Motown Direct Amplifier-inspired Preamp?

Help Support GroupDIY:

untune

Well-known member
Joined
Oct 28, 2016
Messages
214
Location
Manchester, UK
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:
“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.”
“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.”
From Ken Sands, who joined Motown in 1967:
“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.
 

Attachments

  • IMG_5293.jpg
    IMG_5293.jpg
    149.6 KB · Views: 227

untune

Well-known member
Joined
Oct 28, 2016
Messages
214
Location
Manchester, UK
So that’s what I have so far—a list of components. I need to figure out how they might be put together. I've been gathering schematics from the late 50s-early 60s to get an idea of where things may have been borrowed from, if at all. The low gain mic pre has been added on afterwards (likely as a marketing choice) and honestly, I could stand to lose that. I have another chassis that I plan to use for an EF86 mic-pre and it makes sense to do something focused on performing that task well rather than bolting it on here, however cutting that out might mean a change to the power supply etc.

Just to clarify, I don’t aim to recreate the monitoring or multi-channel aspect, just a single preamplifier to line level out.

So, to summarise:
  • Hi-Z -30dBu instrument input to +4dBu transformer balanced line output, 23dB headroom. Primarily for (passive) bass guitar, but a pad for line level instruments such as synthesizers would be a nice addition.
  • 2x ECC83 for amplification duties; I’m not certain how they would be best utilised.
  • 1x 6V6, possibly triode strapped before the output TX?
  • A VU meter for monitoring peak level. Not sure if this would have originally been buffered or just strapped across the signal. I have done this in a previous project and noticed no audible distortion, but included a defeat switch regardless.
  • I like the concept of an efficient, fixed-gain amplifier—but it also feel like it’d be sensible to have more control over the staging. It could still be run maxed out to operate on the same principle, or dialled back given that A/D inputs (compared to tape) won’t be as forgiving when pushing the limit. I think an output attenuator at the very least is a good idea, possibly stepped rather than a pot. I also think some way to control the drive of the preamp valves would be ideal for creative use, with the option to have it set so that it is essentially operating at the most efficient fixed gain as designed. At that point, the instrument output could be used to set the level as originally intended.

I’m sure that there might well be “better” ways to build such a device now, and I suspect the design may be relatively simple (I’m under no illusions that there’s any “secret sauce” here), but since we have so many talented and knowledgeable engineers on this site I’m also curious that, if you were designing a device with the resources and technology available in 1966 (and considering the information above), what would you do? I’m just gathering ideas on what kind of circuit would be best at this point, and as always, any input is greatly appreciated.

Cheers!
 

ruffrecords

Well-known member
Joined
Nov 10, 2006
Messages
13,771
Location
Norfolk - UK
This is fascinating. I love these detective designs and of course I am very much into valves. Do you know if there any pictures of guts of the real thing that might help us? And can you post a link to the commercial unit supposedly base don the original schematic?

6V6 was a fairly commonly used tube in 1940s RCA broadcast mixers for programme amps. Bear in mind the +26dBm into 600 ohms is 400mW. And into 300 ohms it is 1.6 watts.

Cheers

Ian
 

untune

Well-known member
Joined
Oct 28, 2016
Messages
214
Location
Manchester, UK
Hi Ian, I hope you're well!

I'm glad you find it as fascinating as I do; It's an often overlooked and very important piece of music history and is often talked about in bass circles as it was considered a big part of the tone of bassist James Jamerson amongst others (I personally believe tape saturation had more to do with it, but that's another discussion entirely! :D)

Furthermore you gave me an immense amount of help with my last project and I always enjoy reading your contributions to these discussions.

As for pictures that's the unfortunate thing, none exist of the guts as far as I'm aware. Just the outside much like you see attached; the amplifier is apparently still in the Motown museum in Detroit but I don't know if they allow folks that kind of access. I believe the power amp section (a McIntosh MC-30, it was incorrectly presumed to be an MC-60 by Ed Wolfrum in the quote I put in the first post but Mike McLean did correct him on this) ended up on eBay a couple of years back.

I'm happy to link to the modern one; I avoided going into too much detail because I wasn't sure if there were any forum rules about current production equipment and it's a bit of a grey area with this one. I think they make them in small batches rather than a fully commercial venture. The company themselves don't have a lot of info out there on it so I'll provide this link ACME Motown Tube DI so you can take a look. Many of the pictures I've spent weeks gathering from various dark corners of the internet in order to identify the transformers etc!

Much of the info that came from Mike himself was posted on the soulfuldetroit forums way back in 2002. Bob Ohlsson, former Motown enginner, is still active on the gearspace forum and I believe Ed Wolfrum may still be around, but he also had a bit of a falling out with the company producing this preamp a few years back. However I'm not sure if they would know any more than is already in the public domain!

Cheers
Lee
 

emrr

Well-known member
Joined
Apr 12, 2006
Messages
7,875
Location
NC, USA
6V6 in line amps - RCA, GE, Collins, Langevin, Gates......almost everyone.

Some of the various comments from Motown house engineers are wrong, becomes obvious the more you know. I believe I've commented on this amp in other threads here and elsewhere before.

This is a fixed gain tube DI with 6V6 output and very large amounts of negative feedback. There are a bazillion commercial circuits and standard ideas that could be riffed upon.

The output transformer question is easily the thing that imparts the greatest sonic signature, and there aren't a lot of obvious choices in 'high quality' single ended transformers that can take 6V6 current and deliver that output level; you end up with a relatively small list if you know how to compile the data. In fact none of the RCA/GE/Gates single ended 6V6 amps delivered more than +24, they always become push-pull amps when it goes past that point. BUT....that's for full range audio, it goes up 12dBish for guitar, but then we're talking less clean. FWIW.

The shortest path appears to be buying the Acme and reversing it. Just a matter of time before someone does. The museum has probably documented the original as well.

I would personally avoid EF86's and 12AX7's in any new design, NOS simply costs too much and will surely continue to skyrocket. EF86's of mic pre quality are a dwindling resource, too many people are paying tons for tube-rolling in microphones. There are plenty of unused inexpensive tube types of reasonably similar nature, it seems much more fun to explore that territory. Build it with octals, build it with sub-mini, build it with 1930's tubes, etc. Study a tube manual. 5879's are still $10-ish dollars, used in many of the same places EF86's were used. One example.

Summing it up, to me it's either all the way literalist EXACT clone or do something that achieves the same usage goal with only passing concern for what the original was. Many things with gobs of feedback sound just about the same in practice. FWIW YMMV etc etc Have fun.
 
Last edited:

dmp

Well-known member
Joined
Oct 28, 2009
Messages
3,045
Location
Madison, WI
Very interested in this as well. Even if you don't make an exact reproduction (who cares?) it's a type of circuit that can sound phenomenal.
For a single ended 6v6 as the output, you could breadboard cap coupled and transformer coupled versions.
For cap coupled A-24 / LS-50
For transformer coupled A-25 / LS-27
Edit: see EMRR's post above, these probably aren't right for a 6V6
The transformer coupled gives a more colored sound in my experience. The datasheet for the CM-2810 does not mention anything about being gapped or a max DC current, which to me implies it is not gapped. It would be quite a bit more expensive to build a transformer coupled version today, which is probably why it went that way (if the original is that way).
The input would probably be straight into the grid from a gtr/bass. That was typical. Adding an input transformer for a mic input would be fine but I doubt the unbalanced input would have gone through the transformer.
ef86 sounds strange to me - but 12AX7 would be unsurprising with many examples to copy.
I built a DI with the A-10J backwards (similar to the A-12J) and it imparts a distinct character on a bass. It's awesome in it's own way (sounds compressed and glued into a mix). But like I said I would do the unbalanced in straight to grid (which is always awesome). I've never listened to a mic through it. If you were to do a mic you could do an A-10 / LS-10 and not have any regrets.
 
Last edited:

abbey road d enfer

Well-known member
Moderator
Joined
Jan 22, 2008
Messages
13,030
Location
Marcelland
I may be totally wrong, but I am led to understand that the 5-input box was used for guitars only, with the bass using a passive single unit.
Which seems consistent with the fact that ACME sells a single-channel "Motown"passive DI. And also with the fact that solo tracks of James Jamerson are known to circulate.
In the film, the guitar players are all close to each other, but the bass player (Babbit) is further away.
 

ruffrecords

Well-known member
Joined
Nov 10, 2006
Messages
13,771
Location
Norfolk - UK
I may be totally wrong, but I am led to understand that the 5-input box was used for guitars only, with the bass using a passive single unit.
Which seems consistent with the fact that ACME sells a single-channel "Motown"passive DI. And also with the fact that solo tracks of James Jamerson are known to circulate.
In the film, the guitar players are all close to each other, but the bass player (Babbit) is further away.
A lot of this is shrouded in the mists of time and old memory cells but Bob Babbit and Bob Ohlsson seem to say otherwise in this thread:

Soulful Detroit Forum

Cheers

Ian
 

ruffrecords

Well-known member
Joined
Nov 10, 2006
Messages
13,771
Location
Norfolk - UK
I have just watched this video about the ACME Motown tube DI:

and managed to take a screenshot showing the top of the chassis. According to the video, the output transformer is by Ciemag, the mic input transformer is by Triad; both are easily visible in the pic. There is also a mains transformer (top right) with a nearby choke and octal based rectifier tube - sorry no sign of a 6V6. Lastly there are three B9A tubes. One is a pentode for the fixed gain mic pre (according to the video) so the other two are probably double triodes but there are no clues as to the topology used.

I also got a pic of the underside of the chassis from the same video.

Cheers

Ian
 

Attachments

  • ACMEinside.png
    ACMEinside.png
    723.4 KB · Views: 163
  • ACMEunderchassis.png
    ACMEunderchassis.png
    742.3 KB · Views: 180

ruffrecords

Well-known member
Joined
Nov 10, 2006
Messages
13,771
Location
Norfolk - UK
That's a 6V6, not a rectifier, it's described at their website, link in one of the posts above.
You are right. When I saw it in the video it looked much shorter. Also interesting is that the layout shown in this image: http://www.analoganonymous.com/uploads/1/1/0/9/110989005/dsc00598_orig.jpg is different to the one in the video. Maybe an earlier version? The mains transformer is by Hammond and there is also a Hammond 157G smoothing choke.

Seems like a lot of tubes for for the job it has to do unless one is maybe dedicated to a VU buffer.

Cheers

Ian
 

untune

Well-known member
Joined
Oct 28, 2016
Messages
214
Location
Manchester, UK
Hi all,

Happy this is picking up traction! I'm away from my computer at the moment so I'll put a proper post together later, but Ian has done some great research in the meantime and picked up on some great points. I had also grabbed pics from the video he posted as that is the only shot of the inside that exists are far as I know. One gives a clear shot of the mic input end (not much use) and the other gives an out-of-focus shot of the power end (which is where I speculated on the silicon rectifier diodes.) Refer to the component list in the first post for details on the exact power transformer and choke specs etc.

Just to clarify, the reason for the different layouts is because it went through various prototypes and the pics that exist are all spread throughout this process. I believe the latest is the one where the choke is mounted on an L plate beside the power tx. Guessing they reoriented things to minimise hum as it's all packed in quite tightly.

And I also wanted to look into what abbey road said last night because he made a good point, and I wanted to confirm the bass did indeed go through this box and it hadn't just been assumed. I always believed this to be true and this quote from Bob Babbitt himself confirms it did:

More details are that the #5 was for the Bass...
2,3 and 4 were for the guitars and #1 would be used for electric piano/clavinet..
The #1 channel was the closest to the steps and the keyboard would be on the other side of the steps....
The #5 was close to the Bass....
As far as the Monitor system for the Bass when you plugged in to #5 the engineers would ask to have your treble and the volume all the way up and if the VU meter above the channel went into the red when you played a note,you were asked to turn your volume knob down until the meter was not in the red....It was said that Jamerson did not like the treble on...not sure if he complied with the engineers on that subject...and I was told by some of the engineers that as a result his Bass would record distorted...They made it a part of his sound...
This was the only monitor that was used for all of the instruments....Bob

El Wolfrum did indeed design a passive DI box using a reversed Triad transformer, but I believe this was after the Motown direct amplifier was made. The schematic is out there and was also produced recently by Acme, which is probably responsible for the scarcity and cost of Triads now. As mentioned earlier he fell out with them and released his schematic into the public domain.

I believe Ed also created a solid state version of Mike's design a few years later so it could well be that he has some insight into it, if we could track him down.

Cheers
Lee
 

abbey road d enfer

Well-known member
Moderator
Joined
Jan 22, 2008
Messages
13,030
Location
Marcelland
A lot of this is shrouded in the mists of time and old memory cells but Bob Babbit and Bob Ohlsson seem to say otherwise in this thread:

Soulful Detroit Forum

Cheers

Ian
Ed Wolfrum's post mentions Bud boxes with Triad transformers.
The box with preamps and amp/604 is quoted by Mike McLean as the "Motown Five Channel Guitar Direct Amplifier."
Of course, it didn't preclude anyone using the "guitar box" for bass. There may be cases of such doing, but I doubt it was the norm.
The guitar players themselves would not have been too happy with that.
Or, the set-up with guitars and bass on the same box was deemed unsatisfactory and resulted in putting the bass on a separate system, which to me makes more sense.
 
Joined
Oct 4, 2017
Messages
14
Judging from the evidence, and I think it is fair to say we are probably never going to know for sure I would look at the Pultec MB1 with a 6V6 cathode follower output. It's a circuit that was floating around at the time, has around 20-40dB of gain and also satisfies the "lots of negative feedback, and low‑distortion, high‑quality transformers on the inputs and outputs. " criteria.

Of course it could just be the classic resistance coupled amplifier from the back of the RCA Tube Manual or one of a million others, but a pentode power tube output and 'lots of nfb' would certainly make me curious about the Pultecs, Walter Sear loved em for a reason.

Look at the Pultec MEQ-5 and if you add 12AU7 and a volume control up front instead of an EQ you would have a sweet instrument pre.
 

Attachments

  • meq-5.jpg
    meq-5.jpg
    66 KB · Views: 208
Last edited:

Scodiddly

Well-known member
Joined
Aug 18, 2004
Messages
932
Location
Libertyville, IL USA
Wow, this is a cool thing to pursue!

From the "Standing in the Shadows of Motown" book, which came out many years before the movie, Jamerson was reported as adjusting his input gain to get the right amount of "mush", most likely distortion.
 

ruffrecords

Well-known member
Joined
Nov 10, 2006
Messages
13,771
Location
Norfolk - UK
The big question in my mind right now is just how much gain do you need to get a typical 1960s electric guitar output up to 0dBm?

Cheers

Ian
 
Top