New look at tube analog for TDA1541

[regal]

What is the latest opinions for a TDA1541 tube stage?

The only scheme I believe I have seen that doesn't violate the 25mv voltage compliance is the Laszlo's PP:

http://www.diyaudio.com/forums/attac...ac_tubeout.jpg

Unforunately it actually outputs 4Vrms which is too much. But it seems to be a design that could be perferected and more popular. Why has this stage met such obsucrity?

As far as SE, there of course a lot who use an I/V resistor that violate the 25mV compliance. I am not there yet, my own experience with PCM1704 and PCM63K is that this is a big compromise. I have tried a low ohm passive I/V + Cinimag mic step-up + 6n6p with a PCM63K, but the results weren't good, ie a bigger I/V resistor replacing the tranny was much better.


Has anyone built a TDA1541 output stage with a 6 ohm I/V + 6922+6922 parafeed output?

I mean it has become common for 6922 MMC preamps which output roughly 4mV rms. Why not do roughly the same thing but with the 25mV I/V resistor TDA1541a DAC? It seems so obvious but I have never seen any reports or trials, if the 6922 has a noise equivilancy of 300 ohms, I would think with the SSHV that SNR could be fine. Heck the MMC preamps have to deal with the RIAA eq, which wouldn't be required with a DAC, so the results should be better.

Anyone think this is worth persuing? The alternative is of course a zero NFB discrete I/V, but these typically have almost has much 2H distortion as a tube and filtering just isn't as elagant as iron.

[regal]

This is what I am trying to decide on building vs using a 1:10 cinemag input transformer with the passive IV and only one tube gain stage. I guess I would prefer the two gain stage idea simply because I don't have to come up with a scheme to kill the 2ma DC offset to be able to use the step-up transformers.

I know a lot of builders (as in overwhelming majority) just ignore the max 25mV voltage compliance spec when using a tube stage after the TDA1541A, but if one is going to all the trouble to DEM reclock, I2S input, etc, it seems crazy to dump the TDA1541A output into a 50 ohm I/V resistor (I know any design is a compromise though.)

[QSerraTico_Tico]

Try a common grid stage.
Edit: seems not a good idea. Input impedance way too high....
common-grid I to V converter

[regal]

Originally Posted by QSerraTico_Tico

Try a common grid stage.
Edit: seems not a good idea. Input impedance way too high....
common-grid I to V converter

The input impedance is 5 ohms (load seen by the 1541)! Much lower than any common/grounded grid scheme.

This is an order of magnitude lower than a typical tube DAC stage. Not saying it is unique, it is a very typical topology for mm preamps, just can't understand why its not used for DAC's? I mean it is such a common scheme to amplify low amplitude signals, yet I have never heard of anyone trying it with a DAC, why ??



.

[analog_sa]

I don't like the two stage circuit much. Why don't you use a higher mu tube like a D3A, E280F, etc and a cap coupled output?

[regal]

Originally Posted by analog_sa

I don't like the two stage circuit much. Why don't you use a higher mu tube like a D3A, E280F, etc and a cap coupled output?

D3A is not even close for a single stage. There really aren't any single tubes with enough gain to accept a 25 mV input and output 2Vrms and have enough transductance to drive an amp or preamp with the typical 50k input imp.

You would have to find a tube with about gain around 200 and an Rp under 5k, ain't going to happen. I think there is an oddball russian tube that has a triode section of mu=100 Rp~5k and another section is a tetrode, but my understanding is it is inconsistant and not worth the trouble. And the output output would still only be 1Vrms.

I original didn't like the 2 tube solution either, but again most preamps are like this, and I would think there would be some 2H cancellation. Could also drop the cap between stages and DC couple with a higher B+.

But I am not sure this is better than using a stepup transformer, I've played with these 1:10 Cinemag mic input transformers but there just seems to be issues with impedance matching, dealing with a -30dBU signal, primary inductance, Dac i-out output impedance, etc. Most all are designed for either a 50ohm output impedance source or a 200 one (mics.) Multibit Dac chips usually have 1k to 2k output impedance which I think is the issue with the transformer IV trials.

[QSerraTico_Tico]

Originally Posted by regal

The input impedance is 5 ohms (load seen by the 1541)! Much lower than any common/grounded grid scheme.

This is an order of magnitude lower than a typical tube DAC stage. Not saying it is unique, it is a very typical topology for mm preamps, just can't understand why its not used for DAC's? I mean it is such a common scheme to amplify low amplitude signals, yet I have never heard of anyone trying it with a DAC, why ??



.

Yes sure it's 5-Ohm. But it's passive IV-conversion, which I don't like (flat, non-dynamic sound)
I wonder what Zanden is using in their 5000 DAC. They use a single 7308 tube. All other tubes in their unit seems to be rectifiers.
Anyway for common grid stages we must move to FETs. See Nelson Pass, Ecdesigns... Common gate properly named.

[chanmix51]

Hi,

You can also try the super common grid valve I/V converter with an input impedance < 1Ω (theorically <0,1Ω but I can not measure it). Output is 2Vp with 2kΩ output impedance. The good point is you do not need a pre-amp, just maybe a filter to suppress the 44,1kHz noise at output.

greg

[regal]

Originally Posted by QSerraTico_Tico

Yes sure it's 5-Ohm. But it's passive IV-conversion, which I don't like (flat, non-dynamic sound)
I wonder what Zanden is using in their 5000 DAC. They use a single 7308 tube. All other tubes in their unit seems to be rectifiers.
Anyway for common grid stages we must move to FETs. See Nelson Pass, Ecdesigns... Common gate properly named.

I'm guess you don't like passive I-V because all you have heard are big resistances which takes the max voltage compliance on i-out above 25mV (max recommended per datasheet). With a small 5 ohm I-V resistor it is actially a lower impedance than what say a jfet would place on i-out.

I agree that a large resistance on i-out steals dynamics with multibit DAC's. I have heard this with a Jfet version of D1 which had an load of 50 ohms, gave the same sort of flatness that a 50 ohm passive I/V resistor did, That is why I am proposing this tube scheme.

[regal]

Originally Posted by chanmix51

Hi,

You can also try the super common grid valve I/V converter with an input impedance < 1Ω (theorically <0,1Ω but I can not measure it). Output is 2Vp with 2kΩ output impedance. The good point is you do not need a pre-amp, just maybe a filter to suppress the 44,1kHz noise at output.

greg

Greg, that is very interesting, always had an intuition that a pentode could be useful.

But this is designed around a TDA1543, I am trying to get my head around how I would input the TDA1541 Iout to the +3V at the cathode, I know the TDA1541 has 2mA DC offset but how do I adjust this super common gate input to match the TDA1541?