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?

Thanks

Mark

 

  • Mark,

    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.

    http://e2e.ti.com/support/amplifiers/precision_amplifiers/w/design_notes/upgrading-op-amps-in-audio-equipment.aspx

    Regards, Bruce.

  • In reply to Bruce Trump:

    Bruce

    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?

    Mark

  • In reply to mark kovach:

    Mark,

    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.

    Regards, Bruce.

  • In reply to Bruce Trump:

    Bruce

    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.

    Thanks

    Mark

    PhonaticTI.TSC
  • In reply to mark kovach:

    Mark,

    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.

    Regards, Bruce.

  • In reply to Bruce Trump:

    Bruce

    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.

    Phonatic4.4 TI.TSC
  • In reply to mark kovach:

    Mark,

    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.

    Regards, Bruce.

  • In reply to Bruce Trump:

    Bruce

    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.

    Mark

     

  • In reply to mark kovach:

    Mark,

    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. 

    Regards, Bruce.

  • In reply to mark kovach:

    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...