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Interface for use with non-pro gear (pre-pro and amp) Audio Interfaces
Old 21st August 2016
  #1
Here for the gear
 

Interface for use with non-pro gear (pre-pro and amp)

Greetings,

I am needing assistance in seeing if this is even a possibility. I had ordered a Scarlett 18i20 to use but as a noob didn't realize the issues it would cause. I'm using a home theater preamp processor and a HT amplifier. Both are unbalanced. I'm wanting to add an interface to use VST processing on 5.1 content like PEQ and better bass management. When I added the Scarlett to the system I encountered lots of hiss that was clearly audible up to a couple feet (used 1/4" TS adapters). No matter how much I reduced the gain it seems unless I attenuated the signal to the amp I had a problem with it, but this destroyed a good chunk of headroom.

I'm now aware of the general -10dBv of consumer gear vs. the +4dBu of pro gear. I'm returning the Scarlett so thought before jumping into this again I better get opinions.

I'm looking at the Tascam 16x08 which has a -10dBv switch on back for inputs. I believe this would fix the matching to the interface from the pre-pro, but am concerned regarding the connection to th amplifier. The Scarlett had hiss even with all volume controls (hardware and software) at minimum. Would improving the initial signal kill potential SNR issues and reduce hiss, or would I need to implement attenuation as well? Or does anyone know of an interface that would work for 5.1 consumer gear already? My max budget is around $700.

Thanks in advance.
Old 21st August 2016
  #2
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-10dBV vs +4dBu shouldn't be causing hiss.. this alone is just about level matching, so only issue, which this could do is overloading of preamp inputs with -10dBV sensitivity. Also you can freely control volume at any of 18i20 outputs via MixControl, so it definitely shouldn't be issue in this particular case.

Anyway, whole idea of insertion of another AD/DA computer interface with its latency into AV system is somewhat weird to me.
I'd either try to upgrade AV processor to something with better options and bass management.. possibly also complete new AV receiver with power amp.
Or get something from miniDSP, more expensive boxes has also analog I/Os, but cheaper ones can work with HDMI signal.. check their catalog.

Michal
Old 21st August 2016
  #3
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Quote:
Originally Posted by msmucr View Post
-10dBV vs +4dBu shouldn't be causing hiss.. this alone is just about level matching, so only issue, which this could do is overloading of preamp inputs with -10dBV sensitivity.
That doesn't make sense either though, because the hiss will continue even if I turn the pre-pro off entirely with just the Scarlett and amplifier active the hiss persists. Equally they say they support +12dBu and higher input.

Quote:
Also you can freely control volume at any of 18i20 outputs via MixControl, so it definitely shouldn't be issue in this particular case.
So is the unit simply defective?

Quote:
Anyway, whole idea of insertion of another AD/DA computer interface with its latency into AV system is somewhat weird to me.
I'd either try to upgrade AV processor to something with better options and bass management.. possibly also complete new AV receiver with power amp.
Or get something from miniDSP, more expensive boxes has also analog I/Os, but cheaper ones can work with HDMI signal.. check their catalog.

Michal
Most pre-pros that have the features I want would be ~$2K starting, and miniDSP doesn't have have anything that would support 100 parametric filters for room correction. The closest thing is Dirac but this is a better bang for buck for me as there are other VSTs I plan on using. In relation to latency I've used a loopback with a simple soundcard in ASIO4ALL and onboard and for reproduction it was effectively imperceptible.
Old 21st August 2016
  #4
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Quote:
Originally Posted by Shike View Post
That doesn't make sense either though, because the hiss will continue even if I turn the pre-pro off entirely with just the Scarlett and amplifier active the hiss persists. Equally they say they support +12dBu and higher input.
Fine, so levels definitely aren't problem.
According to the description, I'd try to further isolate it and find out if noise levels improve..
when you physically disconnect Scarlett analog inputs from the preamp
when you disconnect Scarlett from USB (connection is not required for interface operation, it works in standalone mode mixer then, so it can be tested this way)

Experiment a little bit with cabling between Scarlett and power amp..
TS->RCA adapters aren't ideal for some types of balanced to unbalanced device interconnection, because it will short neg. bal signal and device ground (ring and sleeve) immediately at source side.. if that negatively can affects the performance depends on particular devices grounding design and also other external connected devices (like computer). Also in some EMI/RFI hostile environments, long unbalanced connections simply won't perform well.

I'd try to locate 18i20 closest to the power amp and create test cable using two conductor shielded cable (eg. the one used for normal balanced interconnection). At source side with TRS jack, I'd connect it like balanced cable (eg. two internal conductors to tip + and ring - and cable shield to sleeve) at receiver side, connect RCA just with the conductors and leave cable shield unconnected.
pic. Sound System Interconnection - connection number 12 at table.
This should work for the most cases and I'd start with that, but couple of other connection schemes exists for different balanced output topologies.

There are also transformer isolators, if you will be really positive, it's grounding issue, but it can be quite pricey for multichannel.

Quote:
So is the unit simply defective?
It might be, but it's relatively unlike to me, this would affect all analog outs. You can test that by disconnecting from everything besides the computer and testing of noise level with some more sensitive headphones or using TRS-TRS loopback to its line inputs and some measurement software or simple DAW analyzer plugin.
If unit is fine, then the issue is there just because combination of balanced and unbalanced equipment and can really affect any balanced audio interface (besides ones with transformer isolation), you'll purchase.

Quote:
Most pre-pros that have the features I want would be ~$2K starting, and miniDSP doesn't have have anything that would support 100 parametric filters for room correction. The closest thing is Dirac but this is a better bang for buck for me as there are other VSTs I plan on using. In relation to latency I've used a loopback with a simple soundcard in ASIO4ALL and onboard and for reproduction it was effectively imperceptible.
Ok. I see, maybe roundtrip latency isn't so critical if you fit that under one frame.
AD/DA roundtrip and computer for 100 parametric filters, I don't know.. still looks like lot of hassle to me, although multichannel computer audio interface is source of fun of course You can jam on guitar along your favorite movies.
I'm not sure about your sources, but for example mentioned miniDSP HDMI box like nanoAVR HD/HDA has 10 filters per each channel and bass management.
https://www.minidsp.com/images/docum...%20plug-in.pdf
Of course it isn't decoder.. so your BluRay player or whatever source it is, has to decode all surround formats to PCM before reaching such box.
Nevermind..

Michal
Old 21st August 2016
  #5
Here for the gear
 

So I tried headphones and they sound perfectly fine with it, so the input section seems to be working just fine.

Made the cable suggested, and problems actually got worse. It went from hiss to hiss and hum.

I then did a loopback and used headphones for monitoring using the RCA + Adapters. The mapping is:

input output
3-4 1-2

Then I took the output looping it back to 5-6, which I then monitored on headphones. I confirmed I'm listening to the loop back by using VSTs outside of the 3-4/1-2 chain.

Absolutely no hiss or buzz, dead quiet. So this must be an interaction between the Scarlett and the Emotiva amplifier.

Any other thoughts or ideas?
Old 21st August 2016
  #6
Lives for gear
 

Hmm.. at least you've partly isolated the problem.
What about that disconnection from the USB, does it improve or it's the same?
Unfortunately when solving such issues, it's typically about trial and error approach and something it's not so obvious.
If it gets worse with mentioned TRS-RCA connection, then Scarlett balanced output stage is most likely cross-coupled. So with that two conductor shielded cable, you've made, you might try to do one small modification, which is connection of ring and sleeve cable at source side. This is that left side optional connection at number 12 in linked RANE document. It's very close, but not exactly same as using plain TS to RCA adapters with regards to cable shield.

As I've mentioned, if that's ground related (most common issue, which doesn't necessary exhibit as typical mains hum with harmonics, especially when computer is involved), another option is audio transformer right before power amp.. but that's another expense.. something like this.
https://www.amazon.com/ART-T8-8-Chan.../dp/B000PJAKXI

Michal
Old 22nd August 2016
  #7
Here for the gear
 

Disconnecting the USB didn't help, but I think I figured out the issue.

It seems that it does sort of relate back to the +4dbu standard of professional gear. Because the Scarlett is running hotter on the output, it seems to also increase the noise by roughly ~10dB that the processor had even when the signal was low. Typically this would only be audible within a foot, but when the signal is hotter it's clearly going to reach further.

Reading on jriver's forums someone added attenuators for -10dB to their 1820i due to, you guessed it, noise issues. Not wanting to make a resistor divider for testing I dropped in an unbalanced NHT passive volume control, set to ~-10dB. I ran one of my processor's outputs to the Scarlett through the NHT then to amp, and another directly from processor to amp. After a little balance checking it the noise seems to be reduced to expected standards (prior to adding Scarlett).

So now the only question is of resistor values for a voltage divider. I'm thinking a simple one with a Rs (series resistor) of 15K and the Rp(parallel shunt resistor) of 4.7K. I'll probably contact Scarlett to confirm the output impedance on the 1820i to make sure these are good value on that side, but to confirm if anyone sees this thread does anyone know of potentials issues with this? I'll probably put them inside the RCA plug on my currently cables.
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