[Cauhtemoc]

I'm thinking about building a beta22 with an integrated WM8740 DAC. How high is the input impedance of the beta22? I was hoping that I could connect the DAC chip directly to the input on the beta22. Is this possible or would I need an OP amp in between?

Does the beta22 have any protection against DC input, or would I need capacitors (e.g. Black Gates) here as well?

 

[FallenAngel]

Well, it has a 50K pot and 1M to ground, so I'm guessing the input impedance is 1M - quite high.

There is no DC protection as far as I know, but I would forget the BlackGates and put some film caps instead.

 

[Nebby]

What board are you going to use for the wolfson dac?

 

[Cauhtemoc]

Quote:

Originally Posted by FallenAngel Well, it has a 50K pot and 1M to ground, so I'm guessing the input impedance is 1M - quite high.

Correct me if I'm wrong here, but a 1M resistor to ground does mean that the input impedance 1M. In fact it lowers the input resistance compared to with no resistor.

I also had not planned to use a volume control, the WM8740 has a built in digital attenuator. It's easier for me to use that.

Quote:

Originally Posted by Nebby What board are you going to use for the wolfson dac?

I will create my own.

 

[Nebby]

very nice! do please keep us updated

 

[Cauhtemoc]

The data sheet for the WM8740 says that the load should have an impedance of at least 1k, ideally over 10k. I have no experience with the 2SK170 or 2SJ74, but doesn't JFETs in general have an input impedance in the megaohm range?

 

[amb]

Quote:

Originally Posted by Cauhtemoc /img/forum/go_quote.gif The data sheet for the WM8740 says that the load should have an impedance of at least 1k, ideally over 10k. I have no experience with the 2SK170 or 2SJ74, but doesn't JFETs in general have an input impedance in the megaohm range?

Yes, the input impedance of the β22 is essentially set by the value of R2 and that of the volume pot. If no volume pot is used, then for all practical purposes it's 1MΩ.

 

[Cauhtemoc]

Perfect, so I can feed the beta22 straight from the WM8740, without any OP amps anywhere to degrade the sound quality.

 

[chris719]

All modern audio DACs should have low-pass filtering on their outputs, keep this in mind. You might be able to get away without one but the measurements get worse and the chance that you make the amplifier oscillate go up.

 

[n_maher]

Quote:

Originally Posted by Cauhtemoc /img/forum/go_quote.gif Perfect, so I can feed the beta22 straight from the WM8740, without any OP amps anywhere to degrade the sound quality.

I think you'll still need to resolve the DC offset issue, right?

 

[FallenAngel]

Considering the size and cost of a B22, I would say film caps are definitely in order, might I suggest Audience AuriCap - I really liked their sound and the price isn't too bad

 

[Cauhtemoc]

Quote:

Originally Posted by chris719 All modern audio DACs should have low-pass filtering on their outputs, keep this in mind. You might be able to get away without one but the measurements get worse and the chance that you make the amplifier oscillate go up.

The WM8740 has an internal filter with a cutoff frequency of 195 kHz. I will probably add an external low-pass filter however.

Quote:

Originally Posted by n_maher I think you'll still need to resolve the DC offset issue, right?

Yup, I will add some film caps just to be sure.

 

[chris719]

If it was my design, I'd use an opamp as a differential to single ended converter with a low-pass filter. This way you can get rid of the caps and have a filter easily.

 

[Cauhtemoc]

Even the best OP amps degrade the sound quality. No OP amps means better sound quality.

 

[cotdt]

Quote:

Originally Posted by Cauhtemoc /img/forum/go_quote.gif Even the best OP amps degrade the sound quality. No OP amps means better sound quality.

as filters they seem to work just fine