Thread: Noise and distortion on with MJS USB Audio Selector?

Originally Posted by mda185

Does anyone have any experience using the MJS USB Audio Selector with their Car PC successfully? I just purchased one and tried to test it for basic function on my test bench. I loaded the drivers and plugged an iPod and a couple of SANSA players as signal sources. I used a cheap set of earphones to listen to the output. I used the software app Mitch includes to check switching function without integrating it into my software front end. (I have both RR and Centrafuse on my evaluation system.)

The selector correctly switches between inputs using Mitch's app. The problem is that the sound is horribly distorted and attenuated when listening to the output on earphones. I noticed that if I 'jiggle' the 1/8" male connectors while inserted in the input jacks, the sound intermittently improves and degrades. It seems like a problem with the jacks themselves. I tried using different sets of cables and ruled out a problem with my cables. I have not tried contact cleaner on the jacks yet.

I looked inside the case and expected to find some small relays. Instead, the design uses semiconductor switching IC's. I wonder if this is part of the problem and I also wonder how good the audio quality can be using IC's to switch the signals. There can't be much signal headroom in this type of design when it is USB powered. Still, the inputs should not be driven into clipping with the output from an iPod or SANSA Fuze. My next step is probably going to be testing the input vs output signal quality using Praxis. I am curious to know if any other forum members interested in sound quality have tried this unit and what their experience has been.

I tried posting a message on MJS website forum but could not do so without registering. I followed registration instructions but have not received an activation email message yet.

If your concern is that you won't get audiophile performance, I think buying a $60 4 channel USB audio switcher might have been a little optimistic in the first place. On the other hand, be realistic about the noise levels and conditions in your vehicle during actual use. I've used a UAS for several years now, and it's been bullet-proof, worked EXACTLY as advertised, and the sound quality has been more than satisfactory for what I consider "ordinary" listening. Unless your doing blind listening tests with the car parked and the engine off, I don't think you're every going to notice whatever limitations the semiconductor ICs are going to introduce. The plugs, on the other hand are critical. You need to use the best quality you can find, and make sure they are secure. I recommend attaching the UAS to a board or surface with screws or bolts so it doesn't move around, then use cable straps to lock all the cables down. A little double sided cushioned tape on the inside of the cable strap should hold the small 3.5 cables firmly.

Cheers,

VegasGuy

Yes, I think I was overly optimistic about the potential for this device at its price point. I was expecting something with simple relays but I do understand that the way Mitch designed this is more economical. I am an old EE and as far back as I can remember, Integrated Circuit (IC) switches were not considered optimal from a sound quality standpoint. In an age where low to medium bit rate MP3 is considered good sound quality by most consumers, I guess this is acceptable. It is not what I was hoping for and the initial experience with noise and distortion out of the box really is not acceptable. I should not have to clean the contacts and play with cables on a new device fresh out of the box.

I feel like I need to explain my viewpoint a little better. I consider myself something less than an audiophile but a serious music listener. By less than an audiophile, I mean I won't spend in excess of $3000 on mobile audio in search of perfection. Car interiors are not conducive to audio perfection with multiple reflective surfaces and tweeters located far from the mid-woofers in most installations. Within those limitations, I still like to get the best sound possible. I have been a DIY enthusiast for over 25 years and have a lot of experience designing and building home and mobile audio speaker installations.

In a typical OEM car stereo installation with the original manufacturer installed speakers in the typical sub-optimal locations, I am sure a device like the MJS USB Audio Selector works and sounds acceptable. In a custom installation with better quality speakers and electronics, I don't think it is the right choice. My primary interest in a Car PC is its ability to store a large music collection, provide a decent user interface to play that music, integrate it with GPS Navigation, and someday, integrate it with reliable bluetooth hands free phone capability. After investing all the time and money it takes to build a decent Car PC with these capabilities, I am not going to limit its sound quality with the input switching device. I am thinking it might be a better sound quality solution to try using Fusion Brain with switched relays or look into something like the M Audio 1010LT PCI card with its multiple inputs and outputs. I think it's time to check out the Car PC case at Mo-C0-So with the PCI card slot.

max level is approx 2v peak to peak on input signal

the IC is of HQ, its designed for AUTOMOTIVE audio selection
its of VERY HQ, and if input levels arnt exceeded should not effect audio what so ever
assuming you have no wacky noise on your usb system
(but you would hear that, and the ic has isolation)

JRC's NJM2750

anyway. want more headroom, it can do it, but it CANT be usb powered
and since PC audio is 1v - 2v pp stuff, it works perfectly

a headphone out from a ipod may drive it into clipping
use the dock fixed level
(or lower the volume on the ipod)

i havent had a bad one yet, so if you got distortion, maybe it bad...
usb power is key, will sound crappy without it

Mitch, I tried registering on your web site Monday to post a question about my experience with the USB Audio Switcher. I have not received an activation email message to enable me to post on your web site. I will be first to admit my knowledge of IC audio switching devices may be somewhat dated but I played around with early IC's like this while still in college. (Long time ago.) They definitely had some issues with introducing noise and distortion back then. I was trying to design a variable crossover with switched resistors to change the frequency. It was a 4th order state variable filter and needed to switch 4 resistor values at once with good matching. It worked well as a switcher but did not sound good enough to pursue past the prototype stage.

I tried lowering the volume on my iPod and Sansa Fuze in the event that they were overdriving the inputs but that did not help. I am positive the Sansa does not go up to 2 volts output. Not sure about the iPod. The only thing that made a difference was jiggling the 1/8" phone plugs on the inputs while inserted in the jacks. I did not look into the quality of the USB power. I was using a Dell laptop and I do remember hearing in the past, that some of them have questionable USB power output. I will try your switcher with a powered hub to see if it makes a difference tonight.

I am extremely impressed with the coding and hardware projects you have done for this community so please understand that I am not trying to put you down. Based on this first look, I just don't think the USB switcher is suitable for a mobile audio install that includes custom speakers, optimized crossovers, time alignment, and high quality amps. The system I am putting together will be using an old school PPI PAR 225 preamp, Zapco DSP6, and old school Soundstream D100/D200 amps with some custom A Pillar pods housing 3" Fountek drivers crossed over to 6.5" mids in the doors at about 300 Hz. There will be a subwoofer in the back. I was planning to use your switcher in front of the PPI PAR 225 preamp. Inputs to the switcher were intended to be a DAC output from the PC, HD Radio, and iPod or other device such as the Sansa Fuze. The 4th input would be for future expansion. The preamp will send a ~ 4-5 volt signal to the amps. Actual voltage to be determined when I set gains on the system for minimum noise. This system would probably cost well over $3K if I paid a pro to install it. I bought the electronics used and will do all the labor myself. If you ever turn your talents to designing a USB controlled audio switching device with relays instead of ICs so that any voltage level could be handled and USB power quality would not be an issue, I will be the first customer in line.

Originally Posted by mitchjs

max level is approx 2v peak to peak on input signal

Ouch, I purchased one of these a while back and haven't gotten a chance to integrate it yet, but knowing that kinda limits my options. You think perhaps putting that information in the manual or on the product page might be possible? (As well as mp3car's product page, that's where I purchased it from)

Originally Posted by malcom2073

Ouch, I purchased one of these a while back and haven't gotten a chance to integrate it yet, but knowing that kinda limits my options. You think perhaps putting that information in the manual or on the product page might be possible? (As well as mp3car's product page, that's where I purchased it from)

If you use MJS device as input to your Car PC that will also be limited to 2 volts Peak in most cases. Very few PC audio devices handle higher voltage. The only exceptions are going to be high end PCI or PCI Express audio cards or pro musician quality USB recording devices and audio cards. I am saying that the MJS switcher is fine to use if you are staying with the motherboard audio or using typical USB audio device. It won't be the limiting factor. If you are shooting for more of a high end audio system, then other options should probably be explored.

keep in mind using headphone outputs of any portable device will be an impeadance mismatch, in most cases they are expected to drive a 32ohm or less load

use line outs

right now, i got my iphone 4(which i think is slightly over 2v pp, playing though a uas4 to a pair of bose amplified pc speakers...
sounds good! clear as it is comming right from iphone direct
need a sub

if you want more head room, its MOD time (and its easy)
cause u cant power it from 5v
and get more headroom
infact this was debated, but you what people want the most, simple!
anyway...


oh, you could also have a ground reference issue...or lack of one...
keep that in mind, your source audio ground is passed directly to the output jack, which then goes to your "amp"
since we dont really know what exactly your connecting to

most people connect the OUTPUT to the LINE in on the PC (another input that only typically wants 2vpp max)

m

Mitch, I owe you an apology. I tried several things based on your recommendations and the sound did improve tremendously.

1. First, I powered it from a USB hub with its own wall wart. This made a huge difference. I have to assume that the USB port on my Dell laptop was not providing good quality power.
2. I connected the output of the MJS UAS to the input of a good quality preamp in my home office stereo system. I use a Chase RLC-1, a very basic preamp out of production for many years. It was loved by audiophiles back in the day for the ability to add a remote control volume to their rigs for casual listening. The Chase has high input impedance, negligible distortion and very quiet electronics.
3. I connected the inputs of the MJS UAS to a USB DAC with line level outputs, an iPod dock with line level outputs, and a Sansa Fuze headphone output.

The Chase RLC-1 was used to drive a pair of Acoustic Research Powered Partner AR 570 speakers, another item out of production for several years but very high quality that blows away most of the powered computer speakers on the market today. They also sound better than most mass market and factory installed mobile audio speakers. The AR speakers are active designs with a 4.5" woofer and 1" tweeter with built in amplifiers and 12 dB/octave active crossovers. They are not audiophile quality but are considered one of the best sounding portable powered speakers ever designed.

My observations:
1. Sound quality and volume improved significantly with good USB power supply. The MJS UAS was even able to drive earphones acceptably when I ran it from the USB powered hub.
2. When the MJS UAS was connected to the Chase RLC-1 input, the sound was crisp, clear, and noise free. If there was any noise contributed by the electronics, I could not detect it when sitting 12-18 " from my speakers.
3. Sansa Fuze headphone output sounded good when the MJS UAS was connected to the Chase preamp. Your switcher should sound fine if used in the car with headphone outputs from a portable MP3 player. It seems that it as long as the output of the UAS is connected to a decent high input impedance preamp section, it can work with any input signal as long as it does not exceed the 2 volt peak limitation.
4. Even though the sound when using headphone output jack on the Sansa Fuze was decent, the sound quality was better when line level input signals were used.

I regret that I started this thread without testing this thoroughly with different combinations of USB power and input and output devices. When used properly with good USB power quality, line level preamp input voltage levels, and a high input impedance preamp connected to the output, the MJS UAS performs well and represents good value for the money. I still wish there was an 'audiophile' version available with more headroom and better quality audio input/output jacks but I realize that I am probably in the minority of Car PC community.

The one aspect I have not investigated was hinted at in one of your posts. I assume the signal ground in the MJS UAS is derived from the 5 volt USB power. This means signal ground is probably around 2.5 volts DC referenced to the USB power ground. I can see where this may lead to a problem with ground loops in a mobile audio installation. You said that this has isolation. Can you explain further? I am going to connect this to a Precision Power PAR 225 preamp which has its own +/- 12 volt switching power supply for the analog electronics. (Anyone over 35-40 years old should have realized by now that I have an affinity for high quality old school mobile audio electronics.) The power supply in my preamp is isolated from the car 12 volt system so this should prevent any problems with ground loops as long as the UAS signal ground also has a high degree of isolation. This is probably also a function of the power supply used for the mobile PC. I am guessing that the only type of installation that would present serious problems is if the USB power for the UAS is derived from the vehicle's 12 volt system with a simple 3 terminal linear voltage regulator. This might be the case if someone connects the UAS to a USB hub and powers that hub with a cigarrette lighter socket adapter.

glad you like it, keep in mind, where i designed this for, pc audio

ok... so if you want to power this off say a 9vdc level
then the headroom is around 2.5volts RMS
( 3.5v p-p)

a simple way, would be to remove T1 from the board
and connect 9vdc from a regulator, 7809 will work...
to C5, note the polarity, + is nearest the IC

thats it, now you have it working on 9vdc, you can go to 12vdc, the max is 13v, the absolute max is 15v (smoke time)
i wouldnt use 12, unless its regulated, and remember a cars voltage is 10-14
so... thats why 9 is a good choice, unless you do a DC-DC converter, a 9-18vin to 12dc

if you need a picture, i can show you one, i set one up to run at 9vdc
with my iphone, i didnt hear any difference from 9vdc, and my USB 4.8v-5.5v

i wish the chip had a charge pump in it... that would allow on low supply voltages, higher p-p signal voltages
but it didnt, and there are other factors in why to use this IC, its designed for automotive enviroment, and 4 to 1 , in 1 package

to do an really high end versions, would not be able to do it for this kind of $, i would need to use dc-dc converter, and good qual op-amps


as for signal ground, NO i does NOT come from the USB ground
the audio ground on the inputs is tied to the audio ground on the outputs
the isolation comes from the ics internal vref
while not the highest form of isolation... there are some more options
but in all my testing i havent had any issues with noise from this device