Audio pre-amp schematic review - Page 1
Forum section: electronics / projects

author: hummusdude
author_rank: Regular Contributor
timestamp: September 08, 2020, 03:35:51 am
content: 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!

author: Etesla
author_rank: Regular Contributor
timestamp: September 08, 2020, 04:01:11 am
content: 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.

author: hummusdude
author_rank: Regular Contributor
timestamp: September 08, 2020, 04:07:55 am
content: 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.

author: MasterT
author_rank: Frequent Contributor
timestamp: September 08, 2020, 12:30:51 pm
content: 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.pdfsaying +-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

author: Vovk_Z
author_rank: Super Contributor
timestamp: September 08, 2020, 02:37:44 pm
content: Quote from: hummusdude on September 08, 2020, 03:35:51 amIt 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?

author: hummusdude
author_rank: Regular Contributor
timestamp: September 08, 2020, 06:12:10 pm
content: The 5V components are powered by the output of the LM2576. I'm using this one because I happen to have it on hand and the efficiency is much higher than using a L7805 linear regulator. I tried using the 7805 but it got quite hot, even when I powered it with 15V from the 7815 instead of 25V from the power supply. I assume that means there is not a small amount of current being drawn by all of the 5V components... 2 74HC595's, 4 TL072's, 2 MCP602's, 2 - 7 segment LED displays and the Nano.

author: hummusdude
author_rank: Regular Contributor
timestamp: September 08, 2020, 06:26:56 pm
content: Quote from: Vovk_Z on September 08, 2020, 02:37:44 pmI thought that as well about the output of the laptop. Even my Moto g6 only shows 200mV pk on the scope using a 1kHz sine wave, same as the laptop with the same 1 kHz tone. My function generator does drive the power amp to max calculated power when I set the signal to 1Vpk. I'm definitely a noob tech so I'm not assuming that I'm not doing something wrong when I measure it. But even when I used the output of the laptop to drive the input of the power amp I still didn't get anywhere near the 29 watts I get with 1V pk input from the function generator. So at this point observed behaviour suggests the output of the laptop/phone is low.So that's why the pre-amp. That, and because I wanted to control the volume digitally.

author: hummusdude
author_rank: Regular Contributor
timestamp: September 08, 2020, 06:28:43 pm
content: Can someone please comment on the use of separate ground paths for power and signal in my pre-amp schematic? I'm very interested to hear if this is an issue given the low power levels overall compared to the power amp.