Advice on power supply suppresion for car line level booster

[andynz]

Hi Everyone

I have a head unit with line level feeding the factory amp pack. It works fine but the output level needs boosting by around 6-10db. The circuit should not be too much problem but any ideas on the power supply for the opamp circuit? What is the best sort of power supply circuit to filter out any alternator noise.

Has anyone built a simple line level amp?

Cheers
Andy

[Perry Babin]

If the factory amp has a balanced input, you may be able to drive it with the speaker level signal from the head unit.

[andynz]

Hi Perry

Thanks for your reply. The level is only about 6db down so I want to try and avoid the quality loss by using the speaker amp outputs. I would also need to passive drop the level.

Hard to know what sort of inputs the factory amps have. It is a Chrysler 300 Boston speaker and amp system controlled via the CAN bus system. You made a good point on the possible balanced input as that would be the 6db difference in level. Best test would be to try driving the - input to the amp and see if I get a signal out. It's th only way I can think of testing it as there is no information available on the 300 amp pack, believe me I have tried!

Even if I build a unbalanced to balanced line driver I still need any advice on alternator noise rejection in a power supply. Do I just simple used a good old fashioned choke prior to the regulator stage?

Cheers
Andy

[Perry Babin]

Noise is rarely caused by minor noise on the supply lines. Noise from the alternator and other sources generally gets into the audio circuit due to a poor quality input circuit on the device.

Many of the newer audio amplifier ICs used in head units are extremely quiet. I've used the speaker level output several times and with the proper gain settings on the amp, the system was extremely quiet. The speaker level outputs are not a 'dirty' signal in most of the newer head units. Take, for example, the TDA7560 (which is likely the part labeled as the PAL007 used in many Pioneer head units). The THD is very low (0.006% at 4w) and the noise on the signal lines is only 35uV. That's approximately a 100dB s/n ratio. If your amp has a balanced input and the gain of the amp isn't too high, the speaker level signal may be the best option.

If you build a circuit, it will have to be grounded to the case of the head unit or through the RCA shields unless you:

A. build an isolated power supply with a transformer
B. use a transformer on the input of the device you build
C. build a noise canceling input circuit into the device.

[andynz]

Thanks very much for the advice Perry, just what I was after. I'll check if the factory installed amp input is balanced and try the head unit amp output with an appropriate attenuator.

Thanks again

Andy

[Eva]

Almost all factory car amps that I had to deal with had balanced inputs intended to be connected to the speaker outputs of a factory head unit. Some of these amplifiers (BOSE style) even use the presence of the 6V bias from head unit chip amp outputs to turn themselves on. That's probably why the signal from a normal pre-out requires boosting, these inputs are intended for 7V rms or so. Premature bass roll-off may also happen with capacitively coupled pre-outs if the amplifier has a low input impedance.

That way, car manufacturers can use the same head unit and wiring both for premium and conventional audio systems. Also, the low impedance and high voltage provided by the head unit amplifier allows to carry the signal trough normal wires without shielding (they are usually twisted for maximum interference rejection).

[andynz]

Thanks Eva

That makes sense. The feed from Head unit to amp module is twisted pair and the amp input has a + & - input with no mention of earth. It does like the best option is to simple use the aftermarket head unit speaker output which is balanced anyway.

Thanks for your help.

Cheers
Andy.