Hi,
I am quite sure this has been discussed before but I did not find it using the search function.
I am going to bulid a DAC using CS8420, DF1704/06 and 2xPCM1704; followed by opamp for I/V conversion, passive HF filter and another opamp as output buffer.
The output buffer will be Analog Devices AD8610, but I am not sure which one would be best for I/V conversion. Maybe AD8065? Any suggestions are welcome.
Cheers
Christian.
I am quite sure this has been discussed before but I did not find it using the search function.
I am going to bulid a DAC using CS8420, DF1704/06 and 2xPCM1704; followed by opamp for I/V conversion, passive HF filter and another opamp as output buffer.
The output buffer will be Analog Devices AD8610, but I am not sure which one would be best for I/V conversion. Maybe AD8065? Any suggestions are welcome.
Cheers
Christian.
The following ICs - OPA627&BUF634 were used in DIY DAC The First SE Mk2. The integrated buffer may be ommitted when a cable is not an output load.
Digital signal which comes from transport should be regenerated with fast TTL gates.
What do you think about AD8610/AD8620? These amps have excellent capacitive load driving capability, much better than OPA627, whitout ringing. Supply voltage +/- 13 V but Peranders use 15 V whithout problems.
Digital signal which comes from transport should be regenerated with fast TTL gates.
What do you think about AD8610/AD8620? These amps have excellent capacitive load driving capability, much better than OPA627, whitout ringing. Supply voltage +/- 13 V but Peranders use 15 V whithout problems.
The best op-amp for an I-V converter is no op-amp. As Barrie Gilbert (one of the top analog designers of all time) explains, an op-amp is in reality just an integrator. This causes all kinds of problems with input stage overload when presented with the step output of a DAC. Anything (transformer, resistor, discrete, etc.) will sound better than an op-amp for an I-V converter.
Charles Hansen said:
Exactly, and I think this is why the AD825 with its higher slew-rate and shorter settling-time performs better than the OPA627 in this application.
Using a simple resistor, you would still be limited by the slew-rate of the transformer or buffer following it. However I would be interested in seeing measurements of simple resistor I/V with the PCM1704. Do you have a link?
I'm glad that you told them first....
Somehow, I suspect that if I had been the first to say it, it would have been dismissed as an extension of my contant ranting about that.
Back around.......'92 I think it was......Charlie sent me a copy of a magazine article titled "The current state of CD reproduction: who is doing it right?" Too bad that the name of the mag that it appeared in was not on the copy, and Charlie can't remember.
They said........the best examples (in I/V stages) were from small American companies that were run by engineers, that used transimpedance amps, and not op-amps for that purpose.
(For those who may be curious.........I was probably the first.....going back to '89.)
Jocko
Somehow, I suspect that if I had been the first to say it, it would have been dismissed as an extension of my contant ranting about that.
Back around.......'92 I think it was......Charlie sent me a copy of a magazine article titled "The current state of CD reproduction: who is doing it right?" Too bad that the name of the mag that it appeared in was not on the copy, and Charlie can't remember.
They said........the best examples (in I/V stages) were from small American companies that were run by engineers, that used transimpedance amps, and not op-amps for that purpose.
(For those who may be curious.........I was probably the first.....going back to '89.)
Jocko
ojg said:
This is kind of like making distinctions about what kind of dog poop you stepped in. It may be soft, or it may be hot, or it may be brown, but it's still dog poop that you stepped in. It's a lot better not to step in the dog poop in the first place (ie, not use an integrator to perform I-V conversion).
ojg said:
Both Muse and Resolution Audio have used the setup you describe for many years now. These have been tested in Stereophile magazine with full test results. Unfortunately neither of these seems to be in the on-line archives, so you will have to find a print copy.
Opa604
That's funny. must be something with my ears, hardhearing.....?
I am using the OPA604 with the TDA1543 NON-OS and it sounds very good to me.
Chris Owen is using the AD8610 in his ACk DACk.
Isn't an integrator just what we need to get rid of the steps (staircase)?
I did not like the AD8610 , tried two dozen opamps.
That's funny. must be something with my ears, hardhearing.....?
I am using the OPA604 with the TDA1543 NON-OS and it sounds very good to me.
Chris Owen is using the AD8610 in his ACk DACk.
Isn't an integrator just what we need to get rid of the steps (staircase)?
I did not like the AD8610 , tried two dozen opamps.
Elso,
Here is the link to an article by Walt Jung that touches on the problems of op-amps as I-V stages, although not in great detail:
http://www.elecdesign.com/Articles/ArticleID/7207/7207.html
The real information was in the articles by Barrie Gilbert that is referenced at the bottom of Jung's article, but these have unfortunately been deleted.
And no, you do not want to use an integrator as an I-V stage.
Have you ever tried a discrete I-V stage like the ones that Jocko has posted? If not, I think you will be pleasantly surprised at the sonic improvement they provide. Maybe even more important than a good clock....
Here is the link to an article by Walt Jung that touches on the problems of op-amps as I-V stages, although not in great detail:
http://www.elecdesign.com/Articles/ArticleID/7207/7207.html
The real information was in the articles by Barrie Gilbert that is referenced at the bottom of Jung's article, but these have unfortunately been deleted.
And no, you do not want to use an integrator as an I-V stage.
Have you ever tried a discrete I-V stage like the ones that Jocko has posted? If not, I think you will be pleasantly surprised at the sonic improvement they provide. Maybe even more important than a good clock....
I'll second that.....
Although the clock is important....especially with 8x, 24-bit stuff that I futz with.
And Charlie should know.......for years he demoed at CES with a cheap CD player that had my I/V inside.
And yes.....it did have a (yuk) TDA1541A and (double yuk) SAA7220 inside.
I hope the ex-employee who walked off with it is still enjoying it. Even though it is too good for him. Schmuck.
Jocko
Although the clock is important....especially with 8x, 24-bit stuff that I futz with.
And Charlie should know.......for years he demoed at CES with a cheap CD player that had my I/V inside.
And yes.....it did have a (yuk) TDA1541A and (double yuk) SAA7220 inside.
I hope the ex-employee who walked off with it is still enjoying it. Even though it is too good for him. Schmuck.
Jocko
Opa604
Thanks for the link. Yes I did try a common base amplifier and I was not impressed. Also tried a simple resitor as IV and was struck by the lack of dynamics and slam.....
Please note I am only using a non oversampling DAC TDA1543. So frequencies involved are not as high as with 8 times oversampling.
Charles,Charles Hansen said:Elso,
Have you ever tried a discrete I-V stage like the ones that Jocko has posted? If not, I think you will be pleasantly surprised at the sonic improvement they provide. Maybe even more important than a good clock....
Thanks for the link. Yes I did try a common base amplifier and I was not impressed. Also tried a simple resitor as IV and was struck by the lack of dynamics and slam.....
Please note I am only using a non oversampling DAC TDA1543. So frequencies involved are not as high as with 8 times oversampling.
Just to give an idea of the problems that can happen when using an integrator (ie, op-amp) as an I-V converter:
a) Looking at the data sheet for the TDA1545 (sorry, I don't have the DS for the '1541), we can see that the analog output settling time is 0.2 uS. This translates to a 5 MHz signal.
b) The open-loop gain of the OPA604 at 5 MHz is approximately 5x. (Remember, it's really an integrator.)
c) Let's assume that you have the op-amp set to give 2 V output. With a gain of 5x (at 5 MHz), that means that the "virtual ground" inverting input is not a virtual ground at all. Instead, expect to see up to 400 mV of signal present there.
Now we can see what kinds of problems can occur. Sending 400 mV into a differential bipolar input stage with no emitter degeneration would cause all kinds of problems. Some op-amps have emitter degeneration resistors or other techniques to reduce these problems, but even here 400 mV is a mighty big number to handle well. The part you chose has FET inputs (presumably not a coincidence!), which will reduce the problem but not eliminate it.
As far as the common base amplifier you tried goes, it's hard to say without seeing the schematic.
a) Looking at the data sheet for the TDA1545 (sorry, I don't have the DS for the '1541), we can see that the analog output settling time is 0.2 uS. This translates to a 5 MHz signal.
b) The open-loop gain of the OPA604 at 5 MHz is approximately 5x. (Remember, it's really an integrator.)
c) Let's assume that you have the op-amp set to give 2 V output. With a gain of 5x (at 5 MHz), that means that the "virtual ground" inverting input is not a virtual ground at all. Instead, expect to see up to 400 mV of signal present there.
Now we can see what kinds of problems can occur. Sending 400 mV into a differential bipolar input stage with no emitter degeneration would cause all kinds of problems. Some op-amps have emitter degeneration resistors or other techniques to reduce these problems, but even here 400 mV is a mighty big number to handle well. The part you chose has FET inputs (presumably not a coincidence!), which will reduce the problem but not eliminate it.
As far as the common base amplifier you tried goes, it's hard to say without seeing the schematic.
Elso -
You edited in the comment about trying a passive resistor while I was composing my reply. I'm not surprised that you found some problems with that approach. It's not too hard to understand what causes them. Sort of a case of "out of the frying pan and into the fire".
That's why guys like Jocko have worked hard to create circuits that don't suffer from these problems, and at the same time avoiding the problems caused by using op-amps. You really should try one of his circuits...
You edited in the comment about trying a passive resistor while I was composing my reply. I'm not surprised that you found some problems with that approach. It's not too hard to understand what causes them. Sort of a case of "out of the frying pan and into the fire".
That's why guys like Jocko have worked hard to create circuits that don't suffer from these problems, and at the same time avoiding the problems caused by using op-amps. You really should try one of his circuits...