The BG1 Stereo is a two channel tube compressor loosely based on the Altec 436 and also the British modified 436. There are two compressors in one 3U chassis, linkable side chain amplifiers. Tubes are pre channel: 4BC8, 6SN7 and 6AL5. Fully balanced circuit with transformer input and output, fuse is a 1 amp slow blow and is located in the power inlet at the back. There is also a select in the fuse holder to switch to 240 mains voltage there too, change the fuse to a .5 slow blow if operating at 240 volts mains. Hook ups are located in the back as well and are balanced XLR.
Input and Output controls: This lets you set the input and output, they are continually variable to allow you to fine tune the compressors. Output is after the output transformer so it is in essence a output pad, don’t be afraid to have this all the way at 10, if you need to turn the output signal down, you can do so with this control.
-600 or 150 ohm input taps. This switch lets you select the input impedance taps on the input transformer. 150 gives you more gain and color/character, also useful in running a microphone directly into the compressor.
-Feedback settings. This is like the BG2, B is for the classic British modify Altec 436 and A is the no feedback stock 436. B gives you less gain with a smoother more Hi-Fi sound of the British compressor. A gives you more gain and more character, lets more harmonics through and gives the compressor more of a sound.
-Compression switch: This will remove the side chain signal making the compressor into a line amp. You can use the different feedback and input impedance settings to achieve different tones with using it as a line amp. Also different distortion character and sounds if you choose to overdrive the circuit.
-Threshold setting: The BG1 has 3 threshold settings, Low is stock, Medium gives you a little more signal going through the compressor before compression, and High even allows you to get some overdrive sounds when driving the compressor hard but still getting compression.
-Attack: 9 attack positions are available from very fast to very slow. 3 is around the stock Altec both American and British and similar to the fast setting on a BG2, 5 is around where the slow setting is on a BG2. 9 is extremely slow.
-Release: 9 release positions are available from very fast to very slow.
-Hold: This allows you to turn on the release, in hold the compressor will compress but not release back to full signal. When you switch back to operate, the release to go back to what ever speed you have the release setting set at.
-Link: This will link the two side chains together, a signal in either side will make both sides compress the same amount. It is like have the side chains in mono but the audio amps still fully independent.
-Meter: there is a trim pot through the small hole by the meter labeled “meter”. This lets you set the zero and calibrate your meters. Different line voltages can cause the meters to rest in slightly different places, use this to adjust back to zero with no audio going into the compressor.
The BG1 was the first compressor I built for John Baccigaluppi at the Hangar studio in Sacramento CA around the mid 2000’s. It was based on the British modified Altec 436. After building a few for the Hangar out of Altec chassises, I built one from scratch out of old parts, this one was sold to John Vanderslice at Tiny Telephone in San Fransisco and remains in studio to this day.
I built a handful of BG1’s in the years after, fast or slow attack time was added to some, also slightly different release times but still 6 release times total. I added the feedback on or off switch on some of these, this turned the British feedback off and made the compressor back into a Altec 436. Engineers seemed to love this switch so I did the same circuit with the BG2. It really changes the circuit into a different compressor. At one point I built a stereo BG1 for Eric Broyhill at Monster Lab Audio mastering studio when he was located in Sacramento. This was fun working with Eric and really trying different subtle changes for a stereo mastering compressor. Monster lab Audio is now in Stockholm Sweden https://www.monsterlabaudio.com.
Moving to Los Angeles as a recording engineer, I starting building the Lisson Grove compressor. Then I started Highland Dynamics and decided to put out the BG2 compressor. I still made some BG1’s here and there and thought it would make a great stereo compressor.
Attack and release: I wanted this to have lots of options here, so about as fast as I could get it to super slow. I’ve been using a pair of BG2’s for a while on my live to two track mix and got into setting them to slow attack and slowest release. Attack 8 is very slow but attack 9 is almost twice as slow. I’ve been using this to mix in a interesting way, getting the compressors to be in heavy compression while on attack 9 and using release 7, 8 or 9 so it just stays compressed but faster notes or drum hits don’t trigger the compression, but it just holds down a overall level.
This BG1 used 6SN7 to drive the output instead of the 6CG7, this isn’t a big difference but the 6SN7 always sounded better to me, so its in this. The two capacitors in the signal path at K40Y-9 paper in old caps. This was something found years ago in the Monster Lab Audio compressor, we went through many different caps in this spot and I just loved the PIO caps. Ever since then, I’ve been a big fan of these and use them in all kinds of things from compressors and preamps to Scott and Fisher hifi amp rebuilds. Power supply on the amplifiers are film. Output transformers are same size as the BG2, just different windings for the 6SN7.