Stumped, amp noise won't stop

Status
Not open for further replies.
T

techrex74

New Member
I built the preamp (2 channel) utilizing the AN6551 chip. the unit works, but it makes noise like I'm touching (with my finger) the input wire. the only way to stop the noise is to touch the ground point (0 volts). regardless of volume Pot setting. I am at wits end now, any Ideas? I've included the schematic. I am using a 15_+ (differential 30 VDC) regulated power supply.

OK I found the unit has to be enclosed in a metal housing, grounded. This stopped the noise.

But now the unit periodically "POPS", as heard through the output. Any Ideas on that?
 

Attachments

  • preamp (Small).jpg
    52.2 KB · Views: 251
Last edited:
I had a similar problem with an amp I built. I changed the input wires to shielded wire and grounded the shield and switched to a regulated switching supply and my probablem went away.
 
mramos1 said:
I had a similar problem with an amp I built. I changed the input wires to shielded wire and grounded the shield and switched to a regulated switching supply and my probablem went away.
Click to expand...

Thanks for the input, no pun intended. Its not the from the input, the volume pot will be turned all the way down, and yet the board still makes noise. this unit uses a dual op-amp package, utilizing the feedback circuit to conture the tonal characteristics. Also I'm using a regulated Power supply.
 
I think you might want to do as Nigel stated and send up the schematic. I had the problem, volume down, I touched the case and buzz city. If you search, the thread is here in the forum. Had a lot of good advise, but those two fixed it. The search function here is a little weak. try amp and then try noise.

If it is in the circuit, you will need to upload a schematic though.
 
I did upload the schematic, it's a thumbnail that can be enlarged. Take a look. When I touch the ground circuit, the amp silences all noise, and works perfect.
 
techrex74 said:
I did upload the schematic, it's a thumbnail that can be enlarged. Take a look. When I touch the ground circuit, the amp silences all noise, and works perfect.
Click to expand...

Which is why I asked for a picture of how it's built - it sounds like you've not grounded it correctly?.
 
Maybe the 0V from the dual polarity power supply is not connected to the circuit's ground.
 
Nigel Goodwin said:
Which is why I asked for a picture of how it's built - it sounds like you've not grounded it correctly?.
Click to expand...

It is a kit, put together exactly as shown in the schematic. In attempt to isolate the problem, I diconnected (via P/S, and input resistors) the Mic amp and Mag circuit.
 
audioguru said:
Maybe the 0V from the dual polarity power supply is not connected to the circuit's ground.
Click to expand...

The CT from the X-former is the same ground to the amp. It also shares the same ground point as the input and output leads
 
If touching the ground leads on the board cures the problem, then it's almost certainly not grounded - as we can't see it, we've no way of knowing if it's connected correctly, but can only assume it's not?.
 
Can you take a jumper wire (clip wire) from CT (the center tap) to ground, clip that on. See if that helps. It sounds like you have a floating ground.

Can you take a picture of the board and upload it.
 






Thanks for all the replies so far. I have uploaded 4 photos of completed unit. The Center Tap from the transformer runs from it all the way to the circuit boards ground point (0 volts) even through the regulated P/S.
 
You may want to research the term and applications of 'star' grounding methods. You have very long common run from the transformer CT to the common on the PCB. Typically an audio application will place the common 'star' grounding point as the common connection of the two main filter capacitors. Here is where you will run wires to all load, input and out connections that require a common. You basically are most likely fighting a 'ground loop' problem that is very common (pun intended!) on low noise audio applications.
 
I see the photos are taken on the floor... does the hum change in different locations? A sensitive amplifier sometimes needs to be shielded - Try setting it on a metal plate (with insulation). Metal plate connected to your signal ground.
 
Hey thanks for the Info. My P/S is using 2, 3300 Mfd caps, one Pos one Neg and in the middle ground or 0 volts. then the board has 2 Caps, 1, 470 uf and 1, other 470 uf (I replaced the originals thinking they were causing trouble), both referenced to ground, 0 Volts.
 
mneary said:
I see the photos are taken on the floor... does the hum change in different locations? A sensitive amplifier sometimes needs to be shielded - Try setting it on a metal plate (with insulation). Metal plate connected to your signal ground.
Click to expand...

I put the unit on the floor just to shoot the pix. The location of operation is on a card table, made of particle board.
 
mneary said:
I see the photos are taken on the floor... does the hum change in different locations? A sensitive amplifier sometimes needs to be shielded - Try setting it on a metal plate (with insulation). Metal plate connected to your signal ground.
Click to expand...

WOW, I'll be damned, I placed the unit in a baking pan, Insulated and grounded, and it is very much quieter, except now it pops alot.
 
Sceadwian said:
Something like that should be in a solid metal housing and well shielded input and output wires.
Click to expand...

it turns out that's what it's going to take, thanks for the input.
 
Status
Not open for further replies.

Similar threads

M
  • Question
Low noise TEC driver design - adjustable linear voltage regulator
2
Replies
26
Views
6K
Tony Stewart
P
  • Question
Linear analog filter with zero amplitude in stopband
Replies
14
Views
3K
Tony Stewart
U
  • Question
Generate continuous random bit stream from noise
2
Replies
20
Views
3K
BobW
B
A
  • Question
200 amp NC 240 AC Relay
Replies
9
Views
2K
rjenkinsgb
Q
  • Question
Input requested for DIY Mixer-Preamp
Replies
16
Views
3K
Quercus
Q
Menu
Log in
Register
Cookies are required to use this site. You must accept them to continue using the site. Learn more…