# Loud noise floor and engine whine on hi-end audio interface. Wtf?



## StabMe (Nov 17, 2011)

Hey!

Tried to make the title as catchy )

So, i have quite a complicated setup. 

First off, it is a Car PC with a MOTU 828Mk2 sound card connected via Fire Wire. It is a high end device geared toward studio use. Has very nice tech specs even for the time it was first released (circa 2004-2005). Here are the details:


10 Analog inputs (Jack 6,25, *+4db*)
8 Analog inputs (Jack 6,25, +4db or -10db)
2 Monitor/Main outs (Jack 6,25)
8 ch ADAT i/o (can also be used as s/pdif optical)
Stereo s/pdif coaxial i/o (rca)
2 Mic/line in (Neutrik combo XLR/TRS) with 48v phantom power switch
2 Firewire 400 ports
Front panel metering for all analog inputs, s/pdif and mic inputs and main outs
Sample rate : 96Khz max

The list goes on. The s/n ratio is rated somewhere near 100db, IIRC.

The card has a complicated power supply so i decided to use a 12V->220V inverter. 

The outputs on the interface are calibrated at +4dB. And there is no way i can change the level - there are no analog control to do it. It is always +4dB. I can make the signal quieter or louder in the digital domain by turning the knobs in the signal chain in AudioMulch, but it always goes at +4dB level. 

I am running a 10 channel active setup. The PC with Centrafuse and AudioMulch act as the source. MOTU 828 acts as an interface. I have an Alpine mono amp for the sub, Eton 4 channel bridged amp for midbass driver (Beyma 8g40), Helix A6 Competition amp that feeds ES HLCD Fullbodies, Audax Midrange drivers and Eton Pro 2-way passive for the rear-fill and a 2 channel Eton bridged amp for the central channel (With beyma coaxial driver).

Here is the diagram:










Now, as you can see, it is an HE system - all the drivers are high efficiency.

I am having a few problems with the set-up:
- first, as soon as the AudioMulch starts, all the speakers start producing very audible noise. Even with the engine off. As i understand, when AudioMulch starts, it activates the outputs of the interface and it starts producing some kind of hiss. It sounds like a whitenoise, like when you turn up the amps all the way amp. Only my gains are set to 30-50% on all the amps. The noise is floor is so loud that it is uncomfortable to drive, actually. *Is it possible, that the noise floor is so audible because of high voltage on the interface's output? It is rated +4db and from what i read on the net, it constantly shows 4.54V on all outputs.*

- second, as soon as i start the engine, i can hear the engine whine. An interesting thing with the whine is - when all 3 RCA pairs are connected to the Helix amp, the noise is most audible. When i disconnect one pair - the noise gets quieter. When i leave only 1 RCA pair connected (dedicated to horns), the whine is barely audible. Wtf? 


I have several ideas to try to combat the noise and whine:

- disconnect the 220V adapter from the MOTU, power it off and connect MOTU to the outlet (i live in Europe and we have 220V here).

- try another sound card. Preferably, with fantom power. Preferably, with the ability to set line levels.

- All the analog outputs on MOTU are balanced and can be used as unbalanced (as i do now). Will it help, if i use the balanced cable on the MOTU side and use a di-box at the amp side?

Going crazy with it, actually


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## StabMe (Nov 17, 2011)

> first, as soon as the AudioMulch starts, all the speakers start producing very audible noise.


The problem with the noise floor was due to "Dither output" option being activated in AudioMulch:

"*Dither output*

When checked, the Dither output checkbox enables Dithering, a process that introduces a small amount of noise to improve the percieved sound quality of very quiet output signals."


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## Elgrosso (Jun 15, 2013)

Awesome HE system!
So if noise floor is fixed, do you still have engine whine, and is it on horns only or all drivers?

What else can you tell us about your setup?
The car, the CC placement, the sub box maybe


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## StabMe (Nov 17, 2011)

Engine whine was still present after fixing Audiomulch and it was LOUD.

I realized that the interface was equipped with balanced outputs. Actually, you can use mono TSR Jacks if you want unbalanced outputs and stereo jacks if you need balanced outputs.

So, we custom made balanced to unbalanced converters with OPA4134 operational amplifiers, replaced all the line level wiring with balanced cables and placed the converters near the amplifiers. This cured 100% of the noise on amps that were located in the trunk. I have a two channel amp under the passenger seat which is used for my central channel and was still a bit noisy. So we had to implement ground lifting (basically disconnecting the shield cable). And it helped for our surprise 

BTW, when using operational amplifiers one can choose the level of amplification when designing the circuit. First, we used 1:1 so that no amplification is happening in the converter. But we noticed that by raising the gain controller on the amp noise/whine level was rising as well. So we used anything between 1:1.25 to 1:75 on the converters and had all the gains on amps at zero level. This has helped a lot. 

I still do have very little noise in the system but it is much much more bearable.


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