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

Get that song on tape! Errr... disk?
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Re: Just built a pair of REDD.47 (EMI/Abbey Rd.) preamps!

Post by OffYourFace » Sun Jun 09, 2013 2:00 pm

awesome, great work!

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

Post by jrjoe21 » Mon Sep 30, 2013 8:51 am

OffYourFace wrote:awesome, great work!
+1 and love those knobs! What are they?

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

Post by øøøøøøø » Mon Sep 30, 2013 9:09 am

some surplus vintage knobs I got from a place in Brooklyn that has a big stash of weird old oddities.

By the way, we've begun ordering parts to make 8 more of these.

It's about to get serious!!!

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

Post by rustywire » Sat Oct 26, 2013 4:37 am

Nicely done. Way to bring them into the 21st century!
The hunt is on, and on and on...

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

Post by Gorquin » Wed Dec 04, 2013 7:28 am

Maybe my brain is fried I don't know but I don't see how the "Make" DI Switch is isolated when in use.

I understand how the "Make" switch works and agree when nothing is plugged in the DI is out of the circuit.
However, when the DI IS plugged in doesn't that put 1/2 of the MIC Input Transformer (The Secondardy, that goes to ground) in Parallel with the 1M resistor to ground (from the DI) unless of course you're using a Mic/DI switch as well?


Great looking build BTW !

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

Post by øøøøøøø » Wed Dec 04, 2013 1:36 pm

the 1M resistor is switched with the "make" switch! The "tip" connection is shielded, and floats when nothing is plugged in.

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

Post by Gorquin » Wed Dec 04, 2013 4:41 pm

OK so the Tip has a 68K in series with a shielded cable to the grid? (Floating when NOTHING is plugged into the DI)
The 1M is connected to the "Make" connection on one end and ground on the other. (Floating when NOTHING is plugged into the DI) This is kind of like a Fender Champ 5F1 input circuit.

So the question is: Is the Mic Input Secondary (Coil, Cap, Resistor to ground) ALWAYS connected to the Tube Grid or is it Switched?

If it's NOT switched and you USE the DI Input, then don't you have the signal coming in putting a small amount to ground through the 1M, going through the 68K to the Tube Grid which then also puts some signal to ground through the Secondary of the Mic input circuit?

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

Post by øøøøøøø » Thu Dec 05, 2013 5:23 am

mic transformer secondary and the Zobel network (R1 and C13) are still connected to EF86 grid when the DI is plugged in. I do understand that this puts an impedance in parallel with the 1M (through the transformer secondary). In practice, it works fine (great, even). I decided to try the simplest approach first, and to err on the side of the mic preamp circuit-- i.e. I wanted a simple implementation that would allow me to use the DI occasionally without ever affecting the mic input operation.

It works great!

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

Post by Gorquin » Thu Dec 05, 2013 6:21 am

OK, well that's the bottom line….How it sounds ! Thanks for the input. I thought maybe I was missing something.

I'm a tech, not an engineer, but I had a small company that did a LOT of tech and refurb repairs for major home and car audio co.'s.
It's surprising the weird things you run across when you do thousands of repairs/year.

Great looking build. I'm giving serious consideration to building one myself.

Thanks

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

Post by øøøøøøø » Thu Dec 05, 2013 6:54 am

We've got 8 more ready to build, but DRIP has been a little slow getting the PCBs out to us this time around. Some issue with their vendor.

We're skipping the 200Ω input T attenuator this time. We've discovered that most mics perform a bit better (in some tiny, nebulous way) with the attenuator out-of-circuit. I misunderstood/miscalculated the reflected impedance through the tube back to the mic-- in normal operation, the input Z should be quite a bit above 200R... more like above 2K. We'll make use of the 34% tap on the input transformer instead-- this will allow us to pad it down while keeping the input Z high.

We're also skipping the DI this time around. While it's nice, we don't need it on all 10 channels. 2 DI inputs in the control room will be quite enough (usually way more than enough, in fact).

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

Post by unvs » Sun Feb 02, 2014 5:41 am

Great job, always been curious about those pres

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

Post by øøøøøøø » Sun Feb 02, 2014 8:24 am

since you bumped it...

Image

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

Post by øøøøøøø » Sun Feb 02, 2014 8:35 am

Only 7 boards are pictured here, but there are actually 8. They're being built in matched pairs, with each component matched to <1%. The first two pairs are probably matched to within <0.25%, truth be told... each capacitor, resistor, attenuator, etc. checked and measured/grouped by hand. I have to find a way to match tubes next.

They're further along than just the boards shown the pic... everything is ready to go as soon as custom enclosures arrive-- they're being professionally made for us by http://www.protocase.com" onclick="window.open(this.href);return false;, who have been exceptional so far. Powdercoated in ANSI 70 light gray with black silkscreening.

A few updates to this revision...

All filter caps are now 630v Solen "Fast" film caps... no electrolytics in the power supply anymore. The only electrolytics in the unit are the filters for the regulated DC filament supply and the cathode bypass caps-- for the latter I used Nichicon KZ "audio grade" electrolytics, increased 2x value for the EF86 and 5x the value for the ECC88, each bypassed with a polypropylene film cap of 1% total value. I have 220µF bypassed with 2.2µF film for the EF86 bypass, and 470µF bypassed with 4.7µF film for the ECC88 bypass. I did a little math and this should get more some more low end extension over the stock values of 100µF for both (on the ECC88 in particular, there's a high probability that the 100µF cathode bypass was causing degenerative NFB below about 30 cycles).

I ditched the rotary input T pad. Instead I've created a tandem of two different switched attenuators-- one that switches taps on the input transformer to reduce gain by 34%, and the other a simple 20dB resistive T pad that keeps input Z at a nice high bridging level (i.e. around 2k or so). The combination of the two input attenuators allows gain settings from 2dB all the way up to maximum, with never a 'gap' of more than 4dB between settings. A little less elegant "user interface" wise, but much more flexible and better sonically-- a tradeoff I was willing to make. It also saved me from having to make those godforsaken attenuators, which took forever.

I added a fourth "voltage gain" position. Voltage gain settings are achieved by varying the amount of inter-stage negative feedback. I added an extra position by allowing provision for opening the interstage loop entirely for a bit more 2nd harmonic coloration and about 3dB additional gain... In practice about an additional 3dB.

I think these are going to sound amazing. the two prototypes didn't have many shortcomings, but I think these will be even better. The two prototypes are moving into the other studio, and these 8 will be in the biggest tracking room. Should be incredible.

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

Post by OffYourFace » Mon Feb 03, 2014 12:00 am

Very Nice!! I'll be in NYC in late June... can I get a tour? ;D

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

Post by øøøøøøø » Sun Feb 09, 2014 8:40 am

Today's progress report.

You don't realize how many EIGHT PREAMPS are until you have them all going at once. Starting to look like a factory in here!!

Image

Faceplate...

Image

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