deveng

Summing amp upgrade
« on: August 19, 2009, 04:29:37 PM »
Gents,

I just finished this upgrade to my Soundcraft 400B console.  The pcb sits inside what was once an empty space in the console meter bridge.  The board replaces the master section's summing amp, fader buffer amp and monitor drive amp circuitry.  The wiring was quite tedious as it all had to fit in the small space available.  The wiring harness snakes down through a hole where the meter wires were originally run so no console frame cutting was required.  It uses my custom opamps with sockets so I can quickly replace or try other discrete opamp options.  Since there was not much room in the back of the console I mounted the output transformers at the back of the console.  There are 2 Jensen transformers for the summing output and 2 Reichenbach transformers for the monitor.  It all tested fine using RMAA.  The original sum amp was a NE5532 and was not bad sounding at all but I wanted some Mojo.  The addition of the discrete opamps and Jensens does just that.  I tested the sum bus prior to and after the mod.  The new summing amp stands up nicely with respect to distortion and frequency response tests and so far I'm very pleased.  The original circuit used a TL072 to drive the monitors unbalanced.  The mod should improve the monitoring to closely match the sound quality of the summing bus outputs.  I've not done any critical listening tests as yet but so far its sounds good.   I recently finished the ground bus mod, the full console upgrade (caps, ICs, mic pre transistors, power supply, etc) and found a bit of extra 60Hz.  I just fixed that today and now have a S/N on the stereo bus at -101dB.  With some additional grounding improvements I hope to improve on that and reach -104dB or a bit better.  As it is now I can crank the monitor volume to maximum and there is only a nice clean hiss.  Next up will be some critical listening. 

regards,
Jeff




plumsolly

Re: Summing amp upgrade
« Reply #1 on: August 19, 2009, 04:34:32 PM »
That looks awesome. Congratulations! I am working on something similar with my Auditronics 110 using Classic Audio Products of Illinois' summing buss boards. Best, Ben

radiance

Re: Summing amp upgrade
« Reply #2 on: August 19, 2009, 04:36:37 PM »
Very nice work! Maybe replace that steel plate with a plexiglas one, so you can see those nice DOA's  ;)
"Knowing that you are dreaming, however, does not automatically guarantee full rationality.
Then again, being awake doesn't ensure good thinking, either." -  Lynne Levitan

deveng

Re: Summing amp upgrade
« Reply #3 on: August 19, 2009, 04:42:24 PM »
Be careful and think about your wiring to and from the summing amp boards.  It's very easy to mess up a perfectly good summing amp especially if its at a distance.  Even after planning my mod carefully, I found some issues with noise that I had to correct before getting the S/N I hoped to achieve. 

Jeff

deveng

Re: Summing amp upgrade
« Reply #4 on: August 20, 2009, 06:48:33 PM »
Just to clarify for anyone working on an upgrade to their summing amps I've listed the measured noise results for my mod.

20-20kHz AES17

2 channels summed:
-99.5dBu  RMS
-101.3dBu A weighted

24 channels summed:
-88.5dBu RMS
-91.8dBu A weighted

I still see a bump at 60Hz (also at 180Hz and 240Hz) with 2 channels summed at about -111dBu.  24 channels summed its at about -93dBu.  At these levels internal grounding and interconnection to external equipment is important. During my debug I found that I had 2 chassis ground points in my power supply and this very small ground difference resulted in almost 2dB degradation of signal to noise.  I hope improve my setup a bit more by increasing the ground wire size from console to power supply.

So if you upgrade your summing amp, again pay close attention to every detail of wiring, grounding, power supply connection etc etc. It can make the difference between a very good S/N and an average one.  When I first started my debug I had only about 80dBu with 2 channels summed!

Oh, and frequency response is quite good too.  It's -0.2dB/-1.5dB  at 20Hz to 43kHz.

regards,
Jeff
« Last Edit: August 20, 2009, 06:58:04 PM by deveng »

SIXTYNINER

Re: Summing amp upgrade
« Reply #5 on: August 21, 2009, 02:03:35 PM »
compliments!
well done
 8)
>>" 1 Picture say what billions words cannot "<< - >>"SMD Kill the DIY STAR"<<
Why in the  forum  if no share

THE BAD DOGS

Re: Summing amp upgrade
« Reply #6 on: September 04, 2009, 01:09:20 PM »
Hi,

Nice work.

I've just buyed a soundcraft 600. Is this possible to have a schematic for this summing amp.

Did you sell your special opamaps.

And so at last which chips, transistors and caps you choos e to upgrade the channels?

Thanks.
www.myspace.com/thebaddogs

deveng

Re: Summing amp upgrade
« Reply #7 on: September 04, 2009, 06:09:11 PM »
Hey Bad Dog,

I sent you a PM. 

The 600 summing amp may be different so my schematic may or may not help.  I don't sell my custom discrete opamps yet  but they may be available in the next few months within my mic preamps that might hit the market  ;)

Regards,
Jeff

Flundran

Re: Summing amp upgrade
« Reply #8 on: September 04, 2009, 10:12:03 PM »
Just to clarify for anyone working on an upgrade to their summing amps I've listed the measured noise results for my mod.

20-20kHz AES17

2 channels summed:
-99.5dBu  RMS
-101.3dBu A weighted

24 channels summed:
-88.5dBu RMS
-91.8dBu A weighted


Hello,
Are these figures from mic input -> main output signal path? How do you set up gain and attenuation controls during measurements (fader positions for example)? I haven't done this myself, I am just curious on how to do it.

/F

deveng

Re: Summing amp upgrade
« Reply #9 on: September 05, 2009, 10:48:07 AM »
Flundran,

My measurements were line in to sum out.  I set the channels faders to zero, set the master fader to -5 then adjusted the line/mic trims to get the proper RMAA level for test.  I've also done line in to direct out THD%+N, IMD and frequency response tests to get on the channels an idea of how the channel inputs behave.  To measure 24 channels summed, all channel faders are set to zero, EQ's switched out, all sends are set to zero level.  Each channel mic/line trim is adjusted individually prior to running the test to ensure the gain is the same.

Regards,
Jeff


deveng

Re: Summing amp upgrade
« Reply #10 on: September 05, 2009, 10:54:44 AM »
One update on this mod.  I mentioned using a large gauge wire to replace the original 18awg ground and chassis ground wires in the power supply harness.  I used a #12 wire and found only marginal improvement in that -111dB 60Hz bump.  It may have only improved it by 0.3 to 0.5dB.  I was quite surprised it didn't improve more.  So, I'll leave these large gauge wires in since they certainly won't hurt and I'll have to look elsewhere to get a bit more improvement.   I suspect at these levels, any improvements will be small and it may be several changes to get any significant (1dB or more) move in the S/N.  I'll keep looking.

Regards,
Jeff

thomasdf

Re: Summing amp upgrade
« Reply #11 on: August 10, 2011, 06:34:15 PM »
Just sent you a PM!

Congratulations for your mod. It's awesome!

Thomas


 

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