Sienna SE Preamp with Line Driver Mod

[sskloss]

From what I can tell, the factory HU in the SE is an all-in-one unit and does not use an outboard amplifier. If this is not the case PLEASE LET ME KNOW, as further research will be needed for the plans laid below. Also, since I have not yet cracked open the HU I am making some assumptions now that may need to be worked around. Again if you find an error or incorrect assumption below PLEASE LET ME KNOW.

This mod will tap the line level audio signals from the factory SE radio giving a full 4ch set of preamp outputs. Doing some homework, I have read that one of the biggest issues is the weak signal level. To resolve this I will also be installing a line level amp/driver inside the factory HU which will boost the signal to 5-8V then provide the drive power to feed active crossovers down the line. Having access to an oscilloscope I will try to compare the waveform of the line level vs speaker level outputs to see if this mod is really better than using a line level converter – and hopefully how much better.

Being somewhat of an audio freak I cannot bring myself to using a line level adapter off of the speaker outputs to feed the rest of the gear I have planned. I really like all of the current functionality and want to retain it as much as possible. Using the factory HU will also save the cost of an aftermarket HU and the time/$$ spent regaining what I already have.

The line amp/driver will use a dual operational amplifier IC (opamp) for each channel. These will be on a single PC board and mounted inside the HU. Power for the amp/driver board will come from the HU itself. I am pretty sure there will be +/-12V rails to use, so there shouldn’t be an issue. If the +/- rails are not there, a simple +12V and Ground will work but with a capacitor or two in the signal path. The RCA outputs will be cables running out of the back of the HU since I am not sure how much real-estate there is to mount 4 jacks.

This will be a progressive mod and will be done over time. I will be traveling soon and only have some weekends to work on this, so please be patient if updates and/or responses are slow.

Steve

[gs3tek]

Sounds good Steve. I'll be following your thread.

[sskloss]

I'll do my best...

Been doing some more research on the current production HU amps ICs and internals and found some interesting info. Even when looking at the high end aftermarket HUs, the best THD figures are just under 1% and going up to 3%. These figures are measured at 1kHz and it only gets worse with higher frequencies and those below about 100Hz. This falls within the midrange and upper midrange frequencies which can make or break an audio system. Furthermore, the crosstalk, voltage noise and signal to noise ratio figures of these chips are not great either. This has given me the fuel to really pursue this mod.

Looking at some service manuals, HUs do not have +/- voltage rails as I assumed earlier. The amp ICs feed off of the 14.4V main power inputs. This isn’t necessarily a bad thing, but does mean that there will be two more capacitors in the signal path than I will like. Furthermore, nice signal caps tend to be large and I am hoping there is enough room inside to contain them on the amp/driver stage.

I have decided to use only one opamp per channel instead of a dual. This will let me keep both Front and Rear of either left or right on one dual IC further reducing space needed and further reducing the chance of induced noise. One opamp is more than enough to boost the signal to a gain of 5 and lower the output resistance to drive whatever comes later.

Steve