How far will the BB OPA2134 Swing?

[huricaine]

As the title says....In the datasheet it says the max supply voltage for the BB OPA2134 is +-18 or 36V. I am wanting to build a single ended supply amplifier with a descreet voltage regulator, a boost converter actually to increase my battery power or transformer from 24V to 36V. I know most op amps swing within about 3V from their rails so if I could boost to 36V, could this get me upto about 30V p2p output? I am using this as a preamp for a class B audio stage with darlington transistors for output and negative feedback and biasing diodes. TIP41 and 42 are my power transistors and I am using BC547 and 557 for the front of the darlington pair. Using my dual 12v battery supply I was able to get 5 watts RMS out of the device...So now I want to go single ended and boost all the way upto as far as I can to get as much voltage and thus power output, possible.

Will the TIP42/42 handle all the potential power that a 36V power supply can create(with a 4 or maybe 8 ohm load) That would be upwards towards 56 watts RMS with a 36V supply(15V/4ohm * 15V). 25-40RMS would also be nice to get from this simple circuit. Will it work?

 

[audioguru]

Audio amplifier outputs are class-AB, not class-B because class-B causes crossover distortion.

An opamp does not boost a supply voltage. You need a power oscillator, inductor and filter capacitors to boost the supply voltage.

TIP41 and TIP42 have a minimum current gain of 15 at 3A so their base current must be at least 200mA but the BC547 and BC557 maximum allowed output current is only 100mA. 3A peak into 4 ohms produces an amplifier output of only 8.5W.

56W into 4 ohms requires a peak current of 5.3A and the supply voltage must be at least 48V. 56W into 8 ohms requires a supply voltage of at least 66V.

It is not recommended to operate an opamp at its absolute maximum supply voltage of 36V, use 32V for safety. Then the output of your amplifier into 4 ohms will be about 21W or 12W into 8 ohms.

Have you considered making a bridged amplifier? It uses two amplifiers for each speaker. Each amplifier drives one wire of a speaker so that the voltage swing (and current) are almost doubled for about 3.5 times the power than from an ordinary amplifier with the same supply voltage. With a single-ended supply then an output coupling capacitor is not needed.

 

[huricaine]

20 watts is not bad at all I can deal with that. when I said boost voltage I meant with a dc to dc boost converter not opamp.

I am using 1 speaker one one channel, I can still bridge an amp to one speaker correct? It would have to be 8 ohm right?

 

[audioguru]

Most ordinary car radio amplifier ICs are bridged so that they produce about 14W per channel into a 4 ohm speaker.
I don't know why you are not using an amplifier IC.

If your speaker is 8 ohms then the power is about half.

 

[huricaine]

I am just expirimenting and learning. Any ideas on a schematic for a single ended design without bridging for now?

here is my circuit:

**broken link removed**

 

[audioguru]

Your amplifier does not have biasing for the driver and output transistors so it will have horrible crossover distortion (buzzing).
The output of the opamp will be about 21V p-p. The output of the driver transistors (if they survive over-current) will be about 19V p-p. The output of the output transistors will be about 17V p-p which is 6V RMS. The power into an 8 ohm speaker is 4.5W and into a 4 ohm speaker is about 8W.

 

[huricaine]

Is that 21V opamp output figure with a supply of 30V p2p? Rightnow on a dual power supply +/-12 I have measured 5 volts comfortably without clipping..And about 6V tops on teh verge or slightly clipping...That was on RMS mode. Maybe the V losses are a little less? I figured 24V P2P - 6V of opamp loss, 1.7 x 2 for VBE, divided by 2 for peak, then multiplied by .707 for RMS = 5.16V. I would hope to get much more voltage...Is there really about a 10V deviation with a supply of 30V? I know the more current I draw the lower the output becomes but if that is the case I can lessen the load on the opamp output?

How do I gor about bridging? Negative of one amp and positive of another to speaker, then tie grounds to eachother? Is brodging pretty much compatible with any amp?

 

[audioguru]

Is that 21V opamp output figure with a supply of 30V p2p? Rightnow on a dual power supply +/-12 I have measured 5 volts comfortably without clipping..And about 6V tops on teh verge or slightly clipping...That was on RMS mode.

No, with your 24V supply.
Your measurement of 6V RMS is (6 x 2.828=) 17V p-p which is low maybe because your multimeter cannot measure audio frequencies. The datasheet for the OPA2134 opamp shows a maximum loss into a 2k ohm load of only 3V p-p.

I know the more current I draw the lower the output becomes but if that is the case I can lessen the load on the opamp output?

The opamp is driving darlington transistors that are not much of a load. Maybe you connected the pins on the transistors backwards.

How do I go about bridging?

Use two amplifiers. One amplifier must invert the signal.

 

[huricaine]

okay you are right I did measure upto 6v rms on my scope with a 24v p2p supply.

I am working on the final amp on a large proto board. I have good size heatsinks and everything and will finish it by tommarow. I am going to boost the voltage of the power suppy with a smps boost converter voltage regulator, do you think 30-32V p2p will be fine? I figure if current is too high I can use output transistors but from what I have learned from speaker design, output resistance alters the speaker's Ts paramaters and reduces efficiency. It changes the Qes. What do you think If I add some output resistance? It would be nice to add resistance to chill things out but idk if it's a good idea or not.

How specifically do I bridge with a single rail power supply? Isn't the signal still VCC - GND after the output transistors, before the output capacitor? And the capacitor does the job of centering the output at 0v and allows swinging below GND? So If I bridge two single rail amps I omit the output coupling capacitor?

one more thing, was your loss calculations through the transistors (2v) a total of both sides (npn, and pnp) I figure it is always about .7 per transistor.

 

[audioguru]

I am going to boost the voltage of the power suppy with a smps boost converter voltage regulator, do you think 30-32V p2p will be fine?

The difference between having a 24V supply and a 32V supply is that the 32V supply allows double the output power. But double the output power is an increase of only 3dB which is a small increase in loudness. 10 times the power sounds twice as loud.
Your weak driver transistors might be damaged. Use 2N4401 and 2N4403 that have a max allowed current of 600mA.

From what I have learned from speaker design, output resistance alters the speaker's Ts paramaters and reduces efficiency. It changes the Qes. What do you think If I add some output resistance? It would be nice to add resistance to chill things out but idk if it's a good idea or not.

Old fashioned speakers used with vacuum tube amplifiers needed to provide their own damping of their resonances because a vacuum tube amplifier had a high output impedance.
But modern speakers are designed to be used with a modern solid-state amplifier that has an extremely low output impedance. It is called "the damping factor" of the amplifier. Usually the damping factor is 200 (many have higher) so the output impedance of the amplifier is calculated at 8ohms/200= 0.04 ohms which damps speaker resonances very well.

How specifically do I bridge with a single rail power supply? Isn't the signal still VCC - GND after the output transistors, before the output capacitor?

Yes, but each of the two wires of the speaker have their own amplifier circuit.

And the capacitor does the job of centering the output at 0v and allows swinging below GND?

Yes and it keeps DC away from the speaker.

So If I bridge two single rail amps I omit the output coupling capacitor?

Yes because the DC output of each amplifier is the same then the speaker gets no DC. One of the amplifiers must invert the signal.

was your loss calculations through the transistors (2v) a total of both sides (npn, and pnp) I figure it is always about .7 per transistor.

It is 0.7V at low currents but increases as the current increases.

 

[huricaine]

nvm...

 

[huricaine]

thanks for you amazing help audioguru I have always liked your help.

So when I bridge the amp I need to invert one of the outputs, what way do you reccomend? Referencing the circuit posted earlier in this thread in a link.

 

[audioguru]

I forgot that you have a dual polarity supply so you do not need an output coupling capacitor. The capacitor is needed when you have a single-polarity supply and a single-ended output because then the output is at half the supply voltage and you do not want DC flowing in the speaker.
When two amplifiers are bridged then they both have the same DC output voltage so an output coupling capacitor is not needed.

I also forgot that your amplifier inverts so it is easy to make a bridged amplifier like this:

 

[audioguru]

I am replying to a deleted post:

Is there a way to combine multiple driver transistors in parallel or similar for higher current handling ability?

Yes, high power amplifiers parallel many output transistors. Each transistor has its own emitter resistor so that the various base-emitter forward voltages are equal. Here is a high power amplifier:

 

[huricaine]

Actually audioguru, I am going with single polarity power supply. I decided to because I was getting offset, and because I wanted to be able to use a mains power supply and boost the voltage with a voltage booster without worying about balancing the rails or anything.

thanks for the info on the paralell transistors. I think I will look at my local store for some 2n4401/3s.

This is the single rail circuit:
**broken link removed**

Any simple way to bridge it with it's single supply?

To boost the voltage I will be using lm2577T ADJ With a center tapped 12v transformer(12-0-12) wired with two diodes as a single rail supply(24v) then boosting that upto about 30v. Does this seem like a good idea? I figure the the more I boost my voltage, the more current the component draws right so I must be carefull with the current rating and not exceed it....My transformer says 2A on it. It dosn't have a VA rating...I got it at radioshack..

 

[audioguru]

With two rectifier diodes, a 12V-0-12V transformer makes +16V. The center-tap is connected to 0V.
With a full wave bridge rectifier (4 diodes) it makes +32V. The center tap is not used.
I do not know how you calculated 24VDC.
Since the transformer can make +32V then a voltage booster is not needed.

If your transformer can supply 24VAC at 2A then it is rated at 48VA. Its peak voltage is 24V x 1.414= 34V. Its full 48W power at 34V provides 48W/34V= 1.4A.

A single-ended amplifier (not bridged) with a 32V supply produces 9W to 12W into 8 ohms.

The amplifier will be about 60% efficient so for 10.5W per channel the total stereo output power is 21W and the total stereo heating is 15W.
Your transformer might be able to provide 48W but this stereo amplifier needs 21W + 15W= 36W. Then a stereo amplifier driving two 8 ohm speakers will not overload your transformer.

EDIT: It is simple to make the bridged amplifier with a single-polarity supply. I show a +32V supply but then the bridged output power is way too high for your 48VA transformer at about 37W into 8 ohms. the heating will be about 25W.

 

[huricaine]

Another good dose of knowledge, thankyou....

I umderstand that if/when I decide to bridge the amp I need a MUCH bigger transformer I am going to buy a larger one when I decide to add to the circuit but not for now.

Also, since this project is a for a fullrange single speaker guitar amplifier I don't need to wory about driving two speakers, just one full-range. Bridging two 20w amps would give 80w right? or close...

about the efficiency, I thought class A/B amps themselves were about 50-60% efficient? Or is this assumed to be a bit less efficient due to biasing the transistors partially to reduce x-over distortion? I am going to go about adding biasing diodes sometime.

When you said the bridged power would be 37w with heating of 25w does this mean the total will be 62w? So I would need around a 70VA or so transformer?

Also, since the 1.2A is peak current at 34v do you think I can get about 20WRMS at 32V and 4 ohm load? I figure the RMS current should be reasonable at 4 ohm if I am careful with the volume controll.

 

[audioguru]

Using a 24VAC transformer, the output power of a bridged amplifier into 8 ohms is about 37W and the heating is about 25W so the total is 62W.
Into 4 ohms the speaker power is about 60W and the heating is about 50W for a total of 110W.

 

[huricaine]

I messedup my radioshack transformer it measures like 1V I had a short circuit by mistake a week ago. I have 2 large toroid transformers, 300VA but idk if I will use one for the project.

If I use a voltage booster with a limit of 1 amp can I combine multiple boosters in parallel for more current handling? Do I need a resistor similar to how paralleled output transistors use? I am thinking of using D cell batteries for 20wRMS combining them mostly in parallel for current and boosting the voltage up...

 

[audioguru]

If you use a voltage booster circuit then it wastes power from the battery because it gets hot.
Less battery power is wasted by using a lower battery voltage and a bridged amplifier system.