Makes sense. My tube pre-amp Calypso has 40k input impedance. I did not check, but I suspect many preamps have a similar impedance.
Is it me only?, but if we refers to Andy's shematic few posts above, what the purpose of the 100k to ground resistor in the I/V stage at the input, or maybe they are not for I/V stage but post voltage output DAC chips which are an option of PCM56 & AD1865 !
Or is that 100K the modelled virtual immpedance to ground to say it is open circuitry ? (litterally no passive resistance there) ?
As we talk about an output current dac ic here but if the voltage option were chosen, then the shematic is not clear, to me at least.
Again, a reading of the paper of the Thermonic valve tubes llinked in the blog spot from miro, first link of first post of this thread iss worthing a read. We deal with current and transimpedance stage at the output of those dac chips in this thread.
with perhaps better results than the ad811 cause the HA can version, it is controversal so needs to be POC-ed VS the AD811. On of the fav op amp according Bob Pease, but he doesn't talkk of transimpedance stage with it when he said so.
You can reconfigure it in Quartus easily, just click Assigments - Pin Planner and here you can reconfigure 😉
It's a standard value so the circuit will work without oscillation, a typical value used on the input of valve stages. If you add 100R resistor in parallel between positive and ground as the I/V resistor then the combined resistance will be 99.9R which is virtually the same.
Thanks,
So what the purpose of the the two // resistors then, as the final value is 99.9R ?The purpose of the I/V resistor is btw it flows the most fastly to the ground, so the standalone single R is a better bet. As your a tube guy, I 'd try here an AB resistor. As youy know carbon comps are good for transcient purpose, whatever we deal with very small current here. I rather put at the output of the dac two fast piggy back diodes to ground to tame the overshoots of such dc current IC outputs,, will protect them at a second purpose (some has that inside the IC)
i can understand you put a carbon comp serie with the grid in this shematic for that osci damping purpose (and certainly 300 to 500R will do) but not the // to ground 100K here at current output of the DAC ic. I never seen yet aa shematic with that for dac current output ics.
I can be wrong, I am not at all a tube guy, I had hard time to make work a mu follower à la Moglia with J113 low fet in lieu of J350 for an E188CC I/V stage.
Also I would reduce the R at the output of the tube to ground and increase the capacitance of the cap whatever you keep the ratio good, tin foil caps migth be more neutral than PTFE cap and less expensive, but a matter of tones taste in relation with your whole hifi setup and your tastes of course.
hope that helps
So what the purpose of the the two // resistors then, as the final value is 99.9R ?The purpose of the I/V resistor is btw it flows the most fastly to the ground, so the standalone single R is a better bet. As your a tube guy, I 'd try here an AB resistor. As youy know carbon comps are good for transcient purpose, whatever we deal with very small current here. I rather put at the output of the dac two fast piggy back diodes to ground to tame the overshoots of such dc current IC outputs,, will protect them at a second purpose (some has that inside the IC)
i can understand you put a carbon comp serie with the grid in this shematic for that osci damping purpose (and certainly 300 to 500R will do) but not the // to ground 100K here at current output of the DAC ic. I never seen yet aa shematic with that for dac current output ics.
I can be wrong, I am not at all a tube guy, I had hard time to make work a mu follower à la Moglia with J113 low fet in lieu of J350 for an E188CC I/V stage.
Also I would reduce the R at the output of the tube to ground and increase the capacitance of the cap whatever you keep the ratio good, tin foil caps migth be more neutral than PTFE cap and less expensive, but a matter of tones taste in relation with your whole hifi setup and your tastes of course.
hope that helps
Last edited:
Hello Iggy! You are free to configure your tube stage in any way you like and with any components you prefer to use.
I won't change my schematic because it uses exactly the components I like to use and have tested over many years. Both the type of components and the values. The 100K on the input is so the valve stage can be tested and used without the I/V resistor. I'm listening to it now just as a gain stage and I like the sound very much. When you add the I/V resistor of whatever value you like, then you can either remove the 100K or simply leave it there. The 0.1uF FT-3 teflon cap is correct for an input resistor of 100K in the following stage, which is the desired input resistor for an 11K output impedance of the ECC40. I put 470K there just as a stop-gap. In practice it would be replaced by the 100K resistor of the following stage. I hope all this is clear!
I won't change my schematic because it uses exactly the components I like to use and have tested over many years. Both the type of components and the values. The 100K on the input is so the valve stage can be tested and used without the I/V resistor. I'm listening to it now just as a gain stage and I like the sound very much. When you add the I/V resistor of whatever value you like, then you can either remove the 100K or simply leave it there. The 0.1uF FT-3 teflon cap is correct for an input resistor of 100K in the following stage, which is the desired input resistor for an 11K output impedance of the ECC40. I put 470K there just as a stop-gap. In practice it would be replaced by the 100K resistor of the following stage. I hope all this is clear!
"Hello Iggy! You are free to configure your tube stage in any way you like and with any components you prefer to use"
WOW ! That's clever input really ! Okay sorry twice for my bad advices, it seems not to be so bad as you asked me a lot in PM more than basic things you hadn't the balls to ask here.
So if you won't change your shematic, why you asked ?
I think you understood a 100K to ground at the current output dac ic to ground is an I/V stage by itsself. "I" being the current, "V" the volts. Good luck with the noise with 100K w/o anything else for a DAC output IC if I cope to what you wrote above. I am not sure you had never tested it with a current output dac ic.
Sorry, I didn't know you had worked a lot of dac outputs stages. It was not a critic, btw. I think your reaction is a little, hummm.... strong !
I thougth it was helping to say the current will flow the least impedance way and it is certainly helpfull it doesn't see anyelse path that create a node and a potential problem (pun intended) in that topology, but you know better. If you want to build a noise factory that will not cope to the bits precision of the dac ic, I am tempted now to answer : free to you.
Okay, bye; Yes perfectly clear to me, both clever condosceension "answers" here in less than one month by two britanics. Better to quite indeed.
Good luck with your DAC, I sincerly hope you to succeed in you dac journey. I think better for me to quite that thread that became a sect of very clever guys. Or soon a mod will think I am he agressor because I'm reacting to such inputs when I try to help with not so bad tips !
WOW ! That's clever input really ! Okay sorry twice for my bad advices, it seems not to be so bad as you asked me a lot in PM more than basic things you hadn't the balls to ask here.
So if you won't change your shematic, why you asked ?
I think you understood a 100K to ground at the current output dac ic to ground is an I/V stage by itsself. "I" being the current, "V" the volts. Good luck with the noise with 100K w/o anything else for a DAC output IC if I cope to what you wrote above. I am not sure you had never tested it with a current output dac ic.
Sorry, I didn't know you had worked a lot of dac outputs stages. It was not a critic, btw. I think your reaction is a little, hummm.... strong !
I thougth it was helping to say the current will flow the least impedance way and it is certainly helpfull it doesn't see anyelse path that create a node and a potential problem (pun intended) in that topology, but you know better. If you want to build a noise factory that will not cope to the bits precision of the dac ic, I am tempted now to answer : free to you.
Okay, bye; Yes perfectly clear to me, both clever condosceension "answers" here in less than one month by two britanics. Better to quite indeed.
Good luck with your DAC, I sincerly hope you to succeed in you dac journey. I think better for me to quite that thread that became a sect of very clever guys. Or soon a mod will think I am he agressor because I'm reacting to such inputs when I try to help with not so bad tips !
Last edited:
Iggy - I had no intention of being critical! Everyone uses their own choice of resistors and capacitors, that's just a fact. I meant quite sincerely that your choice of components is personal and up to you, like mine is. I didn't want to go into resistor/capacitor comparisons on this thread since it's a bit off the subject. I'm sorry if this wasn't clear.
You know a lot more than me about digital technology and DACs, and you have been very helpful to me. I hope this will continue! I have a lot to learn. I don't know what you think of La Perfide Albion but as you know I love France and the French and previously lived in Paris and my house in Ardeche for a few years.
You know a lot more than me about digital technology and DACs, and you have been very helpful to me. I hope this will continue! I have a lot to learn. I don't know what you think of La Perfide Albion but as you know I love France and the French and previously lived in Paris and my house in Ardeche for a few years.
Last edited:
I don't talk of components choice , but electrical values related to the ic you use and the layout ! I am not talking of the "sound" of components but electrical behavior. The inputs about carbon comps and tin foil was a plus, but it was not the purpose of my post. I even don't understand you focus on that !
So cope to the values we are talking to in the conversation : 100K, 1k to grid, 0.1 uF output at DC (okay I understand you don't use low volt input now at the next stage). I perfectly know what is two parrallel resistors; impedance adaptation, etc.
https://electrodac.blogspot.com/p/dac-ad1862-almost-tht-i2s-input-nos-r.html :
References:
So cope to the values we are talking to in the conversation : 100K, 1k to grid, 0.1 uF output at DC (okay I understand you don't use low volt input now at the next stage). I perfectly know what is two parrallel resistors; impedance adaptation, etc.
https://electrodac.blogspot.com/p/dac-ad1862-almost-tht-i2s-input-nos-r.html :
References:
- #5186 Thermionic Valve Analogue Stages for Digital Audio
- PSU comparison Battle of power supply regulators with listening tests
Last edited:
When I have my DAC stage working I may replace the 100K on the input with the chosen I/V resistor. That depends on where I put the I/V resistor. I have the tube stage in another box which I will link with a short connector to the DAC box. If I put the I/V resistor right at the DAC output I would probably leave the 100K in place as a safety measure. It makes very little difference really.
While I agree that Andy has provided his tube I/V suggestions without setting them in proper context / showing how exactly he would use them, and did not explain e.g. that the 100k he has / keeps in it, because for him it is a building block he wants to be able to test isolated, there is no need to get rude.
Yes that's correct. I should have included the I/V resistor in the schematic. I have my tube stage in another box, exactly as the schematic, which I will link with a short connector. I was thinking of putting the I/V resistor in the box with the DAC in, on the DAC output pins. I may eventually put everything in one box but while I'm experimenting it's helpful to have modular units I can switch around. I've been listening to this ECC40 stage for a little while now as a line stage and I really like it a lot. Very curious to put it on the end of a DAC build.
Last edited:
I am trying to use the ad797 for the I/U stage of the pcm1704 DAC. I cannot make it work properly - the sound is louder compared to other opamps and there is a background noise (oscillations?). Is there something I am missing with this opamp? Thanks
Probably self-oscillations. A schematic would also help, maybe the problem is just in the decoupling. Is the AD797 from a reliable source?
Thanks for your answer. The ad797 is from TME, I just plug it instead of other opamps - and add a 2nf between pins 2 and 3 as said in the datasheet.