while looking at OP-amps that I could possibly use to replace the ones in my dac (ad846, used as voltage buffer and not I/V conversion, that part is handled by a resistor) I looked at the ad846 datasheet and noticed it is a "Current-Feedback Op Amp".
how do they work? can you put any OP-amp like the typical voltage feedback ones in its place and it'll be fine (like newclassD, burson, sparkos etc...)? or am I supposed to stay with current feedback amplifiers?
if I need to stay with current feedback op amps, are there some I can simply replace without any modification to the circuit? my dac is not DIY so I would like to upgrade it with making as little changes to the circuity as possible.
looking at op amps like the ad8001,ad811 and lme49713, can you simply replace the ad846 with one of them without any special circuit changes?
thank you in advance!! (the dac I own is the muse model two plus)
how do they work? can you put any OP-amp like the typical voltage feedback ones in its place and it'll be fine (like newclassD, burson, sparkos etc...)? or am I supposed to stay with current feedback amplifiers?
if I need to stay with current feedback op amps, are there some I can simply replace without any modification to the circuit? my dac is not DIY so I would like to upgrade it with making as little changes to the circuity as possible.
looking at op amps like the ad8001,ad811 and lme49713, can you simply replace the ad846 with one of them without any special circuit changes?
thank you in advance!! (the dac I own is the muse model two plus)
As the configuration is a voltage buffer you aren't constrained to only use CFB opamps, voltage feedback opamps will work fine so long as they're unity gain stable. You'll probably need to add a cap across the feedback resistor though or instability is a possibility. The cap will depend on the particular opamp but would likely be in the region of 220pF.
Having said this, are you certain the configuration is purely a unity gain non-inverting one? AD846 can be configured as a transimpedance amp with the I/V resistor connected to pin5 and if that's how its used on the Muse DAC you have, VFB opamps certainly cannot be used. See this post : https://www.diyaudio.com/community/...a-90s-dac-muse-model-two.417596/#post-7792344
Having said this, are you certain the configuration is purely a unity gain non-inverting one? AD846 can be configured as a transimpedance amp with the I/V resistor connected to pin5 and if that's how its used on the Muse DAC you have, VFB opamps certainly cannot be used. See this post : https://www.diyaudio.com/community/...a-90s-dac-muse-model-two.417596/#post-7792344
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this is sourced from a very detailed review of the muse model two:
"By now it should come as no surprise that Muse eschewed the traditional approach for the final gain stage. The Muse Model Two uses a patent-pending design by Kevin Halverson called the “Uni-Block Differential” (UBD) gain stage. In essence, the UBD stage is intended to emulate many of the desirable qualities of a good output transformer. It allows any bipolar power- supply device (whether integrated or discrete, and operating as either a follower or a gain stage) to simultaneously generate both phases of a differential signal. Like a transformer, the differential volt age present at the output is fixed, but the absolute values in relationship to ground are allowed to float.
One attribute of the UBD stage is that it provides complete protection against direct shorts of either terminal. If either leg is grounded, the other simply becomes referenced to the grounded connection. Known as cross—coupling, this concept has only been employed in the past through the use of two separate op-amps operating in inverting and non- inverting configurations. Unfortunately, since these two different configurations have varying characteristics, common- mode rejection can be compromised at the input of a following preamp.
Since the UBD design accomplishes the benefits of cross-coupling with a single gain element, the characteristics of each phase of the signal are identical (just like a good transformer), and common- mode rejection is thus excellent. The UBD gain stage can drive a filly balanced and single—ended signal simultaneously, so the Model Two has both RCA unbalanced outputs and balanced XLR analog connections. (I must admit I don’t fully understand how the UBD accomplishes this cross-coupling with a single gain element. After Muse releases the patent information, I hope to share the particulars with you in a Follow-Up.)
Muse chose the outstanding Analog Devices AD846 op-amp for the heart of this final gain stage. While conventional high-end wisdom of even just a few years ago insisted on avoiding monolithic op amps in favor of custom discrete amplifiers, that’s no longer always true. A number of modern op-amps have incredible performance that even the best analog designers will be hard-pressed to duplicate, much less exceed. Devices such as the AD846, AD797, AD811, and Burr-Brown OPA-206 can produce stellar results in the right hands. The topology in which these op-amps are used makes all the difference in getting the most out of their potential. To fully exploit the 846, a very-low-impedance environment is required, and this is where the UBD (along with the unusual power-supply configuration used in the Model Two) comes into play."
also, I have found this article on analog device's website, could be similar to whats utilized in the muse, even tho it seems like the article shows VFA used:
https://www.analog.com/en/resources...lock-family-simplifies-interface-designs.html
"By now it should come as no surprise that Muse eschewed the traditional approach for the final gain stage. The Muse Model Two uses a patent-pending design by Kevin Halverson called the “Uni-Block Differential” (UBD) gain stage. In essence, the UBD stage is intended to emulate many of the desirable qualities of a good output transformer. It allows any bipolar power- supply device (whether integrated or discrete, and operating as either a follower or a gain stage) to simultaneously generate both phases of a differential signal. Like a transformer, the differential volt age present at the output is fixed, but the absolute values in relationship to ground are allowed to float.
One attribute of the UBD stage is that it provides complete protection against direct shorts of either terminal. If either leg is grounded, the other simply becomes referenced to the grounded connection. Known as cross—coupling, this concept has only been employed in the past through the use of two separate op-amps operating in inverting and non- inverting configurations. Unfortunately, since these two different configurations have varying characteristics, common- mode rejection can be compromised at the input of a following preamp.
Since the UBD design accomplishes the benefits of cross-coupling with a single gain element, the characteristics of each phase of the signal are identical (just like a good transformer), and common- mode rejection is thus excellent. The UBD gain stage can drive a filly balanced and single—ended signal simultaneously, so the Model Two has both RCA unbalanced outputs and balanced XLR analog connections. (I must admit I don’t fully understand how the UBD accomplishes this cross-coupling with a single gain element. After Muse releases the patent information, I hope to share the particulars with you in a Follow-Up.)
Muse chose the outstanding Analog Devices AD846 op-amp for the heart of this final gain stage. While conventional high-end wisdom of even just a few years ago insisted on avoiding monolithic op amps in favor of custom discrete amplifiers, that’s no longer always true. A number of modern op-amps have incredible performance that even the best analog designers will be hard-pressed to duplicate, much less exceed. Devices such as the AD846, AD797, AD811, and Burr-Brown OPA-206 can produce stellar results in the right hands. The topology in which these op-amps are used makes all the difference in getting the most out of their potential. To fully exploit the 846, a very-low-impedance environment is required, and this is where the UBD (along with the unusual power-supply configuration used in the Model Two) comes into play."
also, I have found this article on analog device's website, could be similar to whats utilized in the muse, even tho it seems like the article shows VFA used:
https://www.analog.com/en/resources...lock-family-simplifies-interface-designs.html
Who knows? some sort of pointless differential buffer which could be done with a " single element" fully differential op amp.
Would just blow up if shorted so they floated the power supply. Likely a plain old buffer shown in plenty of opamp design guides
with floating power supply.
AD846 looks pretty impressive for the time, it is Video Driver 80 MHz with 450V/us slew rate
As usual National Semi would beat anything they got. They had 200 MHz 4100V/us slew rate video drivers.
Problem with replacing the AD846 is likely uncompensated to reach 80 MHz and they have a specific pin for external compensation.
So highly likely was used for the magic nonsense buffer. So replacing with something else could be make it unstable.
Since it is likely the design has a external compensation circuit unique to that opamp.