Newbie Question about DI boxes and ampheads

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uncle D

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HI looking to record a Fender mustang v 300w head, I have seen some DI box information that makes it look like you can get a clean (drum and vocal free recording) with a di box and I want the amp effects more that the cab sound. I will be recording to a Tascam dp24-sd. My question is---is there a di box out there that will take the out put of the head with out blowing something in the head, the recorder, or the boxitself, that does not cost and arm and a leg. I am not really a fan of micing up the speaker.

Thanks for your imput
 
Many DIs can handle speaker level input. Generally they'll have a 20-30dB pad that will allow connection to the output of an amplifier.

But isn't that a modeling amp with is own XLR output?
 
The Mustang V is specce'd at 150W into 8 Ohms though it might get close to 300W into 4R. In any case that is somewhat over 30 volts rms. The humble Behringer D120 can handle up to a massive +52dBu, 300volts! So it should be fine for that application.

The DI has a reputation for being noisy but I had one and it seemed ok but at 100W+ noise is hardly a likely issue!

Note that these boxes can ONLY be used with a speaker connected to the amplifier (or a proper load).

You might also want to get a jack plug to two insulated croc clips made up to connect to the speaker or otherwise a simple tap in/splitter box.

Dave.
 
ecc83 said:
You might also want to get a jack plug to two insulated croc clips
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Translated to Yank English: "You might also want to get a 1/4" plug and two insulated alligator clips".

("Jack" is almost any female connector in the US. "Phone jack" can mean an RJ-11 socket or a 1/4" socket depending on context.)
 
Ummm... Most passive DIs have two parallel TS jacks. In from amp, out to speaker, XLR to recorder. What is this alligator clip/splitter thing???

Probably start with the pad at its highest attenuation setting. ;)

Course, if it's got an XLR out...
 
ashcat_lt said:
Ummm... Most passive DIs have two parallel TS jacks. In from amp, out to speaker, XLR to recorder. What is this alligator clip/splitter thing???

Probably start with the pad at its highest attenuation setting. ;)

Course, if it's got an XLR out...
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MOST DIs, passive or active, do not have the necessary attenuation to handle speaker voltages. Peeps must be careful to get the right box.

If a DI box has a properly rated paralleled jack (and the book says you can!) of course use that to "pinch" the speaker signal and scratch the croc lead.

This whole area of signal voltage levels and their nomenclature is a minefield for the noob but it IS the meat and taters of audio and something they fail to learn about at their peril.

Dave.
 
You're right, this side of the power amp is the one place where we can really damage things if we're not reasonably careful. It's really tough to blow things up with line level signals, but speaker levels can be serious.

Transformer DIs used to all be spec'd to do this (and equipped with a proper pad) because it was actually a pretty common thing to do. IDK if I'd trust the cheapest DI I could find on the web to this task.
 
The pad switch on the Whirlwind Director is labeled INST and AMP. The Countryman Type 85 pad switch is labeled Pickup and Speaker (and the 1/4" I/O is labeled INST. and AMP. just to confuse Director users). I've used those on amps more than once.

I recall reasonably beefy PA amps, as in 500W@8ohm, having gain in the mid 30s to low 40s. The DOD 265 Stagehand has a three position 0-20-40dB switch. Rapco's ADB+8 has a 40dB pad. Perhaps I'm oversimplifying it, but that seems like a pretty good match. I'm pretty sure I've used DIs with pads labeled in dB rather than explicitly as speaker inputs on amps as well.
 
500W in 8 Ohms is about 63V. Assuming that's peak-to-peak, a 25db pad gets it down to almost nuts-on +4dbu. 20db sounds like a little less than you might want, but you'll have the transformer step down also, so...
 
ashcat_lt said:
500W in 8 Ohms is about 63V. Assuming that's peak-to-peak, a 25db pad gets it down to almost nuts-on +4dbu. 20db sounds like a little less than you might want, but you'll have the transformer step down also, so...
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Then I would think another 10 or 20 dB more attenuation makes it safe for a input expecting mic level.
 
Most mic pres have a unity gain setting. Set the knob to 0db and go. There's no good reason to attenuate any further. You will add noise on the run to the pre, and that'll get amplified as you try to make up the level.

Or, you know, use an XLRF>TRSM to a line input. :)
 
Something I've always wondered: How does having a DI on the speaker output affect the impedance? I know it really doesn't matter with a solid state amp, but a tube amp is more sensitive to that sort of thing.
 
Farview said:
Something I've always wondered: How does having a DI on the speaker output affect the impedance? I know it really doesn't matter with a solid state amp, but a tube amp is more sensitive to that sort of thing.
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It will do buggerall Jay. The DI box will typically have an impedance of one meg Ohm for an active, at least 100k for a passive. Even a valve amp's Z out will only be a few tens of Ohms shunted by the speaker impedance. So worse case, less than 16 Ohms.

Dave.
 
It's the nature of parallel resistance that as one gets much larger than the other, it has less and less effect on the total, and we normally ignore the bigger one when it gets to be about 10 x the smaller. As Dave says, the DI is more than 10,000x the resistance of the speaker. It's like a pin prick in a fire hose.
 
Awesome, thanks.

I was always wary about that sort of thing. Since I basically stopped playing guitar right about the time the Internet started, it was much easier to say "no" than it was to go to the library and study up on impedance effects on tube circuitry...
 
I seem to recall that the pad lowers the DI's input impedance, but that it's generally still high enough for things that need to be padded down.
 
ecc83 said:
Possibly, but if so that's sloppy engineering!

Dave.
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I can't remember where I got that anyway.

But I just read the spec sheet for the Type 85 and it says in speaker mode it distorts at the equivalent of 1700 watts at 8 ohms.
 
bouldersoundguy said:
I can't remember where I got that anyway.

But I just read the spec sheet for the Type 85 and it says in speaker mode it distorts at the equivalent of 1700 watts at 8 ohms.
Click to expand...

Ha! That's some 116 volts rms or about +43dBu, getting on for lethality that!

Dave.
 
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