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1970s Design Indulgence 
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Graham Slee
Admin Group Pushing the boundaries Joined: 11 Jan 2008 Location: South Yorkshire Status: Offline Points: 7991 
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Correct! Without such precautions you'd end up with a blown amplifier or even worse, a fire!
Edited by Graham Slee  15 Oct 2018 at 1:13pm 

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Graham Slee
Admin Group Pushing the boundaries Joined: 11 Jan 2008 Location: South Yorkshire Status: Offline Points: 7991 
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Here is the (hopefully) final "1970s power amp" and its measured spec. Measured Spec.
Mains voltage: 230VAC Dummy load: 2 x 8 Ohms exact, resistive. Test analyser: Audio Precision APX525 Input sensitivity: 763mV = 0dB this test Results Left/Right (rounded up or down: actual results were slightly better than shown) Output stage quiescent current (Iq): 16.2 mA/17.4 mA S/N: 74dB/74dB ref 28.375dB input, 20Hz  20kHz unweighted S/N: 74dB/74dB ref 0dB input/28.375 dB gain, 20Hz  20kHz unweighted Frequency response: 12Hz  50kHz 0.5dB ref: 50W and 1W Intermodulation (IMD, SMPTE): 0.2%/0.2% ref: 50W; 0.065%/0.07% ref: 1W THD+N (300kHz lowpass filter engaged): 1W (17dB input): 0.035%/0.035% at 1kHz; 0.155%/0.150% at 10kHz; 0.27%/0.26% at 20kHz 50W (0dB input): 0.055%/0.055% at 1kHz; 0.16%/0.16% at 10kHz; 0.57%/0.55% at 20kHz Crosstalk: 62.5dB 

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Graham Slee
Admin Group Pushing the boundaries Joined: 11 Jan 2008 Location: South Yorkshire Status: Offline Points: 7991 
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Forgot to add that it was both channels driven, and each test allowed the APX525 to nullout fully, and so it wasn't a quick snapshot, more like several minutes, and so it should have said both channels driven continuously.


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Richardl60
Senior Member Joined: 04 Nov 2014 Location: Yorkshire Status: Offline Points: 1090 
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Does output vary much with load Graham?


Graham Slee
Admin Group Pushing the boundaries Joined: 11 Jan 2008 Location: South Yorkshire Status: Offline Points: 7991 
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I haven't got a clue because every time I ran it into 4 ohms the output fuse blew at just above 70 watts. I only had 2A fuses and 4 ohms indicates I should use a 5A fuse, but as we found from the fuse chart on page 5 fuses don't always work to plan, so I'll give it a try into 4 ohms shortly using a 4 amp fuse for starters.
Anyway, we should be able to work out what it should do... Maximum "undistorted" output will vary to a degree with load. Early on in this discussion when I was talking about the power supply I commented about how much current was actually required, and if rms watts is rms current times rms volts then how could say a 300VA transformer be used for a 100WPC amplifier? It might seem like 300VA is 300 AC watts of supply and that should easily translate into 2 x 100 watts, after all there'd be 100 watts to spare? But here I am using a 400VA transformer and 50WPC is pulling the voltage down to exactly what it should be, within 0.7 of a volt. This is because 400VA divides down by the 48V secondary voltage to 8.33A, but then the rectifier relationship gives DC current as 0.62 x AC, and so 8.33 x 0.62 = 5.16A. At 50W into 8 ohms the rms voltage is 20V. And 50/20 = 2.5A. There are two channels so that's 5A, so except for the 0.16A, all the current has been used! Looking at voltage we have 65V on load and I had clipping at around 58V pp. I have circuit losses of about 5V, so 58 + 5 = 63V, so where is the other 2V? What about ripple voltage? Reservoir capacitors smooth the supply but current is being drawn all the time, so the bucket is being emptied at the same time as being filled. Making the reservoir a total of 28,200uF (6 rather large 4700uF capacitors) reduces ripple to around 2V, so that's where the other 2V is. So now we should be able to calculate, even though approximately, what a 4 ohm load will do. First of all we can double the ripple to 4 volts, because twice the current will be required. Then we find that the transformer will be overloaded. Yes, a 400VA transformer will be pulled down by us trying to do 100WPC. Food for thought? Let's say the overload will equal its regulation of 7.5%, so it will be doing just over 44V instead of 48V. So in DC we have 62.2 volts less the bridge rectifier's 2.2V, equals 60V. And 4 volts of ripple loss making 56V HT. I have 5V of circuit losses so that's now 51V. Divide that by 2 and take the square root = 18V rms. V squared over 4 gives 81W as the answer. So how do they make a 50WPC into 8 amp do 90WPC into 4 using a 300VA transformer? You've heard the saying "it's all done by mirrors"? And here you can take that literally because such designs use current mirrors (and other techniques) which up the voltage gain, so whereas this design has only 80dB openloop gain, theirs might be 100dB or more (and a lot more transistors and complexity to get there). There is a lot more negative feedback, and so the onset of clipping is "postponed" by around 0.5dB to 1dB, which is sufficient. The dead giveaway would be the 10kHz and 20kHz THD at 90WPC into 4, but I can't seem to find it... And then you have 2 x 110WPC into 4 using a 250VA transformer... it's a miracle! 

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Fatmangolf
Admin Group Joined: 23 Dec 2009 Location: Middlesbrough Status: Offline Points: 4931 
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I wonder if mirage may be a better word! This just shows the simple truth of a good sounding amplifier design like yours over something much more complicated built to create impressive stats. 

Jon
Open mind and ears, whilst owning GSP Genera, Accession, Elevator EXP, Solo ULDE, Proprius amps, Cusat50 cables, Lautus digital cable, Spatia cables and links, and a Majestic DAC. 

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