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Old 4th June 2008, 10:25 PM   #1
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Default Another Gainclone grounding problem

Hello,
I just finished a Gainclone amp (AudioSector) and I'm trying to incorporate an S-K 2nd order x-over, built around OPA 2134's powered by their own supply. The GC alone is fine, but when I wire in the networks, I get a prominent hum from the high pass, and hum plus some buzzing from the low pass sections. The bypass caps (100mf electrolytics and .1mf ceramics on Vcc +and-), and the gnd end of R before pins 3,5 IC-1 and C before pins 3,5 IC-2 go to PG on their dedicated supply. Signal gnd goes to SG on the GC board. Now for the real anomaly. When I connect the x-over PG to GC PG, almost all the hum goes away from the high pass section, IC-1, but hum and signal are silenced in the low pass section, IC-2. Should C and R before pins 3,5 be dis then reconnected to GC PG (SG), or.... Would greatly appreciate some of your expertise applied to this problem.
TIA
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Old 5th June 2008, 08:00 AM   #2
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Mike, I've found active crossovers perhaps the hardest part of the hi-fi chain to get quiet.

I think that we may need a full circuit diagram of the crossovers and amp (including the PSU's) in order to make some suggestions.

Is this set-up in a shielded case? If so is the case connected to ground (earth) and how?
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Old 5th June 2008, 09:05 PM   #3
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Hello NUUK,
It was a relief (in a way) to read your comment regarding active x-overs. In answer to your questions, the amp is in a shielded case. It's in 2 sections, one for the transformers and PSU, the other for the GC and network. They are separated by a metal divider. I don't have an earth ground connected at present (I'm still running the GC through a Variac), but I plan to replace the existing cord with a 3 conductor one, the green wire then bolted to the case, as in a PC. I probably wouldn't have, seeing as all the electronics here in the US seem to now be 2 prong plug and double insulated, but after reading AndrewT's admonitions and seeing as it is for a friend, will do. Also attached is a schematic of the network and it's PSU (please forgive it's amateurness). I didn't include the gainclone schematic as I want to get this posted. Hope this helps, I have more if needed, and thanks again.

PS- All this to keep low lows out of FE166E's (Fo- 90HZ), and of course, roll off highs to the woofers, but maybe there's a better way..
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Old 5th June 2008, 09:55 PM   #4
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OK, when you have your three conductor mains lead, make a star ground system for the crossover section and connect that to the metal case via a 100R resistor and 100nf cap in series.

The amp should be connected to earth as Andrew T has shown somewhere else recently.
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Old 7th June 2008, 04:30 PM   #5
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Hello,
I implemented your suggestion and it seemed to almost do the trick. The low section was quiet, the bass was clean and the network now appeared to roll off the highs as it should, however the high section had a 60Hz hum. I dis then reconnected the boards. The hum was still there, but now it was also in the low section, along with some squeals. And it seems intermittent. I'd remove the x-over boards, do almost nothing to them, put them back and there'll be a change. Could it be the boards? I wired them point to point on Radio Shack perf boards. They've been worked over several times, and though they look OK, they're beginning to show it. This is hardly my first active network, but I've never approached it quite this way. In the past I'd put the network and PSU in it's own case, the grounding accomplished by the interconnect's shield, as with a tape deck or CD player. Here I'm trying to do the same thing but in one case and with (supposedly) more positive grounding. When you said to make a star gnd for the x-over section, did you mean to include the bypass caps or leave them connected to their PSU's PG? I left them connected to the PG, which means that R2 and C4 are the only connections made to the new gnd.
Of course, I don't have to be so radical in attenuating lows to the FE166Es. I could do it with just the right value high quality coupling cap. Maybe that would be better anyway. And for the lows, the network and PSU in a separate case, as described above, though I don't see why I should have to. I'd still keep the earth ground, but now should anything (GC CHG which is now affixed directly) be connected to that point via your RC filter?
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Old 7th June 2008, 04:54 PM   #6
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Quote:
I'd remove the x-over boards, do almost nothing to them, put them back and there'll be a change.
I had similar experiences! It is possible you have got a bad connection after handling the board so much. Apart from that, I can't really suggest anything.
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Old 9th June 2008, 12:22 AM   #7
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OK.. I reworked and cleaned up the boards, hooked them back up implementing your grounding suggestion and, finally, a clean and quiet signal from all sections.
Again, many thanks for your time and expertise.
HAGD
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