living sounds wrote:Hello,
I wrote an email to Joe about a problem with my BA almost three weeks ago, and an email to JLM Audio about not getting an answer almost two weeks ago, but never received any answer back. My problem is pretty specific, so posting it here wouldn't make a lot of sense IMO.
So how can I get an answer?
Thanks!
HI Gregor not sure why you are not getting my replies by email. But I replied the same day for both of your emails. See replies below sent on the dates shown.
5/9/2011
HI Gregor I replied to you last week. See below
29/09/2011
HI Gregor
sorry to bother you, but I'm having trouble getting a Baby Animal to work and despite extensive probing cannot seem to find the cause of the problem.
[Joe] ok
The faulty device is sitting right next to my 1st fully working BA and build to be exactly the same, using a Lundahl LL1538 input transformer and 990 Op Amp. It's passing audio, but the signal is weaker and sounds high pass filtered.
[Joe] Do you have the center ratio jumper fitted to the BA PCB? And is the lundahl mounted 1mm above the PCB?
I've checked every component and solder joint and couldn't locate a problem (I've build and fixed plenty of gear). Measurements with a scope and a sinewave signal show that the signal carrying secondary of the input transformer puts out a significantly weaker signal, while the signal at the primaries is the same between both BAs. Measuring between either signal carrying primary and the signal carrying secondary the faulty unit shows a much higher resistance than the working one (108-115kohm vs. 8-15khohm).
[Joe] This shouldn't be the case in either as most secondaries will be 200R to 1k2 max dc resistance wise. And if the secondary is open circuit the max resistance should be the Rload value. Unless you mean you are measuring this from 0v instead of the half voltage rail?
Also, between the old PCB (bought in 1996 and the new one (manufactured
07/09) there seems to be a change in the transformer hookup, as I measure around 100 ohm resistance between either differential input and one of the two primaries with all switches set the same, but flipped between the old and the new version (so with the new one the phase is actually pre-flipped 180 degrees). The jumpers are set the same.
[Joe] The + & -in with pad out go straight to the primary so should be the same if the transformers are the same.
Putting in a different transformer didn't change anything. All these tests were performed without the op amps in place and no power, since the problem obviously lies in the passive circuit part. And voltages are identical between both units.
[Joe] Sounds like sending me some macro on in focus well lit photo's of the top and bottom with no opamp fitted may help me get this sorted. I will be away at a trade show this week with only a iphone. So if you can end me the photo's this Friday when I get back I will have a look over them and we can work out the problem from there.
Joe