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MANAGING EDITOR Jamieson Rowc, B.A., B.Sc.,

SMIREE

FEATURES EDITOR Paul Grad B.Sc., (Physics)

EDITORIAL STAFF

Rob Evans, CET (RM1T)

Hcnk Muldcr, HTS EE Mark Chccseman

DRAFTING Karcn Rowlands

PRODUCTION EDITOR

Carmel Triulcio

GRAPHIC DESIGNER

Brian Jones

ART PRODUCTION

Alana Horak

PRODUCTION

Kylie Prats

SECRETARIAL Naomi Lenthen

ADYERTISING PRODUCTION

Brett Baker Vikki Patching (Vic)

ADVERTISING MANAGER

Selwyn Sayers

PUBLISHER

Michael Hannan

HF.AD OFFICE,

EDITORIAL A ADYERTISING

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Lelters to the editor

ELECTRONICS AUSTRALIA is published monihly by The Fcdcral Publishing Company Ply Limited.

Typcsct and prinied by Hannanpnni. 140 Bourke Road, Alcxandria, NSW for The Fcdcral Publishing Company Ply Ud. Dislribulcd by Newsagenis Dircci Distribu-lion Piy Lid, 17 Dtn>dy Sirecl, Alexandna NSW 2015

Rcgistcrcd by Australia Posi — publicalion No. NBP 0240 ISSN 0313-0150

•Rccommcndcd and maximum Australiun rciail pricc only.

Loudspeaker kit problems

I was interested to see a letter in the July issiie of EA from a reader “C C. of Ashgrove, Qld” who is not satisfied with the performance of a speaker kił obtained for the Playmastcr 3-way en-closures. C.C. did not state precisely what the trouble was, but I thought it may have been the same type of distor-tion that I experienced with the same speaker kit, which 1 obtained to replace some older speakers in a pair of other-wise good reflex enclosures I wanted to update.

I, too, was disappointed in their performance when they were all instailed, especially at normal liśtening levęls in the home, and often going through a series of elimination checks, suspicion fell on the crossover units.

I knew' the crossover inductances were iron-cored, but as they were all madę up and part of the kit, I used them as supplied. I should have known better than to use them,'for the short-comings of iron cored inductances in ćrossovers has been known for morę than 35 years, by me anyway.

Iron cored inductances in crossovers are prone to distortion as they are un-damped.and can cause ringing, which results in waveform distortion. Their in-ductance also changes with variations in current, due to the iron, so the crośs^ over frequency is known to vary slightly with changes of volume.

When the offending units were opened, they were seen to be connected in a quarter section, parallel connected Circuit which is known also to bc a source of distortion, as the inductances and capacitors can react adverse!y with each other in this modę, which does not occur if the same elements are connected in the series connection. •

So the units as supplied were discard-ed, and two new ćrossovers were madę usirig air cored inductances and connected in the series 'modę with new bipolar electros. The same values of in-ductance and capacity were used, and the performance greatly improved.

There is something else .which may be worth noting.

There is a 90° phase difference be-tween the speakers in a quarter section series connection, but it is not always easy to place the mid-range speaker a quarter of a wavelength (at the ćross-over frequency), backed behind the frontal piane of the woofer in a cabinet. This is no great problem in a quartec section series crossover as the ratę of at-tenuation is less than in a half or fuli section. It just means that the two speakers are in true parallel over a greatęr frequency rangę, and this can help to cancel core resonances. ,

An improvement in response may also be obtained by changing over the connections to the mid-range speaker, but be wary of amplifier instability if this is dohe.

. I changed the connections to the mid-range speaker in my case, and the response was smoother and no instability was encountered. Pm very plcased with the results.

Best wishes to EA and Staff and wel-come back to Jinr Rowe.

R.J.. 0’Dea,

Epping, NSW.

Yintage radios

I have just read the article in your April issue by John Hill, on vintage radio restoęation. At the age of twenty-four I am in business for myself in my home town. I service and restore a lot of valve radios, a lot of them pre-sec-ond world war. It’s amazing the number of valve radios still in use in my area, being used in woolsheds and milking sheds.

. I do have a problem with obtaining valves for radios, and at times I have to give lip on the odd radio because it “beats me” or the customer and myself decide that repair would be too cóstly. Usually when this happens I inherit them for spare parts.

My opinion is that the old valve radio is a great performer, often a lot better than the. rubbish that modern tech-nology is putting on the market place today. One thing I will say the .old radios do have a lovely tonę and ap-peararice.

As long as the radios keep coming into.my workshop, I will try my best.to keep them going to keep the cows and sheep happy.!

P.J. Beeby, B

Balclutha, NZ.

ELECTRONICS Australia. September 1987