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Old 26th June 2005, 08:29 AM   #1
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Default valve i/v with 2.7volt vref

Howdy everyone,

I'm wanting to do a valve I to V converter, and would like to work with chips with current out swinging around the vref (aka vcom2) point and not ground.

I.e the ad1853 and pcm1738

Attached is a circuit that I have used with chips whos current out swings around ground like the pcm63. This I to V converter is known as a Lohoff I to V, the idea comes from the TubeCAD program


I'm no tube guru, but for a dac with current out at vref and not ground, will I be able to eliminate the 2.7v vref voltage offset on the current outs by adjusting the bias on the grid aka "vbias"


OR do I have to float the whole valve circuit at the vref potential and transformer couple its output?, (probably after a buffer of some sort? )

I am sure I have seen this technique mentioned here before, but for the life of me canot find the post/thread

Regards,
Mark
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Old 28th June 2005, 10:41 AM   #2
jormajj is offline jormajj  Finland
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Hi,

I have no answer but I'm facing the same problem.
I use PCM1738 in my CDpro2M player project
and want to add tube drivers.
How can tube I/V be used when output currents are referenced to VCOM2 ?
My alternative plan is OPA627 I/V and tube LPFs...

br. Jorma
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Old 28th June 2005, 02:48 PM   #3
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Mark,

Are you running balanced?

You could take a sample from each cathode of your grounded grid stage using say a 1Mohm resistor and sum into a servo. The +ve input of the servo goes to VCOM. The output of the servo goes back to the grids. It would be be a good idea to fix bias the grids somewhere near their operating point (cathodes at VCOM). The servo will then pull the grids up or down around the fixed bias to trim the cathodes to the VCOM reference. That way you can be sure at start up that your cathodes will be somewhere within the range of VCOM.

All the best.
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Old 28th June 2005, 04:13 PM   #4
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Vref has to be "quiet" --
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Old 29th June 2005, 09:53 PM   #5
AW is offline AW  New Zealand
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Mark,
>I'm no tube guru, but for a dac with current out at vref and not ground, will I be able to eliminate the 2.7v vref voltage offset on the current outs by adjusting the bias on the grid aka "vbias"<

By adjusting "Vbias" you will be able to set the input terminal to the circa 2.5V (my measurement on a PCM1738) that is coming out of the DAC.

It should work OK, and *maybe* even sound great, but it won't be very DC stable - single ended/non-differential tube DC amps drift all over the place - and to do it properly it would need some form of an opamp based servo to hold it all in check.

Allen
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Old 30th June 2005, 01:00 PM   #6
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Quote:
Originally posted by AW
Mark,
>I'm no tube guru, but for a dac with current out at vref and not ground, will I be able to eliminate the 2.7v vref voltage offset on the current outs by adjusting the bias on the grid aka "vbias"<

By adjusting "Vbias" you will be able to set the input terminal to the circa 2.5V (my measurement on a PCM1738) that is coming out of the DAC.

It should work OK, and *maybe* even sound great, but it won't be very DC stable - single ended/non-differential tube DC amps drift all over the place - and to do it properly it would need some form of an opamp based servo to hold it all in check.

Allen
Allen,

The bias stability of a common grid configuration on the DAC outputs was was covered comprehensively in post #3. A solution was also offered which addressed voltage compliance levels at the DAC output during startup, drift and tube replacement situations.

You may have missed it.

Regards, Craig.
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Old 30th June 2005, 10:16 PM   #7
AW is offline AW  New Zealand
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Craig,
I didn't miss it -just thought it extremely complex reply to someone who said he's no tube guru. I intended my reply to be easier to follow and (hopefully) exactly answered his question.

Allen
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Old 1st July 2005, 11:46 AM   #8
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Hi AW,

I thought that Mark was explaining his level of understanding and asking for an opinion of whether he was on the right track. I also believe Mark was looking for a complete solution. He mentioned a couple of options and I presented one which I believed would be a worthy performer.

I don't believe he was looking for a simple solution that "won't be very DC stable" and "drift all over the place". You did however mention op-amp based servo to "hold it all in check." which was already mentioned in post #3.

I thought you didn't read my post because you covered all the ground I had previously but presented a more pessimistic view of obtaining a solution. I believe for the sake of clarity it would be more helpful to refer and/or expand on what was already said than possibly confuse some readers by re-introducing what had been already introduced as something new and a less tidy solution to boot.

I see we do both believe that a servo is a solution for keeping the operating point of the cathodes at the VREF or VCOM level so it is in the voltage compliance range of the mentioned DAC chips.

To get back to Mark's solution - Mark I would recommend if possible to avoid any op-amp in the signal path. As brilliant as the OPA627 is it is not a contender when it comes to a properly implemented discrete transistor or vacuum tube circuit.

If you want any details for the servo or tube I/V feel free to ask. I am sure that many here can also answer that question for you or offer other solutions.
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Old 1st July 2005, 01:57 PM   #9
philbyx is offline philbyx  France
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Hello

I work on a similar project, but for ad1865, then I have a reference at 0v, but the problem is the same.

My first simulation :
1ma give 15volts at the cathode follower output.

I made a regulation for the grid ref, tl431, zener and Diode for stability. This fixes the grid/cathode voltage.

The 2.5 volts ref of the tl431 is used for the css ref too (transistor base).

Only simulation for the moment.

Philippe
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Old 1st July 2005, 02:46 PM   #10
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Hello Craig and AW,

I am indeed looking for a complete solution, and would dearly like to try an all valve I/V (or one with transistor servo), and I too believe an opamp is not the ultimate answer. I have tried discrete transistor I/V's and would like to round out my experience with an all valve version if possible


Dac's having current out points referenced around non ground potential does make it a little trickier. Do you have any suggestions for a method of keeping the cathodes at vref? or any other solutions. I have not based my selection of the Lohoff circuit on any solid facts or arguments beyond the Tube Cad software's suggestions.


Regards,
Mark
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