Hello, this is my first post on DIYaudio in the few months I follow the very intersting topics down here. I want to share the successfull experiment we have made with a valve based «super common grid» I/V converter for a TDA1543.
The idea has slowly emerged from a work with Yves07 and Totof both members of the french forum audiyofan.
The following image illustrates a normal common grid circuit:
The I1 current source represents a TDA analog output. The input impedance must NOT exceed 10Ω to stay within the data sheet specifications of noise and distortion. Here, the input impedance is the cathode's current source which has a very high impedance and the anode load. The anode load is in serie with the internal resistance of the tube and is seen divided by µ+1 which is roughly equivalent to 1/Gm. If we choose a very high Gm tube we would have an input impedance between 30 and 100Ω ... 10 times to high.
The idea is to create a feedback to force the valve's cathod to stay there AND NOT MOVE by a µV !
In this schema, as soon as the cathode moves from a µV, the mosfet gate also does. It amplifies and inverts the change and send it to the valve's grid that also amplifies it. The anode load of 2.2kΩ is seen as 1/(Gmtube * GmMosfet).
In the schema above, the voltage at input varies up to 70µVpp with a 2mApp input which represents an input impedance of 0.04Ω from 20Hz to over than 100kHz. Even OpAmp can not achieve this because their open loop gain greatly decreases with the frenquency. As the DAC current still flows through anode load, the output voltage is 4,4Vpp which means: no preamp ! The output impedance is roughly equal to the anode load: 2.2kΩ (which is ok to me).
You can achieve a full tube circuit replacing the DN3545 with a pentode or a high gain / high transconductance triode, the russian 6F12P seems the best candidate for this purpose. Totof has a working version which he seems to prefer over his TDA1541+OpAmp.
A friend and I bought a cheap TDA1543 board with passive I/V conversion we hacked to use our converter, I will not step back 😀
You can see here my noisy power supply as the converter is managing a 1kHz signal. All signal harmonics are below 90dB. The stereo scene is very large and clear, there are lot of details I have never heard before. I will change my power supply soon 😉
Enjoy !
Chanmix
The idea has slowly emerged from a work with Yves07 and Totof both members of the french forum audiyofan.
The following image illustrates a normal common grid circuit:
An externally hosted image should be here but it was not working when we last tested it.
The I1 current source represents a TDA analog output. The input impedance must NOT exceed 10Ω to stay within the data sheet specifications of noise and distortion. Here, the input impedance is the cathode's current source which has a very high impedance and the anode load. The anode load is in serie with the internal resistance of the tube and is seen divided by µ+1 which is roughly equivalent to 1/Gm. If we choose a very high Gm tube we would have an input impedance between 30 and 100Ω ... 10 times to high.
The idea is to create a feedback to force the valve's cathod to stay there AND NOT MOVE by a µV !
An externally hosted image should be here but it was not working when we last tested it.
In this schema, as soon as the cathode moves from a µV, the mosfet gate also does. It amplifies and inverts the change and send it to the valve's grid that also amplifies it. The anode load of 2.2kΩ is seen as 1/(Gmtube * GmMosfet).
In the schema above, the voltage at input varies up to 70µVpp with a 2mApp input which represents an input impedance of 0.04Ω from 20Hz to over than 100kHz. Even OpAmp can not achieve this because their open loop gain greatly decreases with the frenquency. As the DAC current still flows through anode load, the output voltage is 4,4Vpp which means: no preamp ! The output impedance is roughly equal to the anode load: 2.2kΩ (which is ok to me).
You can achieve a full tube circuit replacing the DN3545 with a pentode or a high gain / high transconductance triode, the russian 6F12P seems the best candidate for this purpose. Totof has a working version which he seems to prefer over his TDA1541+OpAmp.
A friend and I bought a cheap TDA1543 board with passive I/V conversion we hacked to use our converter, I will not step back 😀
An externally hosted image should be here but it was not working when we last tested it.
You can see here my noisy power supply as the converter is managing a 1kHz signal. All signal harmonics are below 90dB. The stereo scene is very large and clear, there are lot of details I have never heard before. I will change my power supply soon 😉
Enjoy !
Chanmix
Here is a better schematic with the mosfet version and the full valve version:
Note Totof's version is equiped with a notch filter at the output to block the 44.1kHz noise created by the DAC. You can also use it with the mosfet version.
I have written a blog post about this circuit with some hints about the power suppy.
greg
Note Totof's version is equiped with a notch filter at the output to block the 44.1kHz noise created by the DAC. You can also use it with the mosfet version.
I have written a blog post about this circuit with some hints about the power suppy.
greg
Hello,
Here is the 1.1 version of the converter :
changelog :
– R3 changed from 2,2kΩ to 2,7kΩ
– R5 changed from 24kΩ to 27kΩ
– HT changed from 190V to 220V DC
– C3 changed from 100nF to 220nF
– Added C10 1µF MK
The result is beyond my best hopes, better bass response, better contrast between instruments, better presence. That's the version I am going to put in a box. The input impedance as seen by the DAC is 0,1Ω, output : 2,5Vp
I have updated my blog with this schematic and the power supply part.
greg
Here is the 1.1 version of the converter :
An externally hosted image should be here but it was not working when we last tested it.
changelog :
– R3 changed from 2,2kΩ to 2,7kΩ
– R5 changed from 24kΩ to 27kΩ
– HT changed from 190V to 220V DC
– C3 changed from 100nF to 220nF
– Added C10 1µF MK
The result is beyond my best hopes, better bass response, better contrast between instruments, better presence. That's the version I am going to put in a box. The input impedance as seen by the DAC is 0,1Ω, output : 2,5Vp
I have updated my blog with this schematic and the power supply part.
greg
Hello oshifis,
C4 with the input impedance creates a low pass filter. As the input impedance is very low (rougly 0,1Ω) only signals at frequency over more than 1MHz are bypassed to GND. This is what we found to ovoid the whole circuit to oscillate at several MHz.
I tried with a 150nF at first and that wasn't enough. The circuit was resonnating at 7MHz with 30Vpp on the pentode's grid. This capacitor is soldered against the 1kΩ gate resistor of the mosfet.
Greg
C4 with the input impedance creates a low pass filter. As the input impedance is very low (rougly 0,1Ω) only signals at frequency over more than 1MHz are bypassed to GND. This is what we found to ovoid the whole circuit to oscillate at several MHz.
I tried with a 150nF at first and that wasn't enough. The circuit was resonnating at 7MHz with 30Vpp on the pentode's grid. This capacitor is soldered against the 1kΩ gate resistor of the mosfet.
Greg
Hello,
Using a 2,7kΩ resistor for I/V conversion led to too much gain and resulted into a very aggressive sound. Here is the version 1.2.
Changelog:
- R3 changed from 2,7kΩ to 2,5kΩ
- Measures on the previous version were wrong
greg
Using a 2,7kΩ resistor for I/V conversion led to too much gain and resulted into a very aggressive sound. Here is the version 1.2.
An externally hosted image should be here but it was not working when we last tested it.
Changelog:
- R3 changed from 2,7kΩ to 2,5kΩ
- Measures on the previous version were wrong
greg
Hi Greg, i'm very interested in your iteration of the I/V converter, but wanna try to simulate it with a jfet instead of the tube (obviously adjusting accordingly the supply voltages) and in LTSpice IV i haven't got a model of DN3545 nor i'm able to put it in the model library so, have you got a model with instruction for placing it in the library?
2nd question, i see that at input you got 3 volts, much higher than the recommended voltage in the datasheet for TDA1543, is this a problem? Anyway i need to try your circuit with an AD1865 that actually has a resistor as an I/V converter.
Thank you very much
2nd question, i see that at input you got 3 volts, much higher than the recommended voltage in the datasheet for TDA1543, is this a problem? Anyway i need to try your circuit with an AD1865 that actually has a resistor as an I/V converter.
Thank you very much
Last edited:
Hello Bonalux,
No problem with sharing our work, you will find the DN3545 LTSpice models and one of the LTSpice schematic I have been working with here.
As far as I know, the analog output pins must be polarized between 2V and Vdd - 1,8V (3,8V with my board) so I found out any value between 2,5V and 3,5V to be nice.
I am interested in your experiments with JFETs, would you please mind letting us know about your work ? 🙂
greg
No problem with sharing our work, you will find the DN3545 LTSpice models and one of the LTSpice schematic I have been working with here.
As far as I know, the analog output pins must be polarized between 2V and Vdd - 1,8V (3,8V with my board) so I found out any value between 2,5V and 3,5V to be nice.
I am interested in your experiments with JFETs, would you please mind letting us know about your work ? 🙂
greg
Hello,
After hours listening, I tried 2,5kΩ, 2,4kΩ, 2,3kΩ and I switched back the pentode's anode load to 2,2kΩ. It fixed a bass problem on lot of records and btw gave a more consistant stereo image.
Changelog:
- R3 changed from 2,5kΩ to 2,2kΩ
- HT changed from +220V to +210V
greg
After hours listening, I tried 2,5kΩ, 2,4kΩ, 2,3kΩ and I switched back the pentode's anode load to 2,2kΩ. It fixed a bass problem on lot of records and btw gave a more consistant stereo image.
An externally hosted image should be here but it was not working when we last tested it.
Changelog:
- R3 changed from 2,5kΩ to 2,2kΩ
- HT changed from +220V to +210V
greg
Hello,
very interesting work.
Have you never tried with AD1865 DAC?
Can you make me any suggestion to try with this dac?
Ciao
Guglielmo
Italy
very interesting work.
Have you never tried with AD1865 DAC?
Can you make me any suggestion to try with this dac?
Ciao
Guglielmo
Italy
Hello guglielmope,
As far as I could see, the AD1865 output current is ±1mA almost like the TDA1543 (±1,3mA). I think you can use the converter as is.
I'd be intersted to find a AD1865 cheap board to try but all I could spot are more than twice the price of a simple TDA1543 + DIR9001 on eBay ...
greg
As far as I could see, the AD1865 output current is ±1mA almost like the TDA1543 (±1,3mA). I think you can use the converter as is.
I'd be intersted to find a AD1865 cheap board to try but all I could spot are more than twice the price of a simple TDA1543 + DIR9001 on eBay ...
greg
But the Vref of the TDA1543 is 2,2V and I think that for AD1865 the Vref is near to 0V.
What do you think about?
Implement AD1865 in a dac is easy, I've looked at this link:
DAC End - the AD1865N-K with single ended vacuum output stage
Can I use in your circuit also D3A tube or 6z11p?
Ciao
Guglielmo
What do you think about?
Implement AD1865 in a dac is easy, I've looked at this link:
DAC End - the AD1865N-K with single ended vacuum output stage
Can I use in your circuit also D3A tube or 6z11p?
Ciao
Guglielmo
Ok, the idea is to set the DAC's ground to another potential on the I/V converter so Vin for the converter would be 0V for the DAC.
6Z11P with its steepness is a really great candidate. In this schematic the grid is fixed at +1,8V potential by the red led. The cathode will be above the grid potential by a value determined by the valve characteristics at approx 150V @ 30mA. This means the cathode (and Vin) will be 1,6V above the grid if you use 6Z11P. Add two diodes (approx 1,4V) between Q1's base and the 150kΩ R8 then set the DAC's ground between the diodes and R8. R6 should remain directly connected to Q1's base and D2's anode.
This solution implies DAC and I/V converter to have separate power supplies, which should be the case because of the HT 😉
Be careful about power supplies, they MUST be very silent including the heaters. I have a small noise (-70dB) at 100Hz because of the heaters circuit today, I have ordered a 47 000µF capacitor to filter it before a LM317.
Thx for the 6Z11P, I did not know this tube.
greg
6Z11P with its steepness is a really great candidate. In this schematic the grid is fixed at +1,8V potential by the red led. The cathode will be above the grid potential by a value determined by the valve characteristics at approx 150V @ 30mA. This means the cathode (and Vin) will be 1,6V above the grid if you use 6Z11P. Add two diodes (approx 1,4V) between Q1's base and the 150kΩ R8 then set the DAC's ground between the diodes and R8. R6 should remain directly connected to Q1's base and D2's anode.
This solution implies DAC and I/V converter to have separate power supplies, which should be the case because of the HT 😉
Be careful about power supplies, they MUST be very silent including the heaters. I have a small noise (-70dB) at 100Hz because of the heaters circuit today, I have ordered a 47 000µF capacitor to filter it before a LM317.
Thx for the 6Z11P, I did not know this tube.
greg
Hello,
Due to a problem on my server, all the URLs of the images below are wrong. Here are the latest versions of the converter and its power supply :
Sorry about that.
greg
Due to a problem on my server, all the URLs of the images below are wrong. Here are the latest versions of the converter and its power supply :
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
Sorry about that.
greg
Hello guglielmope
I just put your attention on the scheme of Chanmix
it's a real I/V converter with Vaccum
Your's is a passive I/V converter with a vaccum gain stage
The Zin from this kind of scheme is a bit different
have a look on the blog of Chanmix for this
you can find there a good deal to explain why i said that
I just put your attention on the scheme of Chanmix
it's a real I/V converter with Vaccum
Your's is a passive I/V converter with a vaccum gain stage
The Zin from this kind of scheme is a bit different
have a look on the blog of Chanmix for this
you can find there a good deal to explain why i said that
Hi!
What is the minimum input impedance of the amplifier must have to work with this converter?
And what are the analogues have DN3545?
What is the minimum input impedance of the amplifier must have to work with this converter?
And what are the analogues have DN3545?
Hi
For the impedance , you can see on the schematic R2
around 100K , normaly any kind of amp can follow
if you needed , adjust R2
For the DN3545 , try to find them
Look to Mouser for exemple Mouser Electronics - Electronic Component Distributor DN3545
Good luck
For the impedance , you can see on the schematic R2
around 100K , normaly any kind of amp can follow
if you needed , adjust R2
For the DN3545 , try to find them
Look to Mouser for exemple Mouser Electronics - Electronic Component Distributor DN3545
Good luck
Hi
My amplifier after the converter has 50kΩ input impedance and simulation shows reducing the output impedance of the converter (with a cathod follower by example) does not really improve the bandwidth so it does not worth the effort.
greg
My amplifier after the converter has 50kΩ input impedance and simulation shows reducing the output impedance of the converter (with a cathod follower by example) does not really improve the bandwidth so it does not worth the effort.
greg
chanmix51/ greg the pictures at xvm-13-41.ghst.net are not available. Can you upload them to this forum?
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