> Hey when do I get off newbie moderation - it's rather annoying!
Took me 10 posts, even being very careful not to offend ANYBODY.
Apparently this forum has a sad history of unmoderated flame-wars. The current moderators worked hard to clean it up, for which we are very grateful.
However, I wonder about this extended newbie-choke. In My Experience (years as moderator at another forum) the newbies do not cause as much trouble as the Old Members, who have been around long enough to think "their" forum is a personal pooping-place.
Maybe your first post should be unmoderated, but your 50th and 100th posts should force a moderator re-evaluation of your behavior? (Boy, I would hate that even worse than the 10-newbie moderation...)
Took me 10 posts, even being very careful not to offend ANYBODY.
Apparently this forum has a sad history of unmoderated flame-wars. The current moderators worked hard to clean it up, for which we are very grateful.
However, I wonder about this extended newbie-choke. In My Experience (years as moderator at another forum) the newbies do not cause as much trouble as the Old Members, who have been around long enough to think "their" forum is a personal pooping-place.
Maybe your first post should be unmoderated, but your 50th and 100th posts should force a moderator re-evaluation of your behavior? (Boy, I would hate that even worse than the 10-newbie moderation...)
Re: OPAx227
Since I haven't played with opamps since school, when it was only with the venerable 741, I am forced to look at the well-known devices first.
Since Burr-Brown (and now TI) is the maker of extremely well-regarded linear ICs, I've decided to examine their line of opamps - OPA27/37, OPAx134, OPA604, and OPAx227/228. The 227/228 intrigues me because it is comparable in price to the others (Digikey) and still labelled as suitable for professional audio.
As an industrial designer, AD's website philosophy leads me to believe that their products may suffer the same poor design.
What do you mean signal conditioning? What kind of conditioning? Audio signals?
ensen.
Bricolo said:
Since I haven't played with opamps since school, when it was only with the venerable 741, I am forced to look at the well-known devices first.
Since Burr-Brown (and now TI) is the maker of extremely well-regarded linear ICs, I've decided to examine their line of opamps - OPA27/37, OPAx134, OPA604, and OPAx227/228. The 227/228 intrigues me because it is comparable in price to the others (Digikey) and still labelled as suitable for professional audio.
As an industrial designer, AD's website philosophy leads me to believe that their products may suffer the same poor design.
JojoD818 said:
What do you mean signal conditioning? What kind of conditioning? Audio signals?
ensen.
check these out
Some interesting chips spec-wise:
opa655 vfb, fet input, look at open loop gain vs frequency
ad9631 vfb, quad core input stage, look at open loop gain vs frequency
ad8021 vfb, drive external buffer by comp pin, bypassing internal output stage
ad8008 and ths3001 favourite cfb opamps
Especially the first two are very interesting. With only 60db open loop gain (it _is_ low for opamps) flat up to 100kHz, and _really_ low distortion, they seem to be very linear, even before feedback is applied. Most other chips I know of rely on >90db of feedback to bring distortion down at low frequencies.
Another thing that makes them stand out is the high quiescent current 15-20mA, wich is a big plus in my opinion.
warning: very high speed parts
Note that I have not tested them yet. They are difficult to find around here.
Has anybody used them? Any comments?
Some interesting chips spec-wise:
opa655 vfb, fet input, look at open loop gain vs frequency
ad9631 vfb, quad core input stage, look at open loop gain vs frequency
ad8021 vfb, drive external buffer by comp pin, bypassing internal output stage
ad8008 and ths3001 favourite cfb opamps
Especially the first two are very interesting. With only 60db open loop gain (it _is_ low for opamps) flat up to 100kHz, and _really_ low distortion, they seem to be very linear, even before feedback is applied. Most other chips I know of rely on >90db of feedback to bring distortion down at low frequencies.
Another thing that makes them stand out is the high quiescent current 15-20mA, wich is a big plus in my opinion.
warning: very high speed parts
Note that I have not tested them yet. They are difficult to find around here.
Has anybody used them? Any comments?
I've only started looking at the data sheets and I think the pattern I'm seeing is that the audio opamps perform very well at f<200kHz while the high bw opamps only start to work well f >= MHz and are quite bad performers f<.5MHz.
It's also interesting to note that the performance drops off dramatically for audio opamps f>200kHz
It could be that regardless of the general specs, the opamps recommended for audio will sound better than the others.
ensen.
It's also interesting to note that the performance drops off dramatically for audio opamps f>200kHz
It could be that regardless of the general specs, the opamps recommended for audio will sound better than the others.
ensen.Hello everyone,
I replaced OP37 with OPA627 this weekend [see schematic in one of my previous posts], and the difference is striking. OP37 wasn't bad, but after the opamp replacement, sound is much more natural and also seems more detailed.
Vocals sound much better, the S-sounds are more controlled.
But I will continue trying out at least AD825 and AD8610. Will keep posting.
What is interesting for me is definately the sound (musicality) and not measured performance - for me, parameters like slew rate, frequency response etc. may be hints as to how a component may perform - not the holy grail.
When considering components with >MHz performance, it might be relevant to keep in mind that (at least playing CD) everything above 20kHz is filtered away anyhow, and what signal might remain in the MHz region is going to be ugly digital noise that noone wants anyway!
/cdl
I replaced OP37 with OPA627 this weekend [see schematic in one of my previous posts], and the difference is striking. OP37 wasn't bad, but after the opamp replacement, sound is much more natural and also seems more detailed.
Vocals sound much better, the S-sounds are more controlled.
But I will continue trying out at least AD825 and AD8610. Will keep posting.
What is interesting for me is definately the sound (musicality) and not measured performance - for me, parameters like slew rate, frequency response etc. may be hints as to how a component may perform - not the holy grail.
When considering components with >MHz performance, it might be relevant to keep in mind that (at least playing CD) everything above 20kHz is filtered away anyhow, and what signal might remain in the MHz region is going to be ugly digital noise that noone wants anyway!
/cdl
Opamps that are represented for "audio " have their specs tailored and optimized for audio usage.
Specs that matter to me (maybe not to everyone, but I am an engineer)
1. Adequate slewrate, and although "fast enough" is good, even fater (contrary to some people's beliefs) does not mean better. For standard opamp voltage rails i.e. +/- 15V 5uV/S or more is adequate and 8 to 20uV/S about ideal.
2. Noise Figure, this is almost as important as slew rate. The amp should have a low 1/f frequency to ensure that noise doesn't increase in the lowest audible octaves. Low noise opamps should have a noise figure under 10nV/sq.rt Freq. The NE5532/NE5534 excel at low noise applications(4.5nV/sq.rt Freq) especially when driven from a low impedence source (like the output of another opamp). NE5532 also has excellent common mode characteristics which allow them to drive unbalanced input/feedback impedences with amazingly low levels of THD, 99% of opamps CANNOT do this. The OPA627 is an amazing all around performer (albeit costly). It uses difet inputs (low offset, high input impedence) but DOES NOT have as excellent common-mode qualities of the NE5532. The OPA627's will only beat the NE5532 (objectively, not subjectively) when the source impedence exceeds 10kOHMS.
3. THD level at 20KHZ driving a low impedence load (i.e. 600 OHMS) this should be in the triple digit region (i.e. .002%) this gives good indication that once the output is loaded, the THD doesn't increase substancially.
4. PSRR (Power Supply rejection ratio) anything around -100dB is excellent and will mean that you won't need that triple regulated all discreet power supply you thought you would. LM317/337' regulators will work just fine.
5. The opamp should be stable at unity gain (unless it's a mic pre or other high gain stage). The opamp should be stable and not oscillate using non esoteric ( conventional ) supply bypassing techniques (i.e. using .1uF close to the supply pins). I say this because I once designed a conventional audio preamp with the Analog Devices AD797 in mind, but they are very "touchy" opamps to work with. They don't like to be configured for low gains and they have a tendency towards parasitic oscillation. Another words, try to avoid some of the more exotic types unless you have personal experience with them. Other people may disagree with me, but this is sound advice, LITERALLY!
This information should help you narrow down your candidates.
Good Luck!!
99% of opamps CANNOT do this
Specs that matter to me (maybe not to everyone, but I am an engineer)
1. Adequate slewrate, and although "fast enough" is good, even fater (contrary to some people's beliefs) does not mean better. For standard opamp voltage rails i.e. +/- 15V 5uV/S or more is adequate and 8 to 20uV/S about ideal.
2. Noise Figure, this is almost as important as slew rate. The amp should have a low 1/f frequency to ensure that noise doesn't increase in the lowest audible octaves. Low noise opamps should have a noise figure under 10nV/sq.rt Freq. The NE5532/NE5534 excel at low noise applications(4.5nV/sq.rt Freq) especially when driven from a low impedence source (like the output of another opamp). NE5532 also has excellent common mode characteristics which allow them to drive unbalanced input/feedback impedences with amazingly low levels of THD, 99% of opamps CANNOT do this. The OPA627 is an amazing all around performer (albeit costly). It uses difet inputs (low offset, high input impedence) but DOES NOT have as excellent common-mode qualities of the NE5532. The OPA627's will only beat the NE5532 (objectively, not subjectively) when the source impedence exceeds 10kOHMS.
3. THD level at 20KHZ driving a low impedence load (i.e. 600 OHMS) this should be in the triple digit region (i.e. .002%) this gives good indication that once the output is loaded, the THD doesn't increase substancially.
4. PSRR (Power Supply rejection ratio) anything around -100dB is excellent and will mean that you won't need that triple regulated all discreet power supply you thought you would. LM317/337' regulators will work just fine.
5. The opamp should be stable at unity gain (unless it's a mic pre or other high gain stage). The opamp should be stable and not oscillate using non esoteric ( conventional ) supply bypassing techniques (i.e. using .1uF close to the supply pins). I say this because I once designed a conventional audio preamp with the Analog Devices AD797 in mind, but they are very "touchy" opamps to work with. They don't like to be configured for low gains and they have a tendency towards parasitic oscillation. Another words, try to avoid some of the more exotic types unless you have personal experience with them. Other people may disagree with me, but this is sound advice, LITERALLY!
This information should help you narrow down your candidates.
Good Luck!!
99% of opamps CANNOT do this
So, PMA, you think I should try altering my prototype to inverting configuration? (see beginning of thread for schematic).
It should be easy enough to revert to correct "absolute phase" by connecting to the inverting rather than non-inverting input of my (balanced) power amp?
Thanks in advance - cdl
It should be easy enough to revert to correct "absolute phase" by connecting to the inverting rather than non-inverting input of my (balanced) power amp?
Thanks in advance - cdl
cdl said:
True, only if you are working only with your own equipment or cables that can be re-assembled inverted. If you are making a unit that may interface with other components, then maintaining polarity within the enclosure is very important.
I tend to scale down the published specs since most are taken with signal at the inverting input. It may be possible to put two inverting stages together, but I do not know if that is cleaner than just using one stage non-inverting.
ensen.Thanks, PMA & : )ensen,
I think I'll stick to the non-inverting design for now then
B.t.w. The sound of the OPA627's has changed a bit after a week of playing - most definately to the better! I have noticed they do run warmer than the OP37 - but that's no crime.
So I don't think i'll change much in the design for now, just check out some more opamps and add more decoupling.
Anyone have ideas on how to check for HF oscillation, RF disturbances etc. etc. without owning an oscilloscope?
Cheers - cdl
I think I'll stick to the non-inverting design for now then
B.t.w. The sound of the OPA627's has changed a bit after a week of playing - most definately to the better! I have noticed they do run warmer than the OP37 - but that's no crime.
So I don't think i'll change much in the design for now, just check out some more opamps and add more decoupling.
Anyone have ideas on how to check for HF oscillation, RF disturbances etc. etc. without owning an oscilloscope?
Cheers - cdl
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