Motown Direct Amplifier-inspired Preamp?

[CJ]

Joe Messina,




On most Motown songs you can barely hear the guitars, they are way down in the mix, so the amp might not he a big as factor as say the instruments and the players. Most of it sounds like a click track tied to the snare to either push the beat forward, keep it the same or drag it back a bit.
It us really hard to hear the guitars on the tracks with a lot of horns, strings, vibes, etc. The best way to hear the subtle effect of the guitars is to listen to the track with and without the guitars.

But I still want to know what was in that box!

 

[abbey road d enfer]

Joe Messina,

You're absolutely correct. I know very well the difference, but fingers did not follow brains.

 

[CJ]

Not you fingers, AI spell check,

 

[olhsson]

Last year I spent a week at the studio with John Windt who had actually assembled the box. It turned out I was wrong about the reverse UTC A-10. It had a high impedance tube input. It may have had a UTC output transformer for each input because it was balanced in the patch bay. Joe's memories match mine. Mike's information in the first post is correct. He designed it and John assembled it.

 

[CJ]

Looks like an LA2a circuit without the side chain would be a good starting point. 1/4' input jack. UTC HA size output,

 

[MidnightArrakis]

But I still want to know what was in that box!

[But I still want to know what was in that box!] -- Is there an actual complete and working schematic of this box somewhere in this thread? I would go ahead and build this thing, even including the VU-meters, if doing so didn't require piecing together 7 different partial schematics from 11 different people!!! Anybody know of an entire schematic of this box or maybe a link to someplace that has one posted or something?

/



/

 

[CJ]

Any schematic would be speculation at this point.

Plus there is somebody selling this as a product.

Does not mean we can't build something similar,

Sweetwater has the tube compliment listed,

I have a hunch that Triad made the OPT,

 

[abbey road d enfer]

I would go ahead and build this thing, even including the VU-meters,

What for?
This was a product built at a specific time for a specific use in a specific environment. In order to make sense, you would need to build a studio around it, with tape machines and not many channels, making records with all the musicians in the studio.
It doesn't exist anymore.

 

[CJ]

Triad HS-50 is a 5:1 with sec taps , it was mentioned that the engineers might need 300 ohms if trying to drive multiple inputs, so the HS 50 could be strapped for that.

The HS 50 I tested only could handle about 14 volts on the primary at 10 hz before saturation, so the 26 db rating is a bit puffed up, it might do that at 40 hz but not 20.

So what we have so far is 2 ea. 12AT7's and a 6BX7 White cathode follower feeding a Triad HS 50 strapped for 233 ohms.

And a vol control.

Now all you have to do is figure out how they are all connected and with what parts.

Unfortunately there is no original sample to compare it with so how will you know if you got it right?

Still fun to try.

I see that the Motown EQ reissue is now available.

 

[DaveP]

I have stopped construction projects for health reasons, but my brain is still intact (opinions differ).

Would readers be interested if I did a full research on the project as if I was going to make it?
There is some tech stuff that needs to be further explored I think.

For example, my first thought when I saw the chassis was, "wow what a lot of heat in that box". I have never seen that much ventilation in a project, there is even a heat shield over the top to stop idiots from leaving their beer on it! Back in the day, a guest singer managed to knock over his beer over my 100W PA and it cracked an EL34, it left an impression on me!

Of course, I won't be able to produce the circuit out of thin air, but I could certainly design some schematics that would do the job.
That could also include some cheaper alternative tubes that are under used at the moment, like the 6BX7 mentioned in the text.
best
DaveP

 

[ruffrecords]

I would find that a very interesting exercise and I would be happy to lend a hand.

Cheers

Ian

 

[DaveP]

OK Ian, we will give it a go!

I am going to work out as much as possible from the specs and establish the gain structure of the original before thinking about a modern recreation.

“Mike McLean was an extremely talented man. He'd listen to Deutsche Grammophon recordings and centred 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.”

Interest in German recordings and 34dB of gain shouts V72 to me, I may be mistaken, but its the right time frame and Abbey Road were using them too at this time.

• #330

At the brain trust last night, John Windt shared this this bit of information.
While it is not a complete schematic, it is a flow chart of the 5 channel amp.
Basically the direct box was a 12ax7 and a 12au7. there was a trim pot between tubes which was used to set level, it's why it is described as a fixed gain preamp. The rest is self explanatory. I didn't get to ask, but when I showed this to a friend he wondered if there was a buffer for the VU meter. I suspect that because of when this was built combined with using a true VU meter, that a 3k9 resistor was just attached at the output and would feed the meter and that it wouldn't cause any increase in distortions, but that is just a guess.

This schematic is just plain wrong, first there is no NFB, second the combined gain of those four tubes would be 50x50x20x20=1000,000 or 120dB, thirdly there is no 6BX7 CF as given by btown2009 in post 122.

I will come back with some calculations
best
DaveP

 

[s wave]

I will just list some links that may be of help on the magic box
https://www.hifinews.com/content/hitsville-usa-page-2

https://soundevaluations.blogspot.com/2019/03/
(the tumblr link didn't post here is what it said
Mythical Motown 5 channel tube DI. Every Jamerson tone from 1967 to 1971. What’s Goin’ On? Last year Mike McLean, the designer and builder, spent months at Acme Audio exactly recreating this original. We are now making 20 of these in a one channel desktop format. The most sought after bass recording device in existence. We opened a list up and it’s filling fast. Contact Bob at [email protected] to hold one of the 20! Audio files soon!
@acmeaudio @rustbeltstudios #motown #hitsville #jamerson #bassguitar
Hello from Bob Ebeling. It is now 2020. For most of 2019 we here at Jext Telez coasted on the overflowing quantity of production we cranked out in 2017 & 2018.) apparently he built a couple dozen of them? You may be right again CJ it seems like there may have been Vintage Triad A-12J Transformers used?

I am having trouble loading a link let's try again - click on the upper left Tumblr Jext Telez image/link maybe that will work... https://lens.google.com/search?ep=g...hLTUzOTQtNDA2My05YWI2LTc2NTRkNDZmODZlNCJdXQ==

 

[DaveP]

Thanks s wave, it all helps.

The calculations:
From what we have been given, the overall gain of each pre-amp is 34dB (-30 to +4dB)

A typical IPT was 600:50k which has a ratio of about 1:9. the OPT we are told was 5:1 (15k:600)
so the IPT gives us 9x and the OPT loses us 5x so the overall gain from the transformers is 9/5=1.8x.

The 6BX7-GT has a spec of mu =10, gm = 7,6 and ra =130ohms.
As a CF, the gain will be 10/10+1 =0.9. Bear in mind that this is in its designed current and we won't be near that so the gain in my experience could be as low as 0.7, but we will ignore that for now. 1.8 x 0.9= 1.62 which is 4.19dB. Say 4dB
This means that our 34dB amp has 4dB of gain contributed by the transformers and CF, meaning the amp itself has only 30dB after NFB.

We now have to guess how much NFB was used, lets say they gave it 20dB, this means that the gain before NFB was 50dB.

50dB is x 316x so over two tubes SQR 316 = 17.7. So now we know that we are talking about two tubes like 6SN7 or 12AU7 ECC82 and any other tube with a mu of 20. If you think it had say 30dB NFB then you would need to find another 10dB of gain from the tubes.

60dB is 1000x so SQR = 31.6 so maybe a 12AT7 or ECC81 , 6DT8 or any tube with a published gain around 40-60.

This is how I go about research for unknown designs, I hope you find it helpful, I owe it all to PRR.
best
DaveP

 

[CJ]

A 12 J is an input, Bob says they did not use an input xfmr.

I would not worry about what ACME is building. Better to go after the original.

We need to break into Hitsville and do a little research. Don't steal anything, just take some notes. We need a good cat burglar.

Maybe the museum is suffering from financial problems and they need money from GroupDIY in exchange for a peak under the hood.

It would be nice to know what the 5 CH box plugged into. That way you could build the whole signal chain, because what came into the both from the 5 CH box was doctored to fit the mix.

One thing I like about the Motown stuff is they divided up the audio band into distinct sections. Then they filled in the sections with various instruments in a way that did not overlap. Not like today where all 56 faders are loaded with junk that creates a wall of crap.

 

[ruffrecords]

34dB gain certainly is reminiscent of the V72 but 20dB of that came from the input transformer so you need 34 - 20 = 14dB gain from the grid of the first tube to the 600 ohm winding of the output transformer so I don't think the numbers stack for the. 34dB from Hi-Z input to transformer output implies 46dB of tube gain assuming 12dB loss in a 9K6:600 output transformer. This in turn implies perhaps 60dB of open loop gain if "significant" NFB is used.


The supposed use of a 12AX7 and 12AU7 certainly brings to mind the circuit of the Pultec MB-1 mic pre with which I suspect they were familiar. It would be straightforward to mix the unbalanced signals from the primary side of the transformers to drive an external amp.

I think it is still possible to visit the studio. I wonder of one of our US based colleagues could go there and take some pics of the guts of the thing. At least that way we might get a better idea of the tube and transformer complement. If there are five balanced output that means there would be 5 output transformers (I cannot remember if the power amp was separate)

Edit: I am happy to make a donation to assist in the acquisition of accurate data. Excellent idea CJ.

Cheers

Ian

 

[CJ]

They had separate outputs so they could balance all three guitars and the bass.

Is this plate or room reverb>

 

[Brian Roth]

They had separate outputs so they could balance all three guitars and the bass.

Is this plate or room reverb>

IIRC Motown had a live chamber in the attic of the studio.

Bri

 

[CJ]

That's right, sheet rocked and painted with shelac. I believe they also had an EMT 140 plate.

Well so much for offering a potentially destitute museum owner money for reverse engineering,

"In October 2016, the museum announced a $50-million-dollar expansion plan in order to create space for interactive exhibits and recording studios.] Since the announcement, the museum has received donations from organizations like The Kresge Foundation, the AARP, and the W.K. Kellogg Foundation to help with both the expansion as well as community programming."

Quick thumb if that is Robert White, could be the Firebird in the neck position, sounds like a pick,

 

[s wave]

I think I remember reading that the one stage was 4 db Dave P - I think you are right. I was in contact with some Gordy last year - I will ask them on another site - maybe we get lucky... I think Silvertone on GS bought a DI Motown preamp(s) a ways back - will check