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Modular phono preamp
Modular phono preamp
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Old 5th June 2019, 06:41 PM   #1
Mark Tillotson is offline Mark Tillotson
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Join Date: May 2018
Location: Cambridge UK
Default Modular phono preamp

I've just finished the build for the first pcb of my modular phono preamp project.

It does RIAA compensation and basic amplification suitable for an MM input.

I intend to design an MC stage pcb, an input connector pcb, an output connector and power pcb (perhaps with battery option), a rumble filter pcb. They will slot together with dupont-style edge connectors, allowing various configurations.

The two channels are independent in this pcb, roughly mirror-image, allowing a star-ground to be used at the power supply end.


Click the image to open in full size.

SMT is used where possible to keep size down. PPS capacitors seem to be the best compromise for low distortion and SMT availability - here a single value, 10nF, is needed.

The RIAA circuit topology is slightly unusual in that each of the poles and zeroes is an explicit separate RC pair. Care has to be taken to keep the opamps stable into capacitive loads in this topology (the 33pF capacitors and 470 ohm resistors).

The nominal component values give exactly 75s, 318s and 3180s time-constants. Alas I could only source 2% PPS capacitors, but 0.1% thin-film resistors are used throughout.


Where DC-blocking is needed I used back-to-back electrolytics of high value with a centre bias to keep them formed over time. The 22M resistors were a guess for this purpose - the time constant is measured in hours so in theory won't have any impact audibly.


Click the image to open in full size.

There is no input DC-blocking capacitor, as my calculations show the tiny bias currents of opamps are not going to have any effect on a MM pickup (despite what a lot of people glibly state). 500nA in a 1k5 winding gives an offset of a fraction of a mV, and a few mA-turns of magneto motive force, insignificant compared to signal levels, especially when the inefficiency of the MM cartridge as a motor is considered. I think I'd need to see some hard evidence to the contrary, such as a plot of THD v. offset current.
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Old 5th June 2019, 06:57 PM   #2
dreamth is offline dreamth  Romania
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ENJOY it now You don't need to explain yourself.That is what most people do out of fear of being judged for their different experience.Yet you won't have any problem with a 4.7uf/16v Nichicon Muse bipolar either as they were already measured for less than -120 db THD when a dc bias less than 400mv...
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Old 5th June 2019, 11:38 PM   #3
Mark Tillotson is offline Mark Tillotson
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Done some more testing now with signal generator and inverse RIAA network, all seems well and the circuit is stable (well, with +/-7.5V rails at least). Time to layout some more pcb's for the project
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Old 10th June 2019, 01:48 PM   #4
rsavas is offline rsavas  Canada
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Looks good Mark, have you listened to it yet? measurements? nice that we can make cheap high quality pcbs for our protos. looking forward to your MC implementation.
I remember one of my first phono amps I built, using a NE5533 dual as shown in the Signetics app note.
What do you have for RFI filtering at the input? I see that Self does not use any either in the Elektor 2012 phono design.
One thought you could combine MC/MM on one pcb since your are isolating channel grounding.
I am not sure of the advantages or finding a way to demonstrate that some of your design choices have merit for but hey why not experiment.
Do not think I have ever seen a audio design where they bias an ecap such as R39,41
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Old 12th June 2019, 11:03 PM   #5
Mark Tillotson is offline Mark Tillotson
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Join Date: May 2018
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The whole point of modular is not to combine functions on one pcb!


The unusual filter topology was just conceit really - its not efficient in opamps, but does make the component calculations trivial. It would be more rational in a general purpose equalization unit as each pole and zero can be individually switched/controlled. Of course the idea is limited to real poles and zeroes only.
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Old 14th June 2019, 01:12 PM   #6
primalsea is offline primalsea  United Kingdom
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Nice!

Where the PCBs made locally or did you use an online service?
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Old 14th June 2019, 11:16 PM   #7
Mark Tillotson is offline Mark Tillotson
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online, ragworm
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Old 16th June 2019, 07:52 PM   #8
Mark Tillotson is offline Mark Tillotson
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Join Date: May 2018
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Testing with two tone signal into inverse-RIAA network (-45dB or so at 1kHz), via modular preamp (+30dB at 1kHz) to home-made FFT engine running from an ADS8885 18 bit SAR ADC.
Click the image to open in full size.
44kHz and 4.8kHz tones. The 0dB level is about 2.8V rms IIRC


The output level is thus about 25mV per peak, corrsponding to about 2mV at RIAA amp input around 4kHz. The peaks are about 82 dB above the IM3 products at the 3rd harmonic frequencies.


The rise in noise floor at low frequencies is mainly due to the greater attenuation of the inverse RIAA network at low frequencies. Since a flat-top window was used this plot's noise floor is rather higher than a Hann window would give, I forgot to do a Hann plot.


Anyway I'm happy the preamp is performing OK from this - the FFT system is very much a work in progress I should point out.


The signal source is a lowly WM8524G, but I'm working on one based on the PCM1796 with better performance.
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