Cheap Surround sound buffer/level shifter

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JR.
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Re: Cheap Surround sound buffer/level shifter

Post by JR. » Thu Oct 08, 2020 9:25 pm

I am about ready to stick a fork in this... the sound is great, but the blinky lights are a little disappointing. No doubt due to the modest transconductance of a compound LTP in an audio path scaled to run in a 5V rail.

I can see activity in the lights responding to program but nothing to send to youtube. I am reminded that back in the FLS days, I added a circuit trick where the feed to the FLS compound LTP came from the BP op amp outputs, but they had anti-parallel diodes in series with the op amp output and the actual BP filter NFB. This way they were two junctions hotter.

To use that trick on my 5v rail design would consume 2 diode drops of signal swing, from an already stingy 5V rail.

So... If it sounds good, it is good... and I can see some lights modulated, so I am happy... 8-)

JR

[update- after living with this another day the loudest channel display is pretty universally indicating Front Left and Front Right as loudest, this seems normal for surround movie mixes, since you want the action localized up front, and surround effects are at reduced levels. /update]
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Re: Cheap Surround sound buffer/level shifter

Post by JR. » Wed Oct 14, 2020 8:06 am

I decided to fire up my TS-1 and confirm all 6 channels were going where they should be, and rocking... I had left and right fronts and backs reversed (left for right) so replugged them correctly. The one channel that was missing was the sub... so back to the bench.

While a little unexpected I found the signal break caused by a missing pad at the now removed pot wiper. This larger than normal pad was anchored by a plated through hole so should have been more robust. I bridged the signal to where it needed to go with a TH resistor wire lead so now it is all good.

My test track for LF is the beginning of "Lucy" movie... perhaps not super low frequency, but even more impressive now with 8dB more signal.

I am still unsure about hitting full output because I haven't seen the red overload LED flash with movie signal inputs, but it gets uncomfortably loud so I am happy... 8-)

JR
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Re: Cheap Surround sound buffer/level shifter

Post by JR. » Thu Oct 15, 2020 10:33 am

Last night while channel surfing I dropped into the middle of a movie I had seen before (The equalizer with Denzel Washington playing a one man army).

The improved experience from a larger screen 65" monitor and dialed in 5.1 surround sound system made it almost seem like watching a different movie... 8-)

JR
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mediatechnology
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Re: Cheap Surround sound buffer/level shifter

Post by mediatechnology » Thu Oct 15, 2020 5:11 pm

Much better than it would have been with Left and Right reversed...
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Re: Cheap Surround sound buffer/level shifter

Post by JR. » Thu Oct 15, 2020 9:39 pm

mediatechnology wrote:
Thu Oct 15, 2020 5:11 pm
Much better than it would have been with Left and Right reversed...
I don't think that was it but found a new bug in the TV monitor... It has a built in "leveling" compression function to boost up quiet programs. This was very useful before I cranked in 8dB more gain to my amps... Now movies sound better with the compression turned off and the gain cranked up full dynamic range.

But today while watching some sit com there was weirdness with the leveling turned "off"... The vocals were dropping out (like a L-R applied to mono signal), while the surround (laugh track) was still at full volume. When I turned on the leveling the vocal track returned to normal so some kind of bug in the DSP...

I am still very happy with it.

JR

PS: I tried another tweak to the blinky lights but still not much blinky action. I can see that most level is in Left Front and Right Front.
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Re: Cheap Surround sound buffer/level shifter

Post by JR. » Fri Oct 30, 2020 9:01 am

I am about ready to stick a fork in this... The sound quality has been good almost from day one, I heard some level related distortion the other day. It sounded like raspy digital overload, but it did not change when I dropped my levels and was only in that one movie. In fact now that my surround system is dialed in I am listening to movie sound tracks more critically, and hearing more flaws or poor sound design choices.

My clever "loudest channel" meter display is apparently not so clever when running a nominal mV level audio path (because of 5V rail). I have been able to massage a little more sensitivity out of the display by shunting the LEDs with fixed resistors to soak up the current when all are not cut off because louder channels just aren't that loud. Vbe difference needs something like 3mV to steer current one dB. After all my tweaks I can finally get some blinky lights when volume is cranked up too loud to comfortably listen to.

I could add a shunt resistor across my 200 ohm output resistors to pad down the output several dB to get more display at modest listening levels but why give away output drive after all this work to get it. :lol: When working the display is pretty boring with Left and Right front channels loudest 99% of the time, so why bother?

Another observation, the built in EQ in both my monitor and my decoder are crude exaggerated presets, perhaps useful for (some) music.

JR
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