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
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
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).
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).
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.
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.
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.
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?.
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.
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.
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.
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'.
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!
Yes, 500 ohms should match just about any board you want to plug in to. As far as the saturation thing, be aware that since the '80s steel has been rolled differently and it saturates differently (info courtesy of the Navy, of all places).
Thanks Keriwena! I think the UTC ones Im looking at are really old with Nickel or Iron so, Ive heard people talk about using them with great results so Ill give it a go. As for the Input Impedance, am I looking at around 100'00 ohm? or more? Thanks again
So you got nothin', huh? Thought so.
It's not a matter of principle, it's a matter of taste. If you have an input impedance much lower than 1 Meg, say 100K ohms, you lose the resonant peak that makes guitar pickups sound alive. Without it, players will tell you the instrument sounds "dead", or "muffled". You can mess all you want with the pot values in the guitar, but you can't get that resonance back. The input impedance of an amp is determined by the L, not the R and C.
It's a matter of basic electronics principles - taste has nothing to do with it.
No one has suggested 'messing with the pot values', I merely pointed out (which you seem unable to understand?) that the high values of the pots in a guitar means you need a high input impedance to feed it to.
I'm fully aware of the effects of too low an impedance, which is actually what YOU were suggesting.
There is no L on the input of an amplifier - where do you get that idea from?. The absolute main determinate is R, C has little or nothing to do with it.
Personally, I think you look a bit silly bending over backwards so far to pretend you didn't understand anything I said.
But, now it's your turn. Please explain to the class these "principles" you claim mean an 7K inductor needs a 1 Meg load because it has a 250K resistor in parallel with it.
As it was utter gibberish (and you still haven't responded to any of my queries about it - presumably because you can't?).
It's not got a 250K 'resistor' in parallel with it, it's got a potentiometer in parallel with it (assuming no tone control), and the output impedance of the guitar varies with the setting of the pot. If you feed it to a low impedance amp, then the pot no longer works, it will become effectively nothing but a 'switch. This is simple ohms law.
Further of course, if the pot is set to maximum, the low impedance amp will severely load the pickup, resulting in lack of output and poor quality.
This is all VERY simple basic electronics, and shouldn't need explaining on an electronics forum.
Taking your last point first, this is NOT an electronics forum - it's a forum for musicians to ask technical questions. If you have to resort to Ohm's law as an answer, you're in over your head.
Well, the majority of guitar players never turn their volume control down because of 'tone suck', so any appropriate answer needs to apply to a simple resistor. Further, the output impedance of the instrument is a red herring. You stated earlier that "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", yet here we're discussing a difference of 100,000 times higher, and as I mentioned earlier, 10,000 times higher isn't high enough. Obviously, something else is going on.
What happened to your curiosity? At this point, any engineer worth his paycheck should be asking "why?". Instead, when given the (obvious) answer, you dismiss it and barricade yourself behind "electronic principles" which offer no explanation at all. Here's a clue - the best amps were designed by repairmen, not degreed engineers. Leo Fender got his start fixing radios. From there he went to making lap steels and amps, and while developing the guitars and amps that now bear his name, he repeatedly loaned them out to local musicians to get opinions on what really worked. Jim Marshall, a drummer who ran a music shop, borrowed Leo's designs and modified them with locally available parts, again depending on the preferences of the likes of Eric Clapton, Pete Townshend, and Jimi Hendrix to produce some of the most sought after amps ever made. The high priests of guitar amp tone, Messrs Dumble and Fischer, were both repairmen. They were intimately familiar with the state of the art, and in a position to make minor tweaks to the circuits and get direct feedback from musicians as to what sounded good and what didn't.
In another thread, you mentioned a player buying a Les Paul and a Marshall to get Jimmy Page tones. (You seemed to focus on the fact the poor fellow couldn't play very well, but that's not the point. You can't learn to drive like Graham Hill (dating myself, much?) in an SUV. You need a car that won't plow in the corners and preferably can hang it's tail out. Likewise, to learn to play like Jimmy Page, you need a rig that will get the tones he got when you hit the strings. Classical and Jazz music are all about playing the right notes at the right time; rock and roll is all about attitude, and screaming Marshalls are a very big part of that.) The problem, most likely (i.e., unless he'd purchased a recent Historic), was the 300K pots that Gibson has been using since the '70s. To sound like Page, you need 500K pots like Page's 'Burst had, as anyone who knows Les Pauls will tell you. There's no "principle" at work here, it's merely a matter of taste and the burden of history defining what we think of as "good tone".
Guitar/Bass tone is the combination of the pickups, controls, cable, and amp input. They all work together and they all have to be within a very limited range to get "good" tone. Your answers are sufficient to get sound from an instrument, but that's like a soccer mom's SUV providing basic transportation. Musicians are performance freaks, sports car drivers for the sake of the analogy, and simply getting any old sound isn't enough. Thus, the specific parts required to do the job properly and satisfy a musician's requirements can't be found in textbooks.
Once again, to be very clear, the input impedance of a guitar amp is determined by the need to maintain the resonant peak of the inductive pickup (see where the L comes from?). Your claim that the pot's values are involved is spurious, as the pots will function perfectly well with a load of 100,000 ohms, but by then the resonant peak has been damped and the result is lifeless and muffled. As I mentioned earlier, you can "mess" with the pot values all you want at this point, trying to prove that they do indeed make a difference, but they don't. It's the resonance of the pickup that matters and while higher value pots will increase the resonant peak, they can only do that when terminated by a high enough load to produce a peak in the first place. (BTW, if you remove the pots altogether, the pickup alone will still require a load of over 200,000K. I didn't pull "100,000 ohms" out of thin air, I know precisely what I'm talking about.)
Hello Fezzle,
My humble advice would be to consider building an active buffer/preamp. I don't want to wade into a huge discussion about benefits, but I never got any decent sounds out of a passive DI. Killed the sound (highs) too much. Having an inductive pickup source loaded by more inductance from the transformer, to me, never was a great idea. Your input impedance of the buffer should be about 1 Meg. That would be the ''hi-Z'' standard input back in the day. Could be even higher but that value should do.
My humble advice would be to consider building an active buffer/preamp. I don't want to wade into a huge discussion about benefits, but I never got any decent sounds out of a passive DI. Killed the sound (highs) too much. Having an inductive pickup source loaded by more inductance from the transformer, to me, never was a great idea. Your input impedance of the buffer should be about 1 Meg. That would be the ''hi-Z'' standard input back in the day. Could be even higher but that value should do.
It's an electronics forum for DIY'ing musicians.
[quote[
Well, the majority of guitar players never turn their volume control down because of 'tone suck', so any appropriate answer needs to apply to a simple resistor.
[/quote]
I think you obviously have a weird idea of majority'
And according to your posts they were all completely wrong, they also stuck to sound basic principles - they didn't just use random connections of components, they took existing HiFi amplifier circuits and altered them for instrument use.
You can't have it both ways - you claim (incorrectly) that you only need low impedance inputs for a passive guitar, then produce a list of the 'best sounding amplifier makes' of all time that all contradict your claim.
I agree on your point Nigel , they started out modding the basic audio amps of the day..and developed from there. A lot of designs of audio and guitar amps also borrowed quite heavily from the RCA and Mullard tube application /data books issued in the 40s. In a way, they needed to base the early designs from readily available stock, most importantly power and output transformers needed to be something existing ( audio amp components and popular tubes).
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