Jun 18th, 2016
I will comment about how my modified Bellari compressor works due to the fact that no other compressor exists that works quite like it. It uses both an optical vactrol style attenuator as well in the same circuit an audio transformer (originally a decent Beyer mic transformer) is used as a variable impedance transformer or “filter reactor” style design to assist in compression and equalization as the signal level is compressed. Another way to explain it is a variable (mutual) inductance transformer interacting with a variable optical cell to create a compression of audio as it is passed through this transformer creating a subtle “iron thickness” in sound. Such a circuit simultaneously generates a slight second harmonic sound enhancement from the tube, as well as a little 3rd harmonic hysteresis from the transformer. A front panel control allows the amount of this distortion to be created. Simultaneously the tonal balance is subtly altered to lessen the loss of inner detail of deep bass and treble clarity when compressing. This is also adjustable.
Audio Pro is now able to take in and modify these Bellari units and make a consistent sounding machine (original unit varied quite a bit due to specification of controls, tubes, resistors etc. which is not uncommon for any tube design). Many people don’t realize (but engineers agree with me), that the circuit design dominated the tube sound not the tube itself. However a 12AX7 will perform differently than a 12AU7, or 12AT7 because the part value needs to be right for such a different type but manufacturers seldom “optimize” the circuit design. Most often the reason why, for instance, one 12AX7 will sound different than another is mostly due to the circuit parts values being more “right” for a certain tube. I have redone the circuit in this preamp and others to make it correct and tolerate for a wider range of tube types. Virtually all manufacturers do not use a few extra resistors to ensure correct grid bias voltage or proper plate voltage stability for instance. Oddly enough the Bellari unit is more correct than many fancy items. It’s circuits are more “complete” needing mostly corrections to get best signal to noise and force the tube into the better range of nice harmonic distortion.
The device has been a work in progress over the previous 10 years. I’ve modified and repaired countless tube preamps. What I wanted (as well many other studio engineers) was a slightly thick tube sound without being muddy, a presence in the voice without stridency (although this unit has the capability to be adjusted to do so.) and especially desirable, was a sense of keeping the character of sound bold, especially in the voice to be slightly forward and warm but not too loose detail. As the compressor works it’s like having a photograph “blown up in size”. Certain other designs do this but seldom the use of harmonic enhancement and dynamic (variable) equalization are done in tandem.
Important note about audible effects of tube: If I may digress a moment, of course many people will have different opinions about such sound character effects. Studio engineers will base their opinion on how it sounds with their favorite mic, how it sounds to analogue tape or digital, or especially and unfortunately how it fits into the mix while listening to their chosen monitor speakers. I say unfortunately because I for one am not impressed with most popular monitor speakers. Most monitor speaker (even the big famous name types) have excessive cone resonances and distortion (known as cone cry) or tweeters which sound a little razzy at the lower frequencies of their crossover range (the 800 Hz to 3 KHz frequency region). Worse yet, such monitors are seldom anywhere near phase correct in the crucial midrange crossover frequencies. This gives the impression of an “unnatural” nasality in the midrange. These monitors will dominate the overall perceived sound character. More over so many of these supposed revered monitor speakers have frequency response deviations of +/- 10 to 20db of peaks and dips!! So a bigger sonic picture can reveal more faults in speaker reproduction, regardless of manufacturers’ claims of being linear frequency response. This character of speaker sound can easily dominate over hearing the nuances of the tube character or the quality of a mic now, or even the sound stage created by the final mix reverb. There are better monitor speakers available which are neutral, honest and will easily reveal the tube character effect and reveal the quality of the entire mix of instruments. Older microphones also have resonance modes, enhancing some frequencies while suppressing others, similar to speaker problems. There are many doubting opinions but from a technology standpoint, many newer microphones are really “cleaned up”. As an overview I tried to “voice” the preamp by assuming to use modern fairly tame monitors and reasonably smooth frequency response modern microphones. I see more of my clients going in this direction. The preamp can work well with ribbon mics, as my design has about 20db more total gain over the noise floor than the original design.
The equalizer section, when switched in, may initially sound as if a little level is lost because passive components are used around a true tube gain stage, but it’s assumed the user will crank up the signal to his taste. I correct the equalizer circuits to a more flat in frequency response as the original units varied quite a bit from unit to unit. Again this is a problem with any tube equalizer. More accurate correct parts are required to improve the circuit accuracy. The circuit has multiple switches to change equalization effects and the labeling is a general effect it gives (“thick”, “voicy”, “presence” or “edgy”). The tone controls are center detent and now can add or subtract equalization from center point. The Eq enhances more than it can reduce by the nature of design. My circuit design can create more euphonic “tube” 2nd harmonic in the equalizer circuits too.
The unit also now features an input high gain tube circuit modification with a switch which effects the first gain stages at the mic/line preamp. If you use the instrument input (phone jack for a high level signal, drum machine keyboard, etc. Make sure to turn the Gain control virtually all the way down (counter clockwise). Otherwise, overloading the front end with high level audio signal will cause weird distortions, some usable and interesting as customers have said.
The unit also now features an analog tape sound done with a “saturate” (of tape) control. Saturate control works only when +0 to +4db as read on the meter is reached. It is a circuit which approximates tape saturation or true tube symmetrical soft “smushing” of audio. Great for creating the overdrive “grunching” but not a “buzz saw” quality, unless you totally overdrive the unit. Contrary to myth, tubes do not naturally overload to produce second harmonic distortion or warmth. It is the circuit which will force them into this performance. The improved design can easily overload the next electronic device connected after it. Many modern reverbs for instance take only -30 to -10db input so they can easily be signal overloaded. A -10 db pad switch on the unit can usually solve the problem, but in general, most pro audio can easily accept a +14db maximum level which the modified Bellari can now do.
The compressor circuit has a switch for easy and severe compression which allows for either a strong obvious compression effect in “severe” mode and a more graceful and forgiving, somewhat over easy compression in easy mode. A bright warm switch at the front end actually changes the tube gain cathode bias allowing an obvious tube warmth sound or a slightly forward present sound character so a bright mic can be warmed up or a warm mic can be brightened.
The cost to modify the unit will vary with the many optional added features. Typically for each circuit change or added switch is approximately $60 to $80, but multiple alterations done at one time will obviously cost less. I’m beta testing the unit with colleagues at their studio to find out the best and most desirable combination of controls. For now if you are interested it’s best to call and I can explain in more detail if needed.
Final Comment: You know these are cute units, admittedly I have changed my knobs to retro type knobs. I’ve owned many “world class” preamps and they are pretty good but none are flexible for generating various tube characters of sound. I do avant garde voice work, trick guitar work, hitting objects stuff as well, lounge singing, etc. into this unit. It creates many tube voicing effects as to be viable for many studios. Bellari did a decent job of layout and gain staging and many years ago was very helpful and interested in some novel ways to alter this design. As an overview, it’s too bad that today’s engineers don’t stick with a design and implement sonically important updates until they have truly made it the best it could be. I’ve seen many electronics devices that with a few critical alterations become remarkable. I like the Bellari stuff because at least they keep making the same thing for many years. This allows updates to be consistent. The irony is their $500 thing can do more than a $2000 thing. Although the other good devices which do similar effects are the Distressor and the Fat Boy, both commendable achievements. These engineers have designed products with important euphonic sound characters which is user adjustable. Such devices can be “tuned’ in for many tastes. This opposes many fancy priced things which simply have an input and output control and don’t do much else.
More to come…