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alvin1118

my New Toy: Digiamp

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hehe, let's have some nice photos to start with :grin:

 

NeoMini Digiamp = USB DAC + RA1 Headamp

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Testing & Experimental stage

I have bad experience with my "very good" Acer laptop USB I/O to Digiamp (built the first Digiamp year ago), it tends to have very high noise floor, unbearable hissing noise are everywhere as long as it is powered up by stock power supply adapter, thanks to lousy laptop SMPS :\

 

As such, i have to trial and error to somehow find ways to reduce the noise such that my laptop can be used as main music source.

 

Here's some pics

 

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Virtual Ground circuit provides precision, and sufficient power supply to RA1. The USB DAC can be powered by USB Internal Power; but then as mentioned, my Acer laptop power supply is not 'clean' <_<

Hence, IC7812 is used to provide +12v for USB DAC +Vin. The intention is to separate the virtual ground power supply with the USB DAC.

 

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Added 100ohms at RA1 R&L Channel Output to reduce hissing noise from my Acer Laptop. It serves as attenuator, which would or rather, believed to degrade the sound quality as the extra 100ohms dissipate a fraction of output current from RA1. Happen to met a electronic professional, he suggested to use a 10uf cap on output to gnd; it's one of the solution to reduce noise. I shall try it out when my hand get itchy again : :think:

 

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Added extra full bridge rectifier and filter caps, hopping to get 'better' Vdc out from the SMPS comes with Digiamp.

 

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Virtual Ground PCB is stacked on the passive components of Digiamp PCB.

 

The hissing noise is significantly reduced, SQ wise is much better than my exception; could be due to the reduce of hissing noise, this 2nd attempt of DIY Digiamp sounded good to me. :cans:

 

 

 

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Added 100ohms at RA1 R&L Channel Output to reduce hissing noise from my Acer Laptop. It serves as attenuator, which would or rather, believed to degrade the sound quality as the extra 100ohms dissipate a fraction of output current from RA1. Happen to met a electronic professional, he suggested to use a 10uf cap on output to gnd; it's one of the solution to reduce noise. I shall try it out when my hand get itchy again : :think:

I don't know anything about amplifier design or anything, but from a theretical standpoint wouldn't putting a cap from signal to ground kill the signal as well? The function of capacitors put in such a manner is usually to reduce upper frequency voltage oscillations. This is also precisely how the noise is being reduce. I guess that if you use a small capacitance, e.g. 0.1uf, you could still avoid the major effects on the signal while getting rid of the high freq hiss, but 10uf? Either way you're still going to inevitably get a huge treble roll-off.

 

Let me know if practice fits theory :grin:

Edited by peanutbutterjam

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I don't know anything about amplifier design or anything, but from a theretical standpoint wouldn't putting a cap from signal to ground kill the signal as well? The function of capacitors put in such a manner is usually to reduce upper frequency voltage oscillations. This is also precisely how the noise is being reduce. I guess that if you use a small capacitance, e.g. 0.1uf, you could still avoid the major effects on the signal while getting rid of the high freq hiss, but 10uf? Either way you're still going to inevitably get a huge treble roll-off.

 

Let me know if practice fits theory :grin:

 

 

 

U put a series resistor 100ohms at the output and then a parallel capacitor 10uF at the output to gnd and what u get is a low pass filter. u can calculate the theoretical cutoff freq and see what u get. :)

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eh, doghead ? the huge volume knob is installed exclusive for my own set... aint it's special ? LOL..

 

Yup, LPF would give u desired cut off frequency, but then again, the SQ will be 'degraded' if the LPF is constructed using passive components.

 

Will need to trial and error on these suggestion, usually, theory != practical; but theory would lead us to better solution :yes:

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eh, doghead ? the huge volume knob is installed exclusive for my own set... aint it's special ? LOL..

 

Yup, LPF would give u desired cut off frequency, but then again, the SQ will be 'degraded' if the LPF is constructed using passive components.

 

Will need to trial and error on these suggestion, usually, theory != practical; but theory would lead us to better solution :yes:

 

 

Hi, i dun exactly understand what u mean by "the SQ will be 'degraded' if the LPF is constructed using passive components", can explain a bit? Thanks.

 

Btw, the low pass filter allows only freq below the fc to pass through. The 10uf is too high here resulting in a very low fc. Also, the opamp may not be able to drive such a large capacitive load at its output. Performance will be impaired and instability and oscillation may occur.

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new solution to the digiamp:

 

1. omitted the virtual ground circuit, replaced by TLE2426.

2. added bead at the output to reduce USB hissing noise significantly

3. replace stock Panasonic FC caps with Sanyo Oscon for digital power supply section

4. added 470uf/50v Panasonic FC caps at main SMPS input

 

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