EuroDriver

I have a Blaupunkt PA 2150 digital power amp which uses a Tripath Chip set.
Input impedance is 10K ohms, sensitivity is adjustable from 6 volts to 0.3 Volts.

When the power amp is driven by 2 different hand held MP3 players the hiss level is very low.

If I turn off the MP3 player but leave it plugged in to the power amp there is a lot of hiss. More hiss from the earphone socket than from the line out socket. If I unplugg the MP3 player from the patch cord but leave the patch cord connected to the amp there is no hiss.

The hiss level is modulated by the amplifier gain control

When I hook up my Concert (non bose) head unit's pre out signal to the amp, there is a lot of constant volume hiss, very audible when switching tracks, even at the lowest power amp gain setting.

It seems like the power amp is very sensitive to the impedance of the source. I called Blaupunkt USA tech support, they were not helpful but suggested to use the Hi-Level adapter that came with the amp. They said it would not sound too different. I suspect the Audi Head unit pre-out signal impedance is lower than the normal practice, but compatible with the Nokia amps they use for the rear speakers. Lower impedance for less capacitative line loss and noise pickup ?

What can I do with the Pre-Outs from the Concert ? Resistor in series, matching transformer, op amp ?

I could used the supplied Hi-Level adapter, but the pre out signal should be better and clearer than tapping into the speaker signal.

The good news is no alternator whine. Amp is mounted in side glove compartment, with very short cables.

I know it would be much easier to just get a new head unit with normal pre out signal levels and impedance, but I would prefer to keep the OEM head for its looks

MacSub

The hiss problem is an inherrent feature of all Audi head units. Just try listening to a CD with silence on it and whack the volume up to full. You'll get all sorts of wierd & wonderful noises. It's just a sh*t design using sh*t components. This problem is just enhanced when connecting a more powerful amp.
The issues with the handheld devices will be because the device doesn't have the same level of shielding as a unit designed to be used in a car. You'll get problems with ground loops as 2 connected devices will be using different ground sources, even if the MP3 player is connected to the cigarette lighter for power.
Change the hea unit it's a piece of sh*t.

EuroDriver

The MP3 players were running off their internal batteries so no addtional ground lead or loop.

The strange part is when the MP3 players are powered off the Blaupunkt power amp produces hiss. When the MP3 players are operating the hiss level in the power amp is very low. I can't remember any other amplifier behaving in this way.

MacSub

between both devices. A ground or earth loop is when 2 devices have a different reference point for ground. The mp3 player needs to have it's ground connected to the chassis of the car, as otherwise when you switch it off the signal into the amp (RCA pre-outs) will be left floating, causing the hiss.
You can get all sorts of noise problems when 2 devices are working from a different earth/ground reference point.

EuroDriver

I tried the Blaupunkt supplied Hi Level Adapter (HLA-1). The results were pathetic. The signal was so weak that the amp gain had to be turned way up and the sound quality was awful !!

Now that the head unit is driving the Focal 130A's directly, I have to say the blending of the sound from the speakers with the stock sub is quite good. System just lacks the clarity it had when the power amp was installed.

Summary : if you want to add a power amp to a Concert HU better, bench test the amp before buying to see if you will have a hiss problem