Hi Everyone,
I have been playing around with this very simple phono circuit. Just two non-inverting opamps with RIAA filter network sandwiched in between. Extremely basic.
My source for the schematic is the Ti LME49720 Datasheet.
Here is my trouble with it: It actually sounds pretty decent (instruments sound correct tonally, timbrally), but compaired to other phono preamps I have lying around, it seems dynamically compressed. I also notice that when I lift the needle at the the end of a record I get a sharp snapping sound from the speakers instead of the usual soft, low "whump." I am guessing that the latter is caused by some hard clipping, and I wonder if my two issues (limited dynamics, and the clipping sound) might be related.
See the schematic below. The design has the gain equally split between the two stages.
I have been pondering a few changes to see if they help:
1- reconfigure the gain ratio between the two stages
2- Add an extra gain stage and split the RIAA time constants between the three gain blocks
3- Add another Op Amp in the first stage and make a cascaded Voltage Amp (each of the two cascaded OPAs run at a lower gain setting to hopefully get more headroom)
4- Get a couple expensive discrete Op Amps for the first stage that can run on higher voltage, thus allowing more voltage swing.
I have never seen option #3 done before with opamps. I assume there are good reasons why, but I don't what they may be.
I am not sure option #1 will get me where I want to go, since I have read many places that headroom is main limiting factor of passive RIAA.
Option #2 is interesting, and easy. I have lots of spare Op Amps.
Sorry for the very basic question. I totally admit to being one of those members with no E.E. background who is constantly "learning." I do suppose it gets old pretty quick.
Best regards,
John
I have been playing around with this very simple phono circuit. Just two non-inverting opamps with RIAA filter network sandwiched in between. Extremely basic.
My source for the schematic is the Ti LME49720 Datasheet.
Here is my trouble with it: It actually sounds pretty decent (instruments sound correct tonally, timbrally), but compaired to other phono preamps I have lying around, it seems dynamically compressed. I also notice that when I lift the needle at the the end of a record I get a sharp snapping sound from the speakers instead of the usual soft, low "whump." I am guessing that the latter is caused by some hard clipping, and I wonder if my two issues (limited dynamics, and the clipping sound) might be related.
See the schematic below. The design has the gain equally split between the two stages.
I have been pondering a few changes to see if they help:
1- reconfigure the gain ratio between the two stages
2- Add an extra gain stage and split the RIAA time constants between the three gain blocks
3- Add another Op Amp in the first stage and make a cascaded Voltage Amp (each of the two cascaded OPAs run at a lower gain setting to hopefully get more headroom)
4- Get a couple expensive discrete Op Amps for the first stage that can run on higher voltage, thus allowing more voltage swing.
I have never seen option #3 done before with opamps. I assume there are good reasons why, but I don't what they may be.
I am not sure option #1 will get me where I want to go, since I have read many places that headroom is main limiting factor of passive RIAA.
Option #2 is interesting, and easy. I have lots of spare Op Amps.
Sorry for the very basic question. I totally admit to being one of those members with no E.E. background who is constantly "learning." I do suppose it gets old pretty quick.
Best regards,
John
I'm another one of those non-EE learners. The folks around here have been very generous in helping me gain a better understanding, along with years of reading, etc. It's a journey, but it's satisfying when things work out in the end.
To answer your question, and off the top of my head, I'm wondering if the signal from the first stage is too 'hot' for the second stage to handle. If a pop/click results in a large transient from the cartridge, it could be hard clipping the second opamp, momentarily.
Perhaps adjusting the values of the feedback loop resistors can result in a more pleasing result. Perhaps reduce the gain of the first opamp by increasing the value of the 150R to 180R? That would raise the noise floor a little, though.
Also, what are you using for a power supply? I think this circuit is meant to run on a regulated +/-15VDC supply, up to +/18VDC.
Hopefully something in there helps somehow. I'm sure you'll be getting better ideas and info soon.
To answer your question, and off the top of my head, I'm wondering if the signal from the first stage is too 'hot' for the second stage to handle. If a pop/click results in a large transient from the cartridge, it could be hard clipping the second opamp, momentarily.
Perhaps adjusting the values of the feedback loop resistors can result in a more pleasing result. Perhaps reduce the gain of the first opamp by increasing the value of the 150R to 180R? That would raise the noise floor a little, though.
Also, what are you using for a power supply? I think this circuit is meant to run on a regulated +/-15VDC supply, up to +/18VDC.
Hopefully something in there helps somehow. I'm sure you'll be getting better ideas and info soon.
Hi! Thanks for the response!
Ah, alas, no scope. Been meaning to get one for years.
Bypassing, yes—.1uF at the pins, 22uF a tad further away.
Cartridge: this will be part of the problem. Shure M44-7. 9mV output.
Best,
John
Hi Rogon! Thanks for your reply!
Well, I have read many of your posts and you have leaned much more aggressively than I. Sad to admit, but I’m much more a solderer. My actual learning over the last 17 years as a member here has been a very slow affair.
To answer your question, Power supply: +/-15VDC (Jung regulators.)
Especially with the Shure M44, I think the goal might be to raise the overall headroom.
Best,
John
One obvious option that I didn’t list earlier…
5. Replace the first Op Amp with a discrete gain block (e.g. the popular FET/BJT Cascode) fed with a higher supply voltage.
Of course this would mean ordering FETs (I have some BC550s) and building a second votage supply. And then I would want to try to make a differential input like Bob Cordell’s. All much more complicated, and a big diversion from the original purpose of building this thing, which was to use up some IC Op Amps. (And I had all the passives in the parts bin.)
But, I guess I have been assuming all along that the trouble is with the first stage. If I’m not mistaken, both Rayma and Rogon are suggesting that clipping, if there is any, may be in the second stage.
However, if I could get enough voltage swing at the first stage to up the gain there, I could lower the gain of the second. Is my thinking wrong, here?
Best regards,
John
5. Replace the first Op Amp with a discrete gain block (e.g. the popular FET/BJT Cascode) fed with a higher supply voltage.
Of course this would mean ordering FETs (I have some BC550s) and building a second votage supply. And then I would want to try to make a differential input like Bob Cordell’s. All much more complicated, and a big diversion from the original purpose of building this thing, which was to use up some IC Op Amps. (And I had all the passives in the parts bin.)
But, I guess I have been assuming all along that the trouble is with the first stage. If I’m not mistaken, both Rayma and Rogon are suggesting that clipping, if there is any, may be in the second stage.
However, if I could get enough voltage swing at the first stage to up the gain there, I could lower the gain of the second. Is my thinking wrong, here?
Best regards,
John
Clipping would happen in the second stage, but that doesn't mean it's not the first stage's 'fault'. The first stage itself is unlikely to be overloading just from the output of the cartridge (unless something is wrong in the wiring or something like that).
Goosed by the 9mV nominal output from the DJ cartridge, output signal peaks coming out of the first stage might be swinging high enough to overload the input of the second stage. Reducing the amplitude of those peaks coming out of the first stage would decrease the chances of the second stage's input being overloaded, thus increasing headroom.
You can lower the gain of the first stage, and the second stage will then see about the same levels it would see if you were using a lower output Hi-Fi cartridge (4 or 5mV nominal output). That would make it less likely that the first stage could overload the input of the second stage.
Maybe try a 10k ohm resistor in parallel with the 3320 ohm resistor in the first stage. That would reduce the gain of the first stage by about 1/3. Then listen and do the pick-up-the-needle test (does it make a tame 'thump' or a wild 'crack'?). If that's not enough gain reduction, try adding another 10k in parallel with the 3320R//10k combination (making 3320R//10k//10k).
You could even do this with a trimpot, perhaps a 10k pot in parallel with the 3320R. Dial the value down until you get the sound you like. Then carefully remove the trimpot without changing its setting, and measure its resistance once it's out of circuit. Then you'll know what resistance you had put in parallel with the 3320R, and calculate the new value of resistance from R//3320R.
Adding more gain from the first stage will drive the second stage harder. I'd try the above experiment, mostly because it's easy to do and it's easily reversible.
Goosed by the 9mV nominal output from the DJ cartridge, output signal peaks coming out of the first stage might be swinging high enough to overload the input of the second stage. Reducing the amplitude of those peaks coming out of the first stage would decrease the chances of the second stage's input being overloaded, thus increasing headroom.
You can lower the gain of the first stage, and the second stage will then see about the same levels it would see if you were using a lower output Hi-Fi cartridge (4 or 5mV nominal output). That would make it less likely that the first stage could overload the input of the second stage.
Maybe try a 10k ohm resistor in parallel with the 3320 ohm resistor in the first stage. That would reduce the gain of the first stage by about 1/3. Then listen and do the pick-up-the-needle test (does it make a tame 'thump' or a wild 'crack'?). If that's not enough gain reduction, try adding another 10k in parallel with the 3320R//10k combination (making 3320R//10k//10k).
You could even do this with a trimpot, perhaps a 10k pot in parallel with the 3320R. Dial the value down until you get the sound you like. Then carefully remove the trimpot without changing its setting, and measure its resistance once it's out of circuit. Then you'll know what resistance you had put in parallel with the 3320R, and calculate the new value of resistance from R//3320R.
Adding more gain from the first stage will drive the second stage harder. I'd try the above experiment, mostly because it's easy to do and it's easily reversible.
Okay, I’ll try lowering the gain on the first stage a bit, see how that goes.
Sorry to ask, but wouldn’t lowering the gain on the second stage also prevent it from clipping?
I was thinking that it would only clip relative to the amount of gain it was asked to provide.
Could there be enough voltage after the RIAA filter to actually swamp the input of the second Op Amp? Or maybe I’m misunderstanding what you wrote.
Thanks again for your replies!
Regards,
John
Sorry to ask, but wouldn’t lowering the gain on the second stage also prevent it from clipping?
I was thinking that it would only clip relative to the amount of gain it was asked to provide.
Could there be enough voltage after the RIAA filter to actually swamp the input of the second Op Amp? Or maybe I’m misunderstanding what you wrote.
Thanks again for your replies!
Regards,
John
Yes, increasing the negative feedback around the second stage opamp would reduce the signal amplitude at its input, so I agree it would be worth investigating reducing the gain of the second stage that way. Worth a try, for sure.
I was thinking that on a loud tick, click or pop, or maybe strong musical peaks, the output from the first stage opamp could be enough to overload the input of the second stage opamp, even after the insertion loss from the EQ section.
I wonder... Is the overload happening in the bass frequencies only? The circuit is DC-coupled. Slight overload at bass frequencies could sound like muddiness or dynamics compression.
Of course another possibility is ultrasonic oscillation from a layout problem.
I was thinking that on a loud tick, click or pop, or maybe strong musical peaks, the output from the first stage opamp could be enough to overload the input of the second stage opamp, even after the insertion loss from the EQ section.
I wonder... Is the overload happening in the bass frequencies only? The circuit is DC-coupled. Slight overload at bass frequencies could sound like muddiness or dynamics compression.
Of course another possibility is ultrasonic oscillation from a layout problem.
With that cartridge, try setting the gain of BOTH blocks to about 24dB each, which is x16 each.
This can be done by reducing the values of both of the 3320R resistors to 2250R instead.
Then this will adjust the levels to about what was intended in the original design.
This can be done by reducing the values of both of the 3320R resistors to 2250R instead.
Then this will adjust the levels to about what was intended in the original design.
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I have only noticed the hard clipping on picking up the needle, which I would assume would create a primarily low frequency signal. I could try AC coupling the stages. Super simple experiment.
Ultrasonic oscillation is a possibility. It is a P2P/perfboard construction. However not having a scope…
It definitely doesn’t sound muddy. The bass is quite good, actually, when only program material is being amplified.
The main thing I would like to improve is a bit more subtle. Even comparing to a similar two Op Amp hybrid, passive/active phono I built, the fully passive RIAA version isn’t as dynamic. Almost as if all notes are being played at the same velocity—that is what I meant by compressed, as if a compressor were being over-applied to the recordings.
I thought that, perhaps, the hard clipping sound when lifting the needle might indicate that less audible clipping might be happening on regular music passages.
This is why I was thinking of ways to increase the headroom in the gain blocks.
Of course, without an o-scope or at least SPICE, I’m just grasping in the dark.
Hi Leoniv,
Thanks for your reply.
Well, passive RIAA has its limits, but advantages, too. For example a passive 75uS TC will extend into the mHz region, while active ones tend to stall just outside the audible range.
Also, (and this is purely subjective) this passive RIAA sounds tonally/timbrally more correct to my ear. Thus the impetus to improve it. Not to mention the fact that it is an interesting (to me) puzzle to work out.
Regards,
John
Hi Rayma,
Thanks, again, for your replies.
I will try that for sure, or I could install a different cartridge, I have plenty of choices at home. But ultimately, I think I would like to improve design, if possible. I think a phono pre should be able to play a hot-output DJ cartridge without trouble. My other phono stages have no problem with the Shure.
Best,
John
So, what about cascading two Op Amps for each gain stage? Is that lunacy?
For example, instead of one OP Amp providing 24db of gain, there were two Op Amps each set for 5db of gain for a total of 25db in combination. I would get the amount of gain I need, but with less strain on each device. Am I thinking about this wrong? I think I must be, otherwise this would be a common practice.
Noise increases, but the noise of these ICs is very low, and LPs have a SNR of around 60db anyway. I’m not saying noise is of no concern, but for this project a reasonable amount of noise is probably fine.
For example, instead of one OP Amp providing 24db of gain, there were two Op Amps each set for 5db of gain for a total of 25db in combination. I would get the amount of gain I need, but with less strain on each device. Am I thinking about this wrong? I think I must be, otherwise this would be a common practice.
Noise increases, but the noise of these ICs is very low, and LPs have a SNR of around 60db anyway. I’m not saying noise is of no concern, but for this project a reasonable amount of noise is probably fine.
Passive RIAA EQ is actually a good idea. There are a lot of room to experiment with:
- Convert the 1st opamp to inverting mode. The input resistor should be 47k in this case, and the feedback resistor is in the Mohm range. The 2nd stage could also be inverting. You need to modify the equalizing network a bit.
- Distribute the equalizing between three opamps. The first part between the 1st and 2nd opamp will do the 3180us/318us bass shelf, and the second part between the 2nd and 3rd opamp will do the treble cut at 75us. The latter will cut the noise of all previous stages too.
- The power supply is very important, it has influence on the dynamics. Use batteries if you can.
- 3320 ohm feedback resistor is probably too low, considering the current output capability of the opamp. Multiply it by 10, and the 150R too.
Hi Icsaszar,
Thanks for your reply.
I have seen mention here and there that there is some problem using inverting Op Amps in the input stage for MM, but I can’t remember if it was explained. Is the Miller capacitance higher? Is the input DC offset higher? Is it a myth?
I will make note of the feedback resistor issue. I have read mention of that, too. I’m actually using 5k6/249R, as that was what I had. Still too low according to your estimation.
Regards,
John
Thanks for your reply.
I have seen mention here and there that there is some problem using inverting Op Amps in the input stage for MM, but I can’t remember if it was explained. Is the Miller capacitance higher? Is the input DC offset higher? Is it a myth?
I will make note of the feedback resistor issue. I have read mention of that, too. I’m actually using 5k6/249R, as that was what I had. Still too low according to your estimation.
Regards,
John
With ±15V rails, you should have ~28dB of headroom, which should be adequate. You could look at using a different opamp i.e. OPA1656 which doesn't invert its output when the common mode input range is exceeded. You could also try putting the 75uS TC in the first stage feedback path (active); that will give you 6dB roll off in gain above 2kHz. If you are getting a "crack", it most likely has higher frequency components that will be attenuated by the reduced gain in the first stage.
Noise for inverting is about 14dB higher, from the 47k input resistor thermal noise.
With a MC loaded by 100R to 1k, the noise level would be much lower than for the MM.
Hi Pyramid,
Thanks for your reply!
Good suggestions. I’m actually using an OPA134 on the input. Though, I have wanted to try the OPA165X for a while now, so maybe I will get some more 8dip adapters and give the 1655 a shot.
For this particular preamp, I want to figure out a way to keep the filters passive.
I’m sure it’s possible.
Regards,
John