Help choosing Audio Transformer for Stompbox

[drumminhands]

Hello all. Noob here. I'm slowly understanding electronics in musical instruments. It's a lot of fun, but a large amount of information is way over my head.

I'm trying to reverse engineer a bass transducer pickup. I've bought one, and it sounds amazing. (DIY : stompbox | DIY | Handmade | transducer | Australia). I'm trying to make a new stompbox, and I thought I'd solder the electronics too.

It uses a small speaker and an audio transformer. I've found the speaker used in the Peterman pickup. 4 ohm and 3W. QUANZHOU FENGQIN ELECTRONIC CO., LTD. | C.C. Feng Chiun Electronic Co., Ltd. (TAIWAN). But I don't understand enough about audio transformers. Can someone recommend a specific audio transformer that will match the impedance ratio for a decent sound pickup? Or recommend what I need to look for?

I appreciate the advice.

[jack briggs]

wrong forum

[J M Fahey]

wrong forum

No Sir, THIS is the proper Forum, since drumminhands is asking about a ***PICKUP*** for an Acoustic Instrument, in this case contact PICKUPS for a cajon type intrument.

Did you even bother to watch the video linked in his question?

Which leads you to Peterman's page?

And ... what do we find there?

Oh, he's just a PICKUP maker, and in that page he offers such unrelated things as:

* End pin jack for Acoustic Guitar Pickup

* DIY Stomp Box Bass sound module Transducer Pro If you are a Stomp box DIY'er Then this is the transducer (pickup) for you. Forget the Piezo guitar pickups that are design..

* External Electric Acoustic Guitar Pickup turn your electric into an acoustic-fender style

Electric Acoustic Guitar Pickup - for Fender style Bridge The New and exciting Acoustic style pickup for electric guitars, .

Back to the OP's question:

Peterman uses a small speaker as a magnetic pickup, in this case more as an acoustic microphone, with speaker cone picking up acoustic vibrations moving voice coil and speaker magnet which together are the actual transducer (pickup) converting mechanical energy into electric (what transducers/pickups do) which can be sent to an amplifier.

2 problems with using a speaker as an acoustic pickup:

1) cone is way too heavy compared to what a real microphone would use, so sound is bassy and dark.
No problem because it's meant to catch just that ; mids and higs are caught by a piezo pickup, each has its own output jack and the bass pickup even its own volume control.

Both combined give wide, extended sound, but the main advantage is that Piezos alone are usually considered thin shrill pickups (not so really, but they are often poorly used) and the speaker based pickup gives tons of larger than life bass (so much so that it needs its own volume pot for taming it down).

2) speaker is a very low impedance pickup, with attached low output *voltage* ; so a transformer is used to boost it to practical levels.

Peterman does not speak much about it for obvious reasons, but it must be a "some impedance" to 8 ohms (or 4 ohms) transformer , used "backwards" meaning you use the speaker as a pickup and use the audio voltage at the other end as the output.
You can also use a miniature 220 or 110V to 6 or 12V transformer, with the low voltage winding connected to speaker/pickup and high voltage windinggoing to volume pot (think 50k or 100k , Audio taper/Logarithmic preferred) .



there are lots of obsolete surplus suitable transformers around, just search for "miniature transformer" and browse what you find.

A couple suitable examples:


1 3FD 212 PSD2 12 Tamura Microtran 8 Pin 115V 230V 12 6V 6 3V Transformer | eBay

or

Microtran PM38F Miniature Transformer with Box | eBay

EDIT: almost forgot, sorry: Welcome to the Forum

There's a couple harsh people around but most are quite cool and willing to share or help

[drumminhands]

J M Fahey, this is such helpful information. *Thank You*. I've ordered some of those products to test them out.

I do have some followup questions as I dive into the electronics knowledge. First, I see much mention that audio transformers only work with AC, not DC. Is a contact mic input AC?

Secondly, what output impedance should I strive for?

1) Sound on Sound discusses the 600 ohm standard, but also that lower impedance can be better for noise levels. And good electric guitars can use 470 ohm.

2) Another source shows a passive electric bass guitar as high impedance, 800K ohm or 100k ohm.

3) I'm currently plugging the stompbox into a Tech 21 VT Bass DI which manual states an input of 1/4", 1 megOhm, instrument level.

I can use the impedance matching formulas here to calculate my desired turn ratios. I just need to know the recommended target.

And thirdly, instead of using a small speaker in "reverse" as the contact mic, what do you think about using an exciter like this? More info on exciters here. In theory if I find a appropriate audio transformer, the sound could be picked up just the same. I'll just need to test if it has an acceptable sound.

And thanks for the welcome to the forum. I understand there are trolls anywhere. But I hope to have lots of fun learning with you all here.

[David King]

I was going to suggest that an exciter would make a better transducer for direct coupling to a soundboard but if you are trying to get your output impedance up above 10K Ohms then starting at 4 Ohms puts you a little further back. There are some very small, inexpensive 500:1 audio transformers that might be appropriate here. Check out that long Low impedance pickup thread:
http://music-electronics-forum.com/t5447/

[Mike Sulzer]

"cone is way too heavy compared to what a real microphone would use, so sound is bassy and dark."

I think there is more to it than that. This transducer detects motion of the wood membrane; the whole speaker moves when the wood moves, but the air tends to reduce the motion of the cone resulting in the relative motion necessary to produce flux changes through the coil. It is the whole speaker that acts as the low pass filter since the velocity of its motion decreases due to its mass as frequency increases.

[J M Fahey]

True.

In fact, there's even more fine detail if we continue analyzing.

Yes, the speaker is glued to the "soundboard" so in theory it should "pick up nothing" being that "both parts are static relative to each other"

But (imperfect?) real parts behave like they behave

1) just guessing: cone and voice coil are floating, not rigidly attached to frame and magnet, so by inertia cone/coil may move different than frame and pick up "something".
Efficiency wold be low but direct percussion on the board will definitely involve high acceleration, so ...
2) there is a closed cavity between glued speaker edge and soundboard, since the latter vibrates we have some air pressure wave which will be picked up
3) the trivial solution: Petersen *is* using and selling them, so they must work, the guy and his place look legit .

@ David King: yes, an exciter is the real deal, of course.
But it's fun using a $1 speaker (although the transformer will add a couple bucks to the cost) and still getting usable results.

@ drumminhands (sorry, only now I connected your user name with this *percussion* instrument.
D*mn Alzheimer !!!! )
Don't worry abot impedance matching, here the transformer is used to boost pickup voltage to a usable level, output impedance will still be low, probably lower than any guitar pickup of the kind usually discussed here.

@ Jack Briggs, sorry if I came out sounding rude, I saw your post as being rude to a newcomer and couldn't resist.
Will now edit my earlier answer if Forum software allows so.

EDIT: sorry, was not allowed to.
But now I think maybe you read the words "stomp box" and thought question was about an effects pedal, which have their own section (I guess) .
It's unfortunate that the percussion instrument whose pickup is being discussed is also called "stomp box" ... although thinking again, such name describes better that instrument that some electronics housed in a small metallic box.

Ok well, the vagaries of Language

[eschertron]

I do have some followup questions as I dive into the electronics knowledge. First, I see much mention that audio transformers only work with AC, not DC. Is a contact mic input AC?

Hi Drumminhands, I'll address this point and hopefully add some clarity. This is the pedantic version
Acoustic energy (music, sound, speech, explosions, et. al.) is transmitted through the medium (we use air mostly) via variations in the pressure in medium.

At the molecular level it's like billiard balls - no wait, more like those big boxes full of balls at kid play centers, the boxes that the kids jump into and 'swim' around in. The event is the person jumping in, the result is that the balls (like air molecules) are displaced. When the person has finished playing and left the box, the balls are on average back to their original configuration. This is quite like a sound in the room. The travels through the air, displacing air molecules as it goes. When the event is over and the room is at its former (quiet?) state, the air molecules in the room are on average back to their original configuration.

So this means that the contact mic (or any acoustic transducer) will generate a 'minus' voltage for every 'plus' voltage so that the signal returns to its original rest state. This up and down of the acoustic waveform will produce the AC (plus and minus) electrical current in the transducer, which in turn is what the transformer is designed to handle.

Short answer: yes. Transformers are used extensively for handling just about any kind of sound/music/stuff, so it's OK to assume that any kind of sonic transducer will produce a signal that works with transformers. The short answer is no proof, just a generally valid assumption.

[drumminhands]

You all are amazing. Thanks for the help for a noob. I'll try the parts recommended and see what else I can find at my local electronics store. The sound quality will dictate the best solution. But you all have given me plenty to chomp on as I troubleshoot. Thanks again. I'll update after I wire something that sounds good.

[David King]

No reason you can't "wind your own" transducer here. Any voice coil and magnet can be adapted with either one moving and the other one fixed.

[drumminhands]

Don't worry about impedance matching, here the transformer is used to boost pickup voltage to a usable level, output impedance will still be low, probably lower than any guitar pickup of the kind usually discussed here.

Great. Then I'll play with some things and see what works. But for my own knowledge, what is the rough range of the output impedance of a guitar pickup normally discussed in this forum?

[David King]

Low impedance would be in the 0.1 - 200 Ohm range,
Medium impedance would be in the 200 - 1200 Ohm range
High impedance would be 1200 - 30,000 Ohm range
That said most of the guitar pickups we talk about here are in the 5k-8k range.
I hope that's helpful, -knowing that there are no hard and fast rules I made it up off the top of my head and I'm sure others may think otherwise.

[bbsailor]

You all are amazing. Thanks for the help for a noob. I'll try the parts recommended and see what else I can find at my local electronics store. The sound quality will dictate the best solution. But you all have given me plenty to chomp on as I troubleshoot. Thanks again. I'll update after I wire something that sounds good.

The real issue with your question is: what is the output voltage of the 4 ohm speaker when used in the stompbox? Most guitar pickups have around 100 milli-volts output or about one tenth of a volt into a high impedance (1 Meg ohm input) device input. Let us assume, for lack of measured data, that the output of your 4 ohm speaker is 1 milli-volt, then you need a transformer with a 4 ohm to 10K ohm rating to get a voltage boost of 50. Divide 4 into 10,000 and get 2,500. Take the square root of 2,500 and get 50. However, since the 4 ohm speaker is being fed into a 4 ohm transformer you will loose half (or 6db) signal by an equal impedance matching and the output will be closer to 25 milli-volts. If you can find a transformer rated at 4 ohms to 20K ohms then you will have an output of about 1 to 70 turns ratio but the output would be about 35 milli-volts. The challenge is finding a high turns ratio audio transformer like 4 or 8 ohms to 50K up to 100K ohms.

A more common transformer is to target using a mic input or mixer input rated at 150 ohms which actually has a 2.4K ohm input impedance to bridge a microphone rated between 150 to 250 ohms. Look for a transformer that is 4 or 8 ohms (including those that indicate CT or center tap) on the low side to about 400 up to 600 ohms (which is more commonly available). Run a 2 conductor shielded mic cable from the transformer higher impedance side into an XLR connector with the transformer output going to pins 2 and 3 and the shielded ground going to the transformer frame and pin 1 of the XLR connector.

Let us know what transformer you find or what voltage you measure on your speaker output so we can help you better.

Search big electronics web suppliers such as Mouser Electronics or Digitech for "audio transformers".

Joseph J. Rogowski

[Mike Sulzer]

However, since the 4 ohm speaker is being fed into a 4 ohm transformer you will loose half (or 6db) signal by an equal impedance matching and the output

Signal tranformers do not have losses locked to their rated impedances because they do not have to be loaded with that impedance. The load on the other end can be much higher than 10K (or 20K or whatever), and then you have some losses in the transformer related to its non-perfect behavior. There is some resistive loss in the windings, and at very low frequencies, there will be some additional loss associated with the current necessary to excite the magnetizing inductance.

In general in audio the load on a signal transformer (as opposed to a transformer intended for power transfer, such as an amplifier output transformer) can be much higher than the impedance rating, and the transformer can be designed to lose as little as possible. For example, that is the nearly universal way of getting good signal to noise ratio.

[drumminhands]

A more common transformer is to target using a mic input or mixer input rated at 150 ohms which actually has a 2.4K ohm input impedance to bridge a microphone rated between 150 to 250 ohms. Look for a transformer that is 4 or 8 ohms (including those that indicate CT or center tap) on the low side to about 400 up to 600 ohms (which is more commonly available). Run a 2 conductor shielded mic cable from the transformer higher impedance side into an XLR connector with the transformer output going to pins 2 and 3 and the shielded ground going to the transformer frame and pin 1 of the XLR connector.

Great info. I was planning on trying to hit the mark closer to a mic input. But do I need to use XLR? I was planning on using mono 1/4" jack so I can use a standard guitar cable to run through a bass DI box.