Level Calibration for monitors

[johnnyscotch]

Hey all,

I have an issue with one of my monitors. I use Focal Twin6BE, and one of them is a few dB quieter than the other. There is no level control on the monitors themselves.
I have a bunch of projects happening and can't really afford to send them out right now, and the monitor sounds normal, it's just slightly quieter.

I was thinking maybe I can add a level control in between my monitor controller and the monitor inputs.
Would this be a simple matter of just putting a couple of trimpots in a box with XLR i/o on either side?
If so, what would be a good value, and can you recommend a good quality pot?

Sorry, I'm just not sure if I'm looking at some other circuitry around it.

Thanks!
Eric

 

[Rochey]

I would buffer as well as put trimpots.

 

[saxmonster]

Have you tried swapping  cables into you monitor control does the other side sound a few db when the cables are switched?  Maybe its just a bad cable?

Dirty connector on the d/a converter to your monitor control?

 

[ruairioflaherty]

Have you tried swapping  cables into you monitor control does the other side sound a few db when the cables are switched?  Maybe its just a bad cable?

Confirm this before taking any other actions.

What is your monitor controller/volume pot?

 

[joaquins]

Have you done proper measurements of them to tell they are fine and only the level is wrong? Have you had any other problems with them before? How much attenuation do you need or have in the one is wrong? I ask this because is a strange failure and may be there's something more behind.

Edit:Maybe inside they already have a level trim pot, so you would just need to teak it, could be moved because of vibrations or a shock, I don't know.

For the solution you are asking I would use a double L pad or U pad, it would be a 2 resistors and a trim pot, one resistor in serie with the hot (pin 2), one in series with the cold (pin 3) and the trim between them, so balance is maintained, using matched resistor would be a good idea. As close as the monitors as possible, if it's inside better. I would use one of this in each, not only in the one that needs to be attenuated, so you keep them matched. Then you trim the one that doesn't needs attenuation to full up, and the other down to match the first one, if you can live with 6dB loss 5k1 resistors and 10k trim pot is a good place to start, if low attenuations are needed and you don't want to loose 6dB you could use lower value resistors and higher trimpots, also for better precision you could have a resistor in serie with the trim pot, a multi turn trimpot or both. If you only need 1dB or 2dB of attenuation there is no reason to loose 6dB in the first place and to have a range over 6dB, so you could use 2k2 resistors, a 2k2 resistor in serie with a 2k trimpot and you have a range between -3.3dB and -9.5dB. With 10 turns trimpot, you could nail 0.01dB if you were able to measure that. You could have less minimum attenuation but the usable range on the trimpot would be in the very lower range which won't be much useful at all.

JS

 

[hazel]

Strange issue,

Follow Joaquins advice and i'd add to measure voltage from your audio interface output just to be sure there's no faulty elsewhere.

Measure Focal's frequency response playing pink noise and mic'em with omni (measurement mic)

 

[joaquins]

Strange issue,

Follow Joaquins advice and i'd add to measure voltage from your audio interface output just to be sure there's no faulty elsewhere.

Measure Focal's frequency response playing pink noise and mic'em with omni (measurement mic)

Good idea to measure the voltage at the output of your audio interface, as well as the output of the monitor controller if you have one.

There is no need for omni, measurement mic at all, omni is better, but any mic will work, since it's a comparative measurement between both monitors, make sure you position them in the same place in the same measurement conditions, make a mark where you put the first and then put the other at the same spot, a couple of millimeters won't be a problem in the position, much better than measuring in the place they are since the room will take a major part in it. The only care with the mic is to have enough freq response, mostly in the low range, you could use separate mics to look at the low end and the top end, big and small diaphragm condensers could be an option. In the case you can buy a behringer ECM8000 measurement mic I will recommend that since it's a pretty good reference for the price, I have one and find it really useful, not a lab grade but really good to make calibration for live performance, measure a room or take impulse response of rooms you like for your IR verbs. But there is no need for this you need now.

Other option would be measure the voltage output of your power stage, which will be much more precise than the acoustic measurement but of course won't show if the problem came from the speakers, which wouldn't be my first guess.

JS

 

[johnnyscotch]

Thanks for all the suggestions, I'm working on it now

Cheers

 

[johnnyscotch]

If you only need 1dB or 2dB of attenuation there is no reason to loose 6dB in the first place and to have a range over 6dB, so you could use 2k2 resistors, a 2k2 resistor in serie with a 2k trimpot and you have a range between -3.3dB and -9.5dB. With 10 turns trimpot, you could nail 0.01dB if you were able to measure that. You could have less minimum attenuation but the usable range on the trimpot would be in the very lower range which won't be much useful at all.

JS

I'm actually just now getting around to this, I finally got some parts and time to get it done.

I've attached a picture of my plan, does it look right? I only need a few dB attenuation: