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August 2010 issue - Page 6

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August 2010 issue
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DAMPING FACTOR

I became acutely aware of your inclusion of damping factor as a measurement in your extensive amplifier tests in the April issue. I find damping factor can be a meaningful spec but not as it is usually presented.


Simply giving a value is mainly useful to compare amplifiers measured the same way but is not very meaningful for an individual amp without further information and usually a lot more of it.


Perhaps you have printed this and I just missed it, but damping factor needs a statement of the impedance used to measure it. I would normally assume 8 ohms for this but in this day of lower impedance speakers perhaps 4 ohms is used and I would appreciate knowing this. Another piece of data needed is at what frequency the value is derived. I’ve seen measurements over 100 Hz and since damping factor implies a degree of bass control this may not be a useful point to take it since at lower frequencies the damping factor can be quite different and is often lower implying lower control.


Finally, I have once in a great while seen damping factor measured across the bandwidth, particularly at high frequencies. This is rare but I find it useful in getting a sense of the basic linearity of the amplifier circuit before the application of corrective negative feedback. If the damping factor is basically the same at low frequencies, mid-frequencies and high frequencies, basically across the audible 20Hz to 20kHz bandwidth the amplifier circuit is probably quite linear and also quite stable and the amp is probably therefore quite good.


Your thoughts on this subject would interest me, given the experience shown in your reviews and particularly given the design experience of many of your writers.

Allen Edelstein

New Jersey, USA

 

Hi Allen. Thanks for writing and your interest. We measure damping factor by switching from 8 Ohms to 4 Ohm and noting the change in an amplifier’s output voltage. Then D.F. is (m-n)/mn-m, where m is the factor by which the load changes (2 in our case) and n is the change in output voltage. You can find the derivation in Mannie Horowitz’s book ‘Measuring Hi-Fi Amplifiers’, Library of Congress Catalogue No. 67-20987. We use 40Hz as the measuring frequency.


You could measure output impedance (inverse of damping factor) across the audio band to assess feedback, but we choose to measure the rise in distortion at high frequencies. This takes into account basic non-linearity as well as the amount of feedback used.


Our experience of using a wide variety of loudspeakers with an equally large number of amplifiers shows that damping factors above about 20 are sufficient to obviously apply control to under damped loudspeakers, whilst damping factors below about 5 have no affect. This, quite frankly, is in line with the general industry view. As Laurie Fincham of KEF pointed out to me long ago, loudspeakers are self damped acoustically and magnetically, as well as electrically by the amplifier. Highly damped loudspeakers do not much need a high D.F. amp. However, under damped loudspeakers like our in-house Spendor S8es boom and sound boxy with low D.F. valve amps, but are fine with high D.F. transistor amps. Our Spendors work wonderfully with Musical Fidelity’s AMS50 but they sound obviously ‘tubbier’ with our Icon Audio MB845 valve (sorry - tube!) amps and do not suit zero feedback valve amps at all.


When taking a view on all this, it’s always useful to bear in mind that every bass unit has a big coil of wire, the low pass inductor, between it and the amplifier, and this will measure 0.5 Ohms or so, according to the core material, wire thickness etc., increasing output impedance and lowering damping factor. This being so it is surprising we hear any differences at all. NK

 

WELL CONNECTED

I thought I would write to tell you about something that occurred after upgrading to a valve amplifier.


I just received my Icon Audio Stereo 40 III (KT88 Version) and after assembling it and connecting up all the interconnects I sat back to listen. I can tell you I am very impressed; whether its this amp or the fact that I am now using valves I can’t tell you, but the sound is a magnitude better than my old transistor amp. All the instruments on my vinyl and CD now sound like instruments; its hard to explain but everything just sounds more real!


Anyway, the point. When installing the new amp I had to move a couple of components around (to allow the amp to sit in a more spacious area, to give better cooling). While doing this I happened to swap a couple of my interconnects to other components. Once I’d finished, I sat back to listen and started with my CD.


Now, although it sounded good, as always, I felt it was not quite as clear and vivid as before. After checking I realised I had used my original CD interconnect (VDH the 1st ultimate) to connect up the tuner. Before I removed anything I swapped to listen to the tuner and after a while thought that was not as good as before also. So I set about swapping all the interconnects back to their original components. Now my CD player sounds great again and my tuner is also back to how I like it.

It seems to be some sort of synergy thing! CD (an original MF X-Ray) just sounds much better with the VDH interconnect, yet that interconnect does not make my tuner sound better, it seems to sound best with its Chord interconnect.


Up until purchasing the new amp I was as sceptical as the next hi-fi listener about the difference interconnects and speaker cables can make (it’s all in the head of the listener etc). But I think as valves let you hear more of the feeling within the source being used, the cable differences are more apparent.


I suppose this may start another cables debate within the Letters page, but I’m only writing down what I have observed. Anyway, it will make a change from the Denon 103 debates!

thanks for listening

Andrew Burtchaell

 

musical-fidelity-ams50

Musical Fidelity AMS50 keeps a tight grip on loudspeakers.

 

 

MICHELL ALECTO

I have just seen the letter from John Rainey in the Philippines regarding his Michell Alectos. Apologies if you were already aware of this, but just to let you know (and it might be worth passing on to the readers) that Graham Fowler at Trichord Research can now supply the Mk2 Alecto boards, so existing owners of Mk1 Alectos can upgrade to new main boards, which can then be further upgraded to 250 watts per channel (I believe an extra transistor is added) and is said to transform the sound. It might be a cost effective upgrade for Mk1 owners such as Mr Rainey.

regards

Robin Cook

 

michell-alecto

Michell Alecto amplifier can be upgraded, Robin Cook points out.

 

KEN DOD

Tonight I’ve been listening to ‘Presenting Ken Dodd’, his first album, a mono LP of romantic ballads from 1962. Once again I’ve enjoyed a lovely sounding mono recording and could hardly believe this one is 48 years old. I won’t be around to see how my CDs fare in 48 years time but I do wonder.


The trip to the record shop in town was well worth it this week. First, I couldn’t resist buying an LP featuring Kenny Ball, Chris Barber and Acker Bilk, the reason being the very same combination are due to appear at Yeovil’s Octagon theatre soon - The Legendary ‘Three B’s’. It seemed a fitting buy.


Then I found the shop had some more 78s for sale. I picked up a few (couldn’t manage to take more because of the weight) and they include three gems. Two Lonnie Donegans and wait for it - a Norman Wisdom recording. Being a fan of his I’m thrilled at owning a Norman Wisdom record on 78.

Well, sometimes you have to go backwards to move forwards...

best wishes,

Melvyn Dover

Weymouth, Dorset

 

Hi Melvyn, I’ll swop my Lady Gaga album for your Ken Dodd album if you want! NK




 
Comments (1)
Nakamichi 600
1Monday, 11 November 2013 04:24
Donald
Hi, I bought a Nakamichi 600 cassette deck head from B&W UK, I think they told me then it was their 2nd to last one. Still excellent. One of the finest pieces of hi-fi machinery.

Yes, Nakamichis were lovely – I have a ZX-9 and love it, but spares are drying up. What a shame. NK

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