+4 dBu to -3dBv conversion - diyAudio
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Old 25th March 2009, 07:39 PM   #1
Cambo is offline Cambo  Canada
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Default +4 dBu to -3dBv conversion

I have a technical conundrum and was wondering if somebody out there could help solve it for me. I have an audio device (tube preamp) whose output is the normal +4dBu running into an amplifier (B&O Icepower) whose optimal input level is -3dBv. My preamp can cause severe distortion and clipping in the amp, and the final level control is SO touchy it's ridiculous - is there an easy way to convert a +4dBu output to -3dBv? Is it just resistors and stuff, can I make a little passive box or converter cable, or is it WAY more complicated that that, requiring power, transformers, heat-sinks, etc?

This is from the preamp makers website:

Output: +4 dBu corresponds to 0 VU. The output is electronically balanced or unbalanced. Output impedance is 75 ohms. The recommended output load is 600 ohms or more. Maximum input level is +19 dB

From the Amp manual:

The input sensitivity on the amp is .70 volts (-3 dbv) for full power. Input impedance is 39K ohms.

Thanks so much. What I need is some simple plans for a converter - this is quite a high-end electric bass system, and I want something that sounds really good, or I should say does not detract from the sound or adds any noise or colour of it's own - it is all in a rack so I don't mind a transformer in a box or something like that, although simpler would be better. I am running the system balanced with xlr connectors. They are Summit Audio pieces, the 2BA Preamp and the TLA 50 compressor, and sound absolutely exquisite, so I'd rather not do any internal modifications, just a converter box or cable if possible. Someone said I may need a step-down transformer...

Right now, the last volume control on the TLA 50 is SO touchy you wouldn't believe it - but I need to convert the input level going to the amp.

I've been getting the runaround for months on this from the preamp maker and just want to get it solved. Any help would be extremely appreciated.


Cheers,
Cambo
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Old 26th March 2009, 11:45 AM   #2
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Those figures should match without any problems, doesn't your power amp have a level control on it?.

If you're getting high distortion then I would suggest you're providing greatly more than +4dBu from the preamp.

I would suggest measuring the output levels, with either a scope or AC millivoltmeter.

But all that's required is a simple attenuator to drop the levels, although I would suggest that finding out why it's too high in the first place would be a better idea.
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Old 26th March 2009, 11:58 AM   #3
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it sounds like the maker of the preamp is using "dbu" as we use "dbv". since in some parts of the world, "U" is the symbol for Volts, it can cause some confusion, expecially if you're used to seeing "dbu" meaning dbuV (db referenced to microvolts). you need to build a 6db pad, or maybe an adjustable one.
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Old 26th March 2009, 12:19 PM   #4
sangram is offline sangram  India
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Is the amplifier hanging off one leg of the balanced connection, or is it also being run in balanced mode?

A step-down attenuator, resistive or with a transformer will work. It's not unusual for pro gear to overload consumer-type inputs of -10dBV, though -3dBV inputs like yours should not have such severe issues. A simple resistive divider will get you there, and it's pretty easy to knock together.

You need a ratio of 1.7:1 for the conversion. I'm pretty sure there will be some transformers around that will downconvert the +4 to -3 level as well, there are plenty for -10 but there may be some for -3 as well. A typical Jensen product that does this is here:

http://www.jensen-transformers.com/pc2xr.html

Transformers do have some low frequency distortion issues, so maybe resistors are the way to go for you. It doesn't look like you'll have impedance issues with a 15K/11K combination (this will get you in the ballpark for a 1.7:1 reduction with the 39K input impedance of your amp considered). You'll need one divider for each leg of a balanced run.

If indeed you are hanging the ICEPower off one leg of a balanced output (or even if it does that internally) there is a 6dB increase in gain and reduction of noise floor. A dedicated summing circuit will remove common-mode noise and bring the gain under control as well.
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