Motown Direct Amplifier-inspired Preamp?

[Winston OBoogie]

Go for broke and shove another 6V6 as a follower in there to drive the meter, peg that sucker!




'tis a mere jest of course.

 

[btown2009]

Meter is across the output.

 

[1000tinyempires]

Another great looking project there—is the VU buffered or just strapped across the output?

Strapped with a bypass switch, no buffer.

 

[abbey road d enfer]

This is because the Ampegs (and some Fenders) are bass amps.

Ampeg G12, Jet, and many others are guitar amps. The first tube operates with grid leak bias with a 5.6 Meg grid resistor. And most Fender guitar amps have 1 Meg grid leaks. The 2nd input, with an impedance of 136k, is notorious for sounding crap with any passive guitar.

 

[TheJames]

Since we were looking for output levels of guitars, I clipped a Rigol scope across a lead plugged into a Boogie Recto and captured some hard strums:

EMG (81/85) loaded ESP produces 4V P2P.
Fishman Fluence loaded Tele produces 5V P2P.
Hot output humbucker (Carvin M22T pickup) loaded guitar produces a whopping 7V P2P.

On basses

Nordstrand Big J Blade loaded 5-string produced an astounding 12V P2P on slaps.
EMG loaded P Bass produced 3V P2P
Stringray HH 5-string produced 3V P2P with flat EQ.

This should give an idea of how much gain we actually would need (probably not much).

I have an interest in a buffer/DI/re-amp box for tracking guitars. After years of using a Little Labs Redeye 3d feeding a Voodoo Lab GCX buffer, I can say that it removes some life compared to plugging a guitar straight-in to an amp and I don't much like the fiddliness of the DI/mic-pre/re-amp situation.

In my mind, I would like a fixed gain DI that can handle any guitar input and send that out as line-level to be recorded. No extra preamp needed. That said, you would probably want the output of the DI/preamp combo to be able to hit a clean +24 or so, but hit no more than say a +18 (and that's maybe pushing it) with the hottest of pickups.

I'm sure people would be a little confused at a really small looking DI signal in their DAW, but with a calibrated setup, you should be able to get exact levels for reamping (you can always add gain ITB or even with a drive pedal after the fact).

Anyway...My 0.02 on the topic.

 

[TheJames]

I might add, that while sorting out how things were done in the past is always a good exercise, and sorting out the Motown preamp/DI solution is a fascinating trail, I would maybe be more interested in the Motown-style guitar recording interface of the DAW era. While tubes are pretty much a divine match for guitars, from a current and future perspective, I wonder if a solid-state version would be a preferable solution for the modern studio. Is it possible to get that same lively, juicy sonic character from a solid-state solution or is that exclusively the domain of tubes?

Another circuit of interest might be the VHT Valvulator tube buffer. I don't particularly like that it hits a gain stage and then a voltage divider and throws all of the gain away (just adds noise in my mind), it is a sonically interesting addition in a guitar rig.

 

[jacomart]

Ampeg G12, Jet, and many others are guitar amps. The first tube operates with grid leak bias with a 5.6 Meg grid resistor. And most Fender guitar amps have 1 Meg grid leaks. The 2nd input, with an impedance of 136k, is notorious for sounding crap with any passive guitar.

The Ampeg amps use a modified Baxandall type tone control, they sound completely different from Fender and Marshall with the tone controls in flat position. Personally I like Ampegs very much. However, the guitarist chooses the one he prefers.
I don't think there is such a big difference in sound using a 500 kohm impedance rather than a 1 Mohm for the DI input, like that between Fender/Marshall (and a billion other brands) and Ampeg amps with their tone controls in a flat position. Without considering that the guitarist will probably act on the tone controls, both of the amp and the guitar and probably also have some stomp, after that the mixing engineer will equalize the sound and the mastering engineer will not back down from doing something. I honestly think we're going off topic.

Cheers
JM

 

[jacomart]

Since we were looking for output levels of guitars, I clipped a Rigol scope across a lead plugged into a Boogie Recto and captured some hard strums:

EMG (81/85) loaded ESP produces 4V P2P.
Fishman Fluence loaded Tele produces 5V P2P.
Hot output humbucker (Carvin M22T pickup) loaded guitar produces a whopping 7V P2P.

On basses

Nordstrand Big J Blade loaded 5-string produced an astounding 12V P2P on slaps.
EMG loaded P Bass produced 3V P2P
Stringray HH 5-string produced 3V P2P with flat EQ.

This should give an idea of how much gain we actually would need (probably not much).

I have an interest in a buffer/DI/re-amp box for tracking guitars. After years of using a Little Labs Redeye 3d feeding a Voodoo Lab GCX buffer, I can say that it removes some life compared to plugging a guitar straight-in to an amp and I don't much like the fiddliness of the DI/mic-pre/re-amp situation.

In my mind, I would like a fixed gain DI that can handle any guitar input and send that out as line-level to be recorded. No extra preamp needed. That said, you would probably want the output of the DI/preamp combo to be able to hit a clean +24 or so, but hit no more than say a +18 (and that's maybe pushing it) with the hottest of pickups.

I'm sure people would be a little confused at a really small looking DI signal in their DAW, but with a calibrated setup, you should be able to get exact levels for reamping (you can always add gain ITB or even with a drive pedal after the fact).

Anyway...My 0.02 on the topic.

Strangely, this page gives substantially different results, we need to investigate.

Cheers
JM

 

[TheJames]

Strangely, this page gives substantially different results, we need to investigate.

Cheers
JM

I can explain that.

I'm a 800lb gorilla and when I play, I hit the strings hard.

Seriously. That guy must be a jazzer or something. I was looking for the maximum output voltage from the guitar/pickup combination and I tried from a variety of guitars with active and passive pickups, strummed and hit hard (just beyond what would be encountered musically) to get a sense of just how much output can be generated.

I remember early in my recording history I set up a drum kit, tuned and mic'd it up, got good levels and tones ahead of the session. The drummer showed up and tore in to the kit and the levels were ridiculously hotter, everything clipping and sounded awful. Lesson learned.

 

[TheJames]

All that said...You might want a switchable gain structure. One level from when monsters like myself show up and will obliterate the input, another level for medium pickers, and another for light players.

 

[jacomart]

I can explain that.

I'm a 800lb gorilla and when I play, I hit the strings hard.

True, you are probably an axe hammer! I'm continuing to search and I always find lower values. Clearly the active pickups have much higher levels. After all, the volume level can always be lowered but if we make a mistake in the gain value and we do it too low then it will not recover anymore.

Cheers
JM

 

[TheJames]

Another comment on Tom's Guitar Projects...His bridge humbucker has a DCR of 8.3K. When you compare that to the DCR of say a Gibson Dirty Fingers at like 16K, there's alot more output capability than the pickup that Tom used.

As for active vs. passive, the hottest output in my tests was (surprisingly) achieved from the passive pickups (Carvin M22T which is a high-output passive, and the Nordstrand Big J Blade which is a passive).

If you design for soft players and weak pickups, when you get somebody with a high-ouput pickup, you're interface is going to go splat.

 

[jacomart]

Another comment on Tom's Guitar Projects...His bridge humbucker has a DCR of 8.3K. When you compare that to the DCR of say a Gibson Dirty Fingers at like 16K, there's alot more output capability than the pickup that Tom used.

As for active vs. passive, the hottest output in my tests was (surprisingly) achieved from the passive pickups (Carvin M22T which is a high-output passive, and the Nordstrand Big J Blade which is a passive).

If you design for soft players and weak pickups, when you get somebody with a high-ouput pickup, you're interface is going to go splat.

For anything heavy we will lower the gain and use large attenuators! However, let's not forget that this DI was made for fundamentally clean funky sounds of the 60's.

Cheers
JM

 

[Winston OBoogie]

Haha, great posts James and Jaco.
Interesting to see the levels you've captured.

There won't be a one size (read gain and input Z) that fits and suits all players and guitars..

From what I read here of the original M'Town gizmo, there were pre-set gains on the 5 channels for the different guitarists/sources. And the rest of the level setting came from the dude setting his volume pot on the guitar.

Probably not ideal for a modern studio box, so just bring those adjustments/features out onto the front panel or box.

 

[abbey road d enfer]

I don't think there is such a big difference in sound using a 500 kohm impedance rather than a 1 Mohm for the DI input,

I agree, but there's a signifiacnt difference between 500k and 250k, that you suggested earlier. Humbuckers will sound muddy and a Telacster will loose its snap.

 

[TheJames]

For anything heavy we will lower the gain and use large attenuators! However, let's not forget that this DI was made for fundamentally clean funky sounds of the 60's.

Cheers
JM

That's fair. And really, my goals are perhaps different than everybody else in here. I've just been following this thread with interest as it kind of fits in with what I am interested in and that's a direct to recorder instrument amplifier with a buffer output and maybe some re-amp capabilities.

Just like the Motown system was developed for their workflow and equipment of the era, I'm interested in something that works for (my) workflow and recording equipment.

The one thing I'm not interested in is knobs. I don't want variables, I want to set it up and let it be. I want to know that when somebody plugs in and plays, no matter how hard or soft they play, I can capture the output in a way that I can play it back directly to a sim or real amp at the exact levels it was played in. It's not like we're dealing with tape any more, and 24-bits of resolution is ridiculous dynamic range.

My converters are currently set to 0dB VU = -18dB FS. So if I want to capture a high-output guitar with a heavy player, I need say 8V P2P input to give me at most +18 dB VU output.

I'm maybe off-target in what I want. I know that I'm no longer happy with using a DI box, mic pre, and re-amp setup for tracking guitars.

 

[abbey road d enfer]

Since we were looking for output levels of guitars, I clipped a Rigol scope across a lead plugged into a Boogie Recto and captured some hard strums:

EMG (81/85) loaded ESP produces 4V P2P.
Fishman Fluence loaded Tele produces 5V P2P.
Hot output humbucker (Carvin M22T pickup) loaded guitar produces a whopping 7V P2P.

On basses

Nordstrand Big J Blade loaded 5-string produced an astounding 12V P2P on slaps.
EMG loaded P Bass produced 3V P2P
Stringray HH 5-string produced 3V P2P with flat EQ.

Such high values suggest the use of an input attenuator.
I have sold thousands of DI boxes where the signal was direct to a voltage follower, then going to an inverter for balancing, and followed by a 15dB attenuator, for a net gain of -9dB.
I never had any complaint about level. Now it was a DI going to a mic pre, with enough gain to set a decent level.
If the object is making a guitar preamp, it must include a more sophisticated gain control than a simple switch.

 

[TheJames]

Such high values suggest the use of an input attenuator.
I have sold thousands of DI boxes where the signal was direct to a voltage follower, then going to an inverter for balancing, and followed by a 15dB attenuator, for a net gain of -9dB.
I never had any complaint about level. Now it was a DI going to a mic pre, with enough gain to set a decent level.
If the object is making a guitar preamp, it must include a more sophisticated gain control than a simple switch.

Why exactly would a "switch" not be advisable? I don't think I want a variable gain but can agree that maybe a general "range" of level would perhaps be wanted. I'm pretty sure I want to be able to create a 1:1 capture of the signal into and out of the DAW. Having a variable control (outside of a say a trim pot to calibrate input/output levels), is a little counter to that goal.

 

[jacomart]

I agree, but there's a signifiacnt difference between 500k and 250k, that you suggested earlier. Humbuckers will sound muddy and a Telacster will loose its snap.

It was 500k (posts 129 and 140)

Cheers
JM

 

[ruffrecords]

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