192KHz 24bit DAC No oversampling and No digital filter

[ThorstenL]

Hi,

To me there is no point in adding a cathode follower after a SS I/V.
...
IME if we want tubes we want an anode follower gain stage.

Then use a DAC that allows sensible output levels with passive I/U Conversion and a low noise tube gain stage.

Ciao T

 

[regal]

Hi,



Then use a DAC that allows sensible output levels with passive I/U Conversion and a low noise tube gain stage.

Ciao T

But that would mean the end to this project (see the thread title.) I don't think we should give up so easily.

With a low impedance load the PCM1704 can sound very good. Possibly a couple transistors between the DAC and the tube and this project could yield fantastic results.

 

[ThorstenL]

Hi,

But that would mean the end to this project (see the thread title.)

I think we need to define what we want.

As the PCM1704 only offers 19 Bit or so resolution, the whole 24bit malarkey is irrelevant anyway.

With a low impedance load the PCM1704 can sound very good. Possibly a couple transistors between the DAC and the tube and this project could yield fantastic results.

If the project yields "phantastique" results with these transistors in front of the tube stage, then it will yield the same with the tube removed and the gain cranked up.

Ciao T

 

[regal]

Hi,



I think we need to define what we want.

As the PCM1704 only offers 19 Bit or so resolution, the whole 24bit malarkey is irrelevant anyway.



If the project yields "phantastique" results with these transistors in front of the tube stage, then it will yield the same with the tube removed and the gain cranked up.

Ciao T

Agreed but the advantage here is the PCM1704 can accept a 24 bit input and sound very good if setup right. With the better 20 bit DAC's you have to dither the 24 bit music first before sending it to the DAC or it will just truncate and have even worse resolution. And if we are going to dither our material might as well stick with the 1865.

I believe the "purpose" of the DAC is to be able to accept 24/96 material convenently in NOS fashion. Obviously 192khz is not possible with an NOS DAC, and I there is little music in this rediculous format anyhow.

 

[Telstar]

Agreed but the advantage here is the PCM1704 can accept a 24 bit input and sound very good if setup right. With the better 20 bit DAC's you have to dither the 24 bit music first before sending it to the DAC or it will just truncate and have even worse resolution. And if we are going to dither our material might as well stick with the 1865.

I believe the "purpose" of the DAC is to be able to accept 24/96 material convenently in NOS fashion. Obviously 192khz is not possible with an NOS DAC, and I there is little music in this rediculous format anyhow.

Exactly my point.

 

[Eric Juaneda]

...Obviously 192khz is not possible with an NOS DAC...

Can you explain what you say?

 

[Telstar]

I quoted all, but I also didnt understand the last sentence (and i dont agree with it).
The very few stuff that is available at 176.4k or 192k is all but ridiculous.

 

[regal]

Can you explain what you say?

The only DAC chips I know of that support 192khz are S-D where oversampling is intregrated to the chip, you can't have a digital filterless S-D 192khz DAC as far as I know.

 

[Spartacus]

The only DAC chips I know of that support 192khz are S-D where oversampling is intregrated to the chip, you can't have a digital filterless S-D 192khz DAC as far as I know.

The 1704 is designed to work after an 8x digital filter = 352.8KHz. It may well work faster still - I'm pretty sure it will support 16x oversampling, but I haven't looked at the datasheet in a while. In other words, it'll support 192KHz fine.

 

[regal]

The 1704 is designed to work after an 8x digital filter = 352.8KHz. It may well work faster still - I'm pretty sure it will support 16x oversampling, but I haven't looked at the datasheet in a while. In other words, it'll support 192KHz fine.

It was always sold as a 24/96 DAC, maybe because it was marketed with the DF1704. I think you are right.

But we still have to come up with a good tube stage. Trust me I worked with the PCM1704 passive I/V and a 47 ohm resistor is just too much it will be mediocre, the PCM63K was much better at handling higher I/V resistors.

 

[Telstar]

The 1704 is designed to work after an 8x digital filter = 352.8KHz. It may well work faster still - I'm pretty sure it will support 16x oversampling, but I haven't looked at the datasheet in a while. In other words, it'll support 192KHz fine.

It can work up to 768khz.

 

[Eric Juaneda]

Hello regal,

(PCM1704) was always sold as a 24/96 DAC, maybe because it was marketed with the DF1704. I think you are right...

The PCM1704 is sold with DF1704 and DF1706.
DF1706 Stereo, 24-Bit, 192kHz 8X Oversampling Digital Interpolation Filter.

...But we still have to come up with a good tube stage. Trust me I worked with the PCM1704 passive I/V and a 47 ohm resistor is just too much it will be mediocre, the PCM63K was much better at handling higher I/V resistors.

It seems that working conditions you describe are not optimum for PCM1704. The PCM1704 is not designed to work with passive resistor. (see #92). Changing working conditions will probably changes results.

Many thanks regal for this return experience.

 

[regal]

Hello regal,


Many thanks regal for this return experience.

I don't have first hand experience but very reputable folks in the digital DIY community have said the PCM1704 comes onto its own with a load equal to or lower than 10 ohms. That would be a good design goal.

 

[Nazar_lv]

I still doubt that the numbers are valid. I think 90% of all Non-Os DAC's out there are based on the same three or four schematics and they are all universally quite bad

trans -tube- trans

And most use TDA1543, which is even worse.

no-no, 1543 it is maybe the worsted dac ever made

Not sure I agree. If I use a simple "flat response" analog stage after the DAC for OS and compensate the Sinc Rolloff for Non-Os and then use the same DAC with optimum powersupplies that keep impedances low enough at all sample rates (including OS) I cannot see this happening.

NOS produce large sample to sample difference and aliases - analog stage should be veery linear
OS - much EMI - very careful layout and proper filtering

I still have a few K-Grade PCM63 stashed. I found the PCM63 the close second after the TDA1541. I did not like the PCM1702 and 1704 anywhere nearly as much. Why, oh why did Philips never make a 20 Bit DAC on the TDA1541 Technology?

agree

The Signal/Noise ratio and/or dynamic range of the PCM1704 are around 120dB A-Weighted, typical

s\n at full scale in 1704 is only 105-110db

For the same reason the 1541 is unusable for correctly dithered 16/44, the DNL spec is just lame.

DNL 1541 is perfect, for say as in 1862

 

[tritosine]

DNL 1541 is perfect, for say as in 1862

These are multibit chips, DNL wont come for free. Read the 1862 datasheet, they used special packahing , and thats because of nonlinearities arise because plastic packaging damages the trimming procedure. Thats why a lot of industrial DAC were ceramic , they pre-heated the die before packaging. Philips wanted to use autocalibration.

 

[ThorstenL]

Hi,

trans -tube- trans
no-no, 1543 it is maybe the worsted dac ever made

Well, tarnation and blimey. I totally agree, but tell that the "dddac" and "dacend" crowd.

NOS produce large sample to sample difference and aliases - analog stage should be veery linear

Yes. Hence I prefer open loop circuits and I ALWAYS use variations on SRPP (including hybrids and my personal favourite, the Gomez Super Totem Pole Stage), which internally linearise the gainstage.

OS - much EMI - very careful layout and proper filtering

Between a rock and a hard place, or as they used to say in old Greece - 'twixt scylla and charybdis of for the americans lacking classical education damned if you do and damned if you don't.

agree s\n at full scale in 1704 is only 105-110db
DNL 1541 is perfect, for say as in 1862

Actually, SNR for the AD1865 is very good too. Who remembers the old Denon CD-Players that used 18 Bit DAC's for the top 18 Bits and a discrete DAC for the lower 2 Bit?

It means we can make a AD1865 into a 20 Bit DAC, Non-OS to 352KHz, use with PDM100 for HDCD in 2 * OS and funk the bottom 4 bit in a 24 Bit word (no way to make a tube stage to do 20Bit anyway, the tube noise will dither the truncation anyway).

I see AD1865 is on lifetime buy now... Take a hint.

Ciao T

 

[tritosine]

http://www.intersil.com/data/fn/fn3113.pdf

 

[QSerraTico_Tico]

Overdriven transformer?

Hi,



Yes, PCM1704 is mostly active I/V only. The problem is that even the best open loop active I/U converters seem to give up something to the simple passive approach. So the PCM63 sounds better because it allows a better sounding analog stage.

As for those who wish to make a high gain, low noise gainstage - I think a D3a is the only game in town and even then you will struggle hugely.

Here is one case where using a low value resistor with the right kind of stepup transformer brings dividends.

If we are boring and use a E180F as gainstage (or C3g, WE 417A etc) we have a gain of around 40, at fairly high Zout. An output transformer with a really good 4:1 stepdown would be a good choice, probably a parallel-feed, superpermalloy cored variant (funny, I just recently ordered some such up for a linestage project). We have remaining a gain of around 10.

If use the Cinemag CMQ-3440 MC Stepup we have a gain of 36, so our resistor can be around 6.8 Ohm. The effective impedance at the tube grid is now 8.8KOhm, high enough that tubes own noise should be removed from the equation.

So what remains is the noise of our DAC and I/U resistors. At 6.8 Ohm we get a Johnson noise of -146dBV.

Our signal is 1.2mA * 6.8 Ohm = 8mV peak or 5.7mV RMS or -44dBV.

So we have a S/N ratio of -102dB remaining after we have used an essentially noiseless Tubestage and a low value I/U conversion resistor.

Hence we have made a 17 Bit DAC at the output of the analog stage, despite all this effort.

Might as well stick to the AD1865 if you want tubes and > 16 Bit resolution. At least the AD1865 tolerates passive I/V very well and we can at least get the full dynamic range it offers (with 220R I/V we can get 119dB SND at the I/U conversion with 150mV out opening up a wide range of tubes to use...

The PCM63 is even better due to +/-2mA Iout, as is the (16 Bit only, sadly) TDA1541.

Ciao T

Would the Cinemag 3440 not be overdriven?