DI, impedances, preamps, phantom, active, passive... what a mess!

[Jevil]

My electronical knowledge is poor (I studied mechanic engineering). So I'm a bit confused about all this shit. When I go to pro-studio, everything gets done and do not worry for these things, but at home is different.

Can anybody post an explanation for retarded people like me?

Or answer these questions.

1. Do I need an active or passive DI Box if I record at home to a soundcard of 9mV to 500mV mic input? No 48 V. phantom preamp. Just TRS "Mic In". Pod Xt works perfect connected to this but have to adjust it's level to minimum.

2. I guess my soundcard (old Terratec) has it's own Mic preamp built-in. It has a knob that let you adjust input level. I can plug my guitar direct into the soundcard and no impedance trouble seem to happen... loading Lepou's plu ings sound awesome... but is this correct or should I use a DI Box?

I could ask many other questions, but don't want to be boring.

 

[Omega_Void]

Is the TRS "mic in" balanced or is it for a stereo input? check your souncard's documentation.

Also check what its input impedance is. Active electronic devices will normally state what minimum load the can be operated into. Just read the manuals.

If it's a stereo jack then a DI box is no help, since the DI output is balanced.

If it's balanced input you'll get cleaner signal from an unbalanced source by using a DI box. There's ones which use a step-down transformer (like 10:1 ratio) for dropping line-level to mic-level signals. This would be useful if you had a synth with unbalanced outputs.

But the PODxt outputs are balanced TRS jacks, so a DI box isn't necessary for recording that.

As for plugging the guitar straight into the card, if it's a passive pickup and it sounds fine then just do it.

 

[Jevil]

Is the TRS "mic in" balanced or is it for a stereo input? check your souncard's documentation.

It is a mono Mic In input. This is the only thing I've found in the manual.

Gain analog: 0 to + 18 dB analog
Attenuation: -45 dB to 0 dB digital
Mic Gain: 9mVolt to 500mVoltl
Please note: the so-called 48 V phantom power supplies occasionally used in the
professional audio sector can not be used with this module.

Also check what its input impedance is. Active electronic devices will normally state what minimum load the can be operated into. Just read the manuals.

No impedance mention.

As for plugging the guitar straight into the card, if it's a passive pickup and it sounds fine then just do it.

Active pickups.


Thanks Omega for your answer!

 

[TheWinterSnow]

From my knowledge and experience, mismatching guitars into lower impednace devices does nothing to the tone but increases the volume (obviously because the circuit will have more current). Most guitar pickups work at a range from about 70-150mV, which means the nominal voltages are fine, you shouldn't have to worry about the use of a transformer to match up levels. And as long as the input to the soundcard matches the correct voltage, the high impedance of your guitar won't play much of a role in the sound since the impedance from a guitar coil has the highest amount of high passing, there will be no subtraction from the tone as the lower impedance by nature has less "resistance" and a smaller rolloff/bandpass

 

[Full_Tilt]

Yeah Ive spent some time worrying about all this kind of thing also especially when it comes to things like balanced unbalanced connections and impedance's .
Some of it is important in certain applications but unless its one of those situations where there's a potential for damaging your gear then let your ears be the judge .
Just keep your input levels at 0 before fading up.

 

[TheWinterSnow]

Just think of this as the rule of thumb, a high impedance source can go into a low impedance device without much trouble as long as you stay in the nominal voltage tolerances of the receiving device. This is how most gain stages work, amplify the signal into a high impedance send it the the next stage of low impedance, High to Low.

Where most applications for a DI are more for voltage leveling though IMO on an electronics technical level you really don't need it and to set a ground loop for balanced to unbalanced devices, if you have an unbalanced output (like a guitar) going into a balanced device (like a recording interface) you will get noise which can be terminated by a ground loop, that trips the ground to the cold terminal or separates them (either way they are both methods of lifting the ground)

 

[Omega_Void]

Just think of this as the rule of thumb, a high impedance source can go into a low impedance device without much trouble as long as you stay in the nominal voltage tolerances of the receiving device. This is how most gain stages work, amplify the signal into a high impedance send it the the next stage of low impedance, High to Low.

Um, no. Typical gain stage has high input impedance and low output impedance. It's necessary for the downstream input Z to be much higher than the output Z of the upstream stage.

I can say this from experience as well. I built a mic-pre (http://sound.westhost.com/project13.htm Rod Elliott's p13, adapted for +/- 9V operation) and the specified load impedance was at least 22k, despite having output impedance of less than 100 Ohms. I tried plugging directly into my souncard, with 10k impedance, just to try. Nothing burned out but it sounded like utter shite. I added an op-amp buffer to present the mic-pre with higher impedance load (either 22k or 47k, I don't offhand recall). Output Z of the opamp is of course very low. Then it sounded very good indeed.

Less impedance = more current, so operating into too low an impedance may cause bad sound as the upstream device fails to provide enough milliamps to develop the correct voltage across the downstream impedance, or device failure as the upstream device passes too much current and heats up enough to suffer damage.

It's usually safe to operate an active circuit into an impedance which is too high, as this reduces the current drawn. But this may also reduce the sound quality. Bipolar transistors get a lot less linear when passing very low signal levels. In general though, the reason to keep input impedances low is to minimise Johnson noise generated in the input resistors.

 

[Omega_Void]

No impedance mention.

That's just shoddy!

Please note: the so-called 48 V phantom power supplies occasionally used in the professional audio sector can not be used with this module.

Sounds like there's no coupling capacitors on the input then! Shouldn't be a problem plugging in commercial products, almost all will have DC-blocking capacitors on the output. A number of DIY designs (like that Tillman JFET guitar preamp) say the output capcitor is "optional" , and there may be a few commercial products which leave it out also. At the least that would cause a dramatic decrease in headroom...

Gain analog: 0 to + 18 dB analog
Attenuation: -45 dB to 0 dB digital

And you can't avoid overdriving the analogue front-end by turning the mic-pre's gain down?

I've never met your souncard and I hate it already.

 

[Metaltastic]

Jevo, just get yourself a BSS AR-133, plug it into the mic preamp on your interface/sound card, and you're guaranteed to have the DI side of things taken care of!

 

[Jevil]

I've never met your souncard and I hate it already.

 

[Jevil]

Jevo, just get yourself a BSS AR-133, plug it into the mic preamp on your interface/sound card, and you're guaranteed to have the DI side of things taken care of!

That's what I'll do. That or any other model... I've seen a H&K that may be okey for 77 €.
Thanks.