# Audio Output Transformer from Radioshack

Discussion in 'The Projects Forum' started by aimanj, Mar 18, 2011.

1. ### aimanj Thread Starter New Member

Mar 4, 2011
13
0
Hi all.I just bought a audio output transformer from radioshack but I'm not sure if it can be used in my project. Well, basically I just want to increase the voltage output from a piezoelectric material. The piezoelectric material is used to capture sound vibration and generates AC voltage.Since the amount of voltage is very low, someone suggested me to use an audio output transformer. So, I bought one from radioshack with this specs:

Impedance: 1kHz, 0 mADC
Input: 1kohms
DC resistance (input): 70 ohms
DC resistance (output): 0.62ohm
Frequency response: 300-10000Hz
Insulation resistance: > 100Mohm at 100VDC

How could I know how much voltage can be gained from this transformer? How to read the specs?

Thanks

2. ### Wendy Moderator

Mar 24, 2008
21,012
2,744
It isn't really a power transformer, though it can be used for such It is meant to be a impedance matching transformer. You didn't give the part number, but I think I have a few I bought for my articles in my parts box.

If it is the unit I think it is it is 600Ω:8Ω

In other words, it will take a high impedance signal from an amplifier and match it to an 8Ω speaker.

The voltage boost will be approximently 1V:8.7V
1V in on 8Ω, 8.7V out on 600Ω, but the current will be much less on the 600Ω side.

Think about it like this, if you are feeding 1W (way too much for the xfmr, but easy to work with on math) it will be the same wattage for both sides.

1W for 8Ω is 2.828V, P = V²/R
1W for 600Ω is 24.5V

aimanj likes this.
3. ### someonesdad Senior Member

Jul 7, 2009
1,585
141
It sounds like you're using the piezoelectric device as a microphone. Any reason why you wouldn't just purchase an appropriate microphone and use well-established audio circuitry?

I'd be surprised if the piezo device could do anything useful with the transformer. If you want to amplify the voltage, it seems to me that the best tool would be an amplifier like what you could make with an op amp -- or a purpose-built audio amplifier chip. Post your circuit's details and operating parameters and there are folks here who would be happy to help you design an appropriate amplifier.

4. ### Audioguru AAC Fanatic!

Dec 20, 2007
9,455
912
RadioShack knows nothing about electronic parts. They spec only the weight of the EL-19 output transformer, not its impedances nor its power rating.

The impedances of the transformer are probably much too low. A piezo transducer is made to have a typical load of 10k ohms.

5. ### SouthernAtHeart Active Member

Jan 2, 2011
124
2
My question is about the very transfomer talked about here, (I think). It's the Radio Shack #273-1380
found here:
Audio Output Transformer

If I ask Radio Shack for specs on it, they'll tell me the size, and the color of wires coming out of it, that's about it.
I just need to know what it's called in the real world, as I want to order one from a parts house like digi-key, that is made to mount on a pcb. It's a line matching transformer that attenuates a 70volt system back down to a speaker level. I'm hoping to find a very small package, only in pcb mount style. If anyone knows of a part number for such a thing I'd appriciate it. If not, I need to know the specs of this one, so I know what to look for.
thanks,

6. ### Audioguru AAC Fanatic!

Dec 20, 2007
9,455
912
See, RadioShack doesn't have a clue about electronics. They sell cell phones.

It is an audio output transformer for an old germanium transistors amplifier.
Nobody knows its max output power.
A few reviewers say it is 1k ohms to 8 ohms so if you use it on a 70V PA system then the speaker will try to draw 4.9W. A 5W audio transformer is probably larger and heavier than the RadioShack one and will not mount on a pcb.

How many Watts do you need? A 0.5W 70V to 8 ohms transformer is fairly small.

7. ### SouthernAtHeart Active Member

Jan 2, 2011
124
2
It's great to hear someone else say that! My thoughts exactly!!

Here's what I need:
I use a 70 volt audio system for carrying sound to outbuildings, as far away as 1000'. So sometimes I need to use an amplifier to boost the power of this signal, in the outbuilding. I convert 70 volt to a low level - line level (not speaker level) to go into the aux input of an amplifier. I've made a few of these, and need some more, so thought I'd actually make a nice little board for my circuit. Hence, I'm looking for a pcb mount 70 volt transformer, but I'm think they don't exist. but I don't know the actual values of a 70 volt transformer, I just know it's used for a PA system to either step the audio level up to, or step it down from the 70 volt line. Size: the smaller the better, I want to get away from a 10 watt audio transformer, to do away with all that extra power loss. Those little audio transformer from R.S. work perfect, just wondered if there's a pcb mount style of them.
Attatched is my schematic for attenuating 70 volt to aux input level. It has been working for a few years, but if there's some major improvement...? Now's the time, as I'm hoping to have a few pcb's made at batchpcb to make the project a little cleaner.

File size:
17.9 KB
Views:
111
8. ### Audioguru AAC Fanatic!

Dec 20, 2007
9,455
912
I don't know why the existing 70V amplifier cannot drive the load, so you need to attenuate the signal to drive more amplifiers.

You show a transformer plus a resistor attenuator to attenuate a 70V PA signal to line level.
Since two resistors can attenuate a signal to ANY level then why is the transformer used??

9. ### SouthernAtHeart Active Member

Jan 2, 2011
124
2
yes.
Trying to attenuate the 70v signal without transforming it back to normal audio sound would cause it to be distorted, no?

If you have or know of a circuit that does what I'm needing I'd like to see it.

Jan 2, 2011
124
2
11. ### Audioguru AAC Fanatic!

Dec 20, 2007
9,455
912
Two resistors make a perfect attenuator. It does not add distortion, it simply reduces the 70V PA level down to 0.7V line level.

The expensive transformer is used when you need isolation. You might need isolation to prevent a ground loop if the amplifiers are far apart.

12. ### SouthernAtHeart Active Member

Jan 2, 2011
124
2
thanks, I probably do need isolation at 1000' distance.

13. ### CDRIVE AAC Fanatic!

Jul 1, 2008
2,223
101
What you're describing is termed "daisy chaining" PA amplifiers. It's the pits! I've recently been asked to come out of retirement to undue nearly 54 years of daisy chaining the PA in a large (started out small) hospital.

Note: The term "PA" has two common meanings. One is "Power Amplifier" and the other is "Public Address". In the case of Public Address it's both but with "Line" outputs.