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1970s Design Indulgence |
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Graham Slee
Admin Group Retired Joined: 11 Jan 2008 Location: South Yorkshire Status: Offline Points: 16298 |
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...not forgetting the direct advice from a well-known kit manufacturer designer, who said the design is inherently stable without compensation. So to prove him right, I took out the compensation and it blew the fuse... Everybody knows best |
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Graham Slee
Admin Group Retired Joined: 11 Jan 2008 Location: South Yorkshire Status: Offline Points: 16298 |
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One day I'm going to have to admit I'm deaf Between 10mA and 300mA quiescent current I can hear no difference! Oh, and the number of different transistors, circuit layouts, transformers - the number of times I've ripped it up and started over - must have cost £10 grand in bits. But power amps are so easy. Just look at the circuits online for just about everything that's been made. Some people will put up with anything! |
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Graham Slee
Admin Group Retired Joined: 11 Jan 2008 Location: South Yorkshire Status: Offline Points: 16298 |
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Latest distortion figures: 0.01% dominant third harmonic 1 watt to 4 watts. All other harmonics nearer 0.001%. Summed: 0.015%. 20kHz THD: 0.045%
There is no audible difference between 0.1% and 0.01%. I only bother to show what can be achieved. S/N 80dB ref -27dB input. Some would quote -107dB... Now, consider, this is a "ripple" power supply. It has no regulation. Stereo loop distortion can be tested by connecting left and right inputs, and listening for hum. What hum? 120kHz -3dB bandwidth, and no slew-rate limiting Miller compensation. But, you see, whatever you do, you will always see noise above 30kHz on the FFT (spectrum analyser). This FFT has 1MHz bandwidth, and there are nothing but spikes coming and going up there, and it doesn't get any better after switching the amplifier on! Got any 1MHz low impedance electrolytics? No? My, I'm so surprised. So, like I say, it's a case of effing about to get a good sound now, just like it is with MC. "doused in the well of the mythical glow, which gives them their legendary sound" huh! The thing is, when you've sufficient people around who can pick up an acoustic guitar. When you own a Strat and have a borrowed Gibson, you can hear what things actually sound like Edited by Graham Slee - 30 Jun 2020 at 10:11pm |
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Graham Slee
Admin Group Retired Joined: 11 Jan 2008 Location: South Yorkshire Status: Offline Points: 16298 |
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Given that ripple current is 10 times load current (rule of thumb design factor), what is the ripple current where the load is a few milliamps (the standing current)?
(answer: a few 10's of milliamps) When used to listen to music the amplifier is switched on. It is then used for a few hours. Later, it is switched off. Over a period of 10 - 20 years, how long was the amplifier on? (answer: say average use is 3.5 days a week for 3 hours a time, which is 10.5 hours a week. Over 10 years, that will be 5460 hours; over 20 years 10,920 hours) During which time, how loud was it run? (answer: in a domestic situation the average is around 80dBA, about 1 watt) What current does the above answer represent? (answer: 1 watt x 8 ohms = 8 volts. The rms value is the square root, or 2.82 volts. 2.82 volts divided by 8 ohms is 0.35 amps per stereo channel, or 0.7 amps in total) What ripple current does that represent? (answer: 7 amps) Now, let us use the only data sheet I can quickly find, which gives us all the information about capacitor lifetime (https://docs.rs-online.com/6f7a/0900766b80d99217.pdf) Let's say the reservoir capacitance is 5,000uF, rated 100V. We will need 5 x 1,000uF at 100V in parallel. Ripple current for each is 2.4 amps. 12 amps in total. Using the chart on page 6, we find IA (7) divided by IR (12) is 0.58. Call it 0.6. The capacitors are operated in an ambient temperature of 40C. The chart indicates a lifetime multiplier of around 65. The lifetime shown for the size chosen is 3500 hours. 65 x 3500 = 227,500 hours, or 26 years. But, over 20 years the amplifier only got used a total of 10,920 hours! So, how come after a re-cap, it sounded so much better? |
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Graham Slee
Admin Group Retired Joined: 11 Jan 2008 Location: South Yorkshire Status: Offline Points: 16298 |
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In 1995 something worrying occurred. Electromagnetic compatibility (EMC) legislation appeared to stop electronic circuits interfering with other electronic circuitry. Before that, interference was something the local radio Ham usually had to sort out. Radio breakthrough was a doddle to fix.
Faster and more prolific communications were at the gates, and the computer had become personal. Soon, everyone would have a PC! Makeshift text documents could be filed to a master computer over the telephone wires, using a modem (and often much frustration), and by 1995 modems were wired and not acoustically coupled. Windows 95 let you browse the "empty web," chat on message boards and get the email. 22 years ago, Windows 98 showed how busy the web had become, and within 2 years of that, the DOS running Windows 2000 wouldn't support most DOS programs anymore. In the intervening 20 years, the mains was filled up by switching power supply hash and the Ethernet to boot! The completely unshielded mains electricity supply has zillions of cellphone burps superimposed on it. They even "flew" internet connections on miles of twisted pair telegraph wires! "They had sex in Pennsylvania. A Brazilian grew a tree. A doctor in Manhattan saved a dying man for free." Meanwhile, the transistor manufacturers were offloading their burden to Shenzhen, seeing they were already manufacturing counterfeit (fake) devices. Welwyn scrapped its UK resistor plant and outsourced it to China, and I stopped buying their resistors because the sound was horrendous. They got rid of the Elna (ROD) Starget in 2006 because it contained alleged hazardous substances. Now the air and all conductors are full of radio frequencies we never dreamt about, getting more powerful and faster, faster but wah! But hey, did you ever hear anything about its effect on hi-fi? |
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peterb
Senior Member Joined: 02 Feb 2017 Location: Cornwall Status: Offline Points: 332 |
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Do I read it right, you are coming to the conclusion this is not going to be an 'always on', design?
Edited by peterb - 03 Jul 2020 at 11:40am |
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Peter
--------------------------------------- Dual 505-1, Cyrus CD T, DIY 80W MosFet amp and PreAmp, 2xKEF 103.2 |
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Graham Slee
Admin Group Retired Joined: 11 Jan 2008 Location: South Yorkshire Status: Offline Points: 16298 |
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Grid-tied inverters place 20kHz on the mains. What is a grid-tied inverter?
The main use of grid-tied inverters is for rooftop solar panel installations, which feed into the grid, giving the home owner discounted electricity. You or a neighbour might have them? The PWM sampling frequency most-often used is 20kHz, and acts as interference on electrical signals. Here it can be seen on the noise spectrum of a 1k ohm resistor. The spikes at 38 and 46 kHz remain a mystery, but I heard mention of these frequencies being harmonics of grid-tied inverters.
I am hoping to be able to run the AP from a remote computer to eliminate the possiblity of the 38 - 46 kHz being the laptop's battery power supply converters. From 100kHz and beyond, is the usual airborne HF crap. |
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