McCurdy AU300 clone - development thread

[amplexus]

Ok so I'm far enough along now that I wanted to put this out into the world. I've been working on these original AU300 tube preamp modules for over a year now for a client who's gone down the rabbit hole- going to far as to have a metal shop fabricate replica sleds so we can build multiple variations of transformer combos that he can swap in and out. He has the original 8 slot rack, the original tube regulated PSU, and I built him a full XLR IO panel, and a control panel with PEC pots, and stepped pads, and input impedance switching on the most expensive greyhills he could buy (customer supplied all the parts... he's _particular_) to give him all the options of the original transformers. Additionally to that he has had 10 sleds fabricated, and even went to an auto paint supplier and had mica blue paint blended and matched exactly to the original McCurdy colour. Suffice it to say that I've become pretty familiar with this circuit and it's construction. So I thought- man these are great sounding amps, why has no one cloned them? I can't swear no one has, but I suspect being that they're Canadian and rather rare to begin with has a lot to do with it. I also know in the past Doug Williams has expressed a lack of love for them- maybe that's scared people off

So 3 ish months ago I started developing the idea to put together a PCB kit and BOM. Carnhill makes a suitable output transformer that I had already used in some of his builds with great success- comparing favourably sonically to the originals- but we had only been using vintage hammond, UTC and Triad transformers on the input as he could find on ebay etc... So i got in touch with Cinemag, as they had a transformer that was close to the right spec- but i needed a centre tap brought out on the secondary. They were extremely accommodating and custom wound and shipped me four units to check out. Over the last few days I've been building up one of the prototype PCB's and power supply PCBs and finally got to basic functional testing last night and they came out beautifully. Very quiet even completely outside any sort of shielded environment, even a bit quieter than the originals. ~45dB up to ~68db of gain (~25db with that will be the supplied input pad), push-pull topology across 3 tubes (2 x 12AX7 1 x 12AU7 driver). At any rate- I'm very happy with how these have turned out at this prototype stage, so I thought I'd put them out in a thread for discussion/opinions/thoughts and to gauge interest in a possible PCB set offering. Ultimately I would like to throw them up as PCB kits, or potentially full kits in the White Market- but they're not quite ready for primetime yet, and I'm still waiting for some clarification on a couple things re: white market membership/procedures/rules. I have a few things on the wishlist to work out for the revision. On the to-do list:

• Shrink the footprint - I had originally laid this out for CDE 150/Vishay MKT1813 caps, but the Panasonic ECQ's sound great and the smaller footprint and lower price is nice, so I think i'll save space on the board
• Need to see if Cinemag will do the same transformer for me but with PC pins instead of flying leads so it can be PCB mounted. If not, I have also tested the Carnhill DI transformer wired backwards and it works great as well and is already a PC pin design. Either way, I need to adapt the layout for a board mounted transformer once i know what the options are
• Might tweak some values to alter the first stage gain- with hot sources it tends to saturate pretty fast. The input pad helps but it would be nice to be able to hit the input transformer a little harder if desired. Also might consider moving the input pad post-transformer
• The current prototype PSU is a 317 based reg for the heaters. It's enough current for 3 channels with sufficient heat sinking, but i'ld like to at least be able to provide for up to 8ch like the original supply. I've seen a schematic using a BUZ11 as a pass xistor that should be good for 5-6A
• Need to get all the various boards together (IO PCB, Main PCB, Pad/Phase PCB, output attenuator PCB, and possible gainswitch PCB if there's interest over a pot) and get a complete BOM assembled
• If i do ultimately put these on the White Market I'll be doing heavier copper layers (2oz), ENIG pads, and will probably do all the PCB's in black or red and in 2MM board if the cost isn't silly.
• May also tweak the connections. I thought about doing card edge and having a single connection point, but thinking about layout inside an enclosure I figured having the euroblocks on either end would shorten wiring runs a bit.
  

So anyhow, that's the story thus far.. Here's an image dump!

Originals in the original rack



Original tube regulated PSU after refurb. Needed to be gutted- all the wiring insulation was cracked and many of the resistors burned.



The originals post-refurb/phantom/pad/impedance installation:




My hastily constructed test jig last night on the new prototypes:


The PSU prototype:



And the original schematic:

 

[thomasdf]

very impressed by your bench ( <3 ) as well as your ultra clean refurb and upgrade.. following closely ! And def interested in a kit

 

[amplexus]

very impressed by your bench ( <3 ) as well as your ultra clean refurb and upgrade.. following closely ! And def interested in a kit

Much appreciated! It’s certainly grown over the years, but I’m actually really happy with the current layout and workflow, what I’m struggling with now is all the space in the shop that’s NOT bench- mostly a lack of it . This stuff is my day job too so it’s paid off to get it suited to efficiency.

I’m hoping to have a new layout rev done in the next week or so, and will keep updating as I go!

 

[JMan]

Very impressive! Looking forward to seeing how this develops and would certainly be interested in a kit whenever that might happen.

 

[fuzzedout]

I am very interested if you make a BOM & PCB kit available.

 

[amplexus]

I’ve been slowed down a bit here by a studio re-wiring, and a couple big time-sensitive jobs in the shop.
This project is still high on my priority list, just need a couple days to get things cleared up.

The holiday should help free up some time, and I’ll be able to get a bom put together for the main board at least, and try to get the next power supply rev done.

 

[punk]

wow - great work - definitely interested in pcb's. you must know shane....

 

[amplexus]

wow - great work - definitely interested in pcb's. you must know shane....

Shane’s reproduction units are my handiwork

He just left my shop about half an hour ago with a bunch of new patchbays I rebuilt for him actually.

 

[punk]

i saw those - absolutely beautiful - you used an 844 on the output - i have used an 842 on mine - i have tried input xformers from a PE1000 on input but the high frequency drops off ~12k if i remember correctly - i see you used A18 on input which i assume is to lower the gain - these things really need pads going in and coming out - i have made one with original iron in and out and the response is flat - but until i sort out pads they aren't getting a lot of work at the moment - but very loud and very quiet otherwise

 

[amplexus]

i saw those - absolutely beautiful - you used an 844 on the output - i have used an 842 on mine - i have tried input xformers from a PE1000 on input but the high frequency drops off ~12k if i remember correctly - i see you used A18 on input which i assume is to lower the gain - these things really need pads going in and coming out - i have made one with original iron in and out and the response is flat - but until i sort out pads they aren't getting a lot of work at the moment - but very loud and very quiet otherwise

We had 8 that are all original input and output iron- i refurbed those but mostly that was just to get them working and reliable for the main rack.

In the 8 frames he had fabbed it was a grab bag of transformers! A11, A18, 844…. He wanted to try a bunch and see how the sound changed. Iirc he liked the 844 output the least. We also both felt the carnhill output tranny was very close to the originals in sound.

the use of the A18 was actually for line input (so yes technically to lower the gain), but i personally found the a18 on the inputs quite fine for mic level too.

On these new kits i’m working on they will have a 20db input pad and polarity switch PCB as well as an output attenuator/fader pcb, in addition to the gain pot- So hopefully that will give lots of options for gain staging!

 

[emrr]

Nice work!

It’s hard for me to see needing output pads with a 24dbm output rating, given the 20db volume control range. 6db, maybe? Depends what it’s feeding.

Yeah 842/844 would be underspec for level handling in direct comparison, distortion surely higher. 850L/N probably better in some respects, inductance surely higher. The original output is what, 2-3x larger physically? Assume original in/out is custom Hammond.

I’ve no idea where these price out today relative to others. Way back when they were grossly overpriced relatively speaking, solely because of the comfort factor of variable gain which 99% of the others don’t have without modifications. Everything else may have caught up.

FWIW it’s probably obvious I’m not a fan of 12AX7 front ends anywhere other than guitar amps. I’ve owned many pre types at some point and haven’t kept a single one with an AX. Others are free to love them! Closest thing I have is probably 6AQ6.

 

[amplexus]

Nice work!

It’s hard for me to see needing output pads with a 24dbm output rating, given the 20db volume control range. 6db, maybe? Depends what it’s feeding.

Thanks! Yea the output fader idea was less for 'padding' more a convenience for trimming output level when sending direct to a DAW for example. On the original 'racking' I did stepped input pads alongside the multiple impedance switching the client wanted. We didn't do anything on the output. If memory serves I did 10, 20, 40dB steps? It worked quite well alongside the impedance switching which allowed some further tweaking of the input 'gain'- but this was largely at the client's request- I think likely a 20db pad switched out front is enough for most use cases.

Yeah 842/844 would be underspec for level handling in direct comparison, distortion surely higher. 850L/N probably better in some respects, inductance surely higher. The original output is what, 2-3x larger physically? Assume original in/out is custom Hammond.

I would say the original is maybe 2x the physical volume of the originals. I'm quite sure the originals were custom Hammond as well. And yes, we found higher distortion and not as much low end extension as the originals. He described them sounding a little 'pinched' which I took to refer to the slightly more limited bandwidth and increased saturation.

I’ve no idea where these price out today relative to others. Way back when they were grossly overpriced relatively speaking, solely because of the comfort factor of variable gain which 99% of the others don’t have without modifications. Everything else may have caught up.

Oh the prices are insane. Like I enjoy these preamps for what they are but the prices are insane. I would absolutely agree they're overpriced esp when they don't come with the original rack or power supply.

FWIW it’s probably obvious I’m not a fan of 12AX7 front ends anywhere other than guitar amps. I’ve owned many pre types at some point and haven’t kept a single one with an AX. Others are free to love them! Closest thing I have is probably 6AQ6.

I wouldn't pick these as a main set of preamps myself. They're definitely big and coloured sounding, and really cool on some sources- esp where you want some hair. But they are FAR from all-rounders. I went into this project because I think they're a fun tool, and a bit of a different flavour than every other DIY Neve and API and RCA out there... And I'd never seen these cloned personally, so I figured, why not! Given the small parts count and a good PCB kit one could probably do up a quad of these for under $1k, the biggest cost being the iron.

I may be an outlier tho, as one of my favourite tube preamp circuits is the UA M610 and those get their share of critiques as well.

 

[emrr]

They get a gold star for being fully push-pull, that’s definitely a minority sound.

 

[ShanePhilips]

very impressed by your bench ( <3 ) as well as your ultra clean refurb and upgrade.. following closely ! And def interested in a kit

Hi Thomas. Hope you are well. My name is Shane Philips. I am the person that got Amplexus Music to start helping with the McCudry Project. I have 3 pairs I am starting to unload if you can build your own power supply. One pair is listed on Ebay and Reverb and I have two other configurations that I jot Josh to build for me. I am willing to give DIY members a really good deal. Cost. I just want to put that money towards my album and fixing other things. Let me know.

 

[ShanePhilips]

I am very interested if you make a BOM & PCB kit available.

Hi There hot you are well. Im the one that commissioned the project. I have the modules for sale if you want. Ill give DIY members my cost plus 5%. You just have to build a psu. I have three versions to chose from. Cheers Shane.

 

[ShanePhilips]

Hi Thomas. Hope you are well. My name is Shane Philips. I am the person that got Amplexus Music to start helping with the McCudry Project. I have 3 pairs I am starting to unload if you can build your own power supply. One pair is listed on Ebay and Reverb and I have two other configurations that I jot Josh to build for me. I am willing to give DIY members a really good deal. Cost. I just want to put that money towards my album and fixing other things. Let me know.

Hi There hot you are well. Im the one that commissioned the project. I have the modules for sale if you want. Ill give DIY members my cost plus 5%. You just have to build a psu. I have three versions to chose from. Cheers Shane.

 

[thomasdf]

Hi Thomas. Hope you are well. My name is Shane Philips. I am the person that got Amplexus Music to start helping with the McCudry Project. I have 3 pairs I am starting to unload if you can build your own power supply. One pair is listed on Ebay and Reverb and I have two other configurations that I jot Josh to build for me. I am willing to give DIY members a really good deal. Cost. I just want to put that money towards my album and fixing other things. Let me know.

DM me with your price please

 

[Allbaldo]

Very excited to see where this heads! Interesting to see mention of the Hammond 842 in here. I have 8 of them I got out of an McCurdy console, and an 850 around here somewhere.

 

[amplexus]

So I know it's been a while- but I have gotten around to completing a single channel prototype build. At this point everything is working really nicely- But I might rearrange some of the connection points on the PCB based on how I have my unit wired- which is probably going to be a fairly standard configuration for most people.

The prototype currently has a pretty basic feature set:

-Stepped input gain, 7 steps from 48dB to 68dB in ~3-4dB steps
-Polarity invert and 20db pad on a switch PCB- Pad is pre input transformer
-H-Pad variable output attenuation - Attenuator is post output transformer
-Cinemag custom input transformer- basically their Quad 8 input transformer but with a centre-tap brought out on the secondary side for P-P Grids. I also tested the circuit with a Carnhill VTB9072 DI transformer wired backwards and had excellent results. I may rework the PCB to board mount this unit, as it's readily available and I like the ease of assembly of pcb mounting it. But the flying leads on the Cinemag were no major issue either.
-Carnhill 2290 Gapped output transformer
-Input and output RFI/EMI protection on xlrs


Gobs of gain and with the pads and attenuators structured this way I think it gives lots of options for gain staging. In a normal Penn-Elcom 2U deep enclosure you could easily fit 2 channels with an internal PSU, or 4 channels if you externalized the PSU to it's own box. I used two toroid PT's for Filament and B+ but that was a cost thing. Could just as easily use anything that has a 230VAC and 15-18VAC tap with the PSU circuit i've built. Heaters are wired for 12V. I haven't added phantom to this unit, but it would be a simple thing to drop in a small SMPS with some extra filtering for the 48V.

Got a few little things to tidy up on the PCB, shrink it a bit, move connections around for ease and cleanliness of wiring etc... It's a little sloppy right now.

 

[shabtek]

gapped push-pull opt?