Directed 1100D

[spooney]

Got a few little issues with this amp I am trying to track down. Biggest issue I think is that I have a little bit of hum in the output. I am thinking that the shield ground in the amp is damaged although I have not found any blown ground traces. Should the rca input shields in this amp be tied to secondary ground? I am not getting continuity between either rca shield and secondary ground. One of the rca output shields is tied to secondary ground. The second issue is that the transformer seems a bit noisy. If I push down on it the noise gets considerably quieter. Is this the sign of a larger issue or do I just need to secure the transformers windings with some silicone or e6000? Maybe a few foam pads on top of the transformer that hit the bottom plate once its installed?

[Perry Babin]

Is there a hum with no RCAs plugged in?

Is the signal source ground connected to the amplifier's power supply ground?

Move the windings around a bit if the noise is an issue. Unless the noise will be a significant problem, don't apply anything to the windings.

[spooney]

No rcas plugged in equals no hum. Low level hum is present with the rcas plugged in and if I touch the speaker terminal it gets a bit louder. The signal source is grounded to the same place as the amplifiers ground. Shouldn't I have about 1 k between the rca shields and the amps ground terminal? Twisting the transformer just slightly took care of almost all the noise there.

[Perry Babin]

This amp uses an active input circuit. In amps that use isolated secondaries to break ground loops, there is often 1000 ohms between primary and secondary grounds. In amps with active input circuits, the resistance between the shields and the primary ground is generally very high (mega-ohms to OL on a multimeter).

[spooney]

Should there be continuity between the two rca input shields? Because there isn't. Should there be continuity between the rca input shields and rca output shields?No continuity there either. Should any of the shields be connected to secondary ground? Only one of the output shields is. Do you have a schematic for this amp ?

[Perry Babin]

The two output shields should be connected to the secondary ground (if I'm not mistaken).

The JBL BP1200.1 (look in the JBL folder) is similar to this amp.

[spooney]

Cool I will look into that when I get home. Thanks.

[spooney]

I looked at the schematic for the jbl and couldn't really find where the shields go. As best I could find they basically just connected to each other through 100k ohm resistors. I have the amp in the car now and it works fine. There is a little noise through the sub but it isn't noticeable unless you have your ear to the sub. If I hadnt already heard the noise on the test bench I probably would never have noticed it. Are some class d's just a little noisy? Seems like I had an earthquake that had the same low level hiss/hum.Maybe my power supply is inducing the noise. Although the amp works fine in the car I still feel there is a problem due to the fact that when it was on the bench and i broke the ground connection between the amp and signal source it made a terrible noise through the sub. I have a viper d1200.1 here which is virtually the same amp and when i break the ground connection between the signal source and the amp there was no change and no terrible noise.

[spooney]

After looking further at the schematic all the shields eventually end up connecting to one of the opamps in the preamp section after making their way through a series of resistors. I think I am chasing a problem that doesn't exist. I guess it would make sense that I was getting feedback through the sub when I broke the ground connection between the signal source and the amp. Two totally different ground references probably isn't a good thing.