untune
Well-known member
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:
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.
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:
“At first, the tendency was to think in terms of installing the five preamps in the control room, and possibly having them equipped with variable gain controls to that the recording engineer could adjust for correct line output level. I didn't like this because gain pots tend to get noisy, and most important of all, it is difficult to maintain the maximum dynamic range in a preamp which must accommodate a wide range of gain. I liked the idea of a fixed gain preamp that was designed for maximum performance at that gain.
The thought occurred to me that it would be hip to make the setting of the preamp output level the responsibility of the musician. If the preamp gain was sufficient to allow a moderate setting loss on the volume control on the musical instrument, then it would be ideal to simply have the musician set this instrument volume control to provide the correct line level at the preamp output. All that was needed was a VU meter in the studio so that the musician would know when the level was correct. The preamp was designed with fixed gain.”
In terms of a technical brief, to quote Mike again:
“The output of a preamp is tailored to provide professional “line level” characteristics as follows:
- Output level: +4 dBu (4 dB above 0.775 Volt, R.M.S. [the voltage required to dissipate 0.001 Watt in a 600 Ohm load resistor] which is 1.228 Volts, R.M.S.: [the level that will cause a standard VU meter to read "0" on the scale with a steady sine wave tone.])
- Output configuration: Balanced and floating output source (free of ground: usually a transformer secondary winding.)
- Current capability: Able to drive a 300 Ohm load (In case the recording engineer doubles up the loading when patching, it is better to provide for full performance with a 300 Ohm load, without overload distortion.
- Headroom: 23 dB of headroom before clipping, which means that the amplifier would clip at an output level of +27 dBu.”
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:
“Motown engineering built the monitoring instrument input box you see in the museum studio today. It used a 15 inch Altec 600 series driver with a Mac-60 power amp and custom built electronics with 12AX7's and BIG UTC transformers.”
Mike himself said, when referring to the Motown studio in general:
“I had built about a dozen preamps with a 12AX7 and a 12AU7 tube in each one.”
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:
“[Mike McLean] said he copied from circuits around that time to make the "Motown preamp" as well as their console and 8 track reel to reel. It's documented by Bob Olhsson that they had Langevin Preamps (Tube), Altec 1567a Tube Mixer, Scully 280 Preamp, Ampex MX-10 Tube Mixer and the Motown Homemade Preamps.”
From Ken Sands, who joined Motown in 1967:“The Ampeg 351 tape amplifier is one example of a design that used a 12AX7 in the front end. McIntosh preamps such as the 175 and 275 had 12AX7's in their front ends. Other studio gear such as the Teletronix LA-2A has a 12AX7 pre in the front end and the Pultec EQP-1A had 12AX7 and 12AU7's. So I guess that there were studio equipment design ideas that they could have borrowed from.”
“Mike McLean was an extremely talented man. He'd listen to Deutsche Grammophon recordings and centre his engineering skills on what the Germans would do. His amp's five channels of guitar level ran from –30 at high impedance to +4 at 600 Ohms. It was very, very clean, with lots of negative feedback, and low‑distortion, high‑quality transformers on the inputs and outputs. There was a VU meter on each input, and a guitar's input would be adjusted at the loudest note to peak zero on the VU, providing maximum headroom. That line‑level output would be patched directly into the tape recorder — there was no mixing path — and this was how we worked with the guitars, bass and, later on, a Clavinet and the Fender Rhodes.”
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:
- Triad A-12J (150/600R:60K) input transformer or a modern reproduction. I’m guessing this is mic-only as there is a toggle switch between mic and instrument. However, the same transformer (or the 11J?) is used in the “Wolfbox” passive DI box, in reverse. I’m not sure it’s possible that the mic input TX could also be utilised for the instrument input.
- 1x EF86 (36dB fixed-gain pentode mic preamplifier). No phantom power.
- 2x 12AX7/ECC83. One spec lists 2x 12AT7 but I’m inclined to believe the original used 2x 12AX7, or perhaps a 12AX7 and a similar, lower gain tube (AT7, AU7 etc.) Mil-spec 5751 have definitely been used.
- 1x 6V6.
- VU meter.
- Cinemag CM-2810 output transformer (4:1, 9K6:600R). I haven’t been able to determine if the core is gapped or not. 50% steel/50% nickel construction which I’m guessing may contribute towards perceived saturation/distortion characteristics. I expect the original would have been UTC/Triad and physically bigger.
- Hammond 369JX power transformer with 120V/240V operation.
- 500 VAC C.T. @ 69mA; 50 VA; 50 VAC bias tap from high voltage secondary C.T.; 6.3V C.T. @ 2.5A
- Hammond 157G choke.
- 30H inductance (+/-15%); 40mA DC current; 595R resistance (+/-15%); Max 400V DC
- 40/20/20/20uF 500v can capacitor. HT filtering I assume.
- Solid state rectification? I think there are four silicon diodes on the board close to the power TX but cannot confirm.
- Input and output pots to control drive/saturation; although it doesn’t follow the philosophy of the original, I like this aspect & I think it’s probably useful in a creative context.
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.