Topic: attenuator help?

[faninor]

I'd like to find an attenuator cable to allow me to record line level signals using my Tascam PR-10 without worrying about overloading the input on this recorder. I'm a little uncertain what amount of attenuation is going to be needed for this.

These specs are in the manual for the input on my recorder:

Input impedance:
200 kOhm
Nominal input level:
-19 dBV
Maximum input level:
-3 dBV

I've done some googling and ran across this formula which seems like it might help me:

dB = 20 * log (V1 / V2)

And also came up with this info:

-3 dBV is about .7 volts
USA professional audio gear has peak amplitude around 1.737 volts

So then I need to attenuate 20 * log (1.737 / .7) = 7.89.... let's call it 10 to be safe... dB? Can someone knowledgeable let me know if my assumptions and math is good?

Thanks much!

[DSatz]

Hi. The usual standard line level voltage in the U.S. is +4 dBu, but that is only a nominal level ("0 VU"); peak levels are considerably higher, e.g. +20 dBu or even slightly more, depending on the particular equipment and the signals driving it. So if your recorder can only take -3 dBV before overload, you'll need more attenuation than you seem to expect. To reduce +22 dBu (= 9.76 V) to -3 dBV (= 708 mV) would require 22.8 dB attenuation, for example.

I don't know the recorder, but if it has that sensitive a line-level input then I assume we are talking about a consumer-type input which is unbalanced, so you will also need to unbalance the signals coming from the professional source equipment. Unfortunately, apart from the use of transformers, no one passive method of unbalancing a signal is appropriate for all possible sources. Some hookup methods that are perfectly OK with some sources will damage others or distort their signals; other hookup methods that are perfectly OK with some other sources can cause no signal at all (except for a tiny bit of noisy leakage) to appear in your recorder with other sources.

In professional situations there are two ways that this problem is typically handled. One is with an active interface (often these are built so that the same unit converts two channels of professional/balanced signals to consumer/unbalanced and vice versa), but that typically requires AC powering. The other approach is with a transformer box (a nice example being http://www.jensen-transformers.com/pc2xr.html), which requires no powering and is the cleanest, most general and reliable solution, although the leads from the transformers to the inputs of your consumer device must be kept quite short.

--best regards

[faninor]

Thanks! Very insightful.

Wikipedia gave me bad advice and said the typical peak would be around +7 dBu, although if I had thought about it for a moment instead of rushing to post my question before shutting down my computer at work I would've realized that the nominal and peak levels don't make sense being that close together.

I have 2 goals really:

1. typical common usage -- get a proper attenuator to use this recorder with my DPA 4060 microphones and MMA6000 preamp or other other DPA power supply
2. possible occasional usage -- recording from soundboard if circumstances allow it (though I have another recorder which would be a better fit for this)

The DPA MMA6000 has a max output of 1.5V before clipping, and the output is unbalanced -- so it seems for that purpose I should only need  6.6 dB attenuation, and something like 20 dB attenuation would be overkill when used with my mics. I don't think there's any other catches I need to consider in this scenario, though I'll have to consider my options for the other situation.

Glad to hear I at least found the right formula to figure out the proper attenuation! Even though I was using a bad figure.

[SmokinJoe]

Certainly everything DSatz says is correct.  Nothing is ever as simple as we hope it might be.  Below are simple answers to your original post, which may or may not work correctly in your situation.

Some people use this http://www.m-audio.com/products/en_us/MicroTrack10dBPad.html

Another option is to use a "headphone volume control".  You can get them cheap at RadioShack and walmart.   I've never used one, but I have heard that they work OK if you need it for the occasional time you get a hot board patch.

[faninor]

I used to own one of the M-Audio 10dB pads, when I owned a Microtrack but before I switched to using the 1/4" inputs on it. I found plugging it directly into the input was too likely to cause problems from sideways pressure to the pad / connector. I had to use the pad as a bridge between two cables, but at that point there were 4 connection points between preamp and recorder which is too much in stealth situations. I want to avoid that again, and the units I'm working with now are about the size of my early gen iPod Nano... way too small for it to be convenient to plug the M-Audio pad directly in. It seemed like a quality part, but with my usage it caused more new problems than it solved.

[mattmiller]

Another option is to use a "headphone volume control".  You can get them cheap at RadioShack and walmart.   I've never used one, but I have heard that they work OK if you need it for the occasional time you get a hot board patch.

I've been using the Microphone Madness 20 dB attenuator cable for a while now and it's worked out fine.  But there is a limitation that I would like to address before it becomes a problem.  I often don't know if I need the attenuator or not, and if I would choose incorrectly and have to either insert or remove the cable once the show starts, I have to break the signal for a few seconds.  I've read nothing but bad things about these cheap adjustable volume control cables, so I don't want to touch one of those.  But does anybody make a GOOD one that is adjustable between, say, 0 and 20 dB attenuation?  And I guess for complete peace of mind, it'd be ideal of the dial could be locked in some way, since I don't always babysit the SBD feed and I wouldn't want a passerby to bump it and have it adjust itself.  If this isn't readily available, is it something that one of our cable-makers could make?  I don't have an unlimited budget, but would be interested in getting a price on something like this.