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A couple of quick circuits
A couple of quick circuits
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Old 15th November 2019, 06:28 PM   #1
Douglas Blake is online now Douglas Blake  Canada
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Default A couple of quick circuits

I've been doing some updating on my existing circuits and thought it might be fun to submit them here for comments...

The first thumbnail is an RIAA pre-amp.
The second is the recording side RIAA curve used for testing in LTSpice.
The third is a pretty standard tone control design.
Attached Images
File Type: jpg PreAmp.JPG (119.3 KB, 235 views)
File Type: jpg InvRIAA.JPG (33.3 KB, 237 views)
File Type: jpg Tone controls.JPG (117.9 KB, 236 views)
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Old 15th November 2019, 10:00 PM   #2
Mark Tillotson is offline Mark Tillotson
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RIAA:

Why the 1k extra Johnson noise on the input? That's going to really affect an MC source.

Why 1.5nF load capacitance when typical MM cartridges want a few 100pF only? Is it meant to be switchable?

Why use a 5532 for MC when its noise-optimized for a few k ohms source? Its best to have a separate head-end for MC due to the very low impedance - its hard to optimize one amp for two very different source impedances.

Tone:


The Baxandall circuit is inverting, why not use the 2nd opamp to re-invert the signal, so that both are operating with zero common-mode distortion, and the phase is correct on the output?


Its recommended to keep bipolar bias currents out of potentiometers to reduce noise when turning the pot - if the track is at all scratchy it interrupts the bias to the opamp and causes large output voltage swings. AC coupling the pot will reduce scratchiness to the amplitude of the signal itself, not the supply rails.

JFET or CMOS opamps have so little input current that the input capacitance smoothes the scratchiness away without special measures.


Hope this gives you some pointers for some of the less commonly understood points of design for these circuits(!) I recommend the works of Douglas Self for learning many of these subtler points, esp. "Small Signal Audio Design" and "Electronics for Vinyl"

Last edited by Mark Tillotson; 15th November 2019 at 10:05 PM.
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Old 15th November 2019, 10:12 PM   #3
Douglas Blake is online now Douglas Blake  Canada
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Hello Mark ...

Thank you, comments noted and under consideration.

As I said these are older circuits that worked with older op-amps but there's always room for improvement.
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Old 16th November 2019, 06:27 PM   #4
Mark Tillotson is offline Mark Tillotson
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BTW the LTSpice directive is hard to copy/paste from an image.
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Old 16th November 2019, 06:33 PM   #5
Douglas Blake is online now Douglas Blake  Canada
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Quote:
Originally Posted by Mark Tillotson View Post
BTW the LTSpice directive is hard to copy/paste from an image.
Ahh, yes, I imagine it would be. Sorry.

The ASC file for it is attached...

This generates a recording side curve, which is what is on the Vinyl.
To use it, include it into your schematic, connect it to your input and tune the pre-amp output to be flat.
Attached Files
File Type: asc inv_riaa.asc (830 Bytes, 5 views)

Last edited by Douglas Blake; 16th November 2019 at 06:37 PM.
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Old 17th November 2019, 12:48 AM   #6
sgrossklass is offline sgrossklass  Germany
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This phono stage has a number of issues.

If you want to power it by anything that isn't a battery, input biasing should be changed from a simple resistor divider (R1/R2) to something more elaborate, like R/RC-R (maybe 47k/(47k||22) - 47k). MM phono cartridge impedance ends up in the double-digit kOhms towards the upper end of the audible range, resulting in poor PSRR.

Note that some NE5532s are only specified from +/-5 V up.

1n5 of input capacitance is unacceptably high for the majority of MM phono cartridges, even more so when in parallel to 100k. Ideally, you want some choice there - sockets may be easiest to do (47p - 100p - 220p - 330p - 470p - 680p should cover most cases). Also stick with 47 kOhm unless you have very good reasons. If RF ingress becomes a problem, employ inductive input filtering.

As mentioned, noise performance is totally inadequate for MC use - input noise is about 20 dB higher than what you want in a good MC amp. It's not spectacular but OK for MM.

For MC use I would suggest adding a dedicated prepre fed by an extra set of inputs. Here's a fancy one that is arguably crying for paralleled transistors or some types with lower Rbb' (MPS8599 may be worth a shot). Satisfactory performance is, however, also achievable with just 2-3 discrete transistors.
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File Type: png phonoprepre-4-3.png (10.3 KB, 164 views)
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File Type: asc phonoprepre-4-3.asc (4.3 KB, 3 views)
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Old 17th November 2019, 01:04 AM   #7
Douglas Blake is online now Douglas Blake  Canada
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Quote:
Originally Posted by sgrossklass View Post
This phono stage has a number of issues.
Thank you for commenting. I will consider your input during the next prototyping session.

Quote:
If you want to power it by anything that isn't a battery, input biasing should be changed from a simple resistor divider (R1/R2) to something more elaborate, like R/RC-R (maybe 47k/(47k||22) - 47k). MM phono cartridge impedance ends up in the double-digit kOhms towards the upper end of the audible range, resulting in poor PSRR.

Note that some NE5532s are only specified from +/-5 V up.
Ahh, the 9vdc spec on the schematic was a test. I was varying the voltage to see what differences I should expect. In the final design, the pre-amp will most likely be run on 24vdc, along with the tone circuit.

Quote:
1n5 of input capacitance is unacceptably high for the majority of MM phono cartridges, even more so when in parallel to 100k. Ideally, you want some choice there - sockets may be easiest to do (47p - 100p - 220p - 330p - 470p - 680p should cover most cases). Also stick with 47 kOhm unless you have very good reasons. If RF ingress becomes a problem, employ inductive input filtering.
I'm actually thinking about dropping the MC support. It would make life a lot simpler.
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Old 17th November 2019, 01:22 AM   #8
Mark Tillotson is offline Mark Tillotson
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Quote:
Originally Posted by Douglas Blake View Post
Ahh, yes, I imagine it would be. Sorry.

The ASC file for it is attached...

This generates a recording side curve, which is what is on the Vinyl.
To use it, include it into your schematic, connect it to your input and tune the pre-amp output to be flat.

Ah, that's useful, haven't played with LAPLACE before. Used it with one of my RIAA circuits, got this nice set of response graphs:

Click the image to open in full size.


"emf" in magenta is the output of the inverse RIAA, "preamp_input" in red is with the resistance/inductance and preamp input load applied to this showing the modest peaking and ultrasonic roll-off. green "output" is after my RIAA amp circuit, and then blue "rumble" is after the subsequent 4 pole subsonic filter.


( I modelled 400 ohms and 500mH for the cartridge into 47k || 200pF - it shows the cartridge reactance is quite a factor in the accuracy of the response, being about 1dB of peaking )

Last edited by Mark Tillotson; 17th November 2019 at 01:28 AM.
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Old 17th November 2019, 02:12 AM   #9
rsavas is offline rsavas  Canada
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Thanks for sharing the Laplace directive usage too.
I can share one MC gain stage that I was playing with a long time ago, it was featured in a AudioExpress mag article a few years ago.

iirc it used mpsa12 and its complement mpsa62 for the o/p, I sub'd those because I did not have or know how to make a model for them.
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File Type: asc PLR_MC_phono.asc (8.6 KB, 7 views)

Last edited by rsavas; 17th November 2019 at 02:16 AM.
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Old 17th November 2019, 02:36 AM   #10
Douglas Blake is online now Douglas Blake  Canada
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Quote:
Originally Posted by Mark Tillotson View Post
Ah, that's useful, haven't played with LAPLACE before. Used it with one of my RIAA circuits, got this nice set of response graphs:
It drove me nuts getting that equation right (which might explain quite a lot, actually )

Here are the results from my circuit in the first post. Gotta love that nice straight green output line. It tells me that the recording and playback curves are near perfectly complimentary.
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File Type: jpg Mag Phono Frequency.jpg (70.5 KB, 21 views)
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