Fun fact! If the open loop gain of the ACA v1.8 were infinity (like a textbook ideal opamp) then the closed loop voltage gain of the amplifier + feedback network would equal (-1)*(R12/R11) = -4.0. In this phony baloney scenario, the input sensitivity would be sqrt(4W * 8R) / (4) = 1.414V.
HOWEVER, the open loop gain of the ACA is certainly not infinity, and that means the closed loop voltage gain of the amplifier + feedback network is certainly not -4.0 .
From the data in #9801 above, we can calculate that the closed loop voltage gain is about -3.3 . {why? because an input of 1.72V, times a gain of 3.3x, gives an output of 5.676 volts. And 5.676 volts delivers 4.0 watts into an 8 ohm load}
AHA so now we have enough information to calculate the open loop gain "OLG" of the v1.8 Amp Camp Amp, when driving an 8 ohm load. We know it isn't infinity (no Nelson Pass amplifier has an open loop gain of infinity!), so what is the actual number?
Miserable EE students who suffered through their 2nd year curriculum of unrelenting nonstop torture, will tell you that an inverting amplifier like the ACA, has a closed loop gain which is equal to
ClosedLoopGain = (-1) * 1 / [(1 / OLG) + (R11/R12)*(1 + (1/OLG))]
Studying this expression: we know the closed loop gain, we know the resistor ratio (R11/R12) so the only unknown is OLG, the open loop gain. "Merely" (!) rearrange this expression to solve for OLG.
And presto, the answer pops out: OLG = Open Loop Gain of the ACA, when driving an 8 ohm load, equals 23.1 times. Not infinity. To be a pragmatist, the open loop gain is "about twenty or so" and is definitely closer to 20 than it is to 200.
HOWEVER, the open loop gain of the ACA is certainly not infinity, and that means the closed loop voltage gain of the amplifier + feedback network is certainly not -4.0 .
From the data in #9801 above, we can calculate that the closed loop voltage gain is about -3.3 . {why? because an input of 1.72V, times a gain of 3.3x, gives an output of 5.676 volts. And 5.676 volts delivers 4.0 watts into an 8 ohm load}
AHA so now we have enough information to calculate the open loop gain "OLG" of the v1.8 Amp Camp Amp, when driving an 8 ohm load. We know it isn't infinity (no Nelson Pass amplifier has an open loop gain of infinity!), so what is the actual number?
Miserable EE students who suffered through their 2nd year curriculum of unrelenting nonstop torture, will tell you that an inverting amplifier like the ACA, has a closed loop gain which is equal to
ClosedLoopGain = (-1) * 1 / [(1 / OLG) + (R11/R12)*(1 + (1/OLG))]
Studying this expression: we know the closed loop gain, we know the resistor ratio (R11/R12) so the only unknown is OLG, the open loop gain. "Merely" (!) rearrange this expression to solve for OLG.
And presto, the answer pops out: OLG = Open Loop Gain of the ACA, when driving an 8 ohm load, equals 23.1 times. Not infinity. To be a pragmatist, the open loop gain is "about twenty or so" and is definitely closer to 20 than it is to 200.
Look at the X-Y oscilloscope photo I posted in reply 274 of this thread. The slope of the line is the open loop gain of the ACA (first version with 19V rail).
See how the line curves ? That means the gain is not linear. Feedback corrects this to some extent, but you'll get a different number when you change the amplitude of the signal used to try to measure OLG.
See how there is a loop in the trace. I think this is thermal distortion where Q1 is at a slightly different temperature on the way up versus on the way down. Not much discussion of that topic here.
See how the line curves ? That means the gain is not linear. Feedback corrects this to some extent, but you'll get a different number when you change the amplitude of the signal used to try to measure OLG.
See how there is a loop in the trace. I think this is thermal distortion where Q1 is at a slightly different temperature on the way up versus on the way down. Not much discussion of that topic here.
2.5vrms just brings the sim to clipping (24v supply). 2.2vrms is just about OK and gives 7.25vrms output.
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Death and taxes
Not in the UK, but in France, so EU: I just received two ACA parts-only kits with 2 Meanwell PS from the DIYaudio store.
Total cost:
$356.00 goods (diyaudio store)
$68.95 shipping (diyaudio store, USPS international)
Eur 62.00 French VAT at 20%
Eur 15.00 Handling charge (frais de dedouanement) by French post office.
So in total (including shipping) about 40% overhead.
Ordered Feb. 17, missed delivery yesterday, picked up at the post office today, Mar. 09. Still waiting for the enclosures from Italy (ordered later). At least there should not be any extra charges there.
Not in the UK, but in France, so EU: I just received two ACA parts-only kits with 2 Meanwell PS from the DIYaudio store.
Total cost:
$356.00 goods (diyaudio store)
$68.95 shipping (diyaudio store, USPS international)
Eur 62.00 French VAT at 20%
Eur 15.00 Handling charge (frais de dedouanement) by French post office.
So in total (including shipping) about 40% overhead.
Ordered Feb. 17, missed delivery yesterday, picked up at the post office today, Mar. 09. Still waiting for the enclosures from Italy (ordered later). At least there should not be any extra charges there.
Shipping was about the same to the US...no VAT or extra handling so other than the government money grab its reasonable imo.
With 8R load the measured OLG is 30.5dB (very close to the sim). With 4R load the OLG is 24.5dB.
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it is always good when the real build agrees with a sim and vice versa.
