I'm building my first fully DIY amp and am having issues with DC offset. Below are some voltage readings:
power gnd to:
pos rail 35.68
neg rail -35.68
input ground -35.12
in variable*
chassis 0
ouput -35.4
across LED 0.35
across safety resistors 0
*The voltage at the input started out around -35V and then fell to essentially zero over around 10 minutes.
I've removed and checked all of the transistors and triple checked the pinouts. An image of my circuit is here: [image]http://i.imgur.com/2B0Yq.png[/image]
Any ideas?
power gnd to:
neg rail -35.68
input ground -35.12
in variable*
chassis 0
ouput -35.4
across LED 0.35
across safety resistors 0
*The voltage at the input started out around -35V and then fell to essentially zero over around 10 minutes.
I've removed and checked all of the transistors and triple checked the pinouts. An image of my circuit is here: [image]http://i.imgur.com/2B0Yq.png[/image]
Any ideas?
Just curious, did you connect the "In Gnd" to the "Gnd" of the power supply? That input seems to be floating...
Peter
Peter
The grounds are not connected. From what I've seen, some leave them disconnected and some connect them with a ~10 Ohm resistor.
What do people think:
1) Leave disconnected.
2) 10 Ohm resistor on board bridging 2 grounds.
3) Wire from RCA in ground to PS ground (with or without resistor)
4) Wire from board input ground to PS ground (with or without resistor)
What do people think:
1) Leave disconnected.
2) 10 Ohm resistor on board bridging 2 grounds.
3) Wire from RCA in ground to PS ground (with or without resistor)
4) Wire from board input ground to PS ground (with or without resistor)
You can not leave it unconnected, you get what you see on your amp. That Gnd gives the DC bias to the input transistor as well to the current source for the diff pair.
You need to ground it to the "star" ground on the PS. People use the 10Ohm, so if they forget grounding the input, like you did, then it would not float and would not upset the amp.
Peter
You need to ground it to the "star" ground on the PS. People use the 10Ohm, so if they forget grounding the input, like you did, then it would not float and would not upset the amp.
Peter
to get correct biassing of the input transistor it should be connected either directly or via a 10 ohm resistor. If you notice on the circuit, 60-80W Power Amplifier , Rod Elliott has the junction of R2 & R8 (ie input ground) connected directly to ground. He designed it he should know! A 10 ohm resistor should only be necessary to isolate a ground loop.
Of course you NEED to connect the GND to the power suplply GND, no question about that. Your LED CCS is biased using the ground, just to say one problem you build up.
About connecting the GND via a 10 Ohm, using the ORIGINAL circuit I do not recommed you to do that. I tried once, and I got a nasty low-frequency oscillation. If you want to do that you should do it like P68 ( I believe some later versions of it) ,where ONLY the low power stages are decoupled with the resistor.
About connecting the GND via a 10 Ohm, using the ORIGINAL circuit I do not recommed you to do that. I tried once, and I got a nasty low-frequency oscillation. If you want to do that you should do it like P68 ( I believe some later versions of it) ,where ONLY the low power stages are decoupled with the resistor.
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