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setmenu 1st June 2004 05:45 PM

TDA1543 nos dac question...
With applications using passive IV conversion, I see that a resistor to ground is used after the output caps.
I use 47K with mine,but note that some people use 100k and
also 1 meg.
And I have read some people do not use one at all.

With the fear of sounding stoopid:xeye: , what is the purpose
of this resistor?



Bricolo 1st June 2004 05:53 PM

I think it's to "load" the capacitor when no amp is connected to the output

till 1st June 2004 06:02 PM

if the cap is a good quality film cap without any resistance, the not connected output is isolated from GDN for DC. You could store some charge DC in the cap and at the moment you connect it to an (of corse turned on, full volume) amplifier, this charge would cause the amp giving a really loud sound POP out of you speakers. People usually donĀ“t like this "pop" sounds or fear it may damage speakers, so they use resistors.

setmenu 1st June 2004 07:03 PM

So as long as there is some DC route to ground , such as the 1meg to ground on the input[s] of my opamp based headphone amp, pops should be minimized?


till 1st June 2004 07:15 PM

i would prefer the resistor at the output of the DAC. If you put a lot of charge / from electostatic voltage, plastic carpet etc. on the cap the 1meg at the amps input will reduce the pop a little, but probably not enough. If you have the 1 meg at the DAC side, the higher value will only cause it needs a second more to bleed out the charge from cap than with a smaller R. (i would use 100k or so)

setmenu 1st June 2004 07:29 PM

Hi Till
The reason for wanting to remove the resistor is that the dac and
amp will be hard wired together inside a Small enclosure, with the
idea of running everything from a single battery pack.[assuming no
terrible sonic penalties of having shared supplies]
As the amp section is referenced to a virtual ground, I felt it
a good idea to remove the resistors that would otherwise connect the amps inputs to -v ?


jean-paul 1st June 2004 07:43 PM

I would avoid shared supplies of a DAC and an amp.

You need the resistors for avoiding nasty switch-on plops as Till explained. I don't understand your remark of virtual ground being connected to V-. Maybe a drawing can help us help you ?

till 1st June 2004 07:44 PM

in this case you will never disconnect and reconnect both and i think resistors are not needed.

jean-paul 1st June 2004 07:52 PM

Except when you change inputs ( assuming you have an input selector on your amp ).

setmenu 1st June 2004 08:28 PM

I have been scratching my head a bit regarding joining the two.

As the amp has everything referenced to a virtual ground [half supply V] and the dac has a ground at battery negative.
So, this would mean from the amps perspective connecting the
dac ground to the amp pot ground would effectively
result in a connection to the amps -v supply ?
And likewise connecting a resistor between the dac ground and amp input would then also couple the amps input to -v?

Sorry if this sounds muddled , I am just blundering along here:cannotbe:

Thus far the amp and dac do sound great with separate supplies:D

Regarding selector switches, there will none as the dac/amp is for
a dedicated portable headphone application.

And as for shared supplies, I would ideally like to hear both approaches as the single supply would save weight ,
but would agree separate is best.



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