saint gillis
Well-known member
Hi, I built some time ago a pair of combos containing each an EQP-1A and an MEQ-5 in a 2 units rack.
The signal goes : 600ohms input transformer > EQP-1A passive circuit > 990C OP AMP stage > MEQ-5 passive circuit > 990C OP AMP stage > Line output transformer
+ a system of relays to be able to use booth at the same time or only one of each EQs.
I realised that on booth units I had a noticeable amount of hum but only on the MEQ-5 circuit, when I would raise one of the 3 pots, the hum would appear like filtered at a low or medium or higher frequency. So I checked my wiring, my PCB, everything looked quite correct, I mean no ground loops, no stupid or illogical wiring, no floating PSU ground.
I remember the best location I'd found for the PSU toroidal transformer was at the back right corner of the rack, screwed vertically. Then I removed the transformer and tried to move it and listening to the hum at the same time (I plugged the EQ in a mic preamp with max gain and listened to the noise), depending on the location of the transformer the hum was higher or lower when raising one of the 3 pots, I mean on some PSU transformer positions the hum was very loud when raising the left pot and almost absent on the right pot, and for instance on another position it would be very loud when raising the center pot and almost absent when raising the left pot.. etc.
On one of the units I found a good compromise when I made a rotation of the toroidal transformer until I found the perfect spot where none of the pots brought any hum. On the other unit I didn't find any good spot, so I used a smaller toroidal transformer (the first was 40 or 50VA, then I used maybe something like 10VA, there are only a few op amps, leds and never more than 1 relay at the same time so it was ok) and with this smaller transformer I managed to find a good spot without hum on any of the 3 pots.
Here's the MEQ-5 circuit : http://www.gyraf.dk/gy_pd/pultec/meq5.gif
I don't have any hum on the EQP-1A circuits. So is the MEQ-5 circuit especially sensitive to magnetic fields? or is there a special trick to make it work more comfortably?
The signal goes : 600ohms input transformer > EQP-1A passive circuit > 990C OP AMP stage > MEQ-5 passive circuit > 990C OP AMP stage > Line output transformer
+ a system of relays to be able to use booth at the same time or only one of each EQs.
I realised that on booth units I had a noticeable amount of hum but only on the MEQ-5 circuit, when I would raise one of the 3 pots, the hum would appear like filtered at a low or medium or higher frequency. So I checked my wiring, my PCB, everything looked quite correct, I mean no ground loops, no stupid or illogical wiring, no floating PSU ground.
I remember the best location I'd found for the PSU toroidal transformer was at the back right corner of the rack, screwed vertically. Then I removed the transformer and tried to move it and listening to the hum at the same time (I plugged the EQ in a mic preamp with max gain and listened to the noise), depending on the location of the transformer the hum was higher or lower when raising one of the 3 pots, I mean on some PSU transformer positions the hum was very loud when raising the left pot and almost absent on the right pot, and for instance on another position it would be very loud when raising the center pot and almost absent when raising the left pot.. etc.
On one of the units I found a good compromise when I made a rotation of the toroidal transformer until I found the perfect spot where none of the pots brought any hum. On the other unit I didn't find any good spot, so I used a smaller toroidal transformer (the first was 40 or 50VA, then I used maybe something like 10VA, there are only a few op amps, leds and never more than 1 relay at the same time so it was ok) and with this smaller transformer I managed to find a good spot without hum on any of the 3 pots.
Here's the MEQ-5 circuit : http://www.gyraf.dk/gy_pd/pultec/meq5.gif
I don't have any hum on the EQP-1A circuits. So is the MEQ-5 circuit especially sensitive to magnetic fields? or is there a special trick to make it work more comfortably?
