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Need help with making an amplifier Answered

First of all I am only considering to make this amplifier I have not gone out and brought any parts yet although I have had a look around. If all goes well I will post an instructable of all the stages required to construct the amplifier. 
I am looking to make an amplifier to be used with my computer and various other devices. This is the amplifier schematics I have had my eyes on.. I am new to all of this really but Ive been trying hard and eager to get this working. 

Amplifier http://www.circuitstoday.com/2-x-60-w-audio-amplifier-circuit 
I have designed this on Eagle PCB, need to double check the connections. Would be useful for some info on the width of the tracks as this was the element I was most unsure about. I am new to Eagle PCB so im not familiar with standard conventions and tricks. 

Pre-Amp http://www.circuitstoday.com/passive-tone-control-circuit
I have designed this on Eagle PCB, on a separate board again need to double check the connections. 

Power supply 
I Have got a basic 35v+/- dc supply schematic, it is based on a toroidal transformer, will probably need a inrush current circuit doing later. I have added the power supply components on the same board as the main amplification stage. 

Frequency Crossover
I have an idea of what I want to use, I was think a 2 way 2nd order Linkwitz Riley filter because I dont want some of the areas of the midrange to be lost, from what Ive seen on graphs, the low pass and high pass dip significantly closer to the crossover frequency. Ive selected some drivers that I liked. The tweeter has an impedance of 8ohms and a frequency response from 1.6khz to 20khz. The woofer has an impedance of 8ohms and a frequency response from 30hz to 4hkz. I read somewhere that it was ideal to crossover exactly in between the two i.e 2.8khz, let me know if you think otherwise. 

Also Ive read up a bit on L-pads, would it be suitable/ worthwhile in this application? is it necessary to have the impedance seen by the amplifier constant, if so where it be? Ive looked up the components on a website supplying crossover and audio devices, there are many types of inductors and capacitors I am not entirely sure which type would be most suitable. 

If somebody could help me out in designing the amplifier and understanding how all the circuits could come together I would really appreciate it. I  would definitely do an instructable of the construction of all the stages of this amplifier.

Thanks in advance. 


You just need one transformer 28-0-28 (or 24-0-24 will do) which will connect to a bridge with large caps (1000uF/50V). This provides unregulated +-35V rails that will power the power amp. These same rails can be used as inputs to 7812/7912 linear regulators (or equivalent) for the preamp. The preamp will be more sensitive to hum than the power amp. If you feel confident of +-15V rails for the preamp, use 7815/7915 regulators, but the preamp output level does not need to be all that much since the power amp has some voltage gain (gain=21). I would suggest you build the preamp and power amp on separate boards and use a star config for grounding (separate ground wires that tie from each board back to the CT of the xformer). I did apps support many yrs ago for National on these types of devices.

Thanks for the comment, I am currently designing the pre-amp and the power amp on Eagle pcb, just routing the power amp now. Then I am going to check all the connections. I have still to sort out a power supply for this project. When I am done I will look for a transformer and try and come up with a suitable schematic, like I said I am not a processional just a student.
Let me know if you would like to see the board layout or schematic I will see if I can send it to you.
Only recently learnt how to use Eagle (Few days ago), so my schematic and board layouts are not tip top, but everything looks good so far.

Yes, I worked in the same group. He was the czar of bandgaps and a bit of an odd bird, but a sharp and colorful character nonetheless. Also worked with Bob Widlar.


Two of the Gods of electronics. I met Bob in the UK five or six years ago on a Nat Semi (RIP) roadshow. What a great character.

Widlar too ? I remember Bob writing about the process of Widlarizing faulty components, and adopted the terminology and the process.


Well, for one thing, a "dual" power supply will be a wee bit more complicated then a simple Pos/Ground PSU. If a transformer is going to supply a 35 v dual line, it will need to be a minimum of 70 volts (35 pos, / neutral or grnd / 35 negative); the range then is 70 volts.

In the link for the power supply what does it mean by a nominal 28-0-28 secondary?
I have considered this transformer -
as it is 300VA, toroidal and 35v x2, as the power supply schematic suggested.

Thank you

That's the normal way to specify a transformer with a center tap. 28-0-28 indicates the voltage swings 28V (+ & -) on each side of the center.

If you don't use the center tap (CT), the total swing on the secondary is 56V AC. A full-wave bridge connected thusly (without the CT), and with filter caps would yield up to approx 78V DC (56 * 1.4).

Add the CT as a ground reference, and one side of the PS would be +39V, the other side -38V. That's before regulation. This link might be helpful...

Note that the power supply on the westhost link isn't just a bipolar power supply--it's TWO bipolar power supplies. Do you need two?

Old-timey electronics used center-tapped transformers because they can achieve a full-wave rectified power supply using only two rectifiers (not 4, like a bridge)--since the only rectifiers were tubes, that made a huge difference in cost and reliability back then. Transformers like 325-0-325 were commonplace.

Is the value 28-0-28 given in terms of AC voltage, if so then the peak voltage required is 39.6v? Would that mean that the transformer i was looking at is unsuitable as it has a secondary voltage of 35v, would i need to consider a transformer with more secondary voltage?
Also what voltage rating would the capacitors need to have? i was thinking about 50v, just to be safe.
Is the bridge rectifier circuit i have drawn in paint correctly connected?



The peak would be about 1.4 times the RMS, so 28 VAC is approx 39V, yep.

A 35-0-35 transformer would yield a peak of approx +-49V, or a total secondary voltage of nearly 100VDC (98VDC). The LM4780 chip amp looks to have a max supply voltage of 84V (+- 42V), so that's a little high.

Antek has some great toroidal transformers for reasonable $$$.

The drawing looks OK (pretty much a duplicate of the westhost link), but I'll still ask--do you need two bipolar supplies? (+A -0- -A, +B -0- -B).

Supply A and supply B are identical, and both draw off the same transformer. Frankly, it would be simpler (and cheaper) to use just one, add more filter capacitors and connect the amp circuits in parallel. There's no electrical isolation between the two supplies as is, nor could the two separate rectified sections together supply any more power than a single rectified section. And any problems in one will backtrack to the transformer and effect the other...

The only compelling reason for separate A & B supplies would be if one (or both) were regulated to a different voltage than the other.

Yep, you should use capacitors that have a higher voltage rating, 50V caps is fine, although probably minimum,  for 39V.

I just noticed that it does not need to be powered by a 35v dual power supply, I misread the notes...
Unfortunately the shipping costs of the Antek transformers is too high in the region of $50. I live in the UK
I have looked around and it seems you can purchase an entire bridge rectifier as one component, so my drawing was pointless.

I have found a bespoke transformer supplier in the UK, i have not enquired yet. http://www.custom-transformers.co.uk/
To me it would seem like this is the only option as it is unlikely that i will find a transformer with 28v RMS and 11v RMS secondary voltages. Not sure about the costs of this bespoke transformer at the moment. 

Forget it, for a one off.

Wind your own. Get a toroidal with a 28V primary, or near enough, and of enough power rating to drive your 11V too. Then wind another winding on the transformer for it - wind 10 turns of anything on it, measyre the output volts, and scale it to what you need - if 10 turns gives you 5 volts, 20 will give you 10, 22, will give you 11.

Don't worry about getting precisely the right voltage.


Thanks for the input but it sounds too dangerous for me to do..

Im just still confused with the specification of the transformer. I had a look at the pre-amplifier properly and it says that it requires a 15V DC dual power supply, how would this change the spec of the transformer? Would it just mean that it would have to have a secondary voltage of 28v, and a 12v dual. Would a 12v RMS be safe as this gives a peak of 16.97v but the max input of the Tl072 is +-15v. I know that some voltage is lost as it is passes through the smoothing circuit or in fluctuations in the power supply. but would it be safe or would i have to go for less RMS i.e 11v.
The custom transformer company require an output current to be specified but i dont see any mention of current in both the power and pre-amp schematics.

Looking at the Pre-amp schematic it says "to other channel VC", would i connect the pre-amp of channel B, i would have to do two of these circuits for both A and B channels right? & there would be connections to pins 5,6,7 for the TL072 IC.

Im unsure of how i would integrate the pre-amp with the power amplifier, is it even possible?

If putting an overwinding on a transformer is too dangerous, then the whole project is probably too dangerous for you.

You are overcomplicating this.

Not pointless--most bridges are drawn that way on schematics, anyhoo. And four discrete diodes in that configuration are still considered a bridge...

Might I suggest you look to scrounging some discarded electronics? Lots of solid state audio equipment has similar voltage requirements. I've got a collection of "recycled" transformers. One from curb-found S.S. organ is 24-0-24, and that's pretty close. 0 USD.