jedblue
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All the decibel and voltage stuff going on the the now closed thread about analog v. digital got me thinking about what was going on inside my BR1600. I knew it didn't have a +4dbu gain structure because it couldn't handle the output of my Revox PR99 tape recorder into any of it's inputs without a lot of gain reduction on the Revox's output pots. So I wondered what was going on with it and exactly where it would clip. The manual say's it's -10dbu into it's line in's and that's about it. Given that I've got all this test equipment for calibrating tape recorders I thought I'd run through the BR1600 and see what it does. I've used a wave generator, a voltmeter, and oscilloscope and this level conversion chart I've showed before...
http://hux.com.au/Soapbox%20Items/World%20Audio%20level%20Reference.pdf
So, set up the wave generator to send out a -10dbm signal @ 1kHz...
Input it to the BR1600 channel 1 and see what it shows up on the BR1600's input metering...
-10db, just as they said it would.
Set the master input attenuation to zero and the master output fader to maximum and see what -10db shows up as on the channel metering..
Hmmm. -4 of whatever the -4's are? Check the conversion chart and realize that there is no standard that equates -10dbm (0.245 volts) to -4 whatevers. The BR1600's channel metering is just some abitrary scale they have decided on. It is not dbm's (or u's or v's or even db's).
Check the volt meter to see what the BR1600 is outputing through it's line out jacks at those settings...
Hmmm, -9dbm, it's picked up +1dbm of gain in there somewhere with -10dbm input signal, zero input attentuation and maximum master fader.
Adjust the master input attenuator a bit to read -10dbm on the line output volt meter when a -10dbm signal is input to the BR1600 with maximum master fader and check that there is a wave form on the oscilloscope...
Good, input and output is now syncronized at -10dbm.
Push the wave generator output voltage up until the BR1600 shows a clipped waveform...
Back it off until the waveform is just about to clip and check what dbm level that equates to...
Hmm... -6dbm, that's only +4dbm above the -10dbm calibration.
Check to see what the BR1600's input metering is saying...
It say's -7db, near enough to the voltmeter's -6dbm for this exercise.
Check that the BR1600's clip light is on at this level..
Yep
Conclusion
1. It doesn't have a whole lot of headroom before it clips - +4dbm above -10dbm when the input and output levels are syncronised to -10dbm.
2. That is 0.388 volts or -30dbfs on the SMPTE RP155 scale.
3. It's channel metering is some arbitrary scale that does not conform to any recognised standard.
4. During operation of the BR1600, don't push the input levels and be conservative on the master input attenuation & channel and output faders or it could easily clip.
4. If I'm going to feed it tape output from my +4dbm calibrated tape recorders, I'd better use the Otari over the Revox because it is switchable between +4dbm and -10dbm but I'll still have to attenuate it back so that the tape recorder doesn't output a signal greater than +4dbm over -10dbm or it'll clip the BR1600.
What a nerdy exercise that was.
I might go through it's mic pre's tomorrow and see how they behave.
http://hux.com.au/Soapbox%20Items/World%20Audio%20level%20Reference.pdf
So, set up the wave generator to send out a -10dbm signal @ 1kHz...
Input it to the BR1600 channel 1 and see what it shows up on the BR1600's input metering...
-10db, just as they said it would.
Set the master input attenuation to zero and the master output fader to maximum and see what -10db shows up as on the channel metering..
Hmmm. -4 of whatever the -4's are? Check the conversion chart and realize that there is no standard that equates -10dbm (0.245 volts) to -4 whatevers. The BR1600's channel metering is just some abitrary scale they have decided on. It is not dbm's (or u's or v's or even db's).
Check the volt meter to see what the BR1600 is outputing through it's line out jacks at those settings...
Hmmm, -9dbm, it's picked up +1dbm of gain in there somewhere with -10dbm input signal, zero input attentuation and maximum master fader.
Adjust the master input attenuator a bit to read -10dbm on the line output volt meter when a -10dbm signal is input to the BR1600 with maximum master fader and check that there is a wave form on the oscilloscope...
Good, input and output is now syncronized at -10dbm.
Push the wave generator output voltage up until the BR1600 shows a clipped waveform...
Back it off until the waveform is just about to clip and check what dbm level that equates to...
Hmm... -6dbm, that's only +4dbm above the -10dbm calibration.
Check to see what the BR1600's input metering is saying...
It say's -7db, near enough to the voltmeter's -6dbm for this exercise.
Check that the BR1600's clip light is on at this level..
Yep
Conclusion
1. It doesn't have a whole lot of headroom before it clips - +4dbm above -10dbm when the input and output levels are syncronised to -10dbm.
2. That is 0.388 volts or -30dbfs on the SMPTE RP155 scale.
3. It's channel metering is some arbitrary scale that does not conform to any recognised standard.
4. During operation of the BR1600, don't push the input levels and be conservative on the master input attenuation & channel and output faders or it could easily clip.
4. If I'm going to feed it tape output from my +4dbm calibrated tape recorders, I'd better use the Otari over the Revox because it is switchable between +4dbm and -10dbm but I'll still have to attenuate it back so that the tape recorder doesn't output a signal greater than +4dbm over -10dbm or it'll clip the BR1600.
What a nerdy exercise that was.
I might go through it's mic pre's tomorrow and see how they behave.
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