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

 

  • In reply to John Caldwell:

    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:

    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.

    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!

    Mark

     

  • In reply to mark kovach:

    Mark,

    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!

  • In reply to mark kovach:

    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.

    Mark

  • In reply to mark kovach:

    Hi Mark,

    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!

  • In reply to John Caldwell:

    Hi John:

    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

  • In reply to John Caldwell:

    Hi John:

    Are you there?  Was the new TINA file OK?  I'm still wondering if the R values around U05, and R25 are acceptable.  I ask this because I see feedback resistors as high as 20K with R ground at 1K for a gain of 21 in some circuit designs and others have low values like in mine.  I'm not sure when to use higher values and when the less noise?(lower values) are acceptable.  It would seem that lower vales, (as long as bandwidth and stability are not compromised) would always be better. Thanks

    Mark

    PS If you want to talk hi-end audio on your time my e-mail is kovach002@verizon.net

  • In reply to mark kovach:

    Hi Mark,

    Sorry for the delay in my reply, I'm actually posting from a tiny little hotel outside of Frankfurt as I'm in Germany at the moment! Surprisingly I have wireless internet!

    Anyway, low value feedback resistors are actually a trade-off between noise performance (which would suggest lower is better) and the loading placed on the opamp. Remember that the feedback resistors are a load on the output in addition to the load that is intended. Low value resistors for the output stage to deliver more current which can cause distortion or limited the output swing in extreme cases. A good practice would be to determine the maximum signal level expected at the output of the amplifier, and to make sure that the output is not exceeding its current delivering specifications into the parallel combination of the load and the feedback network. Or as a simple design rule, most of TI's audio opamps are capable of driving a 600 Ohm load with incredibly low distortion, just keep the parallel combination of the load and the feedback resistors greater than 600 ohms and you should be in the clear. Remember that resistors produce Gaussian or white noise, not the hum your experiencing. Below is a graph that I borrowed from one of my colleagues presentations (Art Kay):

     Notice that at approximately 100 Ohms of resistance, the noise spectral density of the resistor is pretty close to the input voltage noise of the OPA1611. Reducing these values further will most likely not provide much benefit. On the other end of the spectrum, extremely high values of feedback resistors can often cause stability issues as they are large enough to interact with parasitic board capacitances as well as the input capacitances of the amplifier itself, reducing the phase margin of the feedback loop.

  • In reply to John Caldwell:

    Hi John:

    I hope you're seeing some sites while in Germany.

    Thanks for the info.  I've seen the graphs, and the formula that states Rf // Rg < 600 ohms, but my problem, having no formal training is that I'm not sure how to interpret what it means.  I see that and think OK; Rf X Rg / Rf+Rg >600 Ohms, but when I do the math it means that to be "Safe" for a gain of 7V/V that Rf=4.2K and Rg=700ohms, and these values seem far to high from a noise standpoint, especially when the input is somewhere between 0.15 - 0.5 mV, hence my really low values of Rf=500R and Rg (when both are // (250R//125R=83.3R) I thought that the current from the cartridge should be small enough that I can configure the 1611 as I did.  But....

    Add to that the cartridge loading for these low output MC carts from 30R to 100R "grounding" the OPA1611 input, and the thing becomes a nightmare even if I know the correct formulas to apply.

    I recommend to the end user to try 500R as a start for cartridge loading and work down in 100R increments, and to use the jumpers judiciously so that first stage gain settings brings the cart output up to about 1mV, which is then very easy for the remaining 3 stages to deliver quiet, clean gain of 60-70dB, but I'm worried that these low values around the first stage, as you indicated, might be loading down the 1611 when all parameters are in worst case scenario.

    I can do the math if I have the formulas, and understand where and how to implement them with respect to the circuit. Unfortunately for me, much of the information provided in white papers and app. notes presumes specific prior knowledge, which over the years I have been able to figure out, but here I feel like I'm on thin ice, and need you to get me pointed to solid ground; even though it may take a while.

    Mark

  • In reply to mark kovach:

    Hi Mark,

    To start, there is nothing "magic" about the 600 Ohms number, but it's important to see where it comes from. The limiting factor here is how much current the OPA1611 is able to deliver without the output stage beginning to introduce distortion. The output current is determined by the output voltage and the parallel combination of the load resistor and the feedback network. Let's consider the "worst case" for output current, which would be the amplifier swinging all the way to one power supply or the other:

    This is where the 600 ohms number comes from, at full output voltage swing, the amplifier is delivering 25mA, which is well within the capabilities of the amplifier  as shown in the distortion plots. But this is not strictly valid in your application because you will never see 15V at the output of your first stage. From your post it sounds like the maximum output voltage will be 3.5mV (.5mV x 7V/V). Thus, the output current from the OPA1611 in this case is:

    It's safe to say your values are well within the safe output current range of the OPA1611. You can reduce them further if you like. but the white noise produced by the input voltage noise of the OPA1611 (1.1nV * Gain = 7.7nV/rtHz at the output) is going to be dominant here in terms of white noise (remember this is hiss NOT hum).

    I don't think you're excessively loading the OPA1611 at all. I think the issue is that you're picking up 60Hz noise from the cartridge, cable, or possibly even the layout, and then it is being amplified by the pre-amp. Since this is EXTRINSIC noise (comes from the outside world, not the physical nature of the components) changing the resistor values won't make any difference.

    When dealing with EMI issues (electromagnetic interference) we recommend a 3 step approach to all customers. First, find the source of the noise, reducing the amount of noise it is radiating if possible. Second identify the coupling medium and reduce its effectiveness (e.g. how is this noise being converted from electromagnetic radiation to a conducted signal at the input to my circuit?). The coupling medium is often inductive (long pcb traces, large loop areas) for low frequency noise such as 60Hz hum. This stage involves improving layout and grounding, implementing shielding, exploring different cabling topologies,  Finally, reduce the circuit's susceptibility to this noise. This step involves improving filtering at the front end or power supply.

    Tracking down and handling 60Hz issues really takes patience, unfortunately I can't provide a formula to eliminate hum. However, this does give credence to my opinion that the output phono cartridges should use a balanced shielded cable (such as a microphone cable) instead of an unbalanced coaxial cable. I've never understood why modern turntables still use RCA cables?!

  • In reply to John Caldwell:

    Hi John:

    Thanks for the math.  It's so easy once explained.

    For a while, I taught math to College students whose second language was English.  What I found in all cases was that they were failing not because they couldn't do the work but simply because they didn't get the terms or the concepts.  Although the professors weren't teaching abnormally fast, to these kids, hearing it in English was like he was talking at the speed of light. Once we sat down and  I explained abcissa, coordinate, trinomial, cosine, cubed root etc., and they could understand what they were and how to apply the formulas, they all went to A grade students.  It's like that with me sometimes; good instincts plus good explanations equals success.

    Thanks for the EMI refresher course.  Now that my mind is at ease about the circuit design, I have been concentrating on a course of experiments to isolate and eliminate the hum, which is only audible at phoho stage gains of  greater than 58dB.

    So , you believe that all phono cartridges should be wired for 3 wire balanced L and 3 wire balanced R channels, with a differential driver input on the phono stage?

    Back in the last century in my recording studio, we wired the mic's with the shield drain (foil wrap) connected only at the mixing console (star)  So with that in mind would a balanced phono cart wiring require a special cartridge?  I thought that the shield was internally wired at the cartridge.

    Mark