Alpine 3545 power supply noise

Dr Zeus

Member
2010-10-17 5:12 am
This amp is the twin to the other one I'm working on, only this one has a completely different issue.

The amp powers and plays audio well enough, only the power supply near the transformer area has like a white noise sound - almost like theres a small computer speaker just dishing it out in that area. The amp is being powered off a 10A supply without strain. I cant quite pinpoint the noisy area; however if I touch and move the transformer the noise seems to change. The design of this amp allows the transformer to be easily removed with just 10 screws. i was thinking of possibly swapping the transformer in this amp with the other Alpine 3545 but I really dont want to introduce any different problems atm and wanted to get some opinions on doing this first.

If I disconnect the transfomer wires leading to the FMG-33x rectifiers, the white noise goes away, likely because the power supply having 0 load. I removed the amp from the heat-sync and cannot seem to find much in the way of anything obvious (resistors, caps, solder joints) which would cause the problem.
 
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Check the DC on all of the various filter caps. Does the DC remain clean or does the DC become dirty (very high frequency noise, not ripple related to the audio signal)?

When checking the voltage on the secondary, you may need to connect your scope ground to one of the non-bridging speaker terminals.

If everything remains clean (and the amp is producing clean audio), it's likely that the noise is due to loose windings and is not a significant problem.
 

Dr Zeus

Member
2010-10-17 5:12 am
I swapped transformers with the other Alpine 3545 and the probably does not seem to be related to the transformer windings as this amp still makes the hiss/white noise.

I'll start poking around the caps and see what I can discover and if I have any questions. Thanks!
 

Dr Zeus

Member
2010-10-17 5:12 am
What does VR601 do? Its a potentiometer near the 494. When I touch/move it slightly the noise changes a good bit but never fixes; and always comes back after I stop moving it. I've only twisted is ever so slightly...

Also, referencing amp ground, the scope shows excessive noise on most of the screw connections leading fro mthe transformer.
 

Dr Zeus

Member
2010-10-17 5:12 am
I replaced these two caps but the noise didnt change.

IMG_20101218_173016.jpg


Here is what is coming off the power fet center legs with the scope set to 5V/Div

IMG_20101218_172811.jpg


Again with 5V/Div, only time set to 2uS
IMG_20101218_172931.jpg
 
The first waveforms look noisy because your scope isn't triggering properly.

The waveform is probably normal. This is a regulated power supply and will operate at reduced duty cycle which causes the waveform to be different from the normal square wave. See item #25 on the Power Supply Troubleshooting page of the DVD.

The amp doesn't use FETs in the power supply. They are bipolar transistors.
 

Dr Zeus

Member
2010-10-17 5:12 am
I think I found a bad transistor on the driver board. Q618, part number is A114YF-G. its one of those small rectangular transistors. I pulled it off the board and confirmed its not measuring so well, however I compared it to the other amp and they are either both bad or both good. The one on the other amp looks to have been replaced or removed previously. Diode check fails.

Legs
1-2: 4.3M Ohms
1-2:Open
1-3: 62k Ohms
1-3: 62k Ohms
2-3: Open
2-3: 4.1M Ohms

BOLD for COM
 

Dr Zeus

Member
2010-10-17 5:12 am
I could not find any degraded caps in this amp. I went ahead and set the bias for both channels, and the noise from the transformer greatly reduced. I'm thinking about trying the shellac idea next.

This amp's audio output is very clean. I dont think there is anything really wrong with this amp otherwise.
 

Dr Zeus

Member
2010-10-17 5:12 am
I tried searching, but I cand find any information about VR601. What does VR601 do? its a potentiometer just right next to the TL494. I turned it (again) and got the power supply to -completely- quiet down. its totally silent now. How do I make sure this is set properly?

IMG_20101219_102638.jpg
 

Dr Zeus

Member
2010-10-17 5:12 am
104v AC or DC? The meter is picking up both. Right now It measures 98vAC and 112vDC measuring from leg to leg, middle leg of each rectifier.

If I lower the voltage to 94vAC and 104vDC the noise is intollerable for usage in a car.

The noise coming off this amp is pretty loud. Using a free RTA analyzer on my cell phone (Android) it measures about 35db at 5000hz. Its much too loud for usage in a car.
 

Dr Zeus

Member
2010-10-17 5:12 am
ok. I was able to minimize the transformer noise by setting the rail voltage to 106vDC, and the bias to 0.002vDC. Any less than that and the noise is very apparent. The owner of this and the other amp is likely going to install these in his pickup truck which makes the install not very kind to a noisy amplifier.

Thats as good as this amp is going to get I think. I'm test running the amp on the bench and I'll run it hard when I'm satisfied with the current state of things.

The PC1298V is running a little warm though. 150 degrees after playing music for about 10 minutes or so. Thats a little concerning since this is the driver circuit for the outputs. Maybe I'll give it another go at 104vDC and 0.000vDC and see if they heat up just the same.