How to find an audio transformer, step up from 8ohm to 1Kohm

Hi,

I need to find an audio transformer.

I will attach an LM3886 audio amplifier to the audio transformer and the transformer will power a piezo.

The LM3886 doesn't like capacitive loads and the piezo is capacitive, hence the transformer is isolating the two. The transformer is also shunting the piezo with inductance to allow power transfer.

The LM3886 will have an input of 1Kohm and wants an output at 8ohms. Essentially therefore I need a audio transformer that is 8:1000 ohms, stepping up.

I can't find one, and I don't have the equipment or time to make one. Can you help?

I have attached a schematic of the circuit I am building and the datasheet for the LM3886. Also the gain bandwidth product for my use at 130KHz is fine.

Thanks for your time and I will reply promptly to suggestions.

Regards,

Ant.

Hi,
I think you must design it self, than 130KHz isnt more audio + their are out of actuality...
K.

Re: How to find an audio transformer, step up from 8ohm to 1

Hi Karesz,

Thanks for your reply. The amp has a bandwidth minimum of 2MHz, more likely 8MHz. I'll be using it for times 10 amplification, so it will be fine up to 200KHz.

I've been thinking I could get some transformer coils and wrap some Cat5 cable through it. This might be quicker and cheaper. Can you think of anything?

Regards,

Ant.

Hallo ants,
why are you thinking not on copper wire to winding pls?
K.

Re: How to find an audio transformer, step up from 8ohm to 1

I would be inclined to try a different method altogether.

The LM3886 isn't really designed for this kind of operation. My first thoughts are to drive the piezo from a simple transistor oscillator, it's unlikely you need to worry about distortion or exact drive waveform.

Second thoughts are that the LM3886 doesn't like capacitive loads so it probably wouldn't like inductive ones either. It's a low frequency power amplifier when you really need a high frequency voltage amplifier. If the voltage requirement for the piezo can be met from your supply rails, try adding an emitter follower after the LM3886 to isolate it's internal feedback paths from the load.

Brian.

Hi,
absolutly correct, & the next question= is it a drive for what pls?
If you need it for CAD/X-Y table; I think you can not simple apply only a 555 as source for the controlling_ you will need a "usual" motor controller, but with a special supplying... Eventually a PWM?
K.

Re: How to find an audio transformer, step up from 8ohm to 1

Hi Karesz,

I've never made a transformer, so I'm not sure exactly what would be best. I can't see any reason why Cat5 cable wouldn't work, the impedance would be a little unpredictable tho

Hi Betwixt,

Thanks for your reply, I am trying 3 different methods, I have a circuit which I've been changing components on with a breadboard. I've attached a schematic. I can lower the frequency by increasing the capacitance and inductance in the circuit, but the fuses pop on a regular basis. The second angle is to build a circuit from scratch. I do have a few schematics and the parts I need but I haven't started that yet.

My preferred method is to use an amp. The LM3886 amp and transformer seem ok in theory. The reason for this is that I'll be designing/building the piezo myself. This means I'm going to be working with lots of different frequencies all below 200KHz as I adjust the piezo. What I can do with the amp is send a signal generator through the amp and not have to worry about changing components on a breadboard. The datasheet doesn't mention the LM3886 having a problem with inductance. My signal generator can manage up to 10V AC and I need 100V AC so I can't get that from the rail.

It is interesting you mentioned I need a high frequency voltage amplifier and not a low frequency power amplifier. You are absolutely right! Can you suggest a good one?

Hi,
only for some ideas:
A-303 High Voltage Amplifier,Piezo Driver and Modulator,w*w.lab-systems.com/products/amplifier/a303.html
STM Electronics - Circuit Design and Schematics for Amateur, w*w.e-
basteln.de/index_e
WMA Series HV-Amps_w*w.falco-systems.com/products
K.
Apex had good Amps for piezo`s too..

Special transformers and applications: Impedance Transformators; w*w.allaboutcircuits.com/vol_2/chpt_9/7.html
http://www.who-sells-it.com/r/impedance-matching-audio-transformer.html (check on theleft side: "Similar searches")
In all cases; I think too, that it will be not your solution...

Re: How to find an audio transformer, step up from 8ohm to 1

Hi Karesz,

Those are excellent links! Thank you!

The piezo section of the STM Electronics seems to give details of particular amps which could work. This seems to be mostly what I'm looking for. It mentions the OPA445 from Burr-Brown as the opamp in use. I will look it up and see how much it costs.

The Lab systmes high voltage amp, piezo driver and modulator does look very inviting. The cost may put me off but it is certainly worth looking at!

I remember looking up some Apex power amps. They were very good but $400 - $800 each. I will see if they do cheaper high frequency voltage amps.

The Falco systems link didn't work, I'll google for the WMA series.

I've previously enquired about PI's system drivers and they are thousands of dollars.

The stummer.pdf circuit wants a 2KV supply and is too high power.

The ELM electronics chip looks very cool but isn't high power enough.

You have given me plenty to think about!

I will do some research and see what I can find.

Thanks,

Ant.

Re: How to find an audio transformer, step up from 8ohm to 1

At what power will the piezo be driven. Transformer size depends on power it has to whitstand. For 130kHz operating frequency it will have ferrite core.
LM3886 can deliver abt. 38W at 56V supply. Is this you want a 130kHz transformer for 40W with step-up ratio 1:11?

Re: How to find an audio transformer, step up from 8ohm to 1

Hi Karesz,

I think I will leave the power amp behind and look to use a high frequency voltage opamp. I've just been looking at the OPA445.

It looks like it could work. It has a lowish current rating tho, don't you think?

Would 80V and 15mA drive my piezo?

I've attached the datasheet.

Thanks,

Ant.

Hi Borber,

Only just seen your reply. I have to go out, I'll reply a little later.

Regards,

Ant.

ants said:

Hi Karesz,

Those are excellent links! Thank you!
...The Falco systems link didn't work, I'll google for the WMA series...

Click to expand...

welcome!
No,no, it works, you must ignore the text & click on "products"...
K.

Added after 4 minutes:

Apropos,
Trafos hase another problem: it resonats with your piezo`s capacitance:-((, & these value is changing with voltage/temp/load/old etc...
We had so 10 years ago a project for nao drives_piezo elemets_ & it was buid with some HV Amp too(sorry, I cannot more remember, bur it was from some smaller German firm & build discrete)...

Added after 8 minutes:

@ants,
I cannot remember what type your "top secret piezo" is!? You did forget to tell us some over that...
We asked both, with Borber, for that.
The OPA445 need surly some booster & maybe a bridge circuit too(pp10/11)
K.

Re: How to find an audio transformer, step up from 8ohm to 1

Hi Borber and Karesz,

Thanks for your messages. I'm actually going to build the piezo. It is a vibrating plate for atomising fluid. Until I've done so it is hard to know exactly what power consumption it will have. However I can estimate and I'd say 10 Watts is the maximum, if the piezo is built properly it should work with 5 Watts. I don't actually know what I want from the transformer, I'm a novice at electronics. I've matched it for impedance but not power consumption. I want times 10 amplification of the voltage.

If I do the math: 50Vrms at 0.1Arms = 5 Watts

I'd actually prefer to leave behind the idea of using a power amp and find the right high frequency voltage opamp. Do you think that is wise?

The high frequency voltage opamp (OPA445) has the right sort of voltages but an output of 15mA so I can certainly try it but I'd prefer up to 200mA.

The setup would be:

Signal generator 5V AC ----->high frequency voltage amp 50V AC ----->Piezo

The falco systems high voltage amps are out of my price range. The cost of these devices makes me wonder if building my own is going to be very difficult!

The piezo is capacitive. My own build will have a capacitance I estimate at around 4nF. The purpose of the transformer is not only to isolate the piezo from the amp but to provide inductance to shunt the piezo. The accepted practise with piezos is to shunt the capaitance with inductance. The transformer I have been posting about trying to find should have inductance in parallel with the piezo's capacitance producing an LC circuit which resonates around that of the piezo (130KHz).

So I am now looking for a high frequency voltage amplifier that has some leeway to go up to these values: +/- 70V and 200mA that can work at up to 200KHz.

Do you think the right opamp can be bought? And how should I compensate for the piezo's capacitance?

Thanks for your suggestions,

Antknee.

Re: How to find an audio transformer, step up from 8ohm to 1

Power of 5W on 1k load requires 200Vpp or 200mApp. There exist no opa to satisfy this requirements. I can suppose that piezo impedance is 4nF paralleled by 1k resistor. Here must be used an appropriate step-up transformer which will resonate with 4nF load. Important is inductance of secondary and coupling factor. Generator (amplifier) specifications depends on turns ratio of step-up transformer and required power plus losses.
Off course you must make this transformer yourself.

Hi Borber,

The piezo is 4nF in parallel with a resistance. It is hard to say what the total impedance will be, certainly over 100 ohms. But probably not 1K ohm.

You seem to be saying the power amp route with the LM3886 was the right one?

Thanks,

Ants.

Re: How to find an audio transformer, step up from 8ohm to 1

Hallo ppls,
I think a power Amp solution is for you even so good as the must other piezo ppls!
But be car with op amps, you need (as I wrote it over their lines) some boosters (I mean voltage & current), han typicaly are piezos in bridge driven= you have (of course from two cards) the double voltage swing...
W
What you tell "piezo`s capacitance is with inductance to sgunt" = you must find out the systems (piezo with load) resonance frequency & drive it at that frequency, then is your efficience the bet
Search pls for "HV Amps/Piezo driver circuits" >> you will see, its possible to build a relativ simple pwr amp with i.e. +/- 100-150V as pwr amp, this amp in bridge is surly "enough" for your application.
If you has the system measured, you will know better the parameters and it will light to reduce the specs/make compacter the driver (and cheaper) amp for the end solution...
Will be as a (serie) product pls?
K.

Added after 7 minutes:

Power of 5W on 1k load requires 200Vpp or 200mApp. There exist no opa to satisfy this requirements...
...Off course you must make this transformer yourself.

Click to expand...

Hali Borber,
Of course their dont exist as small integrated one, or simple op amp_ but as dedicated from i.e. Apex IT EXISTS!
Other question is the price...
I would not much make with a transformer, their way is to problematic!
Other impedance maching solutions are to select_as i.e. "PI filter"_ her is the capacitance of piezo good to use as same part of hes impedance maching element...
These all will nice much things give to solute & make
K.

Re: How to find an audio transformer, step up from 8ohm to 1

ants, I just used data you provided: 8/1k transformer, 5W power, 4nF capacitance.
Most likely you do not know what you need.
All calculations must be started from piezo towards generator. First define piezo impedance than power it needs for operation and operating frequency. After that you can search solution for the circuit.

karesz, I don't think it make any sense to jump in with solutions because he is just experimenting some ideas.

Re: How to find an audio transformer, step up from 8ohm to 1

For atomizing fluid, I would think the objective would be to drive as much power as possible from the simplest circuit, waveform quality shouldn't matter too much.

May I suggest a two transistor multivibrator idea. If you look at the schematics all over Edaboard for 12V to mains inverters you will see many designs that use just two transistors driving push-pull into the primary of a center tapped transformer. If you use this principle at higher frequency and use a ferrite core instead of an iron one, you should be able to generate the voltage you need with just a few components. By using a push-pull driver you get an immediate advantage of twice the primary voltage so for example, if it was powered from 12V you would only need a 1:2.1 ratio transformer to get 50V out of the secondary.

Brian.