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Hello,
I bought the ventus ez boards. I need the input impedance to be higher, so I swapped all 5kohm resistors for 25k.
My initial tests show audio working pretty well, albeit with a feint high frequency wine. Looking at it with a scope, I am seeing a significant continuous oscillation at around 100mhz with an amplitude of about +/- .6 volt.
Any ideas on how to fix this?
I am only seeing this on the global feedback trace. I'm thinking I need to change CC3 and perhaps add more low value power stiffening .1uF caps. I was a bit concerned that there arn't individual stiffening caps for the opamp and current buffer like the reference design has. I notice the phase shift at 100mhz might be close to 180degrees.
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Hi there..
Is your source by any chance a Buffalo containing a 100MHz clock ?
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This is actually usually the effect of capacitive probes or other measurement anomalies.
In any case your setup is y no means ideal.
instead set it up as a normal non-inverting amplifier. The input impedance will be very high.
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Thanks for the replies, I don't have a buffalo. My source is an apogee rosetta 200 with a 20kohm stepped attenuator before the ventus ez. I can't use a 10kohm input impedance (using 5kohm resistors as in schematic) with the attenuator I have. My power supply is just a 7812, 7912 hooked up to a full bridge rectifier on a 60hz CT transformer, with capacitors. I don't believe this to be a scope probe issue. I measured this signal on headphone output, after the 10uH inductor at about 4mv and with 32ohm headphones attached. The capacitance of the headphone cable should be more than the probe at this point. Measuring before the inductor, or with the headphones disconnected, I see an increase to 600mv - (-43db) what I expect with the LR filter response. I can use a high speed active probe and see if I get different results. Quote:
In any case your setup is y no means ideal.
instead set it up as a normal non-inverting amplifier. The input impedance will be very high.
What do you mean by no means ideal. What is not ideal, is the higher feedback resistor making this too unstable? What do you mean by set up as a normal non-inverting amplifier? If I removed all filter capacitors, I would have a normal non-inverting amplifier, with a current buffer, and balanced to unbalanced converter. Thanks, Edited by user Tuesday, September 3, 2013 8:50:24 AM(UTC)
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I mean. that the feedback resistor values are very high - will produce a lot of noise.
How much gain do you want?
Here is a normal non-inverting SE input setup for 6db gain:
R2 100R R4 100K
R3 1K R5 1K
Wire IN- to GND
Wire your source to IN+ (with signal GND to GND)
Input impedance will be 100.1K.
Cheers!
Russ
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If you want to use it as BAL/SE converter you should try to keep the input impedance <= ~5K to get decent performance. Also grounding the inputs will tell you if its truly an oscillation or an actual signal coming from the source (or via EMI). If your case where you have a stepped attenuator, just increase the gain rather than worrying over the input impedance. Edited by user Tuesday, September 3, 2013 1:25:50 PM(UTC)
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Yes I want to maintain the balanced input, and unity gain. My stepped attenuator is of 20k ohm impedance, so running the ventus at 10k (as per your schematic) will cause some undesirable effects to the shape of the attenuation curve.
I suppose the stability of the amp isn't the greatest with this high feedback resistor. I suppose I could put voltage followers feeding this board, but then I might need to build my own pcb.
I will do some more measurements as you suggested, and post back results.
Also, what is the purpose of cc1? As far as I can tell it's supposed to be a low pass filter, but might actually be an integrator without a parallel resistor.
Thanks!
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Your attenuator has 20K output impedance? That is awfully high... Sounds like you may wish to use an instrumentation amplifier front end, or two buffers.
CC1 should be omitted in this configuration. It is an optional part used only if the buffer is omitted.
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Originally Posted by: Russ White  Your attenuator has 20K output impedance? That is awfully high... Sounds like you may wish to use an instrumentation amplifier front end, or two buffers.
Well the impedance of the attenuator is 20k. The output impedance is generally much less, and varies by different volume settings. About 8ohm to 5006ohm if I am not mistaken. I might like to add the buffers, but then I need a board or some really clever soldering work to fit those tiny opamps. Quote:
CC1 should be omitted in this configuration. It is an optional part used only if the buffer is omitted.
OK! that makes much more sense. You should put that note on your schematic or notes in the BOM. I haven't yet had a chance to hook up the scope again. Thanks.
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Originally Posted by: Russ White
CC1 should be omitted in this configuration. It is an optional part used only if the buffer is omitted.
Looking at the Legato 3.1 (with buffer) schematics does this also applies to c5 / c6 ? Sorry for 'jacking the threat.
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It's a valid question. And the answer is - it depends. :) With the stock parts its fine to leave them there. But it should actually be stable even without them.
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I re-examined the board, with a <1pF Mohm probe on a very high speed scope. This time I saw the oscillation occurring between 600khz and 1.4mhz. It seemed to fluctuate with how I oriented the headphone cable. Shorting the input pins did not remove the oscillation. After removing cc1, it appears the oscillation has completely stopped. 
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Originally Posted by: Russ White It's a valid question. And the answer is - it depends. :) With the stock parts its fine to leave them there. But it should actually be stable even without them. Thanks Russ. One might wonder, as in case kwhubby, the thing actually getting stable by removing the feedback cap, rather than it might be stable without the cap. Do we have a mod here ? Time to dig out the ol' scope.. ;-) (it never ends, does it ?) Edited by user Sunday, September 8, 2013 6:32:11 AM(UTC)
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Quote:CC1 should be omitted in this configuration. It is an optional part used only if the buffer is omitted.
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Yes I can read, but apparently I can't understand. In my case it's a Legato, but the two circuits virtually are identical. My Legato has buffer. And my Legato has c5/c6 populated. So if this is right: Quote: CC1 should be omitted in this configuration. It is an optional part used only if the buffer is omitted. then my Legato logically is wrong. Right ? -- Not that I'm seeing a problem with my Legato, not at all, I just try to conclude in a logical manner. I'll do some measuring later on this evening.
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It should actually be just fine either way. :) Try both ways and see which you like better.
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All that cap does is limit the hf bandwidth that drives the buffer. Depending on the driving opamp this may or may not be necessary, but it should never hurt. :)
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I finally got the thing in my chassis, but now I have this annoying ~420hz squeal coming through any headphones. I'm having trouble locating it with a scope, it's not an obvious oscillation anymore. I'm using a star grounding approach. The only ground connection to each board is with one wire back to a single point, I am omitting the ground pins on the signal side of the board and using the ground via between V- and V+. I can sometimes modulate the frequency by touching the opamp, and I can change the intensity by fiddling with additional grounding. Any suggestions? I'm concerned this may be an issue with the layout of ventus EZ. I'm considering cutting traces to give the recommended separate .1 uF and 10 uF for the opamp and buffer (as seen here http://www.ti.com/lit/ds/symlink/lme49600.pdf). Thanks!
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Can you post a picture of your grounding scheme? I have a feeling this is the source of your noise issues.
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I have torn apart the thing trying to fix the issue, so I don't have a picture of how it was. Here are two drawings of the ways I have tried that should have worked.   Note I have two headphone connectors (1/4" and 1/8") that are connected to chassis ground. Both ways have the noise. Interestingly, the noise changes volume in certain ground configurations. In certain rare cases I can turn on the power and not have the noise, it seems the oscillation takes some excitement to start- switching power or toggling ground can do this. The power supply looks basically like: http://www.paia.com/KRUKits/K83-5/k83-5_schem.jpgI use the lm7812 tab to ground the chassis in grounding scheme 2.
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