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Interesting Soundcraft 1600 mod results
Interesting Soundcraft 1600 mod results
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Old 6th January 2020, 07:57 PM   #1
bwithwings is offline bwithwings
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Join Date: Jun 2010
Cool Interesting Soundcraft 1600 mod results

Hi all

I've been slowly recapping and modding my Soundcraft 1600 console, and have some interesting measurement results I thought I'd share.

I have done all mods in sets of two channel strips, and measured the results with a MOTU 828mk3 interface using Room EQ Wizard.

By far the biggest sound improvement was recapping the power supply. Do this first if you're recapping an old console for sure! Also I would recommend to replace any TO-3 transistor sockets, I had one that was on the way out (weak contact pressure on the pins) and might not have noticed if I hadn't specifically looked at the socket. The sound improvement was noticeably tighter bottom end throughout.

I haven't been able to hear any improvement in sound with any opamp swaps.

Mods done to channel strips are:
* Recapped electros with Panasonic FC (Green traces)
* Recapped with Pana FC, and also recapped film caps with WIMA FKP2 (Blue traces) Some values are not available so used other brand of non-metallized film cap for those
* Recapped with Pana FC, and swapped out input transistors with ZTX951 (Orange traces)

The red traces are an unmodded channel strip with original 80's components.

Click the image to open in full size.
Click the image to open in full size.
where can i upload my pictures online

As you can see, recapping gave me back my bottom end, and everything was calibrated to within 0.1dB when making measurements.

The distortion figures are pretty interesting. Going to Panasonic FCs gave a good reduction in distortion over stock, until we get to the top end which is actually a bit worse.

But recapping the old films to new WIMA films gave a massive improvement, which is pretty audible. It actually IS worth recapping those old film caps in this case.

It looks like while the ZTX transistors have lower noise than the 2N4401 stock, they are a bit more distorty in this circuit.

Hope this helps someone! I'll post the actual cap values I used when I get a chance; I collated several old threads from many sources and ended up with some solid recommendations that improved bottom end response, as shown above.

Cheers
Darren
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Old 6th January 2020, 10:14 PM   #2
Mark Tillotson is offline Mark Tillotson
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Interesting. Were they mylar film caps originally?
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Old 6th January 2020, 11:05 PM   #3
bwithwings is offline bwithwings
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Here's a photo:

Click the image to open in full size.

Some of them are marked "MKT 1.67" on the side, which Google tells me is possibly a Phillips polyester film cap.
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Old 7th January 2020, 04:27 AM   #4
bwithwings is offline bwithwings
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Here's distortion plots for a stock subgroup, and a subgroup recapped with Panasonic FCs:

Click the image to open in full size.

Not as marked a difference, but still an improvement. It also picked up a few dB under 20Hz:

Click the image to open in full size.

The Soundcraft 800b, 600b and 400b are all pretty similar to the 1600 so I'd expect you would see similar results with those desks.
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Old 7th January 2020, 06:46 AM   #5
dotneck335 is offline dotneck335  United States
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If I am reading your graphs correctly, they say that the ZTX951 produced 20 db more distortion than the stock transistors---us that actually correct?
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Old 8th January 2020, 04:26 AM   #6
bwithwings is offline bwithwings
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Quote:
Originally Posted by dotneck335 View Post
ZTX951 produced 20 db more distortion than the stock transistors
You want to look at the green trace (Panasonic FC recap) and the orange trace (same, but also with ZTX Q's).

The ZTX strips had about 6dB more distortion at 10Hz and 20kHz, and maybe 1dB more around the 1kHz mark.

Next two strips I recap, I plan to give the 'trans-amp mod' a go (as well as recapping the electro and film caps) and see what difference that makes in comparison. I have 10x 1600 channels left to recap, and then 10x 400b channels in a sidecar after that, so plenty of room left for experimenting!

Hrm... maybe I need some transformers in there...
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Old 8th January 2020, 12:48 PM   #7
Mark Tillotson is offline Mark Tillotson
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The latest graphs have 50dB/div which is hiding all the detail. Spectra are usually given at 10dB/div so they can be compared by eye meaningfully. And distortion scale ought to normalized so 0dB is the fundamental level too really...
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Old 15th January 2020, 04:29 PM   #8
sgrossklass is online now sgrossklass  Germany
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Seeing a further >12 dB mid-band improvement just from swapping polyester film caps out for polypropylene is quite shocking, really. Mind you, it seems that there are a lot of tone stages using them, and the distortion spec for this mixer at 0.01% isn't that awe-inspiring to begin with...

Worse performance with the Zetex devices may be rooted in lower beta and higher Ccb. All hail the mighty 2N4403! (Quite franky, these transistors with their Rbb' of 17 ohms or thereabouts are amply sufficient for a nominal 200 ohm mic input.)

Going through the schematics, I noticed this and that:

1. Phantom power supply arguably is not as low-noise as it could be - reference zener ZD1 is run a bit on the hot side (5-11 mA), and there's a measly 4.7 F of parallel capacitance (C9). I'd look for a matching low-noise zener of adequate wattage and would up C9 to whatever 25 V part will still easily fit - comparable designs have seen several hundred F there.

2. The headphone amplifier (using a Class B buffer with "cheating resistor" to mitigate the worst crossover distortion) isn't the last word in fidelity.

3. The mic inputs have obviously survived for long enough without major damage, but still not seeing any protection whatsoever makes me a bit uneasy.
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Old 16th January 2020, 12:43 PM   #9
Mark Tillotson is offline Mark Tillotson
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Quote:
Originally Posted by sgrossklass View Post
Seeing a further >12 dB mid-band improvement just from swapping polyester film caps out for polypropylene is quite shocking, really.
Well mixers use high signal levels and polyester distortion goes up as the square of the signal voltage IIRC.
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Old 18th January 2020, 03:45 AM   #10
sgrossklass is online now sgrossklass  Germany
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Which is what you'd expect from dominant 3rd order distortion, as is typical for nonpolar capacitor nonlinearity. 3rd climbs 18 dB per 6 dB of signal in small-signal terrain, or 12 dB relative to fundamental.

Yeah, high levels are a likely contributing factor, but I was still surprised to see the effect being that dominant given the mix of TL072 and NE5532 opamps. They were treating their TL072s sensibly though - if you give them supplies of close to max, keep their output loading at 20kOhm and up and use them inverting if possible, these really aren't too terribly bad. They may still be accounting for the gentle rise towards the high frequencies that is visible now.

Last edited by sgrossklass; 18th January 2020 at 03:53 AM.
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