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Cheap in-line attenuation unit? Do such things exist?

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Cheap in-line attenuation unit? Do such things exist?

Postby Magic Matt » Sat Nov 17, 2012 11:34 am

I have an interface (that I may still replace) which last night I tried using with a direct in from the bass pre-amp "Line Out" - there's no adjustment on the interface for volume. Regrettably this was not a sensible move as for large chunks of the recordings it's brick-walled and distorted (any suggestions on how to clean it welcome).

This got me thinking - rather than the expense of replacing the mixing desk etc which doesn't have enough inputs, is there such a device that could go between pre-amp out and interface line in, and just drop the signal by, say, -10db?
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Re: Cheap in-line attenuation unit? Do such things exist?

Postby Mike Stranks » Sat Nov 17, 2012 12:30 pm

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Re: Cheap in-line attenuation unit? Do such things exist?

Postby Magic Matt » Sat Nov 17, 2012 2:42 pm

Thanks!

Why didn't I think to look on CPC...
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Re: Cheap in-line attenuation unit? Do such things exist?

Postby Magic Matt » Sat Nov 17, 2012 3:07 pm

I don't suppose you could check my maths too could you please?

My output is a +4dBu unit
My input is a -10dBV unit

I used this web page to convert both into a peak-to-peak voltage...
http://www.sengpielaudio.com/calculator-db-volt.htm

+4dBu = 3.47232701
-10dBV = 0.894427191

Then I did one over the other to get the gain I need...
0.894427191 / 3.47232701 = 0.25758725731307201967708680755849

I then worked out the voltage gain for the attenuators listed...
-10dB = 0.316228 gain
-15dB = 0.177828 gain

Since -10dB is still a little too much gain (though probably ok) I think I need the -15dB one, then there's a little extra headroom (which going into a 24bit ADC is no bad thing).

Have I got that right?
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Re: Cheap in-line attenuation unit? Do such things exist?

Postby ef37a » Sat Nov 17, 2012 3:47 pm

Commendable tho it is to check for peak to peak voltages, it is perfectly in order to stay with rms! The attenuation needed is 12dB near enough but 15dB would be a better bet for belts and braces headroomwise as you say. I very much doubt that noise will be a problem!

In any even I doubt that bass pre amp output is "proper" balanced? Most likely impedance balanced which banjaxes the calculations somewhat. Me? I would just put a pot in a tin!

You mentioned rescue of the distorted material? Sony Soundforge has a clipper restoration tool and you can get the whole thing for free for 30 days. I would not hold out a lot of hope for a miracle tho!

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Re: Cheap in-line attenuation unit? Do such things exist?

Postby Magic Matt » Sun Nov 18, 2012 11:35 pm

Thanks!

I'll give Sound Forge a go, thanks. Not expecting miracles, but any improvement would be good as it was a pretty good session that night. I'll keep it anyway, but any improvement I can make is going to improve things. I'm happy for a "first attempt" having not used the gear for so long and only having a 30 minute slot to setup everything, while the rest of the band were also setting up.

The pre-amp isn't just a bass pre-amp, it's actually a general purpose mic/instrument pre-amp with an EQ and compressor built in. It has two properly balanced TRS outputs.

The PA system doesn't say, but as far as we know it's a balanced output on a TRS jack. I'm playing it safe - the noise floor on the interface is very good.
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Re: Cheap in-line attenuation unit? Do such things exist?

Postby 4TrackMadman » Sat Nov 24, 2012 9:22 pm

How about just splitting the bass signal before it goes to amp? Sure you won't get amp coloring but honestly when recording bass getting a DI out from the bass amp usually is not as good as miking the amp or DI-ing the bass before it hits the amp. Does the bass amp have fx send/return? You might try going that route as well. Alternately you could try a cheap preamp like http://www.sweetwater.com/store/detail/TubeMP to correct the output levels or even a guitar pedal eq or the Boss Line Selector.
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Re: Cheap in-line attenuation unit? Do such things exist?

Postby Magic Matt » Sun Nov 25, 2012 2:48 am

4TrackMadman wrote:How about just splitting the bass signal before it goes to amp?

That's what I am doing... pre-amp has two outputs, one going to the amp, one going to the recorder. The issue is that the pre-amp output is pro line level, but the recorder is consumer line level. When I made the recording, I didn't know that, and didn't have time to test it.

I don't use the pre-amp section on the bass amp, because I prefer the separate pre-amp sound. I've got the attenuator and cables now so I'll give it a go next time.
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