dda -2dbu (-10/+4 ref levels)

Posted: Thu Feb 15, 2007 1:26 am

Hello,
I've recently purchased a dda interface console and am trying to figure out the best way to connect it to my 002r.
The direct outs and inserts on the dda are -2 dbu but the ins on my 002 and external adc are -10 or +4dbu.

Here are the work arounds I've come up with. Which would you choose
1. Set adc input to +4dbu and loose a little noise floor at the preamp
2. set ADC input to -10 and run the pre hot (less headroom)
3. Add transformer. I've noticed a transformer option for the direct outs of the dda. Is this where one would add a transformer to step up level from -2dbu to +4 dbu. If so how much will this effect sound. Will i have to spend top dollar on these transformers?

Any and all help is much apreciated
thanks

Posted: Thu Feb 15, 2007 4:16 am

First of all, you may want to differentiate between dBu and dBV because I think you MAY be confusing your +4 and -10 as both being dBu. (I could be wrong, but it's probably a good assumption, since dBV isn't mentioned anywhere. I'm just trying to clarify the situation Wink

). -2dBu DOES seem an odd level.

Unless you are stating the proper references in your question, anyone can suspect that you are actually referring to -10dBV instead of -10 dBu, but they would be guessing, and the answer may confuse you even more. So, here are some things to consider.

From the following Rane website:

http://rane.com/par-d.html

0 dBu Preferred informal abbreviation for the official dB (0.775 V); a voltage reference point equal to 0.775 Vrms. [This reference originally was labeled dBv (lower-case) but was too often confused with dBV (upper-case), so it was changed to dBu (for unterminated).]

+4 dBu Standard pro audio voltage reference level equal to 1.23 Vrms.

0 dBV Preferred informal abbreviation for the official dB (1.0 V); a voltage reference point equal to 1.0 Vrms.

-10 dBV Standard voltage reference level for consumer and some pro audio use (e.g. TASCAM), equal to 0.316 Vrms. (Tip: RCA connectors are a good indicator of units operating at -10 dBV levels.)

Then, once you check which dB reference all your equipment is using, here's a handy little dB conversion calculator (down the page a bit):

http://www.doctorproaudio.com/doctor/calculadores_en.htm

Hopefully, some of this will help answer your question.

Kapt. Krunch

Posted: Thu Feb 15, 2007 6:06 am

The easiest thing is to take option 1 and enjoy the extra headroom. I would often trade headroom for noise floor, but it does depend somewhat on the make/model of your external ADC.

Another option that you don't mention is to have resistive attenuators on the -2dBu console outputs driving the -10dBV inputs of the ADC and the 002r. Don't forget that -10dBV is roughly -8dBu, so you would need only 6dB balanced attenuators. These are available as in-line XLR-XLR adaptors, or you could make them.

Posted: Thu Feb 15, 2007 11:22 am

Kapt.Krunch wrote:

-2dBu DOES seem an odd level.

- Kapt. Krunch

Allen & Heath does the same thing on some of their consoles. They somewhat arbitrarily about 1/2 of their console outputs as -2dBu and the other half as +4dBu, which is really annoying... Its beyond me.... but I think it has something to do with some vestige of [tape] recorders wanting to deal with -2dBu rather than the hotter 4 signal.... I dont know... Typically the so called "tape outputs" are the -2dBu outputs on some of the A&H's.... which really sucks if your not actually using "tape" (what year is this?).

Posted: Thu Feb 15, 2007 11:53 am

It might look as though -2dBu is a compromise half-way level between -10dBV (-8dBu) and +4dBu, such that it could be made to work with either at a pinch without too many headroom problems.

In practice, the -2dBu level was probably driven by engineering and marketing considerations. To achieve direct outs and insert outs at +4dBu with a 20dB headroom (i.e. +24dBu clip level), you need at least +/-20V (preferably +/-24V) rails in your channel strip, giving a peak current of around 30mA into 600 Ohm loads. At the time these consoles were designed, this was a tough spec to meet while keeping distortion figures and thermal dissipation under control in a budget product. Halving the voltage from +4dBu to -2dBu, thereby quartering the power, makes a significant difference to the designer. It also put a marketing distance between these consoles and the fully-professional ones where +4dBu into 600 Ohm is standard on all outputs. Labelling the -2dBu outputs as "tape" outs softens the effect of a weaker spec.

Posted: Thu Feb 15, 2007 12:12 pm

Reference levels can be humongously confusing for most. Everybody's example as set above, is both correct and incorrect. How can that be? Because the Japanese have tried to reinvent the wheel their way!

For instance. The Teac/TASCAM 0 DB, -10 DB output reference level, is NOT referenced to .775 volts! Teac referenced their -10 DB output with reference to 1 volt! And so, their reference levels do not correlate to American/European reference levels properly.

As Boswell indicated, I would go for the headroom, especially for popular recording, where signal to noise ratio is not as big a factor as it is with fine arts classical music. Headroom is the difference between retaining the transients or clipping them, as inexpensive equipment so often does.

Head engineer (who's got the pot??)
Ms. Remy Ann David

Posted: Thu Feb 15, 2007 1:17 pm

RemyRAD wrote:

The Teac/TASCAM 0 DB, -10 DB output reference level, is NOT referenced to .775 volts! Teac referenced their -10 DB output with reference to 1 volt! And so, their reference levels do not correlate to American/European reference levels properly.

That's why we write it as -10dBV, the "V" standing for 1 Volt. The original 0dBm was the level required to push 1milliwatt into 600 Ohms, which came out as sqrt(600/1000) Volts = 0.7746V r.m.s. When 600 Ohm terminations went out of fashion, the same voltage level was retained, but now into a relatively higher load impedance, so the reference voltage level was re-named 0dBu (u = unterminated). There is approximately 2dB difference between 0dBV and 0dBu, making the difference between -10dBV and +4dBu approximately 12 dB or a factor of 4 in amplitude.

Posted: Thu Feb 15, 2007 3:12 pm

Yes and that's why it's so confusing to most. But the unterminated "dbu", value is in fact generally terminated into at least 10,000 to 50,000 ohms. Just not 600 like the front end of the UREI 1176LN's.

You wouldn't believe how many people I've talked to, that couldn't understand the problem with the underwhelming sound they got, while patching their 1176's, LA3's, LA4's, into their inserts on a Soundcraft, Allen and Heath, Mackie or, any console that wasn't an API, Neve, SSL, Electrodyne, Sphere, Quad Eight, OK MCI also, etc.. And that's because those wonderful old limiters can really only be utilized in a control room that was intended for 600 ohm studio operation and many people here don't know that. And none of those inexpensive consoles inserts can be used with any 600 ohm audio stuff. Of course that's not true for the later non-transformer op-amp balanced input 1176/1178 silver faces, which work fine with your kids console.

I'm not even going to talk about the equipment that is supposed to have a 600 ohm resistor to terminate the output transformer, for a proper and flat response, like my Neves and API stuff. Of which there's lots of.

Unterminated at 51
Ms. Remy Ann David

Posted: Thu Feb 15, 2007 3:51 pm

thanks very much for all of your replys. I am learning so much. Really really great stuff
I realize i did leave out dBV when refrenceing -10. My mistake.

So so far everybody votes setting input of adc to +4dBu. I'm not a big fan of using attenuators on -2 dBu console feeding -10 input on 002 and adc becuse i have a lot of outboard gear that's +4 as well as all my outputs during mixdown. What sort of differences in performance are there between +4dBu and -10dBV? Are attenuators more transparent than transformers?My guess is yes. I havn't thought to much about running everything at -10dBV using attenuators. Any opinions on this?

To be honest I'm a bit worried about the extra noise that will be introduced with the level missmatch. I should have mentioned before that I do a lot of recordings with over 50 tracks and as many as 80 (thanks to quadzillas), compressing the hell out of room mics and such as well as some classical and acustic stuff. 6 dbs or so of noise per track really adds up when you get up there in track count.

Any comments on the transformer option. Am I way off here. Would the transformer option on the direct out of the DDA be able to step up level from -2dBu to +4dBu with the right transformer? Or is the transformer option only for sound perposes?

Posted: Thu Feb 15, 2007 4:00 pm

rambelam wrote:

To be honest I'm a bit worried about the extra noise that will be introduced with the level missmatch.

Any comments on the transformer option. Am I way off here. Would the transformer option on the direct out of the DDA be able to step up level from -2dBu to +4dBu with the right transformer? Or is the transformer option only for sound perposes?

Transformers = potential signal distortion but no thermal noise increase.

Resistor pads = thermal noise increase but no signal distortion.

Posted: Thu Feb 15, 2007 7:39 pm

thanks for clearing that up mises. Mises, when you say "potential distortion" is that to say there are transformers for this aplication that won't effect the signal enough for it to be noticable or negative? If so do you know aproximatly how much will I have to spend for this transormer?

I think i should clarify that the DDA console has an option to add transformers in the mixer itself at the direct out not somthing i would be plugging the dir out into.

I'm at a bit of a stand still not knowing if decent transformers are in my price range or if they're are the way to go.

If someone could fill me in to the performance differences between -10 dBV and +4dBu it would be much apreciated.

soon i will start a new thread with better questions now I have more info

thanks again everyone

Posted: Thu Feb 15, 2007 8:47 pm

rambelam wrote:

thanks for clearing that up mises. Mises, when you say "potential distortion" is that to say there are transformers for this aplication that won't effect the signal enough for it to be noticable or negative? If so do you know aproximatly how much will I have to spend for this transormer?

I think i should clarify that the DDA console has an option to add transformers in the mixer itself at the direct out not somthing i would be plugging the dir out into.

I'm at a bit of a stand still not knowing if decent transformers are in my price range or if they're are the way to go.

Transformer designs for audio, to be honest is not my specialty. I do digital communications theory, not really circuit design... but my knowledge of circuits is probably adequate enough for general advice.

Generally the more desireable transformers for audio are quite expensive (rediculously so... but thats the economics of market value).

I'm not even sure if there are any stock transformers out there specifically designed to convert from the -2dBu level up to +4dBu. If their were... I would buy them and I really wouldnt bother worrying about whether they were astounding quality or not.

Good transformers cost upwards of $120 - $200+ apiece (just from what I've seen)... so you couldn't realistically be putting one of those babies on every single channel on your board... though I suppose you could put it on just a few select channels... depends how many -2dBu outoputs you have.

I would not convert downwards to -10dBV... I'd step up to +4dBu. Just my little opinion on that. If I were going to pursue a route.... thats the way I would go, and I would stick with it... and just buy whatever it takes to make that happen. I think stepping down is not really a great idea unless somebody has a counteropinion on that. Most of your equipment wants to operate in that region anyway... so thats why I say go with that.

Did you mention something about your DDA console has an option for some transformers that you can put on the direct outputs (inside the console)? I would investigate what that costs. Forget about "custom" transformers.... Don't go there unless you want to seriously get into that game, and thats just gonna cost mucho bucks.

Posted: Thu Feb 15, 2007 9:51 pm

Your output is -2 at the insert and/or direct outputs largely because, most of the other devices, they intended for you to patch in, have input level controls. So it's not uncommon to take the insert and/or direct output into a compressor or other outboard device at that level.

Gain staging does not keep everything at the same levels. Levels are dropped and boosted throughout the internal signal path of all consoles, recorders, equalizers, etc.. So I think you should do nothing.

Your theories of the loss of signal to noise ratio on the order of +6 DB over 50 to 80 channels does not hold up. Good engineering is good engineering. We managed to do the same thing, all on analog tape without noise reduction for many, many years, with a signal-to-noise ratio far worse than the electronics in your unbalanced DDA console. So, 24 track wasn't just 24 track as we bounced and transferred many things that made some of our tracks, on the multitrack, over three generations down before the mix even started! Automation, noise gates and riding the gain is how we made our stuff sound good.

So again I say, LESS IS MORE and KEEP IT SIMPLE STUPID. You don't need resistive pads to convert to -10 and you certainly don't need transformers unless your audio seems anemic and needs more iron or, a balanced output? Electronics to boost the -2 output level is more electronics you don't need to feed your signal through. The output transformer option for your console is also not completely necessary unless, of course, as you surmise, they included a "step up transformer". It raises the level, when the windings on the secondary are greater than the windings on the primary side (which can also be run in the opposite direction).

API consoles all had transformer outputs that included 3 separate secondary windings. So you had a turns ratio of 1: 1 for a +20 DBm output or, 1: 2 for a +24 DBm output for, 1: 3 or a +28 DBm output drive capability. The other advantage is, as in my API 3124m units utilizes 2 separate isolated outputs from the same transformer. Great to use as a 1 x 2 active splitter or dual isolated outputs.

While Transformers can and do introduce distortion, the largest problem is the low-frequency distortion and the saturation of the core, that occurs with cheap Transformers. Good transformers can actually benefit your audio and/or color it in a pleasing way. Transformers are either specified as low-level microphone or high-level line devices. You wouldn't want to use a line transformer for microphone purposes and vice versa.

If transformers were bad, Neve consoles would never have cut a hit much less a demo. My audio through my Neves go through from 8 to 10 transformers from microphone input to bus output, except in the case of the individual modules which will only force you through 2. And you never heard anybody complain about that.

Tranny heaven
Ms. Remy Ann David

Posted: Fri Feb 16, 2007 5:06 am

If somebody offered me 6dB more headroom at no penalty for most of the type of material I was recording I'd bite his hand off. It's one of the reasons that engineers run the gain trims lower and the faders higher on consoles with limited headroom.

So I would go with driving the +4dBu inputs of the ADC or 002r. You just might consider having a few inline XLR 6dB attenuators in your bag for the rare occasion that you come across very quiet material and need the extra dynamic range. That would be cheaper than splashing out on high-grade 1:2 transformers that you would scarcely ever use.

By the way, the thermal noise of a 600 Ohm resistor is about 129dB below +4dBu over a 20KHz bandwidth, so is not a factor.

Posted: Fri Feb 16, 2007 12:47 pm

Boswell wrote:

If somebody offered me 6dB more headroom at no penalty for most of the type of material I was recording I'd bite his hand off. It's one of the reasons that engineers run the gain trims lower and the faders higher on consoles with limited headroom.

I LOVE all you 'real' engineer types. This is great stuff here. GAFF STICKY THIS!

This Boswell quote I have hi-lighted is a key to gain staging for all you guys and gals with the Mackie/Allen Heath/ Soundcraft(Ghost and below) type of consoles.

I get what I need on my Ghost exactly like this though I do regulate the input voltage at a steady 128 vac. Its within specs so I dont worry about damage per se. The rail voltage comes up a touch and the pres get creamy and punchy while the EQ has an increased bite and the sweep is much more effective.Even surgical if need be.

I gotta agree with Boswell on this entire point. Take the headroom. Buy some outboard(or build) attenuation for needs...Like Remy....dont do nuttin. Your outboard likely has attenuation built into the front ends and makeup gains going out. While you might have to be more astute in your balancing act with this setup, I see no reason it should cause problems in recording materials and certainly with high track counts the added headroom would be to your advantage.

Did I mention that I LOVE you engineer folks?

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