ART PRO MPA Mic Preamp Modification
Sep 12th, 2015
All in one Microphone preamp, Harmonic Enhancement controller,
**Update June 23rd, 2006**
PDF Manual Here
tube compressor design (2 matched channels with metering).
This is a rough version manual we are making available now, however the final version manual will be available soon.
For the last several years I have been working on a concept to create any kind of preamp which actually has variable adjustments to recreate the various tube sonic effects, created by affecting the bias and gain staging of tubes. In essence, I wanted to create a device which could emulate many other products, but in one box. Fortunately at this point I’ve developed a method to efficiently produce this novel product via modification of an ART MPA Pro preamp. Here's the explanation of what I do. Let me first make it clear that op amps, “fancy” capacitors, or wire of any kind will not make the important sonic difference. This design uses only 3 op amps (per channel) anyhow. Apart from the “correct use” of op amps in certain circuit designs, this is not the crucial factor of what my design criteria is about. In actuality my preamp does not have perfectly flat response or linear dynamics, and has measurable distortion that far exceeds any requirement for super spec/special op amp use. However, it beats the fancy stuff cold for producing the tasty vintage, slightly warm but detailed sound character. The design is a combination of circuits which allow the flexibility to preamplify with very low noise and adjust the “voice” (ie harmonic content) of the circuits to better accommodate the new inexpensive high quality large plate condenser mics or even a stereo line level signal in, from a mix or even a CD. It effectively emulates the euphonic slightly veiled, slightly thick sound character heard previously in the better designed vintage units, but with controls the listener can tweak. At each preamping stage (such as at mic in), proper impedance matching is optimized. Subtle use of adjustable tube harmonic distortion in the mid preamp stage (a very novel circuit using a tube and vactrols etc. is done), and a clever way of doing a very gentle gain limiting and gain riding of 2nd harmonic distortion are all looped together but with users ability to adjust these enhancements. This creates a preamp which can easily (with quick adjustment of a few controls) mimic a vintage or modern mic preamp, compressor, transformer coupled sound character. ART’s clever tube gain stage, and incredibly the meter movement hysterisis of ART preamp which is also involved in the feedback loop, emulates a transformer couple effect of the gain riding circuit. (It is Partially influenced by the RCA BA6 compressor, an advanced Neotek, or an API front end). Whoop-dee-doo! I’ll give you some technical details later. I think the BSEE person would have an interest.
By the summer I will be building the full production version of my modified ART MPA Pro. People can buy them, try them and if they don’t like, send them back, but I charge a small shipping and trial fee of $80. No one yet has returned my 8 original prototypes. They are used by some mid-level sound engineers, a few home recordists, and audiophiles who simply use it to enhance their audio playback. Originally I was looking to find a company to buy and implement the design. ART actually did take my advice it appears, and built a little of the design concept into the TPS II preamp (although I correct this to higher quality sounding more usable euphonic character). The ART TPS II is a well built and flexible preamp having one control to vary the tube gain harmonic and equalization characteristics. For the cost of $180 I can make this unit (ART TPS II) more even tempered across the 15 presets when creating the "tube" sound. The original TPS II system is a bit too heavy handed and obvious when processing the preamp signal. The unit is beautifully built (better than some of the fancy stuff) and makes better use of tube character, especially after the easy modifications are done. I also reduce the noise floor somewhat.
The ART PRO MPA when fully modified is $980. This modified version is truly unique for some of the clever adjustment capabilities. The most obvious is the ability to adjust the level and phase of a very euphonic second harmonic distortion which is then gain ridden. The preamp can subtly sound thick, punchy, dense, edgy, silky, warm, strident, etc. It’s not just equalization… it is a subtle alteration and enhancement of the original audio signal through a clever tube stage which is very user friendly to adjust.
The almost unique effect this tube preamp does is to allow a pleasant 2nd harmonic to accent the sound passing through at low levels which is equalizable and adjustable just enough to help “voice” the mic sound. However, when an excessive amount of signal level is present, the circuit reduces the harmonic enhancement somewhat and very softly limits the peaks and sudden volume changes while keeping the inner dynamics of the music intact. The circuit forces the tube to create a very pure second harmonic distortion, more so at lower signal levels than at high levels. This is a crucial effect but seldom done in such a product with adjustable controls. Older vinyl signal compressors acted somewhat this way, bringing out clarity through harmonic enhancement but not allowing the music to “saturate” and blur at higher sonic levels, although the dynamic range of modulated vinyl is the limiting factor. Strangely on another side of this concept were “single ended” tube amplifier designs. This is probably the reason why such small wattage “single ended” tube amps (although fuzzy sounding when out of calibration) are coveted by some audiophiles. Indeed it is a characteristic of a single ended output stage tube amp driving the transformer and speaker load that creates more second harmonic distortion at lower levels while decreasing and “cleaning up” as louder signals are passed. Unfortunately, most of these amplifiers clip asymmetrically and harshly at very low wattage output. With my design it is virtually impossible to hard clip the signal in the ART MPA as I have corrected the tube circuit to soft symmetrically limit only on the exceptional transients. It is an interesting sound effect to truly warm up the “digital” sound (read wide bandwidth and incredible dynamics, but slew limited high frequencies). I don’t personally like too much of this effect unless it’s a trick on an individual recorded instrument. But a slight amount can be pleasant, so adjustment controls installed on the MPA are subtle, but allow it range to enhance a guitar, a piano or even the entire mix. Being an integrated mic preamp tube EQ, harmonic enhancer, gain riding S.O.B. makes it a flexible, cool machine. Many engineers have also used it as a final mix compressor/enhancer.
Recently I have cleaned up the design so I can produce units, or modify clients units faster, and with less calibration fussing. Yes, certain tubes must be used, but now there are more choices for which will easily adapt into the circuit. In fact that was the biggest problem. I tested 12 different tube types, all numbers and only a few could do this trick well. The venerable 12AX7 still won out (in most cases ART’s choice of tube was near perfect). Although others could be matched, installed and the unit recalibrated. 12AU7, 12AT7, 6201, 5751, 6DJ8 were possibly usable. The power supply for the tube circuit is altered to allow the cathode, filament of the tube to warm up before the plate voltage is fully attained. This prevents cathode stripping and increases life and performance of the tube. The tube circuit works on about one half the normal plate voltage. Although many tube buffs (including myself) prefer “full voltage” on the tube plate design, in this circuit it requires less to do the harmonic trick enhancement. I do step up the existing plate voltage higher than the original design though.
The ratio of plate voltage to cathode bias in this 2 stage direct coupled tube design also creates the subtle tube “ka-thump” when dynamic or large amounts of signal hit the circuit. This “D.C.” shot is a common effect in most older tube stages and imparts a sense of pluck or deep bass “ka-thump” when operating at 0db metering levels. In essence, again fattening the sonic picture. Usually this effect is created by at least 4 stages of tube preamping. Ironically, “perfectly” designed tube stages don’t replicate this effect and indeed this may be one of the reasons different manufacturers or ages of tube may or may not create this effect.
Part of this project was to create a mic compressor EQ preamp which was more “at home” friendly too. Since I can’t have access to a big studio, I must play softer but I want the “bigger” sound character in the final recording. We need to cheat a little bit to get the recording to reproduce again with muscle and definition on home HiFi speakers. The unit does effect some dynamic equalization, retaining the bass and treble when compressing slightly.
Here are some technical details of the design. The mic preamp front end uses all discreet transistor design where 3 paralleled transistor on each side of a differential input provide a true low impedance and low noise floor, although this is a well done original ART design. I have added another stage of RFI (radio frequency interference) protection and upgraded input capacitors and gain set capacitors. This does not make a huge difference, but does help to adapt to virtually any mic impedance and possible bad XLR cables which attract radio frequency interference and adds a little more preamp gain and audio bandwidth in the low frequency.
The second stage is altered to obtain another 6 to 12 db of gain before noise and incorporate a novel phase correcting predominance filter circuit. Admittedly this circuit is voiced for typical and now very popular large plate microphones. The subtle use of equalization and phase shift is there to correct for the inherent phase shift coming out of mics with transformers or in fact the effect of coupling capacitors which “add up” in the entire chain of circuits, both in the preamp itself as well assuming the signal will be processed even further into analog and digital mixers. The predominance filter will allow reduction of near field micing “boomy” midbass with minimal reduction of extreme low frequency impulse signals (the pluck of guitar for example). Admittedly the circuit is “voiced” for mics placed 1 to 3 feet away from the instrument being recorded.
Next is a 2 stage tube preamp which has been critically recalibrated with resistors, capacitors, switchable equalization and gain a riding vactrol to get the tube to perform truly in the 2nd harmonic generating mode. A variable cathode bias control on the front panel allows the user to force the tube to create in phase (with the original signal) or out of phase second harmonics. The amount of this subtle distortion is adjustable from .1% to as much as 3%. A gain riding circuit will prohibit the overloading of this tube effect yet also ensures at low levels the tubes 2nd harmonics will be consistently present. This goes against virtually all other designs whereby no harmonic enhancement is created unless the tube is overdriven. This is a very tricky design situation as the tube is often run into a nasty hard clipping situation too easily. My circuit avoids this problem. The harmonics are present more at lower levels and the tube gently soft limits and rounds off the high level signal transients slightly so no hard clipping can be created without a signal overdriving it to unreasonable levels. This is the circuit which does the real work. For example when I am micing my concert grand piano I advance the harmonic control to an in phase harmonic (about 1% to 2%) and switch the gain riding equalization to the “warmer” position. My Kawai KG5 which is slightly bright and “steely/metallic” sounding now takes on a recorded attribute of a more bell like tone, warmer, almost a Steinway sound. When using my acoustic guitars, I will often adjust the harmonics control (or voice) to the negative, out of phase harmonics, switch the bright position (I like old slightly dead guitar strings) and now the sound texture becomes intensely detailed and “sparkling”. When friends get together and I’m micing everyone in the room, bass, guitar, piano, conga, I use the neutral, very low to almost no tube harmonic setting whereby some combination of both in/out of phase harmonics fight to dominate and the neutral gain riding position of the switch is set to make the sonic picture natural. The unit design finishes up with ART’s well done balanced servo line drive amplifier. It’s designed to put out 0db when the meters read at 0dbm (many manufacturer’s do not adhere to this calibration). It’s maximum level out is better than 18dbm, but the meters are now designed to max out softly. Infact the movement of meters show the actual loudness level of the signal and it’s control voltages are used to operate a vactrol, gain and equalizing circuit. It tries to maintain a “big” sound. The truth is most modern microphones are quite linear in dynamic range and don’t necessarily mimic how the human ear perceives dynamics or fidelity. Some engineers prefer microphones which can be somewhat lifeless in dynamics but rich in overtones to create presence. The point is this preamp is kind of my perception of what I want too with modern wide bandwidth microphones. It is not for the perfectionist. It is for the musician who cannot afford or accommodate multi thousands of dollars of specialty vintage gear but desires the vintage sound character on the recording. Especially when used in home recording environments. Another feature is the 20db pad switch is now a subtle phase correction equalization which in fact does slightly equalize down the midrange but over a very broad range, giving the impression of “moving the mics back” but without loss of low or high end detail.
The Modified Preamp is a more convenient, simple “box” which can do most of the tube sound character desired. Plus it is easy and fast to adjust. This slightly softer sound character is not a loss of frequency response or detail. It is again a combination of subtle dynamic equalization with 2nd harmonic enhancement which suits getting the playing of an instrument to sound pleasing at the recording end. Admittedly we all need a little help to get the recorded sonic picture to play back on a home HiFi. More over I will admit the unit is voiced for the serious musician who needs to record at home. I too cannot play loud and at all hours, so now a device that is sensitive to lower sound levels too but can create a “big” sound image is an important recording tool for late night use. This unit can still take the big sound levels too. Use of any mics, (especially ribbon mics) which need more gain and a way to brighten them up a little can be done. I often use Beyer 160 ribbon mics, close field on the piano, but truly the whole sonic quality is best when omni directional, large plate condenser mics are used, in my opinion. Another interesting sever tube fattening effect can be accomplished by running the tube gain high. Watch the now correctly calibrated LED meters go into the red but turn down the output control below 0db metering. Or conversely heavier gain riding can be done by pushing the output control and seeing the VU meters cranked well past 0db. Vocal and acoustic guitars can take on a very upfront, very full and present character by forcing the electronics to gain ride more. Another trick is to process in one channel send it back into the second channel and dial up it’s effect, thereby stacking competing effects. When the compressor effect is occurring, the dynamics equalization circuit does not suppress the extreme high or low frequencies thereby loud passages don’t lose bass impact or treble detail as in most compressors.
As an overview this device was also originally designed to prove the manufactures and the designers are not giving enough control of sound and features for the big money they want. I see too many fancy preamps with 2 knobs and some metering which simply does not do all that much. For $1000-$2000 you get a box that does one thing well. It has it’s place, certainly some of these products are of superb build quality I wish I could put into the MPA. I tried many of these ‘fancier’ premium audiophile preamps and was very impressed, but honestly I found $200 things that seemed to almost do as well sonically, but obviously were not as well built. So you have to pay $1000 or more to get something that will last 20 years. Many preamps regardless of price, still suffered potential RFI problems or very strange sonic effects when certain combinations of controls were moved. That’s another story though, but I must inform you that an unexpected truly obvious sonic defect occurred with cables if I might digress please. Indeed aluminum foil wrapped shielded mic cables were noisy both mechanical (when moved) and prone to RFI and worse created a poorer mic noise floor. I read tech books which said the same but now here was case of RFI noise which even expensive, very expensive and loudly touted cables were just as bad!! I went on a search, testing cables, optimizing RFI suppression, etc. indeed what may be thought of a mic or preamp noise was often a hissy RFI problem invading the preamp. Some cables were far superior at reducing this problem.
So I’ll mention the conclusions we came to testing and comparing cables here briefly. Neumann cables, very old 1940’s and 1950’s balanced cables, Audio Technica cables, and some double pair twisted ‘canare’ cables won out. With the Neumann and Old WWII cables it was because these were truly balanced/tuned twisted pair with a nickel plated copper braid shield. I stress nickel plated (probably 90% coverage) copper braid over a clothe insulator, which seemed impregnated with a plastic substance. Nickel plating really soaks up RFI and is less able to pick up stray electromagnetic fields too. Audio Technica 2nd place has cable which each balanced wire has carbon plastic covering (to eliminate electrostatic and RFI) but then twisted as a pair, covered with cloth insulator and then another fine copper (but not nickel plated) wire shield (about 90% coverage). If it had nickel plated copper it might have been the best. The Canares’ trick had consistently twisted, doubled up low resistance wires and as well as a carbon plastic, or copper shield. Incredibly though also grounding the mic stand to the chassis of your rack equipment reduced deflection RFI into the hanging cables. This was a surprise. By the way I used Power tools, cell phones, remote controls and arcing devices to create these RFI problems. Trial of many mics also proved many were deficient or dumb in the way the RFI was attempted to be suppressed inside their circuits. Many mics still use 1 big capacitor across the balance line, others had no “Faraday” shielding at the first mic gain stage, or worse insufficient ground screening at the mic capsule or first preamp stage. Toughest to find was that some mics just needed better, lower impedance capacitor types installed into their power supplies and audio circuits. It was disappointing to find cheap (or incorrect type) capacitors in some fancier, older (and newer) mics.
Personally I’ve been around some serious vintage engineers and have picked apart some very clever designs with them. Incredibly even HH Scott (A HiFi company) actually made a similar tube harmonic enhancing concept device in the 1950’s and 1960’s. (Another product which influenced the design of these modified MPAs). Again I will mention the “Distressor” product as a clever and valuable tool for enhancing sound and tube character. The Aphex Compellor also gets my approval nod. Here was a multi band compressor voiced for the real world use. A device when a spectrum of musical sound was passed through had controls to compress very carefully , and usable enough to get the final sound right. By the way also the Behringer DEQ-2496, a newer multiband equalizer, dynamic compressor expander also has some very effective capabilities. It’s so clever but it requires a bit of technical thinking. The “Distressor” imparts a wonderful character of tube like sound. The Aphex and DEQ-2496 though are so clean they are almost sonically invisible except for the control equalization and dynamics of music. The Compellor has an obvious sonic signature of being “graceful” to the sonic image. Certainly there are other devices which are worth their money. I’ll discuss them if people continue to call. Another nifty and flexible device was the 1960 Drawmer. Although it’s tube enhancement effect was very limited and non adjustable. (My modified unit had a similar noise floor and a somewhat more obvious tube character.
In closing I would like to thank ART for allowing me schematics and free parts and not prohibiting me in any way to do this project. Some companies are extremely protective and closed minded about such matters. To be honest often their products don’t have much inside anyhow, and it’s a dirty secret. I’m also considering doing a simpler version MPA, (only the very critical modifications and updates which make the critical sound effect). I expect this version to run about $340 to modify, 2 channels to the MPA. Trust me on this one. The ART MPA can be made into truly world class item for short money. Hopefully I have explained things well enough for the pro, the BSEE, and especially for the musicians. Where would we be socially if we could not share our ART, Music, and Science.
P.S. One important point also to be made is the units construction and modifications I do are very reliably installed. The ART MPA circuits boards are a high quality fiberglass with plated through holes with very reliable high quality porcelain tube sockets (with tube shields). Very good quality ground lifted phone ins and outs. Separate in out preamp board from the main tube board. Reasonably well laid out with all parts numbers referenced to ART schematic noted clearly. Although there were a few minor engineering changes done by ART (the best units have multicolor ribbon cable jumping off to the main board). All units are similar to modify now. My 8 prototypes have been in use for about 6 years (since ART first provided this model) and the only problems have been the typical repairs of any well used unit, occasional failed op amp a, a broken control or indeed I had a failure of the vactrol circuit. However all parts are fairly standard parts to find. The vactrol is the common 5C3 still use for 20+ years by so many manufacturers.
I’ll probably include a spare matching vactrol circuit, taped to the back panel. This may be the one part a shop may not have in stock. Anyone who repairs pro equipment or guitar amps will probably have it. But I will include a matching vactrol anyhow. Interestingly although ART uses some decent quality controls, it is easy to install super quality control potentiometers if needed. Admittedly, this is the first part which wears out, as in almost any preamp circuit. In my design only the tube has to be very close to spec. That’s why I redesigned the circuit to extend the life of the tube (only 1 tube failure in the field so far). I possibly may also include a spare matching tube. The other idiosyncracy is I now mount the transformer on the outside case, wrapped in thick electrical tape. Aside of running cooler, it gets the hum fields out of the unit.
Finally for those of engineering bent, here is a list of other modifications done. Installation of copper shield around the transformer and big RFI suppression capacitors of the secondary AC voltage line.
Critically matched high quality resistor and capacitors at equalization stages, phase correcting EQ. Phantom supply voltages for mic is specific circuit to mic with extra RFI suppression.
Separate sub power supply improvement to run meters and tubes and mic preamp section.
Improved grounding and power supply bypass (although ART was pretty good about this situation).
Very high quality little toggle switches, with doubled up contacts for EQ and gain riding effect installed.
Properly positioned wiring throughout (a necessity unfortunately the meter bulbs are on AC and stray hum from untwisted shielded wires does get into the original circuit audio in some situations). A few other minor similar wiring problems are solved.
Improved true metering levels.
Phone jack in can now take up to +14dbm if the unit is used for mix/compressor/tube enhancer, or it still can do an instrument level in too! (bass guitar etc., or I’ll tell you I have one just on my DC player for sound playback, what a lush I am).
Units which I am selling are originally fresh from factory with my 1 year warrantee. Some used units may get the treatment these will cost less (none available presently unless a customer wants his done).
Of course if someone wants their unit super modified it is about $600 and a 4 week possible wait. (that’s $300 per channel for a lower noise mic pre, adjustable tube harmonic enhancer, dynamic gain riding equalization).
Thank you for your time. Love people first. Like your equipment later.
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