Yamaha M1516 Rackjob problem

[mattnj]

Hi,

So I think that Ohm's law just bit me in the butt.  I just racked up 2 channels of yamaha M1516 input channels. The schematic for this can be found on Adam Holt's page herehttp://www.adamholtmusic.com/diy/M1516/M1516ChannelStrip.pdf. It's on page 14 if you should want to look. I've done some modifications so that I can pull the output after the buffer amp after the fader (which I've replaced with a TL072), among some other mods that I do not believe contribute to this problem. 

I am powering both modules with the kit from fivefishstudios http://fivefishstudios.com/index.php?option=com_content&task=view&id=43&Itemid=62 with a 30 VA transformer also from fivefishstudios. 

Here is my problem:
When running frequency response sweeps I found a HPF around 125 Hz.  It seems like there is a lack of power to drive the low frequencies, like if one leg of the xlr is disconnected. (I've checked the "balance" with a minilyzer, and it is normal.) So upon suggestion from a co-worker, I disconnected one channel and ran sweeps (just like how I tested while building) and the sweep is normal. I measured the current draw on the power rails and I am getting around 31 mA with one channel connected.

So my questions are as follows:
1) is my power supply sufficient for my project?
2) could this problem stem from excessive current draw from the addition of LEDs?
3) is there an easy fix such as swapping the power transformer for 50VA or so? or does the entire power supply have to be changed?
4) (a very basic question) if one channelstrip draws 31 mA, do 2 channelstrips draw 62 mA? Since I don't know AC analysis with opamps and transistors, how much current does this channelstrip draw?

Any answers to any of my questions would be extremely appreciated.
Thank you!

 

[owel]

Hey mattnj.

The regulators are rated 1.5Amp on the PSU for each rail. The 30VA transformer is capable up to 880mA. 
I would think there's plenty of power.

>When running frequency response sweeps I found a HPF around 125 Hz.

You found a HPF?  high pass filter? 

Maybe what you're hearing is hum? 120Hz... maybe ground loops that only happen when you connect the second channel?

What other mods did you do? just curious.

Or ...

maybe the other channel is miswired, on your XLR jacks... like you swapped pin 1 for 2 or 3, and the other way around. So you're shorting your output to ground?

 

[okgb]

the freq rolloff is more likely an impedance / capacitor question
check your mods , are they proven ? done by others before ?
do you have the correct values of R & C ?
triple check the work & notes

 

[mattnj]

Hi,

Thanks for the info guys. Owel, the only other mods that I did was cut the traces to the m1/m2 switch and run mic input to this switch and make it a 20 db pad.  I also rewired the phantom and phase switch, since the traces had changed on the m1/m2 switch. I will DEFINITELY recheck all my work. That is a very good idea Owel, that maybe i miswired the xlrs and shorted one leg. There was no hum at all(noise floor was around -80 dbu), just a low end rolloff. My questions were mainly about the power supply which I believe have been answered.  ONly other question is that basic question about the current draw from each channel added togehter. That intuitively seems right, but I just am not sure. 

As far as my mods, okgb, they are experiments suggested by a few members here and are by no means proven, but have been done by other people.  The channels worked very well on the bench with the mods, but I will definitely recheck all my wiring. 

Knowing the current capacity of the power leads me to other problems.  I just immediately expected that I chose the wrong supply for the project.  The second channel is probably wired wrong, so when I connect power to it it screws up the first channel. 

Thanks for helping me in the right direction guys.

 

[okgb]

the trick is to isolate the concerns
i wouldn't think power supply current would be related
to a frequency response issue
but you could break the supply line and measure the current draw
with the unit on .
you could run a signal generator and a scope , traceing where the freq
problem starts

 

[mattnj]

Yeah, the only reason I suspected the power supply is due to inexperience.  Usually when I run a frequency response sweep on preamps and I get a rolloff like that it usually means one leg of the xlr is bad, and the output driver doesn't have the "oomph" to drive those low frequencies into the AP.  So when I checked my XLRs and they were balanced according to the minilyzer I suspected my output section was starving.  That's when my co-worker suggested the power supply thing and I disconnected it from the second channel and the response was fixed.  So I'm not too sure....

 

[mattnj]

I was completely wrong about my problem here. You were all correct in telling me that my power supply was fine.  It was one of my "mods".  I had put an attenuator right after the buffer amp and before the transformer. I guess I didn't carefully select the value and effectively created an L-R filter with the two components. Oh well. It's now fixed.  Silly me..... :-[

 

[mattnj]

I have one more question about this project.  Since I am trying to use as much of the parts that i have on hand, I solved my above problem by placing a dual gang 25K pot after the output transformer and before the XLR.  Like this:

Measuring this with a Sencore SP395  shows flat frequency response at all pot settings, polarity reverse works, distortion is reasonable etc.

Is there anything wrong with an output control like this?

 

[mattnj]

bump