Motown Direct Amplifier-inspired Preamp?

[jacomart]

Let's not lose sight of the fact that the original Motown box expected the guitarist to set his guitar volume control so the VU meter did not wrap itself around the end stop. Do we want to retain this feature?

Secondly, we now know there was a secondary level preset hidden from the guitarist. Howe about we replicate this preset on the front panel in the form of a rotary switch with a number of preset levels for common guitar types?

Cheers

ian

Sounds like a great idea

Cheers
JM

 

[TheJames]

This is perhaps where I diverge from the original Motown design goal.

What did the Motown guys want to do? They wanted to be able to record electric instruments without micing amps and be able to drive multiple destinations with minimal involvement from the engineers. They also wanted to be able to provide some monitoring for the players. There were some technical goals and limitations to work with, and most of the technical details mentioned in the OP cover that.

In their day, most (if not all) equipment was 600 ohm input/output impedance and as such, the output drive needed to be pretty substantial. Motown also wanted to be able to double up destinations and needed to have additional drive capability. Aside from maybe attempting to run a guitar into similar equipment from the time, do we really have a need for that kind of drive? I don't really think I have that need, maybe you do.

I'm also in agreement that I want minimal involvement from the engineer. I just want to have the guitarist plug in, then the engineer (me?) will rec arm a track and go. If I setup the gain structure of the preamp to hit clipping on my converter just a bit beyond where the highest output of the hardest hit strum lands, then I've pretty much optimized the capture of the guitar itself.

I would think by this very nature, moderate playing should land about "0" and heavy playing should hit above that. Soft playing would obviously be a bit lower, but you could either add a secondary or tertiary gain setting on the preamp. For repeatability, I would advise against a pot and instead use a gain control switch.

Everything else that I would like goes well beyond the Motown pre and could totally (and maybe properly) be it's own thread. I pretty much envision a "guitar recording interface."

For instance, I would like the ability to take the recorded signal and feed it to an amplifier for re-amping if desired. If I recorded a guitar part that achieved a 1V P2P level on input, then when I send the recorded signal out of the DAW, I should be able to exactly hit the 1V P2P level on output. I'd rather achieve this with say calibration trim pots than a knob on the front panel.

I also need to "split" or "buffer" an output to feed to an amp while tracking.

Again...What I want is similar, but different enough from the Motown box that maybe I just need to drop out of this thread. I just felt that there was some overlap from what I want and what this thread is about and added my thoughts to it. To reiterate what I said earlier, the Motown guys created what they needed to meet the needs of their workflow and equipment. I think it might be interesting to take inspiration from the Motown engineers and make a box that fits the modern recording setup.

 

[untune]

Let's not lose sight of the fact that the original Motown box expected the guitarist to set his guitar volume control so the VU meter did not wrap itself around the end stop. Do we want to retain this feature?

Secondly, we now know there was a secondary level preset hidden from the guitarist. Howe about we replicate this preset on the front panel in the form of a rotary switch with a number of preset levels for common guitar types?

Cheers

ian

I think that makes sense, it could have a set handful of gain levels, or it could just be set fixed if whoever is building it prefers that. The original had a hidden trimmer like you referred to, how might that have been implemented? Would that trimmer have been in the NFB loop?

 

[ruffrecords]

This is perhaps where I diverge from the original Motown design goal.

What did the Motown guys want to do? They wanted to be able to record electric instruments without micing amps and be able to drive multiple destinations with minimal involvement from the engineers. They also wanted to be able to provide some monitoring for the players. There were some technical goals and limitations to work with, and most of the technical details mentioned in the OP cover that.

In their day, most (if not all) equipment was 600 ohm input/output impedance and as such, the output drive needed to be pretty substantial. Motown also wanted to be able to double up destinations and needed to have additional drive capability. Aside from maybe attempting to run a guitar into similar equipment from the time, do we really have a need for that kind of drive? I don't really think I have that need, maybe you do.

There probably is no workflow or technical reason why you would need that drive capability. But basic physics tells us tubes sound different when driven hard and the exact tone you get also depends on the tube. Although not explicitly stated, I think it is a goal of the project to try to remain as faithful as possible to the sound or the original Motown DI. To do that we need to use much the same topology as they did. Otherwise you might just as well use an op amp based DI box.

I'm also in agreement that I want minimal involvement from the engineer. I just want to have the guitarist plug in, then the engineer (me?) will rec arm a track and go. If I setup the gain structure of the preamp to hit clipping on my converter just a bit beyond where the highest output of the hardest hit strum lands, then I've pretty much optimized the capture of the guitar itself.

The trouble with that approach is that the gain structure will vary with the guitar. If you want a no controls DI then you will need to set it up so your interface does not clip with the hottest guitar and accept a much lower recorded levels with weaker guitars.

Cheers

Ian

 

[ruffrecords]

I think that makes sense, it could have a set handful of gain levels, or it could just be set fixed if whoever is building it prefers that. The original had a hidden trimmer like you referred to, how might that have been implemented? Would that trimmer have been in the NFB loop?

I suspect it is more likely to have been a preset pot at the front end of the preamp (an input pad). Just a 1Meg preset pot. You could fit one in the NFB loop but It would probably not have the as wide a range.

Cheers

Ian

 

[abbey road d enfer]

It was 500k (posts 129 and 140)

Cheers
JM

I stand corrected.

 

[abbey road d enfer]

The trouble with that approach is that the gain structure will vary with the guitar.

Not only the guitar, the player too. Some hit the strings hard, others have a feather touch, some want to crank the guitar's pots, some want to roll-off...
Fixed gain is as unconceivable on a guitar preamp as on a mic preamp.

 

[TheJames]

Although not explicitly stated, I think it is a goal of the project to try to remain as faithful as possible to the sound or the original Motown DI. To do that we need to use much the same topology as they did. Otherwise you might just as well use an op amp based DI box.

Ooof....I think that's one thing (I think) I'm not liking about the Redeye. It's functional...It works fine...It just sounds a little....Limp. Using the instrument input of an ADL600 is much more satisfying.

Not only the guitar, the player too. Some hit the strings hard, others have a feather touch, some want to crank the guitar's pots, some want to roll-off...

Which is exactly why I would want to maintain the proper levels of the performance of the player when I go to re-amp the signal. I don't want to try and guess what that level was...I want to know that I'm hitting it right.

 

[abbey road d enfer]

Which is exactly why I would want to maintain the proper levels of the performance of the player when I go to re-amp the signal. I don't want to try and guess what that level was...I want to know that I'm hitting it right.

What do you suggest in order to do that?

 

[TheJames]

What do you suggest in order to do that?

I think I stated this before...If the absolute hottest level I can get from a guitar is say 8V P2P....I need that to hit my ADC at no more than +18dBFS. One would probably want a little headroom so scale that as needed. On playback, I need to make sure that the signal coming out of the DAW hits the amp at 8V P2P. Unity gain.

If the signal is really low...Then maybe I want to add 6-12 or more dB of gain, but I would need to knock that gain down somewhere on the re-amp side of things to make it hit the amp correctly. Again...Unity gain.

Having that on switches makes it repeatable. Having trimpots allows you to precisely calibrate input/output levels to make sure the levels are "right."

A difference in goal between what I want and the Motown amp is they were trying to hit pretty close to mix levels. My goal is to just capture the nuance of the playing as closely as possible. Right now, while I know I can use X DI box with Y preamp at Z gain and that pretty much hits proper re-amp levels, the tone suffers.

Honestly, there's nothing better than getting things "right" at the source, but sometimes you just miss. More than once the ol' reamp has saved my rear.

 

[jacomart]

I may be wrong, but in the end I think the important thing is to have all the tracks at a consistent level so that you can then mix without pain. Pain you would have if you got an 8Vpp guitar and an 80mVpp bass, I think the philosophy behind Motown DI is just that. Let's not forget that those guys used consoles from the 40s.

Cheers
JM

 

[TheJames]

I may be wrong, but in the end I think the important thing is to have all the tracks at a consistent level so that you can then mix without pain. Pain you would have if you got an 8Vpp guitar and an 80mVpp bass, I think the philosophy behind Motown DI is just that. Let's not forget that those guys used consoles from the 40s.

Cheers
JM

Yeah...If I were looking to mix the actual recorded DI guitar, I'd want to have it at a more consistent level and thus have a gain pot and set it according to the part, player and instrument.

Instead, I want to meticulously document the guitar performance for playback through an amp at a later date.

Different goals.

 

[abbey road d enfer]

I want to meticulously document the guitar performance for playback through an amp at a later date.

The challenge here is not the preservation of level.
What is missing is the interaction between the guitar, cable and amp.
You need to feed the amp with the same impedance as that presented by the guitar and cable.
Many guitar players will say that they don't play the same because this interaction is missing when they record through a DI.

 

[ruffrecords]

Yeah...If I were looking to mix the actual recorded DI guitar, I'd want to have it at a more consistent level and thus have a gain pot and set it according to the part, player and instrument.

Instead, I want to meticulously document the guitar performance for playback through an amp at a later date.

Different goals.

Then you need a different product. I high input impedance unity gain extremely clean buffer to balanced output DI is what you need for starters. If you also want the reamp sound to be the same as if the guitar was plugged in directly then you also need, on the replay side, to emulate the source impedance of the actual guitar used so the interaction of the guitar and the amp is also accounted for.

 

[jacomart]

Then you need a different product. I high input impedance unity gain extremely clean buffer to balanced output DI is what you need for starters. If you also want the reamp sound to be the same as if the guitar was plugged in directly then you also need, on the replay side, to emulate the source impedance of the actual guitar used so the interaction of the guitar and the amp is also accounted for.

And maybe even a double output to record both the wet and dry signal or, at least, send to an amp to make the guitarist hear something similar to the final product while playing.

Cheers
JM

 

[untune]

I suspect it is more likely to have been a preset pot at the front end of the preamp (an input pad). Just a 1Meg preset pot. You could fit one in the NFB loop but It would probably not have the as wide a range.

That makes sense, I found it interesting that Mike made comments about not liking pots in the signal chain, and the revelation of that hidden trimmer was a surprise as it had given the impression that there was no 'variable' gain option.

I saw this project Dual Tube Microphone Preamp which uses switched resistors in the NFB loop and wondered if it might have been a similar implementation, given that an instrument amplifier probably wouldn't need a huge range of gain (especially if the intention is to back off the instrument volume)

I'm intrigued by this mb-1 design (though the meq5 amp seems to be a slightly better fit)

 

[abbey road d enfer]

And maybe even a double output to record both the wet and dry signal

I don't get it; which "wet" signal?

 

[abbey road d enfer]

That makes sense, I found it interesting that Mike made comments about not liking pots in the signal chain,

A rather inconsistent bias, since all guitars have pots in the signal path.

I saw this project Dual Tube Microphone Preamp which uses switched resistors in the NFB loop and wondered if it might have been a similar implementation, given that an instrument amplifier probably wouldn't need a huge range of gain (especially if the intention is to back off the instrument volume)

I really doubt a stepped attenuator could have been described as a "trimmer".

 

[jacomart]

I don't get it; which "wet" signal?

dry: as it is directly from the guitar, wet: treated with any effects; e.g. I would not be able to play on a song that requires crunch or fuzzy sound or any other effect, listening to my sound as it comes out of the pickups. so i would record the clean sound to treat it later with reamp or whatever but i would like to hear something that is at least similar to the final result While plsying.

Cheers
JM

edit: this for the @TheJames needs, not for the Motown inspired stuff

 

[ruffrecords]

dry: as it is directly from the guitar, wet: treated with any effects; e.g. I would not be able to play on a song that requires crunch or fuzzy sound or any other effect, listening to my sound as it comes out of the pickups. so i would record the clean sound to treat it later with reamp or whatever but i would like to hear something that is at least similar to the final result While plsying.

Cheers
JM

edit: this for the @TheJames needs, not for the Motown inspired stuff

You raise an interesting point. At the moment we don't even have a through output so the guitarist can plug into an amp or are we expecting them to listen to foldback headphones or control room monitors?

Cheers

Ian