Project Helios (ten "Type 69" in 3U)

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TwentyTrees

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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!

A black 500 series rack, 3U high, containing ten Helios type 69 style input modules. Each module is a discrete transistor mic preamp, with a passive equaliser. There is an additional module to the far right, with a bank of buttons.

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.

Small 3 - Full Helios rack 3.jpg

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).

Small 7 - 0011 module - EQ stepped gain switch.jpg

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.

Small 9 - 0232 module wiring.jpg Small 8 - 0232 module.jpg

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
 

Whoops

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Hello Andy,
congratulations for your beautiful build, there's many hours of work involved in doing that and the end result looks fabulous.
Wish you can make great music with those units.

Out of curiosity what discrete output buffer did you use?
Is it balanced out?
 

dogears

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Texas
Hey Andy, fantastic work man! It would look even more beyond slick if you put a veneer or something on the ears so they matched the modules! :cool:

@Whoops the output is a simple class-A job as discussed in the original thread, and outputs are impedance balanced.
 

TwentyTrees

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Thanks gents! Very much appreciate the feedback. 😁

Matt, I LOVE the idea of a veneer on the rack ears, that would look sharp as hell... Maybe a thin alu fascia epoxied in place? I'll think on it!
 

Whoops

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the output is a simple class-A job as discussed in the original thread, and outputs are impedance balanced.

Is that the one by Dave Amels used in the Lenny Kravitz console?
The one that member Winston O'Boogie posted?
 

L´Andratté

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Everything about this is great!
Helios pre and eq define the term "musical", that thoroughly beaten dead horse...
And it is well thought out product design, looks absolutly lovely and pro (funny how important that often is, as with people:))
 

fragletrollet

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Oslo
I like the idea of using a 500-rack! I'm building an aux expander to my mixer and have been noodling on this front panel design for a year or two :S It also uses a 500-rack, so each module will line up logically with my 500-modules (albeit my "500-slots" on the expander are stereo modules). Makes for a coherent, streamlined look.

I just ordered some Helios eq-boards from Ian's open source treasury; guess I'm late to the party but I guess that's the name of the game, being into vintage electronics and born in the late eighties :p

Cool idea with the grayhill stepped switch! I got four Elma 06 switches, which are also worth looking into regarding small switches. Mine are double pole 12 position, probably not much wider than the grayhill depending on resistors. There's also this one that's tiny, but not shorting style like the 06 can be.

Where do you get your Grayhills from?

All the best,
-m
 

TwentyTrees

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Yeah, the 500 series is such a great format for this kind of thing. Heck, leave a notch at the top of your module cards to fit ribbon cables and it would be totally possible to use it as the base for a full mixer! Might try that one day... 🤔 In this case, I've already got one TAC 11 slot rack, and as you say having a consistent module spacing really helps. Interested to see your aux expander!

Thanks for the tip on the Elmas, no dual gang Grayhill 56 so that's definitely helpful. I get my Grayhills from Mouser or Digikey, depending which happens to be cheaper - took a while to build these up, what with the shortages.
 

TwentyTrees

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That is an very professional looking result. Well done. What colour is the background of the front panels?


Cheers

Ian
Thanks Ian & John! The CMYK code for the front panels is C10 M05 Y00 K71 - a dark grey, rather than black. I use anthracite grey when I get stuff powdercoated, it doesn't seem possible to colour match that with the Meface process but I'm very happy with the result. Legend are unprinted anodised alu. The panel overall is not as matt as a flat anodised panel, nor as reflective as a glossy powdercoat - perhaps slightly lower contrast than either, but I'm content it should be well legible in the studio!
 

ruffrecords

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Thanks Ian & John! The CMYK code for the front panels is C10 M05 Y00 K71 - a dark grey, rather than black. I use anthracite grey when I get stuff powdercoated, it doesn't seem possible to colour match that with the Meface process but I'm very happy with the result. Legend are unprinted anodised alu. The panel overall is not as matt as a flat anodised panel, nor as reflective as a glossy powdercoat - perhaps slightly lower contrast than either, but I'm content it should be well legible in the studio!
OK, understood. On my computer the panel background colour has a brown tinge, verging on the old Cadac colour. Probably just the combinations of photography and monitor.

Cheers

Ian
 
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