SPL Optimiser

[alexgg]

XLR in , XLR out ,

output is then reduced 10db it seems

with the low input set it boosts the input 10db + output drop is offset,
but filters are driven into clipping if input volume peaks > 6db (clipping circa +17db)

if high input set then output is just 10db lower than input

 

[Harpo]

the output is definitely balanced, I just checked on headphones from the xlr out (phones plugged into patchbay, + same on both ears)

This is an EQ, not a headphone amp. Neither the unbalanced TL071 output stage nor the cross-coupled TL072 output stage of your unit is not designed for this abuse.
Using XLR plugs doesn't prove the wires are connected properly or a wire isn't broken or shorted inside your XLR cable.
Measure all 3 XLR pins for continuity to the opposing XLR pins and no shorts to neighbouring pins.

but .. output is circa -10db

it really matches up well with the 10db of boost from using low level input, this designed for -10db input levels.
on high level input , this not boosted and comes out -10db.
why this is obviously wrong is because the 10 db boost for low level signals is causing clipping leds to light constantly until I reduce the input by 10db from pc output.

dB what ? dBFS, dBu, dBV, ...
A dBFS readout has no relation to the analogue world without a reference. You might be steady clipping your unit.
Your maybe -10dBFS feeding signal might be +8dBu to the analogue world, depending on how your DA converter is calibrated/set up.
There is no boosting input gain stage in your unit, only optional -9.63dB attenuation.
Measure the amount of AC voltage of your converter that your EQ connects to between XLR-pins2/3 with a maybe 100Hz sine test signal. Next connect this cable to your EQs input and measure the AC voltage you are receiving at your EQ output between XLR-pins2/3 without boosting or cutting any band.

 

[alexgg]

yes, there is continuity between corresponding pins of the XLR.
my XLR cables are good too

my dac is outputting +4 dBu

so if i send a signal from pc that is peaking at -3dbfs digital from this dac it should be peaking at +21dBu
from Optimizer manual (attached) input + output line level = +6dbm
the max input signal is 22dBm , clipping LED lights 3db before this = 19dBu
so this would obviously clip (not sure does +6dBm line input mean -2dBu from +4dBu input signal, maybe >> 17dBu in this example?)

anyway, if I send a signal from pc peaking @-6dbfs, thru dac @+4dBu line level,+ set to high signal, it definitely does not light the clipping LEDs , but the output signal is then -10db with the filter gain set  flat.
if I set to low input, it comes out at roughly the same volume, but is massively clipping in filters.

so, if high setting is reducing signal on input, shouldn't there be a corresponding boost on output to level this back up ?

 

[alexgg]

when I say output is -10db in above example I mean -10db from input signal

so -6db dbfs sent through the unit from dac with +4dBu line level + then back through adc again set to+4dBu line level

the signal will measure -16dbfs

 

[Harpo]

so, if high setting is reducing signal on input, shouldn't there be a corresponding boost on output to level this back up ?

Debatable. Looking at the schematic, this circuit doesn't have a corresponding gain stage to make up for this -9.63dB (-10dB) attenuation setting in high input level mode, so this level drop is on purpose.

 

[alexgg]

thanks for working that out harpo,
seems a bit strange though,
the output level is given as +6db nominal in the user manual , the same as the input

nothing in the balanced input before the ref selector that would boost signal before there ?
( I can't really determine what is going on in the circuit math too well here, maybe should read up on it)

the problem with this low level is that it means noise floor is higher for the low output + will have to be boosted

+ the hum when filters set to lower frequencies will make it almost unusable

so I guess this is the problem to address, as to why the unit might be humming so bad with filter set to low frequencies?

 

[alexgg]

if it helps to explain anything, the hum dies down quite a bit while I am holding the side of the case?

 

[alexgg]

hi

I've been looking at this unit again today, I have replaced almost all of the psu + this seems to run ok

can probably ignore the stuff about volume above I think this all ok + as expected

I still have the same problem with 60hz hum though.

whenever any filter is engaged. seems the same if ground lift switch is in or not.

weird thing is, if I hold the unit with hand on each side of the unit the hum reduces.

if I actually lift the unit with hand on each side panel (currently no base or top panel attached but I recall a similar effect), the hum practically disappears.

is this my body working as ground or something else ?

anybody any ideas on what might be the problem ?

just some ground loop ?

would changing the 10ohm resistors exiting the psu have any effect ?, these are currently 2 ohm on mine (I replaced them) as per the printing on pcb differs slightly from schematic here also, (I have replaced psu caps with 50v also as per pcb).

would really appreciate any help, this 60hz hum the only thing holding this unit back from working perfectly.

 

[totoxraymond]

Hi,

I'm no specialist at all, but when u say that you read different voltages between regulator output and close ground and between regulator output and case, I would think you may have sound grounding issue between PSU and case or between earth and case. Either an oxidized contact or a bad cable.

That could also explain why the noise go when you hold the device.

Hope this can help.

Thomas

 

[alexgg]

hi Thomas

thanks for your response

I looked again at the voltages running from the psu + can see that these actually looking ok
-14.6 , + 14.8
its made more complicated because of ground lift switch which when set in connects the psu ground to the earth via the chassis lug.
I set the box down on a table , I think maybe setting on my lap for noise tests was bad possible + find the hum unaffected by lifting the unit.

I would tend to ignore a lot of what I mentioned in early posts, it was one of my first diy repair attempts + I still don't know a great deal about the subject.

also there are a lot of switches on this thing, but by testing I have found the following to be fairly definitive:

the 60hz hum is only active when any filter is active, and only when frequency selection is low.

this is for any filter type setting, combo push buttons set the frequency range of the filter + these seem not to affect the hum.

only the frequency selection knob being set low causes hum in all cases.

set the frequency high + hum disappears.

this is the real symptom I think.

I can say it is almost exactly the same on all 4 filters each set active in isolation

can anyone please offer an explanation for this behaviour?

alex