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Impedance Matching an audio amplifier to a transformer

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riccardo

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Hi,

I have a 40W audio amplifier which is designed to drive speakers down to 4 ohms. I also have a power transformer which obviously behaves nothing like a speaker. If I hook the amp directly to the transformer, it sort of works, but will trip the protection circuits much of the time.

If I add lots of turns to the transformer primary, the amplifier drives it just fine, but due to the reduced turns ratio, the voltage output is not very good.

I think I need to match the impedance so I can drive more current into the transformer primary. Can I simply place another transformer between the amp and output transformer to do this? e.g. 100 turns on the side with the amp, 20 turns on the side which will connect to the output transformer primary, therefore letting me drive more current into it for a better voltage output?
 

MikeMl

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Why add a transformer if the amplifier doesn't need one?
 

Les Jones

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I assume the reason that you want to use an output transformer is to match the 4 ohm output of the amplifier to a different impedance. (Or is it just for isolation ?) What is the power rating of the amplifier, What impedance are you trying to match it to and what is the lowest and highest frequency you want to pass through the transformer. The turns ratio of the transformer will be the square root of the impedance ratio.

Edit.
Sorry. You have already given the power output as 40 watts. (So the maximum output voltage will be 12.6 volts RMS.)

Les.
 
Last edited:

MikeMl

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What makes you think it does not need one?
Because you stated that the amplifier was rated to drive 4Ω speakers, so it will drive any impedance 4Ω or higher...
 

riccardo

Member
The transformer impedance is much lower. Something like <2 ohms, so I am not able to drive it well with the amplifier.

If I drive the transformer directly using a half bridge driver with 12V and 5A, I get a very good HV output. I'd like to be able to drive the transformer with the amplifier instead so I have a simple way of powering it at alternate frequencies.

The amplifier is a TPA3106D1VFPR
 

Les Jones

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What was the original ratings of the power transformer ? (Primary voltage, secondary voltage and power rating.) Also can we assume the transformer was designed for 50 Hz ? (As you are in the UK) When you say the low voltage winding (Which I assume was originally the secondary.) Had an impedance "like < 2 ohms" what was the load resistance on the high voltage winding ? It would be a great help if you would tell us what you are trying to achieve.

Les.
 

spec

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Most Helpful Member
Hi,

I have a 40W audio amplifier which is designed to drive speakers down to 4 ohms. I also have a power transformer which obviously behaves nothing like a speaker. If I hook the amp directly to the transformer, it sort of works, but will trip the protection circuits much of the time.

If I add lots of turns to the transformer primary, the amplifier drives it just fine, but due to the reduced turns ratio, the voltage output is not very good.

I think I need to match the impedance so I can drive more current into the transformer primary. Can I simply place another transformer between the amp and output transformer to do this? e.g. 100 turns on the side with the amp, 20 turns on the side which will connect to the output transformer primary, therefore letting me drive more current into it for a better voltage output?
Hy Riccardo,

I can't figure what you are trying to achieve or why, but it sounds interesting.:)

But to answer your last question, yes you can use two transformers, hopefully both the same. Connect the two primaries in series and connect the two secondaries in series and you should achieve your aim.

If you would like some basic theory about driving transformers with amplifiers and the characteristics of transformers, just say and I will post some information that you may find useful for further experiments.

spec
 

dr pepper

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A power transformer designed to run on the mains 50 or 60 hz will most likely saturate on music as some content will be below 50/60hz, also if you exceed the windings rated voltage the same will happen.
You could filter off low freq's to prevent this if the application allows.
 

riccardo

Member
It is a custom made HV transformer with a ferrite core. And is still fine running over 20kHz. I can rewind the primary easily, but the secondary is encapsulated. As it comes it has 30 turns on the primary and is designed to run from 12V and about 5A-10A.

I'm basically trying to make an adjustable HV supply which retains the versatility of using that amplifier chip (self protection features, simplicity, compactness, and possibly audio). I expect that might need to make a couple of different versions; One to work at lower frequencies (10-200Hz) and another for higher frequencies (kHz). Unfortunately I'm not sure how to work out what is necessary to get a decent amount of power from the amplifier into the transformer in either case.

Yes I would love some more relevant info or a book recommendation so that I could better understand transformer design.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
It is a custom made HV transformer with a ferrite core. And is still fine running over 20kHz. I can rewind the primary easily, but the secondary is encapsulated. As it comes it has 30 turns on the primary and is designed to run from 12V and about 5A-10A.
Your answer is in your post - I've underlined the most relevant parts.

It's a high frequency transformer, not suitable for lower frequencies or audio.

Assuming you're trying to fed it with 20KHz from the amplifier?, then you would probably need to rewind both primary and secondary, as it's likely that only 30 turns is too low.

For audio and low frequencies you would need a MUCH larger transformer, MANY more turns, and a normal iron laminated core.
 

tomizett

Active Member
Nigel is right -
At 30 turns the magnetising inductance will be very low indeed. You need a magnetising inductance for the primary which is far greater at the frequency of interest than the 4Ohm minimum impedance of the amplifier otherwise the amp is expending all of its effort moving energy in and out of the transformer.
Once you have established the number of turns required on the primary, you can work out how many would be needed on the secondary to achieve the voltage ratio you're after.
 

dr pepper

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I didnt know it was ferrite.
Unless your doing something really exotic a ferrite trans is completely unsuitable, it'll probably saturate at any audio freq.
Audio output transformers are usually as nige says iron laminate, and then if the primar is part of the output stage and hgas dc flowing through it, there would need to be an air gap.
A valve amp audio output transformer might be a better start.
 

riccardo

Member
Thanks everyone. What is it about ferrite that makes it no good for low frequency. Where do I find some good reading material about it? (preferably without too much complex maths!)

So the closest I could get to making it work (without making some other circuit or secondary winding) would be to replace the core with iron laminate, and add more wingdings to the primary side?
 

audioguru

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The datasheet for the TPA3106 amplifier IC says that with a 24VDC supply its output is 40 Whats into 8 ohms with horrible distortion (if you use very good heatsinking) or only 25 Whats into 4 ohms (thermally limited by the package).
Maybe your ferrite transformer has a DC resistance of only 2 ohms but a much higher impedance. A transformer with a high resistance load on its secondary is supposed to measure a fairly high impedance on its primary.
Replies here are talking about audio but aren't you using the transformer at 20kHz to produce a high voltage? Then load its secondary properly so its primary impedance is 8 ohms.
 

riccardo

Member
DC resistance is just 0.4 ohms on the primary as it is only 30 turns of copper.
I'm not specifically trying to put audio through it, though it might be interesting to try that if I could.
I really just want to see how well I can make the amplifier drive a HV transformer (or other transformers). It does not have to be great at it. I could just build a dedicated driver for that. I just have these things available and want to explore the possibilities while learning something along the way.
 

crutschow

Well-Known Member
Most Helpful Member
As dr pepper mentioned, an audio tube output transformer designed for a valve amp will likely be the best way to do what you want.
They are designed to handle the power and frequency range you want.
You use it in reverse, driving the 4 or 8 ohm output of the transformer, and taking the high voltage signal from the input winding that normally connects to the valve HV output.
 

audioguru

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A transformer with a ferrite core is used only for high frequencies.
The amplifier IC can drive an 8 ohm speaker or transformer to 40W or a 4 ohm speaker or transformer to 25W. 40W into 8 ohms is an average voltage of 17.9V and an average current of 2.25A. 25W into 4 ohms is an average current of 2.5A. It looks like the current is half what you need.
 

dr pepper

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Google universal transformer equation, you'll get an idea of how frequency affects the core.

Why ferrite works at high freq's is complicated.
One thing about iron or should I say silicone steel which is popular for transformers is that it can take a lot more magnetic flux, power ferrite is around 0.2 tesla, whereas silly steel is around 1 to 1.2 tesla, this alone is advantageous for lower freq's.

Have you thought about redesigning the amp to run off higher voltages.

Another idea I just had is obtain a transformer as used in electrostatic speakers, these might be too high voltage, 2 or 3 kv springs to mind for quad esp 1's.
 

ChrisP58

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Be aware that there are many different ferrite materials. each has different characteristics in terms of frequency, permeability, flux density, etc. While most core materials tend to be optimized for 100kHz, and on up to MHz range, there are probably some that might be usable down to 20kHz or lower.

A quick google search found some sources to look at.
www.mag-inc.com/products/ferrite-cores
www.fair-rite.com/design-tools/materials/
https://product.tdk.com/info/en/catalog/datasheets/ferrite_mn-zn_material_characteristics_en.pdf
www.ferroxcube.com
https://www.nec-tokin.com/english/product/pdf_dl/ferritecores.pdf
http://www.samwha.com/electronics/product/product_ferrite_mat.aspx
 
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