from: http://www.harmoniccycle....sic-26-+4dBu-10dBV.htm

"Introduction

  This is an explanation of how to compare analog sound levels associated with +4dBu professional audio gear and -10dBV consumer or home entertainment audio equipment.

  Professional analog audio electronics gear is often interconnected with cables. The input and output connections are often times described as designed for nominal +4dBu signal levels.

  Consumer and Home Entertainment analog audio equipment is also often interconnected with cables. The input and output connections are usually described as designed for nominal -10dBV signal levels.

Nominal signal level

  The term nominal level describes the audio signal level which any particluar piece of electronics is designed to work with. The analog sound signal is AC voltage. The signal level can be measured on decibel meters that are calibrated to AC voltage levels. Decibel meters are used in an attempt to mimic the nature of sound as its energy level must increase or decrease exponetially for it to be percieved as doubling or halving in listening level.

  When connecting professional and entertainment grade gear it is helpful to know how the nominal signal levels compare.

  The AC voltage difference, in decibels, between +4dBu and -10dBV nominal signal levels is 11.79dB.

  At first glance it may seem there is 14dB difference between +4dBu and -10dBV nominal signal levels but this is not the case. The 11.79dB difference between nominal +4dBu and -10dBV signal levels is evident when the different unit scale values are both referenced to AC voltage.

Unit scale values

  The unit names "dBu", "dBv", "dBV", and "dBm" all have case sensitive spellings and have specific defintions regarding audio signal levels.
Professional analog gear is measured with dBu and dBv and dBm unit scales.

  For example;
0dBu = 0.775 volts AC with an untermninated load.
0dBv = 0.775 volts AC into a 600ohm load.
0dBm = 1milliwatt at 0.775 volts AC into a 600ohm load.

  Consumer and Home Entertainment analog gear is measured on a dBV unit scale. The opper case "V" indicates a different unit scale than the professional lower case "v" scale..

  For example;
0dBV = 1 volt AC regardless of impedance load.

  dBu, dBv, and dBm are considered somewhat interchangeable but, as mentioned above, The use of the units dBv and dBm suggests that the voltage is measured across a 600ohm load. There is very little 600 ohm gear made today, for a variety of reasons, most gear has a higher input impedance. The use of dBm and dBv has been deprecated and replaced by the dBu unit which is measured regardless of impedance.

Compare the volts

  When +4dBu or -10dBV values are reconciled to the other's scale and the actual difference in voltage is compared you can see that there is a 11.79dB difference in voltage levels.


  for example;

-10dBV = 0.316 volts AC

0.316 volts AC = -7.79dBu

-10dBV = -7.79dBu

+4dBu - (-7.79dBu) = 11.79dB difference

in other words, a -10dBv signal is 11.79dB lower than +4dBu



  or for example;

+4dBu = 1.23 volts AC

1.23 volts AC = 1.79dBV

+4dBu = 1.79dBV

1.79dBV - (-10dBV) = 11.79dB difference

in other words, a +4dBu signal is 11.79dB greater than -10dBV

Why -10dBv?

  The history of -10dBV is simply that old hi-fi gear was made to have lower output than professional gear as a cost saving measure.

  When a pro item has a -10dBV input option the intention is to use the gain in the pro item rather than forcing the user to over drive their -10dBV gear into a input that expects a +4dBu signals. The idea is that you use the semi pro gear in it's sweet spot and the pro gear has enough gain to handle the rest.

  An obvious question is to ask whether the *pro* gear is optimized for +4dBu or -10dBV. If the former, it will have adequate head room above and beyond +4dBu. Perhaps +26dBu max or 22dB headroom. If the *pro* gear is optimized for -10dBV then the +4dBu input may simply be padded to bring down the pro grade levels to the semi pro region. This would be done for cost cutting reasons and the headroom above +4dBu will generally be well below 22dB in those devices.

  The only reason to use a -10dBV input is if your gear only works at -10dBV standard levels. In all cases connecting a -10dBv output to the input of +4dBU gear will increase the noise floor... if you do so and need to gain up to +4dBu nominal levels it is often cleaner to do it in the pro side of the connection.

  If you connect a +4dBu output to a -10dBv input then you are sending a hot signal over the line and this will minimize the noise floor but it will also be hot and may overload the input. If you pad it down you may reduce the noise floor a bit but the -10dBV input will have to rebuild the gain if it goes further to another +4dBu device and the noise will likely return at the next stage.

Stuff that is good to know about +4dBu and -10dBV

  If you plug a +4dBu output into a -10dBV input the signal is coming in 11.79dB hotter than the gear was designed for... turn something down.

  If you plug a -10dBV output into a +4dBu input the signal is coming in 11.79dB quieter than the gear was designed for... turn something up."

Good information Mike! At least this is correlating into the real world experience I'm seeing. Unfortunately until today, I thought the opposite in regards to the effect of -10 to +4 and vice versa.

Thanks!

Jeff Evans


Hi Dave. Well I downloaded the manual for this unit and it looks pretty good actually. It says the nominal output is supposed to be +10 dBu and a maximum output of +20 dBu. Now these seem like pretty decent output levels so one could expect to be able to get a decent level going into your soundcard even with the +4dBu input setting which is what I would definitely use in this case. I also see that even the high Z unbalanced outs are sporting exactly the same levels as well.

I see there is no separate output level control so I assume you get this output level by setting the input gain accordingly. Usually when there are balances output connectors on anything the levels there are +4dBu. I assume you are leaving the Hi and Lo controls set flat.

So what happens when you set your soundcard to +4dBu (input) and just keep increasing the input gain until the desired level is reached going into the Delta. The input pad should also be OFF. Is it BTW? If the PAD is on then your output level is going to drop 20dB hence the nominal output goes from +10dBu down to -10dbu which could explain things. Under normal conditions the Peavey should easily drive  +4dBu level nicely into the Delta without any distortion as +4dBu is already 6dB down on the nominal output level (+10dBu) so it should be easy to get there.

If your PAD is OFF as it may well be it might be good to have the Peavey checked out. Sounds like it is working but for some reason your output levels are way low hence the reason you have to go down to -10 on the Delta input in order to get anything going in properly. You should be blasting the Delta even on the +4dBu input setting easily.

Thanks for looking into this Jeff. Yes this unit is well built and performs well. It has almost a cult following that I fully understand. It has a Neve like sound except a little more color that that appeals to some. But it suits my vocal well. I will probably hang on to it until it puffs out green smoke.

To get to some of your points.


I do have the PAD off. I may experiment that to be see the effect, but I will assume what you're reading is correct and it should be left off.


The eq section is labeled High and Low and I do use those to enhance the sound. Again, I can hit the bypass and try some configurations without introducing the eq section.


What happens when I have the soundcard set to +4 is I end up pushing the gain knob up to around 8 on and still do not have quite the gain I need when recording. Once the gain knob is set past 7, the VMP2 starts to add more harmonics (nice word for distortion) than I want for vocals. It is great for bass or acoustic guitar. But not clean enough for the vocal sound I like (warm, round, clear). But even with those higher settings of gain and beyond, the recording levels are weak ~30 db.

When the soundcard is set at -10, I set the VMP gain knob to around 4 - sweet full clear sounding...

I use the 1/4" unbalanced out into the delta. This is highly recommended by several including Jim Williams who mods these and I found one of the original engineers who also recommends this for a cleaner sound that bypasses the output transformer.

Hi Dean,

I'm using an phantom powered AKG 414 B uls plugged direct into the preamp, from there direct to the delta 1010. I use a small soundcraft notepad 124 mixer for monitoring from the delta 1010.

Good point though. For a while I was considering the 414 might be part of the problem, but after adjusting the gains and output settings on the preamp, it sounded great, so I've ruled that out as a problem for now.


Thanks!

Well that is interesting. I get the feeling there is something going on there. It makes you wonder if the unit introduces harmonics (or nice distortion) exactly when in the gain structure and at what output level does that take place. 

I get the feeling that if the Peavey was delivering a nice hefty +4dBu then you should be getting a nice healthy level going into your DAW. Not right down so low as you have indicated. It might be worthy of getting tested. I would put a test tone going in (at mic level) raise the gain slowly and measure the output voltage across pins 2 and 3 of the balanced output. And connect a CRO across the output as well to see the integrity of the waveform. I would expect the output to easily get to +4dBu while the output is still a perfect sinewave (perhaps, depends when harmonics are meant to set in) 

The fact the jacks are bypassing the output transformers sounds feasible for sure. The Peavey has an unbalanced output but the jack inputs on the Delta are balanced TRS INs. How have you wired those. Are you shorting sleeve and ring on the Delta inputs using mono leads. Try connecting the Peavey to the Delta using a balanced female cannon to balanced TRS lead just to rule out any problems that might be occurring using just the jack leads.

-10 is actually -10 dbv not dbu. There is a different reference level involved. The dbV setting relates to 1V as the ref level where as dbU uses the 0.775 v as the ref level. -10 dB V is actually -7.8 dBu just to add to the confusion. +4 dBu always relates to the dBu ref level.

Thanks for that correction, Jeff. You are, of course, absolutely right. Now, as long as we're aiming for accuracy, isn't the "B" supposed to be uppercase, e.g. "dBv"?

A thought: could there be some signal loss due to an impedance mismatch between the 1/4" output of the preamp and the Delta's 1/4" input channel? The transformer output, which the designer intended as the primary output, is going to have a lower output impedance than a 1/4" TS. 

I'd try an experiment, bringing a test tone into the preamp and recording it simultaneously from both the 1/4" and XLR connections. That would tell you right away if there are significant level differences between them, and if you used a white noise test signal you could also compare their spectral content and see if there really is a downside to going through the transformer.

@ Jeff / Bit

I just ordered a couple of custom cables to use/test the balanced outs of the VMP2 per your suggestions. 

Thanks Gentlemen!


  

The manual does not say what the output impedance is but the XLR output is quoted into a 2K load which means the output impedance is lower than that. The unbalanced outs are specified into a 10 K load so looks like unbalanced output impedance is higher than the XLR. But I would be surprised if an impedance mismatch would cause this much voltage drop. It has to be pretty bad before that much level is reduced.

There would not be anything wrong with the transformer outputs either. They would simply contain the sound of the output transformer which would well be nice and should be included in your options for sounds from this pre amp. A transformer sound from the XLR and a cleaner more transparent sound from the unbalanced outs. If it was in a rack I would wire both outs to the patch bay. I would tend to start using the balanced XLR outs from any gear first. It could be argued it is what the manufacturer intended you use to represent the total sound from the unit. 

Balanced XLR (male and female) leads to balanced TRS leads are handy to have around to be able to patch things when and if they come into your studio. In your case the first hookup I would have tried would be the balanced signals connected from the preamp to the Delta. When they put balanced TRS connectors on things I always try to connect from a balanced output. You get an extra 6dB of gain just by using the balanced outputs. 

You make some excellent points Jeff. At one time I used to use the balanced outputs, but there was a A&H mixer in that process which was also adding color - too much. After some research I start using the unbalanced line-outs and stuck with that - even after I had taken the mixer out of the loop. I owe it to myself I think is what you're saying to try that sound as well.

I didn't know about the extra 6db if gain using balanced inputs. I should have known that, but...

Looking forward to the new cables arriving to try the unbalanced outputs.

Thanks Again!