spl optimiser eq hum

[alexgg]

hi I thought it might be better to start a fresh thread because a previous thread I wrote on this last year seems confusing.

basically I have an spl optimiser eq. I have attached a schematic.

there is a 60 hz hum problem.

I totally replaced most of the psu board including all electrolytics, it outputs +14.9v -14.75v. the hum remains

its possibly linked to grounding. the unit has a ground lift switch that either connects or disconnects the internal psu 0v ground rail to the input iec cable earth.

with ground connected to earth it seems most stable (get some confusing changes by holding the chassis elsewise).

the problem is quite high amplitude 60hz hum appears when you set the frequency of any one of the 4 filters to low setting.

the potentiometer for frequency is connected on one side to ground via 470ohm resistor. on schematic the frequenz pot is shown as 2 x  10k  in the unit it is actually a 2x 100khz dual pot.

so setting this to low, means the wiper is at low resistance to ground. this causes hum in audio. I am not sure why.

also could removing a 10ohm resistor + small cap in the switch to ground earth possibly help (ie straight 0v >earth)?

would be grateful if anyone can help to explain this behaviour.

 

[Andy Peters]

hi I thought it might be better to start a fresh thread because a previous thread I wrote on this last year seems confusing.

basically I have an spl optimiser eq. I have attached a schematic.

there is a 60 hz hum problem.

Well, for starters, the schematic indicates that the XLR pin 1 is connected to the circuit ground, so I suspect a Pin 1 problem.

\-a

 

[jwhmca]

Does it hum with input disconnected?

 

[jwhmca]

Cracked pots from a fall-impact-face scenario? Does the EQ work except the hum? Do the lower freqs etc... function okay, there is just hum in them?

Resistance between the 470ohm resistors (ground side)and IEC ground?

 

[alexgg]

thanks for the response so far.

a bit more information :

it does still hum when the input is disconnected.

it does not hum without any filters active (each filter has bypass switch)
- this different from just straight in > out bypass, where again it does not hum.

so, only hums when any filter is engaged, all filters have the same effect.
hum present when frequency is set to low.

doesn't actually cumulate with say 2 filters set to low.
max hum level when just 1 filter set to low, set another to low and it stays the same, maybe + a bit.
hum barely visible when active filter set to highest frequency, increases as frequency is decreased.

also if you see in schematic there are 2 buttons above the frequenz pots which set the range for the eq, the hum actually decreases as the range is decreased by these buttons given the frequency pot is set at its lowest.
can see there is output from range buttons combining with frequency pot outputs after 10k resistor just before it enters the tl71 opamps. cannot explain how this is working though.

all filters seem to work as they should.

 

[alexgg]

another weird behaviour I have noticed is that when I try the unbalanced TS outputs instead of balanced XLR I find that the level is dropped quite significantly + still contains the hum, this reduced by the general level change which might seem in line with just running unbalanced.

when I then switch back to XLR the audio output level is dropped somewhat. ie if I put the unit ref level to low, it normally outputs around the same level. if I use unbalanced outs it drops this level because its unbalanced, when I then switch back to balanced its still lower ?????
. its like something has been drained. I cant quite explain this , it goes away if I switch the unit off and on again.


not at all sure about the pin 1 problem i'm pretty sure from reviews this unit must have worked originally  so I guess something has gone wrong since.

 

[ruairioflaherty]

How old is the unit?  Have you looked at the PSU on a scope or a decent multimeter? 

 

[alexgg]

How old is the unit?  Have you looked at the PSU on a scope or a decent multimeter?

its early SPL , 20 years old I think. sos review posted in 1994.
http://www.soundonsound.com/sos/1994_articles/nov94/sploptimizer.html

I have a fluke 75iii. I checked voltages coming out of psu  - 14.75 , +14.9

I am not sure what else to check there. I  have replaced all diodes + electrolytics.

I'm not that great at electronics so cant see how this hum is introduced ?

 

[Andy Peters]

I'm not that great at electronics so cant see how this hum is introduced ?

it really helps to look at the power supply at the regulator output as well as on the op-amp pins with a 'scope, set to AC coupling and maybe 200 mV/box vertical. The trick is to know where to connect the 'scope probe ground.

 

[alexgg]

I'm not that great at electronics so cant see how this hum is introduced ?

it really helps to look at the power supply at the regulator output as well as on the op-amp pins with a 'scope, set to AC coupling and maybe 200 mV/box vertical. The trick is to know where to connect the 'scope probe ground.

hi, I'm thinking about getting a scope, should I get a 2 channel or1 is ok ?

would I look for ripple in supply voltage ?

just watched that Australian guy on youtube explain the importance of care with earth referenced units so connecting ground probe to grounding lug probably best right?

alex

 

[jwhmca]

Resistance between the 470ohm resistors (ground side)and IEC ground?

 

[alexgg]

hi resistance there is 10ohm

there is a 10 ohm resistor in the connection between pcb ground + the ground lug >> iec ground

you can switch this connection off using the ground lift