Anyone with an aftermarket head unit?

[spinedoc]

Posted this in the audio forum, but no replies. I have a Pioneer aftermarket unit in my 96 SL500 and am quite happy with my stock speakers, no desire to upgrade them at all. My problem is I get a lot of clipping (bass cuts out) on the front speakers at high volume. I dont know if this is an issue with the ohm difference between the head unit and the german speakers, or if they head unit simply overpowers the front speakers. The sound is actually quite excellent, but just not playable beyond a medium audio level. Anyone else have this problem?

My solution will be to just add a couple of capacitors to the inline of the front speakers to cut out some of the bass. My stereo installing days are long behind me, anyone have any suggestions for what size capacitor I need to cut out bass at a reasonable level, I am thinking 80-100mhz.

[LeaUK]

Spinedoc

I would investigate the problem at source if I were you. Fitting capacitors in series is really a no-no as the physical size and capacitance of electrolytic caps makes makes this almost unpractical for low frequency's you mention 80-100Hz.


Does your head unit plug into a Sony amp located in the passenger footwell ie the amp powers the speakers, or is it directly connected to the speakers?

Whats the impedance of your speakers - are they stock? They should b
e 4 ohms and I suspect there should be a crossover network somewhere in the door allowing the separation of audio frequencies to each bass, mid and tweeter driver.

My car has no rear but three in the front - bass, mid & treble - what's your configuration?

Lea

[Roger65]

You can get bass blockers designed just for this at www.crutchfield.com. The blockers the sell are the type that install in series with the speakers. I've bought a lot of gear from them and their service is top notch, although their prices can be high. I think the bass blockers you need are around $15 a pair, not too bad in this case.

[spinedoc]

Thanx guys, awesome as usual. I was under the impression that the MB system was a different resistance, I think 2 ohm, than traditional aftermarket systems which are 4 ohm.

The head unit is wired directly to the in dash harness via a harness adapter, so I assume it is outputting to the MB amp, then to the speakers. That is why its hard to lower the bass at the source, MB splits left and right only to front r/l and rear r/l, so if I cut out bass at the source it cuts out both front and rear, including the subwoofer. So somehow I have to cut the bass out at the speaker level itself.

[LeaUK]

It could be 2ohm but that's unusual.

After reading your post again I can see what's happening. Whatever the impeadance is (and maybe it is 2R) your amp is switching off to protect it's output circuitry. It's going into output protect mode and so clips at high output currents. Of course low bass demands the most current from the amp.

Have you got a Digital Volt Meter (DVM) as you could use this to determine the your speaker impedance.

[tecqboy]

Impedance is not resistance and cannot be measured with a VOM. Many non-techs confuse the two and use the terms interchangably. Impedance is frequency specific. Therefore, a 4 ohm impedance is measured at a specific audio frequency, never DC as would be the case of a pure resistance. So the impedance of a woofer speaker should be measured and speck'd at it's natural resonance point, probably about 150 Hz for a small size driver. A "bass blocker" is really a LC high pass filter, with a -3db turn over point and a -6db or greater slope per octave. -3db is the half power point. It's all logrhytmic too.

Early SL's with Becker audio systems did indeed have 2 ohm speakers. I have them in my 1991 SL. I do not know for certain what the impedance is of the later sytems with the Bose speakers, but I suspect 4 ohms, as it is more the "norm" in later production speakers. Of late, I've seen speakers speck'd at 6 ohms, just to add to the confusion.

When I put a new head unit in my 1991 SL, I used the original door 2 ohm speakers, as they are of excellent quality, but I bypassed the power amp under the seat. Then I just changed the value of the crossover networks in the door speakers to match the higher 4 ohm output impedance of the new head unit. I calculated the values of the crossovers to be 120 Hz for the woofer and 500 Hz for the midrange. I left the tweeters alone. The sound quality is very good. I added two 8 inch subs to the rear storage compartments and drive them with the rear outputs of the head unit through passive low pass filters. The head unit is rated at 40 watts distributed, which is more than adaquate for a car this size.

[Djenka]

Puting a bipolar cap (or two polar caps in opposite direction but twice the desired capacitance) in series with front speakers ONLY can give you desirable efect. However, original Merc speakers in most of the cars have insufficient power handling and that is your problem #1. Upgrading speakers (very possible and not so difficult) will give best result on long run.

~2-4uF cap will usualy cut off anuthing less than 1000 cps but for lower crossover lookup for refference on crossovers at google.com


Warm greetings from South Africa and Serbia...
Summer time... and the living is easy...

[LeaUK]

tecboy - technically you are quite correct, but we must remember that most here are not familiar with the subtle differencies between DC and impedance let alone the J axis! However, by using a DVM you can assertain the speaker's nominal DC resitance - it's impedance at 0Hz, which in the majority of drivers is close to it's impedance at resonance and is often used as a 'rule of thumb' technique.

It's unusual to find drivers with such low impedancies before around '95 and so I'm surprised at the SLs configuration, but if they are 2R and your new head unit's drivng them direct, that's probably the reason - head units are typically only 4R stable. However most modern amps can drive as low as 1R ;-)

Passive crossovers are rearly the best option - if you can use active you will achive a significantly greater roll off (especially if you use digital designs) and remain at maximum efficiency. This means simply - subs sound FAR better using high roll-offs, and in my opinion the filter should be a minimum of 12dB @ 80Hz for 10" and above

Spinedoc - change the amp for a 2R stable version