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Old 24th February 2010, 05:08 AM   #11
Pashley is offline Pashley  Canada
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Default Variable impedance amps

Rod Eliot's site has an article on variable impedance amplifiers, mainly how to modify the feedback in a transistor amp to get whatever impedance you need:

Variable Amplifier Impedance
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Old 24th February 2010, 04:26 PM   #12
djonins is offline djonins
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Smile Different wide range speaker

In my opinion, the Fostex speaker is inferior because it is matching the amplifier power. It should be tested using the 12 inch wide range speaker with above 30Wor more rated power and 99-100dB 1W/1m efficiency. Then you would have neglectable cone movement with big surface and the mass of the speaker magnet and housing would easily compensate for vibrations transfer to the baffle. There are various speakers with these features in the market, for example "Audio Nirvana" etc.

Alltogether, this is the fantastic scientific-engineering approach to show in praxis how it really works. Thanks for all your efforts.
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Old 11th April 2010, 07:41 PM   #13
nuvistordave is offline nuvistordave  United States
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Excellent article. I only saw a couple of gaffes. First, the best way to increase the plate resistance of your SET amp is to put an un-bypassed (power) resistor in the cathode circuit. This also lowers distortion. Second, use carbide router bits for your MDF cutting; they last practically forever and are well worth the extra cost.
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Old 14th April 2010, 08:29 AM   #14
Globulator is offline Globulator  United Kingdom
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Very good article, I especially liked the bits about diffraction, driver placement and panel construction, bracing and damping.

Not sure about 93Hz as a cut off though, I think I'd like some bass from a speaker, with a 42 litre vented box you'd get down to 50Hz without any trouble.

I think a lack of any bass at all is often mistaken for lack of woolly bass when people box high Fs drivers into sealed containers.

I have a sealed box that works, but that has a B139 (Fs = 25Hz) and a 72 litre box.
In addition a subwoofer will not be any more precise than a (properly designed without a big bass hump) vented box in my experience.

For stuff without bass however I bet they sound crackingly good, I always like the sound of a whizzer cone driver too for some reason.
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Old 25th April 2010, 03:28 PM   #15
Martin Colloms is offline Martin Colloms  United Kingdom
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Default a generous contribution from Morgan Jones

Some fine stuff here and generously given to the community, thanks

..a few observations

The 6dB spl diffraction step... is not set in stone, as the power response is maintained...

.....and the step relates to the change, in a free field environment , from spherical radiation at low frequencies to an approximation to 2pi baffle radiation at higher frequencies.
A listening room is not free field and so returns a fair measure of the off and rear radiation at low frequencies back to the listener as both pressure and reverberation, depending on frequency. Typically the step correction need only be 3dB and one may adjust to taste and local acoustic.

I agree with that contributor, that if you can afford it marine grade Baltic or Russian ply is a fine panel choice over MDF which is a bit hard sounding and characterless, but I am nitpicking here.

Bracing is a complex matter as you need to know the shape of the resonant panel modes and then brace according to amplitude and how that mode shape is radiated .

Ironically a brace can reshape a mode so that it is louder due to increased radiation efficiency.

Concerning the Zobel this is a nice touch to maintain the match to a 6.5ohm source.

May I suggest that the magnet is a magnetically and vibrationally noisy location, and that film capacitors are surprisingly microphonic. It would be worth trying them in a quite place eg at the terminals and not on the driver.

Even for a sealed box such as this I would favour rolling up the stuffing and tying it down in a relatively central location. Builders can also tweak the Zobel values to act as a tone control.

The reason the 4'5 ohm resistor addition is possible for a lower output, high Z amplifier is that valve- tube amps happily increase their voltage swing in to higher impedances. So a 10W output into 4 ohms may well swing a 10W equivalent drive voltage into 8ohms; the load line simply rematches.

Even quite high NFB tube amps behave this way.

Some heavy work on RFI reported in the latest issue of HIFICRITIC has confirmed the radio aerial properties of the loudspeaker and its connecting cable.

Some improved results have been reported with 10n 250 V or higher (high quality pp film , even Mundorfs have been used) in series with 76 to 160 ohm ( try them out) wired as a high frequency Zobel across the speaker terminals.

Commercial devices of this type can cost $200 a pair !

The purpose is to gently snub the RF in the speaker and wiring where the cables etc are typically 50 -150 impedance at RF , eg above 1MHz. bigger caps don't seem to work, you need to consider their self resonance frequency.

This RF gets in the amp and for many types we have tried at HIFICRITIC , subtly adds grain and reduces transparency.

RFI is a big issue and as we begin to try and control it , source to load , all cables including mains, and up from the supply , we are finding it a much greater factor in high fidelity than we expected.

Martin Colloms

HIFICRITIC, audio review magazine, hi fi critic
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Old 26th April 2010, 06:34 PM   #16
EC8010 is offline EC8010  United Kingdom
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First, a "Thank you" to Martin Colloms for his observations. As he's the author of "High Performance Loudspeakers" (now into its 6th edition, and thoroughly recommended), we can assume he knows of what he speaks. His comment about baffle step and 3dB is spot-on, since I only needed 2.4dB in my living room. As for positioning braces, I'm ready to believe that there could be better or worse positions, but the central position seems to work and gives the baffle around the drive unit support. I agree that capacitors can be microphonic, but my reasoning for glueing the capacitors to the magnet was that it has a lot of mass and is rigid - which is probably more than can be said for anywhere else in the box! Still, it's an easy enough change to experiment with.

I think the stuffing observation is valuable and well worthy of experimentation. The one thing we do not want is to shake the mass of stuffing up and down. I would expect changes here to affect the bass and low midrange.

I did initially do some calculations on the palliative effect of the flatter loadline allowing greater voltage swing into the loudspeaker plus series resistor, and therefore restoring some of the power you'd expect to lose, but the effect wasn't as great as I'd hoped and the article was already getting quite long, so I left it out.

RFI is insidious - especially if you consider loudspeaker cables to be aerials connected to an amplifier's input stage via a global feedback loop. I seem to remember A N Thiele writing an AES article about this effect and suggesting RF filters at the output of the amplifier. The RF Zobel idea seems to be more aimed towards preventing the loudspeaker cable acting as an incorrectly terminated (and therefore resonant) transmission line.
The loudspeaker: The only commercial Hi-Fi item where a disproportionate part of the budget isn't spent on the box. And the one where it would make a difference...
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Old 14th October 2010, 12:52 PM   #17
Joe Rasmussen is offline Joe Rasmussen  Australia
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Hi Morgan

This project reminds me of an experience I had circa 1975 or 1976 here in Sydney. Allen Wright showed me this Jap 6" fullrange drivers and I went to the importer to get a pair. The brand is no longer available, it was Foster, I believe same as Fostex. As I was paying for them I was told that I ought to get in touch with a chap at Sydney University who had measured these drivers and who might be helpful to me. I did phone him and he promised to send me some computer print-outs. I was shocked that they argued for the addition of series resistance (resistors) to overcome too damped alignments when put into various box volumes as per a computer print-out-table. I then got back to him and started to blurt out about the effect this would have on damping factor blah blah blah. He would have none of it - it was the alignment that mattered and clearly damping factor was no issue with him at all - it was the alignment that mattered, period! Took me a long time and a lot of thinking and finally realised he was right. The gentleman's name? Richard H. Small. Yes, I was arguing against the one and only Mr. Small - what a silly dill I was. :-)

Cheers, Joe Rasmussen - BTW, still getting to ETF regularly? Like to get back there next year.
"Elsinore Project" DIY Speaker System
- "Don't take anything I say as an affirmation, but as a question." Niels Bohr [i.e. be humble]
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Old 14th October 2010, 12:55 PM   #18
Joe Rasmussen is offline Joe Rasmussen  Australia
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BTW, anybody interested in this topic, non-Voltage drive of loudspeakers, might also want to click on the link below: "The Linear Current Loudspeaker".
"Elsinore Project" DIY Speaker System
- "Don't take anything I say as an affirmation, but as a question." Niels Bohr [i.e. be humble]
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Old 3rd February 2011, 07:48 PM   #19
elmer12 is offline elmer12  United States
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Hello, I am looking at this article now (Feb. 2011) and wondering what speaker to use with it since the Fostex 166e seams hard to find.
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Old 3rd January 2013, 02:01 PM   #20
stevensoosai2527 is offline stevensoosai2527
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When experts churn out details, its always cause jaw-droping effect...! Thank you
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