Jun 3rd, 2015
ART Pro Channel
AudioPro modification with the THE – Tube Harmonic Enhancer
1. Transformer equalizer mod for vactrol compressor: $140
Signal level should meter at about -5dB or greater on the VU or tube drive LED’s, then adjust the compressor to allow the “gain reduction” lights to read -4 or greater. Now you can adjust the newly installed THE control (below the compressor knob) to voice your music. The transformer effect activates with the THE control at about 25%, thickening the bass subtly and adding harmonics and presence in the high end. At about 75%, you will notice less overall compression, thicker bass, and a slightly “plumpy” bass transient quality. Too much THE can be blurry sounding in some situations. Ultimately, the compressor now does not lose as much presence or power in the music, and vocals will be pulled back a little. NOTE: This version modification is not exactly the same as previously done, so it will not match units done from 2010-2014. This version modification can be installed into virtually any opto circuit! It is a more clever design that does not require a level control to optimise the compression equalization and transform harmonic enhancement. These new modifications will allow just a little transformer equalization at the start of compression and a very gradual effect as the opto circuit is run harder, until the opto is turned on hard, and then only does the transformer saturate. Very nice now to just flavor the signal or the mix. Technically a 60db dynamic range of music will only have a few db of harmonic enhancement unless opto is pushed to full limiting. The compressor circuit will be prejudiced to not compress bass or treble (below 200hz or above 5khz) as much as midrange by a 1 to 1.25 ratio approximately. As of 2015 this mod is available to do.
2. Variable Cathode Bias (VCB) control for last output tube stage: $128
The VCB control installed under the master volume level knob. When this control is centered, tube harmonics have very little enhancement. Moving the control clockwise or counterclockwise from center creates in phase or out of phase (respectively) second harmonics up to about 2%. It takes about three to five seconds for the tube to reset its bias and the change to be audibly realized. Typically, positioning the VCB control about 50% off center produces the most pleasing result.
3. Tube information:
Substitution of other tube types in this circuit is not recommended without careful recalibration using a distortion analyzer to set the proper in/out phase control voltage. Some people do try other tubes, but then the circuit will not generate accurate phase effects. One tube type might create a stronger tube character effect than others, but modern tubes are best for performance.
Should the third tube on the right side of this circuit fail, it is best to swap a tube from one of the other two sockets, which are very close in spec and function. We can then supply you with a replacement tube for the empty socket. In fact, this circuit design uses a calibration trimmer next to the output tube to adjust these modern tubes very accurately.
Now go use this device and HAVE FUN! Reread our ART Pro MPA mod article for more information about how in/out phase harmonics affect the sound character of instruments.
Pricing:
We charge $140 plus shipping back for the transformer equalizer modification and $128 plus shipping back to install the VCB, or $220 for our complete modification (local service) $240 also includes the cost of return shipping, provided that you box and pack sufficiently! In any case, we will also update the filament supply to draw equally from the +/-15v rails as well as clear up any minor problems that may exist in your unit.