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

[xxxchange]

Hi there, I have been carefully reading the two posts on diaphragm pickups, or scroll / baby bass ampeg pickups.

I have been considering building my own, due to 1) lack of original or even copied ampeg baby basses in my area (overseas, out of the reach of the "salsa" zone)

2) the fact that this is the standard for latin music bass sound

3) the pickup that originally comes with my cheap Chinese upright completely sucks.

I am not however a purist, and I have been considering doing some departures from the original design. I am curious to know what
knowledgeable people might think about them.

I have even recognized some of what I thought were my own ideas in some of what has been covered in this forum (like attaching something to the diaphragm, thus extending the idea of "rivets" from the original design).

* First possible departure: do we really need two separate diaphragms? How about using one big diaphragm covering a big portion of the top plate of the bass, however keeping the original idea of sensing the movement at the two bridges feet?

Would this in your opinion increase or decrease the canceling effect of having one foot "out of phase" with the other? The "big diaphragm"
would in certain modes vibrate so that one half goes up as the other goes down, thus cancelling the signal generated on the pickups, is that right?

I would love to hear your opinions about this matter.

* Second point of possible departure:

I have always thought of "improving" the original design, if not in sound quality, at least in output level.

A lot of water went under the bridge since the 1960's when these basses were made, and for instance nowadays neodym magnets are available.

In electrical guitars or basses having a magnet that is way too powerful would disturb the string's ability to vibrate...
but on an upright, with many pounds of tension on each string, I guess the effect of additional "muting" by using powerful magnets would be
barely noticiable.

* Second point of departure, extended:

Then, since neodym magnets are available that are very strong, and yet don't have a lot of mass, why don't we couple them with the vibrating part of the system?

I thought of two possible desings, but since I don't know too much about magnetism, I don't know which could work better.

In both designs there is one ring magnet attached to the vibrating diaphragm (by means of a bolt going through the axis of the magnet), and another magnet, static, attached to the coil.

The difference between model-A and model-B is that in the first one the vibrating magnet is thin and long and fits inside the coil, while the static magnet surrounds the coil. In the latter, the vibrating magnet is the opposite, like a coin, and vibrates on the top of the bobbin, and another magnet, static, is placed at the bottom of the bobbin.

If I'm right, this pickup (in either model) should produce much more output than the original, since the magnetic flux would be increased considerably.

Any comments would be much welcomed.

[xxxchange]

Further clarification of the graphic

I had some bad experience in trying to convey ideas with my graphics (sometimes you think something is very clear to you, but it looks rather confusing to others), so I thought I would add some more text to explain it further, and improve the original graphic by adding a legend that explains what is what in the graphic.

We see the two models I described in the first post. The cut is diametral, as if you were slicing a cheese in two parts. The magnets used are small neodym ones, of the ring type (a cylinder with a hole in the middle), probably around 5 to 15 grams in weight for those attached to the diaphragm, and a little bit more for the static one that is attached to the bobbin.

The magnets have been axially magnetized. The difference between both designs is about the orientation of the magnets.

[Bruce Johnson]

Hello xxxchange;

From my experience with diaphragm-style pickups, I think your ideas are fine and workable. You're going to have to build test models and do experiments. As I mentioned in the other threads, almost everything with the tonal characteristics of a diaphragm pickup is in the mechanical characteristics of the diaphragm mechanism. The coil and magnet play a very small part.

Regarding single vs dual diaphragms, the main reason Ampeg used two small diaphragms on the Baby Bass was to keep the pickup system compact. A single diaphragm to fit under both feet of the Baby Bass bridge would need to be roughly 7" (178mm) diameter. You can certainly build a system with a single large diaphragm, but it will take some experimenting with the spring rate vs the travel. The bigger diaphragm could potentially capture lower frequencies, but it also may have much more pronounced harmonics problems. You're going to have to work with it.

The "mystery" pickup in Ampeg's horizontal Scroll Basses (AEB-1 & AUB-1; 1966-1968) uses a single diaphragm. It's about 5" (127mm) square, and has an effective diameter of 4 3/4" (120mm). It's made of 2 layers of 0.030" spring steel.

On your magnet and coil designs, just don't go too far trying to make them powerful. If you create too large of a signal level, the amp is going to clip and distort. And to properly load the amp, the coils should have a resistance in the range of 5K to 10K ohms.

Just to give you a reference point, the system in my AUB-2 Scroll Basses has two coils with a total of 10K ohms resistance, and two 1/4" x 3/4" Alnico V magnets that move within the coils. That gives plenty of signal level in a passive circuit. So, don't go crazy adding magnets! I really doubt that you'll need the stationary magnets that you have shown in your drawings. The moving magnets, particularly neo's, should have all the field strength that you need.

[bbsailor]

Hello xxxchange;

From my experience with diaphragm-style pickups, I think your ideas are fine and workable. You're going to have to build test models and do experiments. As I mentioned in the other threads, almost everything with the tonal characteristics of a diaphragm pickup is in the mechanical characteristics of the diaphragm mechanism. The coil and magnet play a very small part.
Regarding single vs dual diaphragms, the main reason Ampeg used two small diaphragms on the Baby Bass was to keep the pickup system compact. A single diaphragm to fit under both feet of the Baby Bass bridge would need to be roughly 7" (178mm) diameter. You can certainly build a system with a single large diaphragm, but it will take some experimenting with the spring rate vs the travel. The bigger diaphragm could potentially capture lower frequencies, but it also may have much more pronounced harmonics problems. You're going to have to work with it.

The "mystery" pickup in Ampeg's horizontal Scroll Basses (AEB-1 & AUB-1; 1966-1968) uses a single diaphragm. It's about 5" (127mm) square, and has an effective diameter of 4 3/4" (120mm). It's made of 2 layers of 0.030" spring steel.

On your magnet and coil designs, just don't go too far trying to make them powerful. If you create too large of a signal level, the amp is going to clip and distort. And to properly load the amp, the coils should have a resistance in the range of 5K to 10K ohms.

Just to give you a reference point, the system in my AUB-2 Scroll Basses has two coils with a total of 10K ohms resistance, and two 1/4" x 3/4" Alnico V magnets that move within the coils. That gives plenty of signal level in a passive circuit. So, don't go crazy adding magnets! I really doubt that you'll need the stationary magnets that you have shown in your drawings. The moving magnets, particularly neo's, should have all the field strength that you need.

Bruce,

In a diaphragm-style pickup the mass of the diaphragm and whatever is attached to it will play a major role in determining which harmonics are emphasized.

The xxxchange design proposal is a classic moving magnet type of design. Another way to try this with less mass (a moving coil) is to make a very light, pancake type of coil, single layer, glued to the diaphragm with a fixed, adjustable, strong neo magnet underneath each bridge leg. The coil could be in the approximate range of 4 to 8 ohms impedance and then connect to a small transformer 4 or 8 ohms to 20K or 50K to bring the level up to the 100 to 200 mv level for an amplifier input. The coils can even be series or parallel humbucked to help reduce hum.

Since it is only the vertical motion of the coil that induces the most voltage, xxxchange can try nonmagnetic diaphragms that have the least mass and enough strength and compliance to produce a good tone and harmonic balance.

Just some food for thought. What do you think?

Joseph Rogowski

[xxxchange]

Hello xxxchange;

You're going to have to build test models and do experiments. As I mentioned in the other threads, almost everything with the tonal characteristics of a diaphragm pickup is in the mechanical characteristics of the diaphragm mechanism. The coil and magnet play a very small part.

First of all, thank you for your contribution.

I'll take your word, since you have been already experimenting with these pickups, and concentrate on the "mechanical" side of the question, rather than the electronics.

I had been considering other materials for the diaphragm, since the use of magnets will make the use of steel as a diaphragm material no longer a pre-requisite.

But frankly I haven't got any great ideas. I don't know a lot of physics, so I don't know which materials might be better, I don't know what physical properties it has to fulfill... I guess springiness... and lightweightedness are things too look for. Some kind of silicone rubber, maybe? But it won't withstand the huge pressure from an upright bridge.

What I was considering is, since I've read somewhere that the idea behind a diaphragm pickup was advertised as if the steel would "mimic" the vibration of the fiddle plate... well, to use the classical lutherie technique of two cuts at the side of the diaphragm, to make the plate vibrate more freely (just like the "f-holes" in a fiddle).

Another thing one can learn from the 500 year old violin design is to have the diaphragm arched, so that it can withstand more pressure without loosing flexibility.

Regarding single vs dual diaphragms, the main reason Ampeg used two small diaphragms on the Baby Bass was to keep the pickup system compact. A single diaphragm to fit under both feet of the Baby Bass bridge would need to be roughly 7" (178mm) diameter. You can certainly build a system with a single large diaphragm, but it will take some experimenting with the spring rate vs the travel. The bigger diaphragm could potentially capture lower frequencies, but it also may have much more pronounced harmonics problems. You're going to have to work with it.

I only had the chance to try an ampeg for about 20 seconds, so I don't know well enough its harmonic problems.

From what I have gathered, the diaphragm system adds some weird harmonics quite low at the spectra, so that basically you need to pass the sound of the bass through a lowpass filter, and leave that weird metallic resonance out.

The frequency for that filter is quite low, so the usable range of the baby bass is very limited. But perfect for salsa music, which needs a percussive, fundamental and boomy sound.

I don't know if this is true, I am basing myself in what I've been reading, because of lack of first-hand experience.

I don't know if that applies either to the scroll-bass. At any rate, maybe the use of a bigger diaphragm could worsen the problem if the weird resonances appear even at lower frequencies... because the usuable range could be even more limited.

Just to give you a reference point, the system in my AUB-2 Scroll Basses has two coils with a total of 10K ohms resistance, and two 1/4" x 3/4" Alnico V magnets that move within the coils. That gives plenty of signal level in a passive circuit. So, don't go crazy adding magnets! I really doubt that you'll need the stationary magnets that you have shown in your drawings. The moving magnets, particularly neo's, should have all the field strength that you need.

Allright, I'll follow your advice and don't "overpower" the desgin, although I thought about using more powerful magnets because I was planning on winding a heavier gauge of wire, and thus less turns, because I have no experience in winding and the idea of handling hair-thin copper wire scares me (and the idea of soldering the two ends, is even scarier to me ).

Thanks again for the advice!

[xxxchange]

Thank you for your contribution to this thread.

Bruce,

In a diaphragm-style pickup the mass of the diaphragm and whatever is attached to it will play a major role in determining which harmonics are emphasized.

I grant you that. The problem is that normally one has to have a clue of what could work, and then experiment. And in my case I really don't have much of a clue.

What I figure, I could isolate the parameters one by one, bore a diaphragm size, and start experimenting with different materials, all of the same diameter.

First to come to mind: well, steel of course, the original design... and the reference for others to compare. Maybe I could also try wood, as in the even more original design of the renaissance, but I don't know if it could work with such reduced dimensions.

The xxxchange design proposal is a classic moving magnet type of design. Another way to try this with less mass (a moving coil) is to make a very light, pancake type of coil, single layer, glued to the diaphragm with a fixed, adjustable, strong neo magnet underneath each bridge leg. The coil could be in the approximate range of 4 to 8 ohms impedance and then connect to a small transformer 4 or 8 ohms to 20K or 50K to bring the level up to the 100 to 200 mv level for an amplifier input. The coils can even be series or parallel humbucked to help reduce hum.

Thanks for your idea, it certainly would leed to a purer sound... but I would rather stick with a passive design. As a musician I know that I hate active instruments.

The idea of the moving coil and static magnet (an not the other way round) would be the correct one, I grant you that, but it also needs someone with more skill than myself to construct. That would be the usual dynamic-mike or speaker approach, more or less.

I reversed that because I thought that just 5 grams of added mass at each bridge foot won't affect the sound too much, but maybe it does.

Since it is only the vertical motion of the coil that induces the most voltage, xxxchange can try nonmagnetic diaphragms that have the least mass and enough strength and compliance to produce a good tone and harmonic balance.

Yes, that is certainly a good idea, to leave the usage of steel as the material for the diaphragm as optional.

But, do you have any ideas so as to which materials could be great for the task?

I would gladly hear about that. Better options for diaphragm material.

It's also a matter of preference, I guess. E. g., I know I don't like the sound of hard woods for bass construction, but rather the resonances of ash and maple alone.

At any rate, it seems that I cannot do without some --ehem-- tedious experimenting, if I am set to find "the sound".

Just some food for thought. What do you think?

Well it's being digested right away . Thanks for the post.

[Steve Conner]

How about titanium for a diaphragm?

Re the low-pass filter thing, to get a fundamental boomy sound: Harmonics are what allow you to distinguish one instrument from another. If you're going to filter them out, then you can start with any sort of bass guitar at all, and the result will sound the same.

[David Schwab]

Yeah, real upright basses have a lot of harmonics, and not all that much fundamental. But I guess the baby bass doesn't really sound like an upright... it's more of a thump.

[bbsailor]

Thanks for your idea, it certainly would leed to a purer sound... but I would rather stick with a passive design. As a musician I know that I hate active instruments.

xxxchange,

The use of a transformer is passive.

Joseph Rogowski

[Bruce Johnson]

But I guess the baby bass doesn't really sound like an upright... it's more of a thump.

Well, yes and no... An upright has a complicated sound; it starts out with a pronounced pop and warble, and then settles into a tone with a lot of harmonics and overtones. The pop and warble are a result of the strings being plucked hard, and the structure flexing, and the bridge doing a quick little snap to the side. This all happens at the beginning of the note, before it gets into the mechanics of the tone created by the interaction of the top, soundpost, back, etc.

The thing is, when an upright is played in a band, the pop and warble is the main thing that you can hear over the roar. It's percussion with tone. For the Salsa guys, that's what the bass player does and wants.

The diaphragm pickup of the Baby Bass is made to create that pop and warble, which you can't really get out of typical electric bass with a magnetic pickup. The Baby Bass does a good job of the pop and warble, but no, it's steady tone isn't nearly as rich as a real upright. It's not going to compete with an upright in solo studio work. But, on stage in a club, playing Salsa and heavy rhythmic music, the Baby Bass is the king.

It's funny, because when Everett Hull and the Ampeg guys developed the Baby Bass in the early '60's, their intention was to make a portable replacement for the upright, for use in light jazz combos. That's the music that Everett loved and played. But, over the decades, the Baby Bass found its own niche market.

[xxxchange]

Thanks again for another valuable contribution.

Well, yes and no... An upright has a complicated sound; it starts out with a pronounced pop and warble, and then settles into a tone with a lot of harmonics and overtones. The pop and warble are a result of the strings being plucked hard, and the structure flexing, and the bridge doing a quick little snap to the side. This all happens at the beginning of the note, before it gets into the mechanics of the tone created by the interaction of the top, soundpost, back, etc.

Yes, most musicial instruments begin their sound with some kind of noise. You cannot take that away without taking the charm of the instrument with it.

I have seen one 18th century book on mechanical instruments, in which the author compares the sound produced by musical instruments with syllables: there's always a consonant (i.e. , pure noise or mixture of noise and sound) followed by vowel (i. e. sound, tone). Different instruments or playing techniques and articulations, don't produce only different "vowel" sounds, but also different "consonants".

All happens at that unsteady, chaotic instant in which the instrument sounds begins. Then everything sort of gets stabilized.

It's percussion with tone. For the Salsa guys, that's what the bass player does and wants.

Yes, you cannot describe it any better than that: it's percussion with sound. So simple as it sounds, and sorry if I'm a little yet it's hard for musicians trained in the European classical tradition -like myself- to reverse the dogma. Let me clarify: all the classical musicians I have met whenever presented with the choice of playing off rhythm or off tune, they wouldn't hesitate to sacrifice rhythm... even the very best classical orchestras have an intonation standard that is well beyond their rhythmic standard.

It takes a lot of practice to reverse that paradigma. And yet however you cannot play well latin music until you really understand that. The very best latin musicians have an acute sense of rhythm that -IMHO- no other musicians can rival at. Listen to the pianists Peruchin, Chucho Valdes, or bassist Cachao, or Tito Puente, or any other master and you'll get the idea. The notes happen right where they should, they never settle with anything less than rhythmic perfection (yet it never sounds mechanical).

The downside is that as great as they are in rhythm, well, intonation is not the house special. I remember one Cuban bassist, as a bass guitar player I could tell he was a double bass player that took a bass guitar. He confirmed my suspicion as I had a talk with him after the concert. He had taken the bass guitar to avoid the trouble of traveling with a double bass, but he hated it. In his words: whenever you play the double bass I can stop strings carelessly, but with this instrument I have to really watch out for where the notes are.

So for the latin bass player the harmony changes are just an excuse for producing different "colors" of "thumps", "pops" and "warbles"... (not that I'm there myself, I'm still strugling to get rid of unnecessary decorations, like anything too melodic sounding).

The diaphragm pickup of the Baby Bass is made to create that pop and warble, which you can't really get out of typical electric bass with a magnetic pickup. The Baby Bass does a good job of the pop and warble, but no, it's steady tone isn't nearly as rich as a real upright. It's not going to compete with an upright in solo studio work. But, on stage in a club, playing Salsa and heavy rhythmic music, the Baby Bass is the king.

Yet I heard that it supposedly has a very decent arco sound, better than any magnetic pickup. Gary Karr -classical double bassist- was supposed to be an endorser of the Baby Bass.

Regarding the competition between an acoustic double bass and a Baby Bass, I think there shouldn't be any at all. From what I've seen, people never ask a Baby Bass to produce acoustic sounds, and they don't play them with a conventional double bass technique. At least not the bassist I have seen live: he had very low tension strings -no steel- on it -nothing like real double bass strings- and deflected the strings so much it nearly looked like someone doing archery. Plus, you could only guess within the minor third which note he just played, but the "when" he played was something you just could not ignore. What you may call a slap-in-the-face sound.

It's funny, because when Everett Hull and the Ampeg guys developed the Baby Bass in the early '60's, their intention was to make a portable replacement for the upright, for use in light jazz combos. That's the music that Everett loved and played. But, over the decades, the Baby Bass found its own niche market.

It certainly did, and quite early also, I think latin music has been played on those creatures (or copies) for the last 50 years or so.

Anyway, taste is something that depends on culture. And even the "shortcomings" of certain designs might become desirable in time.

[xxxchange]

xxxchange,

The use of a transformer is passive.

Joseph Rogowski

Sorry for my utter ignorance on the subject of electronics.

I should have payed more attention to the physics professor, but I just couldn't stand him.

I am trying to catch up, but I think I missed the train already. Could you recommend me some sources to understand the theory behind the instruments' electronics?

[xxxchange]

How about titanium for a diaphragm?

Thanks for the tip on titanium. What are the physical properties that make titanium an elegible material, by the way? I don't know if it's comercially available over here, I'll check.

Re the low-pass filter thing, to get a fundamental boomy sound: Harmonics are what allow you to distinguish one instrument from another. If you're going to filter them out, then you can start with any sort of bass guitar at all, and the result will sound the same.

Well, as Bruce Johnson explained, the magnetic mike is unable to pick up that initial percussive sound.

And I emphasized in my last post why it is so important, more than any harmonic spectrum in the steady sound, for latin music.

Now, as I recall the Baby Bass produced quite an -unexpected- good sound when bowed. I guess the pickup dissipated the sound very quickly -while playing pizz- and that's why it produces such a sudden but short burst of sound, with less than optimum note pitch definition and harmonic richness.

But if you are constantly feeding it with more energy, like when bowing, it might -as said, haven't heard it myself- sound well.

[David Schwab]

Well, yes and no... An upright has a complicated sound; it starts out with a pronounced pop and warble, and then settles into a tone with a lot of harmonics and overtones. The pop and warble are a result of the strings being plucked hard, and the structure flexing, and the bridge doing a quick little snap to the side. This all happens at the beginning of the note, before it gets into the mechanics of the tone created by the interaction of the top, soundpost, back, etc.

I actually have an 85 year old King upright. The point was about them having a big hollow tone from the body. The initial attack does fade out pretty quickly, just as with the baby bass.

That's the music that Everett loved and played.

And played at low volumes! He hated loud music.

[Joe Gwinn]

Thanks for the tip on titanium. What are the physical properties that make titanium an elegible material, by the way? I don't know if it's commercially available over here, I'll check.

Where is "over here"? Please update your user profile to give the city and country, as it helps greatly when answering many kinds of questions.