Alpine MRV-F307 Tweaks

[tsmith1315]

Perry:

They probably use the 20 ohm because it's virtually 100% guaranteed to prevent problems.

Along a similar line of thought, it's closer to the load the HU's internal amplifier is designed to drive. I would expect better all around performance (SQ), even if marginally so.

Ryan:
My experience with the older Alpine speaker inputs was that they worked every time and sounded surprisingly good. The only others that consistently worked as well for me were on the ADS 642 crossover.
You could certainly DIY a better quality component, but it's questionable whether you'd hear the difference when the source is an IC amp in the HU.

As Perry suggested, tapping off the HU would yield a better quality signal. I've done it a few times in the 500mV days with good results.

What kind of factory unit is it? Perhaps someone here has had experience.

As far as cat5 goes, Adire did a little research into that and found medium quality coax delivered something like 10dB better performance against inductive coupling and 30dB better against radiated (engine) noise. I've got the .pdf somewhere if you're interested.

[RyanW]

Thanks for the feedback guys; I am interested in any documentation that will help me learn what is going on. I currently have the 4 channel amp under the driver seat with the HU connected to a star ground at the same point. This is where I would connect the transformers/LOC. My car is a 2005 Mazda 3... Any feedback on the Headunit is much appreciated. I like having steering wheel controls and factory appearance to prevent thievery.

I am fairly cetain that the problem I am having with my current setup can be attributed to the fact that I am using the low quality speaker level inputs onboard the amp. When I pull thngs apart, I will either upgrade or eliminate this portion of the amp.

If I were to tap off the HU, would you recommend a small opamp circuit? Perhaps a pair of OPA 2132's would suffice?

[Perry Babin]

Visit a couple of the various mazda forums. Someone should be able to tell you what you need to know about the head unit's sound quality.

The speaker level inputs on some amps are very low quality. Generally, the problem is engine noise. In the past, I've found that the LOCs generally worked better.

If you go into the head unit and cannot find a point in the circuit that has sufficient signal, you would need to use an op-amp. You don't need to use expensive op-amps. A TL072 will do the job. If you mount the op-amp in a socket, you can experiment with various ICs.

[I Am An Idiot]

You have to be old to know why the 20 ohm resistor is better than the 100 ohm in a Line Out Converter. If you had a radio with a 2 channel output chip and a resistor/heatsink fader, and you connected 2 LOCs with a 100 ohm resistor the fader would not function properly. You would always have output from both converters. With the 20 ohm resistor when faded to front the rear would have no output.

[RyanW]

Thanks for the info Idiot.

In the new age of digital controls and having a true 4-channel headunit, this should not be an issue. Would there be any other reason that I should not use a larger value resistor here? It seems pretty reasonable to say that, within reason, a higher load impedance will be better.

Can anyone recommend a good signal level audio transformer? I have checked out digikey, but there doesn't seem to be much available. Many state the freq. response as 50Hz to 10khz... not very good. I'm thinking a small transformer with a ~5:1 ratio would do the trick. Are there any North American distributors that might have a better selection?

[Perry Babin]

Jensen may have something:
http://www.jensen-transformers.com/

Before you spend too much money on transformers, you should determine if the sound quality of your current head unit is acceptable. At least do a frequency sweep to see if the output has a flat response or has equalization built in (to compensate for OEM speakers with poor response).

[RyanW]

Well, one thing is certainly clear... higher quality transformers are very expensive. At $100 - $200 per channel, it is definately not economical. The cheap $10 transformers have very poor frequency response. Looks like the most logical thing to do is to tap out the unamplified signal... as you guys have suggested. I'll have to hit those Mazda forums before re-inventing the wheel.

[RyanW]

I've spent a bit of time on the Mazda3 forum and learned a few things.

1. I obtained a datasheet for the IC amp, so I now know which pins carry the pre-amplified signal.

2. I found a person who had tapped the pre-amplified signal from his Mazda 6 headunit. He forwarded me pdf instructions c/w pictures.

For the small effort required, I think that this is the best way to achieve good SQ with a stock headunit. Why screw around with overpriced LOC's when you can, for ~$4.00, bypass the dirty cheap IC amplifier chip and send real signal level to a good amp.

[Perry Babin]

Did he measure the output voltage at the point where he tapped into the circuit?

Did he tap off of the inputs of the amplifier IC?

[RyanW]

Perry,

Yeah, essentially it looks like he tapped off of the traces leading up to the IC amp chip. He cut a couple traces and replaced the coupling caps with 10uF bipolars. He did not make any mention of the signal level... but I presume that it worked for him without any signal amplifier. I have e-mailed to ask if he knows what the pre-amplified signal level is. If it is low, then I will use a couple of OPA2134's to boost it a bit.

The only part I will need to research is creating the artificial earth for the op-amps. Rod Elliot has posted schematics and a pretty good article on how to do this.

It would be nice to find a schematic of the headunit... I wonder if the signal could go higher than what is required to push the power-amp into clipping. (ie. If the power-amp is disconnected maybe just cranking the volume control higher would push the pre-amp within it's safe operating limit) Not sure if I am conveying my point correctly; but if the pre-amp is on +-6v rails, then although as it is currently set up it never needs to exceed perhaps 500mV, this doesn't mean that it is not capable of delivering a 2V signal. I mean, they most likely used a 4-channel smd op-amp for the pre-amp. Any op-amp I know of can deliver 2V without issue. Am I out to lunch?

Ryan