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Audio pre-amp schematic review

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hummusdude:
I'm building a pre-amp and tone circuit for a power amp that I built. The power amp is courteous of Scott Campbell at Circuit Basics.com. https://www.circuitbasics.com/design-hi-fi-audio-amplifier-lm3886/ It's a 40 W design using an LM3886 gainclone chip. I've built the two channels and the power supply and they work great, very quiet and getting about 29 W into 8 ohms ( his design assumes 6 ohm speakers).

But I ran into a small problem when I tried to drive it with my laptop headphone jack. The output of the jack is only about 1/5 of the input source he designed the input stage around. I know I could have tweaked the input to boost the sensitivity, but I also wanted some kind of volume control to throttle the amp because I also use my phone a lot and sometimes when I listen to podcasts I have to turn the gain all the way up. So the volume control is also to prevent accidental full-volume start ups if I forget to turn down the volume on the phone. So not wanting to introduce any changes to Scott's circuit I decided to build a simple pre-amp and found a circuit online that looked simple enough. I also added a Baxandall tone circuit from Elliot Sound Products for a little bit of EQ. For fun I added digital control of the pre-amp gain using a digital pot and an Arduino Nano. And for even more nerdy fun I hacked an old HDD (Thanks to Great Scott!) to use as a rotary encoder input to the Nano.

I've breadboarded all of the control circuitry and everything works great. I'm getting close to ordering a pcb and I was hoping to get some feedback about my schematic. I haven't yet done the board layout...waiting until I'm sure about what I've done so far.

Some explanation about the design choices...

The power supply output is 25 VDC and I'm tapping that to drive the digital circuits, hence the 7815/7915 linear regulators for the audio  signal op amps and the LM2576 to power the 5V components.

I haven't yet added any power ground paths because I'm not sure it's necessary. The LM2576 is a switching regulator, so maybe that's a problem. But it also doesn't enter the signal path...only provides 5VDC to the Nano and some control chips. I did get some pretty nasty high frequency oscillations when pushing the pre-amp close to it's limit. Not sure if that's the buck converter or perhaps just the result of having it all on the breadboard...see pics. But considering the rather low levels involved I'm not sure a separate ground path is needed. But that's one more reason to post here and ask.

Appreciate any input or comments!

Etesla:
For what it's worth, I've tried to use those digitally controlled potentiometers on two occasions, and on both occasions I found them to be too fragile and inconsistent to work out. Just a heads up, you might want to put unused footprints on your PCB for real potentiometers in parallel with the digital ones as a backup for the first REV of the board.

hummusdude:
I've had mixed results with them as well. I've only been able to get the bare DIP chips to work, never the small break out boards you can find on Ebay/Amazon for a few bucks. I guess I always assumed the cheap imports were of questionable reliability but good to know your experience. Thanks for the suggestion to add parallel connector points in case I have to re-work that some day.

MasterT:
You can't use +5V digital pot in circuits running from +-15V. Data sheet is confusing,
https://www.renesas.com/us/en/www/doc/datasheet/x9c102-103-104-503.pdf
saying +-5V in specification table & +-8V as absolute maximum. Usually voltages at any pin shouldn't go below 0 and above +5V supply voltage, so this applicable only with audio OPA running in the same range, single power +5V. Like LME49721

Vovk_Z:

--- Quote from: hummusdude on September 08, 2020, 03:35:51 am ---But I ran into a small problem when I tried to drive it with my laptop headphone jack. The output of the jack is only about 1/5 of the input source he designed the input stage around. I know I could have tweaked the input to boost the sensitivity, but I also wanted some kind of volume control to throttle the amp because I also use my phone a lot and sometimes when I listen to podcasts I have to turn the gain all the way up.

--- End quote ---
It is a bit strange to hear about small laptop output. I guess it gives at least 1 V. A gain of power amp from the link is 20k/1k + 1 = 21 (typical for any IC power amp, +- 50%). That means everything has to be ok with output voltage level.
If laptop gives about only 0.5V AC max then power amp output will be 0.5*21 = 10.5 V AC. That's not few, but not too much.
If laptop gives about 1.0V AC then power amp output will be 1.0*21 = 21 V AC. That's ok.
(Given that power amp has 20k input impedance) you can use a 2k-5k potentiometer for volume control (without any preamp).
Or may be I don't understand a problem?

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