1 Topic by samzen_simzen (edited by samzen_simzen 2012-11-29 18:55:38)

  • Registered: 2012-11-29
  • Posts: 2

Topic: Babyface Unbalanced Input w/ baad Noise

Ok, here's the problem:

I experience very bad, clearly audible noise when the input gain on the unit is cranked up to about 90%. Normally, this shouldn't happen.

This is how the background story goes:

I am trying to use the Babyface Preamps to amplify a phono signal, which for archiving purposes I need to record 'flat' (no equalization -> the records in question are from the pre-RIAA-era, so they need to be analysed for distortion curves and will then be eq'ed digitally).

This is the setup:

Fujitsu Lifebook AH530
Win 7
RME Babyface (duh;)
Sommer RCA Cables
RME Unbalanced Breakout Cable
Stanton STR8-150 turntable drive (fyi: it has a built-in ground switch)

I am able to reproduce the Problem on 2 identical systems. Hooking up the Babyface to a different laptop model reduces the noise somewhat, but it's still audible.

At first I thougt the noise came from the power grid (I am currently in a middle east country), but we have an online-uninterruptible power supply ('clean' power directly from the battery) set up and running.

The problem even persists when the laptop is running on batteries (power supply unit disconnected), and I have no sources at all connected to the breakout cable whatsoever.

Also, the noise occurs when I hook up XLR to RCA-cables (yes I know, those aren't good, I'd never use them in a permanent setup) to the balanced breakout box.

The problem does not occur when I set up the balanced breakout box and connect the turntable through a Sonifex Redbox RB-PA2 balanced out Preamplifier. Only problem then is the Redbox built-in RIAA-Curve, which I actually do not want.

This all does very much sound like the USB-Port of the Laptop is interfering with the Babyface Inputs, or am I missing something?

I am going to try routing the unbalanced signal through a Palmer-PLI02 DI-Box to the Babyface symmetrical Ins, but this will take a couple of days at least, since the units are not at my disposal atm.

So, just to be sure: is this a known issue at all? (yes I've tried the 'Suchfunktion', but could not find any entries describing a similar situation). Are there any other factors I might have to take into account?

Thanks
Sam

PS. One more thing: I am using Audacity for digitizing the records. I know it doesn't support ASIO, but it serves my needs. The Hum/Noise is audible and visible both on the device itself as well as in TotalMix. It peaks at about -32.5dB when the unit's gain is set to about 80-90%. Normal ground noise (through balanced inputs) at this gain level is at about -56dB, which is barely audible.

2 Reply by MC

  • MC
  • MC
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  • From: Germany/Thailand
  • Registered: 2007-04-06
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Re: Babyface Unbalanced Input w/ baad Noise

The Babyface has balanced microphone inputs that are not suitable to be used as phono inputs. Phono is unbalanced and requires a much higher impedance (47 kOhms) than what the BF has (2 kOhm).

I would recommend to use a phono preamp where you either can disable the RIAA curve, or let the Redbox be modified by a technician. Removing RIAA is quite simple.

BTW, our gain pots are labelled in dB, not percent...

Regards
Matthias Carstens
RME

MC's Website

3 Reply by samzen_simzen (edited by samzen_simzen 2012-11-30 09:11:10)

  • Registered: 2012-11-29
  • Posts: 2

Re: Babyface Unbalanced Input w/ baad Noise

Thank you for your suggestions.

Ok, I admit it - this is obviously a noob problem - I should have considered the mismatching impedances beforehand. But I'm not a technician...still, never too late to learn..

Just to be sure:

If I understood this right, a DI-Box would not solve the problem, since the average DI-Box will be designed to connect a high-impedance source (line level, instruments) to a low impedance target. A phono output at 47kOhm not being a high impedance source, this setup will most likely fail to correctly match source and target, by incorrectly altering the turntable signal's frequency response? Unless the DI-Box was specifically designed for phono-impedance, the signal it puts out would be significantly changed?



To anyone into digitizing old records:

There is a very affordable 'flat' preamp manufactured by tracertek (sorry for the ad, but this makes digitizing shellac vinyl with a babyface really easy, and also comparably cheap wink. Usually, these types of preamps would cost thousands of dollars.

I'll have an engineer look into the redbox to see wether the RIAA_EQ is bypassable at all. Considering the redbox's blueprints it might work, the EQ is wired right behind the input.

I'll keep you updated on wether this worked or not.



yours
Sam

PS. Oh yes, and the Babyface DOES indicate the gain potential in dB, NOT in percent wink