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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only.

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Old 1st Jan 2003, 10:58 pm   #1
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Default Improving Bush DAC90A sound quality.

The DAC90A suffers from rather muffled sound, especially on LW. This is partly due to sideband cutting in the high Q aerial coils, together with a rather poor audio HF response. For better sound quality, disconnect the 10nF capacitor across the primary of the output transformer and add a 1.5 microfarad polyester capacitor across the cathode bias resistor of the UL41. This has the effect of boosting the treble by progressively removing the feedback introduced by the formerly unbypassed cathode resistor as the frequency rises, thus increasing the gain. (The more normal bypass of about 50 microfarads bypasses the resistor across practically the whole of the audio band, giving a level response).

Old 4th Jan 2003, 5:14 pm   #2
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Default Re: Improving Bush DAC90A sound quality.

I've restored at least 8 DAC90A sets and found no specific problems with LW sensitivity once the sets are properly aligned - both mixer and IF stages.

Regarding their 'muffled' sound, this can be due to the IF being too 'tight' thus cutting the sidebands and reducing the higher audio frequencies. I doubt the aerial/mixer stages have such hi-Q that they restrict the sidebands, even on LW.

Re-alignment with wobulator if possible will help. (LW/MW signals are 9 kHz appart. Strong daytime sigs should be at least 18 kHz appart so IF bandwidth can be about 10 kHz for reasonable audio quality without too much splash from adjacent channels.)

A number of Bush sets have an IF bypass capacitor from V3 to earth - 0.003 - 0.005 usually. Try reducing this as it is also a treble cut. Reducing the 'tone corrector' across the audio output transformer is also a good idea. But beware when reducing these capacitor values - too great a reduction may lead to various forms of instability.

BTW - less treble is transmitted by broadcsater on LW/MW these days for various good reasons. What was reasonable treble response in the past now makes many sets sound 'woolly', so altering capacitors to improve this is a reasonable practice.
Old 4th Jan 2003, 8:40 pm   #3
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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
Old 21st Apr 2004, 4:09 pm   #4
Join Date: Apr 2003
Location: Salt Spring Island, BC, Canada
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Default Re: Improving Bush DAC90A sound quality.

I had a similar muffled sound problem with my DAC 10 which I believe has a similar circuit to the DAC 90A. The problem was that the cap from audio amp V3 to earth was 0.004uF (4000pF), which really cut into the high audio frequencies. I replaced it with a 390pF which is adequate to filter off the IF frequency and the sound from the radio really sparkles now.

I suspect that the draughtsman tidying up the engineers "back of an envelope" circuit diagram, made an error and then too large a cap went into production!


Last edited by Paul Stenning; 27th Dec 2004 at 12:36 pm. Reason: Fix link or code for vBulletin
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Old 21st Apr 2004, 11:28 pm   #5
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Join Date: Mar 2003
Location: Birmingham, West Midlands, UK.
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Default Re: Improving Bush DAC90A sound quality.

I sometimes wonder - occasionally you often come across a set where a minor (and cheap!) alteration would result in improvements in sound quality or sensitivity. I'm tempted to believe that designers were well aware of the potential of these alterations, but wanted to make sure that there was a noticable difference in performance between the cheaper sets in the manufacturer's range and the luxury models!
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