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Discussion Starter #1
Hopefully this isn't retarded:

I want independent volume control for the Master (total system), rear fill, subwoofer, and front stage and sub. I bought my own pots, cable, RCA connectors, PCB's and everything else I need to make a bunch of RCA level controllers (3 total for current iteration). The Audio Dynamics DSP has its own included level controller/wired remote that will be nice for the front stage and sub, my knobs will independently control the Master system volume, rear fill and sub.

My problem is, and the reason I have come to the below design, is that the Kenwood DMX906S head unit that I want does not have a volume knob or a wired remote with a knob option for head unit (Master system) volume control. I must have a physical knob, I will not use UP/DOWN buttons to modulate Master system volume, nor will I strain my arm reaching to the head unit on the dash to get to them, hence the first pot you see in the signal tree below. I can put my pots wherever I want, and will be building a couple to have 3ft leads so I can build them into carbon fiber tubing and toss them around if I want, or mount them on clips somewhere convenient and easy to reach when I'm somewhere else in the vehicle besides the captain's chair.

I will use front RCA outs on the HU into the first pot, where I will then split the signal to the downstream pots.

Is this utterly ridiculous, or am I a genius with the ultimate power and control over my system?




System volume control.png
 

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Maybe just get a different HU that has a knob on it?
 

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Discussion Starter #8
It's around 8p in AU...You finishing off a Foster's 6er or what? What about my system question??? FOCUS!
 

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It's around 8p in AU...You finishing off a Foster's 6er or what? What about my system question??? FOCUS!
I may not be in Au at the moment?


Well it might work with pots, but it would probably be best with OP amps to make the output impedance low. Without that then the output impedance will go very high as the volume goes low.
There could also be cross talk and ground loop problems, if one is not careful.

It seems like it should be tested on a bench to see how well it works?
 

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Discussion Starter #11
Well it might work with pots, but it would probably be best with OP amps to make the output impedance low. Without that then the output impedance will go very high as the volume goes low.
There could also be cross talk and ground loop problems, if one is not careful.

It seems like it should be tested on a bench to see how well it works?
The signal is only being split once to one amp. I don't know what an "OP amp" is. Is that an OZ thing?

You're referring to combined resistance to the last amp considering upstream pots were at any value below max, correct?
 

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The signal is only being split once to one amp. I don't know what an "OP amp" is. Is that an OZ thing?

You're referring to combined resistance to the last amp considering upstream pots were at any value below max, correct?
Google "OP amps".

If the output impedance is 600 ohms and you go through a pot and reduce the output by 10dB, then the impedance will likely head towards 10k ohms...
The input impedance of amps is normally ~ 10 to 100 k-ohms... so effectively you are creating a filter... or making it likely that you could create a filter.

This is why passive preamps are rare.
 

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Discussion Starter #13
Google "OP amps".
If the output impedance is 600 ohms and you go through a pot and reduce the output by 10dB, then the impedance will likely head towards 10k ohms...
The I put impedance of amps is normally ~ 10 to 100 kohms... so effectively you are creating a filter... or making it likely that you could create a filter.

This is why passive preamps are rare.

I just wanna turn the volume down, why you gotta make this so difficult?
 

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Discussion Starter #15
I'm glad you brought it up. I hadn't considered it. Will this filter effect create an issue or is it just something to consider?

So what's the concern for me, then? If I have the HU and DSP normally at max, and control entire system volume with the Master System pot, then the only amp downstream that would be doubled up on pots would be the sub amp. Wouldn't this essentially be the same as signal attenuated by a head unit into a bass knob? Isn't a HU volume adjuster also a pot?
 

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Discussion Starter #16
Addendum to the above: I'm not worried about the 4CH fill amp being doubled up. It will most likely be at max most of the time as well. I figured I'd use it to tone down the rear when I only wanted the clarity of my front stage say, if I had good, lossless media or something. Most of the signal running through this system will not be audiophile quality, but I'll have the gear to enjoy it when it is.
 

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I'm glad you brought it up. I hadn't considered it. Will this filter effect create an issue or is it just something to consider?

So what's the concern for me, then? If I have the HU and DSP normally at max, and control entire system volume with the Master System pot, then the only amp downstream that would be doubled up on pots would be the sub amp. Wouldn't this essentially be the same as signal attenuated by a head unit into a bass knob? Isn't a HU volume adjuster also a pot?
The HU knob is likely a pot attached to an OP amp...
The gain knobs on amps are also likely attached to OP amps...

If it is only for the sub, then you do not need multiple pots... you may only need one pot?

And any low pass filtering on the sub channel is probably not going to be an issue.


However if your DSP allows multiple tunes... then I would suggest using that approach would be the easiest, and the approach which is the least susceptible to dramas.
 

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Addendum to the above: I'm not worried about the 4CH fill amp being doubled up. It will most likely be at max most of the time as well. I figured I'd use it to tone down the rear when I only wanted the clarity of my front stage say, if I had good, lossless media or something. Most of the signal running through this system will not be audiophile quality, but I'll have the gear to enjoy it when it is.
Passive preamps work great when run mostly wide open.
I would still be using good cables, and pots which have low cross talk specs.

Some high end home preamps have individual left/right channel volume knobs... just to keep those parts away from each other to minimise cross talk.
 

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Discussion Starter #19
Got busy for a couple days. Can't wait til this holiday crap is over. Thanks, Holmz, for sharing your knowledge! Unfortunately, most of what you're talking about is a tad over my head. Consequently, often times the price I pay for not being very smart is learning things the hard way. So unless you foresee major flaw or impending doom with my schematic, I'm going to go ahead with it.

I guess I should ask more simply; do you see the increased resistance of the signal at the amps after passing through doubled-up pots as being a serious issue? How detrimental are we talking?

I am using 50k audio taper pots. I picked up 5 of them from mouser. Quick specs on this page:

 

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Discussion Starter #20
And how would I go about utilizing Op amps in the circuit if you think they will be necessary?
 
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