TwentyTrees
Well-known member
Hi all,
This is quick build report on a special project I completed yesterday, which has been many months in the planning and execution: 10 channels of Helios-style preamp + EQ in a custom 500 series rack!
The modules are based on board layouts by Matt from Iron Age Audioworks (member Dogears), and I'm indebted to him for his help with this project and his extraordinary generosity in sharing his work. They're the standard Helios "0011 upper module" design (2128 amp as mic pre > passive EQ > 2128 for makeup gain), though with Jensen input transformers and a discrete output buffer, and of course they sound fabulous. Metalwork was by Meface.
Two of the modules (9 + 10, with white caps on the +48V switch rather than red) are matched for stereo work. I matched all the EQ capacitors and inductors as tightly as possible, and then replaced the EQ gain pots with Grayhill series 56 rotary switches (solder tag version), to which I soldered resistor ladders using 0.1% thin film 0603 SMD resistors. Definitely a job for a clean magnifying glass, sharp tweezers, and a steady hand! That said, once I got the rhythm down I actually found it a lot quicker than my standard through-hole resistors and a Lorlin, and it's the only way I found to get close to replacing a 9mm pot with a rotary switch! Now the low and mid bands boost in 1db steps, very tightly matched (mid band cuts are much shallower, due to the way the cut / boost works in the Helios, but that works for me as I tend to cut in software for the greater control).
The rack itself is also worth mentioning. It's a modified TAC 511 Advance from Total Audio Control (member Sahib, whose advice was invaluable). It has an additional auxiliary TRS input per module, debalanced by a THAT1240 and bypassing the Jensen mic input transformer. A custom module in slot 11 routes relay control voltages to switch each module between its main input and the auxiliary input (and also filters the external SMPS before feeding the main power rails). The plan is to use the main inputs for mics, and the aux inputs as "tape" inputs fed directly from converters. The big orange button simultaneously switches all modules to their aux inputs, overriding the individual channel switches - switching from tracking to mixing modes at the touch of a button. There's also a passive mult of the output XLR to an additional TRS. I've labelled the switching / power filter module as "0232", after the Helios module numbering system for a module "containing various switches"! Metalwork again by Meface.
I'll finish with a huge THANK YOU to Matt (Dogears) and Cemal (Sahib) for all their help in bringing this beast to life, as well as to Ian (Ruffrecords) for his many insights into the Helios EQ, Whoops for sharing the Musicland Helios schematics, and many others besides. This unit is going to be the heart of my studio going forward, but is also designed to be relatively portable - hence the SMPS. I'm itching to use it in the field - can't wait to take it to a session with my band, once that sort of thing is an option again... We're releasing our debut album next Friday 25th June, and album no.2 is definitely getting tracked and mixed on Project Helios!
Cheers,
Andy
This is quick build report on a special project I completed yesterday, which has been many months in the planning and execution: 10 channels of Helios-style preamp + EQ in a custom 500 series rack!
The modules are based on board layouts by Matt from Iron Age Audioworks (member Dogears), and I'm indebted to him for his help with this project and his extraordinary generosity in sharing his work. They're the standard Helios "0011 upper module" design (2128 amp as mic pre > passive EQ > 2128 for makeup gain), though with Jensen input transformers and a discrete output buffer, and of course they sound fabulous. Metalwork was by Meface.
Two of the modules (9 + 10, with white caps on the +48V switch rather than red) are matched for stereo work. I matched all the EQ capacitors and inductors as tightly as possible, and then replaced the EQ gain pots with Grayhill series 56 rotary switches (solder tag version), to which I soldered resistor ladders using 0.1% thin film 0603 SMD resistors. Definitely a job for a clean magnifying glass, sharp tweezers, and a steady hand! That said, once I got the rhythm down I actually found it a lot quicker than my standard through-hole resistors and a Lorlin, and it's the only way I found to get close to replacing a 9mm pot with a rotary switch! Now the low and mid bands boost in 1db steps, very tightly matched (mid band cuts are much shallower, due to the way the cut / boost works in the Helios, but that works for me as I tend to cut in software for the greater control).
The rack itself is also worth mentioning. It's a modified TAC 511 Advance from Total Audio Control (member Sahib, whose advice was invaluable). It has an additional auxiliary TRS input per module, debalanced by a THAT1240 and bypassing the Jensen mic input transformer. A custom module in slot 11 routes relay control voltages to switch each module between its main input and the auxiliary input (and also filters the external SMPS before feeding the main power rails). The plan is to use the main inputs for mics, and the aux inputs as "tape" inputs fed directly from converters. The big orange button simultaneously switches all modules to their aux inputs, overriding the individual channel switches - switching from tracking to mixing modes at the touch of a button. There's also a passive mult of the output XLR to an additional TRS. I've labelled the switching / power filter module as "0232", after the Helios module numbering system for a module "containing various switches"! Metalwork again by Meface.
I'll finish with a huge THANK YOU to Matt (Dogears) and Cemal (Sahib) for all their help in bringing this beast to life, as well as to Ian (Ruffrecords) for his many insights into the Helios EQ, Whoops for sharing the Musicland Helios schematics, and many others besides. This unit is going to be the heart of my studio going forward, but is also designed to be relatively portable - hence the SMPS. I'm itching to use it in the field - can't wait to take it to a session with my band, once that sort of thing is an option again... We're releasing our debut album next Friday 25th June, and album no.2 is definitely getting tracked and mixed on Project Helios!
Cheers,
Andy