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Re: DIY DAC

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In Reply to: Re: DIY DAC posted by mamu on January 1, 2005 at 08:10:52:

Hi,
> could you please comment one the TDA1545 DAC from diyparadise.
In general fine, but Yeo made a hash with his "improvements" to the DAC Current scaling (BTW, I have always advocated to operate the TDA1545 at 2mA Full Scale Current, as far back as the late 90's).
As long as you operate the DAC with nominal Supply voltages you need to be very carefull how you arrange the passive I/V, in fact, the TDA1545 is non too well suited to passive I/V.
The DAC on +V supplies is strictly limited to a 3V Peak-Peak output Swing centered around a 3.5V Midpoint. In other words what you get out is 1V RMS.
If you bump up the current by connecting 11K from the 2/3 +V node (NEVER EVER use 1/2 +V) to 2mA Peak-Peak your I/V conversion resistor must be below 1K5, simply to not exceed the output voltage compliance and also be connected to 2/3 +V.
You can of course operate the DAC at overvoltage, in which case each Volt overvoltage increases the Output Range by 1V Peak-Peak, the midpoint is always at 2V + ((Vdd - 3V)/2).
But of course, if you go above 5V you live dangerously and on your own risk.
Other than this BIG caveat things should be peachy.
I probably would still recommend my Valve Output Stage to be used after the TDA1545, making sure to get 11K from 2/3 +V as reference and the adjust the nominal 375R cathode resistor untill the output voltage on the DAC is as close to 3.5V as possible.
Ciao T

Follow Ups:

Re: DIY DAC - mamu 10:43:16 01/10/05 (2)
Hi T,
many thanks for your comments!
Any "glimmer" if Yeo's DAC will sound better than Scott Nixon's TDA1543 based one?
-Matthias
- Re: DIY DAC - Thorsten 13:25:22 01/10/05 (1)
  Hi,
  > Any "glimmer" if Yeo's DAC will sound better than
  > Scott Nixon's TDA1543 based one?
  Reasonably well implemented the TDA1545 will sound a little smoother and relaxed, but I am not sure how well the kit does.
  BTW, with diodes from my "junk box" I measure around 0.65V at 50mA current, this suggests that 9 diodes are best suited, giving 5.85V +Vdd.
  If we tap off the nominal 2/3 Vdd at the 5th diode (NOT the 6th) we get 3.25V as common voltage. We can swing 1.25V towards the ground and 1.6V towards +Vdd, pretty much as good as it gets.
  Now we can swing 1.25V max and the worst case current is 2.2mA. Thus we would need around 1K2 I/V conversion resistance. It seems advisable to make the Reference resistor adjustable (at least intially) and to adjust it with a 'scope and a 0dbfs sinewave to give just no clipping with a 1K2 I/V resistor, a 22k multiturn trimmer should do.
  One might consider experimenting with the exact settings for the I/V resistor a little more and seek the best compromise between dynamic range and clipping, but I suspect that 1K5 will be the absolute limit.
  One would really have to build a DAC and test it to get this down to a "T".
  Ciao T
  - Re: DIY DAC - mamu 22:37:27 01/10/05 (0)
    Thanks again!
    Thinking about to order Yeo's DAC and doing some testing and tweaking.
    -Matthias

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