a contender for the AD825/AD797?

jackinnj · 26th March 2004, 01:59 AM

just got a blurb from TI about the OPA725 -- take a look at the THD specs on TI's site -- and it's relatively inexpensive -- and some in this chip family have a very useful "Disable" function.
http://focus.ti.com/docs/prod/folders/print/opa725.html

Calimero · 26th August 2004, 01:11 PM

In the search for improvement of the opa627BP within the I/V section of my Ack!DAC, I have looked at the datasheet of this chip. The reason for trying it as replacement for the 627BP, which IMO performed the best of some opamap I tried amongst them AD8210, AD8065, OPA134, LM8161 (didn't survive, because they can't drive the capacitive load (0.33uF output caps).

Also 627 performed the best, I was less satisfied about the bass control, grainiest in the mids and clarity of the hights could be better. It is one thing to put ones finger on the sore spot, it is another to heal the problem.

Nevertheless I decided to keep the ack!DAC foundation, batery supply, CS8414-TDA1545, and opamp I/V unchanged and go for another opamp, that could better handle the capacitive-load, I choice the opa725 + BUF634 combo.

725 because of the fast settling-time and low-noise and BUF634 to reduce the capacitive-load on the performance of the I/V-section.
As the opa725 between V+ anvd V- is maximaly 12V and the reference voltage of the TDA1545 is setup as: 3.3V. I have decided to run the 725 from a +12V supply only. I used SOIC-packaged versions as I could solder those on the PCBB and use the 8DIP-foorprint for the BUF634 (after cutting some PCB-tracks).
I also removed the biasing Fet's, as the 634 has its own biasing scheme (I use it in BW-mode)

The result was worth the work, I can gladly say. Most of the grainiest in the mids has been gone, the hights has opened up and very much improved is the bass control. (to be expected with the 634, however I couldn't hear a significant improvement in bass in combination with the opa627BP)

I still have to work on decoupling the 634. I think of using quality 0.1nF for V+ and V1 and a 100uF PanasonicFC between V+ and V-.
Any comments of the experts?

Is there anyone who has tried this combo? If so in what way?

Henk

jcx · 26th August 2004, 03:02 PM

opa725 data sheet shows a 1/f noise corner ~ 50 KHz, the 1KHz noise ~= 20nV/sqrt(Hz) - maybe bearable with line level input but not generally useful for audio

26th March 2004, 01:59 AM	#1
jackinnj diyAudio Member Join Date: Apr 2002 Location: Llanddewi Brefi, NJ	a contender for the AD825/AD797? just got a blurb from TI about the OPA725 -- take a look at the THD specs on TI's site -- and it's relatively inexpensive -- and some in this chip family have a very useful "Disable" function. http://focus.ti.com/docs/prod/folders/print/opa725.html

26th August 2004, 01:11 PM	#2
Calimero Banned Join Date: Aug 2003 Location: A farm on the prairie (protcted from foxes)	opa725 as i/V converter In the search for improvement of the opa627BP within the I/V section of my Ack!DAC, I have looked at the datasheet of this chip. The reason for trying it as replacement for the 627BP, which IMO performed the best of some opamap I tried amongst them AD8210, AD8065, OPA134, LM8161 (didn't survive, because they can't drive the capacitive load (0.33uF output caps). Also 627 performed the best, I was less satisfied about the bass control, grainiest in the mids and clarity of the hights could be better. It is one thing to put ones finger on the sore spot, it is another to heal the problem. Nevertheless I decided to keep the ack!DAC foundation, batery supply, CS8414-TDA1545, and opamp I/V unchanged and go for another opamp, that could better handle the capacitive-load, I choice the opa725 + BUF634 combo. 725 because of the fast settling-time and low-noise and BUF634 to reduce the capacitive-load on the performance of the I/V-section. As the opa725 between V+ anvd V- is maximaly 12V and the reference voltage of the TDA1545 is setup as: 3.3V. I have decided to run the 725 from a +12V supply only. I used SOIC-packaged versions as I could solder those on the PCBB and use the 8DIP-foorprint for the BUF634 (after cutting some PCB-tracks). I also removed the biasing Fet's, as the 634 has its own biasing scheme (I use it in BW-mode) The result was worth the work, I can gladly say. Most of the grainiest in the mids has been gone, the hights has opened up and very much improved is the bass control. (to be expected with the 634, however I couldn't hear a significant improvement in bass in combination with the opa627BP) I still have to work on decoupling the 634. I think of using quality 0.1nF for V+ and V1 and a 100uF PanasonicFC between V+ and V-. Any comments of the experts? Is there anyone who has tried this combo? If so in what way? Henk

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26th August 2004, 03:02 PM	#3
jcx diyAudio Member Join Date: Feb 2003 Location: ..	opa725 data sheet shows a 1/f noise corner ~ 50 KHz, the 1KHz noise ~= 20nV/sqrt(Hz) - maybe bearable with line level input but not generally useful for audio

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