Halving mid frequency on 12-48 and 828

[jason_tsh]

Hi Ted,

I was wondering if you could tell me how I would go about halving the mid frequencies on the EQ sections in my Alice 12-48 and 828 mixers? They both have a lowest mid sweep frequency of 700hz, whereas my Alice 2008 goes down to 350hz, which is perfect for me, for cleaning up muddy sounds. Actually, being able to go down to 200 or 250 would be even better, but I am guessing that maybe it’s easier to halve the value of the mid frequencies, rather than changing it to some random frequency?

It’s probably obvious that I don’t know much about electronics, hehe, but I am handy with a soldering iron, so changing out some components for different value ones should be within my grasp, as long as I know what I’m replacing with what.

Any help would be greatly appreciated, as this is the only thing stopping me from using the 12-48 at the moment.

Thanks,

Jason

[Ted Fletcher]

Hi Jason,

It is possible to alter the range of the mid control on the 1228, (as long as the version is Version 2, which is most likely).On the channel circuit board near IC4 there are two capacitors, C14 and C15. The original value is 6n8 (6.8 nanofarads). If you change these capacitors to say 15n, that will reduce the operating frequency of the mid control and should not affect anything else.

[jason_tsh]

Excellent! Thanks very much Ted.

The mixer is a 12-48, which you made a little later than the 12-28 I think. It doesn’t have transformer balanced preamps. I will check the circuits on my 828, 2008, and 12-48 for comparison, and will try this on one channel and go from there.

Thanks so much for your help.

Jason

[jason_tsh]

I’ve just finished modifying the mid-sweep frequency on the 1st channel of the 12-48 mixer, and it has worked a treat! I’m very happy.

I will gradually do the same to the other 11 channels over the next week.

Now, how do I go about narrowing the Q on the mid-sweep a bit? It is very wide (and smooth!) at the moment, and I’d like to try narrowing it a bit. When the sweep control is set to the lowest frequency (around 400hz now), with maximum cut (15db), the dip spreads all the way from 100hz up to 4khz. Ideally I’d like to reign it in a little bit. I don’t need surgical eq by any means, and I love how smooth it sounds, but it would be great to narrow the Q a little, so that when the sweep is set to 400hz and cut fully it leaves everything above 1khz alone.

Ted, it was indeed 6n8 caps (which I changed to 15nf as you suggested). Would you be able to suggest what component/s I need to change to narrow the Q slightly?

Thanks ever so much for your help so far. I’m extremely happy to have even got this far!

[Ted Fletcher]

The mod to change frequency is relatively simple, but altering the Q of the circuit is a lot more difficult!

I’m afraid that particular circuit has a fixed Q….. but it is very smooth!

(Making it sharper would cause phase problems at the transitions…. no, if you need something sharper you will have to use a plug-in.

[jason_tsh]

In practice, I’ve found that it works absolutely fine, and yes, it does sound very smooth.

Thanks for all your help Ted. I really appreciate it.

[jason_tsh]

One more question I’m afraid Ted. 😯

Is there a reasonably simple way to disable the global phantom power from the mic pres on the 12-48?

I makes my Shure 545d mics hum and sound thin. They work perfectly (and sound great) on preamps that don’t have phantom, and generally I don’t need phantom power from my mic preamps or mixers, as I have separate phantom power units and power supplies for my condenser mics.

I don’t mind modifying each channel if that’s necessary. Or is there a global phantom power supply that I can disable?

[jason_tsh]

Woop! My apologies; it turns out that two of my Shure 545d mics were internally jumpered to High impedance, and I didn’t realize. Just flipped the impedance and hum-be-gone!

I would still like to disable the global phantom power on the 12-48 if possible though, as I tend to mainly use ribbon and dynamic mics, and valve mics with their own power supplies.

By the way, I had lots of fun doing that mid-sweep modification. Thanks again for your help Ted.

[Ted Fletcher]

Looking inside the 12-28 there is a single busrail that runs across the channels with a connection from the power supply board. Turn the mixer on, measure the voltage on the busrails….. this will make sure that you have the right one! Then just disconnect the feed from the power supply. Tape up the end of the loose wire so that it does not short on anything and there you have it.

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