is a re amp box needed?

[dmmi]

Why couldn't someone measure with a multimeter Voltage and resistance:
- Guitar jack output
- interface output
- re-amp output.

I think that would put all arguments to rest.

 

[MarcusGHedwig]

Regardless of any theory behind impedance matching or loading or whatnot, even if you lower the fader to get a reasonable level, the fact remains that the signal coming out of an interface is still way lower impedance than the signal directly outputting from a guitar, and isn't the point of re-amping to mimic the sound of the guitar --> amp chain as closely as possible? Don't expect me to bail on my Redeye any time soon

 

[TheWinterSnow]

I just wanted to make a few corrections :

"impedance matching does not mean that you are matching the load to the output impedance"

Actually it does, that is the definition of impedance matching. The long winded definition of impedance matching to an electronic engineer ( such as myself !! ) is that the load impedance is equal to the complex conjugate of the source impedance. If there is no reactive component to the source/load impedance then the load impedance will equal the source impedance. The correct term you were searching for was "impedance bridging".

"The only thing that a reamp box is doing, its not matching impedance, its mirroring the impedance by changing the voltage to current (decreasing voltage, increasing current)."

No it isn't. The input current flowing into the guitar amp is determined by ohms law just like every electronic circuit.

"a reamp box is doing nothing more than attenuating the input volume to that "SIMILAR" to pickup, but since you don't know the actual output of your pickup, it doesn't really matter."

Actually it does really matter because the input stage of a guitar amplifier was designed for the signal level of a typical pickup.

"The buffer acts as a current amplifier, giving the voltage the kinetic energy it needs to drive into the next stage."

No, a buffer acts as a driver than can source high output currents. A current amplifier is a device that receives a current at the input and applies gain to it to produce an output current. In addition, the term kinetic energy is a term from kinematics not electronic engineering. One Volt is a measure of electrical potential measured in Joules per coulomb while kinetic energy is related to the mass and velocity of a moving body.

"pulling the volume down won't degrade the signal like an A/D conversion goes, as the output voltage from a D/A conversion will be the same, the allocation of output voltages will be transposed across the matrix so to speak,"

Yes it will. If you reduce the range of the digital codes going into a DAC then you reduce your effective resolution. What is this matrix that you speak of because it has nothing to do with how a Delta-sigma DAC operates.

"even though line level is massively higher in voltage than a guitar pickups output, I think its a current to voltage relationship that is important to keep."

The relationship between voltage and current at the input stage of the amp is determined by ohms law so if the amp sees a voltage higher than a pickup then the current will be higher than a pickup.

Summary
--------
You should use a reamp box instead of using the line out because this will ensure that the input stage of the amp is
receiving the level that amp designers intended. If you don't use a reamp box then it may still work on YOUR amp but
it will not work properly with ANY amp in general.

way beyond the scope of the situation, other than the fact most of the stuff you called wrong was corrected only with a different way to say what I already said.

just a couple things, matching simple at a voltage divider point of view, is the resistance from the output to ground, a load which will be in parallel will have a loading effect. To mitigate that by a marginal number, the input impedance (or load) is recommended to be AT LEAST 10 times larger than the outputs impedance. If the output impedance was equal to the load, the effective impedance will be dropped in half, so, literally, having the output impedance of one stage equal the input impedance of the next stage would actually be a mismatch causing an incorrect lower voltage and increased current on the DC parameter. The input impedance must be at least 10 times (preferably 100 times greater if possible) than the output impedance of the previous stage, that is a match.

Volumes of a typical guitar for a tube amp (which is a voltage controlled amplifier) are specified around the output voltage of a guitar pickup. The output volume of an interface can be trimmed to the appropriate volume.

A typical buffer is a common collector/drain amplifier and the have a unity voltage gain with a current gain >1, pretty self explanatory. Kinematics, Voltage - Potential Energy (That is what is is sometimes referred to potential voltage) and well you can't have a potential source without having a kinetic force (aka current).

Yes Kinetics is apart of physics and kinematic equations, however, the volt and resistance measurement are all based on equivalent forms of releasing energy into force. Kinematics from the point of Issac Newton's discoveries on physics where the basic laws that transposed a system to measure electrical increments with. Hence the conversion from Force, to Joules, to Coulombs and essentially, voltage (potential electromotive force) and current.

Lets not even go into DACs as the only thing we need to know is that for every bit in the system there is a specified output voltage that never changes. As long as the volume is not attenuated to the point of bit squashing, where essentially you ran out bits to reproduce the signal, the signals degradation will be minor. To the severity that has been talked about, bringing your faders below unity in the mixing process would hurt the quality of your recordings because you are compression the bit rate. The only noticeable difference in lowering the DAW volume would be a higher noise floor, thats where the calibration of DAW volume and amp gain is needed, however eve a simple pot in between can easily help.

And with the voltage to current relationship, I was referring to the proportional difference on the secondary coil on a transformer, as even with a reamp box the power dissipation will not change, so essentially while you have the same output power, you proportionally changed the voltage and current relationship to maintain equal power.

Regardless of any theory behind impedance matching or loading or whatnot, even if you lower the fader to get a reasonable level, the fact remains that the signal coming out of an interface is still way lower impedance than the signal directly outputting from a guitar, and isn't the point of re-amping to mimic the sound of the guitar --> amp chain as closely as possible? Don't expect me to bail on my Redeye any time soon

for correct impedance matching the input impedance of the next stage needs to load the previous stages output much greater to prevent a loading effect. Thats all impedance matching is doing, is ensuring that all the stages of amplifiers are operating within a %10 current a voltage levels of being unloaded. So having a lower output impedance from the interface to the amp prevents the effective impedance from significantly dropping the voltage and increasing the current from the previous stage. Its not the amp that is getting effected, as it is so high in impedance nothing will change how it will operate, its the amp that changes how the other devices that plug into it work. At the end of the day, the only thing you are doing is matching up the interface to give you the same voltage that your guitar pickup does (or within the ballpark) and be high enough of a load to make the interface operate correctly.

 

[deLuther]

I tnink that reamp box is good thing because it provides transformer isolation (not active reampers) and allows usage of long balanced cables.
According to impedance matching it seems that reamp box is not so good because of 1:1 transformers (at least according to Jensen schematics).
From my experience with Radial ProRMP, soundwise it is similar to ghetto (direct from interface) reamping, but less noise, maybe due to transformer isolation.

Here is results of my experiments with ProRMP, including comparison clips of ghetto reamping vs ProRMP:
http://www.ultimatemetal.com/forum/equipment/568415-radial-prormp-modification.html

topsoul182
You can try Art MP preamp as reamper, slightly better than direct from interface (from my experience), plug interface into balanced input, use unbalanced output (it can be used to send signal to amplifier according to manual). Just try it and judge yourself.

 

[TheWinterSnow]

technically to the same degree a cap will provide isolation for much cheaper.

 

[deLuther]

Can you post some schematics?
Or you mean just some kind of high or low pass filter?
And what with balanced to unbalanced conversion (not at interface output)?

 

[TheWinterSnow]

All I was saying is that a Cap is a coupling device to isolate low frequency signals like DC the same way that a Transformer does, both can be used but a transformer is really only used as a coupling device in RF devices.

the output to the guitar would would have the the ground through a capacitor then connected to the cold terminal, that would defeat the ground from a DC point of view, but not the AC and from the amp, the cold and ground are the same point (0v) where the cold should be to begin with.

 

[ScootBaloo]

Den, in my post I wasn't really refuting what you were saying but I had some issues with the language you used to describe it.

 

[TheWinterSnow]

well we are in different countries, I know the EE terminology and the way of teaching is a bit different. Even over here terminologies change from EE to EE depending on how long they have been around, the old guys are doing things differnt from the more updated way, regardless of which is more correct or not, since that is more of a personal opinion than anything.

When I am at a loss for explaining, I usually hit my books and that consists of the majority of what I say, paraphrasing of course, but really its coming right out of a published book.

 

[Let it burn...]

This whole thread just saved my 180 EUR as I was about to buy an X-amp . It's really eye-opening. Thanks for all the contributions guys, this is why I love this forum!