Alumnitone: need help reducing distortion in bass

[dcoyle]

(sorry for the misspelling in the title, I can't edit it)

I have a Dean EAD fretless acoustic bass and I removed the in-bridge pick up and made a new bridge of bone. I'm running Fender tape wound electric strings on it. I bought a strat sized Alumitone pickup because I wanted to get as wide a frequency response as I could, a blade style to cover the 4 string spacing, and a narrow sampling window for harmonics. Another consideration for me was that I wanted to cover as little of the sound hole as necessary, as I had tried a larger PU embedded in wood, and it muffled the sound coming out of the hole.

There is no complaint about the Alumitone, I'm sure in a strat it would be fine.

However, in my use it craps out in the lowest octave and seriously distorts. Again, this is not a complaint about the Alumitone, I realize I am asking the xfmr to pass frequencies it was never designed for. I also realize I am married to that transformer and core, the core is in the current loop.

I searched this forum for Alumitone and read BBSailer's gift idea to Lex at Lace about a secondary secondary core shunted by a pot. It got me wondering if there was a way to improve the ability to carry the low frequency flux in my pickup by CA gluing a little empty transformer core to end of the one in the PU. It wouldn't be efficient because it is not in the transformer's winding loop, but when I've played with audio air core chokes I found that adding ferrous material in the area seems to increase their inductance. Perhaps the increase in inductance would help the transformer accept the bass better, as in audio output xfmrs? It seems to me it would not change the leakage capacitance, although I could see it making the stray inductance worse. It seems to me I would be adding flux lines for transfer, even though not in a optimum geometry. Any thoughts/critique about this idea? Any suggestions on where to source a tiny xfmr core?

Any thoughts about how to improve the performance of the existing transformer would be greatly appreciated.

Thanks,

Dan

[Mike Sulzer]

I think you are right that the inductance is too low. I doubt that you can increase it enough by adding ferromagnetic material outside the core, but I am pretty sure that you will increase the hum pickup.

There is nothing inherently broad band about a low impedance pickup stepped up to high impedance with a built in transformer. You can increases the bandwidth of a normal pickup by taking off turns. Reducing the signal level is not a problem for bass since you do not play with a huge overdrive factor.

Back to transformers for a minute. Back in the dawn of time when solid state audio amps were first built, audiophiles recoiled in horror. Yes, the early SS amps were not very good, but those who listened carefully noticed that on the very deep bass, say pipe organ, the best SS amp was better. Thus SS amps were used in tri-amped systems for the sub woofer pretty much from the beginning. Audio transformers are full of compromises; avoid them when possible (except when they are part of the intended "sound"), especially for bass.

(sorry for the misspelling in the title, I can't edit it)

I have a Dean EAD fretless acoustic bass and I removed the in-bridge pick up and made a new bridge of bone. I'm running Fender tape wound electric strings on it. I bought a strat sized Alumitone pickup because I wanted to get as wide a frequency response as I could, a blade style to cover the 4 string spacing, and a narrow sampling window for harmonics. Another consideration for me was that I wanted to cover as little of the sound hole as necessary, as I had tried a larger PU embedded in wood, and it muffled the sound coming out of the hole.

There is no complaint about the Alumitone, I'm sure in a strat it would be fine.

However, in my use it craps out in the lowest octave and seriously distorts. Again, this is not a complaint about the Alumitone, I realize I am asking the xfmr to pass frequencies it was never designed for. I also realize I am married to that transformer and core, the core is in the current loop.

I searched this forum for Alumitone and read BBSailer's gift idea to Lex at Lace about a secondary secondary core shunted by a pot. It got me wondering if there was a way to improve the ability to carry the low frequency flux in my pickup by CA gluing a little empty transformer core to end of the one in the PU. It wouldn't be efficient because it is not in the transformer's winding loop, but when I've played with audio air core chokes I found that adding ferrous material in the area seems to increase their inductance. Perhaps the increase in inductance would help the transformer accept the bass better, as in audio output xfmrs? It seems to me it would not change the leakage capacitance, although I could see it making the stray inductance worse. It seems to me I would be adding flux lines for transfer, even though not in a optimum geometry. Any thoughts/critique about this idea? Any suggestions on where to source a tiny xfmr core?

Any thoughts about how to improve the performance of the existing transformer would be greatly appreciated.

Thanks,

Dan

[dcoyle]

Thanks for your time and thoughts, Mike.

On a different but related subject, I wonder if you, or anyone else, could recommend to me how many turns of what gauge wire on a jazz bass or p bass bobbin might make a broad band pickup? What step up xfmr would be good to use?

Thanks

Dan

[David King]

For a low to medium impedance pickup you'd want 150 to 1200 Ohms of a slightly heavier gauge wire. Probably AWG38 to 40 would be about right. You probably wouldn't need a step up transformer at all with a 1200 ohm pickup though the output would be a bit low and not ideal with a noisy gain structure. At 150 Ohms you could try a cheap mic to instrument transformer which had an XLR on one side and 1/4" male on the other. Less than $20. A85F Line Matching Transformer | Shure Americas
These transformers are 300 ohm to 40,000 so about 1:133

As I understand the problem with the alumitone now the bass frequencies are saturating the core and the result is something like square wave distortion as the tops of the wave forms are getting lopped off? I'd love to see what this actually looks like on an o-scope. Passive distortion always seems like it would impossible by definition. Does it sounds really bad or is it more like a soft-kneed compressor?

[madzub]

i'm not sure why it would be distorting, but i do recall it was observed that there is some low frequency roll off, i think Helmuth Lemme pointed it out here, due to some stray inductance or something, i forget. i had some of those single coil alumitones as well as a humbucker sized model, and the single coils were definitely weak in the low end, and that was on guitar. the humbucker was better, though i'm not sure if it was due to the larger primary or maybe there were more winds on the secondary.

sounds like transformers arent the way to go, that instead you should use a low-mid impedance pickup and use a preamp to boost the signal. That will give you full frequency range, and you could use capacitors to tune the resonance in the high end, or active eq.

[Mike Sulzer]

Ther is another factor that can help keep the response flat with more turns: resistive loading.
Discussion on extended response bass pickups:
http://music-electronics-forum.com/t31003/

Thanks for your time and thoughts, Mike.

On a different but related subject, I wonder if you, or anyone else, could recommend to me how many turns of what gauge wire on a jazz bass or p bass bobbin might make a broad band pickup? What step up xfmr would be good to use?

Thanks

Dan

[dcoyle]

Thanks for that info, David.

I am very embarrassed but happy to say I now think that it was the amp, not the Alumitone that was distorting. I have not yet made a mount for the pickup and was holding it in place, freehand. Last night I was using a 5w SE amp that had worked fine with the previous (Markley) pickup. It dawned on me after I posted that it was a pretty steep Q if the xfmr was fine on 80 hz and crap at 40. So, just now I tried it on my bass amp and I couldn't reproduce the noise. The pickup is still not mounted, so I need to see how it does when I can dial in it's distance, but right now the Alumitone may be fine.

Sorry to be a dolt, thanks very much for your help, Mike, David and Madzub.

[dcoyle]

Thanks again for your help, Mike, and for thinking about my problem, further.

As I said above, I am embarrassed and happy to report I now believe it was my amp that was distorting, not the PU. Operator error.

I am very interested in winding some low Z pickups myself and have collected several magnet bobbin assemblies to do this, for my solid body basses. Long, long ago I had a Gibson LP low Z bass, and it has intrigued me ever since.

I will read with interest the link on resistive loading.

Thankyou, everyone.

Dan

[bbsailor]

For a low to medium impedance pickup you'd want 150 to 1200 Ohms of a slightly heavier gauge wire. Probably AWG38 to 40 would be about right. You probably wouldn't need a step up transformer at all with a 1200 ohm pickup though the output would be a bit low and not ideal with a noisy gain structure. At 150 Ohms you could try a cheap mic to instrument transformer which had an XLR on one side and 1/4" male on the other. Less than $20. A85F Line Matching Transformer | Shure Americas
These transformers are 300 ohm to 40,000 so about 1:133

As I understand the problem with the alumitone now the bass frequencies are saturating the core and the result is something like square wave distortion as the tops of the wave forms are getting lopped off? I'd love to see what this actually looks like on an o-scope. Passive distortion always seems like it would impossible by definition. Does it sounds really bad or is it more like a soft-kneed compressor?

David,

The 1:133 you mentioned above is the impedance ratio but the turn ratio is the square root of this or about 11.5 or 1:11.5 but the published value is 1:12, close enough!

The real challenge in making Low Z pickups is understanding and balancing output voltage and signal to noise ratio. When the noise potential is low, you can anticipate a lower number of coil turns to produce an acceptable output. However, sometimes you want to voice a current transformer based pickup in a certain way that may require breaking some long established rules to use the characteristics of the Low Z single or multiple few turns of the primary string loop to enhance a particular application. One example is using a current transformer (CSE-187L) and a K&J Magnetics neo magnet .25" wide X .125" thick X 2" long to pickup an acoustic set of strings where the only ferrous part of the string is the core. The bass strings need a little boost so it can be designed into the mismatch design to passivly allow the lower primary frequencies to be at the peak of this circuit's resonance.

I am only one of a few people who share this information, but the Current Transformer characteristics (Core type, permeability, turns ratio, string loop type and resistance) all go ino making a passive filter that can be designed to tune a current based pickup to favor certain broad frequency bands.

If the best MEF minds would jump in, they could share how coil Q, DCR, primary turn ratios, and inductance affect what we hear out of these strange things we call current-based pickups (or Alumitones).


Joseph Rogowski.

[dcoyle]

Having installed the PU and having used it a bit, I'll share my reaction.

Mike Sulzer said:

" There is nothing inherently broad band about a low impedance pickup stepped up to high impedance with a built in transformer. "

I can only add "Amen".

Again, for any one reading this because of a curiosity about Alumitones, the pickup was not distorting, it was my amp. The results for acoustic bass guitar are not what I hoped, however. The bass is fine, but the top end is a little too closed in for (IMO) an accurate reproduction of the harmonics.

I think my next step is to put an iron bar across the bottom of a strat PU core and wind on about 150 ohms of #38 and put it through the Gibson AD-1 step up xfmr I have.

Thanks to all for your help.

Dan