| rampage101 |
I've just build my first ever amp, a Gainclone.
As it is, I used the Non-Inverted schematic from BrianGT's pdf for his pcb's. Now I turn the amp on and have music playing, then after like 2 seconds, I get a loud hum. My speakers a 8ohm 2-way Sanyo speakers, that about all I know about them. If I can I'll put some pics up of my circuits, so you can see how I did it.
Any ideas as to what I could do?
Also, the power supply 0v, if I connect the a +ve from one rectifier and -ve from the other rectifier, I get like 50vdc. My toroid is 18v - 0v, 18v - 0v. I've got 1 pair of secondaries to 1 rectifier, and the other pair to the other. then filter the dc with 470uf 63v Rubycon caps. This gives me 25vdc from each, which is what I want, to run 2 lm3875 chips. So how do I get a 0v to connect to my grounds or whatever? |
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| matjans |
if you would have searched the forum thoroughly, you would probably have found this thread . Read through it, there;s a lot of advice there and (if i remember correctly) there are hints on how to connect any type of transformer to any type of rectifier (be it discrete or integrated bridge)
if you don't connect your star ground (or whatever you'd like to call it) to V0 you'll get this strange humm & hiss stuff.
;) |
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| BlackDog |
| quote: | Originally posted by rampage101
after like 2 seconds, I get a loud hum.
Also, the power supply 0v, if I connect the a +ve from one rectifier and -ve from the other rectifier, I get like 50vdc. My toroid is 18v - 0v, 18v - 0v. I've got 1 pair of secondaries to 1 rectifier, and the other pair to the other. then filter the dc with 470uf 63v Rubycon caps. This gives me 25vdc from each, which is what I want, to run 2 lm3875 chips. So how do I get a 0v to connect to my grounds or whatever? |
The hum sounds like a grounding problem. read on.
I have a similar transformer arrangement. Use the +ve of one bridge rect. as your +ve supply, and use the -ve of the other bridge rect. as your -ve supply. Your 0V point is your leftover connections (ie one +ve & one -ve from the bridges) connected together.
Not sure how you could build an amp without doing this (ie without a ground) , unless using the single supply schem from the datasheet.
Steve |
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| rampage101 |
OK, Well now that I've got the 0v rail connected to my ground point, its all working properly from what I can tell, although the chip is getting hotter quicker than it did before.
But this now only gives me 2 DC rails, so can I just parallel my other chip of the same connection? Its a 300va 18VAC Toroid, so it should be amp to handle the load OK.
So how do I go about putting in a volume control? I know I need something around a 50k pot, but I connect it between ground and what else? |
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| Nuuk |
| quote: | | So how do I go about putting in a volume control? I know I need something around a 50k pot, but I connect it between ground and what else? |
The success of any project, not just DIY hi-fi, is closely linked to the amount of research done beforehand!
As Matjans says, read through this forum, or perhaps start with
this . ;) |
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| peranders |
| I say first of all: Read the datasheet of the LM3875 and look for the typical connection. To get a LM38xx going is real easy so I think you have forgotten someting major. Check the power supply voltages. Check the feedback network. Check the non-inverting input (should be grounded) if you have made an inverting amp. Signal ground and power ground should be electrically connectec. |
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| rampage101 |
| Its all good guys, thanks heaps for all the help. It was just the 0v rail missing. I've already read the FAQ and some of those other threads suggested, except I just couldn't get my mind around the concept. But, I think I've got it now. |
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