ADC0804 (to PANOS29)

[gogo]

Dear Panos29,
Do you have the actual schematic for ADC0804 (you use this chip in your Aleph-l preamp)? From the PCB-layout of your design I can`t explain to me how this think work...Completed schematic for your Aleph-l will be best explanation.

Thank you in advance!

[panos29]

the actual schematic of the volume controler is the same as the aleph-p V1 controler with the exception of the use of seperate supply for this circuit(simple 12V and 5V taken from a seperate winding and using 2 regulators in series), other difference is the use of small transistors (bc550c) instead of tip30. Moreover I added another relay (total 7 instead of 6) so if somebody wants can have 128 steps, although not recomended from me.

[gogo]

>Moreover I added another relay (total 7 instead of 6) so if >somebody wants can have 128 steps, although not recomended >from me.

But why you don`t recomend using 7 relays?

[stadams]

Panos29,

I understand that you are using the circuitry that was developed by Pass Labs however, have you given any consideration to using a sample and hold IC on the digital output of the ADC0804. This would latch the output of the 0804 to remedy the fluttering LSB. Or you could just upgrade to an ADC that has sample/hold built in.

Later,

[panos29]

Dear Gogo,
I am not using a seventh relay as the adc0804 is already a bit unstable with 6bit resolution, so with 7bit things will get even worst.

Dear stadams any help will be indeed very welcomed and needed! But, as am not at all a digital wizard and my digital experience is very small, so I would love to have some more info on the subject of sample and hold ic you recommend or/and an upgrade. Any suggestions?

Best regards,

Panos

[stadams]

How long are your analogue inputs? What is their proximity to the digital clock lines in the circuit? Read section 2.3.4 Noise, in the National Semiconductor spec sheet.

Later,

[till]

I made the ADC0804 circuit from the alephp 1.0 schematic on veroboard, one evenings work. I changed the powersupply, i use 7805 for digital and 7812 relais. It is stablre at 6 bit, i swicht a seventh bit via push/pull - switchfuntion of the volumepot manualy. I have no problem with this circuits 6bit+1bit manual resolution, but a microcontroller based circuit would have the advandtage of less noise when tuning the volume - if someone would post how to implement the right make before break algorithm.

[jackinnj]

there's no "good" reason that an ADC0804 should suffer from not being able to read 8 bits. I used these chips quite extensively at one time.

The reference voltage must be noise free -- consider using an LM4040 or LM4041 reference diode, bypassed with a 1uF mylar film capacitor. If you're too cheap to buy a reference diode use an LM317LZ (100ma) adjustable regulator, there's a little overhead, but post-regulating the VRef input will take the noise down to a few millivolts. The output of the LM317LZ should have a 1uF tantalum.

The analog ground and digital grounds should connect at only one point.

Decouple the inputs to the ADC with a 51 ohm resistor. You can also use a small RC network on the ADC inputs.

Place a 4.7uF tantalum capacitor on the V+ of the ADC.

Decouple the gain stages with a 10 ohm resistor to V+ and V-.