Once again--How do I properly connect outboard preamps---

D

dcontento

New member
I am pretty much a complete newbie other than playing around with a TASCAM Portastudio I bought a year ago. I've seen a few threads dealing with my particular problem, but I am still not sure what I need to do. I know this is a long, detailed post, but please bear with me.

I have a couple of outboard preamps that I used with the porta-studio by adjusting the portastudio trim level to zero. It sounded ok, but I wanted better quality and am now recording with my computer and Reaper. I purchased an audio interface with the idea of running the outboard preamps through the interface. There are no separate inserts in the interface unit. The chip used in the new interface has a special provision for 0db gain setting which basically converts the circuit to a unity gain buffer. According to the manufacturer this means that the output voltage in the interface preamp is the same as the input. This sounded like a perfect solution, but the problem is that the interface unit has a stated maximum input level for 0 dBFS of +12dBU. Both of my outboard pres provide in excess of 70 db gain and need to be driven a bit harder than +12db in order to provide the color I am looking for. I am afraid that I would seriously overdrive the interface if I used reasonable gain on the outboards.

Hoping I could solve the problem (by throwing more money at it- haha!), I bought an expansion unit with 8 preamp channels that connects to my new interface via ADAT. Now here is where I become confused:

The expansion unit has 8 audio input jacks on the back. They are combo jacks. The manual states that if an XLR cable is used, the preamp switches to microphone level. If a TRS jack is used, the preamp will automatically switch to line level. (Inputs 1 and 2 can be switched to "instrument" for high impedance input so I won't be using these for preamps).

Now for my questions: Since the expansion unit has no inserts, and I run the outboards through the expansion unit via TRS to preamps set at line level, aren't I essentially no better off then I was with the portastudio? Am I correct in assuming that I have to set the trim to zero on the expansion unit just as I did with the portastudio? Am I mis-interpreting the "max input of +12dBU" spec in the interface manual? Isn't there a way to utilize the AD convertors in the interface without compromising the signal from the outboard preamp?

I would be extremely grateful to anyone who could help me sort through this or who could lend advice in any way.

One last thing - For reference, the equipment I am using is listed below:

Computer Interface: RME Babyface
Expansion Unit: Focusrite Octopre MKii
Outboard Pre 1: Golden Age Project Pre73DLX
Outboard Pre 2: Warm Audio WA12
 
Can't you, using Total Mix, specify the type of input it is?

I have the RME UCX and I'm pretty sure I can alter the input parameters of any given input (combo on front or line in on front or back) to whatever it needs to be.... and that software comes with the Babyface too, I believe.

I run an external channel strip (Mindprint Envoice) through input 3, set us as line input, into Reaper all the time... no problems. You're sure you're not getting a bit confused on the gain issue? I'm not the expert in the numbers but I don't think the 70dB of gain of your pre and the +12dBU input level mean what you think they do.

Someone with more knowledge than me may correct me, however.
 
Thanks Armistice. I'm not sure, but I think you probably hit the nail on the head concerning my confusion with output gain and +12dBU input max. You know, I bought the Babyface 10 days ago and just received the Octopre mkii two days ago. I have yet to connect the outboards to either one as of yet. I was reading all the literature first, thinking that I would return the gear if it didn't fit my needs. I saw the "max input" thing in the spec section of the Babyface manual and thought it might be a problem. I have to admit , the pres in the babyface sound great by themselves, but I really want the option of the Neve and API clones. Based on your reply, I'm going to connect everything and see what happens. Thanks for taking the time to respond.
Please, anyone else who has experience with this gear, please chip in.
 
Caution gentle readers! Hobby Horse about to come amongst ya!

Your problem dcontento is caused by your mixture of "prosumer" and actual, no prisoners taken, "pro" level gear. Excellent tho' the Babyface is (I have never read anything bad about one) its operating levels must be constrained in the interest of being "compact and beejoo" and bus powered.

Fortunately the problem is easily solved but this is where the equestrian content comes in. You need to make up some attenuators but unfortunately many, many home recording bods have no electronics skills whatsoever and worse! Many seem to despise the idea of getting their hands sullied with solder and iron? But not for nothing were the guys at Abbey Road called recording ENGINEERS!

Most top line gear, pre amps, mixers, have a max output of at least +20dBV (10V) and many hit +22-24 dB. Professional, standalone converters have a sensitivity to suite, i.e. around +20dBV for 0dBFS but most are switchable/programable for lower levels.

So, get some soldering kit together (including a 50mm table clampiing vice) and buy a resistor development kit and a decent digital multimeter (you are not going to learn the resistor colour code overnight!). Thus armed you can record some tones, most DAW software will generate sines, squares and noise, at precise dBV levels and the meter will tell you what the various voltages are coming out of various bits of kit*. Simple arithemetic to work out the attenuators you need and there are many sites with calculators where you just plug in dBs and in and out resistances and out pops the resistor values. You can chose balanced or unbalanced attenuators.

*But! You will need to makeup some adaptor leads. More "proper" recordist experience!

Dave.
 
The 70dB of gain available from the pre does not specify the actual output of the pre-amp. It just tells you how much the input signal will be amplified - in this case, I calculate a gain of approximately 3000.

12dBu, however, tells you that your converter will clip if you hit it with more than about 3.1V rms or about 8.7V peak to peak.

Now, here is a solution to your problem.

The way to run your pre-amps harder while not clipping the converter is this is by using a "pad".

A pad serves to reduce a signal by a pre-determined amount.

Your Focusrite looks to have a built-in input pad of -10dB, so try that. Have the Focusrite gain set at minimum.

If that does not reduce the input signal sufficiently, then buy or build an inline pad and connect it between your pre-amp and the Focusrite. You might want to get pads in a few values and see what works best; -12dB, -18dB, -24dB for example.

That should allow you to drive the preamps as hard as you want while keeping the converter suitably below clipping.

(The Focusrite specs state that line input gain is from -10dB to +55dB, but it does not say if that is with pad engaged, so you will just need to experiment).

Hope that helps.
 
Dave and PRHunt, my thanks to both of you. You each suggested an attenuator of sorts and I remembered seeing a device called a Nano Patch Plus. It is a passive attenuator designed primarily for controlling volume from your computer audio interface/soundcard to your monitors. It claims to have no active circuitry, buffers, transistors, batteries, or power supply of any kind, is completely transparent, and is adjustable form 0db to 60db. If this device would do the job, I would be able to apply attenuation as required for each of my current preamps as well as any I might acquire later on.

Photo of unit: http://www.smproaudio.com/images/products/monitorcontroller/nanopatch/Nano_Patch+_hires_2.jpg

Info on unit: http://www.smproaudio.com/index.php...rollers/nano-patch-plus#features]SM Pro Audio

The unit costs around 60USD, not cheap, but as you guessed Dave, I AM somewhat awkward with a soldering iron.

If I may impose upon the two of you once again, would you mind checking out the links above and let me know if you think this might do the job without effecting the quality of the signal?

Best Regards
 
I strongly suggest before you buy anything else you connect your mic pre up to your Babyface as I mentioned and see what happens.

Your problem appears theoretical at this stage, and I do exactly what you're trying to do with gear from the same manufacturer that uses the same controlling software without the slightest of issues...
 
dcontento said:
If I may impose upon the two of you once again, would you mind checking out the links above and let me know if you think this might do the job without effecting the quality of the signal?
Click to expand...

Well, it looks like it would do the job. I have *not* seen or used the device in question, however. If in doubt, you could contact the manufacturer.

I had a simpler fixed attenuator in mind, perhaps something like a Naiant MPD Inline Attenuator

Good luck!
 
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