Help choosing Audio Transformer for Stompbox

[J M Fahey]

Ok, then the Bass DI box takes care of the XLR problem.

And don't overthink it, you are not designing something on a clean sheet of paper but "reverse engineering" a commercial product, which as seen uses a cheap as dirt speaker as transducer and very very very probably a cheap as dirt transformer, either a surplus "transistor radio output" one or, more probably, what I suggested, a 240/120 to 6/12V power transformer wired backwards.

Very much doubt a $65 Jensen or $150/200 Lundahl microphone transformer works better there.

[bbsailor]

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.

Read this web link: Understanding Impedance. This article will explain output impedance and input impedance, matching output impedance to input impedance and bridging impedance.

I hope we have helped you in your search for finding the right parts required for building a stomp box transducer as well as understand the consequences of using the impedance values of the parts that you ultimately find.

Joseph J. Rogowski

[Mike Sulzer]

Read this web link: Understanding Impedance. ...

Joseph J. Rogowski

But note that his short section on why a guitar output needs a very high load impedance is incomplete. The inductive pickup coil rises in impedance with frequency as he implies, but the most critical frequency range for maintaining the high frequency response is near the resonance. Here the impedance is a few times higher than that of the inductance alone because of the effect of the inductance and the capacitance (mostly cable capacitance, partly coil capacitance) acting together.

[bbsailor]

But note that his short section on why a guitar output needs a very high load impedance is incomplete. The inductive pickup coil rises in impedance with frequency as he implies, but the most critical frequency range for maintaining the high frequency response is near the resonance. Here the impedance is a few times higher than that of the inductance alone because of the effect of the inductance and the capacitance (mostly cable capacitance, partly coil capacitance) acting together.

Mike,

I do not disagree with your point but it is not fully related to finding a good matching transformer to convert the output from a 4 ohm speaker to feeding a bass BI box with a 1 Meg ohm input impedance and a potential active gain of 12db or 4 times. This thread is about finding a matching transformer to boost the output of a 4 ohm speaker acting as a transducer to convert the sound from a foot stomp on a wood box to an appropriate electronic level to feed the bass DI box. Your reference to why a guitar output needs a high impedance amp input impedance so as not to load down the highest impedance that occurs at resonance is valid but no so much in this situation. The graphs of your high inductance SD pickup shows a peak inductance of about 800K ohms at resonance. By using the rules of bridging impedance you would need an input impedance of 8 Meg ohms or 10 times the output impedance to minimize the input loading. Some people say that 5 times the output impedance is also an acceptable but lower limit of the bridging impedance value (or 4 Meg ohms in this case). Here is where your ear is the final judge.

If drumminhands wants to build his stomp box and put a 500K ohm volume control in the wood stomp box, this would lower the impedance to about 333K ohms (500K in parallel with 1 Meg ohm). Then, using the bridging impedance rules would put the maximum transformer output impedance at about 40K ohms (actually 333K ohms). This transformer would then be 4 ohms to 40K ohms and have a turns ratio of 100. This might be hard to find but since the DI box has a gain of 4, a lower turns ratio might be more easy to find. A 4 ohm to 10K ohm transformer would have a voltage boost of 50. The appropriate transformer turns ratio is related to the output voltage of the 4 ohm speaker loaded and boosted by the chosen matching transformer turns ratio which is why I used the reference to the "understanding Impedance" web link.

Mike, how is your reference to guitar pickup resonance related to this stomp box transducer design?

Joseph J. Rogowski

[Mike Sulzer]

Mike,
Mike, how is your reference to guitar pickup resonance related to this stomp box transducer design?

Joseph J. Rogowski

Joseph,

It is not.

This is, however, a forum on electric guitar pickups. Incomplete discussions of the effects of their impedances could be confusing.

By the way, I did not say that the input impedance of the guitar amp needs to be high enough so that the pickup is not loaded down at resonance. It is, as you said, loaded down some with the standard 1 M resistor, and there is nothing wrong with that, but it is important to understand the reason why it is loaded down, and for that a discussion including the pickup-cable resonance is necessary.

On the other hand, this stomp box transducer design is a a kind of casual enjoyable project, and I do not think that a detailed understanding of the impedance standards in the audio world is all that necessary unless that is what you want to do.

[J M Fahey]

Guys, again, do not overthink this
Fully agree on studying minute differences between regular guitar pickups, those resonances at different frequencies with different peaks and reacting to different preamp impedances make all the difference in the World, and Musicians swear by one or another, but here we are studying an instrument made of basically a junk clothes or kitchen cabinet found on the gutter, with a flapping unglued bottom, plus a nailed on strip of roof quality galvanized sheet, picked up by a $1 speaker where output voltage is boosted by a wall wart or clock radio quality backwired $1 power transformer ... and there lies its beauty

In fact I believe wiring the speaker leads, floating (ungrounded) to pins 2 and 3 of an XLR connector would provide more than enough signal if plugged in a PA mixer or acoustic guitar XLR Mic input, while the Piezo would perfectly match the high impedance ... uh ... Piezo input and drumminhands could well explore that possibility too.

Only problem?
It would probably be way too Hi Fi ,perhaps too refined for such a crude instrument.

The original idea, of course, is to plug Piezo into a regular *guitar* amp input, and voltage boosted speaker transducer into another.
How much voltage boosting?
Not that important , anything is more than enough, see that Piezo goes direct while speaker has a volume pot (attenuation only).
That alone backs my idea.

Impedance matching?

You must be kidding
Would you wear highly polished shoes, white gloves and a Tuxedo to eat at the corner greasy burger place?

[drumminhands]

By the way, I tried out a few parts today. The Exciters sound good. And I used some of these audio transformers http://www.mouser.com/ds/2/449/XC-600133-202475.pdf

Thanks again to all the help given here.

[J M Fahey]

Not so easy, kid

Now we need some MP3 or a YT video , we want to *hear* it