There are line drivers and pre-amp EQs out there with balanced outputs. I have used an old school PPI PAR-225 with a lot of success. They also made a PAR-245 IIRC. These are very high quality units with on board DC-DC converter power supplies and isolated, balanced outputs. I think I paid $75 for mine used. Audio Control makes a line driver and a pre-amp EQ that would probably work well. (You only need one or the other.)
Another way to minimize noise issues is to power the minidsp from its own dedicated power source that uses a dc-dc converter to generate voltage that is isolated from the vehicle's ground. If you don't want to roll your own, there is a fellow over on mp3car that developed a nice unit that supplies 5 VDC for USB hubs that would work well. I bought one but have not installed it yet. My initial purpose was to use it for a USB sound card for my car PC install. It should also work well with minidsp. It was about $80.
Im interested in one of these units for a 2 way front setup and let the HU do the sub crossover. I emailed miniDSP about the noise problem and they suggested I use the balanced unit, but those inputs look alien to me. Ive googled around a little bit but im still baffled with those. Can someone point me in the right direction? I'll have 2 sets of RCA's run (front/sub) if that makes a difference.
Im interested in one of these units for a 2 way front setup and let the HU do the sub crossover. I emailed miniDSP about the noise problem and they suggested I use the balanced unit, but those inputs look alien to me. Ive googled around a little bit but im still baffled with those. Can someone point me in the right direction? I'll have 2 sets of RCA's run (front/sub) if that makes a difference.
You have to make your own custom cables. Probably easiest to cut the ends off your existing rca cables and solder on the connector that should come with miniDSP.
You have to make your own custom cables. Probably easiest to cut the ends off your existing rca cables and solder on the connector that should come with miniDSP.
You can't just plug an rca into a balanced input and call it balanced. An RCA is coaxial because the signal is shielded by the ground wire. A balanced signal is the signal and its opposite, with no ground, and is generally used in a twisted pair. You have to have a balanced source signal. I suspect if you plugged an RCA into a balanced input you would be -3dB and not get any noise benefit, but I don't know if it would be harmful in any way. Some OEMs, like BMW, use balanced outputs from their head units (which I suspect is more to defeat the audio aftermarket than to defeat noise). Zapco uses balanced inputs but they use a converter that's powered by the amp.
I just hooked up my DSPs in my car and I have crazy noise when the car is running, so I suspect it's power supply noise; I'm going to try a few things and see what works.
You can't just plug an rca into a balanced input and call it balanced. An rca is coaxial because the signal is shielded by the ground wire. A balanced signal is the signal and its opposite, with no ground, and is generally used in a twisted pair. You have to have a balanced source signal. I suspect if you plugged an RCA into a balanced input you would be -3dB and not get any noise benefit, but I don't know if it would be harmful in any way. Some OEMs, like BMW, use balanced outputs from their head units (which I suspect is more to defeat the audio aftermarket than to defeat noise). Zapco uses balanced inputs but they use a converter that's powered by the amp.
I just hooked up my DSPs in my car and I have crazy noise when the car is running, so I suspect it's power supply noise; I'm going to try a few things and see what works.
See my post # 226. There are ways to convert your head unit output to balanced. BTW, the proper description of the input circuit would be to call it a differential input. You can still get benefit from this input design even if you run a single ended signal in. It only amplifies the difference between the + and - signal inputs and greatly reduces common mode noise. Most electrical noise in the car is common mode. Most decent quality amplifiers have a differential input that accepts the single ended output from head units without difficulty. The amp manufacturers would not do this if there was not some noise benefit.
I am well acquainted with what twisted pair wiring does. If anyone is reading this that does not understand the reference to twisted pair wiring, look up some of my old posts. I am a strong believer in using twisted pair for preamp level signals but it is not essential. You can get away with coax RCA cables if you use a good preamp with differential outputs (ie: balanced) near your head unit and a signal processor or amp with differential inputs at the other end. Of course, if you use twisted pair and equipment with the differential output and input circuits, you will be even better off.
BMW uses differential (balanced) outputs on its head units because their cars are noisier than most. Putting the battery in the trunk or under the seat results in large currents flowing through the car chassis from battery to the alternator. This creates stronger EM fields in the car than a front mounted battery configuration. The balanced outputs on BMW head units also offer an advantage in terms of signal to noise ratio (SNR). The output voltage is on the order of 5 volts instead of the usual 2 volts from other head units. With a constant noise level, the higher voltage output results in better SNR. BMW also uses unshielded twisted pair wiring from its head units to the rear mounted amplifiers for noise reduction reasons.
This noise issue is the single biggest reason I have not ordered a miniDSP.
How does running the balanced input help if all you have is rca cables to be able to supply signal?
And please....dumb it down for me.
I will do my best to explain but you may have to do a little homework on your own. If there are terms or concepts you don't understand, Google is your friend.
First, you have to understand where the noise comes from. Electrical noise in mobile electronics usually is related to the alternator. This is the noise that goes up and down in frequency as you rev your engine. This gets into the audio preamp signal path in two common ways.
1. The first is through ground loops created when two or more pieces of equipment are not at the same ground potential. For example, your amplifier is grounded to the chassis in your trunk and the head unit is grounded to the chassis near the dashboard. The resistance between the rear grounding point and dashboard grounding point is 1.5 ohms. This means there is voltage difference between the two ground points and current will flow through your rca preamp cable shield between the front and back. This current will have a ripple voltage on it that is tied to the alternator. In a typical coax cable, the voltage will be electromagnetically coupled onto the positive signalwire in your RCA cables as well. If equal amounts of alternator noise are present on both the + and - wires, we call this common mode noise voltage.
2. The second common noise path is from electromagnetic fields that are present in your car. When large currents flow through a wire or the car chassis, there is a magnetic field set up around the current flow. This field will cause a voltage to be developed on signal wires that run through the field. Coaxial cable construction is supposed to reduce or eliminate the noise voltage that gets into the preamp signal because the outer shield is supposed to be tied to the vehicle ground. The problem is that if the coax shield is not adequately grounded, it no longer has the intended shielding effectiveness. If the equipment connected by the coax cable is at different ground potentials, the shield is largely ineffective because current is flowing through it.
Twisted pair wiring helps this situation but only if the signal processor or amplifier input has differential (balanced) circuits. This means the input amplifier will only amplify the difference between the + and - signals. Any noise that is present on both lines, gets subtracted out. This works with a common single ended head unit output but not as well as it does if the head unit has differential (balanced) outputs. RCA cables can have twisted pair wiring construction - they have been commercially available for years. You don't need a +, -, and separate ground connection in your preamp cable for this to work. Pro audio equipment usually has a separate braided ground conductor with the twisted pair inside the braid because that reduces noise better than unshielded cable but - this is not essential for mobile audio. You still get good to very good results with twisted pair cables that have RCA connectors on each end and no shield.
The miniDSP is probably suffering from noise problems for a combination of reasons. First, it probably does not filter out alternator noise on the 12 VDC + line completely. Powering it from a supply that is isolated from the car's electrical system will help with that. This is what all high quality preamps and amplifiers do. They have switching power supplies that generate the necessary voltage without being tied to the vehicles ground. A poor man's approach to this would be to install a filter choke in the 12 VDC + line instead of a separate power supply. I am into car PC's and will use an isolated power supply from that system. Powering the miniDSP from an isolated supply should also eliminate any possibility of ground loop currents flowing through the cable shield if you are using coax cable and a single ended preamp output signal.
The second step is to use the miniDSP with the differential (balanced) input circuit. It would be most effective if you also used a preamp or line driver that converts the head unit output to differential signals but may work well enough with single ended anyway. It will require experimentation. I have not ordered my miniDSP yet and can't tell you exactly what works yet. I can tell you that I have been doing electrical instrumentation for jet engine test cells for much of my career and the principles behind what I am telling you were learned in my day job. These methods have also worked well for me in over 20 years of playing with mobile electronics and doing my own installs. The automobile noise issues with miniDSP can be solved. It is just a matter of how much effort and money it will take. It may be cheap and easy or it may require purchasing a preamp or line driver with differential outputs. I do recommend using twisted pair wiring no matter what method is used.
I will do my best to explain but you may have to do a little homework on your own. If there are terms or concepts you don't understand, Google is your friend.
THANK YOU!! That was very informative. I followed ~80%? of what you were saying and can google the rest.
I am still on the fence about the miniDSP. I remember noise issues with my first system(s) and having to ground the shield of the rca to the head unit with a little piece of wire. This seemed (to an uneducated kid) strange to say the least and I always felt that their had to be a better way. Adding balanced differential pre-amps and other hardware makes the economics that originally made the miniDSP so attractive shrivel up.
I am staying tuned to this post to see how you and others solve noise issues.
Adding balanced differential pre-amps and other hardware makes the economics that originally made the miniDSP so attractive shrivel up.
I am staying tuned to this post to see how you and others solve noise issues.
I agree that adding differential pre-amps and separate switching power supply eats up a lot of the cost advantage. I don't think miniDSP was designed with the mobile audio end user in mind. If you are integrating it into a system where you can control the quality of the power and the signal path such as home audio, it makes more sense. Still, for those of us that already have invested in high quality pre-amps or something like the Zapco Symbilink, the cost of trying miniDSP is lower. If you are a hard core DIY person, you can buy a DC-DC switching power supply on a chip from companies like Mouser or Digi Key and mount it in a box with the minDSP. I may do this.
The used equipment market is helpful. I purchased my PPI pre-amp used for $75 about 4 years ago. It is > 20 years old but good high performance equipment like that is probably better than 90% of the stuff on the market today. There is also a PPI half DIN preamp on the DIYMA group buy that is a screaming deal if it has differential outputs. Another option would be to look for a used Audio Control Matrix line driver or one of their half DIN pre-amps. All have differential outputs.
Even if you don't go for miniDSP, I recommend installing a good pre-amp or line driver for a lot of people. It eliminates a lot of potential noise problems if your amplifier also has differential inputs. Most, if not all of the decent amplifiers on the market today have differential inputs. All of the good Old School amps have them.
I don't get it - an unbalanced input is basically a differential input that compares line to ground instead of line to line. What's the difference between a differential and non-differential unbalanced input?
I don't get it - an unbalanced input is basically a differential input that compares line to ground instead of line to line. What's the difference between a differential and non-differential unbalanced input?
In a true differential input design, both the + and - inputs are isolated from powerground by anywhere from 500K to several Mega ohms. In addition, the op-amps on the input are powered from a bipolar power supply with something like +/- 15 VDC. The ground potential between them is completely isolated from the vehicle 12 VDC ground. In an unbalanced input, the negative (-) signal line is tied to the 12 VDC ground and not isolated. The true differential input has much better Common Mode noise Rejection Ratio (CMRR) than an unbalanced input. If you use twisted pair wiring that pretty much ensures the noise on both signal lines is equal, the true differential input works extremely well.
With a typical single ended, unbalanced head unit output signal, the performance of the differential amp input is not as good but still superior to an unbalanced input typical of less expensive designs. That is why good amp and signal processor manufacturers still use differential inputs.
I don't know if the name Nelson Pass is familiar to you but he is one of the most respected amplifier designers of he past 30 years. He had a hand in the original Soundstream amps and the original top of the line ADCOM amps that are so respected. He once commented in an online post that the fact that the mobile audio industry did not embrace balanced signal designs as a general rule said a lot about the market and the customers. I am paraphrasing but I believe he was saying that the majority of the mobile audio crowd was willing to put up with mediocre designs and mediocre sound as long as the equipment was cheap in price. I think it can be argued that the mobile audio market has been dominated by lower cost designs for most of the last 20 years because the profits were there and most customers did not demand better performance. Getting way off topic now.
I have no noise, zero, in my 95 BMW M3 powering 2 unbalanced units. I got 2 12v to USB adapters from monoprice.com and extra USB cables but didn't need them. The adapters were like a buck each. Gotta love monoprice. It's low quality construction but you .aren't going to be moving them around.
I power the adapters off the remote powerwire and ground them to the amps ground which happens to be the battery post because the battery I'd in the trunk right next to my amp.
I think people give up too quick with the noise issue. It's all about location because if I use the 12v adapter in the actual cig lighter I get mad alt noise but change the power and ground locations and use the USB for power and you are good to go.
Last edited by TAMUmpower; 03-17-2011 at 10:02 PM..
You can't just plug an rca into a balanced input and call it balanced. An rca is coaxial because the signal is shielded by the ground wire. A balanced signal is the signal and its opposite, with no ground, and is generally used in a twisted pair. You have to have a balanced source signal. I suspect if you plugged an RCA into a balanced input you would be -3dB and not get any noise benefit, but I don't know if it would be harmful in any way. Some OEMs, like BMW, use balanced outputs from their head units (which I suspect is more to defeat the audio aftermarket than to defeat noise). Zapco uses balanced inputs but they use a converter that's powered by the amp.
I just hooked up my DSPs in my car and I have crazy noise when the car is running, so I suspect it's power supply noise; I'm going to try a few things and see what works.
That doesn't sound too bad to do if I had to. what kind of connections are those? they look to me like the screw type used on your amplifier?? Im hoping they are a plug type connector so they're not permanent. Most importantly, are the benefits of the balanced kit worth it? I'm pretty new to quality car audio so chasing a noise problem would probably kick my ass.
That doesn't sound too bad to do if I had to. what kind of connections are those? they look to me like the screw type used on your amplifier?? Im hoping they are a plug type connector so they're not permanent. Most importantly, are the benefits of the balanced kit worth it? I'm pretty new to quality car audio so chasing a noise problem would probably kick my ass.
It sounds like mda185 knows more about balanced inputs than I do, I'd let him chime in before you plug one into an rca input. You wouldn't want to put both sides into the rca input because then you'd be shorting one side of the HU output to ground. If anything, you might be able to use one output wire and simply ground the input jack to the chassis.
Back to the DSP noise problem, this morning I put a miniDSP in my signal path and powered it from a 9V battery instead of the car. Completely silent! So now I just have to figure out how to quiet the power supply from the car.
I have no noise, zero, in my 95 BMW M3 powering 2 unbalanced units. I got 2 12v to USB adapters from monoprice.com and extra USB cables but didn't need them. The adapters were like a buck each. Gotta love monoprice. It's low quality construction but you .aren't going to be moving them around.
I power the adapters off the remote powerwire and ground them to the amps ground which happens to be the battery post because the battery I'd in the trunk right next to my amp.
I think people give up too quick with the noise issue. It's all about location because if I use the 12v adapter in the actual cig lighter I get mad alt noise but change the power and ground locations and use the USB for power and you are good to go.
That is a good example of ground loop noise. When the 12v to USB adapter was powered from the cigarette lighter, the ground potential was not the same as the amplifier ground in the trunk. Changing the grounding point solved the problem.
TAMUmpower, this is great info. I am curious to know what head unit and amplifier you are using.
I was under the impression that pretty much any regulated 5v supply would work better than the on-board 12v inputs. I'm using ONE OF THESE CELL PHONE CHARGERS from PE for mine. I just butchered the cigarette lighter end and run 12v/gnd leads directly to the input of the charger so I can hide it out of view. Has anyone else done this? It seems like an easy solution for $3.00
~Greg
"Nobody's really going to be free until nerd persecution ends." -Lewis Skolnick
It sounds like mda185 knows more about balanced inputs than I do, I'd let him chime in before you plug one into an rca input. You wouldn't want to put both sides into the rca input because then you'd be shorting one side of the HU output to ground. If anything, you might be able to use one output wire and simply ground the input jack to the chassis.
I am not sure I understand the question. On most aftermarket head units, the (-) signal for left and right channels is tied to the chassis ground of the head unit. Chassis ground is tied to 12 VDC vehicle ground. This is a single ended output. Every Alpine and Sony Mobile ES unit I have owned was this way. It is very rare to find an aftermarket head unit with pre-amp level differential outputs. (Speaker level outputs are differential.)
On many factory OEM head units, the (-) signal from head unit to amplifier, is floating with respect to ground because they have a differential output that can drive speakers directly or serve as outputs going to an amplifier. This makes it economical for auto manufacturers to offer base audio systems and upgraded ones using the same head unit. If you short the (-) output of one of these head units to ground, you will fry some output chips. When connecting the differential output of this kind of head unit to a single ended device like miniDSP unbalanced, you should use the (+) output for each channel and tied the (-) signal lead of your cable to the head unit chassis ground. In this situation, I would much rather use the miniDSP balanced kit and not connect the (-) signal lead of the cable to ground. It should yield much better noise rejection.
I have no noise, zero, in my 95 BMW M3 powering 2 unbalanced units. I got 2 12v to USB adapters from monoprice.com and extra USB cables but didn't need them. The adapters were like a buck each. Gotta love monoprice. It's low quality construction but you .aren't going to be moving them around.
I power the adapters off the remote powerwire and ground them to the amps ground which happens to be the battery post because the battery I'd in the trunk right next to my amp.
I think people give up too quick with the noise issue. It's all about location because if I use the 12v adapter in the actual cig lighter I get mad alt noise but change the power and ground locations and use the USB for power and you are good to go.
Yea thats what I got. In the long run I have actually changed back to 12v. I had 0 noise with the 5v setup but after like 5-10 min some sort of charge would build up and start to come through the system as a buzz.
So in the course of changing power and ground locations I found a power(remote wire) and a ground (chassis ground behind rear seat, that provides no noise on startup or after the car has been running for awhile.
Re: MiniDSP: An excellent alternative!
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