Need opinions about this tube circuit

[gaetan8888]

Hello

Maby I've post it in the wrong section, since there is no reply, I ask my question here in the tube section.

I have done a I/V tube ouput for a Nos TDA1541A dac, I use 12AX7 tube because I have many of this tube.

Any suggestions about my schematic, is my circuit are ok ?


http://www.diyaudio.com/forums/attachment.php?postid=1876699&stamp=1247210460

Thank you

Bye

Gaetan

 

[DougL]

Three questions,

How much gain do you need?
What is the input impedance of the stage you are trying to drive?
Or what is the output impedance are you targeting?
How long a cable / how much capacitance are you driving?

12AX7 may or may not be appropriate, depending on your answers.

HTH

Doug

 

[nigelwright7557]

I would AC couple the input.

I think the cathode 200R resistors are way too low for a 12ax7.
I use 1k2 with 12au7 that will drive much more current.

 

[Wavebourn]

Yes, about 1K is needed. Also, I would use much smaller value of a feedback cap (like 30K) connecting it after a coupling cap from the upper cathode, then one more coupling cap to output.

Also, consider grounding grid operation feeding input signal to cathode, all above apply.

 

[SY]

Why an SRPP in this application? The voltage swings are low and the load isn't particularly reactive. There's a distortion penalty to using this topology.

 

[gaetan8888]

Three questions,

How much gain do you need?
What is the input impedance of the stage you are trying to drive?
Or what is the output impedance are you targeting?
How long a cable / how much capacitance are you driving?

12AX7 may or may not be appropriate, depending on your answers.

HTH

Doug

Hello Doug

I need between 40 to 50 times gain.

The input impedance of the stage I am trying to drive are arround 35k ohm.

This tube stage will be in the same case than the power amp, so the cable will be a max of 4 inch long and will be video type cable.

Thank

Bye

Gaetan

 

[gaetan8888]

Why an SRPP in this application? The voltage swings are low and the load isn't particularly reactive. There's a distortion penalty to using this topology.

Hello SY

I did chose SRPP in this application because it was seem more simple to do. But it can be another type of circuit.

Thank

Bye

Gaetan

 

[gaetan8888]

Hello

There is this other tube circuit, but I suspect that the resistors values are not right.

Suggestions ??

Thank

Bye

Gaetan

 

[nigelwright7557]

If you look up the datasheets for 12AX7 and 12AU7 then they give examples of circuits with resistor values.

If you want advice on circuits then you need to tell us what the signal comes from and where it goes to so we can work out input and output impedances.

 

[Wavebourn]

Try this one. Choose a gate stopper accordingly to the device you use.

 

[Wavebourn]

Oops... Drawing errors corrected.

 

[gaetan8888]

If you look up the datasheets for 12AX7 and 12AU7 then they give examples of circuits with resistor values.

If you want advice on circuits then you need to tell us what the signal comes from and where it goes to so we can work out input and output impedances.

Hello

The signal comes from a Nos TDA1541A dac and go straight to the power amp input.

Thank

BYe

Gaetan

 

[DougL]

I calculated Values using Tube Cad

It looks like it would work.

This is just FYI.

Doug

 

[gaetan8888]

Try this one. Choose a gate stopper accordingly to the device you use.

Hello Wavebourn

I do not have any fet in my parts bin, so maby the circuit calculated by Doug would be easyer for me to do.

Thank a lot

Bye

GAetan

 

[gaetan8888]

I calculated Values using Tube Cad

It looks like it would work.

This is just FYI.

Doug

Hello Doug

I have redone my circuit according to your Tube Cad values.

Can I still have a 150k (or higher value) NFB resistor, to have just a bit of NFB to reduce the distortion, ?

I presume that I need to take account of the Rin 10K resistor for the calculation of L and C in the input 20khz low pass filter ?

I include the image of the new circuit according to your Tube Cad values.

Thank very much

Bye

GAetan

 

[Wavebourn]

I do not have any fet in my parts bin, so maby the circuit calculated by Doug would be easyer for me to do.

Sure it would be easier.

Good luck!

 

[Wavebourn]

Can I still have a 150k (or higher value) NFB resistor, to have just a bit of NFB to reduce the distortion, ?

150K / 200 Ohm would be like a mosquito bite of an elephant for a stage with initial gain of 30.

Speaking of "just a bit of NFB", you would reduce inaudible distortions adding audible ones such a way.
Here is the topic that explains it in details:

http://www.diyaudio.com/forums/showthread.php?postid=1874886#post1874886

 

[gaetan8888]

150K / 200 Ohm would be like a mosquito bite of an elephant for a stage with initial gain of 30.

Speaking of "just a bit of NFB", you would reduce inaudible distortions adding audible ones such a way.
Here is the topic that explains it in details:

http://www.diyaudio.com/forums/showthread.php?postid=1874886#post1874886

Hello Wavebourn

Do you know (Approximately) how much distortion this circuit would have without NFB ?

Thank you

Bye

Gaetan

 

[jerluwoo]

Unless I'm mistaken, I thought plate to cathode would be positive feedback. I also believe that the AX7 won't be happy trying to drive 35k load even as a cathode follower.

Have a look here http://www.fortunecity.com/rivendell/xentar/1179/theory/vasfda/vasfda.html for info on using this dac with a tube output.

 

[Wavebourn]

Hello Wavebourn

Do you know (Approximately) how much distortion this circuit would have without NFB ?

Less that with NFB as drawn, Jerluwoo is right. Amount of distortions will depend on the signal level, you may estimate it calculating transfer function values for different input voltages from curves and a load line. Curves for 12AX7 were published many times in many places. Just google them. Measure by a ruler, put results in a table, then calculate how far they are from straight linear function.

Unless I'm mistaken, I thought plate to cathode would be positive feedback. I also believe that the AX7 won't be happy trying to drive 35k load even as a cathode follower.

Have a look here http://www.fortunecity.com/rivendell/xentar/1179/theory/vasfda/vasfda.html for info on using this dac with a tube output.