Hilton

The starting point - about the crossover

Below is a quick diagram of the front crossovers.

The markings are:

R1: RYH4 - 2R2 5% 4W
C1: WEGO ELKO Glatt 23VAC 3.3uF W10 TFZ/TM bi-polar
L1: BLM 9030
L2: BLM 2050

For reference, the measurements of the components are:

High pass:
R1: 2.237ohm
C1: 3.871uF (17% higher than spec)
L2: 202uH

Low pass:
L1: 0.934mH

Hilton

At this stage, without more planning, I'm thinking about doing one or more of the following:

1. Replacing old 3.3uF electrolytic cap with newer polypropylene cap of same rating
2. Swapping the inductors between the high and low pass filters
3. Possibly switching to better modern inductors with lower resistance
4. Maybe replacing the stock crossover completely with something that has a more robust design (e.g. two 12db order filters)
Item 1 above seems to be pretty much a no-brainer based on quick forum reading, and at $2 per cap, a cheap option. The rest of the ideas need further research and more importantly, measurement of the stock driver performance, hence the question in post 1.

I'd like to figure out how to make my own meaningful measurements e.g. using a laptop soundcard and mic. so that will come in along the way. Any advice on how to setup relatively easy/cheap recording equipment for measuring frequency/impedance output of audio stuff would be appreciated too!

Arnoud

Hi Hilton,

Have a read on the following interesting related tip, especially the information towards the end regarding the rears and it's factory installed - and wrongly wired up - cross-overs: http://www.nichols.nu/tip512.htm
It turned out that this was also wired up in this default wrong way on my own car, which is a MY 1994 (I therefore wonder if it has been wired up wrong in all cars). I modified it as per that tip by wiring it up correctly, and then taking out the 6.6 ohm resistor and putting that in parallel with the 3.3 ohm one (making it therefore 2.2 ohm) and putting a wire instead of the 6 ohm resistor. This all helped sound improvement wise for the rear.




During 2012 I did changed to Hans superb sounding front speakers & passive cross-overs solution and disconnected the rear's, as per my post #285 here: https://rennlist.com/forums/928-foru...uction-19.html I'm very happy with that setup ever since.

By the way: have a look at GoldWave software http://www.goldwave.com/ , now on V5.70 and free to download and try. This has been around for 20+ years, and not bad at all for doing basic digital audio recordings + analysis & manipulation...No affiliation, as always, just passing on what works for me.

Hilton

Thanks Arnoud - interesting, I hadn't seen that tip, but have come across another forum poster who found the same, and who also designed implemented a 3-way crossover to power the rear woofers/mids/tweeters off the same channel. I'll be checking the harness when reassembly time comes to make sure the tweeter and mid are the right way round!

https://rennlist.com/forums/7574000-post9.html

I'll look at goldwave - I haven't scratched the surface yet of what's needed to sort out frequency response and impedance curve measurement, so any recommendations which reduce the internet forum (diyaudio.com mostly so far) and blog reading is welcome

Hilton

Just checked the source of that nichols tip - it was chris ford, whose site is still up and running including the pics, schematics etc.

http://928s4.com/tips/StereoMods.htm

I drew my own version of the rear crossover circuit, so will check it against Chris'

edit: just replaced the pic below - I had the wrong value for a resistor. Now matches what my rears are in my 89. Also interesting - the board for the rear crossovers in my car has hand-drawn traces covered in solder, and a solder-embossed part number. My 89 left the factory in Oct '88 IIRC - I have a letter from Porsche somewhere, so presumably they were still waiting on the circuit boards like Arnoud's pictured above.

edit2: just noticed that the piece screwed on top of the crossover to protect the traces is actually one of the same boards used to make the crossover, with the exact same drillings for pins etc., just without the hand-made traces. Pic added

siscogts

This insieme real Treasure for me, i haven't never seen those posts And links before, And It is just what i was searching for:updating stock system retaining original parts as much as possible... Thank You,i am starting To update sound system tomorrow.
Francesco from North Italy

Arnoud

Many thanks to Chris Ford (I wonder if he still has a 928 ?) for finding this out and writing about it as he did, and nice to see this kind of pre-production PCB version and cover of the rear crossover in your car Hilton - thanks for posting those pictures.

For completeness please find hereby the cover protection plate on mine:

Hilton

I just ordered a bunch of capacitors for the first pass.. swapping the stock ones for polypropylene ones.

I ordered 6 of these - http://www.parts-express.com/dayton-...citor--027-420 - I bought extras so I can measure the capacitance and match the left and right sides to each other (I have an old but good Tenma LCR meter)

They won't be here for a couple of weeks.. so I'll find other stuff to do

siscogts

one question, do you think this mods to rear crossover will work also with the 2 aftermarket woofer I bought(4Ohms instead 2 ohms)?or is it unrelated?
Francesco

Arnoud

Unrelated, as the factory rear crossover's are only connected up to the rear mid-range and rear tweeters (as per the above information and the car electrical schematics).

More importantly for your question: do you have your 2 aftermarket 4 ohm's woofers connected up to the car factory amp (which you reconditioned with new capacitors etc., IIRC), or to your head-unit and/or other external amplifier?

danglerb

As I understand it the factory crossover has an error that should be corrected. Unlikely the same error would be in some aftermarket crossover.

Hilton

Francesco, the rear crossover only feeds the rear tweeters and 4" mid-range speakers. The large rear woofers are on their own channels from the factory amplifier, which has its own low-pass filter applied in the amplifier (I'm guessing around 150Hz?).
The link to another thread I placed in post #5 above is to a post by another user who added another filter in parallel with the rear crossover, so that all three sizes of rear speaker could run off a single channel.

My own intention is to run the woofers on their own amp channel, like the factory setup, with a 150Hz low-pass filter applied. This is because they're 2ohm (my amp can drive 2ohm loads), and I want to adjust the gain on the woofers separately to match the other speakers.

As an aside - I discovered today that the antenna cable from the roof antenna to the head unit had no continuity on the signal pin. It was a break in the head unit connector - but I was surprised to find out in the process that the signal wire is hair-thin, so I'm contemplating replacing the whole 4m of antenna cable with RG62 before I put the headliner back in. (RG62 is low-capacitance, 93 ohm cable - better for AM radio reception than more common RG58/59)

siscogts

Thank you for the informations.didn't noticed the direct wirings from the amp, and forgot about wirings , despite I refoamed old woofers some months ago...

I cut the "useful" part of the old woofers to install new ones(Phonocar 2748) , they are still wired to the original amplifier, amp IC components can lead 2 ohms speakers and can deliver about 28-30w rms , anyway no problem also with 4 ohms(can deliver 23W), I calculate that 2 4-ohms speakers need to be rated for more than 40-45w at least to stand the power output....

I replaced , 6 months ago, every capacitors and every part inside the amplifier, every electronic part that can age ....with good results about sound quality.


Here you can compare old capacitors dimensions to the new ones(left old, right new ones, hoping that bigger is better):



today afternoon I will investigate and swap green and yellow wires of rear woofer connections.
finger crossed....
Francesco

Francesco

siscogts

Does anyone know correct wiring/pinouts from The Din cpnnector coming from The amp?