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Old 4th Jan 2003, 8:40 pm   #3
Leon_Crampin
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Default Re: Improving Bush DAC90A sound quality.

Thanks for your comments. I am assuming of course that the alignment is correct, although the narrow IF bandwidth you describe can be caused by simply peaking the coils sucessively and then iteratively, whilst applying the signal at V1, rather than following the correct Bush procedure. If aligned to the maker's spec. and the coils peaked once only, the bandwidth is correct. After all, these same transformers are used in all the Bush sets of this period, and the bigger sets don't cut the sidebands on LW!

I would not recommend removing the capacitor from V3 anode to earth, as this is an IF bypass. The AF stages can be saturated with HF and distortion and possible instability will result.

The aerial coils on this set have a very high Q (air cored, backed on polystyrene, large diameter and litz wire on LW). This gives the set its good sensitivity, but if you do some calculations, the sidebands are definitely attenuated on LW where the required bandwidth is a much larger percentage of the carrier frequency than on MW. Remember also, that on LW, the signal is lower in frequency than the IF, so the aerial coils' bandwidth can easily become the limiting factor. You can prove the point by shunting the LW aerial coil with a resistor to lower its Q, but of course this wrecks the sensitivity. Also, when calculating the Q bear in mind that these coils are not significantly damped, as would be the case with a conventional transformer coupled front end using an external aerial. Bush even took the trouble to separate the LW and MW coils, further reducing the loading.

I much respect Bush as designers, but in this case I think that the need for maximum gain (who ever turns one of these sets around for maximum signal?) and a customer preference for "mellow tone" probably won the day here.

Incidentally, I agree with various comments on this site about R4 LW quality, but it is, after all, our fallback nationally receivable radio station. The volume compression and overmodulation used by the BBC serve to increase the range and intelligibility under adverse reception conditions, but don't do us any great favours!

Last edited by Paul Stenning; 27th Dec 2004 at 12:32 pm. Reason: Fix link or code for vBulletin