudspeakers just as David has said.

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Old 6th Aug 2020, 11:47 am   #1685
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Default Re: The Audiophoolery Thread.

A few numbers might illustrate what is needed. Suppose we need an amplifier to deliver 100W into an 8 ohm driver. That is 28.3V rms or 40V peak. Add a Vce(sat) of a volt or so, 1V of capacitor ripple, and a volt of diode drop and get to power supply rails of +/-45V, or a +/-35V transformer. That will deliver a bit more than 100W (130W).

So let's respecify this amp at 130W into 8 ohms. The peak current requirement will be the peak output voltage divided by the loudspeaker resistance, or 5.7A. Of course this amp will have to work into 4 ohms too, and need to supply 11.4A peak.

In practice a nominal 4 ohm driver might be closer to 3 ohms so the peak current becomes 15.2A.

The output stage would need to be specified as 130W into 8 ohms (and not quite twice that into 4 - say 200W, because of Tx losses and capacitor ripple), and a peak current of 15A.

Heatsinks would have to cope with that, because although most listening is at an average power of ~1W, (a) reviewers tend to test continuous output into a dummy load and (b) modern massively compressed rock music played loud might get close to full power sine wave tests.

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Old 6th Aug 2020, 11:50 am   #1686
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Default Re: The Audiophoolery Thread.

I am sure I have seen an article about speakers with cone position feedback.
It was before powered speakers became common. Perhaps Phillips?
It would be impractical to have position feedback in speakers without built in electronics.
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Old 6th Aug 2020, 11:53 am   #1687
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Default Re: The Audiophoolery Thread.

Yup. I could probably find the references. And I agree - any practical implementation would need amps integrated with the speaker.

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Old 6th Aug 2020, 11:59 am   #1688
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Default Re: The Audiophoolery Thread.

Philips made a series of speakers that they called 'Motion FeedBack' (or MFB). These had a piezo-electric transducer on the bass driver unit that sent a feedback signal to the (built-in) amplifier. IMHO the smaller ones are more impressive (it's not too hard to get reasonable bass response from a large speaker cabinet).

Service manuals for them are on the web somewhere if you want to see what they did. The units themselves are not too rare and don't command 'audiophool prices'. I paid a few 10's of pounds for a pair of the smallest units that I must try out sometime...
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Old 6th Aug 2020, 12:43 pm   #1689
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Default Re: The Audiophoolery Thread.

Interesting. Very early then, in the 1970's. And with paper cone woofers by the look. I really question how many of the smorgasbord of Klippel distortion mechanisms motional feedback can deal with.

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Old 6th Aug 2020, 1:08 pm   #1690
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Default Re: The Audiophoolery Thread.

Everything you ever wanted to know about Philips MFB speakers - https://www.mfbfreaks.com/
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Old 6th Aug 2020, 1:10 pm   #1691
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Default Re: The Audiophoolery Thread.

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AM also did reviews of amateur radio equipment. Amongst some of the retrospective comments from his ex-assistants were comments that he was not at all a pleasant person to work for.
Without doubt, he wasn't perfect - and I have had personal experience of both the good and the bad. When it comes to the particular story, the actual suppression is attributed to John Crabbe, another Aunt Sally of the new wave reviewers, which is curious, considering that John Atkinson cited one of his reviews as a model of the art. I remember being quite impressed with E70s at the time - I heard them at a show, I think - and I don't recall them being unduly criticised in the review. I must look it up...
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Old 6th Aug 2020, 2:20 pm   #1692
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Default Re: The Audiophoolery Thread.

Philips used a piezo accelerometer on the cone, so it was acceleration feedback not position, so with the second derivative under control there would be uncontrolled constants for initial position and initial velocity. I presume they were 'leaked' to zero with long time-constants.

Interesting and said to work quite well. I once heard a pair of the small ones in Wood's in Huddersfield, but not under good listening conditions and not with suitable material.

Driving speakers is interesting. One view is to voltage drive them, and another is to current drive them.

If driven from a perfect current source, the parameter being controlled is the force on the turns of the voice coil. Not all force is available for Newtonian acceleration, some goes into flexing the spider and surround, these are not necessarily linear and can give hysteresis as well.

If driven from a perfect voltage source, the current is not controlled, but then the speaker is not a passive victim of the urges of the amplifier. It is not a pure motor, it is also a generator. Cone motion produces back EMF in the voice coil, proportional to velocity. An ideal speaker would take whatever current necessary to get itself to the dictated velocity where the back EMF cancelled the EMF from the amplifier. Essentially velocity feedback for free. It's not without its imperfections, because the cirrcuit loop includes the DC resistance of the voice coil, wires, crossover inductor and the outpur Z of the amplifier. If all is well, though and these resistances are linear, the speaker velocity is controlled, but winds up as a linear fraction of what you'd expect given the amplifier output voltage, with the loop resistance effectively compromising the loop gain of the effective feedback process.

Let's shoot a sacred cow. What gets called Damping Factor in an amplifier isn't. If you designed an amplifier with zero output impedance, it would be as perfect a voltage source as you could want. It is impossible to dissipate energy in a zero resistance, so it CANNOT do any damping at all. THe damping of cone motion at resonances come from the resistance of the voice coil, crossover windings, wiring.

On the whole, you want a nice low output impedance from an amplifier so you know where you stand, and it's probably what the crossover designer took into account and equalised the drivers with. Also speaker impedanes can vary widely (and wildly) so quoting damping factor against a pure and notional 8 ohms, is a bit thick.

Going the opposite way to a pure current driver, the amplifier has infinite output impedance, and presumably plenty of compliance. To the rest of the circuit, cables, crossover, driver, it looks like an open circuit with current appearing by telepathy or something. There can be no response of the loop current to the motion of thespeaker, no damping of resonances so the tendency is to leave the speaker with more of its imperfections showing. Resonances will be less damped and take longer to decay.

The same thing could happen in the zero ohm output amplifier case, but only if the resistance of the crossover coils and the voice coil were trivial.

But with normal voice coil wire resistance and the coils in the crossover, the low impedance amplifier trying to act like a perfect voltage source, along with low resistance wiring is closer to ideal than would be a current driver amplifier.

David
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Old 6th Aug 2020, 2:59 pm   #1693
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Default Re: The Audiophoolery Thread.

An interesting experiment with MFB 'speakers is to put ones ear close to the bass driver and then turn it on, the usual rumble of the world gets quieter. Demonstrated to me in a PYE shop 1979/1980.
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Old 6th Aug 2020, 3:22 pm   #1694
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Default Re: The Audiophoolery Thread.

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Well, a significantly non-flat frequency response and unnecessary sensitivity to speaker impedance swings immediately disqualifies an amplifier as halfway decent in my opinion ...
I suspect you're not among the target readership of today's HiFi mags David .

VB
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Old 6th Aug 2020, 3:35 pm   #1695
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Default Re: The Audiophoolery Thread.

I sometimes buy items from Hi-Fi Collective who I find excellent. Most recently, I bought some silver mica capacitors from them, of which they have an excellent range. In browsing the website came across something to drive audio homebrewers to the heights of ecstasy. Namely, for those for whom only the best will do, a nice line in resistors.

Specifically:

1) Audio Note 2W Niobium Non-Magnetic Resistors @ 19.50 each + VAT, so 23.40 each.

https://www.hificollective.co.uk/cat...resistors.html

2) Audio Note Silver Tantalum Non-Magnetic Resistors:

0.5W: 19.00 each + VAT = 22.80
1Watt: 29.50 each + VAT = 34.40
2 Watt: 41.06 each + VAT = 49.27

https://www.hificollective.co.uk/com...resistors.html

The specifications can be found at the above links.

They also do a nice line in high end capacitors, but let's not go there!

Binding posts? 76.25 for two pairs + VAT = 91.50:

https://www.hificollective.co.uk/bin...ds-copper.html

Just don't click on 'Buy Now' in error!

A nice line in amplifier kits too.

Not for me to question how other people spend their hard-earned cash or to mock them, but the thing I always find a bit sad, is that those who have a high enough disposable income to be able to afford to pay eye watering prices, will generally be middle aged or even later in life. It's a biological fact that from our teens onwards, our hearing declines - particularly our ability to hear higher frequencies which - earlier in life - would have been audible, but are no longer, so however good audio equipment actually is, or is perceived to be, no amount of money can buy the essential item which is needed to enjoy the full benefit - namely, our hearing.

But then as with cars or whatever, while we like to think we make objective and rational purchasing decisions, it's much more about emotions - pride of ownership, 'bragging rights' and so forth. But let's be honest, there are many who mock 'audiophools' but who buy expensive laboratory grade test gear, the specification of which goes far beyond their needs as hobbyists.

In a cheerless world, anything legal which puts a smile on anyone's face and makes them happy is no bad thing.

'The only difference between men and their boys is the price they pay for their toys'.

(The late Liberace).
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Old 6th Aug 2020, 3:59 pm   #1696
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Default Re: The Audiophoolery Thread.

I'm expecting a phone call in a few minutes from a customer who will, I suspect, want me to replace the four rather ordinary 100k anode load resistors in his valve pre-amp with four posher ones. He recently asked me to put some Shinkohs https://www.hificollective.co.uk/com...resistors.html into another piece of his kit and he says he prefers the sound of them. It's his kit, his money and his enjoyment. Unfortunately 100k Shinkohs are pretty much unavailable (although I will look). The AN resistors David's mentioned are targeting the market gap that the Shinkohs have left.

Cheers,

GJ
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Old 6th Aug 2020, 4:06 pm   #1697
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Default Re: The Audiophoolery Thread.

On the 'feedback' part of this thread - we (I was at Salford) used sub-miniature piezo accelerometers glued to drivers, to derive signals to feed adaptive filters running on boards installed in 286 computers - it was all a rather long time ago. The really difficult thing was establishing a target for the filter adaptation - we thought we wanted to reduce the spatial, freq. and time domain artefacts associated with the way the driver excited the room at low frequency, but working out a cost function was not at all trivial, and doing so whilst not measuring the room to death with large microphone arrays, even more so. Well, they don't call it Permanent Head Damage for nothing

B&O had a rather nifty product which tried to do something like this by measuring its own radiation impedance - the acoustic impedance (pressure / velocity) looking into the room at the location of the source - using not a mic and accelerometer, but a single mic which moved out on a motorised 'stick', to provide an impedance estimate based on a two-microphone finite difference velocity approximation. I suppose this would be around late 1990s, and if I remember correctly, it was on sale in shops.

The accelerometers we used were apparently developed to encourage anti-personnel mines not to waste their time blowing up for a passing tank, I was disturbed to find out much later.
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Old 6th Aug 2020, 4:47 pm   #1698
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Default Re: The Audiophoolery Thread.

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Let's shoot a sacred cow. What gets called Damping Factor in an amplifier isn't. If you designed an amplifier with zero output impedance, it would be as perfect a voltage source as you could want. It is impossible to dissipate energy in a zero resistance, so it CANNOT do any damping at all. THe damping of cone motion at resonances come from the resistance of the voice coil, crossover windings, wiring.
I'm glad you pointed that out David. The impact of amplifier output resistance has almost no impact on a loudspeaker's performance.

It is one of the figures of merit quoted by amplifier manufacturers that isn't one at all.

From the guy who coined the term in the first place. He refutes this in a letter in August '47 WW

" Loudspeaker Damping"
IN your April issue D. T. N. Williamson refers to electromagnetic damping of a baffle- loaded loudspeaker, through low output resistance of the amplifier, as being important. I used to think so myself, and was the first to use the
word " damping factor " but my belief was much shaken by the following argument. If a loudspeaker can be represented by an equivalent circuit consisting of a resistance in series with an " ideal " loudspeaker of 100 per
cent efficiency, then the damping must be applied across the input terminals. In this case, even if the amplifier output resistance is zero, the damping is limited by the series resistance which, for 5 per cent efficiency, would be twenty times the resistance of the ideal loudspeaker. This extreme
simplification, of course, leaves out the reactive components of the speaker impedance, but the argument still holds qualitatively.

Can any reader of Wireless World point out any error in this argument? If it is true, there is very little gained by attempting to achieve excessively low output resistances.
F. LANGFORD- SMITH.
Sydney, Australia

Actually, following Small, the Q including amplifier output resistance includes a term in (Rg + RE) where Rg is the generator resistance AKA amp output resistance and RE is the electrical resistance of the voice coil (+ any crossover components). If RE is say 10 ohms and Rg is 0.02 ohms the effect of the amplifier output resistance is tiny (it makes a 0.2% change in overall loudspeaker Q!)

So the term "damping factor", as Langford-Smith realised 63 years ago, is a nonsense specification.

Craig
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Old 6th Aug 2020, 5:32 pm   #1699
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Default Re: The Audiophoolery Thread.

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Quote:
Originally Posted by stevehertz View Post
... One of the biggest things seems to be this infatuation with an amp having to 'match' a pair of speakers ... most, even half decent amps have very straight frequency response curves so it's plain ridiculous ...
What you would call 'even half decent amps' don't cover the whole range of amps available these days. There's quite a large number of single-ended triode amps without any negative feedback out there. Their frequency response into a pure resistance may not be all that flat and, more significantly, their output impedance won't be low either. Given the wild swings in speaker impedance with frequency, there's quite a bit of scope for different combinations of amps and speakers to sound different.

Cheers,

GJ
Right. I am talking about common or garden solid state hifi amps, not esoteric valve stuff which are greatly outnumbered by the former by a massive magnitude. So, for me, given what I've just said, amp speaker matching is largely (not uniquely) an audiophoolery thing.
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Old 6th Aug 2020, 5:39 pm   #1700
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Default Re: The Audiophoolery Thread.

The output impedance of the amplifier has no direct importance, so long as it's reasonably low.


But it does have an indirect effect. Designers of crossovers have to pin the tail on the donkey and pick a value they assume their speakers are going to work with. This then has an effect on how well they equalise the responses of their drivers (oh and damping factor just took another hit in the midrange and tweeter from the resistors they added to pad their relative sensitivities to match)

So it makes sense to get the amplifier output impedance into the ohm region, but going any further yields very rapidly diminishing returns.


I had a look at those boutique resistors and the others on the site. Not one had a spec for excess noise.....

David
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