SW-VFA-01: Audio power amplifier video series

[sandrohv]

Hey guys,
Happy New 2020 to you all!

With this post I wanted to announce that I am starting Youtube channel on the design of audio power amplifiers. The content will be completely educational, and will cover the foundations of amplifier design and as well as the design of a best in class audio power amplifier. Searching Youtube, I noticed this has not been done before, so I wanted to give it a shot.

The plan is to keep updating this first post as an index of all the material as it gets published. The plan is to crank a video a week, so we'll see. I am pretty excited about this, so let's see how we fare.

GOALS FOR SW-VFA-01 AMPLIFIER (Specs):
- Power: 150 Watts
- THD: 0.001% worst case, 20KHz, max output power, 8ohm load
- Noise: 3.3nV/rtHz spectral noise, 3.2uV/rtHz
- SNR: 110dB SNR un-weighted at full power (redundant, but why not)
- Freq. response: 1.5Hz - 500KHz
- Input sensitivity: 1V_RMS
- Systematic DC Offset: <1mV

VIDEOS:
1.1. Audio Amplifier Design Fundamentals & SW-VFA-01: Introduction, Amplifier Specifications, Goals
SW-VFA-01 Video 1.1

1.2. Audio Amplifier Design Fundamentals: Measuring amplifier parameters with LTSpice
SW-VFA-01 Video 1.2

2.1. Audio Amplifier Design Fundamentals: Amplifier frequency response, Input Network & Power rails
SW-VFA-01 Video 2.1

2.2. Audio Amplifier Design Fundamentals: Input network design and test using LTSpice
SW-VFA-01 Video 2.2

3.1. Audio Amplifier Design Fundamentals: Fundamentals of negative feedback
SW-VFA-01 Video 3.1

3.2. Audio Amplifier Design Fundamentals: Measuring Loop gain and Open loop gain in LTSpice
SW-VFA-01 Video 3.2

4.1. Audio Amplifier Design Fundamentals: Two Stage voltage feedback amplifiers
SW-VFA-01 Video 4.1

4.2. Audio Amplifier Design Fundamentals: Two Stage voltage feedback amplifier model in LTSpice
SW-VFA-01 Video 4.2

4.3. Audio Amplifier Design Fundamentals: Measuring the loop gain of the major and minor loops of an amplifier in LTSpice
SW-VFA-01 Video 4.3

5.1. Audio Amplifier Design Fundamentals: Introduction to distortion in audio amplifiers
SW-VFA-01 Video 5.1

5.2.1 Audio Amplifier Design Fundamentals: Measuring harmonic distortion in LTSpice - Part 1
SW-VFA-01 Video 5.2 Part 1

5.2.2 Audio Amplifier Design Fundamentals: Measuring harmonic distortion in LTSpice - Part 2
SW-VFA-01 Video 5.2 Part 2

UPDATES:
1/07/2020: Just bought a microphone (Samson Meteor) - Thanks SGROSSKLASS! Starting video 2.1, audio quality will go up .
1/25/2020: Will be launching parallel video series: LTSpice: How to.
2/25/2020: Forgot to put the model file for the HoneyBadger amplifier. It can be found on post #63.
3/1/2020: Uploaded my complete version of the HoneyBadger amplifier (schematic, symbol, test bench and models). It can be found on post #72.

Any feedback, comments, encouraging support and constructive criticism is welcome.

Thanks guys!
- Sandro (SW Audio)

 

[sgrossklass]

This is quite interesting.

First thing I noticed - your audio (of all things) needs work! Muffled, way too much room reverb, and noise reduction enabled by the sound of it. (Sounds like a typical omni laptop or webcam mic. Not good.) I'd suggest scouring the Podcastage channel.

Do you have a decent line-in or at least mic-in available? Then I would suggest, at the very least, something like a Neewer NW-3 (or a Marantz Pro MPM1000 for more than the bare minimum), a Behringer Xenyx 802 (or 502 on a tight budget), plus some decent-quality XLR mic and RCA - 3.5mm cables. For onboard audio, recording at 96 kHz is recommended, making sure that everything is set up properly.
If you don't, ditch the mixer for a Behringer UMC202HD or something (that said, the mic preamps in a cheap mixer are likely to be better than those of a cheap audio interface, just having +/-15 V available helps).

I am not a huge fan of inexpensive USB mics, their converters just don't tend to sound too good, often a bit thin and harsh (not to mention the general lack of preamp gain control that could offset the limited ADC dynamic range). By contrast, for speech there really isn't very much wrong even with the $20 NW-3 XLR-only mic. (Even this is so far removed from what was common 20-25 years ago it's not even funny.) A Samson Meteor is quite decent if you put it on a stand though, I also came across this MAONO AU-PM401 which seems to sound perfectly acceptable as well.

Again, these are the budget options for new stuff. If your budget will stretch to an AT2035 or Rode NT1 on either a Focusrite Saffire 2i2 or a Mackie 402VLZ4 feeding an Asus Xonar of yesteryear (which is what I have), sure, go ahead. Plenty of options here, really.

 

[sandrohv]

Hi Sgrossklass, thanks for the feedback. I used the laptop microphone and after listening to the recording during the editing phase, I do agree that the audio could definitely be better.

Current constraints:
+ My laptop does have a mic-in, so 3.5mm is available.
+ Budget: ~$50-75, more would be pushing it.

Your suggestions:
+ Turn off the noise reduction effect (I did have it on)
+ Record on 96kHz
+ Mics to look at in order of goodness:
- Line-in: Neewer NW-3 (~$20), Marantz Pro MPM1000 (~$50), AT2035 (~$150), Rode NT1 (>$200)
- USB: Samson meteor (~$55), MAONO AU-PM401 (~$50)
+ Mixer: Behringer Xenyx 802

Questions:
+ The recording was done in AAC 44.1 KHz. I am actually stuck with this unless go to a more involved solution that would make editing more difficult. Would this be ok even though it is not 96KHz?
+ While I agree the audio is not great, do you think I should re-record the video, or live with it and upgrade for the next one?
- Very interested on this one, since I already recorded video 2. Therefore, would be interested if I need to re do it, or record fresh with a new mic starting video 3.
+ Finally, any comments on the content?

Thanks so much!
- Sandro

 

[sandrohv]

VIDEO 2 is up

Hi all!
Video 2 is up: 1.2. Audio Amplifier Design (SW-VFA-01): Measuring amplifier parameters with LTSpice

Audio is still mediocre since I will be getting my mic tomorrow, but I already had this video recorded, so might as well put it up. Next video will have better audio.

Attached are the files of this video.
BTW, the forum uploader does not like .asy files (LTSpice symbols). This is a problem. Anyone know how to fix this? (I am zipping the .asy file)

Take care,
Sandro (SW Audio)

 

[PRR]

...+ Record on 96kHz...

No!

Speech is fine at 40kHz sampling (or even lower).

I can't listen here. sgrossklass' remarks say much.

PLACEMENT!! Getting a good distance and direction is KEY to any recording task. (We used to have a funny cartoon in the studio....)

For Concert Hall Sound: where would you put your mother? Usually near 3rd row center, but listen around.

For Lecture: the TV News/Talk folks have this figured out. Tie-clip lavaliere. It's actually not a good sound, but good tieclip mikes correct for the lost treble.

A good alternative is a desk-mike 12"-24" from your mouth. But be careful you do not get off-mike when you turn to your guest, model, or sketch.

Noise reduction can be useful but can be awful obnoxious! Record in quiet! Most homes will be quiet enough for speech.

In-laptop mikes run from good to "hey it has a mike". Few aim for much better than video-chat quality, when leaning over the keyboard. And you can't aim a laptop. You want to place your mike separately from your controls and your camera.

 

[sandrohv]

I can't listen here. sgrossklass' remarks say much.

Hi PRR, thanks for the comments. I just my new mic today, per Sgrossklass suggestions, I got a Samson Meteor. Therefore, expect much improved audio starting video 2.1. One nice feature of the Meteor is that it has monitor out, so I can listen in real time how I sound and be aware immediately if audio is poor. Finally, video 1.2., while recorded with the laptop's mic, it does not have noise reduction, so audio should be a bit better.

Question for you:
- Do you think it worth re-recording the first video, 1.1? Not my preference, but if it has to be done, I'll suck it up and do it.
- Any comments on the content? Since I just got the series started, would be good to course correct as early as possible.

Thanks for the help,
Best, Sandro

 

[PRR]

> has monitor out, so I can listen in real time how I sound and be aware immediately if audio is poor.

Good; but you can't really judge your own sound while talking. (And if inexperienced, hearing your own voice back may upset your speaking.)

Record a page and then listen-back on your regular YouTube playing system. Then again on a complicated page where you turn your head or move around.

 

[sandrohv]

Hi PRR, so I just recorded today about half of video 2.1. The improvement with the new microphone is drastic. The monitor out function was not as useful as I thought it was going to be though. If it is at normal levels, I can't really tell between it and my own voice. If it is too high, it is just an annoyance.
What I am doing is record one page at a time, listen to it, edit it and then move on. Process seems to work well. I'll try to post video 2.1 tomorrow so you guys can give me feedback on it.

Best, Sandro

 

[drinkingcube]

Just watched the first video, and as a young person who has learned a lot from YT videos without perfect recording setups, the audio seems completely fine.
I see you've invested in a new mic, and that's fine, it'll make it just that much better.

In terms of content, I really like the pace and depth you're going at.

In the last year I've done my own exploratory journey regarding most of these concepts, but to go back and reinforce them and check them against what you're saying is very helpful.
Thanks a lot, and keep up the great work.

 

[sandrohv]

Just watched the first video, and as a young person who has learned a lot from YT videos without perfect recording setups, the audio seems completely fine.
I see you've invested in a new mic, and that's fine, it'll make it just that much better.

In terms of content, I really like the pace and depth you're going at.

In the last year I've done my own exploratory journey regarding most of these concepts, but to go back and reinforce them and check them against what you're saying is very helpful.
Thanks a lot, and keep up the great work.

Thanks Cube!

 

[sandrohv]

Video 2.1. Audio Amplifier Design: Audio amplifier frequency response and power rails

Hi all!
Video 2.1 is up: 2.1. Audio Amplifier Design (SW-VFA-01): Audio amplifier frequency response, Input Network & Power rails
SW-VFA-01 Video 2.1

Audio is now fixed. Using the Samson Meteor to do the audio recording did wonders.
In this video I discuss amplifier frequency response, the amplifier input network and the value of the power supply rails.

Take care,
Sandro (SW Audio)

 

[naf]

thank for update video, i like to watch and learn

 

[tommy1000]

Very good. Thank you.

 

[sandrohv]

Thanks guys, in the next video we are assigning values to the network and doing component selection.

 

[jony]

Thanks for the videos. And keep up the good work.

 

[sandrohv]

Video 2.2: Input network design and test using LTSpice.

Thanks Jony!

Hi all, Video 2.2 is up: Input network design and test using LTSpice.
The associated file for this video is here.

For the next video: I may actually start a parallel series on LTSpice How To's. I think by now I am very comfortable with the software, so I wold like to share the learnings.

Enjoy!
- Sandro

 

[naf]

thank you

 

[gannaji]

Thank you, dear sandrohv! Subscribed !

 

[sandrohv]

Thank you Gannaji!

 

[PB2]

Very nice work Sandro, I too thought that the audio in the first video was good enough but I
do like the improvement with the new mic.

Your LTspice intro was excellent, I've used it for many years and learned a few new tricks.

I'm watching 2.2 now and have a few minor comments. There are some tube preamps with
quite highish output impedance, the Dyna tube PAS line is an example. I suppose you could just state
that it is not designed for high output impedance amps given the low values for R1 and R2.

R2 (when R2 >> R1) sets the midband input impedance of the amp and it would either
be driven by the spec or by what's needed to support the diff pair circuitry. I'm leaning
toward JFET diff pairs so that an amp can have a very high Zin. Most SS amps without
JFETs are around 20K Zin as you probably know.

From memory I thought you said to use an NPO (ceramic) for C2 but then later you said
film (what I expected) when selecting a cap at Digikey.