Drip V6 LA-2A Problem - No Bass.... SOLVED!

[Grimace]

Hey folks. Long time lurker, first time poster.

I'm having some pretty intense bass roll-off on my Drip Opto 6.  I've tried turning the stereo adjust and the limiter response screws, to no significant effect. I've confirmed the C1 through C4 capacitor values are correct. I've checked everything with the gain reduction set at 0 and also with it raised. I've removed the T4b's. All with no significant change.

I've noticed the problem when going...
Mic --> Preamp --> LA2A; also when going
Dynamic Mic --> LA-2A (no pre); and also when reamping pink noise by going
EMU 1212M --> LA-2A.  So I'm pretty sure it's the LA-2A at this point.

Here's what I get with the pink noise generator. The first Freq Spectrum is the pink noise, the second is with the LA-2A.

As you can see, I basically have nothing below 200 Hz. There's also a bit more HF content. I even notice that when recording spoken voice, that you can hear and see additional HF content above 2K or so. Sooo... any thoughts on this?

As you can see, I've searched, I've read. I've followed up on the most common suggestions, but I'm still a bit stuck.

 

[Bowie]

Could be any number of things.  Hate to be the one to say this but I'd go back through and check the value of every component.  Don't trust that something is correct because you checked it before.  90%+ of the time it's an incorrect component.  With the heavy high passing going on, I'd start by checking your capacitors.

 

[pucho812]

go over everything, could be a part, could be a solder connection, could be wiring.

 

[tmuikku]

my first project looked like that years back. turned out that my computer interface had TRS sockets altough being unbalanced. ring wasn't connected to anything... the unit i made had output transformer and i hooked it with balanced cable to my interface => other end of transformer secondary was not connected.. i spent hours trying to debug my diy unit  8)

so, just in case try with other cable and test with mixer and headphones

 

[Grimace]

Initially, I had a problem with the 2K diode drop resistor (I'm using diodes). I wasn't getting any compression. I bypassed the drop resistors and fixed that problem. But in the course of figuring that out, I went through the wiring with a fine-toothed comb- I'm quite confident in it.

I'm looking over the capacitors right now. For the mica caps, I have 500pF's at C13 and parallel with the NE-2. I have 150pF's at C4 and C14.

I have .1 uF WIMA caps at C3 and C2. But this is odd... The Drip BOM lists six .1 uF capacitors, but as far as I can tell, only 2 are used (C2 and C3). Can someone confirm this before I go any farther?

my first project looked like that years back. turned out that my computer interface had TRS sockets altough being unbalanced. ring wasn't connected to anything... the unit i made had output transformer and i hooked it with balanced cable to my interface => other end of transformer secondary was not connected.. i spent hours trying to debug my diy unit  8)

I really thought that was the problem at first, tmuikku. I too am using a "TRS --> XLR connector --> Pad" on only the left side of the stereo line outputs when reamping for this test. However, I have the same problem when I go Dynamic Mic --> LA-2A --> ADA8824 --> DAW. And I'm seeing the same loss of bass clearly in the Spectrograph in the DAW in either test. Unless you think I'm missing something...

And just to be sure I wasn't, I did just route my LA-2A output to a monitor. Same cables. Same problem-- A noticeable lack of bass with the LA-2A. But I use those same cables with my preamp into the monitors and it sounds fine.

It was a good suggestion though. 

And yeah, an o-scope may wind up being a necessary investment. For financial reasons, I'm really hoping I can figure it out without one though.

 

[tmuikku]

just tought to share try to get your hands on an oscilloscope to find the problem area.

you could use the audio probe trick as well, just use proper high voltage capacitor.

 

[Grimace]

Alright. Props to Pucho and Tmuikko. I got it sorted.

As Pucho said, it was a cable... sort of.  One of the screw terminals had come loose, so effectively, I had an unbalanced signal.  So it was a wire, not a cable, just as Pucho said.

I kind of have a love/hate thing with the screw terminals. I love how easy it is to connect it and remove the board if need be... But it's also hard to ensure everything is connected, especially when you're putting more than one wire into the terminal, which is what my problem turned out to be. I soldered them together, so now the problem won't repeat itself... until it does, of course.

I really wanted to believe the unbalanced cable theory, based upon this old thread that I found http://groupdiy.com/index.php?topic=22031.0

It has a lot of useful troubleshooting in it and the symptoms looked incredibly like mine. And sure enough.  From now on, I'm going to inspect and tighten those screw terminals every time I go into the case.

The problem was also exacerbated by the fact that I thought it sounded fine before, but on my first *critical* application, it was falling on its face. So I partly wasn't sure if I was just now noticing it or if it had always been like that. I was doubting myself. But I think it just had come loose.

Thanks everybody. I'll change the thread title to "Solved!" now.    ;D