A few months ago, a fellow Orion owner asked if I would recalculate the new treble shelving filter in version 3.2 to increase the treble level a bit. (See the trace labeled “Version 2” in the last graph on this page.) After upgrading his Orion crossover to version 3.2, he found the treble somewhat lacking. A SPICE modeling exercise showed that within limits, small changes to R400 changed the depth of the treble shelf above 10kHz without significantly altering the response below about 1.5kHz. I suggested that he add a 1k resistor in series with R400 to increase the treble by 0.4db.
He found that this relative boost in treble added openness in the harmonics and made the sound stage appear deeper with more air. A friend of his concurred that 3.2 sounded better this way.
I decided to listen to variations to the treble shelf filter in my own system. I found the 0.4db boost to be a big help at low listening levels, but it was too energetic for me at louder levels. I installed a 5k pot in series with R400 and played. Reducing the boost to 0.3db seemed about right so I listened to it this way for some time. Most of my more important recordings sounded more like they did prior to revision 3, but unfamiliar material was often too bright and tiring. Ultimately, I decided to turn this filter into a tone control since its utility changes from recording to recording. A 6 position 2 deck switch now provides from 0 to 0.5db of treble boost (relative to the -3.2db shelf) in steps of 0.1db. It is turned off most of the time, but sometimes it makes a dull, muffled, or lifeless recording much more enjoyable to listen to.
Filter Response Study
The shelving filter is formed by the series RC network, R400 and C400, added parallel to R4. Without this network, the resistors R1-R4 form a voltage divider that drops the input level to U1A by half. The new RC network causes the impedance of the bottom half of the divider (R4 in parallel with R400+C400) to change with frequency. The resulting filter is a 1.8kHz, -3.2db shelving low pass filter.
The following graph shows the response changes at the input to U1A when R400 is changed from 5.62k to 6.34k. Adding 700 ohms changes the response by +0.3dB at 10kHz and 20kHz, and -0.03db at 700Hz compared to Orion 3.2.
It is possible to make the new curve track the old even closer in the midrange by lowering C400 to 12.5nF but that’s a lot of bother for 0.03dB.
This graph shows how changing R400 from 5.62k to 10.62k in 500 ohm steps changes the response. This is where the investigation started.
Here are some of the level differences at 10kHz and 700Hz when R400 is increased.
change to R400 response change (in dB) at (ohms) 10kHz 700Hz +500 +0.2 -0.02 +1000 +0.4 -0.04 +1500 +0.55 -0.06 +2000 +0.7 -0.08 +2500 +0.8 -0.09 +3k +0.9 -0.10 +4k +1.1 -0.13 +5k +1.3 -0.15
The level changes at 20kHz are almost identical to 10kHz until about 3kohms is reached.
Zoom in on the top end
I replaced the 5k pot with a surplus six pole two deck rotary switch. The switch increases the treble above 10kHz by 0.1db per step. It’s connected so that between 0 and 5 resistors are added in series with R400. SPICE modeling showed that 249, 267, 287, 309, and 332 ohm resistors would give roughly equal 0.1db increases. The resistors could just as well have been around 300 ohms each.
The switch is installed on the front panel of the preamp/crossover box. A three pole switch could be used to turn on an indicator LED when treble boost is active but this switch was at hand.