Calrec PQ1549 hum

I finally started using my stereo PQ1549 more and realized I have a low hum going on with both channels. I am using the Mini PSU w/18v regulators. Moving the toroid has no affect on anything. I also disconnected one channel to make sure it wasn't some kind of ground loop between the two.

Now on the Gyraf's Mini PSU, I have no connection to the 0 (ground?) on the AC input side. Should that be chassis grounded, or some kind of center tap to my toroid? (I'm only using one out of two windings, the other winding has no connection) The output is of course using +/- 18VDC and ground to the PCBs. On the scope I do see a little bit of sawtooth waves. Eq does work great, just some annoying hum. Thanks for the help.

-kdawg

Do you have enough voltage before the regulators? Should be atleast 4 V higher then the wanted regulated voltage.
My thumb roll is to use the same AC voltage as wanted regulated voltage. like 2 x 18V AC for +/-18V DC

I will check it tomorrow, but I believe I have a TE62054-ND toroid from Digikey, which is 2x18 VAC. Am I supposed to parallel the secondaries in this case (I left the 2nd pair disconnected)? I thought if I had them in series that I would have too much DC (36VDC+) for the LM7818/LM7918 regulators.

-kdawg

Hi,

here's what I did and I don't have any hum.

use both secondaries. join together the loaded wire of one secondary pair and the un-loaded wire of the other secondary pair. this goes the the 0V input of the mini-psu. the remaining secondary wires go to the ac-inputs of the psu. this way you create the needed center-tap.

corneel

Am I supposed to parallel the secondaries in this case

Click to expand...

Yes, you are! What's the difference between +18 V and -18 V? 36 V - so you need 18 V per regulator, that makes two 18 V windings in series.

Samuel

Am I supposed to parallel the secondaries in this case


Yes, you are! What's the difference between +18 V and -18 V? 36 V - so you need 18 V per regulator, that makes two 18 V windings in series.

Click to expand...

Yes, that does make two 18v in series, but the question is whether they should be paralleled, & the answere to that is no, they should be in series.

Ouch! :grin:

Samuel

Thank you - finally works right now. Without a center tap it was just floating ground and wasn't getting enough voltage to rectify it to +/- 18v.

-kdawg