Context matters. What I meant, and you know it, is that in general, using an op-amp of any kind to drive a 10 Ohm load is dubious at best and generally a non-starter in terms of good design practice unless you are forced to because of other constraints. That is the dog, not the part number. Manufacturers do this in most pedestrian media players and phones because they are cheap and space and power are at a premium, and just hope I won't notice when I'm driving even my tame 32 Ohm Philips that I have to crank the volume all the way up to get any sort of reasonable volume out of them on quieter recordings. This is a well noted and chronic problem. This is why for quality DAPs stronger headphone drivers are must, and probably why the OP wanted to build a headphone amp in the first place- because the sources as they are (with an op-amp output) are inferior at driving the low impedance IEMs that he indicated. Yes, this option is being chosen here primarily because it is a first project, and keeping it simple. Everything is a trade-off. I fully support that, which is why I suggested your route. I don't know if 10 Ohm IEMs are a serious goal, but if they are, or a higher level of fidelity is desired, I'm sure once he is a little more confident he will probably circle back around, and then learn that a couple of transistors are nothing to be intimidated by, especially for the huge leap in performance.
You understand how good this circuit is, having built one. I love your quote here, because it is my experience with this circuit as well (speakers too)-
"I've thrown everything I have at it (Senn HD600, Beyer DT880, AKG K601, Shure SRH940) and it drives them all with aplomb, as effortlessly and transparently as my Mytek Stereo DAC 96. In fact right now I'd have to give the edge to The Cabito, it sounds slightly "cleaner" to me, but it's my baby, I just made it and I've seen the impressive numbers, so expectation bias is almost guaranteed and I'd bet good money that I wouldn't be able to tell them apart in a blind test."
Did you ever get back to measure real distortion figures? I bet they would be stellar- in your simulations I'm seeing a awful LOT of zeros!
You understand how good this circuit is, having built one. I love your quote here, because it is my experience with this circuit as well (speakers too)-
"I've thrown everything I have at it (Senn HD600, Beyer DT880, AKG K601, Shure SRH940) and it drives them all with aplomb, as effortlessly and transparently as my Mytek Stereo DAC 96. In fact right now I'd have to give the edge to The Cabito, it sounds slightly "cleaner" to me, but it's my baby, I just made it and I've seen the impressive numbers, so expectation bias is almost guaranteed and I'd bet good money that I wouldn't be able to tell them apart in a blind test."
Did you ever get back to measure real distortion figures? I bet they would be stellar- in your simulations I'm seeing a awful LOT of zeros!
Sorry gentlemen, was busy yesterday. It seems that both of you are agreeable that transistor amps excel op-amp. And I'm willing to try, as mentioned in one of the earliest posts, the intention was to start off easy using op-amp. My concern was the lack of experience and not wanting to trouble you too much. However, I see true brotherhood here that wants the best for me as well. So let me do some reading on transistors and get back in a few days.
With that being said, what kind of transistor do you recommend? JFET? BJT?
@wparks you mentioned that you have built the Rod Elliott Project 113, do you still have the design files (gerber, pcb BOMS)?
With that being said, what kind of transistor do you recommend? JFET? BJT?
@wparks you mentioned that you have built the Rod Elliott Project 113, do you still have the design files (gerber, pcb BOMS)?
Do you have a plastic breadboard like this one, you can knock together quick projects for testing? These are invaluable- you can knock together several different circuits, and optimize them before you ever have to solder anything. Op-amps plug right into them.
I do still think there is some value in you first prototyping op-amp only circuits before you move to the op-amp with the transistor output. Come up back up to speed, have the confidence of something that is stable and working, and you can proceed from there.
The amplifier in ESP Project #113 uses the complimentary transistor pair BD139 (NPN) and BD140 (PNP). These are excellent, new production, currently commonly available excellent transistors that are bread and butter for learning on. Both of us are intimately familiar with the design, so please ask lots of questions and we can give you a LOT of detail here. Even though the circuit might look a little intimidating, it is very simple. The transistors form a complimentary, non-inverting, unity gain follower or buffer amplifier if you will, no voltage gain, but a LOT of current gain from the bases to the emitters. You can read up on a common emitter follower circuit, and it's a lot like two of those mirrored above and below. This is "class AB" operation, because as the signal transitions up and down one or the other transistor will be mostly on, the other mostly off, trading back and forth. This is how all class AB power amplifiers work, so it's good learning applicable to a wide range of audio circuits you may study in the future.
R5, D1, D2, and R6 conduct a small amount of current from positive supply to negative supply, and set up the DC voltages on the bases of Q1 and Q2 to turn them both on just a little bit. Consequently, a small DC current (called a bias current, or a quiescent current) flows down through Q1, through R7, R8, and Q2 that is there all the time just to keep the transistors on and ready to move with the signal. Your input signal is amplified by the op-amp, which drives the bases of Q1 and Q2 through the two capacitors C3 and C4. The transistor follower reproduces this same signal at the output, but with much stronger drive strength (much lower output resistance). The output signal is tapped and brought back through R4 to the negative input of the op-amp, so the op-amp can correct any possible error seen between the amplified input and the output. This is the same negative feedback that all of the op-amp circuits use, but here we call it "global feedback", because the output buffer is inside this feedback loop. This is very powerful, because it functions like the output buffer is an inherent part of the op-amp, it's behavior can be governed as precisely by the op-amp, as the op-amp itself, so the output ends up having phenomenally low distortion, and extremely large bandwidth, despite having much stronger output power.
One trouble with this amplifier is that it is sensitive. Because it amplifies high frequencies so well, and is so fast, and strong, it is probably not the best circuit to lay out on a plastic breadboard unless you are really careful to avoid output to input parasitic coupling, and try to keep the gain really low at first (with a small value of R4 as discussed.
I don't use printed PCB's. I hand wired mine on a chunk of perf board, not quite as fancy as what cabirio did, with the built-in regulated power supply, so be sure to check out the build he linked you to, it's excellent. I built mine into a little junk network router box, with the transistors heat-sinked flat to the chassis, because I wanted to use this as a test amp to drive speakers, which it does quite well. I use a larger +/-15V @ 1A central power supply to power all of my various op-amp based audio accessories that I built into an old ammo box that connects via the red/blue RCA connectors with braided cables. I can provide you a scan of my perfboard layout- it's quite dense, and I am sure cabirio can as well.
I do still think there is some value in you first prototyping op-amp only circuits before you move to the op-amp with the transistor output. Come up back up to speed, have the confidence of something that is stable and working, and you can proceed from there.
The amplifier in ESP Project #113 uses the complimentary transistor pair BD139 (NPN) and BD140 (PNP). These are excellent, new production, currently commonly available excellent transistors that are bread and butter for learning on. Both of us are intimately familiar with the design, so please ask lots of questions and we can give you a LOT of detail here. Even though the circuit might look a little intimidating, it is very simple. The transistors form a complimentary, non-inverting, unity gain follower or buffer amplifier if you will, no voltage gain, but a LOT of current gain from the bases to the emitters. You can read up on a common emitter follower circuit, and it's a lot like two of those mirrored above and below. This is "class AB" operation, because as the signal transitions up and down one or the other transistor will be mostly on, the other mostly off, trading back and forth. This is how all class AB power amplifiers work, so it's good learning applicable to a wide range of audio circuits you may study in the future.
R5, D1, D2, and R6 conduct a small amount of current from positive supply to negative supply, and set up the DC voltages on the bases of Q1 and Q2 to turn them both on just a little bit. Consequently, a small DC current (called a bias current, or a quiescent current) flows down through Q1, through R7, R8, and Q2 that is there all the time just to keep the transistors on and ready to move with the signal. Your input signal is amplified by the op-amp, which drives the bases of Q1 and Q2 through the two capacitors C3 and C4. The transistor follower reproduces this same signal at the output, but with much stronger drive strength (much lower output resistance). The output signal is tapped and brought back through R4 to the negative input of the op-amp, so the op-amp can correct any possible error seen between the amplified input and the output. This is the same negative feedback that all of the op-amp circuits use, but here we call it "global feedback", because the output buffer is inside this feedback loop. This is very powerful, because it functions like the output buffer is an inherent part of the op-amp, it's behavior can be governed as precisely by the op-amp, as the op-amp itself, so the output ends up having phenomenally low distortion, and extremely large bandwidth, despite having much stronger output power.
One trouble with this amplifier is that it is sensitive. Because it amplifies high frequencies so well, and is so fast, and strong, it is probably not the best circuit to lay out on a plastic breadboard unless you are really careful to avoid output to input parasitic coupling, and try to keep the gain really low at first (with a small value of R4 as discussed.
I don't use printed PCB's. I hand wired mine on a chunk of perf board, not quite as fancy as what cabirio did, with the built-in regulated power supply, so be sure to check out the build he linked you to, it's excellent. I built mine into a little junk network router box, with the transistors heat-sinked flat to the chassis, because I wanted to use this as a test amp to drive speakers, which it does quite well. I use a larger +/-15V @ 1A central power supply to power all of my various op-amp based audio accessories that I built into an old ammo box that connects via the red/blue RCA connectors with braided cables. I can provide you a scan of my perfboard layout- it's quite dense, and I am sure cabirio can as well.
I screwed up on my post and cannot correct it- The emitter follower circuit I wanted you to research is a "common collector", not common emitter. My bad.
you can see it on Wikipedia (although their explanation is headache inducing =painfully= technical) and many other places. Basically, the collector is connected directly to the supply rail, and the emitter signal exactly follows the base signal (unity gain), with much higher current output.
Here is the datasheet on the BD139/BD140 transistors. These designs are optimized for these, while many other BJT's could be used if desired. These are great ~medium power, very durable, inexpensive, and high performing transistors that are useful in a LOT of applications so a good starting point to learn about.
Rod makes a little money on the side selling PCB's for his projects, and one is available for project 113 at this page if you are not completely comfortable custom building on perfboard/protoboard. This amplifier has been built hundreds if not thousands of times to almost universal success. As a first pass, I would recommend sticking with Rod's basic design, then as you learn more venture out into the many many modifications possible, such as the one that cabirio did. Rod's basic design is very well understood and a great performer as is.
you can see it on Wikipedia (although their explanation is headache inducing =painfully= technical) and many other places. Basically, the collector is connected directly to the supply rail, and the emitter signal exactly follows the base signal (unity gain), with much higher current output.
Here is the datasheet on the BD139/BD140 transistors. These designs are optimized for these, while many other BJT's could be used if desired. These are great ~medium power, very durable, inexpensive, and high performing transistors that are useful in a LOT of applications so a good starting point to learn about.
Rod makes a little money on the side selling PCB's for his projects, and one is available for project 113 at this page if you are not completely comfortable custom building on perfboard/protoboard. This amplifier has been built hundreds if not thousands of times to almost universal success. As a first pass, I would recommend sticking with Rod's basic design, then as you learn more venture out into the many many modifications possible, such as the one that cabirio did. Rod's basic design is very well understood and a great performer as is.
Done.
(The Moderation Team account isn't routinely monitored. Best to report a post that needs action and then we (mod team) all get to see it 🙂)