I/V using opamps

[Trident]

On Wednesday Farnell ordain in the units required, and will try this weekend.

Thanks, and Merry Christmas

[Mooly]

Your welcome, lets hope its a good result.

And Merry Christmas.

[Trident]

Hi,

After studying the mounting understand that this is a Howland current pump, modified where the first operational amplifier acts as voltage and input impedance adapter (infinite), and the second operational amplifier makes the voltage at constant stream, with a ratio 1/100, with a low impedance Zo...... It maintains the output current constant, varying only the voltage, irrespective of variations in the load, minimizing signal distortion


In this way the op amp that is used is not important. What is important is the relationship of voltage and current in the circuit. The key point to remember is that an Op Amp will use all his considerable open-loop gain to maintain both their investment and non-inverting inputs in line with each other, ie, in the line voltage. If the positive input of the OpAmp see 1V, then its negative input 1V to do well. If the input signal is 0 V, then the output voltage of the circuit should also be above 0 V as the voltage only SRAM maintains two inputs to the same voltage accurately.

[Mooly]

I'm not convinced

I still think the current drive ability of the opamp is important although I can't see the rest of the circuit.

In your other diagram, (which I'm going to have to delete because its under copyright from Rod Elliots site, Elliott Sound Products - The Audio Pages (Main Index) you show the voltage output of the opamps "across" the low impedance bridge and so the opamps driving ability must come into the equation. If the second opamp output is at 10 volts then you can either say that 10 volts is across the low impedance of Rload and R3 or, if you say that the first opamp contributes to "raise" the voltage at the junction of Rload and R3 then that first opamp drives a low impedance load.

I think it matters given the impedance values in the circuit

[Trident]

Yes, you sure you're right

The operational capacity to supply current must be important in this circuit, especially because the designer chose originally an operational with capacity to deliver 50mA, which is beyond the capacity of 2604.

[Mooly]

So you are still going to try the TLE2072 ?

[Trident]

Hello,

Yes, I just ordered eight units in Farnell.

[Mooly]

I'd be interested to know your impressions when you have fitted them

[dirkwright]

Originally Posted by abraxalito

I agree George, opamps for I/V just do not deliver transparency. I have changed my view based on experience, because I used to go for them a few years ago. Try a passive filter (steep one, based on inductors and capacitors) before discrete I/V and prepare to be surprised (Listening to passive I/V, 7th order elliptic filter now, can't go back).

In a few places I read that it is important to isolate the DAC output from reactive loads like LCR filters. I do not know the details.

[Trident]

Hi Mooly

These two weeks, I've been testing, measuring ... and many drunkenness

These are my conclusions, and a final thought:

- The OPA 2604 has been for a long time, my first choice when it came to stage I / V in dual SMD circuits operational. But this project has been inappropriate ... Why?? ... I do not know.

- The TLE2072, did not fulfill my expectations, not bad, but not quite like it sonically, I can not say it was worse than the 2604 bid, just different... not for me.

- From there, I decided a radical change, I decided to ride in stage I / V the AD8620ARZ, and in the next step the AD8066, a total of 8 chips with 16 operational ... radically changed the sound better: great dynamic, balanced bass, and high frequencies present without fanfare .... a revelation.

At the same time, I was riding a TDA1541 dac of analogmetric, but using only the PCB, and original components, and of the highest quality that money can buy... no counterfeit Chinese components or factory rejects, made ​​in china.

All bought from Farnell to make sure they are not fakes. And instead of riding the OPA627 expected, I tried to make out with 6SN7 tubes (NOS Sylvania from 1954 year) in SRPP assembly, and the sound was more compact, not worse ... distinct, pure, and liked it, but it made me think ....

Why operational circuits used to color the sound?

My first option if it is a mounting DIP8, no SMD is the OPA627, but in this case the tube output is better.

I'm thinking the end of the tube DAC, and use it for the CD player, different sounds are the player's DAC with four BB PCM56, and the TDA1541 with tubes ... I suspect that everyone will be more appropriate for each type of music I listen to at all times.

The TDA1541, SAA7220 and Marantz players come from, and the CS8412 also original (not Chinese) I bought in little diode