Ok, I'll try with just the initial buffer then, and see how that works. I intend to power this with an otherwise unused large toroidal transformer, on which I have wound about 25 turns of wire for each secondary (some of the original enamelled copper). This will only give me 5 or 6 volts, which I hope is enough.
remember that the LM3886 is also an OpAmp and you can use it to implement your filter (baffle step in this case) -- unlike PM, i'm always trying to avoid devices in the signal path if i can. People seem to be getting good results with a buffer in front of their GC so you could also just use the active BSC as a buffer
Article on active BSC
http://www.t-linespeakers.org/tech/bafflestep/bstepcompo.html
Attached is a schematic that illustrates the level control addition (ignore the summing stage up front & substitute your XO for the LR EQ section)
dave
Article on active BSC
http://www.t-linespeakers.org/tech/bafflestep/bstepcompo.html
Attached is a schematic that illustrates the level control addition (ignore the summing stage up front & substitute your XO for the LR EQ section)
dave
Attachments
It looks interesting Dave, nice diagram actually. But for now the baffle step correction is temporary, so I won't be changing it.planet10 said:
I made a buffer with TL071 (in a socket) last night, on veroboard. I used + and - 6.7v, off the shelf bridges, 15,000uF at trafo. end and 1,000uF on the board. I decided to test it by placing it between my volume pot. and the lowpass filter I made.
To make the thing, I shorted OUT to -IN and connected the output from my volume pot to the +IN pin. I placed a 22k resistor from its psu 'GND' to the input. Otherwise GND comes straight from LM3886 (main power amp) along a wire where it meets the 22k to the buffer's psu GND and carries along to meet the lowpass filter's GND.
This works, but produces CONSIDERABLE hum. I guess I have the basics right, but have ******ed up the grounding scheme. My problem is that schematics often don't show what to do with ground connections. And not really understanding much electronics, things tend not to work quite right...
Should I introduce a resistor in series with ground from one circuit to the next? Should I remove my 22k resistor to ground in the buffer circuit? Does ground in the buffer circuit need connecting to anything at all?
I think I'm nearly there with this. When it works properly the big job will be finding room for a third transformer in the amp and wiring it all up for good....
edit: I should have said that my hum is mains frequency and coming through the subwoofer. Also, if it helps diagnose the problem - the subwoofer's amp is not connected to house earth. Nothing in my system currently is, although I sometimes connect my CD player to house earth via an added terminal.
Lol! - eeking like your screen-namesake!pinkmouse said:
Relax dude, most commercial hi-fi doesn't use house earth it seems, at least nearly none of the kit I've ever bought or used.
I added an earth terminal to my cd player to remove the 'tingly' chassis problem, and hopefully improve the sound.
My friend was terrified at the possibity of connecting the 'ground' of an LM3886 to house earth - he thought it would introduce 35vdc to the earth or something, but I think he's somewhat off with his understanding! Still, I've been too scared to do it! On that particular amp (our first effort, to power a floor-shaker) we did earth the metal chassis to house earth at least.
I'm checking out that application note now... ta!
Hi Jennice,Jennice said:
I have a supply for left and right seperately in the amp - is there anything more apt? I have a supply for the buffer, and a separate supply for the filter. The reason they do not share is becuase the filter will have it's own box with controls and live elsewhere on the rack. Otherwise I'd have had the op-amps sharing the same juice.
I have not had a chance to experiment yet, but things were previously connected in the same way minus the buffer with acceptable hum levels (of course there were 'impedance' problems like this).
Amp, buffer, filter are all unshielded. Hookup wires are twisted tinned copper and some twisted cat5. CD player is shielded. Sub is at the other end of the room connected by a 4m (roughly) coax wire. I do not have radio pickup problems (we are in a poor area for radio/tv signals), just a hum problem.
I think for now to root of your problem is that you have so many supplies. Properly grounded, the number of supplies will be much less of an issue but when grounding is poor it shows up like a nasty rash. I very much doubt that shielded wires anywhere will make a blind bit of difference, so don't worry about that.
I haven't had time to look at those PDFs so can't properly comment, but I hope that reading them you will get a better understanding and sort out your problem. Feel free to ask questions the PDFs may raise on here
I haven't had time to look at those PDFs so can't properly comment, but I hope that reading them you will get a better understanding and sort out your problem. Feel free to ask questions the PDFs may raise on here
richie00boy said:Feel free to ask questions the PDFs may raise on here
Here's one for you:
What would happen if I grounded to house earth my buffer AND filter AND amp? Any safety reason not to?
In the meantime I might try removing the 22k resistor I added from buffer input to ground, as this might not be needed, I really don't know. No-one told me to put it there
Jennice, you're the man!!!Jennice said:
I had huge hum through my sub, and some noise and hum thru l-ch of lm3886 - when turned loud problems happened. Connecting a crocodile clip lead from psu gnd of buffer to psu gnd of filter SLASHED the hum! Maybe a 20db reduction
The current situation is perfectly acceptable (if improvable) and I had a very loud Madonna - Immaculate Collection test before going out for the evening. The sludge effect on synth notes has really gone, even if I turn the sub too loud. With some tweaking I think I'll have about the best monopole bass my tiny room will allow!
Thanks, Simon!
I will wear my "You're the man" crown with pride this week-end. (Yuck... this sounds SOOOOO righteous I could puke *grins*)
Anyway... To me this sounds like you've got a power supply problem. My guess is that (at least) one of your power supplies relies on the signal ground wire to maintain it's balance ( a +/- supply). Obviously this shouldn't be the case, so I would re-check your power supplies for correct function. Otherwise, you might have a very poor signal ground wire connection (?). Bottom line is: the current peaks of one of your PSU's flow through places it shouldn't (signal ground maybe) ! I guess now you know what you should be investigating this weekend, Sherlock Holmes.
Good luck -
Jennice
I will wear my "You're the man" crown with pride this week-end.
(Yuck... this sounds SOOOOO righteous I could puke *grins*)Anyway... To me this sounds like you've got a power supply problem. My guess is that (at least) one of your power supplies relies on the signal ground wire to maintain it's balance ( a +/- supply). Obviously this shouldn't be the case, so I would re-check your power supplies for correct function. Otherwise, you might have a very poor signal ground wire connection (?). Bottom line is: the current peaks of one of your PSU's flow through places it shouldn't (signal ground maybe) ! I guess now you know what you should be investigating this weekend, Sherlock Holmes.
Good luck -
Jennice
Wear your crown with pride!!Jennice said:Thanks, Simon!
I will wear my "You're the man" crown with pride this week-end.
Anyway... To me this sounds like you've got a power supply problem. My guess is that (at least) one of your power supplies relies on the signal ground wire to maintain it's balance ( a +/- supply). Obviously this shouldn't be the case, so I would re-check your power supplies for correct function. Otherwise, you might have a very poor signal ground wire connection (?). Bottom line is: the current peaks of one of your PSU's flow through places it shouldn't (signal ground maybe) ! I guess now you know what you should be investigating this weekend, Sherlock Holmes.
Good luck -
Jennice
Could my problem lie with my home-wound transformer secondary?? As this is the psu that when added caused unhappiness. One secondary/rail is 0.1v higher than the other. Would this cause a problem? If so, I can fix this easily by replacing the trafo. No way I'm using that whopping thing (must be a 200+va core) in my chassis - it won't fit, and it's a waste or iron! I need a teeny little one. What va rating should I look for, please? And should I be aiming for about 10-13vac per secondary?
Regards
Simon
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