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Discussion Starter #1
I had a simple plan: Put a DSP in between my factory head unit and my factory amp.

I’m using the DSP purely as a stereo EQ to cut the upper mids a touch and boost the highs a touch. Nothing drastic because I already think the system sounds pretty good and this is all I wanted to do.

Inevitably someone is going to comment that the factory amp's signal processing will defeat the purpose of the DSP by undoing anything it does. If you believe this, you fundamentally misunderstand how serial audio processing works. But I'm not really here to discuss that.

I ended up choosing the C-DSP 6x8. I really didn't want this installation to change my overall output level, but it did. Prior to this installation, I never turned up my volume higher than 10. After the installation, I'm finding myself needing to turn it up past 10 all the time. This matters because my car, like many others, will equalize the signal differently the the higher the volume value is, most noticeably making the bass sound weak. The gain reduction from installing the DSP is enough for me to start hitting that "bass reduction" threshold when I never did before and that totally blows.

FYI, I can't just turn up the output level with the DSP's software because the DSP is receiving a fixed gain signal from the head-unit that will clip the DSP if turned up any more. (The head-unit's volume control does not adjust the level of this signal, it just tells the amp where to set ITS volume.)

To be honest, prior to this installation, I didn’t realize line level outputs could be different from one another. I thought they had a standardized maximum voltage. Whoops, I was wrong. Had I known about this, I probably wouldn’t have bought this product, whose output voltage is advertised as 2Vrms. Some miniDSP products have jumpers inside of them that can be adjusted to allow for a higher output level but according to customer service, this is not one of them. Unfortunately, this DSP is unreturnable and cost me $275. The C-DSP 8x12, which has a higher output voltage than the 6x8 is $500. (And I’d only be using it as a stereo EQ if I got it, which feels kind of ridiculous given the price. I’d only even consider getting it if I could sell the DSP I currently have.)

At this point, I’m kind of just hoping for an easy, relatively cheap solution to make the signal a few decibels louder before hitting the factory amp. But I’m not super hopeful. I tried a quick search for a cheap EQ/DSP with a higher output voltage than 2Vrms but couldn’t find anything.
 

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I guess I don't fully understand the problem. With the input and output levels on the miniDSP to 0dB the only changes in output would be from EQ cuts, and if you make enough cuts to reduce the overall volume, then you should be able to safely bump up the output level a couple of dB without a problem.

Volume dependent processing, as you know, tappers off as you increase the volume. This is primarily to improve low end response at low volume levels, due to the equal loudness contours. These features work really well most of the time, but they are designed around the user taking advantage of the full volume range. For example: If you head unit goes up to 50, the volume dependent loudness features will be functioning at low levels, but will turn off as you increase the volume. You don't want that processing to still be taking place when you hit good listening levels, if your gains are set so that you only use a small bit of your head unit's volume knob, your gains are too high, and you're never reaching a point at which the processing turns off.

You should have the gains on the amp low enough that you still use the majority of your head unit's volume range when you are listening enthusiastically. Otherwise, if your gains are too high, you never reach a point on the head unit volume for the EQ to be defeated. You only want volume dependent EQ at low volumes to account for the equal loudness contours.
 

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So is the issue that the input signal is too low? Why not boost the output a bit?
If not, how about a line driver after the DSP?
 

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Discussion Starter #4
I guess I don't fully understand the problem. With the input and output levels on the miniDSP to 0dB the only changes in output would be from EQ cuts
MiniDSP's websites lists the specs of the DSP as such:

Input/Output Max Levels:
IN: 8 Vrms/4 Vrms (4 Vrms for me because I'm using the low level inputs)
OUT: 2Vrms

I did a little test:

When playing a 0dbFS sine wave through the system, the input meter of the DSP reads -4.5 dB. Since the input maxes out at 4 Vrms, this means the signal going into the DSP is about 2.38 Vrms, according to this calculator, and this is the same signal that would be going directly into the factory amp had I not installed the DSP. The DSP's output only maxes out at 2 Vrms so the DSP cannot make a signal as loud as the original signal that came from the head-unit. It comes about 1.5 dB short. This doesn't sound like much, (to me it kind of is), but the situation is further exacerbated as I'll explain.

if you make enough cuts to reduce the overall volume, then you should be able to safely bump up the output level a couple of dB without a problem.
It doesn't actually work out this way for a few reasons.

The first being that cutting only reduces the level of the frequencies you cut. If frequencies you don't cut are nearly making the audio clip, bumping up the output level will certainly make the audio clip.

The second reason is that cutting without even bumping up the volume can still make the audio clip. It's incredibly counter-intuitive but if you don't believe me, this can easily be verified in Audacity or a DAW of your choice. It will be more obvious with an uncompressed or lossless audio file of a song that's mastered loudly (with most peaks coming very close to clipping, which is how 99% of modern music is mastered). While making cuts with an EQ reduces the RMS level of the signal, not all peaks are reduced, and some actually end up higher than before, so when you're dealing with a song that's mastered the modern way, with every peak coming very close to clipping, many of those peaks are going to clip, even though you're making the audio audibly quieter.

The third reason is that I'm not just cutting. I'm also boosting.

All this being said, the EQ changes I've made, even though it's mostly cuts, require another 1.4 dB of gain reduction to avoid any clipping I've deemed severe enough to matter. In total that puts me about 3 dB below where I was originally. And then the EQ itself makes the audio sound quieter since it's mostly cuts, so in terms of human perception, it's probably closer to 3.5 or 4 dB quieter than it was originally.

If I remove a subtle upper-bass boost I did, I can squeeze another decibel out of it, putting me 2 dB below where I was originally (again, quieter if you factor in the EQ itself).

Volume dependent processing, as you know, tappers off as you increase the volume. This is primarily to improve low end response at low volume levels, due to the equal loudness contours. These features work really well most of the time, but they are designed around the user taking advantage of the full volume range. For example: If you head unit goes up to 50, the volume dependent loudness features will be functioning at low levels, but will turn off as you increase the volume. You don't want that processing to still be taking place when you hit good listening levels, if your gains are set so that you only use a small bit of your head unit's volume knob, your gains are too high, and you're never reaching a point at which the processing turns off.

You should have the gains on the amp low enough that you still use the majority of your head unit's volume range when you are listening enthusiastically. Otherwise, if your gains are too high, you never reach a point on the head unit volume for the EQ to be defeated. You only want volume dependent EQ at low volumes to account for the equal loudness contours.
I subjectively like how this system sounds more when the volume value is lower than how it sounds when the volume is higher. I understand the theory, but I see no reason to intentionally make myself need to reach higher volume levels if I don't like how the system sounds when the volume is that high.
 

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Another idea, leave the settings as is for your factory stereo pass through and add at USB DAC with a digital output for music from your phone. This option avoids the EQ cuts as the signal will be flat coming from the DAC. Use the preset selection on the control to select the input option.
 

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So a minidsp doesn't adjust for volume adjusted bass roll off of the factory system. Would a different brand be able to do this like JL, Audison or Helix?
 

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Discussion Starter #8 (Edited)
Another idea, leave the settings as is for your factory stereo pass through and add at USB DAC with a digital output for music from your phone. This option avoids the EQ cuts as the signal will be flat coming from the DAC. Use the preset selection on the control to select the input option.
Sorry, I don't think I understand what you mean by this.

So a minidsp doesn't adjust for volume adjusted bass roll off of the factory system. Would a different brand be able to do this like JL, Audison or Helix?
You can make the miniDSP adjust for this roll off. I'm only using the DSP as a simple stereo EQ. If it isn't clear, I intentionally don't want to flatten the signal going to the speakers, which is what a lot of people use DSPs for. I want to, for the most part, preserve how the stock system sounds.

It's true that I could make bass adjustments using the miniDSP purely to, for example, make the system's "volume 12" sound like how "volume 8" used to sound. I'd have to make these adjustments purely by ear since in my setup the miniDSP is receiving a flat signal. The amp controls all the factory EQ. I think trying to do this will get a little messy and it's kind of like hitting a moving target since the DSP's EQ dictates how loud the DSP can be turned up which in turn dictates how I need to EQ it, and so on.

Then there's the option of leaving the HU at a fixed volume and using the miniDSP's remote control as volume control. Then I can tune the DSP without aiming for a moving target. The thing is, I want to retain the functionality of the HU volume and steering wheel volume.

I have to take a step back and remember that my original goal was to not change anything that I didn't want to change about the system. The most direct way of achieving that seems to be to just get back that lost voltage.

But I'll continue to think about my options.

EDIT: Just ordered an Install Bay IBR67 Line Driver. Cheap at $20, but no one seems to be complaining about it? Wish me luck! Thanks to everyone who responded.
 

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You can always use something like REW to take measurements of what the factory head unit is doing -- just take the line out of the head unit and use that instead of a microphone. Might make it easier to see what's going on.
 

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Sorry, I don't think I understand what you mean by this.
I was suggesting that your low output from the dsp is due to the corrections you're making. By using the digital input you would have a flat signal that does not require EQ. This might make up for your low output.

That said, the line driver might be the better option
 

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Hmmm. After buying a Dayton 408 DSP, but not having installed it yet, I was concerned that it might not have enough voltage output. It was only about 2.8 volts, with zero clipping. But as it turns out, that seems to be plenty for my amps to work with. My 4 ch only has the gains set to about 50% My mono amp is at about 60%

Not sure why yours isn't enough ? Sounds like an issue somewhere...
 

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Will ... is replacing the factory amp a possibilty? I understand that you were trying to minimize the physical impact while doing this, but an aftermarket amp would play nice with the 2V output of the DSP. Should be able to find something to fit the location and power requirements.
 

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Not sure why yours isn't enough ? Sounds like an issue somewhere...
He's using the factory amp.

You may have found your solution. If not...

Since you said you were using low level these old eq's are all around. Several brands made them. The eclipse ones are 10v line drivers. Other brands are 8v line drivers. Could probably pick them up cheap as they at all used at this point.
 
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