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Old 20th Aug 2018, 9:37 am   #1
Keith
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Default DAC90A - poor audibility at low volume

I'm currently using a DAC90A as a bedside radio and need to listen to it at low volume in the mornings to avoid waking others. I know my hearing is not great these days but I find that speech on Radio 4 long wave is difficult to follow under these conditions. The problems are hum, lack of treble and a form of distortion in the mid frequencies. I have managed to reduce the hum by adding another 32uF to the reservoir capacitor (fed via a 1K resistor to avoid overstressing the rectifier). In an attempt to increase the high frequencies I have removed C17 and C20 and reduced C18 to 0.047uF. I would be grateful for any other suggestions.
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Old 20th Aug 2018, 9:45 am   #2
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Default Re: DAC90A - poor audibility at low volume

The distortion at low volume, is the speaker coil rubbing on the magnet, a common cause of distortion at low volume.
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Old 20th Aug 2018, 10:01 am   #3
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Default Re: DAC90A - poor audibility at low volume

Hi Keith, worth trying another speaker if you have one. If there's no improvement then you'll need to try something else. I would recommend disconnecting the existing HT capacitor because if it is leaking it will pull down the HT voltage and reduce the remaining life of the rectifier. A measurement of HT voltage might also give a clue.

Cheers, Jerry
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Old 20th Aug 2018, 2:44 pm   #4
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Default Re: DAC90A - poor audibility at low volume

Thanks for the suggestions - I'll try an alternative speaker first. The apparent distortion was improved somewhat by changing the UL41 for one which wasn't microphonic! Incidentally, whilst looking through my DAC folder I came across the attached. I can't remember where it originated (possibly this forum?). It looks as if the modifications are also partly to do with improving the treble response. I'm intigued by the RC network between the UL41 cathode and HT - presumably a hum-bucker.
Attached Files
File Type: pdf DAC90a.pdf (116.5 KB, 112 views)
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Old 20th Aug 2018, 3:44 pm   #5
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Default Re: DAC90A - poor audibility at low volume

Kalee 20 of this parish drew up those modifications and they work well.

You can do them in isolation to see what the results are, can make a big difference to the "boomy" box sound of a DAC90a.

The full thread is here-
https://www.vintage-radio.net/forum/...ad.php?t=35784

Have you changed the audio coupling cap on the UL41 grid 1? It has to be perfect to prevent the valve or worse, the output transformer, going phutt.

Sam.
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Old 20th Aug 2018, 4:10 pm   #6
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Default Re: DAC90A - poor audibility at low volume

You mentioned hum ... from Sam's link above, I was surprised just how much difference this mod made when I recently restored one of these.
https://www.vintage-radio.net/forum/...d.php?t=148835
UL41 LEAKAGE
Although the grid coupling capacitor had been replaced, my UL41 showed a reading of about 0.4V positive on its grid. As it happens, the anti-parasitic stopper resistor is connected between the grid tag and another, unused, nominally spare tag on the UL41 valveholder. So, any internal leakage which develops within the UL41 button-base has its effects augmented. Configuring the layout to avoid connecting grid-related circuitry to sparee pins resulted in a drop to 0.15V positive on my UL41 grid – which I was contant with
.

John
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Old 20th Aug 2018, 4:10 pm   #7
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Default Re: DAC90A - poor audibility at low volume

Most radios of this type do hum a bit, and this can be very obvious when listening at low volume in the dead of night. They don't really make good bedside sets.
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Old 20th Aug 2018, 4:28 pm   #8
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Default Re: DAC90A - poor audibility at low volume

If only used at low volume how about an attenuator twixt output and 'speaker, keeping a load on the output by say 4 ohms in parallel and 100 in series with the 'speaker.
 
Old 21st Aug 2018, 9:30 pm   #9
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Default Re: DAC90A - poor audibility at low volume

I'd definitely check the HT first - I've had a few of these sets sounding like Daleks courtesy of low HT .......................
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Old 22nd Aug 2018, 4:17 am   #10
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Default Re: DAC90A - poor audibility at low volume

Rather than removing c17, I have found that reducing the value from 0.003uf to 470pf gives a marked improvement in the treble response.
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Old 22nd Aug 2018, 8:43 am   #11
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Default Re: DAC90A - poor audibility at low volume

Quote:
Originally Posted by merlinmaxwell View Post
If only used at low volume how about an attenuator twixt output and 'speaker, keeping a load on the output by say 4 ohms in parallel and 100 in series with the 'speaker.

I like this idea as it also attenuates the hum as well
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Old 24th Aug 2018, 5:57 pm   #12
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Default Re: DAC90A - poor audibility at low volume

Thanks for the further suggestions - the attenuator is definitely a possiblity. I tried replacing the speaker with a modern Japanese 4 ohm but no difference. Reducing the values of the coupling/roll-off capacitors helped considerably, but Kallee 20's hum cancellation circuit only increased the hum (R values from 10K to 5K tried).
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