Hi all this is my first post here but ive been a reader and heavy searcher for some time of this great forum, and now I have the need to start my own thread about my project...

Ok my project is a audio power distribution amplifier the basic idea is I have a set of speakers in a few rooms of my house that I wish to drive from the sound card output of my PC, they will all have the same "song".

The speakers will all be independant in reguards to volume and being turned on or off, these are handled by either simply switching that part of the circuit off and the volume controlls on the power amplifier side of my design..

The picture shows one channel of the design the top half is the pre-amp / distribution part "TL074" and the lower half is the power amplifier "TDA2003", the other outputs from the distribution half will go to copies of the power side for the other rooms ect.

The picture is a collection of application designs I took from several data sheets.

From my eduction and extensive searching of this forum Im faily sure about the power amp and the components in it and what they do with reguards to its operation, the part Im not sure about is the input TL074 op-amp and what the components around it are for, I can have a good guess as some of them ie the 1M ohm feed back resistor and the decoupleing capacitor at the input but the others Im not too sure about,
Could I just use the voltage follower arrangement as the input stage instead as well and so save those parts except for the input capacitor

I will also be driving this design from a PC power supply hence the TDA2003 for the power amp but the TL074 requires split rails and I dont want to dig into the PC for those rails I was intending on powering it from a single molex connector so I would only have +12v +5v and GND, so the problem is the missing -12 or -5v rail for the op-amp I was going to solve this problem by either using two resistors to create a virtual split power supply or some voltage regulator such as a ICL7660SCPA CMOS Voltage Convertor, would this be an ok solution.

I would also greatly appreciate any comments and suggestions on improvments or alternative ways of doing this project.

Thankyou for reading my rant of a first post and for any repiles.

For anyone quickly looking for ideas or designs I will post the latest design below here as well as in a reply...

Originally Posted by clipo

I will also be driving this design from a PC power supply hence the TDA2003 for the power amp but the TL074 requires split rails and I dont want to dig into the PC for those rails I was intending on powering it from a single molex connector so I would only have +12v +5v and GND, so the problem is the missing -12 or -5v rail for the op-amp I was going to solve this problem by either using two resistors to create a virtual split power supply or some voltage regulator such as a ICL7660SCPA CMOS Voltage Convertor, would this be an ok solution.

Your diagram is a confusing combination of split, and single ended, supplies?. I would suggest you use a single ended supply and a similar resistor network as shown to generate an artificial split-supply. A couple of points though, you need blocking capacitors on the outputs on the opamps to the volume controls. You show a gain of ten for the first opamp, this probably isn't required as the output of a sound card is fairly high anyway, and I don't like the way the feedback connects back to the voltage divider.

Hi Nigel thanks for your quick reply,

I too was very confused with the input to the first op-amp and was hoping some one could explain what this configuration was for.

I have edited my diagram to remove the confusion now, is this the sort of modification you were meaning.

The split supply is a problem Ive been thinking about a great deal and have searched these forums for a solution and the only one I have come with is the attached design that some other memember posted so that you could bias a op-amp for single ended supply.

"sorry who ever made this drawing I cant remember your name for the credit"

I would suggest you use a single ended supply and a similar resistor network as shown to generate an artificial split-supply.

Is this existing resistor network the one I have circuled in blue.

If you or anyone else have any alternative ideas or sugestions then please feel free to post them this project is very much in the design phase and open to any changes.

Thankyou again for your reply.

Originally Posted by clipo

Is this existing resistor network the one I have circuled in blue.

No, that's the feedback for the power amp.

If you or anyone else have any alternative ideas or sugestions then please feel free to post them this project is very much in the design phase and open to any changes.

You're not generating a split supply for the opamps, check the top right in your second attachment - you need the front part of that (three resistors, one capacitor).

You don't need a separate opamp to feed a volume control in each room. A single TL071 opamp can drive a 2k load which is 5 of your 10k volume controls in parallel. The output from your sound card might also have enough power to drive the volume controls in 5 rooms in parallel.
You probably need a two-resistor combiner to convert stereo into mono.
I originated that opamp supply sketch.

The soundcard could probably do even more as soundcard outputs are designed to drive headphones to high volumes aswell.

Originally Posted by clipo

how did you come to the conclusion that the TL071 can drive a 2K load did you use the table from the datasheet ive attached, I might include one if I think it will over load the sounds card in prolonged use.

Many spec's including distortion are spec'd with a minimum of 2k load resistance.

You said a two resistor combiner to convert stereo to mono have you got any links or more information on this, since one of the versions I'll be building will be mono for a friend but my version will be stereo output.

A 10k resistor from the L channel and a 10k resistor from the R channel join at the input of the 1st opamp. Its output will be mono.

Looks pretty good, but I would suggest adding a capacitor from the junction of the three 100K's to chassis (that is across the bottom 100K). Also (which you may also be doing but not showing?), a capacitor across the supply rails near the opamp (10uF should be OK in both cases).

Originally Posted by ulot

1)if i'm not wrong the 3 10uf 25v caps are for coupling if so, what determines their values?
2) the 470uf coming from the inverting terminal of IC1 appears to be coupling also
3) c4 and c5 are filter caps but i'm not sure why one is polarised and the other not
4) i'll guess c4 helps filter out some stuff and i don't know what R4 is doing there
5) i'll say c3 is coupling also
3)

Hi "ulot" i'll just jump right into the explanation...

1) The 3 10uF are coupling caps to determin there vaule you need to think about what frequencys they need to couple and so make sure they impose little resistance to those frequencys with "Xc = 1/2*pi*f*c" and in the pre-amp stage to first cap forms a high pass filter with the 100K resistor with its 3db point so not to attenuate wanted signals.

2) The 470uF cap is another decoupling cap so only the AC signal from the power amp is passed to the inverting input and amplified due to the potential differance formed by R2 R3 and in this case ive take the application design from the TDA2003 data sheet.

3) Im not 100% sure about the reasion behind these and why some are electrolitic and others are not, what I understand so far is they are of a value to allow any interferance/frequencys that are unwanted to be grouneded and so "removed" ...Im reading up on this currently"

4) R4 and C4 for a network to ballance out the impedance of the speaker I dont currently understand fully this either, its forming a network called a zobel network, do a search on this forum some one has answered this question

5) C3 is another coupling cap.

I would recomend you read up on high, low and band pass filters only 1st order ones for the moment since lots of the theory used is derived from them,
and I would sugest downloading data sheets to any parts you come across and looking at the application diagrams in them they can be useful
and look up operational amplifiers and there use and the golden rules since a lot of that is used here too...

hope this helps and if Ive got anything wrong I hope the others can correct me and answer you questions so I can learn too...


Nigel Goodwin: thanks for your quick reply, I will be adding a capactor across the powersupply to the opamp and will add a 10uF one you recomended could you also explain why you chose this value to me and help "ulot" in the process"..
And what value capacitor would you recomend for the lower 100k potential divider resistor ive been looking into this and have got a few conflicting ideas currently in my head...

Thank you once again for your time and help

Originally Posted by clipo

Hi "ulot" i'll just jump right into the explanation...

1) The 3 10uF are coupling caps to determin there vaule you need to think about what frequencys they need to couple and so make sure they impose little resistance to those frequencys with "Xc = 1/2*pi*f*c" and in the pre-amp stage to first cap forms a high pass filter with the 100K resistor with its 3db point so not to attenuate wanted signals.

2) The 470uF cap is another decoupling cap so only the AC signal from the power amp is passed to the inverting input and amplified due to the potential differance formed by R2 R3 and in this case ive take the application design from the TDA2003 data sheet.

No, C2 is a feedback coupling capacitor - this allows different values of AC and DC gain, the DC feedback sets the DC gain at ONE, and the AC gain is set much higher - about 100. The capacitor is a large value to get good bass response.

3) Im not 100% sure about the reasion behind these and why some are electrolitic and others are not, what I understand so far is they are of a value to allow any interferance/frequencys that are unwanted to be grouneded and so "removed" ...Im reading up on this currently"

C5 and C6 are supply decoupling capacitors, essentially the big capacitor decouples at low frequencies, and it's good practice to place a small capacitor to decouple at high frequencies - where the big capacitor won't work very well. But the large one is by FAR the most important!.

4) R4 and C4 for a network to ballance out the impedance of the speaker I dont currently understand fully this either, its forming a network called a zondal network or some name like that I cant remember exactly, do a search on this forum for a few power amplifier chips some one has answered this question

It's called a 'zobel network', it's to help stop the amplifier going unstable if feeding difficult loads.

5) C3 is another coupling cap.

Sort of?, essentially it's a DC blocking capacitor, to prevent DC reaching the speaker - it has to be a VERY large value for good bass response, but it does indeed 'couple' the signal to the speaker.

I would recomend you read up on high, low and band pass filters only 1st order ones for the moment since lots of the theory used is derived from them,
and I would sugest downloading data sheets to any parts you come across and looking at the application diagrams in them they can be useful
and look up operational amplifiers and there use and the golden rules since a lot of that is used here too...

hope this helps and if Ive got anything wrong I hope the others can correct me and answer you questions so I can learn too...


Nigel Goodwin: thanks for your quick reply, I will be adding a capactor across the powersupply to the opamp and will add a 10uF one you recomended could you also explain why you chose this value to me and help "ulot" in the process"..
And what value capacitor would you recomend for the lower 100k potential divider resistor ive been looking into this and have got a few conflicting ideas currently in my head...

Like most values, they aren't at all critical - 10uF would be fine - I usually choose components based on what I have!

The vast majority of components can vary massively, the trick is knowing which ones you can alter, and which you can't - which comes with experience mostly. There's really nothing at all critical in that circuit, that's a very nice byproduct of using opamps!.