Tomás Mulcahy
active member
Joined: 25/04/01
Posts: 2431
Loc: Cork, Ireland.
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"Aliasing" noise in Spendor designed speakers.
#748312 - 28/06/09 05:23 PM
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I have a pair of Lentek S4 speakers, apparently designed by Phil Swift, who also did the
Spendors. They're a very similar design.
I'm working on treating the room, and
a sine sweep reveals that the speakers produce alias tones when the sweep gets to around
the 10k region.
Anyone hazard a guess as to what might cause that? The speakers
are 35 years old. The only thing I changed was the crossover caps.
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madtheory upright pianos
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Hugh Robjohns
SOS Technical Editor
Joined: 25/07/03
Posts: 12527
Loc: Worcestershire
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Re: "Aliasing" noise in Spendor designed speakers.
[Re: Tomás Mulcahy]
#748326 - 28/06/09 06:35 PM
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Intermodulation distortion somewhere -- probably in the amplifier, but assuming your sine
sweep is from a digital source it could well be the D-A converter.
hugh
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Technical Editor, Sound On Sound
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Tomás Mulcahy
active member
Joined: 25/04/01
Posts: 2431
Loc: Cork, Ireland.
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Re: "Aliasing" noise in Spendor designed speakers.
[Re: Hugh Robjohns]
#748373 - 28/06/09 10:37 PM
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Quote Hugh Robjohns:
Intermodulation distortion somewhere -- probably in the amplifier, but assuming your
sine sweep is from a digital source it could well be the D-A converter.
hugh
Thanks Hugh. The Amp is an
equally vintage Quad 404 with some mods to bring it to better than mark II spec.
The DA is an M Audio Firewire 1814. The aliasing happens even if I run the DA at 96kHz.
I don't know how they've implemented the outputs on the M Audio, corners could have been
cut.
I guess I could 'scope the outputs of DA and amp. I suspect the DA though!
I can't think of any other way to test it.
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madtheory upright pianos
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oggyb
Joined: 09/02/08
Posts: 975
Loc: Leeds, UK
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Re: "Aliasing" noise in Spendor designed speakers.
[Re: Tomás Mulcahy]
#748387 - 29/06/09 12:26 AM
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Quote Tomás Mulcahy:
Quote Hugh Robjohns:
Intermodulation distortion somewhere -- probably in the amplifier, but assuming your
sine sweep is from a digital source it could well be the D-A converter.
hugh
Thanks Hugh. The Amp is an
equally vintage Quad 404 with some mods to bring it to better than mark II spec.
The DA is an M Audio Firewire 1814. The aliasing happens even if I run the DA at 96kHz.
I don't know how they've implemented the outputs on the M Audio, corners could have been
cut.
I guess I could 'scope the outputs of DA and amp. I suspect the DA though!
I can't think of any other way to test it.
Some possibly useful info:
When I run my M-Audio
Transit interface at 48K I get no intermodulation within the range that my speakers can
reproduce accurately (approx. 35-18K).
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Jobless Graduate Bum;
www.ogonline.org
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Tomás Mulcahy
active member
Joined: 25/04/01
Posts: 2431
Loc: Cork, Ireland.
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Re: "Aliasing" noise in Spendor designed speakers.
[Re: Tomás Mulcahy]
#748398 - 29/06/09 04:47 AM
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OK, thanks!
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madtheory upright pianos
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Hugh Robjohns
SOS Technical Editor
Joined: 25/07/03
Posts: 12527
Loc: Worcestershire
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Re: "Aliasing" noise in Spendor designed speakers.
[Re: Tomás Mulcahy]
#748410 - 29/06/09 07:53 AM
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How are you generating the sweep tone?
It would be worthwhile trying a
different pair of known good speakers on the same amp to prove whether it is the source,
D/A, or amp -- intermodulation effects certainly can occur in speakers (or anywhere where
there is a non-linear process).
Best to work this through logically, I think,
clearing each stage of the chain as you go.
hugh
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Technical Editor, Sound On Sound
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Phil Ward
Joined: 04/08/06
Posts: 199
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Re: "Aliasing" noise in Spendor designed speakers.
[Re: Tomás Mulcahy]
#748457 - 29/06/09 10:34 AM
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Thomas,
Either Spencer Hughes or his son Derek were responsible for all Spendor
models up until Phil Swift bought the company a few years ago. I wasn't aware that Spendor
were involved in the Lentek S4, which I always thought was a Phil Swift design from his
pre-Audiolab period. I think it's unlikely that Spendor had anything to do with it – just
doesn't look like something Spencer or Derek would have done.
It was a pretty
good speaker in a very polite BBC sort of a way. Very insensitive though with its heavy
bextrene cone bass/mid unit. And it's drivers, from Audax, were not the kind of units that
time will have treated very well.
Anyhow, the "aliasing" noise could be a
failing tweeter. It's unusual but I have heard that kind of thing when a tweeter has a
serious mechanical problem causing distortion to rise to the point where it is audible as
a separate tone. If it is a dead or dying tweeter however I suspect you're a bit stuffed.
Ebay's your best bet for a couple of Audax 6 x 9, 25mm fabric domes. You might have a long
wait though.
Phil
PS. I can't remember if the Audax tweeter comes
apart easily. If it does, very carefully remove the front-plate (to which the dome/coil
will be attached) from the magnet and clean out the gap. Use a blast of air and then some
sticky tape down the gap all the way around to catch any loose bits of detritus.
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Freelance audio product designer/writer
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Tomás Mulcahy
active member
Joined: 25/04/01
Posts: 2431
Loc: Cork, Ireland.
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Re: "Aliasing" noise in Phil Swift designed speakers.
[Re: Phil Ward]
#748470 - 29/06/09 11:07 AM
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Thanks Phil! That's more info than I've ever been able to find with hours on Google 
Quote Phil Ward:
I wasn't aware that Spendor were involved in the Lentek S4, which I always thought
was a Phil Swift design from his pre-Audiolab period.
You're very likely precisely right, that's why I said "apparently". I
should've qualified that with "read it on a hi fi forum on the internet". As you say,
they've a polite BBC sound, like the Spendors. I was under the impression that the woofers
were Kevlar, the earliest ones Audax did. What's Bextrene?
Righto, I'll test
with another pair of speakers, then maybe I'll clean the tweeter. Not much point in
replacing them, I might as well get new speakers I think. I'm wondering if the Quad would
do justice to a pair of PMC DB1S+...
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madtheory upright pianos
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Tomás Mulcahy
active member
Joined: 25/04/01
Posts: 2431
Loc: Cork, Ireland.
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Re: "Aliasing" noise in Spendor designed speakers.
[Re: Tomás Mulcahy]
#748529 - 29/06/09 01:04 PM
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OK, Bextrene seems to be a version of polystyrene.
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madtheory upright pianos
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Tomás Mulcahy
active member
Joined: 25/04/01
Posts: 2431
Loc: Cork, Ireland.
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Re: "Aliasing" noise in Spendor designed speakers.
[Re: Tomás Mulcahy]
#749012 - 30/06/09 07:28 PM
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OK, tried a different speaker, no intermod distortion. That's good, at least I know the
Quad is good. Time to start saving for PMCs!
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madtheory upright pianos
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