The first 1176 and the granddaddy of them all was the Bill Putnam designed Revision A. Based upon the 1108 preamplifier, the pre and line amp stages used FETs as the first active element (later revisions used bipolar transistors in each of the amplification stages). Only a small number of units were made; serial numbers seem to suggest that just 115 of the Rev A units were ever manufactured.
Coveted by a fortunate few big name mixers and producers, the only opportunity to hear one of these things in action has been on major label releases. The DIY Rev A board hopes to has changed all that.
The power supply has been taken from the later revisions eliminating the need to source the hard-to-find power zener. Also a trimmer potentiometer has been used in place of the selected, fixed resistor (R44) in the original. This allows quick, easy adjustment of the Gain Reduction Meter Driver Amplifier without resorting to hand-selected resistor values and by eliminating the need for soldering-in parts after the unit is completed.
Precision resistors from the later revisions have been selected for the Gain Reduction Control Amplifier section. This eliminates the need for hand-selecting and tedious sorting of resistors and insures good performance using now-common off-the-shelf 1% resistors (these were quite rare and very expensive when the Rev A was first designed).
The input circuit is identical to the revisions B through F though one source reports that a Peerless transformer was used in some units. But for the vast majority of the 1176's life, a UTC-O12 was used. You can try to source one from the used market, but luckily Ed Anderson has arranged for reproductions to be made and Hairball Audio is now supplying them along with the special output transformer.
The physical layout of the board closely resembles the arrangement of the schematic diagram and each circuit block is clearly outlined and labeled. This is to promote a clearer conceptual understanding of the circuit to allow each circuit block to be analyzed in isolation, if required, and to make it simpler to trace the audio and control signal paths of the circuit.
Stuffing The Boards
Confirm which version board you have by looking at the lower left corner of the main PCB. Use the correct BOM for your board linked in the left hand column of this page. Currently supplied boards are V1.2.5; earlier versions were based on serial numbers 101-124 of the original units while the boards supplied now are based on serial numbers 125 to 216.
Some have found the changes to the later serial numbers have resulted in a more controllable limiter so these modifications have been incorporated into the current V1.2.5 boards. You can see how to modify an earlier PCB here or by comparing the schematic from the original pre-serial-number 125 units and post 125 in the manual downloadable from the JBL Pro Service site. Changing the V1.2.5 boards to an earlier S/N is just as simple - just use the earlier BOM for the values, move a couple of components in the Signal Preamp stage, change the ratio resistors and eliminate the feedback components in the Signal Line Amplifier.
I would suggest that you test each component before stuffing it in the board. It only takes a few seconds to verify a resistor value and the time you spend beforehand will - one day - save you hours of finding that faulty component after all is said and done and you're faced with a non-working piece of equipment. Even if your eyes are very good and you know the resistor codes backwards and forwards you can usually count on a bad component if you don't check before inserting it into the board.
All of the components needed for the printed circuit boards are currently available. The bipolar transistors are manufactured by Central Semiconductor Corp. and the FETs are available from Fairchild and others. All of the on-board parts should be available from Mouser. If substitutions are required or desired please note the pinouts of replacements carefully. These original "trailing edge" bipolar transistors have a different pin assignment than newer devices. Similarly, "European" BC devices have different pinouts as well. Read the data sheets carefully if using substitutions.
As indicated on the original schematic, some details regarding the transistors should be noted. Select Q12 and Q13, matching hFE within 10% (the absolute value is not critical). This will allow proper calibration of the meter driver circuit and correct operation once calibrated.
Q7 through Q10 must be selected for an hFE of 250 or above. Additional transistor footprints have also been provided to allow for the substitution of alternate devices for Q3, Q5, Q7-10, Q12 and Q13 such as the 2N5088 used in the Purple Audio MC76. The 2N5088 typically has a higher beta than the 2N3707 in the samples I've tested. Another alternative is the 2N3708 which are selected for high beta.
Matching Q1 and Q11 may ensure accurate tracking of the gain reduction metering circuit that reflect the actual behavior of the audio gain reduction. However, keep in mind that the meter circuit provides a visual indication of the audio signal, so accurate matching of these parts might not be essential. You may wish to select FETs for a particular threshold range.
Capacitor footprints were selected to match the dielectric type suggested by the schematic and are sometimes based on knowledge of the original circuit. Non-polar capacitors of a typical range covered by modern plastic-cased film types have 0.2" pitch as does the range covered by ceramic types, while electrolytic footprints were chosen for pitch relative to capacitance value and voltage.
In addition, pads for wound construction film coupling capacitors in the audio path are provided. Sprague "Orange Drop" caps were used in the prototype with good results. The part numbers used were 225P15491SD3 (current part appears to be 225P15491XD3) for the 0.15uF and 225P10591YD3 for the 1.0uF values.
A footprint is provided to either wire a link or solder in a three pin header to put the Tracking Adjust trimmer in or out of circuit. The trimmer is normally out of circuit so you will have to link the rounded rectangular pads together once the initial Null Adjust trimming is done, otherwise your gain reduction meter circuit will not operate correctly. The square third pad is provided as a place to put the shorting plug temporarily if you use the header scheme.
Please refer to the manual from JBL Pro linked in the sidebar for a full explanation of the calibration procedure or use the relevant videos from the Rev D page. Alternatively, you can refer to a post by Studio Electronics' guru David Kulka: here for discrete meter circuit calibration in a nutshell.
Wiring Details
All external wiring points on the main board terminate at the edges of the board and are placed on 0.2" centers. This makes it possible to use screw terminal blocks, simplifying wiring and allowing alterations to be made without de-soldering and potentially pulling up printed circuit board traces. For long term reliability though, soldering wires directly to the board is probably the best option. Edge placement helps keep the wiring neat and tidy by eliminating routing of hookup wires over the center of the board surface.
If you do use screw terminal blocks, make sure you use the correct version top screen layer PDF linked in the left hand column as a reference. Some of the pad designations are obscured by using terminal block and using unconfirmed wiring drawings that refer to earlier PCB versions can cause unintended consequences. Use the correct PDF for your board version.
The AC input pads have been offset from the output transformer connections to provide a clear visual demarcation between them and prevent catastrophic wiring errors from occurring. Output transformer designations are for the Cinemag replacement transformer. Colors are marked on the board and on the PDF overlay screen layer page as well as the schematic. Be sure you are using the correct version PDF for the board you are using to confirm pad designations for external wiring.
Generally, wiring the Revision A is the same as wiring a rotary version of the G1176. One subtle difference is that the GR (Gain Reduction) meter setting doesn't ground one terminal of the VU meter in the discrete version as it does in the IC circuit and special rotary control PCBs have been designed with this in mind. Please refer to the original schematic if you have any doubt about the wiring (note that the ability to read a schematic is a prerequisite for building this project).
Revision A V1.2.5 24.07.10 Errata:
Main PCB Screen Layer;
R12 marked 1k should be 2.2k
R16 marked 47k should be 100k
R30 marked 120k should be 150k
R76 marked 8.2k should be 18k
C11 marked 47pF should be 10pF
Meter Board;
3.6k resistor marked R16 should be R18
Revision A V1.2.5 Documentation has been updated to reflect the above corrections.
Revision A V1.2.5 05.12.10 (currently being supplied):
No known issues.
Please refer to overlay, BOM and schematic linked in sidebar for your particular version/date code of board set.
