OPA1604: TIDU034 questions and possible upgrade solution

Hi Alex.

So there is nothing to be done in terms of performance? Or overall design?

How about moving U1B after U1D stage just to be able to put OPA1622 in his feedback loop? Does it make any real diffrence? 
Most buffers (standalone like LME49600 or typical opamps) are mostly used with global feedback, rather than standalone.

Could you tell me how much I degrade THD and noise floor when I replace OPA1604 to OPA1644? This chip is somewhat competetive in price but has higher voltage noise (which will affect dynamic range I supose).
For this circuit it will be better to stick with hi performance JFET chip or should I look for bipolar one with very low voltage noise? Like OPA1604, or 2x dual ICs like OPA1612 or something else?

Another question is how low can I go with potentiometer value to not overdrive opamp? ATM I'm using 4.7K so worst case resistance is ~ 1.2K.
I assume that opamp will be able to handle even lower value pot am I right?

How about placing 2 gang pot? Output of baxandall will be connected to another gang which will work as a typical (passive) pot with it centre (2 pin) connected to ground.
It will give me zero output noise when pot will be absolutely muted and will improve control law.

But how about overall noise of that configuration and THD perspective? If I'm going to use that kind of configuration (which is kinda simple to achive with this desing, it's only a matter of pot) should I put some buffer after passive control or maybe I should leave buffer as it is?

I know that balancing both inputs will cure DC offset, but I read some desing notes and books about common mode distortion I were thinking about possible solution to avoid it.

First, I have to offer a couple of corrections to my post above. 

1. My simulation of the output impedance of the volume control was in error. While there is some change in output impedance, it is very small. Output impedance simulates well below 100mΩ across the audio band. 

2. Adding a feedback resistor can improve common mode distortion, as long as your source impedance remains constant. However, in this application the most susceptible amplifier is U1C, which sees a varying source impedance depending on the position of P1. You could add a feedback resistor there, but it would really only be effective at a single volume setting. 

Buffers inside vs outside feedback:

A standalone buffer such as the LME49600 typically operates as an open-loop device, which can add considerable distortion. Placing such a device inside a larger control loop will reduce distortion as long as the loop has sufficient bandwidth.

The OPA1622 has low enough distortion as a standalone buffer that I wouldn't expect significant improvement by wrapping it in a larger loop. 

Substituting other parts: 

Either a FET or a bipolar op amp could be used here. The resistor values are small enough that the increased current noise of a bipolar shouldn’t contribute much to the overall noise of the circuit. A FET input op amp will have higher voltage noise, in general. Both OPA1604 and OPA1644 have very low common mode distortion, and all 3 options have very low THD+N in general.

Our models for OPA1602, OPA1622, OPA1644, and OPA1612 all model noise accurately. To optimize this design for noise, you might want to consider running noise simulations in TINA-TI. We do not model distortion, unfortunately.

Potentiometer values:

We have curves for THD+N vs frequency for different load resistances in the datasheets for all of these parts. Keep in mind that the amplifier may be able to drive a smaller resistance, but its distortion will increase while doing so.

Additionally, at lower resistance values, thermal effects in the potentiometer may need to be considered. It’s possible for these thermal effects to introduce distortion within the resistor itself. 

2-gang pot:

Placing a 2-gang pot here would work, but would negate the benefit of using a Baxandall configuration, as you would no longer have a linear-in-dB response for the volume control. If you don’t need that linear behavior, this design could be greatly simplified.

So I assume that I can go even lower than 4.7K, to something about 2.5K (~ 600R worst case scenario) am I right?

When I was talking about 2 gang pot I was refering to this picture :

If output impedance of U1D is very low then there is no need to use additional buffer, but due to low output impedance of U1D it's not a bad idea to move U1B stage right after U1D, overall gain would not be affected.

How about making input impedance equal of U1A stage? I will do it just in case :)

My version is not so diffrent, I just changed some values.

So here is the question.

If I've got 220R in series with signal, and then two resistors which parallel impedance is equal to 10kohm + I will include output resistor from DAC which is 50-100R (in most cases).

Then I assume I need to add all those values and it will give me my RB, right?

How about CS? I've got those TVS diode which is ~ 1.5nF and then a second LPF with 1nF.

I've got another question which is related to differential amplifier desing.
I where thinking about using it to feed enable pin of OPA1622, but it's a good idea to work with those with gain less than one?

I want to feed both inputs with my power supply voltages (symetrical 18V, or I can use some voltage dividers) to achieve output voltage less than 1V.

Hi Mateusz,

Again, you may be able to reduce the potentiometer value, but you'll likely be trading a small amount of thermal noise for some amount of distortion, due to increased loading on the op amps.

The dual-gang potentiometer circuit is a valid circuit, but does defeat the purpose of using the Baxandall circuit, since your volume control won't be linear-in-dB anymore. If having a linear-in-dB volume control is not important for your application, using a circuit like the following may be a better option.

OPA1612 and OPA1622 in this schematic could easily be replaced with another op amp if desired.

Regarding common mode distortion, John Caldwell goes into more detail in the following two articles: 

http://www.ti.com/lit/an/slyt595/slyt595.pdf 

https://e2e.ti.com/blogs_/archives/b/precisionhub/archive/2014/07/15/resistors-in-the-feedback-of-a-buffer-ask-why 

So long as your source impedance is low, it likely wont be an issue. In this case your source impedance would be the sum of your DAC output impedance and your series resistance. 

The TVS diode should be included if the input of this circuit connects to the outside world (in the application note, an RCA connector provided the input signal from elsewhere). If your DAC and volume control are on the same board, there shouldn't be a problem with leaving this out. 

Similarly, the input filtering in the application note was provided to block environmental EMIRR, you may need to add a reconstruction filter on the output of your DAC, but this may follow a different design than in the application note. Your DAC will likely provide a recommended application circuit in its datasheet. 

I am not sure I understand your question on differential amplifiers and the OPA1622. 

So if I understand rigth - if I've got 50-100R resistor in series with DAC output opamp and then followed by 220R in amplifier (due to LPF) then my RB (according to 1st picture in my thread) will be in range of 270-320R am I right?

How about CB then? Let's use values from your last picture.
I've got some free space, so adding two passive components to balance impedance of U1A stage is not a problem.

But it will match hi frequency impedance when I add those capacitance, am I right?
BTW should I use some resistor in series with U1D stage? Or it's not needed?
Traces to OPA1622 are pretty short anyway :)

If I move U1B stage after U1D (as a inverting buffer) will it lower my output impedance in comparision to U1D stage?