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Overload margin in Phono pre-amp
Overload margin in Phono pre-amp
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Old 17th June 2020, 10:24 AM   #1
Mark.Clappers is offline Mark.Clappers  Netherlands
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Default Overload margin in Phono pre-amp

Hi all,

I'm designing a flat pre-amp to interface with my turntable (Pioneer PLX-1000 with an Audio-Technica AT-XP5 ). This is going to feed a TI PCM4222EVM which will feed a miniDSP openDRC-DI. RIAA and subsonic filtering will be done inside the DSP. My line level "preamp" is a Topping DX7pro feeding DIY active speakers.

The flat pre-amp will be based on the AD8429 chip, so setting gain is just a matter of putting in the right resistor value.

The AT-XP5 has an output of 5.5 mV (at 1 kHz, 5 cm/sec). So about 55mV @ 20KHz. If I set the gain of the AD8429 to 40dB that gives a maximum output of 5.5Vrms or 3.89Vpk. The AD8429 can ouput a maximum of about 13Vpk on +/-15VDC rails giving me a headroom of 9.12V. This is about 7.4dB headroom.

Is this enough or should I lower the gain of the AD8429 to increase the headroom and avoid clipping on pops / scratches? I would like as much gain in the first amp as possible to get the best possible noise performance.
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Old 17th June 2020, 10:27 AM   #2
KatieandDad is offline KatieandDad  United Kingdom
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Some vinyl has a dynamic range of about 70dB.
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Old 17th June 2020, 10:38 AM   #3
Mark.Clappers is offline Mark.Clappers  Netherlands
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Hi KatieandDad,

Thanks for the reply.

70dB dynamic range is not something I'm really worried about with this ADC. It's going to run @ 48KHz/24bit. 12 bits should be enough to capture the full dynamic range of 70dB. The ADC is capable of much more. Even when running @ a maximum input to the ADC of -20dBFS for best THD performance there are 8 bits or 48dB dynamic range to spare.

So I think I`m good in that regard. It's really the analog front end that I need some guidance on.
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Old 17th June 2020, 10:42 AM   #4
Mooly is offline Mooly  United Kingdom
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Overload margin in Phono pre-amp
Quote:
Originally Posted by Mark.Clappers View Post
If I set the gain of the AD8429 to 40dB that gives a maximum output of 5.5Vrms or 3.89Vpk.
That doesn't sound right. 5.5 volts rms is 7.8 volts peak.

Fwiw, a Pioneer A80 I used to own had the (discrete) phono stages running on -/+45 volts.

I'm a great believer in empirical testing... have you looked at the output on a scope when playing something 'noisy and scratchy'? Sometimes you can research stuff and still not be convinced or come up with a conclusive answer wheras a quick real test can help put things in perspective.
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Old 17th June 2020, 10:43 AM   #5
MarsBravo is offline MarsBravo  Netherlands
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Try to calculate the required slew rate @ 20kHz. With a dynamic range of 40dB you're needing >500V/μsec up to the fifth harmonic (cmiiw). That's why phono pre's correct while amplifying.
Or your DSP must have an extreme low noise floor.
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Old 17th June 2020, 10:46 AM   #6
MarsBravo is offline MarsBravo  Netherlands
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Quote:
Originally Posted by Mooly View Post
...Fwiw, a Pioneer A80 I used to own had the (discrete) phono stages running on -/+45 volts...
A Technics phono pre (unknow which model) runs it on +90Vdc.
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Old 17th June 2020, 10:50 AM   #7
Mooly is offline Mooly  United Kingdom
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Overload margin in Phono pre-amp
90v single ended would be similar and so plenty of headroom with those.
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Old 17th June 2020, 10:51 AM   #8
billshurv is offline billshurv  United Kingdom
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Overload margin in Phono pre-amp
Hi Mark,


That is veyr similar to the setup I will be ending up with. I need to check the TI EVM specs but ISTR 4V for FS. So you work back from that. A good rule of thumb is to set 5cm/s to be IIRO -23dBFS on the ADC. That will deal with all but the most extreme clicks. There is a school of thought that says you can clip on scratches as long as the amplifier recovers quickly.



There are more mathematical ways to calculate this, but there will always be a bit of suck it and see due to variability in records.
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Old 17th June 2020, 10:57 AM   #9
Mark.Clappers is offline Mark.Clappers  Netherlands
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Hi Mooly,

You are right, I miscalculated. So the headroom with 40dB gain would only be 4.4dB.

Right now I'm in the design phase of this little project (actually I got my Basket filled with parts @ Mouser, not ordered yet). So no measurements possible yet.

I will of course order a couple of 0.1% resistor values to play around with.
+/-45V rails will give a lot of extra headroom but that's not possible with this project. I hope to keep it simple (I'm hooking up a some evaluation boards and HW I have laying around).

Hi MarsBravo,

Slew rate @ 22KHz /13Vpk is about 1.8V/uS. This is the worst case situation. The AD8429 can do 22V/uS so plenty of headroom in that regard I think.

The Opendrc DI noise floor (from miniDSP forum)
digital I/O noise floor........................... 126.07dBFS due to asynchronous S.R.C. dither noise
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Old 17th June 2020, 11:09 AM   #10
Mark.Clappers is offline Mark.Clappers  Netherlands
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Hi billshurv,

Good point, so to get the best THD performance the maximum input signal should be about 0.4Vrms to the EVM.

With 55mV maximum from the cart the gain should only be 7.3X or 17.3dB.

That will insure plenty of headroom in the pre-amp stage. Noise performance will suffer a bit, probably around 4nV/SQRT Hz.
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