Impedance balancing 828 outputs?

[mattstevens87]

Hi Ted,

I’ve just come across an 828 on ebay, and have read that the majority of these have unbalanced direct outputs. It would be nice to get a little better SNR out of it, but I don’t want to trouble the power supply with a glut of active balanced line drivers, and a set of output transformers would probably cost more than what I’ve paid for the desk! Would it be feasible to “pseudo-balance” the direct output, and would you be able to tell me the output impedance? I suppose I could use some 1k trimmers and calibrate it on the AP at work.

Also, could you send me some schematics for the mixer?

Very many thanks for your generosity and the support that you’re providing for these desks!

Matt Stevens

[Ted Fletcher]

Yes, the circuits for the later versions of the 828 are available.

[mattstevens87]

Ted,

Are the schematics posted on the website somewhere? I’ve been unable to find them using google, could you email them to me?

I ended up using 100R resistors to impedance balance the direct outputs. I was going to do some measurements and try using trimmers, but this was a quick fix and allows me to use the balanced jack loom that I have to hand!

The version of the 828 that I have doesn’t seem to have the discrete mic pre, it looks like the transformer (a belclare) goes straight into a 5534. Sounds better than the preamps on my soundcard either way. It also seems that four of the channels direct outs are taken pre-fader, and the other four are taken post fader. I noticed a resistor on the channel PCBs that was soldered into a different position that correlates with this. Would the desk have left the factory like this or was it modified afterwards? I like the extra flexibility it provides, given that the mic amps don’t have a pad.

I have noticed that with phantom power switched on, the power transformer acoustically buzzes. The phantom supply still works, and measures correctly, but I take it that this isn’t normal. I intend to recap the desk soon, and will take a closer look at the PSU then.

Many thanks,
Matt

[Ted Fletcher]

Yes, I agree with quasi-balancing the outputs…. but I would not bother with trimming unless you are thinking of driving huge long lines. My bsolution would be to run the outputs through a series 100R, and connect the return leg to ground through a 100R and a 100uF cap. This gives all the advantages of balanced working…. you lose a little level but +20 should be enough for all but the most extreme of old US systems.
The buzz from the mains tx is fairly normal… the phantom power on those old desks was a bit primitive.
On the channel boards there was provision for pre or post fader feeds and we made a number of ‘specials’ so it’s quite likely that the arrangement you have is original.
I have had many questions about re-capping and I always advise against it unless there is a real problem with performance. In normal use the capacitors should last for about 30 years unless the operating temperature is very high… as it was with old valve gear; in that case the coupling caps tend to dry out….. Basically, don’t bother unless you have to!

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