Hello all,
First a quick disclaimer, there’s a lot of background info here but if you prefer to skip it and look at the design goals I’ve put them in the next post.
I’ve been getting the itch to tackle another valve project lately and given that the Motown Direct Amplifier was the single piece of equipment that originally got me thinking about valve electronics in the first place, I’m thinking about attempting to recreate it in some way. Sadly, I just recently found out that the designer of said direct amplifier, Mike McLean, passed away last November. His schematics were never made public as far as I am aware, and there is a company producing a modern version of his circuit (allegedly from the original schematic) whom I very much doubt will ever make it available. Therefore, to build a picture I thought it best to consolidate all the information that was put forth by Mike himself along with his esteemed colleagues. Given that the aforementioned recreation is/was made in limited numbers, info is very scarce and I’ve had to study a handful of pictures and brief discussions to work out (roughly) which components were used. While I doubt that it will have been made 100% faithful to the original, it might be enough to give some ideas on topology and make some educated guesses in order to design something more of an homage than a straight copy.
By way of brief history for those who aren’t aware, the Motown Direct Amplifier was an amplification device (pic from theproaudiofiles.com attached) which was designed and built in-house and used at the Motown Hitsville USA Studio; it was intended to remove the need for artists to lug multiple heavy amps in and out of the studio when time and space was at a premium. The unit sat at the back of the room and gave five musicians (usually the bassist & guitarists) the ability to plug in their instruments, set their recording level, and monitor themselves in the live room. Importantly, it also satisfied the need for “a high fidelity, high performance preamp between the guitar and the line input of the console mixer position.”
To quote Mike further:
In terms of a technical brief, to quote Mike again:
I expect that modern line input standards might modify the point about driving a 300 Ohm load, but the rest would stand. Any input on the above is appreciated.
Mike didn’t give any indication of topology, but various sources give suggestions. We know that they were vacuum-tube preamps, and Ed Wolfrum said:
Mike himself said, when referring to the Motown studio in general:
There was also speculation online that some of the designs may have been borrowed or expanded upon based on existing equipment at the time, but it’s impossible to say how true this is:
From Ken Sands, who joined Motown in 1967:
That’s also pretty much the extent of what is known via the original designer and those who worked with him. Again, it’s hard to say just how accurate these details are. The rest is speculation based on the appearance of the modern recreation; note that this also has a mic preamp included that was not part of the original unit (although apparently is based on Motown designs.) I’ve briefly broken down the components in notes that I have gathered:
The thing that stands out is the use of the 6V6, which (in my limited understanding) should probably be driving a speaker rather than a line level output? I have triode-strapped an EL84 for a line output in a previous project and that worked, but I was led to understand it is very uncommon. My first thought when I saw it was that it was a rectifier tube; I struggled to identify it, before later finding confirmation that it was a 6V6 (or a variant.) Perhaps it was common in guitar amplifiers at the time (I think the Fender Champ has a similar topology) and was incorporated for some reason? They do seem to crop up in certain Ampex preamps (or more specifically, speaker amplifiers e.g. 620, 622), but always in push-pull. Again, I’d be interested to know anyone’s thoughts on this.
