AD1955 I/V stage/tube version

[SunRa]

Ok, thanks... I'll search in the morning, maybe that's a place to start..

[Cauhtemoc]

Originally posted by aparatusonitus


Where are you gonna find a opamp that can work (preferable at unity gain) as error amplifier at Jung SupeReg on 2V39-2V9?

Take a look at my regulator. It should offer performance similar to the Jung regulator and be able to work at voltages as low as 2.5V.

[aparatusonitus]

I mean an monolith opamp, not discrete one...but someone can play with this one as well

http://www.diyaudio.com/forums/attac...amp=1139437025

[SunRa]

Cauhtemoc, aparatusonitus,

Thanks for your reply's. In fact I was allready watching cauhtemoc's thread. It's for sure a viable option and a nice source of informations.

For now I have bigger troubles trying to figure out if I can make a transimpedance amp without an op-amp. Valve operation in this configuration is out of discusion I guess as I don't think it's possible to mentain in the same time the advantages (different signal and power loops, aka insulation between the psu and the signal, and the overall simplicity and ingeniosity of the circuit I referd to)

I guess the other option is to use an op-amp or discrete (if it's whorthy) based I/V converter followed by a tube stage to take care of the amplification. However for know I'm in a dead end, as the transimedance articles where to revelatory I guess . (thanks analog_sa)

[David Davenport]

Hi SunRa,

It is really pretty simple - don't make it more complex than it is. Start with a basic design and then refine it after the basic is working.

There are three parts to it: the current source and bias, the I/V, and the output stage. A simple current source is just a resistor. The bias point can be a voltage divider from an available voltage. The I/V is a resistor. The output stage can be something like that in my article. You need to bias the input to the output stage to allow for the 2.9V DAC bias. So your cathode bias could be something like 5V or 6V to cover the bias that the tube needs above the DAC bias.

Then you can get fancy with constant current sources and regulated bias supplies.

Dave

[analog_sa]

Originally posted by David Davenport


The I/V is a resistor.

Ok. But how big?

[SunRa]

Ok. But how big?

I supose you ask this because the output of the dac needs to see an impendance as close to 0 possible. Mr.'s Davenport active stage has the advantage of transformer coupling and so the impendance seen by the dac could be reduced by the ratio of the transformer.

This way the resistor can be big enough to provide the voltage needed by the tube amplifier.

I am somehow skeptical however because I don't fully understand the way the transimpedance amplifier works, and if satisfies some requirements of the DAC I'm not aware about. I'll just have to take a more in-depth look at those articles before asking some more questins

Mr. Davenport, thank you for the reply,

There are three parts to it: the current source and bias, the I/V, and the output stage. A simple current source is just a resistor. The bias point can be a voltage divider from an available voltage. The I/V is a resistor. The output stage can be something like that in my article.

I thought the same at the begining of this thread but then it occured to me that the datasheet topology is not just a simple resistor with an op-amp amplifier. Scaning the articles regarding the transimpedance amp made me to reconsider this design.

You need to bias the input to the output stage to allow for the 2.9V DAC bias. So your cathode bias could be something like 5V or 6V to cover the bias that the tube needs above the DAC bias

Ok, so asuming I select a bias resistor to provide 5-6V (depending on the operation point I choose for the triodes), how should I bias to the 2.9Vof the DAC? By conecting the ground of the output stage with the 2.9V from say a voltage divider? This would be at the other end of the cathode resistor? Or at the common point of the I/V resistors? I guess the second is better as the cathode resistor should be conected to the power ground. It's a little tricky with this ground conections...

Then you can get fancy with constant current sources and regulated bias supplies.

Well, I've mentioned the jung regulators just to show that I want to take care of some of these details. The first posts where directed to them although they are not my primary concern. However the contributions regarding them where welcomed as they atentioned me to some dificulties I may encounter.

[David Davenport]

Hi SunRa,

Let me first talk about the I/V. No matter what kind of fancy names that are given to output stages, the current-to-voltage conversion is always done with a resistor somewhere in the circuit. Sometimes it is hard to see which resistor is doing the I/V, but in the case of a simple resistor directly on the DAC chip or connected through a transformer, the I/V resistor is obvious.

How big is the resistor? Let's take an example of a simple tube amplifier with the I/V resistor on the grid. Start at the output of the amplifier and work backwards. First decide the maximum output level you want from the output stage, for example, 2VRMS. Second, determine the gain of the amplifier. This gives the voltage needed on the input of the amplifier. So for example, if the amplifier has a gain of ten (20dB) and you wanted 2VRMS out, you would need 0.2VRMS in. From here it is easier to figure in peak-to-peak, so that would be 0.57V p-p. Now go to the specifications for the DAC chip and determine what the maximum peak-to-peak current is. This is generally something like the 0dBFS current minis an idle, or no signal, current. Let’s say it was 10mA, to pick a round number. You now have the voltage and current to calculate the resistor value. 570mV / 10mA = 57 Ohms. This is your starting point for figuring the resistance. It never works out perfectly because of variations in the circuit so start with a 56 Ohm resistor and build the amplifier. Then use a test CD with a 0dBFS signal to see what the final output from the amplifier is. Let’s say it was 1.8VRMS instead of the 2VRMS that you wanted. Your final resistor value is this ratio times the resistor used. 2VRMS / 1.8VRMS x 56 = 62 Ohms.

Now, how to connect the bias to the amplifier. Definitely connect the cathode resistor to power ground. Then connect the bottom of the I/V resistor (common point of the two I/V resistors) to the bias. Then connect the reference for the bias voltage to the same ground that the cathode resistor is connected to. See the article here: http://www.raleighaudio.com/installation.htm Keeping in mind that the DAC chip there keeps a DC level on the I/V resistors and the REF is at ground. Your case will be different where the REF will be at 2.9V and the connection to ground will be from the reference (ground) of the 2.9V supply. It is tricky, so think it out ahead of time.

Dave

[audiodesign]

see this good doc.
http://www.ti.com/lit/an/slaa399/slaa399.pdf

[audiodesign]

DAC preview of AD1955 with passive I/V

USB DAC 2 - Hi-Resolution System