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krn01 22nd May 2012 04:30 AM

Integrating automotive OEM Class-D amps with after-market automotive amps

I am lookimg for advice/thoughts from the community here regarding integrating
automotive OEM Class-D amplifiers with after-market automotive audio equipment.
I have not been able to find any similar questions (or answers) on any
automotive forums so I thought I would try here.

The basic question is: What should be considered when connecting an automotive
OEM Class-D amplifier to an after-market amplifier? Specifically, when
the Class-D amplifier's output drives the signal input for the after-market
amplifier? In this case, the speaker load is removed from the Class-D amp and
replaced by the after-market amplifier's input. This is a common scenario for
integrating after-market equipment since it is difficult to find a low-level
signal with the "closed" systems found in many newer cars.

Presumably the after-market amplifier's input impedance is much higher then
a speaker load and I would think that could affect the operation of the
OEM Class-D amplifier driving it. I also wonder if a Class-D amplifier
designer would enginner for this scenario?

My main concerns are:

Damaging the OEM Class-D amplifier in the case where some output channels
have no load at all (a four channel Class-D amp where only two channels
are needed and two are left "open load").

Potential issues with EMI since modern cars have a lot of microprocessors and
controllers scattered about them controlling everything from the throttle
to air bags and stability control systems. Here I'm concerned the wiring
between the Class-D amplifier and the after-market amp could act as an
antenna if the output filtering of the Class-D amplifier is affected.

So what do you think? Is it as simple as placing a 20 watt/8ohm resistor
across the speaker outputs on the Class-D amp and leave it at that? Are there
Class-D designs where that would be a bad idea? Other considerations?


Saturnus 23rd May 2012 05:56 AM

You're shooting flies with nuclear missiles here. All you need is a headphone preamp.

theAnonymous1 23rd May 2012 02:18 PM

High to low level converters have been widely available since before I was born; you can get them on ebay for pennies. As a matter of fact, I have a couple right here you can have.

JZatopa 25th May 2012 05:05 PM

Could you post what you are planing to do, it sounds like you are making things harder for yourself then they need to be. Tell me your car, the gear you want to use and what goal you are trying to achieve.

ric-paul 25th May 2012 09:10 PM

Interesting post.

I agree with you that the input impedance of even a high level input on an aftermarket amp (often e.g. 100 ohm) would be too high. Due to the nature of the class D -signal, the outputcoil will be optimised for a certain impedance, like between 1 and 4 ohm. replacing this with a 100ohm load would mess things up considerably. Or so my gut tells me.

The 8 ohm resistor would work, but I agree that you are taking a route there I would advice against.

Most car specific systems will be cracked eventually, even the OEM systems that drive the amps with digital signals :D so if at all possible, I would prefer finding a low level signal somewhere. Google and carsites are your friend (e.g. , though I don't go there anymore or

And if you are installing an aftermarket amp, you will often do so because of a lack of quality. Using just about all steps of the 'old' chain to feed it into a new amp sounds like a recipe for disappointment.

krn01 27th May 2012 05:04 AM

Thanks everyone,

The car is a 2011 Volvo S60 with a 4 channel Class D amplifier.
The head unit connects to the amp via a MOST bus. See:

MOST Bus - Wikipedia, the free encyclopedia
MOST - Home > Home

The digital signal includes the audio, controls for fader, tone, voice for the phone,
signals from parking sensors, etc. Other devices are on the same bus (SAT
receiver). It won't be easy to crack :) So high level input for anything audio
downstream it is.

The after-market equipment is a JBL MS-8 signal processor. It accepts multiple
inputs, sums them, digitizes the signals and provides surround sound processing,
automatic equalization using supplied mics in a headphone, and some other nifty stuff.
The MS-8 has build in amplification and can also drive other amplifiers via
good ol analog/RCA connectors. There are many reports of the MS-8 doing a
great job of improving the stock system even when driven by high level
signals (and yes, I agree, not the best way to go).

The main thing to improve is the stock system has a terrible problem with the door
panels vibrating. The MS-8 can limit the frequency range sent to the doors and
supply the bass frequency on a different channel. Crossover frequency and
slopes are all user configurable allowing a lot of room for experimentation to
find something that works. I have installed woofers in the rear deck to handle
the bass relieving the doors from handling that. So the
goal is to mostly address that shortcoming and add some surround sound
signal processing.

I certainly don't think it will hurt anything to put some load resistors across the
speaker output from the stock Class-D amp and then tap off that to feed the JBL-MS8 (which
then would be powering the door speakers and suppling a low level signal
to a second mono amplifier to power the woofers in the rear deck).

I did find a reference to a line out converter that actually used loading resistors.
See: Line Output Converters
But all of the ones I could find were high impedance voltage dividers. It was
interesting to see someone had taken that route at some time (although I suspect the
transformers used in that design would have limited frequency response).

Sorry if this wandered off-topic for this site. I meant to explore the issues with no-load and high
impedance loads on Clas-D amplifiers in automobile applications and did not
intend it to be a auto stereo install thread.

Thanks again.

Eva 28th May 2012 04:39 PM

There are essentially two types of class D amplifiers: The ones that take output feedback before output filter and the ones that take it after output filter. Pre-filter fb amps are the most common due to their simplicity, but suffer from variable output filter peaking depending on load impedance, they are intended to drive a certain HF impedance. Post-filter fb amps don't care about load impedance.

So a dummy load is required to get the 10Khz to 20Khz range right almost in every piece of class D consumer equipment, but since no loading is required at low frequencies, the load resistor can be wired in series with a capacitor to reduce wasted power, approx. 10uF for 4 ohm or approx 4.7uF for 8 ohm (low voltage film caps or bipolar electrolytic caps for speaker crossover). In fact, excess top octave treble can be cut with a lower resistor value (or boosted with a higher value).

Some low-pass filtering is desirable too, because the output of a class D amplifier contains components above the audio band, highest content happens typically from 200khz to 400khz. A transformer based high to low converter can do this filtering, roll-off in the top octave can be compensated by changing dummy load resistor value, and roll-off in bottom octave depends only on source resistance, which is output impedance of the amplifier and should be quite low (and distortion in transformers only happens with high levels of LF signal, provided that the primary is AC coupled).

ric-paul 28th May 2012 10:59 PM

It gets even more interesting, and more beyond my field of expertise. (My Volvo V70 luckily has an aftermarket HU :D)

So I'll just relax and enjoy the show from here on out...

If you were living nearby I could throw you some (what I think might be) intercom signal transformers for experimenting (frequency range=unknown though), but no use as buying them local might be cheaper than the postage.

JZatopa 29th May 2012 11:33 PM

Your MS-8 has a dummy load internally, you don't need to be adding your own dummy load. If you are having problems it is most likely not setup correctly. A lot of your questions would be better answered at

Personally it sounds like you need to dynamat your doors and possibly add some ensolite. This will stop pannel resonation and absorb some road noise. The internal amp of the MS8 is very weak (18wrms a channel), it will work if that is your only option but I would recommend using external amps. This will keep you from clipping and greatly improve the sound in your car.

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