From the left...
You experimented with the valuesBy keeping the ratio of Rf and Rin the same then the voltage gain stays the same. Rin determines the actual input impedance.
I think I'm getting the idea of input/output impedance. It seems to be a real game trying to get as much impedance on the input as possible to make the output from the prior section efficient. But then the last one, that drives a tiny 8ohm load, has to stand that idea upside down. So it is a bunch of tweaking to get the efficiency where you want it?
I'll have to noodle this a bit to understand it. I'm trying to grasp the current gain concept. For some reason the voltage gain seems easier to understand for me right now.
I was actually very close on this but I didn't post it because I wasn't sure. I did notice the glitch was when the transistors "lost power"... I didn't put together that the opamp itself would then power the output directly. That's amazing. When the transistors are providing current gain, wouldn't *some* flow backwards to the base via the resistor?
I'll run these tests tonight. I'd like to also build this out on my breadboard, any reason that wouldnt work? Someone said this has 1.5% distortion. Is that "really bad"? I know that is a very tough question. I'm just trying to get some idea of what it *should* sound like when I'm done.
Mooly thanks SOOO much for working with me. You've taught me a lot!
Input impedance isn't so much of an issue as long as you understand the basics which is that the lower it is, the more of a load it places on the source component feeding the amplifier. So if you take a CD player or tuner or any source component like that and look at the specifications it will probably say something like "maximum output 2 vrms 600 ohm". An ancient cassette deck might be 700mv at 10K ohms.
Those figures mean it can supply that voltage across that load impedance. Make the impedance lower and the output voltage will fall away and possible distort too. So the input impedance has to be at least equal to and preferably higher than the minimum value the component can drive.
Another way of thinking of it... a large 12 volt battery can supply plenty of current into a load such as a bulb or motor. If you add a 10K resistor in series with the positive battery terminal there is still 12 volts on the end of that resistor but it won't now light a bulb or run a motor. The "output" impedance of the battery is now 10K and limits the current. Output impedance of a component is similar. So a 10K load will cut the 12 volts down to half and so on.
So we make the input impedance of most amplifiers high relative to the source. In practice that means 10K and above.
Well the opamp has current gain too, just not very much. About 10 to 20 milliamps is the most that common opamps can supply. So if the load on the opamp draws more than that, then the opamp just limits and the output voltage is reduced and distorted.
That glitch is called "crossover distortion" and is the number one problem with all class B and class AB amplifiers. The resistor across the base and emitter is a quick fix but it's not the best way. Current doesn't flow back because the opamp is always maintaining the bias in the "right" direction.
When a transistor is used as an emitter follower the emitter voltage follows the base voltage less around 0.7 volts. The transistor has a "high" input impedance on it's base and low output impedance on its emitter. The current gain is determined by the property called "hfe" and for a small transistor is around 100 or more. That means with 1 milliamp flowing into the base and emitter junction, a curent of 100 milliamps will flow from collector to emitter. The emitter current would actually be 101 milliamps as Ie = Ic + Ib
Absolutely... get it built, get it working, then it can refined and elaborated on. Is it bad ? Try it and see.
No problem... and it give me a chance to play with LTspice which is a bit of a steep learning curve.
I always have to think when I see the old Leak troughline... and I just had to Google that and found I'd spelt it wrong because I always think it must be throughline. It's trough as in pig trough though. Quiet and quite is the other one that crops up all the time.
So I need to put together an order for parts to build these two circuits. I have the opamp (a 741) and BJT (2n3904/2n3906) but not the emitter follower transistors. I'm looking for suggestions for a few other things to buy that you folks think I might quickly find I want (upgrades) as I'll have to pay shipping and need to get my order up over 10$ anyway. Any tips would be great. I'm thinking:
10 - 2n5401
10 - 2n5550
10 - 10uF caps (ceramic?)
10 - 1000uF caps (ceramic?)
I was thinking of getting some of the TIP31 and TIP32C units also as they are used in RJM1's post.
I have a few LM741s - is there a "upgraded one" I might want to order also?
I also have a large assortment of 5% 1/4 and 1/8 watt resistors. The reason for the capacitors on the order is all of mine are the electrolytic and they are strange values compared to what these circuits are using.
Thanks all!
10 - 2n5401
10 - 2n5550
10 - 10uF caps (ceramic?)
10 - 1000uF caps (ceramic?)
I was thinking of getting some of the TIP31 and TIP32C units also as they are used in RJM1's post.
I have a few LM741s - is there a "upgraded one" I might want to order also?
I also have a large assortment of 5% 1/4 and 1/8 watt resistors. The reason for the capacitors on the order is all of mine are the electrolytic and they are strange values compared to what these circuits are using.
Thanks all!
Electrolytic caps in 10, 47, 100 uf are always handy. Depending on what voltages you are working perhaps aim to get 63 volt working. Small film type caps in smaller sizes are useful. Say 0.1uf and 0.22 uf. Anything over these sizes are going to be electroylitic for the circuits you are looking at.
Opamps, it's worth getting a couple of TL071 FET ones.
Transistors as you mention plus say BD131 and BD132 and TIP41 and TIP42. These are medium and high power devices... a bit old but easy to work with and easily available.
If and when you build a specific design, then that is the time to think of getting specific parts, otherwise common generic parts will allow most circuits to be built and experimented with.
Opamps, it's worth getting a couple of TL071 FET ones.
Transistors as you mention plus say BD131 and BD132 and TIP41 and TIP42. These are medium and high power devices... a bit old but easy to work with and easily available.
If and when you build a specific design, then that is the time to think of getting specific parts, otherwise common generic parts will allow most circuits to be built and experimented with.
Thanks again. My order should arrive today (tomorrow at latest) so I'm excited to build. I was thinking through this in my head today and realized I dont have a volume control.... As I'm testing I may not want this blasting at full volume all the time. Any tips to satisfy this? Or should I control the volume via the input device (ie. blackberry) for now and wait to get into a volume control later?
You can use the input device for volume control or use fixed resistors at the input to make a divider. There are reasons why a volume control is normally better though. The output of most devices with software controlled output deteriorates at low level while the noise stays the same. So if you amplify that you get a poorer output than setting the device to full output and then attenuating it with volume control which gives no real loss of quality.
Molly . Item 31 did you ever get it to sound nice ? I have used it for motor drives . There is a classic design like that which is used to teach current dumping ( Quad 405 ) . Could you simulate it becasue I don't have one using 100 R feed forward resistor ( not 1 K ) and a 3K pull up resistor to - V supply from the op amp output ( pull down if - ? ) . I've got a hunch it will work and it it does I will tell you a story dating back to 1977 . If you have time that is . Looking at 0.01% distrotion I think if it does what I suspect .
Hi Andrew . I was told about my friends degree at Oxford which was this very project . He is now PHD and worked at Cern . He never showed me the circuit , it was verbal . I suspect this was the starting point . He used 741 . He lost interest in all of this and is an Opera singer . He gets me out of trouble in electronics .
He took his PHD in electrical engineering as electronics bores him ( I suspect ) . He designed some of the power transformers that are used in high voltage generating ( certainly in the team that did that ) . Looks promising to retread his steps . He never lifts a soldering iron . That's his problem . Sorry Hubert your secret is out .
He took his PHD in electrical engineering as electronics bores him ( I suspect ) . He designed some of the power transformers that are used in high voltage generating ( certainly in the team that did that ) . Looks promising to retread his steps . He never lifts a soldering iron . That's his problem . Sorry Hubert your secret is out .
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Hi Nigel,
In my early days I played a lot with circuits like this. The circuit is post #31 is very limited in its output capabilities (9 volt supply and all) but it does "what it says on the tin" and is a great starting point to get a working circuit that allows experimentation. That's really important when starting out, to get something up and working.
I've seen this type of circuit referred to as "current dumping" but the Quad technique uses a bridge to derive an error signal. Its a lot more sophisticated than just using a resistor to "fill in" the missing low level part of the output.
Elektor did a circuit in the 1980's ??? using a 741 with its output essentially grounded and darlington transistors in the rails to the 741 so that the current drawn by the opamp was used to control the outputs. I think 100watts output was claimed.
You would have to draw what you mean regarding the circuit... can't quite decipher it all
And whose Molly ?
The other details as yours
Hi Molly . I was thinking Sandman and the one you mention . Even sticking to your 9V can we have a go at doing it and use virtually what you gave out . I am trying to prove a point in another forum and used your example . I did say where and apologized I will have you know . I was told feed forward was discredited as the Quad was not the best example of it . Seems to me people didn't like the patent either is my guess . Let's pretend we are just starting out and see if we can tweak it . We don't leave the member who asked the question behind I would add .
BTW the other forum said your 1K was wrong !!! They didn't understand . One guy who is a top amp designer asked if it had to be split supply ? Not mocking him as it is a correct question .
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I think this is what you mean.
Adding a resistor from the output to either rail allows one of the transistors to sink the current flowing in that resistor and allowing Class A operation up to a value determined by the current. The distortion is still present when the output increases over that level.
Aubrey Sandmans "Class S" circuits were debated over many many years in Wireless World yet I believe he never offered a fully worked example. All the circuits he gave, although showing the principle, were designed using higher than normal impedances. Some of the Technics Class AA offerings were based on the idea I seem to remember.
Split supplies... for a beginner I think single supplies and AC coupled is best. It makes no difference to the basic operation of the circuit though.
Do we get to see a link to this other forum
Adding a resistor from the output to either rail allows one of the transistors to sink the current flowing in that resistor and allowing Class A operation up to a value determined by the current. The distortion is still present when the output increases over that level.
Aubrey Sandmans "Class S" circuits were debated over many many years in Wireless World yet I believe he never offered a fully worked example. All the circuits he gave, although showing the principle, were designed using higher than normal impedances. Some of the Technics Class AA offerings were based on the idea I seem to remember.
Split supplies... for a beginner I think single supplies and AC coupled is best. It makes no difference to the basic operation of the circuit though.
Do we get to see a link to this other forum
Thanks Mooly . Soldering is a problem for me now . I would have just soldered it up in the past . In have tried a capacitor to replace the resistor . I must learn this simulation stuff . I have never asked my friend to see that circuit . I imagine he remembers it as it got him his BSc . From what he said it was basically that . I have knock it together many times without success . I have found a properly biased class A or B works well with an op amp . Often I don't use loop feedback . I fancy in chips they have mastered this idea . It is usually said to be a distortion cancelling mechanism with no details ( OPA 604 if I remember . ) . I will do valves in future as I should be able to solder them . DIN plugs are out .
http://audio-database.com/SANSUI/amp/au-alpha607nraii-e.html
http://audio-database.com/SANSUI/amp/au-alpha607nraii-e.html
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Ok, and to be clear, when you say "volume control is normally better", you mean the fixed resistors and the input creating a divider that you mention?
I was thinking at first a simple pot on the input would work, but I'd actually be changing the value of Rin and thus the input impedance of my circuit if I did that correct? Even the divider would have to be considered in my impedance?
Mooly's amp
Many will not realize that Mooly's amp was the most controversial amp of the 1980's and a little before that .1928 was when the idea was spoken of first . Here is something to read . Basic amps sometimes create a life in Audio for people who build them . It did for me .
Current Dumping Article
Good choice . Wish it existed when I started .
Many will not realize that Mooly's amp was the most controversial amp of the 1980's and a little before that .1928 was when the idea was spoken of first . Here is something to read . Basic amps sometimes create a life in Audio for people who build them . It did for me .
Current Dumping Article
Good choice . Wish it existed when I started .
Rin sets the input impedance of the amplifier circuit. Anything we add "to the left" of that such as a volume control or divider is effectively in parallel with Rin.
In practice it's not a worry at all and you can see that the "total value of Rin" would be different for each and every setting of a volume control. It's not a problem because any source component (MP3 player, CD etc etc) can easily drive the impedances we are seeing.
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