Steinmusic cdm12.1

[iansmith14]

Originally posted by mhuber
Hi Brownlow,

I have built a prototype CD player using this drive:



Works like a champ but I still prefer vinyl over polycarbonate

I use the DF1704/PCM1704 combo. The low jitter clock (thanks Guido!) is positioned next to the DACs and is feed back to the drive. The CDM12 drive electronics are on an large old style pcb which makes modification to the circuit simple.

I do the I/U conversion by use of a simple 22R resistor. The output stage consists of a step up transformer followed by a single tube stage and an 1:1 output transformer (picture shows an older version with different output stage). My $2000 commercial CD player has not spun a single CD since I first listened to this mess of parts.

Manfred

Hi Manfred,

What range of resistors did you try for I/V conversion and what was the affect?

Thanks,

Ian

[mhuber]

Hi Ian,

what range of resistors did you try for I/V conversion and what was the affect?

with my own unit I have only tried a value of 22R. This was because 22R gives a good match for the Sowter 8347 transformer.

I have built an other unit for a friend which uses a 47R resistor, a third order passive L-C-L Bessel low pass and a single tube stage for amplification. This one sounds quite different but It is a completely different setup, not just a changed resistor value.

In general there are two things to take into consideration:

The D/A converter's output is protected by a pair of diodes against electrostatic discharge etc. These diodes will start to conduct when the output reaches about 600mV. We have to keep the I/U converting resistor low enough to prevent this from happening. In my case the D/A converter outputs max. +-1.25mA of current and the resistor is 22R. This means that the peek output voltage will be 22R*1.25mA=27.5mV. I guess this is a save bet.

The other thing to consider is that I/U conversion done by a simple resistor will generate some 2nd order distortion products. This is because the output of the converter is not a real current source.

The output resistance of the converter chip at max output level is Vref/(2*Iout_max). In the case of the PCM1704 this is 5V/2.5mA=2000R. At min output level all switches in the R2R ladder network are open and there is no current flowing at all. The output resistance is indefinitely high. Let's see what this means for the outout current:

The min output current is Imin=-1.25mA. This is the current set by the
internal bias current source. There is no current through the R2R network in this case.

The max. output current is Vref divided by the sum of the resistances of the R2R network and the I/U conversion resistor less the bias current of 1.25mA:

Imax=Vref/(2000R+22R)-1.25mA=1,2228mA

This is 2,2% of deviation from the ideal of 1.25mA. The 2nd order distortion calculates as:

K2(%) = 100 * (Imax+Imin)/2/(Imax+Imin)

In our case:K2 = 100*(1.2228-1.25)/ 2/(1.2228+1.25) = 0,55%

This is quite a bit of distortion but it is purely 2nd. order and there is a near linear relationship of distortion and level. The calculation was done under the assumption of max output level. If your CD is recorded at an average level of -20dB then there will also be about 20dB less distortion. In our case this means 0,055% of K2. Good enough for me.

I hope this helps.

I also hope I didn't make too many errors

Regards
Manfred

[iansmith14]

Hi Manfred,

Thanks for the detailed response, do you have documentation for the CDM12?

Regards,

Ian

[mhuber]

Hi Ian,

Originally posted by iansmith14

Thanks for the detailed response, do you have documentation for the CDM12?

It came with some information on hook-up etc. but that wasn't much.
I guess Holger Stein will send you the schematic if you ask nicely.

Regards
Manfred

[Elso Kwak]

Hi Manfred,
May I ask what is the masterclock frequency the CDM12 uses?
Last Friday I saw a expensive Euro 4000 costing transport using the CDM12 with the crystal hung on the microprocessor.

[mhuber]

Originally posted by Elso Kwak
May I ask what is the masterclock frequency the CDM12 uses?

11.2896MHz

Last Friday I saw a expensive Euro 4000 costing transport using the CDM12 with the crystal hung on the microprocessor.

You know, the high end industry considers a 4000 Euro transport being a "mid price" product. There has to be some room for improvements

Manfred