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Application Notes

Tips on how to get the most out of your D.W. Fearn products

1. Lo-Z Position of the Input switch on the VT-1 or VT-2

2. To Pad or Not to Pad - some guidelines for using the -20 Input position

3. LP-1 Line Pad -- What it can do and cannot do

4. Paralleling Inputs on the VT-3 or VT-I/F Instrument Interface DI

Lo-Z Position of the VT-1 or VT-2 Input Switch

Background

In the early days of "transformerless" output mics, there were some microphone designs that exhibited an extremely low output source impedance. The Schoeps transformerless mics are a good example of this.

Although there is nothing wrong with this type of microphone design, the unusually-low source impedance often produced less-than-optimum results with typical mic preamps. In order to improve compatibility with these mics, an impedance-matching network could be inserted between the mic and the preamp. Both the VT-1 and the VT-2 provide this matching network, which can be selected with the "Lo-Z" position of the Input switch.

Which mics need the Lo-Z position?

Not all transformerless mics require or will benefit from the Lo-Z position. Although I do not have a comprehensive first-hand knowledge of all the transformerless mics, I have either heard or had feedback from VT-2 users that indicates that the majority of current-production transformerless mics do not need the Lo-Z position. Specifically, all of the Neumann TLM-series mics have been designed with the proper source impedance and do not need the Lo-Z position.

However, the sound does change slightly with the Lo-Z position regardless of the source impedance of the mic and you may find that using the Lo-Z position will give you a minor change in the sound character. With 150 ohm mics, the difference is subtle and generally not particularly useful.

Some vintage mics had taps on the output transformer that could be selected to provide (typically) 50, 150, or 200 ohms. Many versions of the RCA 44 and 77 series mics, for example, could be set this way. Microphones used in radio and TV in the U.S. and worldwide were often set to 50 ohms. Re-setting the taps to 150 ohms will almost always improve the sound and operation of these mics. However, the Lo-Z position of the VT-1 or VT-2 mic preamp will probably provide a better match and improved sound if the mic has been set to 50 ohms and you do not (or can't) change it.

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To Pad or Not to Pad

Background

Most consoles and outboard mic preamps feature a switchable 20 dB pad on the input. In the early days of solid-state mic preamps, this was a necessity because the preamps could very easily be driven into an ugly distortion. Many of today's solid-state designs have considerably more headroom, but the 20 dB pad still is necessary in many mic'ing situations.

Vacuum tube (valve) preamps rarely had input pads. Tube condenser mics had very high output levels, and the close-micing used today was done back in the 1950s as well. Didn't the preamps overload?

Undoubtedly they did, but there were so many sources of distortion that the contribution of a bit more in the mic preamp was likely to be insignificant in most recording situations.

Why then does the VT-2 have a pad?

There are a couple of reasons. One is to provide a gross adjustment of the level so that the Attenuation control is neither at the top or bottom of its range. More importantly, the pad often gives you the ability to obtain two different sounds from the same mic and mic placement. A high-output condenser mic close to a loud sound source will provide a lot of signal. In the "0" position of the Input switch, there is likely to be considerable overload taking place in the VT-1 or VT-2. Fortunately, this overload generates lots of second harmonic distortion, which is not generally very audible as distortion but rather as adding fullness and body to the sound. (See Russell Hamm's paper, Tubes vs. Transistors -- Is there an audible difference? for the definitive description of this phenomenon.)

By setting the Input switch to -20, you can effectively raise the threshold of this second harmonic distortion by 20 dB. The sound retain its crystal-clarity that the VT-1 and VT-2 are known for.

So you have a choice: "-20" for the greatest clarity and realism, or "0" for a "bigger-than-life" sound. Use whichever is more appropriate for the recording.

What about pads on the microphone?

Most condenser mics have a switchable 10 or 20 dB pad built in. This is used to extend the dynamic range of the mic since the internal solid-state electronics will overload if the sound is loud enough. The distortion produced is usually not very pleasant or useful.

When recording with a condenser mic that has a pad, you have four combinations of settings with the VT-1/VT-2 to chose from. All four will sound different.

In my experience, I find that if the mic electronics are likely to overload, it is best to use the mic's built-in pad. Almost anything close-mic'd probably has the potential for overload under some conditions.. Use the mic pad first, before the VT-2 "-20" position.

These days most of the recording I do myself is of choral or orchestral performances. I most frequently use a Neumann SM69 as the main pick-up, and B&K omni's for ambience. On many sessions, the SM69 output is high, but very clean (the mic does not have a built-in pad), and the Attenuation controls on my VT-2 are set around 9 o'clock if the Input is on "0," and around 3 o'clock if the Input is set to "-20." After several years of recording this way, I almost always leave the Input on "0". The distortion has never become audible as such, and the recording has a fullness and power that appeals to me.

Pads and Noise

If you use the "-20" position, the noise floor of the recording will be higher. More often than not, this is not audible. The VT-1/VT-2 is an extremely low-noise preamp and the slight additional noise is rarely a factor.

However, if the mic level is very low, using the "-20" position (with the Attenuation control wide open) is just not good engineering practice. Don't use the pad unless you need to.

Pads affect the sound of some mics

Inserting a properly-designed input pad should not, in theory, change the sound of the mic at all. It should just drop the level. But pads do affect the sound, more often than not. Measurements show no change in frequency response, distortion, etc., but yet a microphone sounds different padded than straight in. I suspect that this has to do with the design of the output transformer (or electronics) of the mic and its interaction with the mic cable and the preamp input. On many mics, the difference is extremely subtle, or even nonexistent. The sound of other mics change more noticeably when switched to "-20." Good or bad? Better or worse? You have to be the judge of that. But to my ears, "0" almost always sounds better than "-20" with many mics.

The VT-2 is not alone in this phenomenon. Some preamps sound like a different mic was plugged in when the pad is inserted. The VT-2 is considerably more subtle.

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LP-1 Line Pad Applications

Background

The D.W. Fearn LP-1 Line Pad allows the VT-1 or VT-2 mic preamps to accommodate a line-level (+4 dBm) input signal. It is used to modify a line-level sound in a subtle way and is particularly useful in "knocking off the roughness" of a less-than-ideal sounding track. The LP-1/VT-2 combination can be used on an individual track, on an overall mix, or in disc mastering.

Can the LP-1 be used as a "direct box?"

Generally, no. The LP-1 is designed to take a line-level signal at +4 dBm and reduce it to about -40 dBm for the VT-1 or VT-2 mic preamp. A typical musical instrument output (such as an electric bass guitar) has an output level and impedance that requires totally different characteristics. (The D.W. Fearn VT-3 or VT-I/F Instrument Interface performs this function.) The LP-1 could possibly be used as a "direct box" if the musical instrument has a "line-level" output.

Using the LP-1/VT-2 on an individual track

A track recorded with a solid-state mic preamp can often be improved using the LP-1/VT-2. Many of the characteristics of the VT-2's "sound" can be applied to an existing track this way. The output of the recorder is patched into the LP-1 input and the LP-1 output is connected to the VT-2 input. The VT-2 output is connected to the mixer input (or perhaps to a limiter or equalizer input first).

Using the LP-1/VT-2 on an overall mix

The mixer left and right outputs are connected to the LP-1 A and B inputs and the LP-1 A and B outputs are connected to the VT-2 A and B inputs. The VT-2 outputs are connected to the recorder inputs. You will have to determine a method to monitor the processed sound since the console mix bus monitor will be before the LP-1/VT-2. Perhaps you can monitor the input to the 2-track recorder. (Alternatively, the LP-1/VT-2 combination could be inserted into a patch point in the mixer stereo output bus if the mixer has this feature.)

Level setting and the VT-2

The LP-1 has a loss of about 45 dB, which will provide a fairly "hot" signal to the VT-2 when the VT-2 Input switch is in the "0" position. The "tube sound" will be more evident in this position. Switching the VT-2 Input switch to "-20" may be more appropriate for a subtle modification of the sound.

(Click here to go to the LP-1 product description page)

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Paralleling Inputs on the VT-3 or VT-I/F Instrument Interface DI

Background

The D.W. Fearn VT-3 or VT-I/F Instrument Interface DI was designed as an accessory to the VT-1 or VT-2 Mic Preamps. It provides the proper level and impedance match for all electric instruments such as electric bass, electric guitar, electronic keyboards, synthesizers, samplers, drum machines, etc. The output of the VT-3 or VT-I/F DI is at microphone level and impedance, e.g. about -40 dBu at 150 ohms. The output level is somewhat higher than most microphones, requiring the use of the "-20" position on the Input switch on the VT-1 or VT-2. (This was done to accommodate all types of instruments, from a low-output pickup on a classical guitar, up to an amplified bass.) This higher-than-normal level has usefulness in adjusting the sound of the instrument.

How to do it

When the VT-3 or VT-I/F DI is used with a musical instrument such as bass guitar, the user has the choice on the VT-1 or VT-2 mic preamp of selecting the "0" Input position and getting a powerful, punchy sound, or using the "-20" Input position and obtaining a very clean and accurate sound.

Sometimes the desired sound is somewhere in between, and the user of the VT-3 or VT-I/F DI and VT-2 has a useful option for capturing a sound in between the "punchy" and the "clean." Here's how to do it:

Plug the instrument into the "A" input of the VT-3 or VT-I/F DI as usual. Then place another guitar cord or other quarter-inch to quarter-inch cable from the "A" "AMP" output to the "B" input. Both "A" and "B" outputs are then run to the VT-2 "A" and "B" inputs.

On the VT-2, the "A" channel should have the Input switch on the "-20" position, while the "B" channel Input switch is set to the "0" position. The VT-2 outputs are then run to two channels of the recorder, or to the mixer.

The "A" output will be very clean and natural sounding, and the "B" output will have significantly more punch and a sound that is more powerful and raw. The balance of the two outputs will provide the engineer with a wide range of sounds. Limiting on one or both of the channels may be helpful too.

Thanks for taking the time to read these Application Notes. I plan to add more over time. If you have a suggestion, an application you would like to share, or question, please send me e-mail. I'd be happy to include any useful information in this section.
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