PCM56K iv resistor value?

[georgehifi]

Another question, if the output impedance of the PCM56p is 1.2k in current output mode for a single chip. What does the output impedance become when you have 4 in parallel? The same?

Cheers George

 

[QSerraTico_Tico]

Another question, if the output impedance of the PCM56p is 1.2k in current output mode for a single chip. What does the output impedance become when you have 4 in parallel? The same?

Cheers George

devided by 4 perhaps????

 

[georgehifi]

devided by 4 perhaps????

Is that a guess?

Cheers George

 

[Bernhard]

For n x R parallel:

R = R/n

 

[georgehifi]

OK, I asume N means the number of dacs which is 4 .
So if I want to use the output impedance of the 4 dacs as one of the elements for a simple low pass which would be 300ohm, an 8000pf cap to ground after it will give me a -3db at 66khz correct?

Cheers George

 

[Bernhard]

For the calculation of the low pass you need the total impedance:

the impedance R of the DAC chips is parallel with the R of R_I/V.

 

[georgehifi]

Ok, can't get my head around this, you've got 4 dac chip at 1200ohm series output resistance each, divided by the 4 gives 300ohm series output resistance?
300ohm series with 8nf to ground which would give a -3db @ 66khz

So are you saying Benhard that I cannot use the output impedance of the dacs (300ohm) as the series resistance with a cap to ground to make a 2nd order low pass filter?, I cannot have a low pass filter here because the input of the i/v stage is close to zero?

Cheers George

 

[Bernhard]

Do you want to keep the staircase away from your I/V ?
I don't know if it is a very good idea to have the filter before the I/V.

You can try.

Still, I would use passive I/V with C and a buffer with gain.

You can sell that zap.

 

[Gabdx1]

Hey , I have some news, Ill try tomorrow the mod : 375R i/v and 50nf // cap ! I found some 15nf i might try later.

Test with multimeter shows no dc , no standing waves, buzzing or nasty sampling noise ! I get around 300mv voltage swing from music Tomorrow = test first on cheap system , then good amp cheap speaker, then the real thing !!!

 

[Gabdx1]

It works, bass is good.

I will reduce the resistor to 200 ohm for i/v , there is too much audible distortion still at only 375 R.

Bernhard , how do I calculate the Z and the value of the cap for filter ? what cap size I should use to filter iv at 375 R , and i/v at 200 R ,

I wish I knew how to calculate this !!!! help.

 

[Bernhard]

I measured the distortion with 500 ohm and it is < -96db.

 

[Gabdx1]

Probably my system sounds like crap and the distortion is from my system. I admit it !

Practically speaking, I really want to know in numbers the amount of db attenuation a 15nf cap will give at 150khz using the 200 or 375 R i/v.

Thank you.

 

[georgehifi]

This is the way I see it correct me if I'm wrong.
If your 15nf (15000pf) goes to ground after the dacs output, the the dacs series output resistance is 1200ohm, this with the 15nf (15000pf) to ground after it, gives you a -3db @ 8846hz (8.8khz) which in my opinion is way to low, you won't have any highs.

Halve the cap 7.5nf (7500pf) and you get -3db @ 17692hz (17.6khz) this is still a bit to low.

Half it again to 3.75nf (3750pf) and you get -3db @35384hz (35.4khz)
Cheers George

 

[Bernhard]

No, R_I/V is parallel to the DAC impedance.

I use the equivalent of 250 ohm per chip.

I'm not at home, please wait a bit for the right value.

The simple RC filter has only 6 dB / oct, so if the corner freq is 20k, @ 150k it is only -18dB.

 

[georgehifi]

Ok Bernhard if the series output impedance per channel of a single PCM56 is not 1200ohm, (which I think is what the data sheet says), are you saying to us that it is 250ohm? and how did you measure that?
I know you have the 1200ohm internal resistance drawn going to ground,
http://www.diyaudio.com/forums/showthread.php?postid=1807567#post1807567
but I have always believed that the output impedance of any amp, voltage or current output is it's series output resistance resistance, not what a DMM can measure to ground?
Cheers George

 

[aparatusonitus]

This is a Wadia Digimaster 64.4, I take it that means it oversamples 4 times.

No, 64 is for 64x oversampling (16X4, linear interpolation) and 4 stands for 4 dacs.

 

[aparatusonitus]

I run my 2 x 4 PCM56P K's into the Transimpedace amp attached, it is hard to find out the input impedance of it, the designer says close to zero

Input impedance is Re for a given transistor and current, my guess is less than 10R.

 

[Gabdx1]

I finished the passive output for pcm56 and the result:

- Loss of deep bass,
- Frequency 1 kHz - 8kHz get a huge gain
- Imaging and detail is loss
- Total absence of realism in the sound

It's a disaster and I don't recommend it to anyone.

 

[georgehifi]

Input impedance is Re for a given transistor and current, my guess is less than 10R.

I have a feeling it's more like 25ohm, as when I placed a 100ohm in series with the input of the Transimpedance I/V amp, it droped the output at the output rca's by 2v, from 9v to 7v.
Was the 100ohm in series that I instaled acting as a voltage divider with the input impedance of the Transimpedance I/V amp?
Cheers George

 

[Bernhard]

I finished the passive output for pcm56 and the result:

- Loss of deep bass,
- Frequency 1 kHz - 8kHz get a huge gain
- Imaging and detail is loss
- Total absence of realism in the sound

It's a disaster and I don't recommend it to anyone.

You must be either a troll or unable to put the simplest things together.