LM3875...made changes...screwed it up

[chromenuts]

Hi all

So I built an Audiosector LM3875 premium kit quite a while ago as my first DIY amp. I had some grounding issues at first, but worked through them with some help and had a perfectly good working amp.

Since then, it was used on and off in different systems both in a small apartment and my bigger system at home with no issues at all.

I moved on to build ACAs and am now partially through F2Js and the gainclone sat unused.

Genius that I am, I decided it might be cool to take a sidetrack and add a few features to the gainclone and make it useful again as a sort of integrated unit for a bedroom system. Maybe even take it with me to BAF. NOT.

The goal was to add a pot for volume control along with rewiring it to allow switching between two sources (an iPhone cradle and the bedroom DVD/TV).

I also wanted to run a parallel signal out of the amp that could be plugged into a small active sub to help the bottom end out of the system which only has some fullrange 4 inch Fostex drivers in bass reflex cabinets.

Everything I did was a compromise to add these features given that the original layout of the amp was a bit strange.

The original amp tried to isolate the amplifier section, input and output as much as possible from the power supply while still having only one chassis. It turned out putting the transformer and connections on the top of the chassis plate was the only way I could think to do that.

In order to add the new features I had to reroute both input and output wiring through the power supply section of the amp in order to place the connectors on the back of the amp where they could be used. I did twist both signal and speaker wiring, but did not install shielding.

All the modifications I made appear to work properly. The problem I am having is that I now have a random crackling/static type of noise coming from the amp that I can not figure out the cause of.

Hopefully this link to a video I made of it happening might work and be helpful:

LM3875 - Google Photos



I will attach some pictures of the amp inside and out. If needed I can dig up older pics from before I made the changes.

The changes I made included:

Adding a cradle to the top of the amp that has a lightning connector.
(This required an Apple lighting pigtail and a mini stereo cable modified to wire to the source selector switch)

Adding RCA input to the rear of the chassis.

Adding a DPDT switch to chose between these two sources.

Adding an ALPS 20K potentiometer for volume control.

Adding wiring between the signal selector switch and pot, as well as the pot and amp boards. Grounding inputs at pot.

Wiring a parallel line level output from the amp boards input to the back of the chassis to the RCA out for the subwoofer. (A 100 Ohm resistor isolates this from the input of the amp board.)

Relocating the speaker connections to the back panel of the chassis.

Adding a SPST switch to control power supply to the amp boards to the front of the chassis.

Rewiring the LED so that it indicates power to the amp boards instead of the rectifier board.

This amp uses a surplus transformer I sourced from Apex Jr that has a single primary and dual secondaries. It produces about 21 volts AC.

There is a single rectifier board powering both amp channels at just under 29 volts DC.

All speakers used are rated at 8 Ohms.



I thought there might be an issue with the lightning/mini stereo cable wiring with a bad ground or otherwise.

The noise is unpredictable. It can happen when either of the inputs are used.

It happens from a variety of phones I have tried at both inputs using lightning adapters or directly plugging into an older iPhone mini stereo port.

At this point I do not know where to look. I was concerned that perhaps I had a bad solder joint on the amp boards since they were floating and moved around a bit while I made modifications. I reflowed all the soldering on the amp boards.

I question whether the method I used to create a parallel output for the sub could be causing a problem.

I was mimicking a method that was recommended to me in order to have parallel outputs on my DCB1 by adding the 100 Ohm resistor. However, I am not sure if tapping the input pads on the amp board for the signal was a mistake.

Any insight as to where to look next is greatly appreciated.

 

[chromenuts]

I reconfirmed the DC offset at the speaker posts. It’s the same regardless as to whether the inputs are shorted or not. Only .5 mV on one channel and -5 mV on the other.

Really baffled by this situation. When I was testing with different sources it did seem like the older iPhone caused the most instances of this static-like noise. It also seamed that it occurred more often through the wiring I set up for the cradle.

I tried turning the amp on with no source and sat and listened for a good 15 minutes without hearing a peep.

I also pulled out an old CD player yesterday and tried playing a few tracks through both inputs . Normally the noise occurs within the length of a single track. I didn’t hear any noise at all.

I’m wondering if the iPhones have an inherent problem in there grounding or signal transfer. As I said it happened through the older iPhones lightning port and it’s mini stereo headphone port.

 

[avtech]

I can't see a protective earth connection from your mains inlet.

It might be that the CD player is providing the ground and making it quiet, whereas the phone is a floating connection.

Anything less than 50mV DC at the output is generally ok. I wouldn't worry about half a mV or 5mV.

I've had noise issues with phones in the past and found that they would produce static between tracks being played. Hard to describe but there would be an audible cutoff between <noise>-<track play>-<no noise>-<track stop>-<noise>. Very strange.

Also, I wouldn't be too hard on yourself. Your build looks good - there are heaps worse out there for sure! My only tip would be to use some PCB cleaner to get the flux off the board. Leaving flux on the board doesn't matter too much for short term prototypes etc but definitely good for longevity.