Just built a pair [now TEN!] REDD.47 (EMI/Abbey Rd) preamps!

Get that song on tape! Errr... disk?
User avatar
øøøøøøø
PAT. # 2.972.923
PAT. # 2.972.923
Posts: 5984
Joined: Sun Jan 06, 2008 8:26 pm
Location: Los Angeles
Contact:

Just built a pair [now TEN!] REDD.47 (EMI/Abbey Rd) preamps!

Post by øøøøøøø » Sat May 25, 2013 3:47 am

Just buttoned these up today, and want to show them off! I cheated for this project and used the newest iteration of the Drip Electronics PCB (not yet on their website), with a few changes, additions, and modifications. It's a ridiculously nice PCB, so "why not."

These are based on the original REDD 47 preamp design, but I did a few things to optimize them for stand-alone rackmount use (i.e. not in a console) to make sure the input and output transformers were always loaded with proper impedance, etc.

I have a stepped T-attenuator on the input which always provides constant 200 ohm impedance, and a continuously variable T attenuator (in rotary fader format) on the output which also provides constant 200 ohm impedance. The british-style transformers want to be loaded with 200 Ω at all times to minimize ringing/overshoot. I also added switchable regulated +48v phantom on its own power transformer, a polarity/phase switch, and an isolated DI input.

In order to keep them cool and house them in a 2U case, I skipped the tube rectifier and used a pair of UF4007 ultra-fast silicon diodes instead. The heater supply is a super burly regulated DC filament supply. There are dual 0A2 regulator tubes, and I used NOS Valvo EF86 and GE 6DJ8/E88CC.

I went a little silly (but not too silly) with the caps. I tried to avoid electrolytics. Power supply filters are mostly big Solen film caps (not too overly huge or pricey). A Rel-Cap AudioCap tin/polypropylene is the coupling cap, and a NOS Vitamin Q paper-in-oil 1µf is the output capacitor. Resistors in the audio path are mostly carbon composition, with metal oxides and metal films in the power supply.

The DI is stone-simple... I used a "make" switching Switchcraft jack and shoulder washers (isolating it from the chassis). A 1M resistor sets input impedance for the DI, and there's a 68k grid stopper inserting the DI signal right on the grid of the EF86, as physically close to the tube as possible. When nothing is plugged in, the "make" switch breaks, and the DI is completely out-of-circuit.

I settled on a scheme with two star grounds, one for the power section and one for the audio amp grounds. All internal signal runs are shielded, with the internal shields connected together and terminated at pin 1 XLR (except DI, which has the shield tied to the 'cold' of a shielded pair and terminated only at the jack itself, other end of the shield floats). The thing is dead quiet even wide open and at unrealistic monitor levels.

The graphics are done with waterslides of my own design, intended as a sort of tribute to the look of the original modules. I used a pewter gray base and then clear-coated over the waterslides.

They sound freaking fantastic. The first test was with a 4038 overhead and it was a great pairing. The DI sounds great as well... very clean and "hi-fi" sounding, not artificially big as you may expect a tube DI to be. Awesome for synths, guitars, and if a bass is supposed to sound really clean and punchy (not so much vibey, actually!)

...and without further ado, here they are in the racks!

Image

For reference, here is a pic of the cassette-format (external power supply) original modules (second module from left). My aesthetic concept was more a "tip of the hat" than a straight clone, since these would obviously be mounted in a big 2U case with onboard power supply (instead of a small cassette).

Image




All in all, a worthwhile project!
Last edited by øøøøøøø on Sat Feb 15, 2014 5:57 am, edited 1 time in total.

User avatar
øøøøøøø
PAT. # 2.972.923
PAT. # 2.972.923
Posts: 5984
Joined: Sun Jan 06, 2008 8:26 pm
Location: Los Angeles
Contact:

Re: Just built a pair of REDD.47 (EMI/Abbey Rd.) preamps!

Post by øøøøøøø » Sat May 25, 2013 11:00 am

by the way, I opened one back up today and grabbed a gut shot.

Transformers are Sowters. The big radial electrolytics over on the right are the filter caps for the DC heater supply. For these "prototype" units, I miscalculated the number of Solen film filter caps I'd need, so I threw a few of those box-shaped Wima film caps on a Mouser order I already had to make, and just zip tied them down, running wires to the holes on the PCB. Not super elegant, but definitely functional and secure.

The little daughter board on the back panel is for regulated phantom power. The stepped T-attenuator on the front panel was an undertaking in and of itself! The output fader (continuously variable) is a Bourns T-attenuator as sold by Hairball Audio to be used in their 1176 clone. It's originally a 600 ohm impedance attenuator, but a few resistors on the back and it's a very smooth-tapered 200 ohm attenuator.

Image

User avatar
OffYourFace
Mods
Mods
Posts: 13681
Joined: Thu May 03, 2007 2:59 pm
Location: Los Angeles, CA

Re: Just built a pair of REDD.47 (EMI/Abbey Rd.) preamps!

Post by OffYourFace » Sat May 25, 2013 11:21 am

Awesome! Thanks for taking that pic!
What are you using for the (gray) shielded cable, Canare?

Oh and where did you buy your Valvo ef86?

User avatar
øøøøøøø
PAT. # 2.972.923
PAT. # 2.972.923
Posts: 5984
Joined: Sun Jan 06, 2008 8:26 pm
Location: Los Angeles
Contact:

Re: Just built a pair of REDD.47 (EMI/Abbey Rd.) preamps!

Post by øøøøøøø » Sat May 25, 2013 1:31 pm

Canare install cable for the shielded runs (we just had a pile of it laying around), and Valvo EF86s (NOS, NIB) came from eBay.

eggwheat
PAT. # 2.972.923
PAT. # 2.972.923
Posts: 5151
Joined: Sat Mar 03, 2007 8:30 am

Re: Just built a pair of REDD.47 (EMI/Abbey Rd.) preamps!

Post by eggwheat » Sat May 25, 2013 1:53 pm

Look great! how does it sound? My dad worked in the audio dept at E.M.I in Hayes for years.. :)

User avatar
øøøøøøø
PAT. # 2.972.923
PAT. # 2.972.923
Posts: 5984
Joined: Sun Jan 06, 2008 8:26 pm
Location: Los Angeles
Contact:

Re: Just built a pair of REDD.47 (EMI/Abbey Rd.) preamps!

Post by øøøøøøø » Sat May 25, 2013 2:07 pm

Haven't had a chance to give them a thorough workout yet, but here's what I know so far:

1) With Coles 4038 mono overhead above low-tuned Ludwigs in our big room, it was a great pairing. We tried a Neve 1073, an API 512, and an Eisen Audio preamp with a Hardy 990-inspired circuit. The REDD was the clear winner on this particular source-- big, wide, lifelike, silky... extended-seeming top end AND bottom end when compared to all other options. The API was actually our least favorite in this application.

2) the front panel DI is fucking awesome. Syrupy and almost "amp like" on a guitar... not sterile at all. Glassy, too. Not exaggerated low end... great on bass too, but more 'hi fi' as opposed to subby/dubby, so perhaps not for everything. Great to warm up synths a bit, but watch the level... will distort anywhere the front end of a guitar amp would distort, basically. Something with almost line-level output will clip the front end for sure.

3) Plugging into the front panel DI and then daisy chaining the two preamps in series (using the first to overdrive the second) with an ES330 gives you the exact same fuzzed-out guitar sound as "Revolution." Exactly. Massive amounts of NFB in this amp make it break up rather suddenly and completely!

4) Plugging pre-recorded music into the front panel DI sounded a lot more exciting and lifelike than doing the same with the front panel DI on API 512 pres.

Haven't tried them as a pair yet (except on the program material), though we have a nice new-to-studio nickel-capsule/AC701k Telefunken SM2 (single point stereo) on a stand waiting for this sort of experimentation. We'll see.

User avatar
jimboyogi
PAT. # 2.972.923
PAT. # 2.972.923
Posts: 2083
Joined: Tue May 27, 2008 2:38 am
Location: Melbourne
Contact:

Re: Just built a pair of REDD.47 (EMI/Abbey Rd.) preamps!

Post by jimboyogi » Sat May 25, 2013 2:13 pm

Brad, congratulations!

I don't understand most of what you have written, but the builds look terrific, well done!
øøøøøøø wrote: 3) Plugging into the front panel DI and then daisy chaining the two preamps in series (using the first to overdrive the second) with an ES330 gives you the exact same fuzzed-out guitar sound as "Revolution." Exactly.
This I can understand, and it makes me very happy :)

Love to see the schematic of the pre.

User avatar
øøøøøøø
PAT. # 2.972.923
PAT. # 2.972.923
Posts: 5984
Joined: Sun Jan 06, 2008 8:26 pm
Location: Los Angeles
Contact:

Re: Just built a pair of REDD.47 (EMI/Abbey Rd.) preamps!

Post by øøøøøøø » Sat May 25, 2013 2:17 pm

Here's a clean version of the schematic for the original REDD. Mine sticks to this one literally except the other stuff I added (DI, input and output attenuators, phantom, and phase switch), and the fact that there is no rumble filter (on the secondary of the input transformer), to extend the bottom end.

Image

Here's the original REDD document in .pdf form (a little more detailed).

http://www.ladislavbrezovnik.com/diy/dr ... ematic.pdf" onclick="window.open(this.href);return false;

User avatar
fuzzjunkie
Expat
Expat
Posts: 7276
Joined: Thu Oct 12, 2006 11:32 am
Location: Seattle

Re: Just built a pair of REDD.47 (EMI/Abbey Rd.) preamps!

Post by fuzzjunkie » Sun May 26, 2013 3:49 pm

Damn impressive, even more than your U-47 build. Always in awe of the Redd 47. Only ever seen a pair before and never thought to build one. I know there is a clone or two out there, but this build has a much better aesthetic!

User avatar
Joeleo
PAT. # 2.972.923
PAT. # 2.972.923
Posts: 818
Joined: Thu May 13, 2010 7:55 pm
Location: Detroit, MI
Contact:

Re: Just built a pair of REDD.47 (EMI/Abbey Rd.) preamps!

Post by Joeleo » Sun May 26, 2013 6:23 pm

Beautiful! There is no sexier gear-porn than pre-porn.

User avatar
ziess
PAT. # 2.972.923
PAT. # 2.972.923
Posts: 1602
Joined: Sun Nov 26, 2006 10:55 am
Location: Near Glasgow

Re: Just built a pair of REDD.47 (EMI/Abbey Rd.) preamps!

Post by ziess » Thu May 30, 2013 9:17 am

Those look really cool. Where did you source the faceplate from?

User avatar
ziess
PAT. # 2.972.923
PAT. # 2.972.923
Posts: 1602
Joined: Sun Nov 26, 2006 10:55 am
Location: Near Glasgow

Re: Just built a pair of REDD.47 (EMI/Abbey Rd.) preamps!

Post by ziess » Thu May 30, 2013 11:53 am

Check you out! Looks really good. Been wondering about some preamp designs for a while, might take a look at making one of these, or maybe a stereo unit.

User avatar
ziess
PAT. # 2.972.923
PAT. # 2.972.923
Posts: 1602
Joined: Sun Nov 26, 2006 10:55 am
Location: Near Glasgow

Re: Just built a pair of REDD.47 (EMI/Abbey Rd.) preamps!

Post by ziess » Thu May 30, 2013 11:57 am

In fact, now that I think about it I have the PCB half done for a G7 so maybe I should finish that first!

User avatar
øøøøøøø
PAT. # 2.972.923
PAT. # 2.972.923
Posts: 5984
Joined: Sun Jan 06, 2008 8:26 pm
Location: Los Angeles
Contact:

Re: Just built a pair of REDD.47 (EMI/Abbey Rd.) preamps!

Post by øøøøøøø » Tue Jun 04, 2013 8:12 am

New heat sinks on the DC heater supply regulators and a few extra ventilation holes in the top of the chassis, and both units are happy now... full 10 hour day yesterday and no funny business. I think I can officially call them "done."

They were really, really good on saxophone yesterday. Can't wait to give the pair a workout on things like piano and drum overheads.

User avatar
cauger
PAT. # 2.972.923
PAT. # 2.972.923
Posts: 460
Joined: Sun Jun 01, 2008 3:13 pm
Contact:

Re: Just built a pair of REDD.47 (EMI/Abbey Rd.) preamps!

Post by cauger » Wed Jun 05, 2013 3:40 pm

Awesome! I'd love to hear some clips of it in action if you ever get a chance.

Post Reply