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. » Tue Aug 25, 2020 4:28 pm

Finally on order...
plusten.jpg
This feels a little like giving birth... I ordered 10 pc proto run (minimum), and a stencil so I can screen on solder paste...

The row of pots across the top are input trims between digital surround decoder and gain stage. I incorporated 1.5x gain from the wiper, doubled with unity gain inversion for total gain of 3x.. This will allow my 1V decoder output to drive Hypex amps to full scale (they need 2.5V AC).

The round puppies across the bottom are old school LEDs in case the SMD LEDs are hard to see...

The 6 green LEDs are one lit at a time for loudest channel.. the 7th red LED is for about 3dB below amp full scale for any channel (this could actually be an improvement patent to my old Peavey FLS patent).

I can not imagine doing another project using this old tired PCB software (eagle version 4.03)... It still indicates about 250 warnings on the schematic engineering rules check.

The PCB design rules check only poots out about 50 warnings related to the SMD LED footprint. Life it too short to be warning free....

JR

PS: I am really looking forward to getting full output from my hypex class D amps. Since this last decoder upgrade I am hearing new improved surround effects from old movies even at -8dB FS.
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mediatechnology
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Re: Cheap Surround sound buffer/level shifter

Post by mediatechnology » Wed Aug 26, 2020 5:36 am

Who makes your boards?
https://ka-electronics.com

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Plandemic Series: "Plandemic - Doctors In Black" https://proaudiodesignforum.com/content/Plandemic.mp4

CDC: "For 6% of the deaths, COVID-19 was the only cause mentioned. For deaths with conditions or causes in addition to COVID-19, on average, there were 2.6 additional conditions or causes per death." https://www.cdc.gov/nchs/nvss/vsrr/covi ... /index.htm

crin
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Re: Cheap Surround sound buffer/level shifter

Post by crin » Wed Aug 26, 2020 8:28 am

sorry to chime in so late… but which Hypex modules are you using? they vary quite a bit in terms of RMS input for full power output…

i assembled a pair of NC400 monobloc amp kits last year. the DIY kits contain everything neccessary to build a complete amp, including a SMPS for each amp module. the amps are rated at 400/200 W 4/8 ohms. i measured them at 8 ohms and noticed that, despite the 200 W rating, they're actually 70 watt amps… they'll put out the rated power for a couple of minutes and then go into thermal shutdown. 70 watts is what they can put out continuously into 8 ohms. i don't know if it's the SMPS or the NC400 module that shuts down – but one of them does.

also, their inputs are very sensitive to DC. initially i used them directly with either the outputs of my metric halo or a DIY line driver – just to have them shut down and set the inputs to nFatal. i don't remember exactely how much DC i had at the input (the metric halo has DC coupled outputs), but it was very little. and divided by the the load i calculated (IIRC) less than 0.2 mA of DC current that caused the input protection to trip.

i ended up installing 1:4 input transformers right next to the amp modules. ever since, no more nFatal. and as the speakers driven by the amp have 98 dB/w/m, i don't need the full power i.e. the voltage swing at the amp input is high enough even after the 1:4 step down.

long story short: make sure you don't have a lot of DC after your buffers…
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that shouldn't be in charge of brooms
(graham parker, "Don't Let It Break You Down")

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JR.
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Re: Cheap Surround sound buffer/level shifter

Post by JR. » Wed Aug 26, 2020 8:38 am

mediatechnology wrote:
Wed Aug 26, 2020 5:36 am
Who makes your boards?
I used https://dirtypcbs.com/store/order/20607 ... f012f739e6.. Thay have a pretty good interface, with a gerber viewer right on their website to confirm the release package is OK before they bundle the order to a cheap chinese board maker.

In hindsight since all these proto services want to make 10x I could have saved money by designing these as 6 smaller individual per channel boards but too late now. :lol:

JR
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JR.
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Re: Cheap Surround sound buffer/level shifter

Post by JR. » Wed Aug 26, 2020 9:21 am

crin wrote:
Wed Aug 26, 2020 8:28 am
sorry to chime in so late… but which Hypex modules are you using? they vary quite a bit in terms of RMS input for full power output…
UcD34MP, apparently now obsolete.
i assembled a pair of NC400 monobloc amp kits last year. the DIY kits contain everything neccessary to build a complete amp, including a SMPS for each amp module. the amps are rated at 400/200 W 4/8 ohms. i measured them at 8 ohms and noticed that, despite the 200 W rating, they're actually 70 watt amps… they'll put out the rated power for a couple of minutes and then go into thermal shutdown. 70 watts is what they can put out continuously into 8 ohms. i don't know if it's the SMPS or the NC400 module that shuts down – but one of them does.
Most likely power supply limited. Real music is not continuous.

For TMI back in the 80s I designed a small power amp (AMR PMA70+) that incorporated cap doublers, bootstrapping the amp's power supply rails for momentary 2x voltage swing, 4x peak power. My little amp only put out 2x35W continuous, but 2x 60W for 15 seconds (limited by a PTC in the PS boost circuit), and more than 100W for mSec peaks (limited by reservoir caps). That little amp sounded more like a 100W amp than 35W, while 35W is all your bench test would ever find without tone burst testing.
also, their inputs are very sensitive to DC. initially i used them directly with either the outputs of my metric halo or a DIY line driver – just to have them shut down and set the inputs to nFatal. i don't remember exactely how much DC i had at the input (the metric halo has DC coupled outputs), but it was very little. and divided by the the load i calculated (IIRC) less than 0.2 mA of DC current that caused the input protection to trip.
I asked them about input DC common mode range and they told me... NO DC. I hoped I could drive their differential + and - inputs from my buffer stage op amps biased up to 2.5V DC to avoid cap coupling, but they advised against it. It was hard to get good information on the modules from the WWW but I contacted customer service and they gave me a link to a proper data sheet https://www.hypex.nl/img/upload/UcD32_07xxxx.pdf. Apparently the UcD modules were intended for OEM use, thus the modest voltage gain (only 12.5 dB) and low input impedance (1800 ohm). Typical stand alone power amps are higher gain and higher input Z, but they expect the OEM customer to handle the front end.
i ended up installing 1:4 input transformers right next to the amp modules. ever since, no more nFatal. and as the speakers driven by the amp have 98 dB/w/m, i don't need the full power i.e. the voltage swing at the amp input is high enough even after the 1:4 step down.
I hope you are driving that transformer from a low impedance source (or have much higher input impedance than I do).

My UcD module input impedance is 1.8k, A 1:4 step up transformer would reflect a 1.8k input impedance down by a factor of the voltage ratio squared (1/16) so 112 ohms at input side of transformer. I am pretty sure my cheap digital surround convertor can't drive 100 ohms.
long story short: make sure you don't have a lot of DC after your buffers…
I am using 33uf electrolytic coupling caps (-3dB @ 2.6Hz) that I just purchased brand new because all the old electrolytic caps in my back lab have been sitting around since the 1970s... :lol:

I have been using these amps in my surround system for almost a year at -8dB below amplifier full scale (also using 8 ohm speakers). I only ocassional have to turn down the volume. I am really looking forward to finally being able to turn the volume up and making full power with these puppies.

JR
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Re: Cheap Surround sound buffer/level shifter

Post by JR. » Sat Aug 29, 2020 9:05 am

OK I had nothing better to do this morning so I looked up a spec sheet on the NC400. The differential input impedance is listed as 104k, so the 4:1 step up input transformer will only reflect that down to a 6.5k ohm effective input impedance. 6.5k input Z should not be heavy lifting. The nominal voltage gain of 25.8dB is 4x the gain of my modules.

It appears the NC400 is designed to be used as self contained fully functional audio amplifier, not an OEM module (like my UcDs) that needed to be integrated into a system involving some glue circuitry for input conditioning.

JR
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Re: Cheap Surround sound buffer/level shifter

Post by JR. » Mon Sep 21, 2020 12:11 pm

I'm starting to get anxious waiting for my PCBs after 3 weeks... I just checked the tracking number and it says they were in Chicago yesterday, so should only be days more.

I hope I remember the design. :lol:

JR
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Re: Cheap Surround sound buffer/level shifter

Post by JR. » Thu Sep 24, 2020 10:26 am

Boards and stencil finally arrived. The stencil is a bit of an extravagance for an only one off DIY project, but I decided to give it a try.

Sweet. 8-)

JR

[edit- I figured out it was possible to try to line up the stencil, upside down... layout was symmetrical enough that it looked close. :lol:

I'll wait until tomorrow to push some paste thru the stencil... My solder paste is pretty old but I feel lucky. /edit]
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Re: Cheap Surround sound buffer/level shifter

Post by JR. » Fri Sep 25, 2020 4:43 pm

I am too old for this sh__. First try at using the stencil was a fail... Perhaps the fact that my solder paste is almost 10 years old didn't help. The second try with a lighter hand made was less of a mess.
buffer.jpg
Don't look too closely, this is without any touch up...The hot air wand blew some small parts around. I also had a beer screaming in my ear for me to drink it. :lol:

Tomorrow I will touch it up, and pop the through hole components. There are some extra footprints that may not be used. If the smd LEDS are not bright enough from my viewing angle I will load in some old school TH LEDs.

Another observation, in the weeks it took for these boards to get fabbed and shipped across the globe, I forgot some my design decisions. :o This was never breadboarded so a certain amount of component values are guesswork, but I think I got it sorted.

JR
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Re: Cheap Surround sound buffer/level shifter

Post by JR. » Sun Sep 27, 2020 4:12 pm

I got the SMD touch up sorted... one of the op amps was missing the pads by a mile, one other was shifted over...

I think I need to use a nozzle on the heat gun want with a bigger opening so it doesn't blow parts all around. Maybe next time.

JR
[update- found another op amp that needed a shift and a few R's that needed reheating. OOPS I found one missing trace, a shorty connecting two SMD parts so easy to bridge across. Moving forward... /update]
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