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|13th March 2007, 10:26 AM||#1|
Newborn LM3875 BrianGT Kit (Rev 3) Running!
After over a year with the kit sitting in the basement I finally got the thing assembled in an old case I had laying around.
I've been running tests on it and I'm fairly pleased, but perhaps you can help me with some initial quirks.
Here are my test results and issues:
Currently testing one channel of BrianGT LM3875 PCB /w Heatsink, No Zobel
Accoustic Research Fun Partner, 8 ohm
* Toroidal secondaries showing +-28.4VAC. I thought that was way off the 5.5% regulation that Avel claimed until I measured my main voltage at almost 125VAC. Computing the stepdown ratio for 115 / 25 yielded 4.3. I computed 124.5 / 4.3 and that is what the secondaries were running. Well within the approx 6% regulation on no-load.
* After the bridge and diode losses, my rectifier board output was right around 34v.
* First amp power-up test with shorted signal inputs yielded about 22mv DC bias on speaker outputs. Quite acceptable for a first try. As expected the 40w lightbulb glowed momentarily during powerup because of the toroids inrush current.
* Connected CD player output to inputs. Connected amp output to AR 8 ohm speaker. Sounded pretty decent right at the start. Given the limited range of the AR's and the limited quality of the CD player I was impressed. The one thing I really noticed was its ability to really control the dynamics of the music. Nothing flabby. An acoustic bass snapped when the player's fingers came off. Bells and other percussive instruments were rock solid. Frequency response was what I expected from the ARs. Dynamic V+ input into the amp during playback varied between 33.1 and 34.2 volts (as much a you can get valid multi-meter results with varying signal). Anyway the average was around 33.5v which is right at the peak output for 6 ohm load and decent for an 8 ohm.
* Some surprises: At moderate listening levels I was hearing some hiss coming from the speaker. The hiss became more apparent as I reduced the volume and it decreased somewhat as I brought the volume level on the CD player low. I shut down the amp, disconnected the CD player and shorted the inputs. After power-on there was absolutely no noise whatsoever. Dead silence. I think it likely that the noise is being generated by an impedance mismatch between the CD player out (which probably expects a pair of crap headphones) and the amp, though I don't know what the input impedance of Brian's kit is. I suppose I could do a search.
* More surprises (though I guess it shouldn't be): I got a nasty, loud 60-hz ground hum if I (1) powered off or opened up the CD player (2) turned the volume on the CD player all the way to zero (sometimes) or (3) disconnected the source while amp was still on. Is this normal? I think the amp is looking for a signal and when it doesn't find one, it *really* goes looking. As long as there was even a minimal signal on the inputs there was no hum at all. Is there any easy way to minimize this effect (some way to short the inputs if there is nothing plugged in?)
* Other than the slightly weird surprises, I installed the other channel and measured its DC offset. It came in around 27 mV. Connected the other speaker and powered the whole thing up. Put on a Pat Metheny concert and burned the amp in. After about an hour or so I noticed it really was quite nice even on the relatively cheap but good ARs.
* 3rd Weird Surprise: I have an old Class A Forte Audio pre-amp that I tried with it last night and nothing special happened other than a weird crackling sound when I changed the volume selector (it doesn't use a pot, but a 100 position rotary switch). I assume this is the switching noise, but I would have thought that Forte' would have (a) filtered out the switching noise or (2) my old amp had input filters that threw away crap like selector switching noise and I never noticed it. Perhaps (and more likely?) the Forte de-bounce caps have degraded and the noise is going through.[/list=1]
* Has anyone else heard hiss during initial tests on an LM3875 rig?
* Is the 60Hz ground hum with no-input or powered down inputs a common occurrence? What is the easy solution?
* Anyone else find weird transients coming through their new amp that they haven't seen before (like knob or selector switching noise)?[/list=1]
When I tweak this thing a little more and make it more civilized I'll document the whole process, pictures -- the works.
Thanks everyone. Great kit Brian!
Don't believe everything you think!
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