I agree it was looking weird and connected power supply was very unusual, but I try to learn 😎
So I made new schematic and it looks much better now, isn't so?
So I made new schematic and it looks much better now, isn't so?
All looks good to me. Do not see a point for R4/R8 if you have a regulated PS. If it was my pre i would omit either the input or output capacitors and most likely both 🙂
I have to say I look also on Yamaha A-S500 service manual and they are using input and output caps 🙂 Sadly, in 97% I don't know why schematics are like that... It looks for me too sophisticated!
Ah yes, you are right! I changed it into 100.
Here is newest version:
I am just wondering what it the average amplification for main amplifiers. 10x? 20x? My targed is (I think) simple: just "high fidelity" pre-amplifier with silver plated PCB.
Fairly typical power amp gain values are 26-29 dB (so app 20-28 times amplification), however this can vary. You also need to remember that if you are driving a unity-gain power amp that the 15V rails on the opamp will be a constrain.
That's cause they are not too worried about the sound of coupling caps and i am 🙂
The opamps they use have also a higher input offset voltage of 0.3mV vs 0.1mV for the 49720.
It is certainly not the first integrated amp with a needless excess of coupling caps. As if they know those cheap caps will become leaky in a few years 😛
I believe many here use line stages with 0db gain and for a good reason. The vast majority of power amps need 1 - 2v at the input for full power. Practically any digital source, bar some PC soundcards provides at least as much. Being on the safe side a gain of 6-8db appears more than sufficient for a line stage and avoids the annoyance of getting excessive sound volume at the 9 o'clock position.
With C9 and C10 being 4.7 uF, be mindful of your load impedance. With a 100-ohm build out as shown in your current schematic that's terminated into 600 ohms (for example), the -3dB corner frequency is approximately 48 Hz. Scaling C9 and C10 to 47 uF moves the corner to 4.8 Hz. Of course, the higher the load impedance, the lower the low-end corner frequency.
I think your last iteration is getting close to being a reasonably good design. I would still limit bandwidth in the feedback loop, but that's something for experimentation later on if you like.
To nail this down right, it would be helpful to know something about the devices on both sides of your circuit.
Also, don't be dissuaded by those who suffer from an acute case of "capacitor paranoia." Certainly Yamaha does care about their audio circuits and generally, there's a good reason why their designers choose specific components and circuit topologies that go into their products. There's absolutely nothing wrong with judicious use of high-quality capacitors in the audio path if you know when to use them.
If you're new to op-amp circuit design, don't neglect three key pieces toward achieving stable circuits: (1) component layout and routing; (2) input/output/DC power conditioning; and (3) grounding technique. What looks good on paper can yield disastrous in the real world. The major semiconductor manufacturers all have good application notes on these topics. I've always considered the first couple chapters of Walt Jung's Op-Amp Cookbook as a must-read for any beginner.
Hello RFengineer,
Thank you for your attribution. I noticed that caps C9 and C10, but also C1 and C5 are in Yamaha amps usually 10uf/25V. It is quite frustrating not to know why. And I don't know anything about corner frequency. 🙁
Connected devices can be any any 🙂 It can be my phone, handheld voice recorder, mp3 player, as amplifier after low pass filter. The output can be my Yamaha A-S500 or it can be built into my retro amp Yamaha AX-10. I would be happy if other people will enjoy this preamp too.
Thank you for Walt Jung's Op-Amp Cookbook recommendation. I will check it.
I did PCB, it should be one side silver plated, audio grade caps, dale resistors.
You're asking the right questions. Since your circuit will be used as a utility amplifier with various devices connected before and after it, then you were absolutely right to include the input and output coupling caps. One of the dangers in seeking advice from others is in the assumptions other people are making when they give you an answer. That's why I asked the question.
Calculating the -3 dB "corner" frequency, coupling capacitor values, and bandwidth limiting RC networks isn't as difficult as it may seem. I'll work through an example and explain the result.
In a prior message, I indicated that if your circuit was terminated into a 600-ohm load (e.g., a 600 ohm transformer primary), the low-frequency amplitude would be limited to about 48 Hz before further rolling off at a rate of 6 dB/octave. A transformer load isn't purely resistive but for this purpose, let's assume it is.
The 4.7 uF output coupling capacitor together with the 100-ohm build-out resistor and 600-ohm load is what creates that low frequency limitation. The more a capacitor is loaded by a resistance, the more frequency response is affected with respect to other audio frequencies.
If the load wasn't resistive and was purely a capacitive reactance, then there's no corner roll off point but ALL frequencies then become attenuated by the same amount. In effect, that condition creates a capacitive voltage divider. Never forget it's the resistance that creates the corner frequency.
The output of an op-amp is not a zero impedance, but it's very close -- close enough that we assume it's zero. We have to include all resistances in the capacitive coupling loop. If the output impedance of the op-amp was 1K-ohm we would then have 1,000+600+100= 1700 ohms as the R value. If our 600 ohm transformer also had a series resistor, we would need to include that too.
Here's the general formula to determine the -3dB corner frequency:
F(-3 dB) = 1/(2Pi*R*C)
where, R is in ohms, and C is in FARADS, not microfarads, not picofarads, etc.
So, using a hypothetical load of 600 ohms and your design, we have a total load of 600+100 = 700 ohms. The series coupling cap is 4.7 uF or 4.7E-6 Farads. Calculating, we find F(-3 dB) = 48 Hz. The roll off starts before we reach 48 Hz, but that's the point where the response is down by 3 dB and as stated earlier continues to roll off at 6 dB/octave. Using the concept of "scaling," if we multiple the value of the capacitor by 10x, then the corner frequency also drops by a factor of 10x. As such, if we replace your 4.7uF cap with 47 uF in this example, the corner is now 4.8 Hz.
Since your amp *could* be used into a 600 ohm load because it's a "utility amp," I would change the output coupling caps to 47 uF or even 100 uF. Since you won't be running your circuit directly into a low-Z speaker, you will find that nearly all audio inputs of other devices will be at least 600 ohms (often 5K, 10K and higher) so that's a good assumption when choosing the output coupling cap.
I also mentioned bandwidth limiting your circuit in the feedback loop. Each feedback resistor is 10K-ohm. We can use the same formula to find the point where the high-end rolls off. In this case, let's shoot for a target bandwidth limit of 10X the maximum audio frequency of 20 kHz = 200 kHz. You know the R value, now compute C using the SAME formula and tell us the answer where the response is down -3 dB at 200 kHz.
Notice that in the first example, the capacitor is in series with the load. In the feedback example above, the cap is in parallel with the 10K feedback resistor. Generally, when there's a series load to the capacitor, we're looking for a low-end limit. When there's a parallel load, we're looking for an upper response limit.
You can now go back and analyze the Yamaha schematic. You'll have fun calculating the frequency response of each stage. The more you do, the more proficient you'll become.
You will encounter situations where you must conduct the same analysis but you'll see series/parallel combinations of resistance also connected to the capacitor, sometimes via other components like inductors. You'll get to a point where you'll figure that out. For now, stick a few basic audio circuits, then move on.
For example? A power amp with a 600ohm input impedance is rarer than the proverbial hen's teeth. Why even mention such an eventuality?
Because he's constructing a universal utility amplifier for use with many devices and such devices may have substantially different source and terminating impedances. To not take that into account is design neglect.
Low-Z inputs on pro equipment are more common than you realize. We're not talking about a major re-design to ensure there's an adequate low-end response when feeding his circuit into a Lo-Z input. It's simply a matter of scaling the caps. So, are you saying he shouldn't pay an extra 2-cents to go from 4.7 uf to 100 uF caps -- and then know there's not a frequency response limitation?
Excluding the output coupling caps is not a viable option due to the unknown input topology of the following stage. There may be a day when he connects the circuit to an unknown input impedance that's clamped with a high DC offset -- then wished he had the coupling caps if he decided to exclude them due to "minimalist" circuit advice he received.
Here's an example, I am sitting in front of a Urei 1176LN compressor/limiter that uses a variable 600 ohm T pad right at the input. The nominal input level is +4 dBu which in this case is also +4 dBm. It uses a 600-ohm input like hundreds of legacy dynamics processors, EQs, and amplifiers. If he chooses to use his circuit as a preamp to attain line level from a consumer-grade device with -10 dBv IHF levels, then the output caps should be sized to ensure that they aren't the limiting factor in attaining good low frequency response. An argument against properly sizing the caps shouldn't even be up for discussion.
Hello RFengineer,
First of all, thank you VERY MUCH for this explanation. Your time I really appreciate! If you will need set up WordPress with eshop, I will be happy to help you (free of charge), I am good with that 😉
Sometime it is little bit difficult to decide what the best solution is. I see lots of experienced people here and everybody has an opinion. I am always preferring the opinion, which has explained reason. Then I see service manuals of my two devices I own, Yamaha AX-10 from 1995 and Yamaha A-S500 from 2012 with service manuals. I did re-cap in both devices and I want always to improve them.
The service manuals has some parts with some values, eg mentioned input/output capacitors, so this is why I want to understand what’s going on in designs. If somebody don’t like Yamaha, I am ok with that, I don’t like Sony, lol.
I have no problem to use good caps, actually I have only audio caps Elna Silmic II and Nichicon MUSE, FG, UKA, UKW as I am only into audio stuff.
I decided to make good pre-amp and I am glad it is getting nice shape and I know without help I would be lost. So yes, I am thankful to everybody, even to people who don’t agree with something.
I hope I will find some time very soon and I will post new updated scheme.
Actually I have just two (I guess simple) questions:
1) Is power transformer 15V with 200mA for LME49720 enough? When I was checking the actual current in AX-10, there was used just 110 mA transformer.
2) I have for amplification R3 / R2 + 1 values 10000/680. Why higher resistance is usually used? What will happen if the resistors will be just 100/6.8?
Of course, my question is for anybody who would like to contribute to my “superior hi-res ultra fidelity pre-amplifier”.
> Is power transformer 15V with 200mA for LME49720 enough?
Did you read the datasheet?
Look at page 5, IOUT and IS. 12mA to live. 20+mA of load current, if needed, but in audio less than half the time. So 20mA max per chip. 40mA for stereo. Double the DC current to shop for an AC Amps rating. 80mA. You may find that you can't buy a respectable transformer that small.
Did you read the datasheet?
Look at page 5, IOUT and IS. 12mA to live. 20+mA of load current, if needed, but in audio less than half the time. So 20mA max per chip. 40mA for stereo. Double the DC current to shop for an AC Amps rating. 80mA. You may find that you can't buy a respectable transformer that small.
I did PCB, it should be one side silver plated, audio grade caps, dale resistors.
Don't spend too much on parts until you fix the PCB layout :roll eyes:
Milan, I am honestly fine with helping people and answering their questions as are the others I am sure, but that you keep asking questions without appearing to make any effort to read the datasheets or finding information yourself is a little bit annoying, sorry. A basic opamp-circuit like this has been analysed app. a million times or more and all of this information is available online - you just have to look for it!
When you've finished reading the data sheet part on power consumption and power supplies as PRR suggests, I respectfully suggest you move on to the section about PCB layout, especially the supply decoupling and the length of the feedback loop 😀
You should use a series output resistor to isolate capacitive loads . Try 100 ohms in series and a 1meg shunt to ground on the output
I do read data sheets and I do googling! 😉 The question is if I understand what is written in datasheets.
I asked about power current - and it look like I didn't read datasheet. But there is too many currents: bias current, output current, offset current, circuit current, quiescent current 😱 So I have to ask...
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