Neve 4 channel 1290 100hz hum?

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Spencerleehorton

Well-known member
Joined
May 12, 2012
Messages
4,035
Location
Felixstowe, Suffolk, UK
Hi all,
I dont seem to be able to get rid of this 100hz hum when 48v is on?
I have made sure I have an audio gnd and chassis gnd, the 48v gnd is only attached to chassis gnd and I have a 10R 2 watt resistor from the audio gnd to chassis gnd.

Im going to test only 2 channels and see if its the psu, I have volkers psu pcbs here and will change, failing that I've run out of ideas??
 
Psu I've built has 7824 for 24v and 317 for 48v,  uses 3 x 1000uf 63v and a 220uf 100v on the 48v.
Has 220R and 8.2k on 48v with 0.1uf caps 1st and last.
0.1uf caps are 1st and last on 7824 as well.

Getting g 23.5v on 7824 and 47v on 317.
Setting each channel on tr3 collector to 22.66v and N is 18v.
 
I've got some 783's, let me show my psu schematic  and see if something is wrong with it as just changing the 317 and a resistor would be the easiest solution.

If I'm using a 8.2I and 220R on 317 what would I use for the 783?
 
Just checked the toroidal as I'd incase in mumetal , the toroidal is 24v 0.83A and it's not centre tapped.
Could this be my problem?
What's a good toroidal to be man enough for 4 channels of neve with 48v?
 
Spencerleehorton said:
I've got some 783's, let me show my psu schematic  and see if something is wrong with it as just changing the 317 and a resistor would be the easiest solution.

If I'm using a 8.2I and 220R on 317 what would I use for the 783?

The same I believe, try it out.
 
What raw dc volts do yo have at the input to the 317? Is there enough volts drop across it for it to regulate properly?

Your toroid can provide 24VAC at 0.83A. Rough rule of thumb your total dc output current draw should not be greater than 0.83/1.6 = 500mA thereabouts. That should be plenty to supply 4 x 1290 and some phantom power.

My suspicion is the raw dc into the phantom regulator is not high enough.

Cheers

Ian
 
Voltages after diodes are 63.3v for the 48v and 37.90v for 24v.

I've moved the toroidal out the box and 100hz still present.

These tests are just with 2 channels connected but all 48v are connected.
 
Have put in 783 and I have 8.2k and 6.8k and getting 61v so need to change resistors, also noticed the caps that feed the 48v diode bridge are only 100uf not 1000uf and the cap which is on the 24v is only 220uf.
Dont know if I needed to change them all for 1000uf?
 
Spencerleehorton said:
Have put in 783 and I have 8.2k and 6.8k and getting 61v so need to change resistors, also noticed the caps that feed the 48v diode bridge are only 100uf not 1000uf and the cap which is on the 24v is only 220uf.
Dont know if I needed to change them all for 1000uf?

The dc voltages seem about right and my next question was going to be have you got a scope so you can look at the ripple on the raw phantom supply. But now you mention those cap values my guess is the ripple is far too high for the regulator to operate. You definitely need to up those capacitors. 1000uF for the phantom diode bridge should be right. The 24V I would be tempted to raise to 2200uF which should give you about 1V pp ripple with a 200mA load.

Cheers

Ian
 
Ok ian, I was swaying towards that and need to up the voltage on them as the 48v supply only had 1000uf 50v, I'm tempted to do the two 1000uf 50v with the 2 x 22k config for 48v.
Or I might as well wait until I have all the bits for volkers psu as it has that config.
Need to learn a bit more with Psu's as they get me every time!!!

Thanks again for your wisdom.
 
A couple of rules of thumb you may find useful.

1. For a capacitor input bridge rectifier the AC current rating of transformer should be 1.6 times the expected dc load.

2. Choose reservoir capacitor for about 5% ripple - that's why I picked 1V for your 24V supply.

3. Calculate the value of the reservoir capacitor from C (uf)  = (1,000,000 * Idc/(f * Vripple)

where Idc is the desired dc load in amps, f is the ripple frequency (100Hz here in the UK) and Vripple is the desired ripple voltage. Plugging in the values for the 24V supply gives C = 2000uF.

Cheers

Ian
 
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