I am interested in trying to find out the output (to clipping) of my NAKSA 80 amps.
I created a 4ohm load - using 5x 20ohm, 10w ceramic resistors in parallel.
Can anyone tell me the current and power output represented by the following scenario:
* increasing the signal into the amp (from my sig-gen - set to 1kHz), the CRO screen shows 40v p-t-p.
(This is actually the max I can read on my CRO. 🙁 And the output signal is not clipping!)
I thought that V = IR means that 10a must be being output into a 4ohm load - but this is a lot more than I would've thought the amp was capable of outputting (particularly as I'm not yet at clipping!).
Can anyone enlighten me as to the correct calculation?
I created a 4ohm load - using 5x 20ohm, 10w ceramic resistors in parallel.
Can anyone tell me the current and power output represented by the following scenario:
* increasing the signal into the amp (from my sig-gen - set to 1kHz), the CRO screen shows 40v p-t-p.
(This is actually the max I can read on my CRO. 🙁 And the output signal is not clipping!)
I thought that V = IR means that 10a must be being output into a 4ohm load - but this is a lot more than I would've thought the amp was capable of outputting (particularly as I'm not yet at clipping!).
Can anyone enlighten me as to the correct calculation?
Incorrect, p2p is 2.828 * rms, rms = 0.353 * p2p. peak to peak is twice the peak (amplitude).
40V p2p is 14.1Vrms.
The amplitude is 20V, so the peak current is 5A, and the rms current 3.53A
40V p2p is 14.1Vrms.
The amplitude is 20V, so the peak current is 5A, and the rms current 3.53A
Thank you, Mark. 👍
So does that mean rms power being delivered at the 40v ptp reading ... is I^2 * R - ie: 3.53 x 3.53 x 4 = 50w?
That's just mean power, no squaring or square-rooting if you are talking power, that's only for volts or amps.
mean power = Vrms^2 / R = Irms^2 * R
mean power = Vrms^2 / R = Irms^2 * R
Thanks again, Mark.
To re-phrase my original question ... what power is being output into 4 ohms, if I am seeing 40v ptp on my 'scope?
Thanks. Unfortunately, that equation makes no sense; it could be just as wrong as this one:
... which Mark said was in error.
This equation is correct and the one I use routinely because it's so handy and easy. Here's the algebra:
P = (Vrms)^2/R (Given a sine wave.)
Vrms =Vpk/sqrt(2)
Squaring the above Vrms to Vpk conversion:
(Vrms)^2 = [ Vpk/sqrt(2)]^2 = Vpk^2/2
Substituting the squared conversion into the first equation:
P = Vpk^2/2R
P.S. Yes, what @wg_ski said. 😉
Thank you very much, everyone. 👍
OK, so 40v ptp into 4 ohms (not clipping) is 50w. (Vpk is 1/2 * ptp = 20v; 20^2 / 2*4 = 400/8 = 50w.)
I changed the load to 8 ohms and this time found clipping ... at about 65v ptp.
For convenience, say 66v ... so Vpk = 33. Power is 33^2 / 2*8 = 68w.
Am I correct, here? The clipping ptp seems about right - given the DC rails are +/-35v (so 70v in total).
Yes. BTW ac voltages and currents are almost always quoted as rms, since then the equations for resistive loads are the same as for DC which means you don't have to remember multiple sets of equations (for instance a 120V mains supply is 120Vrms i.e. 340V peak-to-peak). If its peak/amplitude or peak-to-peak always make that clear. When you see dBV that is 20*log10(Vrms). So your 65V ptp into 8 ohms might also be described as 27.2dBV
BTW the symbol for volt is V, not v, for watt is W not w. Symbols are multipliers are all case-depedent (compare m for milli to M for mega, f for femto, F for farad, h for hecto, H for henry).
BTW the symbol for volt is V, not v, for watt is W not w. Symbols are multipliers are all case-depedent (compare m for milli to M for mega, f for femto, F for farad, h for hecto, H for henry).
Sadly this does not mean much.
No, you are using Vpp
You need either Vp which is half that or V RMS which is Vpp/2.83
No it's not.
You have +/-42V rails so you might put out up to 84Vpp in Fairy Land, in practice some 70-75Vpp in Real World, considering supply drop under load and transistor drop, some 3-4V at each rail.
To extend your CRO range you need an input attenuator.
Nothing critical here since we are within the Audio band, drive impedance is very low and scope impedance is high .
10k resistors are fine here.
I suggest 2:1 and a 3:1 attenuators, you must multiply Scope display by 2 or 3X respectively.
The Math was posted above, although some suggestions were sort of funny 😲
Where do you get the idea that I have +/-42V DC rails JMF?
Yes, the standard NAKSA 80 has +/-42v rails but mine are 'down-rated' - on Hugh's advice ... as the amps were built to drive my Maggie mids & ribbons (3 ohms & 2 ohms). Hence, my power traffo has 25v secondaries - not 30v.
Obviously, they wouldn't, maaate.
But as I explicitly quoted my DC rails:
... if you thought I was wrong, you could've said something like: "+/-35v? A standard NAKSA 80 is +/-42v." - rather than just barrelling in with this comment:
Just in case anyone is interested ... I just now went back to a 4ohm load - clipping occurred at about 50v ptp.
So if ptp V = 50, pk V = 25; power at clipping is therefore 25^2 / 2*4 = 78w.
So, with your 50v peak to peak, convert it to RMS by dividing it by 1.414 = 35.36V RMS.
Then square your 35.36V = 35.36 x 35.36 = 1250.33 . Then divide that by your OHMS > and you get WATTS.
1250.33 divided by 4 (ohms) = 312.5 Watts.
1250.33 divided by 8 (ohms) = 156.3 Watts.
( these power levels are RMS )
Then square your 35.36V = 35.36 x 35.36 = 1250.33 . Then divide that by your OHMS > and you get WATTS.
1250.33 divided by 4 (ohms) = 312.5 Watts.
1250.33 divided by 8 (ohms) = 156.3 Watts.
( these power levels are RMS )
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