Here's a thread for questions regarding Impedance, Signal Levels, D.I., Preamps, Buffers, Cab Simulators, Interfaces, and the like.

Of all the above I know little to nothing. Maybe someone can help me out.

1) I get that some things want to see a high impedance input while other prefer low. What's all that about? Some old amps have different inputs for that, right? Now how do the following sound sources fit into this scale:

• Guitar signal (passive magnetic pickup vs whatever active pickups do),
  Your common microphone types, Keyboard/synth signal,
  Guitar or other preamp,
  Portable recorder output (like my old stereo Tascam DR 07 recorder)

Let's take an example. I want to use some piezo contact mics to record weirdo sounds (or tape it to an acoustic instrument) into my little tascam. I hear piezo contacts sound like crud without a preamp, so I whip up one of these (Barcus Berry 3000 clone) and run it through first. Will that signal play nice with my recorder?

2) What the hell even is a preamp anyway? Very loosely, I get that it is practically any distinct amplifying circuit appearing before the final or power amplifying stage. Maybe. I don't know. Functionally, it seems there are two breeds: shit that has the label "preamp" and the shit that makes your signal the correct whatever it needs to be to do its job. I'm asking about the latter.

Let's say I want to run my guitar signal into a little live mixer (I've got a cheap Mackie Mix 8 for smushing up synth sources) which will then run to my little stereo recorder. Is this where you'd run some sort of Sansamp box? Do you need another box on top of that? What makes the difference between the Sansamp or the other thing and something that merely mimics the circuits of some classic guitar preamp? Note that the Sansamps are supposed to be able to run direct in and sound good into a guitar amp; how is that?

Now, if that all takes care of things but it all sounds a bit off, is that when you pop in a cab simulator?

3) Any decent, high utility Direct In circuits one could DIY without spending a kidney on a transformer, and why would or wouldn't I want to use one now and again?* You probably need a damned nice transformer, don't you? But damnit, I ain't making no Steeley Dan album here. I just want to fuzz out some squirrel chatter or some such nonsense. Anyway, what sort of features/options for such a circuit should one look for?

*I mean, if you have anything to add not covered in the video I liked to.

Anyway, non of this makes any damned sense to me. If you wanna make sense of it, that'd be cool. If you have questions regarding similar things that don't make sense to you, post them here, I guess. Isn't it something how folks can be really smart at somethings yet really dumb at others?

I would highly recommend a copy of of "The Sound Reinforcement Handbook" that Hal-Leonard publishes. It is a wealth of knowledge and Is a must even for audio novices.

Excellent suggestions. Thank you very much.

Mickey242 wrote: Sat Sep 11, 2021 2:08 am I would highly recommend a copy of of "The Sound Reinforcement Handbook" that Hal-Leonard publishes. It is a wealth of knowledge and Is a must even for audio novices.

I second this recommendation.

I was given a copy of this in the 90's and I am grateful.
The guy who gave it to me photocopied it from the version that came with his old Yamaha console.
Ended up buying an actual copy a few years ago just to have a proper one around.

VaticanShotglass wrote: Fri Sep 10, 2021 1:03 am 2) What the hell even is a preamp anyway? Very loosely, I get that it is practically any distinct amplifying circuit appearing before the final or power amplifying stage. Maybe. I don't know. Functionally, it seems there are two breeds: shit that has the label "preamp" and the shit that makes your signal the correct whatever it needs to be to do its job. I'm asking about the latter.

Let's say I want to run my guitar signal into a little live mixer (I've got a cheap Mackie Mix 8 for smushing up synth sources) which will then run to my little stereo recorder. Is this where you'd run some sort of Sansamp box? Do you need another box on top of that? What makes the difference between the Sansamp or the other thing and something that merely mimics the circuits of some classic guitar preamp? Note that the Sansamps are supposed to be able to run direct in and sound good into a guitar amp; how is that?

Now, if that all takes care of things but it all sounds a bit off, is that when you pop in a cab simulator?

A preamp, generally, could be defined as a device that modifies and prepares a signal to be received by a following stage, usually a power amp. In a guitar amp the preamp is taking an instrument level signal and increasing it to a line-level signal*. The power amp then takes a line-level signal and increases it to a speaker-level one.

*the preamp can also serve other functions. tone control is a big one, as is phase inversion in the case of a push-pull amplifier.

Dr Tony Balls wrote: Mon Sep 13, 2021 10:11 amA preamp, generally, could be defined as a device that modifies and prepares a signal to be received by a following stage, usually a power amp. In a guitar amp the preamp is taking an instrument level signal and increasing it to a line-level signal*. The power amp then takes a line-level signal and increases it to a speaker-level one.

*the preamp can also serve other functions. tone control is a big one, as is phase inversion in the case of a push-pull amplifier.

Right. That is what I've always assumed, particularly regarding guitar amps. What I'm more confused about is when you are taking the traditional amplifier out of the picture. Let's say I want to just plug my guitar signal into the mixer to listen to it on a monitor, maybe run a line out from there to a little recorder. Most of my guitar signal are self made tweaked amp-ish overdrives (like those amp-in-a-box pedals).

What sort of circuit or stage is needed in one of those boxes or following it to get it to line-level? Would it make sense to just make a little box with a FET buffer mimicking the input of a SF/BF Fender input through which you could run your standard Fuzz boxes, etc.?

A recording line input wants to see a standard line level, nominally either:
•+4dBu which = 1.228V. “Pro” line level. The dBu (“u” for “unloaded”) scale is referenced to .775V, as dBu effectively superseded dBm (“m” for “milliwatt”) where .775V represented 1 milliwatt of power dissipated in 600-ohm load.
• -10 dBV which = .316V. “Consumer” line level. dBV is referenced to 1V.

To interface your guitar with a line level recording input, you would either use an instrument preamp (SansAmp, Rusty Box, etc etc…there’s a gazillion) plugged directly into the recorder input from an appropriate line level output on the preamp, or you’d use a DI (active or passive) to connect your instrument to a microphone preamp (console or outboard) and then connect the mic preamp’s output to o the recorder.

You could probably use your amp-in-a-box pedals with some success depending on your recorder, but keep in mind that while +4dBu is nominal “pro” line level, most recording inputs will take a much hotter peak signal, generally well above 20dBu. 20 dBu is 7.75V, and 24dBu is 12.3V…dB scales are logarithmic and so will increase very quickly.

The simplest way IMO to hear your guitar chain through your mixer/monitor setup is to plug the guitar signal chain into a passive DI connected to a mic input on the mixer.

Hey, thanks! That is all very helpful. Sorry for the late reply, I've not had time to be online lately.

Adam P wrote: Tue Sep 14, 2021 8:05 am A recording line input wants to see a standard line level, nominally either:
•+4dBu which = 1.228V. “Pro” line level. The dBu (“u” for “unloaded”) scale is referenced to .775V, as dBu effectively superseded dBm (“m” for “milliwatt”) where .775V represented 1 milliwatt of power dissipated in 600-ohm load.
• -10 dBV which = .316V. “Consumer” line level. dBV is referenced to 1V.

To interface your guitar with a line level recording input, you would either use an instrument preamp (SansAmp, Rusty Box, etc etc…there’s a gazillion) plugged directly into the recorder input from an appropriate line level output on the preamp, or you’d use a DI (active or passive) to connect your instrument to a microphone preamp (console or outboard) and then connect the mic preamp’s output to o the recorder.

You could probably use your amp-in-a-box pedals with some success depending on your recorder, but keep in mind that while +4dBu is nominal “pro” line level, most recording inputs will take a much hotter peak signal, generally well above 20dBu. 20 dBu is 7.75V, and 24dBu is 12.3V…dB scales are logarithmic and so will increase very quickly.

The simplest way IMO to hear your guitar chain through your mixer/monitor setup is to plug the guitar signal chain into a passive DI connected to a mic input on the mixer.

No problem. Everything I mentioned assumes you’re familiar with the decibel, which is a bit presumptuous, so:

A decibel (dB) is a way to express the ratio of two power values, using a logarithmic scale. So it can be looked at as an expression of gain (+) or attenuation (-).

The dB value will be the same magnitude for a pair of power values, with + or - depending on which value is considered first. In other words, going from 10w to 100w is +10dB, whereas going from 100w to 10w is -10dB.

You calculate dB by taking the base-10 logarithm (usually shown on a calculator as log10) of the ratio, and multiplying the result by 10: log10(100w/10w) * 10 = log10(10) * 10 = 1 * 10 = 10dB.

You can also use dB to express voltage gain instead of power, simply by taking the log10 of the ratio of the voltage values and multiplying by 20. Happy to explain why 20 instead of 10 if you’re interested in the math.

bishopdante wrote: Sun Sep 26, 2021 2:47 am Really, . . .

Thanks! I'm bookmarking this stuff for the nights I get to plan out projects.

Right now, related projects I have in the pipeline include a little contact mic pre amp and DIY contact mics. Might make it a stereo deal to run two mics at once into my stereo portable recorder. Plan to bang on some fence wires and lumber and stick them to a washing machine, you know, standard stuff to figure out how they work.

Then it'll be one of those instrument to line level deals like you mentioned. That should do me for utility boxes for now until I plan out a nice parallel pedal looper.