Hello,

in my 1972 914 I recently installed a remote mounted amplifier (Sony 222w) and hooked up the original radio (Motorola mono FM) along with a bluetooth receiver. Everything worked great for a few days, until something weird started happening. If I run the audio system with the engine off, everything is perfect. However, if I run it while the car is running, every few minutes the audio will cut off completely for a few seconds (sometimes even one minute), and then everything will come back to normal again.

Also, once in a while I get some low frequency hum through the speakers that will last for 5-10 minutes. When this happens other events that affect the electrical system (for example, using the blinkers) will produce noise in the audio system. This only happens while the hum is going on.

Any suggestions? It feels as if there are maybe some electrical spikes in the electrical system, and the amp must be having some protection that shuts it off temporarily.

Thanks,
Andrei.

First, check to make sure ALL of your ground connections are clean and solid.

What is the current draw (amps) of the audio amplifier?

Your battery acts as a capacitor (condenser) that filters ignition noise on automotive audio systems. It's possible that your alternator and battery aren't providing enough amperage. Also, when installing a high-powered amplifier, you should install a large audio capacitor to help with the spike in current draw, especially from the low frequency (bass) notes.


https://www.youtube.com/watch?v=/C082c66cSmM

Which component is turning off? How did you connect the radio to the power amp? Have you checked ALL of your wiring? You should connect a PI filter(s) & a fuse(s) as close as possible to both components. Batteries do not isolate ignition noise. Nothing in the car should be creating a hum. The input power buffer capacitor shouldn't really be necessary, but it's not a bad idea. I've never had to use one. Maybe your radio or power amp has intermittent issues. What's the power amp's INPUT current rating? BTW:

power = voltage x current

1 watt = 1 volt amp

The amp is not very high powered. Officially it could do 222w on spikes, but continuous it is rated at 35w x 2 channels. They say a maximum current draw of 12A at max output. But I get this turning off at any output power setting, even at low volumes, so I don't think it is a problem with high power draw.

The way things are connected is as follows: amp power comes from the fuse box; amp connected to speakers in the only possible way; bluetooth receiver goes to the RCA plugs, and is powered from the same line as the amp; radio output goes to the high level input of the amp. BTW, even disconnecting the radio from the amp the amp still cuts off randomly. Interestingly, the bluetooth receiver keeps working even when the amp cuts off, so the amp and the receiver must still be getting power.

I suspect the low freq hum comes from the radio, and it is a separate problem from the amp cutting off. So I'll need to fix it one problem at a time. My current suspicion is that the amp has some protection, either against overvoltage or against a short in the speakers. I don't know which is more likely. I was planning on connecting today a voltmeter to the power cables that go to the amp, and checking if there are voltage spikes.

Depending on the efficiency of your amplifier, it has the potential to draw more than the rated output.

However, I looked at the specs for the SONY 222W amp and it stated that the current draw is 12 Amps (4Ω, 35 W x 2). 12 amps is 1/3rd of a 914's stock 45 amp alternator's maximum capability.

Instead of connecting the amp to the fuse block, it would be more efficient to connect the amp directly to the battery (with an appropriate in-line fuse). Both the (+) and (-) wires, as choosing the body(chassis) ground is susceptible to deterioration over time. Again, check and freshen up all of your ground connections anyway.


As for the low frequency hum ... it's possible that the input signal cable's shield(s) may be the culprit.

... and yes, a flooded-cell battery starting to loose it's capacity to store energy will exhibit ignition noise in an automotive audio system. I know when it's time to replace my trucks' batteries when I start experiencing ignition noise on my HAM radio(s). A battery is a voltage storage device, just like an electrolytic capacitor (filter). In fact, some people are now installing 'super' or 'ultra' capacitors in lieu of a standard automotive battery.


https://www.youtube.com/watch?v=/JAT_8H23iGI

It will draw more than the rated OUTPUT. The rated INPUT will always be greater. Hum is usually thought of or heard as a 60 Hz noise component. Car's don't generate 60 Hz. Does the pitch of the hum change with engine RPM? Finally, perfect shielding, direct battery connections, local fusing, & the use of a Pi noise filter are all very important. The Pi filter consists of a resistor in series with the load with two capacitors in parallel connecting the load to ground.

Very interesting about the batteries. Thanks.