I want to hook up an aftermarket head unit (Sony XAV AX-100) in my 1999 XJR and want to use the Harman Kardon amplifier in the back, but am not sure if it is doable signal wise. The XJ Series Sedan 1998 Electrical Guide has helped me identify all the cables and connectors, so making new physical connections to fit the aftermarket head unit is sorted.

However, I am still confused about the audio signal voltages. The electrical guide states that the active voltage for the HK amplifier input pins is 0-30 MV (millivolt), Which seems way too low for the output voltages of any aftermarket head unit. To me in fact, that 0-30 millivolt range seems extremely low anyway for in-car audio, as this is microphone-level voltage territory and I find it strange that the HK amplifier is designed for this, but apparently that’s the case. I haven't found a post yet that talks about these voltages or signal compatibility.

There are two ways to connect the Sony head unit audio signals to the HK amplifier:

1. by using the RCA-jacks (labeled pre-amp out) on the back of the Sony head unit. In this case there is a separate sub-out RCA jack on the Sony head unit, which will remain unused since the HK amplifier does not have a dedicated sub input. The output voltage of these pre-amp outputs is 2V according to Sony, so I assume a 2V peak-to-peak. I would also make an adapter to connect the RCA jacks to the Jaguar wire loom that’s already fitted, so I don’t have to run new wires through the car.
2. via the regular rectangular connector at the back of the Sony head unit. The pins here are suited to drive speakers directly, so I assume the voltage range is greater than that of the pre-amp RCA jacks. Is that correct? I cannot find any information on the internet about typical voltages from these main output connectors, not for this particular Sony head unit, but also not for any other head unit in general.

Is both my assumptions are correct, then will the output voltage of the Sony head unit will be way too high for the HK amplifier to handle. To my understanding this will either do damage to the HK amplifier or result in massive signal clipping. It's not like this Sony head unit is special in respect to the voltages it puts out for its audio signals, so this would be a more common problem I suppose. If it isn't possible, then I might buy a new amp, but it's worth the try with the HK amp already fitted in my opinion.

Has anybody encountered this problem before and can anyone help me with understanding how I can get this to work?


Premium in-car entertainment power amplifier pin-out information out of the XJ Series Sedan 1998 Electrical Guide.

 

I have not done the kind of work that you are planning but it is possible that the Jag "Power Amplifier" also has a pre-amplifier and receives the same signals from the head unit as are sent to the head unit's pre-amplifier stage when it operates in "Standard" mode. If this is the case, the concept would be a bit strange - sending so low level signals all the way to the back of the car.

The only signals from your new Sony that you could try with would be those from the RCA jacks but that could mean sending signals coming out of one pre-amp to another pre-amp (not very good). There is, however, a possibility that the info in the Jag manual is somewhat misleading, i.e. not clear what it actually means, and that the Jag amplifier could accept the output from the Sony pre-amp. I would buy another Jag Power Amplifier, plug it in first to verify that it is good and then try with connecting the Sony RCA outputs to it keeping the volume at "0" and then move the volume up very little to see what happens. You can also try it with your existing Jag amp if you do not mind if it is damaged (by overvoltage at inputs).

 

Thank you for your reply!

I found some time to try and measure the voltage coming from the original Jaguar head unit with a portable oscilloscope. For that, I unplugged the cable from the head unit to the HK amplifier at the amplifier side and connected the oscilloscope to the Right Front channel signal pin (black wire). I played audio from the CD-changer input on the head unit, and I tried four different speaker volume levels on the head unit. Of course, during measuring, there was no sound coming from the speakers, but I verified the loudness of the speakers with the different volume levels afterward. I measured the root-mean-squared voltage (=Vrms) with the oscilloscope, which corresponds to a peak-to-peak voltage (=Vpp):

Volume setting and the measured audio signal voltage:

• 0 (turned all the way down): 0 mVrms = 0 mVpp
• 5 (normal volume): 10-20 mVrms = 28-56 mVpp
• 10 (pretty loud): around 50 mVrms = 141 mVpp
• 15 (way too loud): around 150 mVrms = 424 mVpp

So as you say, the info in the electrical guide is indeed a bit misleading. It takes signals above 30 mV, but whether the HK amplifier can handle signals in the 2000 mVpp (i.e. 2 Vpp) range is not clear yet. But, even if it wouldn't damage the amplifier, it looks like you don't want to go near that, since there is no way to separately control the attenuation of the amplifier in order to not be deaf in that case. Interestingly, I did not notice any distortion or clipping through the speakers at the loudest volume level. I might have just been oblivious to it, or I probably haven't reached the limit of the input signal voltage into the amplifier.

Following your suggestion, I'll maybe try to connect the Sony head unit via the pre-amp outputs to the HK amplifier and increase the volume in small steps from "0". Now I know that a small increment of the volume on the Sony will probably not immediately damage the amplifier as implied by the electrical guide.

I'll report back what I did.

A sidenote: the perceived loudness at the volume levels can be different for everyone, and that loudness can also be influenced by the impedance of the speakers you have installed. You can also have CD's that are 'quieter' than others, eventually causing a lower output voltage from the head unit. The voltages that I list above are to be taken as a rough estimate, in my opinion.

 

As I suspected, the Jag Power Amplifier also has a pre-amplifier. I am sure it has certain overvoltage protection but the question is up to which level. According to your measurements, the input levels into the Power Amplifier are in the range up to 150 mV, maybe up to 200 mV at max (never used) volume. Perhaps one solution for the use of your Sony's output of up to 2000 mV could be to insert a fixed 10:1 voltage divider. Example: