New designs for a diaphragm pickup (yet another ampeg baby bass thread)

[Bruce Johnson]

Anyway, the rocking motion of the bridge, however important or not it might be, can never be sensed by the Baby Bass PUs.

I don't have time to write a couple of pages right now, but here are a few bits for you to think over. You're getting warmer now, but you have it backwards.

The two Baby Bass coils are wired out of phase from each other. This means that most of the output signal comes from the rocking motion, not the up-and-down motion. This is intentional.

The rocking motion of the bridge is the dominant source of the pop and warble on an upright bass. The bridge rolls about an axis defined by the shape of the top, the soundpost, and the bridgebar.

One of the keys to replicating the pop and warble in a percussive pickup system is in is getting the correct balance between the spring rate in the rocking direction and the spring rate in the vertical direction. This is where it can get tricky.

[Joe Gwinn]

Well, actually I'm not very fond of German beer, it's too bitter -IMO-.

How can you say that? There are more German beers than German towns - it's impossible to try them all.

There's one unfiltered one, hefeweizen, which is sort of acceptable -always my own personal opinion-, other than it's so thick you don't know if you should have it with a spoon.

Knife and fork.

It's not the beer man talking here -more like the red wine in my case- but the ones I like the most are Belgian. I've been to Brugge for a concert once, and the Belgians have some really tasty beer (and quite strong, also).

I'm partial to the Trappist beer from Belgium. I'm sure that there are many others, but one can get the Trappist stuff here in the USA.

Moving to French food, well, certainly not bad at all, but overrated. I like France a lot, but if you go to Italy you can eat a thousand times better for the same price.

Well, I like them both, but you are right about the cost, at least in restaurants.

Anyway, I don't need anything too fancy looking for dinner. Having been born in Argentina, I'm used to eating meat twice a day, with only salt as seasoning... let's better change the subject or I'll get homesick.

OK. German food and French wine, with Belgian beer as a chaser. Or should that be Italian wine...

[xxxchange]

I don't have time to write a couple of pages right now, but here are a few bits for you to think over. You're getting warmer now, but you have it backwards.

The two Baby Bass coils are wired out of phase from each other. This means that most of the output signal comes from the rocking motion, not the up-and-down motion. This is intentional.

So it turns out it is EXACTLY THE OPPOSITE of what I thought ... I feel dumb.

In an acoustic instrument the rocking motion of the bridge is accentuated by the leverage effect created since one bridge foot is sort of stationary...

I thought that the absence of soundpost would make the two feet vibrate sort of in unison.

Well, I guess you could wire the coils so that the pickup is more sensitive to the up-and-down motion, but that would not yield the sought-for Baby Bass sound, according to your experience.

The rocking motion of the bridge is the dominant source of the pop and warble on an upright bass. The bridge rolls about an axis defined by the shape of the top, the soundpost, and the bridgebar.

You mean the shape of the top of the bridge? Bridgebar is the bassbar? I don't quite understand, you're using acoustic double bass nomenclature now, but the Baby Bass lacks all of those, well, not a bridge, but a soundpost and bassbar...

One of the keys to replicating the pop and warble in a percussive pickup system is in is getting the correct balance between the spring rate in the rocking direction and the spring rate in the vertical direction. This is where it can get tricky.

I sense that this is like the cornerstone in the diaphragm pickup design . You cannot sense the two movements, so you have to choose one.

Actually, even some other bridge movements are present, although not efficiently transfered to the instrument plate normally... you could eventually build a pickup system to sense those...

What is the spring rate? Sort of the "springiness" or "elasticity"... the bridge's tendency to either favor one or the other movement?

What kind of measures would you take to ensure you favor the rocking motion? Is there anything you can do from the standpoint of design in order to bring one or the other to the fore?

For instance, I've read somewhere, I don't recall where -I will have to look again-, that the distance between the bridge's feet was essential to tune the frequency response. This might play a role here as well.

Which parameters affect in your opinion the tendency of the bridge to favor one or the other way of vibrating?

For instance, having the feet close together, and a high bridge is better I guess, than having a short bridge with the feet further appart... ?

Sorry to ask so many questions, but you really got me thinking with that last piece of information. I guess that's why they designed the pickup with two separate diaphragms, instead of a big one. That would also sort of "favor" the rocking motion... whereas my "one big diaphragm" design would have a penchant for moving up and down, and thus, a lot of signal cancellation from the out of phase PUs...

[David King]

The string's movement feeds the bridge movement which feeds the top's movement and vise versa all the way back to the string. You'll have inertia and drag and resonances acting and interacting at each step. I think it's going to be very hard to think this one though without getting dirty first. Let's remember that the baby bass was designed first and foremost to be easy to mass produce at a good profit margin. I don't doubt that the designer knew what he was doing but I doubt he knew exactly what he was going to get on the first try. They may well have adapted a few aspects to meet expectations and they may also have hit some dumb luck along the way.

Are you equally concerned with arco as with pizz sound?

[xxxchange]

The string's movement feeds the bridge movement which feeds the top's movement and vise versa all the way back to the string. You'll have inertia and drag and resonances acting and interacting at each step. I think it's going to be very hard to think this one though without getting dirty first. Let's remember that the baby bass was designed first and foremost to be easy to mass produce at a good profit margin. I don't doubt that the designer knew what he was doing but I doubt he knew exactly what he was going to get on the first try. They may well have adapted a few aspects to meet expectations and they may also have hit some dumb luck along the way.

Are you equally concerned with arco as with pizz sound?

Well, I guess most people using the Baby Bass are more inclined to pizz, but I might be doing some arco as well. Supposedly, the diaphragm pickup has a decent arco sound.

I agree with you, at the end of the day, you have to "get dirty", some questions cannot be answered based on theory alone, I guess. There are far too many intervening factors interacting with each other to be able to discern which would be the optimum construction beforehand.

Now, regarding the original design, I don't know if it was dumb luck, but they certainly created something with appeal for bassists looking for a percussive and fundamental bass sound at ampeg.

Well, I already bought the steel plate, wire and magnets are on the way (sort of, I have to decide the gauge of the wire and the exact shape of the magnets). I will see how it turns out. Too bad that I don't have an original to compare it with, but I'll post the results here whenever they're available.

I don't need a lot of luck to beat the cheesy sound that my Chinese electric upright produced right out of the box (lousy piezoelectric pickup).

Thanks for all the feedback

[Bruce Johnson]

Yeah, I mistyped.....I meant Bassbar, not bridgebar.

The point I was trying to make is that the upright bass, as well as the other violin family members, were designed to be played with a bow. The tone from bowing action is, of course, a complicated thing by itself. But basically, it's a chattering action between the bow and the instrument. The resulting tone is a combination of the vibration of the bow frame (based on it's tension, diameter, wood character, etc) and the vibration of the instrument. How the instrument vibrates depends on many things, but the important thing here is that the main physical movement is parallel to the bow strings.

When you push the bow across the string, the force you are applying is trying to rock the bridge sideways. In engineering terms, it's rolling (that is, rotating) about an axis that runs approximately parallel to the neck. To turn that bow motion into acoustic sound, the body structure of the bass (or violin) is designed to use that rolling motion of the bridge to flex the top. The biggest motion of the top is in the sideways twisting of the center section, between the F-holes. Notice how the bridge is taller than it is wide at the feet. This is to provide a larger lever arm to use to twist the top.

This twisting of the top is controlled by the soundpost and the bassbar, and other things. The soundpost forces the top to twist about the point that it contacts, which is on the treble side behind the bridge foot. Because of the geometry, this makes the effective spring rate much higher on the treble side than on the bass side. (Spring Rate is the "strength" of the spring, that is, how hard you have to push on it to get it to bend a certain distance). In this case, the top deflects with less force on the bass side than on the treble side. It's all because of the geometry; the lever arm created by the distance from the pivot point (the tip of the sound post). The purpose of the bassbar is to reinforce the area of the top around the bass-side bridge foot, and force more area of the top to move with it.

The reason for all of this spring rate tuning stuff is to tune the body so that it resonates at the very low frequency needed to get this chattering action going with the bow. This chattering frequency needs to be very close to the fundamental note that the string is tuned to. However, the frequency you need to resonate at will change depending on what note is played, and on which string. That's the reason for the varying spring rate thing. It creates a graduated range of low frequency resonance that varies from the bass side to the treble side.

The important thing is that this is a unique feature of the violin family instruments. They have a built in sub-low-frequency chattering action, in which the bridge rolls side to side. It's in there on purpose to accentuate the bowing action. It's that super low rumble that makes your hair stand up when you hear a bow drawn across an upright bass.

Note that a typical flat top acoustic guitar doesn't have this in its construction. The top bracing is biased to the treble side to some degree, but the twisting of the top is only a small part. An archtop guitar, however, is sort of a hybrid. It borrows some of the violin family design, and uses the twisting to boost the attack profile on the notes.

How does all this relate to Salsa and Baby Basses? Well, when you pluck an upright bass hard, this built-in chattering action feature gives you a bonus on each note. There's a quick loud "pop" followed by a couple of cycles of "warble" at something like 2-5hz, before it settles into the tone of the note on the string. Think of it as POWwowowowowhmmmmmm. The warble in particular is a distinct characteristic of a plucked upright bass, and is caused by the built-in low spring rate rolling motion of the bridge. You won't hear it in an electric bass or an electric upright with a solid-mount bridge.

Everett Hull and his guys were specifically designing a pickup that would work with a bow and gut strings. The two diaphragms, the height and width of the bridge, the out of phase coils, and all that were made to detect and enhance the chattering action with the bow. Once they had that, it also did a pretty good job of replicating the pop and warble during plucking. Through an amp, it was a reasonable representation of the sound of an upright bass, at least from a distance. From there, the Baby Bass was a stick with strings. They added a crude scroll headstock and thin foam-filled plastic shell body to make it more visually appealing, and a legend was born.

The "mystery" pickup on Ampeg's later horizontal Scroll Basses (which are my own area of expertise) is a derivative of the Baby Bass pickup. It has a single large diaphragm, but it still has two out-of-phase coils underneath. It has a unique sound, but the pop and warble is more subdued. It also has a lot of weird harmonic problems.

For the percussive pickup (that's what I call it) in my own model Scroll Basses, I didn't bother with diaphragms. I went with a suspended framework of piano wire rods, which allows me to really tune the motions and spring rates in all of the axes. The system I'm using now, I spent about three years developing. Lots of work on mule instruments. I think that it's quite a bit better sounding than the Baby Bass pickup, even though it's squeezed into the limited space of the horizontal bass. I am working on a more complex version percussive pickup for my own model electric upright bass that I intend to start offering eventually.

Since someone will ask, no, I don't currently sell the percussive pickup components from my Scroll Basses. It's unlikely that you could get them to work on your instrument anyway. Percussive pickup installations have to be designed and tuned for each particular instrument. I've thought about someday offering an adjustable percussive pickup kit for home builders, but I haven't gotten to it yet.

Also, one of the reasons I'm so familiar with Baby Basses is that I've done a fair amount of repair work on them in recent years. I generally don't do repair work here, because I'm so booked up on my own new instruments. But I have done some work on Baby Basses for a few customers, just because they are "cousins" to the Ampeg Horizontal Scroll Basses, which are my specialty. Most of what I've done are necks; rebuilds, structural repairs, and a few special replacement necks from scratch. But word has gotten around that I can make hardware and pickups, so guys have been begging me to offer replacement pickup parts for Baby Basses. There are a lot of Baby Basses out there, and no real source for parts anymore. So, back in the spring, a customers' Baby Bass was here for some neck work. With his permission, I completely disassembled the pickup system and reverse-engineered it. I've got drawings and pictures of every piece. Again, I don't have time right now, but I may eventually offer replacement parts and upgrade kits. Please don't write to me asking for parts now, because I don't have them!

[xxxchange]

Well, that was a nice chunk of information that you share with us.

Trying to put all the pieces together:

- the mechanical aspect is the most influential for diaphragm pickus.

- therefore one should spend more R&D time trying to find out the correct bridge / diaphragm configuration rather than experimenting with magnets, wire and coils.

- particularly important factors are the spring rate in all axis. Since the pickup is sensitive to rocking motion of the bridge, one should dimension the bridge and diaphragm (or any other spring mechanism) so that this particular motion is unimpeded.

- something to watch for: harmonics and resonances introduced by the spring mechanism.

I would like to thank you for your contributions to this topic, Mr. Johnson, and I would love (and I think everyone else in this forum would feel the same) if you share with us some of your ideas for an improved upright pickup.

[xxxchange]

The reason for all of this spring rate tuning stuff is to tune the body so that it resonates at the very low frequency needed to get this chattering action going with the bow. This chattering frequency needs to be very close to the fundamental note that the string is tuned to. However, the frequency you need to resonate at will change depending on what note is played, and on which string. That's the reason for the varying spring rate thing. It creates a graduated range of low frequency resonance that varies from the bass side to the treble side.

Carefully re-reading your last post, some questions arose:

do you think it could be a good idea to have different spring rates at the two bridge's feet? This is the case, apparently, with acoustic instruments. However, if one uses two identical diaphragms, this wouldn't be the case.

By the way, the f-holes, a feature that is present in all bowed string instruments, even before the violin-family beginnigs (sometimes rather "c-holes", e. g. medieval fiddle, rebec, pochette) then could also be a feature that favours the rocking motion, as they flank the bridge in both sides. Also, the way the grain of the wood runs at the instrument plate might be another factor (wood with its asymmetric bending properties, particularly the ones used for soundboards of all instruments).

The important thing is that this is a unique feature of the violin family instruments. They have a built in sub-low-frequency chattering action, in which the bridge rolls side to side. It's in there on purpose to accentuate the bowing action. It's that super low rumble that makes your hair stand up when you hear a bow drawn across an upright bass.

Especially if you bow the lowest string of a five-stringer! That sounds really mean.

Now, if I understand it correctly, to have a super-low frequency chattering, you need low spring rates--since high ones would produce higher frequencies, right?

The "mystery" pickup on Ampeg's later horizontal Scroll Basses (which are my own area of expertise) is a derivative of the Baby Bass pickup. It has a single large diaphragm, but it still has two out-of-phase coils underneath. It has a unique sound, but the pop and warble is more subdued. It also has a lot of weird harmonic problems.

I see, the subdued effect is because the bridge is much flatter, and therefore there is less leverage, right? That narrow and high aluminium bridge from the Baby Bass design would rock a lot more on those two thin steel discs, compared to the Scroll Bass design.

And still Everett Hull didn't want to give away gut strings! At that time, that was the only electric bass in which you could use non-magnetic strings.

For the percussive pickup (that's what I call it) in my own model Scroll Basses, I didn't bother with diaphragms. I went with a suspended framework of piano wire rods, which allows me to really tune the motions and spring rates in all of the axes. The system I'm using now, I spent about three years developing. Lots of work on mule instruments. I think that it's quite a bit better sounding than the Baby Bass pickup, even though it's squeezed into the limited space of the horizontal bass. I am working on a more complex version percussive pickup for my own model electric upright bass that I intend to start offering eventually.

So instead of a diaphgram you use some sort of lattice, as the spring part of the pickup. It's easier then to have different spring rates on every axis, by adjusting the size of the rods.

And there is a lot of experimentation required to find the best possible configuration for any particular given instrument, with so many factors involved (particularly bridge design, scale and string configuration).

Also, one of the reasons I'm so familiar with Baby Basses is that I've done a fair amount of repair work on them in recent years.

Well, I wouldn't be here trying to find out how these instruments work if there were any Baby Basses readily available in this part of the world... and hadn't these instruments been used for all classic salsa recordings.

[mauroconga]

hi ...
i suggest see the original Ampeg pickup patent :
ELECTRICAL STRINGED INSTRUMENT - Google Patent Search

and :
- use 42 awg wire gauge for this ...you most use this caliber ...!
(the resistance is 5000 ohm. for each pickup)
- diaphragms in silicon steel (like the electrical transformers ...)
- if you preffered the salsa sound, need a wood bridge ...
- a smooth magnet ... the alnico is the best ... not are recommendable the neodimiun for this ...
- i suggest a .22 Mf condenser for a "dark" sound (the original is .15 Mf / 400 V. )

[xxxchange]

hi ...
i suggest see the original Ampeg pickup patent :
ELECTRICAL STRINGED INSTRUMENT - Google Patent Search

and :
- use 42 awg wire gauge for this ...you most use this caliber ...!
(the resistance is 5000 ohm. for each pickup)
- diaphragms in silicon steel (like the electrical transformers ...)
- if you preffered the salsa sound, need a wood bridge ...
- a smooth magnet ... the alnico is the best ... not are recommendable the neodimiun for this ...
- i suggest a .22 Mf condenser for a "dark" sound (the original is .15 Mf / 400 V. )

First of all, thank you for your input.

I am aware of the existence of that patent, but thanks for pointing it out anyway. Actually I think if one looks hard there are similar ideas of the people who sold their instrument design to Ampeg - I don't recall their name - to be found on google/patents.

Now, I understand that as a maker of baby basses you have to be quite conservative. Most customers want an exact replica of the old models. It's quite understandable, it happens over and over, throughout the history of instrument making. Old models always become a "fetish", especially when they are out of production. Ruckers harpsichords.... Amati violins... on and on and on... to Fender guitars/basses, Ampeg baby basses, etc.

Since I am more willing to experiment, and I don't have to deal with customer's whims, as I said at the beginning of the thread, I am a little bit less tied to the original design of the pickup. However, it might well be that the original design for the baby bass is actually the very optimum in deed.

In fact, the departures I had in mind originate from the fact that the materials used in the original design seemed hard to find, at least for me, over here. For instance, there's no AWG 42 wire, but metrical approximations to that gauge. Now, my total lack of experience in the winding coils business pointed in the direction of getting something a little bit tougher than AWG 42, because I fear I couldn't handle that thin wire without ruining the whole thing after a few hundred turns. Not to mention the soldering of the terminals.

For instance, I've tried to locate "silicon steel", with absolutely no luck. Fortunately enough, many people say that the material of the discs is not THAT important.

On the contrary, you seem to adhere quite closely to the original design of the pickup. You certainly have a point that in order to duplicate THE SOUND of the baby the best approach should be to build a standard baby bass pickup.

Did you have bad experiences by using different specs for the diaphragm pickup, or is it on a hypothesis level that you think that striving away from those values would lead to a bad result?

Thanks for your post, and any other comments would be always welcomed,

P. X. L.

[mauroconga]

hi ...

is very important the materials in the magnets (ferrous like the alnico and smooth in the magnet power) ,and the CORRECT POLARITY for a good sound .
if you not apply the correct steps ,the sound be too ...."petit"

listen to me : the theory about "the coil an the magnet are not important ... The magnetic coils really don't have that much to do with it....." in other posts , sincerely ,not apply for obtain the classic BB ampeg sound ...the real point for the desired sound is :

- appropiate magnets
- correct position (magnets ,and coils)
-the correct wire gauge (42 AWG)

.

[paolo]

AEB-1 Scroll bass

Hi guys,
I'm a newcomer to the Ampeg bass world, just got me a 66 AEB-1 scroll bass. It's from Sonic Circus and has an added p-bass pickup. The tone with this pickup is really nice, smooth and p-bass like (I really love p-basses). The mystery pickup however does sound quite ugly, with a very pronounced ugly resonance somewhere around 1-2 kHz, and even backing of all of the tone knob it still can be heard: nothing like the magic bass thump I was expecting from this baby. By the way, do you know the value of the tone capacitor (Mauroconga says 0,15µF, right ?) - perhaps the guy who installed the p-bass pickup has changed it for the usual 0,05... I have read the explanations in Bruce Johnson's very interesting very strange instruments website, and I will try adjusting the tail piece tonight. I sent him an e-mail, but it keeps coming back with an unable-to-send notice: anyone knows how to reach him ??
I was wondering about the strings: do long scale strings fit to this bass or are special extra-long ones needed ? I am intending to put some good flatwounds on it (the bass came in with infamous roundwounds), and I was thinking of Labellas or Thomastiks: any recommendations you could give me ?
Cheers from Belgium, Paolo

[mauroconga]

Hi guys,
I'm a newcomer to the Ampeg bass world, just got me a 66 AEB-1 scroll bass. It's from Sonic Circus and has an added p-bass pickup. The tone with this pickup is really nice, smooth and p-bass like (I really love p-basses). The mystery pickup however does sound quite ugly, with a very pronounced ugly resonance somewhere around 1-2 kHz, and even backing of all of the tone knob it still can be heard: nothing like the magic bass thump I was expecting from this baby. By the way, do you know the value of the tone capacitor (Mauroconga says 0,15µF, right ?) - perhaps the guy who installed the p-bass pickup has changed it for the usual 0,05... I have read the explanations in Bruce Johnson's very interesting very strange instruments website, and I will try adjusting the tail piece tonight. I sent him an e-mail, but it keeps coming back with an unable-to-send notice: anyone knows how to reach him ??
I was wondering about the strings: do long scale strings fit to this bass or are special extra-long ones needed ? I am intending to put some good flatwounds on it (the bass came in with infamous roundwounds), and I was thinking of Labellas or Thomastiks: any recommendations you could give me ?
Cheers from Belgium, Paolo

I consider ,the BB sound is created by the strings (flat) ,and specially for the doublebass technique , and the big signal received for the 2 diaphragms ...

i invite to see my BB website :Baby Basses M.O.G

[Bruce Johnson]

Hello Paulo;

I'm here in my shop, as always. I don't know why you haven't been able to reach me by e-mail. The address you should be sending to is right at the top of my web site: brucejohnson100.....at...ATT.....dot Net. I'll be glad to answer all of your questions about your AEB-1.

Quickly:
Yes, there are some adjustments that you can make to the "mystery" pickup, which will improve the sound from it.

The stock tone capacitor on an AEB-1 is a 0.1mfd 200vdc yellow electrolytic. When I repair vintage Ampegs, I usually replace them with a 0.1mfd polyester capacitor. The original electrolytics will often fail over time and short out. That may be part of your problem.

The AEB-1 requires special strings that are longer than anything stocked any of the manufacturers. Even "Long Scale Plus", "Extra Long Scale" and "Super Long Scale" are not quite long enough. I stock strings that I have specially made up by D'Addario to exactly fit the Ampeg AEB-1. Unfortunately, I'm the only real source. These strings are Chromes flatwounds, and they will typically last 10 years or more. The cost is US$25 per set, plus shipping.

[Bruce Johnson]

Paulo;

I got your phone message on my answering machine, thanks! I still don't understand why you are having problems e-mailing me. I've got regular e-mails coming in from all around Europe.

Try this: Post a PM (Private Message) to me here on this forum, and include your e-mail address. Then I'll try writing to you.

Hi guys,
I'm a newcomer to the Ampeg bass world, just got me a 66 AEB-1 scroll bass. It's from Sonic Circus and has an added p-bass pickup. The tone with this pickup is really nice, smooth and p-bass like (I really love p-basses). The mystery pickup however does sound quite ugly, with a very pronounced ugly resonance somewhere around 1-2 kHz, and even backing of all of the tone knob it still can be heard: nothing like the magic bass thump I was expecting from this baby. By the way, do you know the value of the tone capacitor (Mauroconga says 0,15µF, right ?) - perhaps the guy who installed the p-bass pickup has changed it for the usual 0,05... I have read the explanations in Bruce Johnson's very interesting very strange instruments website, and I will try adjusting the tail piece tonight. I sent him an e-mail, but it keeps coming back with an unable-to-send notice: anyone knows how to reach him ??
I was wondering about the strings: do long scale strings fit to this bass or are special extra-long ones needed ? I am intending to put some good flatwounds on it (the bass came in with infamous roundwounds), and I was thinking of Labellas or Thomastiks: any recommendations you could give me ?
Cheers from Belgium, Paolo