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Audio Amplifiers Forum
some OPA627 questions
Was the architecture of the OPA627 changed sometime ago? I see that the BP version has better specs all around than the AP, costs more and is less available. Does the BP version have a different design than the AP or is it simply better quality control? Will the BP stay current?
I am using the OPA627BP extensively in a 4 stage phono preamp. Recently, in addition to the second and third stages, we tried the 627 in the first stage. This stage's impedence can vary from 30 ohms to 47K ohms depending on the cartridge. We noticed that at about 300 ohms the 627 starts to feel the burden of the lower impedence and it's output is reduced proportionally to the lower input impedences.
Now, it still sounds very fine; even at 30 ohms/reduced output, but I'm wondering if I am playing with fire in the long run. I considered the OPA1611 but the 627 just sounds so good. Given that;
1. Do you think the 627 will be OK under a low impedence condition long term...and
2.Do you recommend the 1611 or another opamp that would be more acceptable of input impedences that can range from 30ohms -47kohms?
All the versions of the OPA627 are identical designs. The differences are in the test limits for DC accuracy such as offset voltage and drift. These issues are generally unimportant in audio applications.
I don't understand your comments about reduced output with lower source impedance. This does not make sense to me but maybe I don't understand your application circuit. The OPA1611 is an excellent choice for very low source impedance applications. Above a source impedance of 1k or so, you would do better to move to another amplifier as the input bias current noise will dominate.
Here is a link to a short discussion on audio op amp selection that may be helpful.
Thanks for the response. I'm not sure why offset V and drift are unimportant in audio apps; is there a paper you could direct me to, or at least a quick definition?
As for the gain decreasing with load, perhaps I've made a mistake on the input stage, could I send you the input part of the schematic? I have it in TINA TI, or in a word RTF format. Which is best?
Audio is an AC signal with a lower frequency limit in the range of 20Hz. DC offset is 0Hz and cannot be heard.
Sometimes offset voltage can pose design issues but generally it's not a critical issue for audio signal paths. Even the lower performance grades of the OPA627 have very good DC precision (offset and drift), better than most commonly used "audio" op amps.
You can attach a TINA schematic file to a posting.
Included is the first half of the schematic. I'm mostly concerned witht the input, where the impedence of the cartridge can range from 25 ohms to several hundred, and the loading/damping resistance (as suggested by cartridge manufacturers) can be as you see as small as 30 ohms (R04) from pin 3 of U05 to ground. Any value below 500ohms seems to load down the OPA627 and decrease the output. Any suggestions?
For me DC offset is a huge consideration if it makes its way to the speaker's voice coil. As this entire circuit can have a total gain of 70bB (3100X) Would the better spec OPA 627's make a difference here? I'm concerned with how hard the servo (AD8610) needs to work to null the DC offset from the larger gain settings.
Any TI op amp that would substitute for the AD8610? I would like something with the same low voltage / current noise specs but that would work at +/- 14 or 15 VDC instead of 12VDC as does the 8610.
This looks like it's a preamp for a moving coil cartridge. The 30 ohm input resistor is not loading the input of U05. It is loading your cartridge. It creates a voltage divider with the source impedance of the cartridge, reducing its output voltage. U5 is merely amplifying the signal it receives and is an insignificant portion of the load on the cartridge. There may be benefit to the cartridge response by loading heavily in this manner but it comes at the cost of reduced output voltage and increased noise.
For a moving coil cartridge I believe you would get lower noise with an OPA1611 for U05. Also, more gain in the U05 circuit would improve noise performance. If you decide to increase gain, do it by reducing the value of R18 and increasing the value of R17. For conventional moving magnet cartridges the OPA627 would be okay for U05.
The servo circuit appears to have some incorrect resistor values entered if you are trying to run TINA simulations. An "m" indicates milli-ohms and you must mean "M." You should not worry about the servo "working too hard" as it won't get tired. If the output of U03 is within its linear output range it will properly correct the combined offset voltage of U05 and U01. You could substitute OPA627 for the AD8610.
I'll also offer my personal opinion... DC coupling throughout many stages in the signal chain often creates more problems than it solves. Properly chosen coupling capacitors to block DC offsets do not cause disease.
Thank you much for your help. Without any formal training, I learn from inciteful comments like yours. I've included the entire left channel with correct R values for U05. (I don't know why those others were in place!)
Thanks for the catch on R's 14, 15. I thought that I had improperly imported the spice file from AD, and froze TINA.
Speaking of which, could you elaborate on the servo problems that could arise? As you see, I have two in place.
Caps are a strange animal in the hi-end audio community, with their own "Sonic signature" which I would not have believed had I not designed 4 different speakers without inductors (only caps) in the crossovers (naked for all the world to hear); so to sell a product that has zero caps in the signal path is a good thing if one can make it work. My original design had coupling caps which are very large and expensive.
This phono pre was designed to acommodate both MM and MC cartridges; with as much flexibility as possible in both gain structure and cartridge loading. From a purely design point of view, if you had to choose, would you recommend the OPA627 or the 1611 for U05? In other words which would be the best compromise for all contingencies?
A while back, I put the AD8610 in place of the OPA627 (for cost considerations) in the servo. Now I was hoping for a similarly priced or less expensive sub for the 8610 from TI, but can't find it.
Servo designs to control DC offset create a low frequency roll-off just like capacitive coupling. It is not correct to assume that the capacitors in the servo "are not in the signal path." They are in the signal path. Your signal path is not "DC-coupled all the way to the speaker." It just looks more like it. There can be advantages in a servo to control DC offset because circuit values can make the selection of capacitor types more attractive. The amplifier in the servo loop can also affect audio performance, depending on the circuit topoloty and values. This sometimes leads to a conclusion of "I went to a lot of trouble to eliminate capacitors so what I hear must be better."
I would not make a compromise on the op amp for the moving coil cartridge. I think a better approach would be to go straight into the OPA627 for moving magnet cartridges and only add the U05 preamp into the signal path (an OPA1611) for moving coil cartridges. Different switching arrangement could accomplish this.
The OPA1641 would be a good alternative for the OPA627 in the servo, or for that matter, in the signal path. OPA827 is also a great op amp for the signal path. In my opinion, better than the OPA627 but it has not yet earned its stripes in the audio community. Neither of these newer op amps is available in DIP package which may be a consideration.
As for the capacitor issue, there are many myths out there bases on poorly controlled comparisons (very common in the audio community). That is not to say that capacitors cannot cause problems. They certainly can. Proper selection for the particular circuit function and value is the key.
Thanks again for the excellent comments, you sound like an audiophile; at least at heart!
In double blind tests (The only kind I will do) with the 10 pairs of ears that I trust most, the original design ( minus U05) had 2 versions; OPA 627 and OPA827, and the OPA 627 was preferred by 8 of the 10 myself included. Unlike most audiophile grade testing, as you mentioned which is suspect, these tests are done in many sessions over months. I recently found another manufacturer (on TI's website) of headphone amps who also preferred the 627 to the 827.
I think I may have shot myself in the foot. The last schematic was as you suggested; jumpers to include or exclude U05, but board size constraints made that impossible. This is why we have a jumper over R24, allowing U05 to be a unity buffer for MM cartridges. (Yes, what was I thinking...?)
With the new PC boards arriving tomorrow, we are scrambling to find a way to kludge a version to include/exclude U05 as before. However, if that doesn't happen, how terrible would it be to use the OPA 1611 at unity gain for MM cartridges @ 47k inpedences or a little less? It seems like it should be OK, but sometimes things get critical at unity gain; something I have yet to understand.
Assuming that you change to an OPA1611 for U05, a jumper across R25 does not deal with the full issue for MM cartridges. It will bring the gain down to a more appropriate level. The issue on my mind is the current noise of the OPA1611 reacting with the higher source impedance of the MM which creates additional noise. The source Z of a MM is in the range of a few k-ohms. In a quick calculation, the effect is not quite as bad as I estimated but you would do better to completely eliminate U05 from the signal path for MM.
Note the comment on source Z of MM cartridges. The 47k you cite is the commonly used load impedance for the cartridge but the source Z is much lower.
As for OPA627 vs. OPA827. The OPA627 is, without doubt, a superb audio op amp. I can cite many cases, however, where an added "something" that does not belong in the signal is perceived as "better sounding." So I'm always a bit skeptical of purely subjective tests, particularly on op amps. Nevertheless, I congratulate you on your attempt to do well-controlled comparisons as you have. You have one important point in your favor in this particular comparison--the OPA627 and OPA827 are very much of the same general type. It makes me crazy when two very different op amps are compared in the same circuit thus leaving one (or possibly both) to be inappropriately used.
As one of Bruce's colleagues in the Precision Analog group in Tucson and a fellow audio enthusiast I thought I throw in my two cents, mostly just because I'm excited there is a phono pre-amp post on e2e!
1. Bipolars on the front end of phono pre-amps (e.g. OPA1611): This always made me uneasy because without AC coupling the cartridge to the phono pre-amp, the large input bias currents of the bipolar are now drawn through the cartridge itself. I'm not sure if this would provide a deflection of the stylus or a pre-loading effect or even worse change the frequency response of the cartridge by changing the properties of the moving magnet over time (applied opposing magnetic field?). This weekend while reading Douglas Self's book Small Signal Audio Design he mirrored these sentiments but also noted that no one had done an academic study of these effects yet. This is why I chose a FET input opamp on one of my recent phono pre-amp builds (personal project). Also, the increased current noise of bipolar parts is not really noticed at lower frequencies because the cartridge loading impedance is shunted by the cartridge DC resistance, however at high frequencies the cartridge inductance makes the noise contribution from the 47kOhm input resistor much more significant. This is exacerbated by bipolar amplifiers which employ input bias current canceling (OPA1611, LT1028, etc) because this circuitry injects two additional correlated noise sources into the inputs.
2. AC Coupling Cap vs. DC Servo: I will completely agree that capacitors have the potential to wreck an otherwise low-noise, low-distortion audio chain. I've seen many customers use high-k ceramic capacitors in active filter and data converter circuits and be quite surprised at the massive level of odd-order harmonic distortion these parts create. Electrolytics have the potential to do this as well, however in both cases this behavior is due to the voltage coefficient associated with the part's capacitance value (their capacitance is actually changing with the applied voltage). Therefore, the distortion created by a capacitor is completely dependent upon the voltage across it. If an AC coupling cap is sized properly however, there should be negligible voltage across it and therefore negligible distortion created.
However, my beef with DC servos is more that they represent an additional failure mode in the system. Compared to a capacitor, an opamp is a fairly delicate circuit element, any number of unfortunate events may compromise the opamp used in the DC servo and therefore pass DC directly to the output. But a decent film capacitor doesn't care what happens to the power supplies, ESD events, solar storms, etc, it will always block DC from showing up on the output. I believe that preventing a failure from propagating through the signal chain is an important concern for any piece of equipment.
4. OPA827 vs OPA627: This is a tough one to settle. All I'll say is that I personally REALLY enjoyed listening to my latest phono preamp prototype which used OPA827s and completely passive RIAA equalization. But then again, it was built on a breadboard, sitting on top of a bench power supply, sitting on the floor of my living room. I'm not sure the opamps were the limiting factor ;)
Please excuse the rather modest speakers, the last set I built are currently in a state of being "upgraded". As I'm sure you know, building a set of speakers only satisfies you for a few months, and then the bug returns...
Analog Applications Engineer
PA Linear Apps
Hello again Bruce and welcome John
My original design tested the 1611 as well as the 627, 827 and a few from other manufacturers. We found that the 1611 sounded fairly good with MC, but horrible with MM. This is when the design used 2 op amps in the signal path and we were configuring them for a total gain of 60dB. In that design the 627 easily out performed the 827 in double blind listening, and so we went with the 627.
This new design employs 5 op amps in the signal path and so before I saw either of your posts yesterday, we swapped U05 from a 627 to a 1611. This op amp was a recent add on to the design and is supposed to make a step up XFMR unnecessary for cartridges of 1mV or less. (sort of a pre preamp. ) Here, the total gain is about 5V/V max (15dB) where cart loading is usually less than 3K. I did this because of a new breed of carts like MI and some other exotic designs where outputs are between 0.5mV and 1.2mV but loading is suggested in the 1K to 4K range. I am the owner of one such cart called the Sussaro from Soundsmith, and with the 627 I was loading at 2.5K with 220pF and the sound was magnificent.
Well, we were pleasantly surprised with the results from the 1611. Was it musical-YES! Did I notice anything out of place or exaggerated-NO! This is all I need to know for a first eval. I'll need to wait 1 month for the 1611's to break in before more critical listening...just kidding; but that's the kind of crap that's perpetuated in the hi end audio community and what I have to deal with when trying to convince people to try less than 47K on their MM carts.
Times have changed. Carts and electronics are more sophistocated. Why do we still need to stick with 47K? Why not 30, 20 or even 15K? Give it a try... but I digress.
The 1611 was cleaner with tighter bass. Usually when this is the result, I listen for lost harmonics or attenuated harmonics, which tend to give the illusion of a cleaner open sound, but I could not find fault with the timbre as all vocals and instruments sounded natural.
I think that the 627 will still be the choice for 5K and above compensated carts and so I now will need to try and kludge my just delivered PC boards to allow for the 1611 on some carts and the 627 for others. So in essence, Bruce, John and Mark seem to be in agreement about the 1611.
We thought long and hard about the possibility of disaster regarding op amp servo's and ESD, which is why there are two of them in the design, and we also listened. Even with some of the "Highly regarded" industry caps in place Auricap, WIMA, Clarity and a few others, everyone loved the "Natural sound" of no caps. Again, double blind tests-one person at a time (so there was no influence) "Which do you like better?" "Are you sure?" "Then let's see you do it again..and again" is basically how I conduct my tests for what it's worth!
I believe the values of the caps were 5uF and 10uF, but I'd need to check my notes. If either of you guys wants to suggest a cap(s) value and location instead of the servos, I'm always willing to try more experiments.
Not having any formal education, I get scared when I read things like:
So I try not to read too much like the above until after I've put something together and it sounds like crap or blew itself up.
Hey John; didn't your momma tell you not to breadboard opamps? That phono pre has a WAF (wife acceptancy factor) of about -11! Hah!
I'm glad that you were pleased in your evaluation of the OPA1611! All my comment really meant to convey was that my engineering intuition just didn't feel right with a bipolar on the front end of my pre-amp, so I went with a JFET input type (the OPA827). I had a few reasons for doing this, noise and input bias current mainly. The same goes for my reasoning on AC coupling caps vs DC servos, the engineer in me tends to fear the worst, and liked that added safety provided by an AC coupling cap.
WAF is not an influence for me at the moment, that's why in my house I get to have an electronics lab and a dedicated room for speaker measurements instead of a "study" and a "guest bedroom". In my defense, the pre-amp wasn't on the breadboard (and floor) for long, and is currently being transitioned to a pcb and a well built enclosure.
Good luck with your project!
Hello Bruce and John:
The new PC boards were delivered and we built a Phonatic phono stage as configured in the TINA schematic enclosed.
It's loaded at 2.5K and 100pF using a moving iron cartridge at 0.5mV / 3cM.
With U05 at unity; total phono gain = 55.7dB it's very quiet, and wonderfully musical. As we increase the gain of U05, we begin to hear an underlying hum. Even at U05=3V/V this hum becomes audible, and increases with gain.
I would like to eliminate poor circuit design on my end as a reason, first. Then I can proceed to poor board design.
What I think is the real culprit; an induced problem in the line from the cartridge, could be easier to find once I'm sure that the above are correct.
If not too much trouble, would you take a look at the feedback R's around U05, and also R25 which is now 100R, but was originally 50R. I'm trying to keep noise down and hence the low values, but would welcome any suggestion that might be more suitable, especially if these values may be causing my hum.
This first stage will most likely be used with 100-2.5K loading and 20-400pF
Thanks for the help.
I'm glad to hear your project is still moving forward. Just to clarify, were you referring to the schematic previously attached in this thread or an updated one? I ask because I don't see a schematic attached to your post. That said, when I hear descriptions of noise I generally think:
"Hum" --> 50/60Hz and its associated harmonics.
"Hiss" --> Gaussian/random intrinsic noise of the circuit itself.
Remember that the feedback resistors will contribute Gaussian noise consisting of their intrinsic noise (calculated from their parallel combination) as well as a second noise term that is due to the input current noise of the amplifier multiplied by their values in parallel. The noise sources are a vector sum (root sum of squares) because they are uncorrelated. At this level of gain, I think 60Hz noise pickup from the cartridge and cabling will absolutely dominate.
Going along with your description it sounds like your picking up mains noise which is always a difficulty with phono pre-amps, especially on ones with gains approaching 60dB such as yours. As always, there are many opportunities for noise ingress from the environment: The cartridge may be acting as an antenna, the cabling from the cartridge to the pre-amplifier may be picking up noise, the PCB layout may be non-ideal, or the power supply may be inadequately filtered. Here are some PCB tips I put together to get you started (click on the image):
With noise it is always extremely useful to view the frequency content of the output signal. There are numerous freeware programs available online which allow you to utilize a computer soundcard as a spectrum analyzer. This would allow you to track the amount of 60Hz noise through the entire signal chain (I recommend building a simple AC coupling and buffer circuit to protect your soundcard). One such program allowed me to view the noise spectrum of the prototype phono preamp I showed in an above post:
The red trace is the noise floor of the soundcard and the blue trace is the output of the phono circuit (calibrated to 0dB = 1Vrms). By viewing the frequency spectrum of the output noise it was very easy to identify noise sources in the circuit. You can determine if this hum is coming from the cartridge by shorting the input to the pre-amp and monitoring the level of 60Hz hum on the output. Also, if you haven't already, one test would be to power the phono preamp from alkaline batteries and examine the output spectrum to determine if 60Hz noise is coming from the amplifier's power supply itself (the relay coil may also pickup noise, just a thought).
Every engineer feels your pain, high gain amplifiers pickup noise from everywhere!
Thanks for the reply. I included a new TINA file last mail; must be lost in space. I will resend it now. NOTE U05 is OPA1611
The noise is a hum (60/120) perhaps, not gaussian which doesn't really appear troublesome until signal levels would be too loud to listen comfortably
I believe our P. supply design is sound. What you won't see on the schematic is 100n caps at all U(xx) +/- V supply pins (about 1-2mm from the opamps) plus 6.6uF caps usually no more than 0.5" distant.
Regarding Rf, the question for me is how minimum (in terms of actaul total trace length) is acceptable? In this design I had to make the distance a little longer than I liked for some other considerations, which can be changed if necessary.
We have a sophistocated analyzer(SpectraPlus) computer, external soundcard and special mic (for speaker FFT analysis.) Someday I'll try to dig up the PS chart and show you-it's very quiet.
Do you have any way to look at a PCB file such as a gerber editor or reader?
Mark0535.PhonaticMC 12 06 16.TSC
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