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Author Topic: help with issue in my amp  (Read 8687 times)

duclon

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help with issue in my amp
« on: June 27, 2007, 08:32:25 AM »
I've built a small 12v DC practice amp. Here is a simple issue, for someone other than me. i searched but did not find an answer. When i have nothing plugged into it, there is a loud hum, or buzz. when i plug in an instrument it goes down, not away. when I touch the instrument jack or the metal that all the pots and jacks are mounted to it pretty much goes away. So I know this has to be simple but this is my first amp and i am missing it.
Any help?
Mike

LJ King

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Re: help with issue in my amp
« Reply #1 on: June 27, 2007, 09:53:54 AM »

It sounds like you are either missing a ground connection or have an improper one in the amp. Check the input jack connections first.

jfetter

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Re: help with issue in my amp
« Reply #2 on: June 27, 2007, 09:56:49 AM »
Buzz is generally broadband noise. If you run on battery only and unplug from AC is it still present?

Is the amp "Class D' ?

duclon

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Re: help with issue in my amp
« Reply #3 on: June 27, 2007, 10:06:36 AM »
Buzz is still there when run on DC battery and it's a bridged amp circuit if that matters.

duclon

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Re: help with issue in my amp
« Reply #4 on: June 27, 2007, 10:11:20 AM »

It sounds like you are either missing a ground connection or have an improper one in the amp. Check the input jack connections first.
my first thoughts are a ground issue, it goes away when I touch the metal plate everything is mounted on. I'll check later when I get back to my workshop.
for more explanation, It's also a stereo amp, 2 inputs go to 2 preamps, then to 2 tone stacks, into a opamp adder circuit, then to a bridged amp circuit, which was just a little kit I bought.
i've connected all grounds from the separate sections together.

tonyharker

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Re: help with issue in my amp
« Reply #5 on: June 27, 2007, 11:17:05 AM »
You could have a ground loop.  The grounds should all go separately to one point ideally on one of the smoothing capacitors.  If you have connected the grounds to one another then that is the problem.  Also the input jack ground should be isolated from the chassis and go directly to the input ground connection.

Do you have a schematic/diagram you can show us?

Tony
« Last Edit: June 27, 2007, 11:18:07 AM by tonyharker »

duclon

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Re: help with issue in my amp
« Reply #6 on: June 27, 2007, 12:27:22 PM »
You could have a ground loop.  The grounds should all go separately to one point ideally on one of the smoothing capacitors.  If you have connected the grounds to one another then that is the problem.  Also the input jack ground should be isolated from the chassis and go directly to the input ground connection.

Do you have a schematic/diagram you can show us?

Tony

Thanks Tony, This sounds like it might be the cause. one question from your reply is on the input ground connection, where do I run it to, the preamp or back to the power source. I'll try and explain the circuit;

i've really combined 4 circuits to make this. , actually before the op-amp adder there are 2 of each of the preamp, and tone stacks for a stereo instrument.
The total circuit is very simple, I use a jfet preamp to a fender tone stack design, then to the small adder and on to the power circuit. The preamp circuit has a ground, the tone stack circuit has a ground, the op-amp adder circuit has a ground and the power amp circuit has a ground for power and input. the power amp part was a little kit I bought, the others I put together from schematics I got on the net.

I kind of hooked up some of the grounds together and then connected over to more grounds which were tied together, then to the ground of the power source with one wire, 9v battery. So it is kind of a big mess of grounds. I put a diode, and 2 caps at the 9v source after i read it might reduce any noise from the power source, i guess the caps are the smoothing caps you mention and I should run all grounds separate to one of those caps?

The only thing I'm not sure of from your comment is where to run the input jack ground to.

I'm at work now but when I get home I'll take the circuit diagrams I used and put them together as one and post it they way I have it wired.

Mike
« Last Edit: June 27, 2007, 12:35:14 PM by duclon »

teemuk

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Re: help with issue in my amp
« Reply #7 on: June 27, 2007, 12:46:38 PM »
http://www.ssguitar.com/index.php?topic=288.0

Edit: Since it's pretty hard to get DC-powered systems to buzz now matter how bad the layout is I would check that you

a) have a proper common reference in all your inputs - especially when nothing is plugged in. In other words, there should be a resistor that shunts the inputs to common.
b) have properly bypassed the opamp dc supplies with small value capacitors in as close proximity to supply pins as possible
c) do not play in presence of computer monitors, fluerescent lights etc.
« Last Edit: June 27, 2007, 12:54:26 PM by teemuk »

duclon

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Re: help with issue in my amp
« Reply #8 on: June 27, 2007, 01:16:29 PM »
http://www.ssguitar.com/index.php?topic=288.0

Edit: Since it's pretty hard to get DC-powered systems to buzz now matter how bad the layout is I would check that you

a) have a proper common reference in all your inputs - especially when nothing is plugged in. In other words, there should be a resistor that shunts the inputs to common.
b) have properly bypassed the opamp dc supplies with small value capacitors in as close proximity to supply pins as possible
c) do not play in presence of computer monitors, fluerescent lights etc.

thanks, I think a might be the answer. I don't have a resistor to common on the input jacks and when I touch the jack base the noise completely goes away. I'll do some digging and find what value I need to add and where and I'll try that first, then look closer at the ground wiring if it doesn't solve the issue, but the more I think about the grounds, I pretty much have each ground from each section terminating at two locations then going to the common from each, so as I read in one of  the tutorials it is close to star grounding except I extend from the points where they all join to the common with 2 inch wires.
« Last Edit: June 27, 2007, 01:18:26 PM by duclon »

teemuk

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Re: help with issue in my amp
« Reply #9 on: June 27, 2007, 01:51:27 PM »
I assume your input stage is something like this:


Convert it to something like this:


You can preserve the Rs.

Something close to 1 Meg is usually pretty good value for this "input" resistor (Rin at bottom picture).

duclon

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Re: help with issue in my amp
« Reply #10 on: June 27, 2007, 02:54:33 PM »
i don't know what i was thinking but I thought you meant I needed something at the input jack. it looks like I have a resistor at the input of each of my preamp sections. heres the preamp I'm using, 2 for stereo. i tried this with the MPF102's and it didn't seem to work, I may have had bad jfets, so I put in a couple j201's and it worked great for the preamps.
I'll pursue the ground wiring suggestions when i get back to my house and post back the results later today.
thanks again for all the help, i think once i get back to the circuit I'll narrow it down.
« Last Edit: June 27, 2007, 02:56:00 PM by duclon »

duclon

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Re: help with issue in my amp
« Reply #11 on: June 28, 2007, 09:55:23 AM »
i re-ran all the grounds, as to the tutorials on the site, star ground. I added another resistor to ground on the input. Still have a big hum until i touch any metal like the base of the input jack. I tried another amp circuit and it does the same thing. I also tried a different preamp circuit, pulled out the tone circuit, still the same..
So the circuit is now on a flat piece of metal that I was going to mount to the top of a small combo box I built, holes for the pots and jacks. could it be that I need to enclose the whole circuit in a metal box? would that help?
Mike

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Re: help with issue in my amp
« Reply #12 on: June 28, 2007, 11:06:04 AM »
i re-ran all the grounds, as to the tutorials on the site, star ground. I added another resistor to ground on the input. Still have a big hum until i touch any metal like the base of the input jack. I tried another amp circuit and it does the same thing. I also tried a different preamp circuit, pulled out the tone circuit, still the same..
So the circuit is now on a flat piece of metal that I was going to mount to the top of a small combo box I built, holes for the pots and jacks. could it be that I need to enclose the whole circuit in a metal box? would that help?
Mike

It's always a good idea to put your amplifier in a metal box because it cuts down on interference.
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teemuk

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Re: help with issue in my amp
« Reply #13 on: June 28, 2007, 02:21:06 PM »
Stuffing the circuit into a metal box helps but IMO amps shouldn't buzz annoyingly unless the source of interference is right next to them. How is your wiring? Is it excessively long? Do you have wire loops that act as antennas? Does your PCB have similar loops? Can the wiring capacitively couple some interference? Do you run low amplitude signal wires near high current wires? Is input jack near output jack or the supply wiring? Do you have very short wires from input jacks to the first gain stage. Do you use shielded wire where it's needed?
...You know, the usual stuff.

Also: Check that DC supply of yours. Make sure that every connection related to it is OK and tight. I've had nothing but troubles with my 12VDC practice amp that is powered from a wall wart. The DC sockets and jacks of those things are always a bit "loose" and therefore sometimes begin to create infernal buzz or shoot sparks.

duclon

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Re: help with issue in my amp:Finished
« Reply #14 on: June 28, 2007, 03:08:57 PM »
Stuffing the circuit into a metal box helps but IMO amps shouldn't buzz annoyingly unless the source of interference is right next to them. How is your wiring? Is it excessively long? Do you have wire loops that act as antennas? Does your PCB have similar loops? Can the wiring capacitively couple some interference? Do you run low amplitude signal wires near high current wires? Is input jack near output jack or the supply wiring? Do you have very short wires from input jacks to the first gain stage. Do you use shielded wire where it's needed?
...You know, the usual stuff.

Also: Check that DC supply of yours. Make sure that every connection related to it is OK and tight. I've had nothing but troubles with my 12VDC practice amp that is powered from a wall wart. The DC sockets and jacks of those things are always a bit "loose" and therefore sometimes begin to create infernal buzz or shoot sparks.

Thanks a lot for those comments. and thanks to the others who posted back also. My board wiring looks good, but my wires tying it all together don't. I need to really clean it up, I did that on the ground wires, and I'll add shielded wire on the input and see if that helps. It's about 3 inches long and there are 2 of them.
I really appreciate the help from you guys, This is my first amp. I've got a few books, and am working through them, but they are either very basic or really deep, so it's taking me time. I search the net, go to any sites related to this stuff, but i guess I try and cut corners by just building the circuits without looking at the basics of good design and practices. You know the new-bee i want it now thing.
I'll take a really close look at this thing, clean it up as for the wiring and any thing else suggested and then post back in a few days with the results and pics of the completed project.
Mike

EDIT: I did all these things, and after doing that it works great. No Buzz or Hum and no need to put it in a metal box. Here's a pic of the outside fo the box,
http://www.softseas.net/9voltamp.jpg
I'll take a pic of board later. It's 10 watts at 8 ohms at 18 volts and 5 watts at 9 volts stereo inputs mono out. run on a 9 volt or a wall wort. It's my  first wood working and amp project.
Thanks again for the help...



« Last Edit: July 01, 2007, 04:10:37 PM by duclon »