Impedance matching for DI to Mic Pre - diyAudio
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Old 10th February 2012, 07:31 PM   #1
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Default Impedance matching for DI to Mic Pre

Hi people,

I am in need of some advice on impedance stuff, I'm very new to this stuff so please forgive me if I dont make sense. I have mic preamps that state a 5k input impedance, is there an optimum impedance to input to that for best sound..?
I am trying to get a passive DI box built for Active + Passive bass guitars... does this mean I would need two different transformers..Ive heard an active bass has a very low impedance compared with a passive one.. Ideally I'd like a solution where I could have both options with the best matching for each to my 5k impedance Mic preamp. Any advice would be most appreciated!

Thanks for reading

Dave
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Old 11th February 2012, 10:25 AM   #2
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You need to first be aware of what 'matching' actually means - it's VERY rare for matching to be required (and is mostly a very bad idea), the major exception being loudspeakers on valve/tube amplifiers where matching is essential.

Almost universally you should feed a low impedance output to a high impedance input, with the input needing to be at least five times as high as the output it's fed from.

So a passive DI for a passive guitar will work perfectly well with an active guitar, while it doesn't need to be as high an impedance, it doesn't do any harm. I'm a bit confused why you want to 'have one built'?, they are commonly available and pretty well just a transformer and a few sockets in a box.

However, you should be able to feed an active guitar directly to your mixer, no need for a DI at all - unless you're using it to tap off the guitar signal going to a guitar amp (which I would recommend an active DI for anyway, passive DI's aren't really high enough impedance to not affect the sound).
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Old 11th February 2012, 03:34 PM   #3
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As to active basses having a lower impedance than a passive ones, I'd double check that. There seems to be a similar belief that EMG pickups are low impedance, but they're not. Every spec I've found lists their output at 10K, and that's medium high for a passive guitar. I think people are being fooled by pot values, which have almost no effect on the total.
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Old 11th February 2012, 03:42 PM   #4
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Quote:
Originally Posted by Keriwena View Post
As to active basses having a lower impedance than a passive ones, I'd double check that.
Nothing to double check - passive guitars and basses require input impedances preferably in the mega-ohms, the output impedance of active guitars/basses is in the low kilo-ohms or less, as it's the output of an opamp.

If nothing else, the values of the controls in a passive guitar/bass mean the following amplifier has to have a very high impedance input.
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Old 12th February 2012, 04:56 AM   #5
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Quote:
Originally Posted by Nigel Goodwin View Post
If nothing else, the values of the controls in a passive guitar/bass mean the following amplifier has to have a very high impedance input.
OMG! You're so wrong.



Fezzle, Nigel is right about one thing - since active instruments are designed to plug into amps meant for passive instruments, there's no need for special treatment in terms of impedance.
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Old 12th February 2012, 10:41 AM   #6
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Quote:
Originally Posted by Keriwena View Post
OMG! You're so wrong.
Perhaps you would care to give a reason for your incorrect assumption?, and why you think every manufacturer of bass guitar amplifiers has been totally wrong for over 50 years?.
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Old 12th February 2012, 01:30 PM   #7
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The general rule of thumb is for a passive instrument that an active DI box is a better choice with their generally higher input impedance (couple of hundred thousand to approaching meg ohms depending on make and model) For active preamped instruments a passive DI with basically a transformer works fine and depending on make and model will offer better isolation. Generally a passive DI will have an input impedance in the low hundred thousand ohm range. Some active DI's use a transformer on the output side while having the active input buffer others are active input to output.

Your mic pre amp input impedance looks a little high generally there around a couple of thousand ohms.

As was mentioned the classic 600 ohm to 600 ohm current based matched input / output interface has long since given way to the voltage based low impedance output to high impedance input interface.
A lot of equipment still rates it's output level into 600 ohms I guess for the chance it gets connected to low impeadance input, there are still some out there!


Not to be confused with single ended high impedance in the 10k range.
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Old 12th February 2012, 05:07 PM   #8
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Quote:
Originally Posted by Nigel Goodwin View Post
Perhaps you would care to give a reason for your incorrect assumption?
I guess it's the demeaning, dismissive way you respond to my posts that makes me assume you're a male Chauvinist.

As for the rest, when you contradict me by making an absurd statement like, "the values of the controls in a passive guitar/bass mean the following amplifier has to have a very high impedance input" the burden of proof is on you.
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Old 12th February 2012, 07:25 PM   #9
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Quote:
Originally Posted by Keriwena View Post
As for the rest, when you contradict me by making an absurd statement like, "the values of the controls in a passive guitar/bass mean the following amplifier has to have a very high impedance input" the burden of proof is on you.
So in other words you can't explain what you claim because you don't have any idea of the principles involved, or you wouldn't consider that accurate statement as 'absurd'.
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Old 12th February 2012, 07:31 PM   #10
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so would an ideal impedance out of the direct box be somewhere around 500ohm? The reason I want to build one is I've heard a few old school direct boxes that use UTC transformers that have seem to have a lovely tone quality, Ive been told depending on the core material and when its approaching its max level handling one can get certain qualities of saturation. This of course may be very subtle but nonetheless could bring something to the table!
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