PG Music Home Forums General Discussion Recording, Mixing, Performance and Production Poor vocalist

Perhaps and perhaps not. Here is why

Certain things need to be known. What is the impedance of the input device?

If you want to calculate a load resistor that’s tailored to your preamp/board’s actual resistance, it’s easy to do. The formula is:

1/Rg = 1/Zd – 1/Za

where Rg is the resistor to be used in the Gizmo, Zd is the desired total load impedance, and Za is the actual load impedance of the input.

A Mackie board with XDR Pro preamps has an input impedance of 1300 ohms; plugging the numbers into the equation, we get:

1/Rg = 1/500 – 1/1300 = 0.002 – 0.000769 = 0.00123

Pushing the “1/x” button on the calculator, the answer is:

Rg = 812.5 ohms

The nearest value in the 1% tolerance series of resistors is 806 ohms, so that would be the one to use.

I did not download a schematic for the VTB1 but Sweetwates shows specifications as Switchable Impedance between 50/200 Ohm which would not work for a 600-ohm mic.

I will try to find a variable impedance device or schematic.

A little electrical stuff. A microphone produces an alternating current (AC). So...things get a little more complex as opposed to direct current (DC) like coming out of your flashlight. Impedance is not really easy to understand.

Take a simple circuit, battery connected to a resistor.


The battery pushes current through the resistor, which "resists" this. Thus, the higher the resistance offered, the lower the current through the circuit.

But there are also AC circuits that involve capacitors (C) and inductors (L).


Each of these also contributes to the overall "resistance" offered by the circuit, except this combined "resistance" is called impedance for the simple reason that it involves not just the resistors, but also the capacitors and inductors in the circuit. It "impedes" the flow of current through the circuit. The term reactance is used to refer to the non-resistive component of the capacitors and inductors.

Yes, as I said it is not exactly simple.

If I knew the impedance of your input device and knew the impedance of your mic I could add a resistor into the circuit in five minutes.

It is also possible to change the impedance of you input device but that can cause other issues.

Billy

EDIT: I found this video pretty well explains this subject in an easy-to-understand way. It also shows how to build a variable impedance device for an SM57. I will go build one and check it out.

Re old bell 600 ohms.
there are several variable imp devices that address the problem...eg ?

https://artproaudio.com/product/rp-1-mic-preamp/

specs say imp variable tween 150 ohms – 10k ohms.

useing this would mean also one doesnt need to crank the
pre on the usb audio interface.

see you tube for tests of this type of gizmo made by various manufacturers...down to 40 bucks ive seen them.


om

Here is a short article by Rob Jones about this subject.

https://www.microphone-data.com/media/filestore/articles/Mic%20impedance-10.pdf

Rob Jones deals with technical aspects of
marketing at Focusrite Audio Engineering
Ltd. He is a graduate of the Tonmeister
course, University of Surrey . Once upon a
time he used to be an actor but then found
music to be far more interesting.

A SM57 will work in any interface. How it will sound is the question. Changing the impedance is the easiest way to change how it sounds. Running it through a tube pre will add distortion in the 1st, 2nd, and 3rd harmonic generally considered by most people to add a "warm" sound to the mic. Low-cut filters can be added for cheap giving even more control.

For those of you who are unsure about soldering, it can be learned in a very short time with many instructional videos on youtube. It is really not hard to learn to do. It is a very useful skill if you own a guitar or a microphone as something is going to fail sooner or later.

Billy

There's no such thing as "can't sing."

Singing is a natural part of the human voice. If you can speak, you can sing.

https://youtu.be/58hISyYdt5E

https://youtu.be/RY_dYVTZkfI

Hey Bryon,

Excellent stuff on your site. I particularly liked "Princess Of The Forrest". Very nice work.


Regards,

Bob

The specs on Sweetwater don't tell the full story, the actual impedance used is different than the specs you observed.
From the manual (http://studioprojects.com/pdf/vtb1_manual.pdf)page 3
"Mic Input (Rear switch in 200 ohm position): 2000 ohms
Mic Input (Rear switch in 50 ohm position): 300 ohms"

So with the switch at 50 ohm position, it yields 300 ohm .. not sure why there is a difference in this, which is why I asked.
I am NOT an electrical engineer type guy (I don't even like soldering, except for stuff like copper pipe; my electronics soldering has always ended dismally)
To me the VTB1 makes both my LCD and my SM57 sound better than the Focusrite or other devices.
I was honestly asking if this may be why ..
In other words, my original question wasn't meant to be rhetorical, but rather quizzical in nature so I can learn more, so I asked
.. and yes the VTB1 does have an adjustable amount of tube added (12AX7 that can be replaced with a non-stock one), so it becomes more complicated at that point
It allows hybrid tube/solid state mix

My usual setting is about 40% tube, and I have no idea if this would increase/decrease the end result impedance, like I said it gets complicated, but I do like it and wonder if there is a empirical explanation
Never looked for the schematics, but I'm sure they are out there .. they would mean absolutely nothing to me

Electronic DIY hobbyists are changing out the opamps for "better" ones in the VTB1. I mention this because they are asking for schematics with no response which is very often the case for devices like this.

This is all the technical information I could dig up.

The microphone front end is a solid‑state Class AB circuit, which feeds a TRS jack insert point, allowing additional processing to be inserted into the signal path. According to the circuit description, a balanced, current‑source‑fed parallel transistor arrangement feeds into a bipolar op‑amp. The transistors provide up to 45dB of gain and the op‑amp a further 15dB. Both the mic input and line output are on balanced XLRs. The line switch is on the front panel next to the instrument/line input jack.

A FET is used in the line input circuit, to present a high input impedance when the device is switched to instrument mode. (In this mode, the input impedance is 1.5MΩ.)

Unusually for a preamp in this price range, the input also offers variable input impedance, via a rear‑panel switch, to suit both 50Ω and 200Ω microphones.

The actual input impedance of the VTB1 is 2kΩ in the 200Ω position and 300Ω in the 50Ω position, as it is normal to feed mics into an impedance an order of magnitude greater than their own impedance.

Whatever theory suggests, your best bet is to try both positions for each application and see which sounds best to you; usually, you'll hear a subtle difference at the low end.

This unit is certainly not designed to accommodate all mics but of course, it will work. I think a SM57 is about 310 Ohm actual, rated at 150 Ohms. Most reviews I have read say this is a pretty good device for a low-cost device of this type.

Without a schematic or the actual unit setting here to take apart and reverse engineer, it is impossible for me to say anything more than what I have read from a couple of electronic DIY sites.

The simple answer to all these impedance issues is to buy or build a variable impedance device. Under $50 to buy and around $20 to build yourself.

All the best,

Billy

EDIT: This describes the impedance issues with microphones in an understandable way. https://mynewmicrophone.com/what-is-a-good-microphone-output-impedance-rating/

This may or may not be helpful...

A dynamic microphone uses a diaphragm to capture the air-pressure (sound) waves and convert that into a physical movement that in turn moves a coil in a magnetic field to create an electrical. The system is the exact reverse of a conventional loudspeaker, which changes an electrical signal to air pressure waves.

Amongst the challenges with these processes is getting the mechanical and electrical parts to behave as cleanly as possible, with no resonances to cause peaks or notches in the sound.

The mic/speaker designers will do their best to minimise any such resonances, but there will always be some.

Resonances in this context are like a weights on springs. When you put energy into them they bounce around, and once they start, they can continue for some time unless there is something to take the energy out again.

A car has the same problem and car makers solve it with shock absorbers "damping" the bounce.

In a mic/speaker, the diaphragm, its support and the air around them will form part of that damping. There's also the coil within the microphone that acts as a kind of spring in an electrical sense. There are actually several different little resonances in every such assembly.

One of the few places left after that mechanical damping for a microphone to get rid of the energy in that resonance is to dissipate it into the load resistance, so the load resistance is a significant part of the damping.

The microphone designer will plan that the final resonances will be damped as best they can be with a particular load resistance, but as is obvious, load resistances vary, so all they really can do is to aim for a "best chance" load.

Years and years ago, telephone designers discovered that telephone wire pairs have natural impedance(*) of around 600 Ohms, and as early microphones often had quite long cables from mic to pre-amp, it made sense to use that natural impedance as best one could.

For ideal signal transmission, the three components, mic, cable and pre-amp, would all have the same impedance, but in practice that's not really necessary and it also happens that matching too well is usually not ideal for sound quality.

A kind of consensus formed for a microphone impedance of around 300 Ohms, the cable at around 600 Ohms and the pre-amp around 1500 to 2000 Ohms.

The final piece of the equation is how much of that damping the listener wants. Should the microphone sound bright (lower damping) or dull (higher damping)?

Condenser microphone have their own pre-amp, so little of the above applies to them.

(*)Impedance is approximately analogous to resistance, but accounts for the effects of capacitance and inductance in the circuit.

Thanks,David!