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Old 2nd August 2003, 01:59 AM   #11
SY is offline SY  United States
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OK, I think I'm visualizing this. If you're doing what I think you're doing, run the XO board's ground to a point on the case near the input jacks. Run the PS ground there, too. That's the only place they should connect. If the XO board has large electrolytic bypasses, their ground leads should be isolated and returned to the single ground point.

Now, isolate the input and output jack grounds, and return them to that same point via 10 ohm resistors.

This will be a bit of a pain, but should clear up the hum. If you can post any pictures or sketches, that would be cool. You can do that via the Attach File option on the screen you use to write your post.

And it's not annoying at all, except for my rueful thoughts that the 'Net would have been a great thing to have around when I was getting started and making all my easy-to-fix mistakes.
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Old 2nd August 2003, 02:12 AM   #12
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Ok cheers for all your help its way too late to fiddle around with anything my bedroom is adjacent to parents (im 18 going to study acoustics at uni YAY for me) bedroom and I dont wanna risk a thump or click to wake em up.

So I'll try out this all tomorrow well thats really today now but I'll keep you posted with my results.

Over and snore Matt
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Old 2nd August 2003, 02:35 AM   #13
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Good luck!
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Old 2nd August 2003, 01:53 PM   #14
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I've done half of what was prescribed, this involves the single point near the input jacks. Coincidently my input jacks have tags to solder onto for the ground, is it OK to use one of these as the point. I only have two 10 ohm resistors lying around with a few others within two or three ohms of ten are these alright to use also??

The two 10 ohms I had went from the input grounds to the star point. Now I cant turn the volume down to zero, it goes close to it but no complete cut. This wont really be a problem but is there a way to kill it completely. Also doing what I've said I've done, both the inputs and the single star point, not the outputs. It cuts down on the hum a bit but not by much.

With the input and output grounds (shielding on the wire) where should I connect the other end. One is to the case and the other doesnt go anywhere, should this be altered??

Once again Matt
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Old 2nd August 2003, 02:16 PM   #15
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10 ohms is an arbitrary figure. 15 or 8 would work just as well.

If you isolate your input jacks, the tag won't be at ground; you'll need a separate point for the star ground. Run the 10 ohm (nominal!) resistors from the input jack ground tag to the star point. Your real gain will come, I'm guessing, when you separate the PS and signal grounds and return them to the star ground separately.
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Old 2nd August 2003, 07:25 PM   #16
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OK well here goes ive also added a diagram hope this helps.

Now, heres the latest. I have the left channels treb section running silently zero hum, the bass section has some hum which is too loud but much quieter then before.

The right channels trebble section hums but quietly (all this is audible from the listening position) and the right channels bass is the loudest out of the three that hum.

On the vero board which the xvoer is on there is a split in all the tracks mid way along the length that separates the left and the right channel completely in the audio sense. They all link up via the power supply lines and the earth from the star ground on the case.

Each side of the board is identical including the way they are earthed.(doing various things with the earthing doesnt affect anything) So one section is silent (left treb) so that must mean the way everything is attatched is fine, and its corresponding right channel is wired up identical but hums (grounding identical too the right channels grounding wire to star is even shorted then left).

The only thing that isnt totally the same is the PS into the board, it enters close to the left treb pins (see diagram) and then each other section is wired in parallel from this as shown. The only thing left for me to do is power each opamp directly thru hardwireing to the PS. Also regulate. Which I am going to try anyway.

One plus to this tho is that it sounds, hum asside, simply beautiful (OPA627 on treb and buffer then OPA2134 doing bass). Better then when off the batteries, this might be to with the +- rails being identical in magnitude and running at 17.5v, close to max spec.

Cheers again Matt
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Old 3rd August 2003, 02:18 AM   #17
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Try removing the jumpers between bass and treble grounds, then running separate wires to the star point.

If that doesn't fix it, keep the separate grounds, then un-daisy chain the supply feeds. Bring the plus supply rails to a point, then run separate wires from each board to that point. And do the same for the minus rails. Note that your quietest channel is the first one in the daisy chain, the noisiest is the last one.
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Old 3rd August 2003, 02:26 AM   #18
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This is kinda more complicated then it seems, the trebble is easy as it only has one point to ground but each bass has at least four points to be wired to ground. Still anything is worth a shot.

The hum issue became much less pronounced when I added the big smoothing caps which wouldnt affect the ground, at least I dont think it would. The hum is only at 100hz, which corresponds to the ripple frequency produced by the bridges. So this could be not earth bound but on the supply lines. Its strange tho that the treb left is silent but the rest not. Anyway I am going to regulate Im going to order the components tomorrow and I'll see what that does. I also have another two 6800 caps I could try them aswell giving like 15000uf smoothing for a 60mA current draw. OTT if you ask me, but if it works it works.

Matt
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Old 3rd August 2003, 02:33 AM   #19
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Well, I'm assuming you don't have too much ripple on the supply lines, and at 100 Hz, the PSRR of most opamp circuits is pretty good. The silent treble channel is a good clue that this is indeed the case, but a quick check with a scope may put your mind at ease.
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Old 3rd August 2003, 10:59 PM   #20
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Ok here goes second attemp after comp crashed right.

Well I was getting annoyed at nothing working so I decided to have another (like the fifth time) look at the vero board for any irregularities. I found that an un-used track on the right treb opamp was connected up to summit on the bass where it shouldnt be. Anyway I broke the offending article and this made the right treb silent and decreased the right bass hum to the same level as the left.

I am now convinced the two channels are identical (I thought that before right but I hope they are).

Well I started to pull out opamps and see what happens. That way I can isolate specific areas to see where the small amount of hum is being induced. Anyway there are two OPA2134s per bass channel, one does the level set and baffle step, the other does the 24db roll off. Basically on removing the 24db roll off the hum dissapears, and when you put the output from the first opamp directly to the ouput the whole lot runs silently albeit without and xover on the bass.

So Im guessing that maybe isolating the 24bd from the rest and giving it its own star point to earth might solve the problem. I hope that I will get it totally sorted out sometime tomorrow tho.

Anyway cheers again Matt
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