Cheap Surround sound buffer/level shifter
Posted: Tue Jul 28, 2020 2:54 pm
I decided to move this here as I soak this design in beer at night.
To present the design brief, and catch up everybody on current status,,, First I am a packrat so had enough old speakers sitting around to support a 6 channel system, front L&R, rear L&R, dialog center, and sub... I had to recone the woofers in one 3 way 8" pair of AMR studio monitors, and the woofers in my old 2 way 6" hone hifi speakers. The recone kits were relatively painless. The woofer surround remained intact.
For amps I started out with 3x cheap stereo amps. I miss the light show from the meters but not the mediocre sound. I have long been a fan of Bruno Putzey's class D amps (Hypex) so bit the bullet and purchased a 4 amplifier module and a 2 amplifier module.
I am cheap so went through a few, too cheap to work digital surround decoders. First upgrade was to one with a master volume control, recent upgrade even has a remote volume control.
I had some drama getting the surround channels working but a helpful customer service guy kept nudging me toward the solutions. Both my ROKU TV monitor and the Direct TV satellite had sound settings that affected operation. After that was sorted next problem was a low level hum/buzz that the previous digital decoder did not suffer.
I resolved the low level noise by powering the digital decoder from a 5V supply in the amp (a nice feature).
This leaves one last functional improvement (driving amps to full level), and one feature improvement (more blinky lights). I miss the meters from the cheap stereo amps.
The Hypex amp requires 2.5V input to realize full scale output. This is a little problematic as the cheap digital surround decoders running from 5V wall warts make 1V max output cleanly. Adding a simple op amp buffer with a second inverted differential output gives me a quick +6dB so almost there (2.5V) and reasonable from 5V rail.
I suspect I could grab enough rail voltage from the amplifier power supplies to generate a regulated +/- 15V but that is more effort than justified to make 2.5V. Alternately I could repurpose the now surplus 5V wall wart to provide a -5V rail... Old school op amps like i have laying around in my back lab are still challenged about swinging close to rails.
OTOH modern CMOS op amps easily git er dun with a single 5V rail.
The cons of using a single 5V rail is that I need to bias the audio up to 2.5V and cap couple in and out. The pros from +/-5V rails is I could DC couple inputs and outputs.
I am leaning toward KISS with just one single 5V rail. I am pretty confident I won't hear a few extra capacitors in series. I am leaning toward the opa 2156 TI cmos amp, not cheap but for a one off project why skimp?
Before I resolved the noise floor issues I was leaning toward a balanced or at least a differential stage between the decoder and amp grounds. Now that they are the same ground I can eliminate that stage. I am thinking of using pads on every decoder output to balance levels, while in over a year of use I have never needed to trim level balances yet.
==
Next design decision is about the blinky lights. I have already decided against full meters (like on the old hifi amps), further the only one 5V rail makes stacking LEDs in series ineffective. At a minimum I want one LED for signal resents, say down around -20FS... Then a second LED up around -3dB FS could indicate head room.
After too many beers I have thought about a third LED, a variant on my old FLS invention (The LEDs that light above the EQ slider). I could have a third, middle LED that illuminates for the loudest of the 6 channels. So Signal present and headroom for all channels and a loudest channel LED that moves around.
Any thoughts or suggestions?
JR
To present the design brief, and catch up everybody on current status,,, First I am a packrat so had enough old speakers sitting around to support a 6 channel system, front L&R, rear L&R, dialog center, and sub... I had to recone the woofers in one 3 way 8" pair of AMR studio monitors, and the woofers in my old 2 way 6" hone hifi speakers. The recone kits were relatively painless. The woofer surround remained intact.
For amps I started out with 3x cheap stereo amps. I miss the light show from the meters but not the mediocre sound. I have long been a fan of Bruno Putzey's class D amps (Hypex) so bit the bullet and purchased a 4 amplifier module and a 2 amplifier module.
I am cheap so went through a few, too cheap to work digital surround decoders. First upgrade was to one with a master volume control, recent upgrade even has a remote volume control.
I had some drama getting the surround channels working but a helpful customer service guy kept nudging me toward the solutions. Both my ROKU TV monitor and the Direct TV satellite had sound settings that affected operation. After that was sorted next problem was a low level hum/buzz that the previous digital decoder did not suffer.
I resolved the low level noise by powering the digital decoder from a 5V supply in the amp (a nice feature).
This leaves one last functional improvement (driving amps to full level), and one feature improvement (more blinky lights). I miss the meters from the cheap stereo amps.
The Hypex amp requires 2.5V input to realize full scale output. This is a little problematic as the cheap digital surround decoders running from 5V wall warts make 1V max output cleanly. Adding a simple op amp buffer with a second inverted differential output gives me a quick +6dB so almost there (2.5V) and reasonable from 5V rail.
I suspect I could grab enough rail voltage from the amplifier power supplies to generate a regulated +/- 15V but that is more effort than justified to make 2.5V. Alternately I could repurpose the now surplus 5V wall wart to provide a -5V rail... Old school op amps like i have laying around in my back lab are still challenged about swinging close to rails.
OTOH modern CMOS op amps easily git er dun with a single 5V rail.
The cons of using a single 5V rail is that I need to bias the audio up to 2.5V and cap couple in and out. The pros from +/-5V rails is I could DC couple inputs and outputs.
I am leaning toward KISS with just one single 5V rail. I am pretty confident I won't hear a few extra capacitors in series. I am leaning toward the opa 2156 TI cmos amp, not cheap but for a one off project why skimp?
Before I resolved the noise floor issues I was leaning toward a balanced or at least a differential stage between the decoder and amp grounds. Now that they are the same ground I can eliminate that stage. I am thinking of using pads on every decoder output to balance levels, while in over a year of use I have never needed to trim level balances yet.
==
Next design decision is about the blinky lights. I have already decided against full meters (like on the old hifi amps), further the only one 5V rail makes stacking LEDs in series ineffective. At a minimum I want one LED for signal resents, say down around -20FS... Then a second LED up around -3dB FS could indicate head room.
After too many beers I have thought about a third LED, a variant on my old FLS invention (The LEDs that light above the EQ slider). I could have a third, middle LED that illuminates for the loudest of the 6 channels. So Signal present and headroom for all channels and a loudest channel LED that moves around.
Any thoughts or suggestions?
JR