Glide On Fade Awesome Switch
I'm pleased to offer a very handy switch for converting a neve 1272 amplifier to a mic pre. There is much discussion about this topic on the internet and naturally, since it is the misinformation superhighway, quite a bit of not so good info. Fortunately finding a Neve 1272/1073 schematic is relatively simple so while you can chose to believe whatever you like, the facts of the case can be verified with a schematic.
Some background, in a nutshell:
Old Neve consoles used a passive mix buss for combining all the channels on the console into the stereo output. For this application, Neve designed the 1272 amplifier which stock, contains, a 283AV amplifier, a 10468 input transformer, a LO1166 output transformer, a fader pot and power supply filtering wired point to point between the connectors.
In these same consoles, Neve used a 1073 EQ/mic pre. In that module, besides the EQ circuit, you have the same basic elements of a 10468 input transformer, LO1166 output transformer, 283AV amplifier and a very fancy switch designed to switch between a mic (10468) or line (31267) transformer, pad or boost the 283AV amplifier up to 50 ish dB and then switch in another amplifier stage, BA284 for gains above 50 ish dB. So far as amplification is concerned, the 1073 and 1272 are built from completely identical parts. Don't take my word on it, you can compare these facts on a schematic for yourself.
Neve applied the 1272 as a line amp for make-up gain on a passive mix buss and while it is a fact that the module was used as a "line amp", there is more beneath the surface which many people who claim the 1272 is "just a line amp" seem to overlook. The buss impedance used on these consoles was somewhat unique in that it was set at an impedance close to the output of a microphone. So while symantics will tell us that Neve used a "line amp" for their makeup gain on the mix buss, when you look at a schematic you'll see that the "line amp" is set for 150ohm input impedance. Thats a mic pre. The mic input impedance on a 1073 is typically set for 150 ohms, same as a 1272. The console requires a mic pre for the buss make up gain, and thats exactly what they used, a mic pre at a fixed gain with a fader for trim. They labeled it "1272".
1272's have earned a lousy reputation as a mic pre not because they are not a mic pre but because implementing the gain switching is somewhat complicated or at the very least, somewhat expensive and many folks racking these or cloning these have elected to make cost cutting decisions which yield a crummy sounding mic pre. They don't sound bad as pre's because they were designed as "line amps", thats a total bunch of nonsense. They sound bad as mic pre's because people set them up wrong, period.
I've come up with a solution to this problem that saves hand wiring a very expensive switch and thrown in a few options along the way. Properly installed, the Glide On Fade Awesome Switch will allow your 1272 to perform *IDENTICALLY* to a 1073 in regards to gain staging the 283AV amplifier up to 50 ish dB for both mic and line inputs. This switch will only do the padding/boosting on the 283AV amplifier as per the 1073 design, so this switch only uses the 1272 for about 50 ish dB of gain. If you need more than 50 ish dB of gain from your mic pre, you should be using a mic pre capable of delivering the gains you need instead of wrenching every bit of gain possible out of a 1272. A 283 AV *WILL* produce 70dB of gain (or more), but in the process of doing so, the bottom end changes. If you want to stay with what Neve traditionally designed for this amplifier, this switch follows the switching arrangement they specified. If you need 80 dB out of a 1272, there are plenty of bad ideas out there, this is not one of them.
This switch additionally allows provisions to use the 1272 as a traditional LINE input with two choices:
The neve 1073 EQ used two different input transformers, a mic input transformer and a dedicated line input transformer. The switch in mic mode would pad and boost the 283AV wired to the 10468 input transformer and in line mode would switch the 283 AV to a 31267 input transformer for line level impedance.
The Neve 1063 EQ used a single 10468 input transformer, in mic mode the switch pads and boosts the 283AV and in line mode, a pad shunts the 10468 for use with line level impedance.
The Glide On Fade Awesome Switch can be configured to switch between two transformers or use one to accomodate a line level input. If you have a 1272 and do not wish to add an additional transformer, order the switch configured in 1063 mode, if you wish to add a line input transformer, order the switch configured in 1073 mode.
The switch itself is a 12 position precision grayhill switch, which if you've never used has a very positive action and feels like real quality in your hands while changing positions. There are 5 line input positions and 7 mic input positions. The switch can be mounted inside the 1272 module right on the front panel, see the wiring instructions for more detail.
If you have raw 1272 modules or if you are DIY'ing a mic pre based on the 283AV amplifier, this is a convenient, cost and space effective solution for gain switching but most importantly, preserves exactly the circuit design that was originally specified for this amplifier.
Switches are $85 and include two additional resistors and a capacitor which need to be installed directly on the 283AV edge connector inside the module. Please refer to the wiring instructions for easy to follow wiring instructions. Please specify if you would like your Awesome Switch configured for use with one input transformer or two. Its not a simple jumper on the switch, the boards are stuffed with different value components, so please be specific as to which you'd like to order.
Please note I have refered to total gain available as "50 ish dB". The transformers present in this design present gain to the output signal. Depending upon how you have the transformers strapped (or which you use if you are DIY'ing) there is a theoretical gain of about 60dB available stock with this switch. The list below is "Neve speak" for the 283AV gain staging available from this switch, but you may measure 60dB depending the transformers in your unit.
1 = +10 (in other words, a 10dB pad)