Re: Mysteries of the "premium" DSP / AMP (archive)

The complete automotive resource for buyers, sellers, and owners like you.

Great information! Thanks for posting it. I have been dying to get my hands on one of these systems to investigate it for some time, but it has been hard to find any "take-outs" yet.
I am very familiar with the Philips chip, as I evaluated and rejected it for a project I was working on some time ago. There is hope however for those who do not want to go inside the DSP "box" to get a reasonable signal to drive a sub amp. If the speaker output of the chip is unloaded by driving a high impedance instead of the specified 8-Ohms, then the distortion drops dramatically until it is in the <0.1% neighborhood. Therefore, connecting it to a typical external amplifier with an input impedance above 10kOhm should give good enough results for subwoofer use. The problem is that many of the "line output converters" that people insist on using simulate a speaker load, so this distortion reduction is often not realized in practical applications. A direct to amp connection without converters should work well with any amp with sufficient input level range. Is BMW using a bridge configuration, or is it single-ended? I would imagine single ended is the way to go with this chip since it's not all that stable. Is it the same for all the channels?
While in there, did you notice if they happen to have the operation software on a plug-in EPROM?

Best regards,
Bob Hazelwood
VP Product Management, a/d/s/
'93 525i 5-sp. Sharked, BL/ss'd, H&R + Bilsteins
BMW CCA (Boston)

: In the name of science :-), I took apart the premium DSP/EQ/AMP in my E39, and learned alot about what it is and isn't.

: At the heart of the system are two Motorola 56004 DSPs. These will each handle 6 separate audio channels, so the pair handle 12. 6 up front: tweet, mid, bass. 4 in back: tweet, rear deck. 2 sub channels.

: The 56004's have two digital inputs. One is from the Alpine head unit, converted to digital through an Analog Devices 16-bit stereo ADC... the AD1877. The second digital input is directly from the CD-changer.

: The 56004's handle the 7-band EQ, variable "loudness", volume, balance, fade, and crossover functions. Unfortunately, the crossover function is hardwired into the program, so you can't adjust this. For example, if you wanted to go to a 2-way setup up front, you're stuck with the x-over adjusted for the factory 3-way setup. 12 separate digital streams are generated, which feed into 6 stereo 18-bit DACs (AD1859).

: These low level signals are then fed into 12 "power" amps. These are actually a joke. The mids and bass are amplified by a Class-B/H chip made by Philips: TDA1560Q. This part is rated 40W into a 8-ohm load at 10% THD. It's half that at 1% THD. Below 100Hz, the power rating drops precipitously.

: I experimented by tapping the output of the subwoofer DAC and feeding this low level signal directly to an external amp. The result was great. Very clean. You avoid the distortion of the TDA1560Q this way. However, you need to be comfortable with soldering small wires onto the PC board. Worth it if you want a really clean result.

: It was a fun and educational project. Disappointing to find that the $1000 option had less than $100 worth of parts. It could have been so much more... sigh.

Questions, comments, or problems, please visit the Roadfly help desk.