E30 Audio Overhaul - My Project Details

[bmwman91]

Might as well post it up

Life is pretty busy right now so it will be a while before I can make detailed drawings of the 12 main parts & another few for the assembly. I'll try to get something up.

I have really been lagging on the car build too. I am in a competitive outrigger canoe club and we are in full race season, so 3-4 days of the week are spent practicing, and race weekends are basically useless for anything else. I really really want to get this done though. My next plan for some home speakers has been picked and I am excited.

(the one on the left...I discussed the modded ones to the right with the designer & they would either do nothing or screw up the response)


It'll use a Fostex 8" full-range driver and a Fostex tweeter crossed over with a capacitor at ~10kHz. This time around I am going to build my own amp. The sensitivity is ~95dB at 1W, so I really don't need too much power. Bass extension is flat to 50Hz or so. These bastards are 62.5" tall haha! The plans come from www.frugal-horn.com.

[5Toes]

^ I was like "Damn he's really into audio if he is designing cabinets like these!"

Haha but you just used some other plans. Damn 95db at 1 watt... that is super efficient

[bmwman91]

^ I was like "Damn he's really into audio if he is designing cabinets like these!"

Haha but you just used some other plans. Damn 95db at 1 watt... that is super efficient

There are some MathCAD worksheets out there that people use to design stuff like this. It takes a lot of experience & intuition, as well as trial & error, to develop stuff like that. I am far from that level, but luckily some who are there are also willing to share for free.

And yes, the efficiency is nice. Horn-loaded enclosures can easily obtain 100dB+ from 1W, with other response trade-offs. The main trade-off is size & complexity though (these get worse by a factor of 10 once you marry from what I can tell lol).

[Danny]

I don't know what's going on here but I like it.

[5Toes]

Yeah because horn enclosures are natural amplifiers.

[reelop19]

As soon as you mentioned canoes I got a whole lot less interested:)

[rooster]

I don't know what's going on here but I like it.

im with danny... nice looking boxes

[bmwman91]

Update time:

I think I am done with the amp mods for now. There is one nagging issue left, but that will probably take more time than it is worth. I'll explain at the end of the post.

To start, I wanted to bypass the CH5 crossover so that the nice Audio Control unit I bought could handle that. Crossing over at 220Hz is maybe a little uncommon, but I want to give it a shot since simulations look promising with this crossover frequency. I also like to try to distribute the acoustic energy in decades to reduce distortion in the drivers from nonlinear excursion.

I traced out the input stage with a DMM. Painstaking work...4 layer boards are a pain.


After some jumping & rearranging a resistor...


While I was in there, I replaced the opamps with some true audio-grade ones. The stock ones are MC4558's. Their replacements are OPA2134's which are well regarded for their performance characteristics while still being of a reasonable price.


I replaced 7 total. All you need is some desoldering braid, tweezers & a soldering iron with a needle tip.



The other big project was getting the gains matched & locked in for all of the channels. I don't trust cheap potentiometers because a)vibration makes them shift value, and b)double pots are notorious for having poorly matched resistances (gains varied 8% between CH3 & CH4 with the stock pot). So, I got out my trusty scope & signal generator & did this for all channels. After giving myself a little margin to avoid clipping (outputs generally don't get ALL the way to the rails) I removed the pots & installed some resistors. Yeah, it is sort of ugly, but it is how you get exact odd-ball values.

The gains were set to provide maximum unclipped output with the maximum input signal (13.5V peak) received from the 6XS.



Crazy resistor mishmash goodness!


I was glad to be done with all 5 channels. What a pain!


Just one more close-up of some mods. I did not replace ALL of the opamps since the ones related to the crossovers aren't getting used at all. Maybe if I go & sell this someday I will replace them all if the buyer doesn't want to use an external XO. Spot the "better" ones!


So I had mentioned one annoyance at the start of the post. Well, channels 1, 3 & 4 are perfectly matched. For a given input, they are within 10mV of each other. For some reason though, CH2 behaves differently. The gain in the power stage must be a bit higher than the others, and maybe its biasing is tweaked (probably due to tolerances stacking in cheap components). Its output can be up to 2-3V higher than the other channels at high outputs for the same input, and it clips 3-4V higher (it goes ALL the way to the rails while the others don't). I spent some time with a DMM trying to map out the stage, but it is too much work since I cannot remove that board from the case to see the bottom traces (I could, but I would mess up the transistors' heat sinking). It is probably just a bias resistor that it out of whack. Still, the difference is really only prevalent with really high outputs and I don't plan to operate there much anyway. It might be time to leave well-enough alone!

[harv]

I feel like I'm in Physics class again. Very interested, but lost in trying to wrap my head around what I'm reading! This thread ROCKS!

John

[Danny]

What the fuck is going on!!!!!!!!!!!!!!!!!

[dirtysix]

My head hurts.
But I'm lovin' this thread.

[bmwman91]

I feel like I'm in Physics class again. Very interested, but lost in trying to wrap my head around what I'm reading! This thread ROCKS!

John

What the fuck is going on!!!!!!!!!!!!!!!!!

My head hurts.
But I'm lovin' this thread.

Haha don't worry too much about it. This stuff is all totally excessive & there's no way I will hear a difference when driving on the highway. It's more "practice" for when I do my next home audio build...building some of those monster double horns AND my own amps. One of the virtues of DIY audio is being able to totally overbuild everything beyond reason. It's good experience...audio touches on all sorts of mechanical & electrical engineering principles. It's also a ton of fun to build & mod things (which I think is very much in the spirit of the E30 lifestyle)!

[M42Power]

I love this thread. I always felt that the good stereo was ment to be left at home to play vinyls and the e30 was hopeless. But now there is some real sound system going in! I am gonna need to overhaul mine.

[BeirBrennerE30]

yeah this is crazy. I really need to brush up on my audio/amplifier design. This is just straight awesome.

[reelop19]

If you need an amp to work on I have a 4 channel of which only 1 channel works. Let me know if you'll fix it for me