Lower than expected output
I just completed my ESP P101. It sounds great other than a bit of rectifier hum on the left channel which I'll be taking care of this evening (bad routing). The problem I'm having is that even though I've set the bias to Rod's maximum settings, it's still very underpowered. Loaded, I'm still producing 138VDC. I don't think it's a power issue. I've contacted Rod, but I understand he's a busy guy, so here I am.
I've fed it now with the line level output from my CD player (adjustable line level out) and with the line level from my MP3 player. I don't think input voltage is the issue, but I'm rather new at all of this.
I should be producing roughly 180 watts per channel RMS. That would put my output voltage somewhere around 37 Volts on the output right? I'm seeing peak values of only 9-10 V!
Something is wonky here.
your gain has little to do with your output power nor with your supply rail voltages.
Since you mentioned 37V I assume you have quoted 180W into 4r.
Measuring peaks of 9V to 10V, again I am jumping to the assumption you have measured rms on a DMM, rather than actual peaks from an oscilloscope, indicates you are driving your amplifier hard.
Music signals usually have a true peak voltage about 10db to 20db above the average music level.
If you are measuring rms peaks then you have probably added about 6db to your average.
10V is only 11.5db below maximum power. adding that 6db guess indicates your average level is only 17.5db below maximum.
With the power you have available and the sensitivity of your speakers you have little margin to increase the SPL much higher. If you do, you are moving into the realms of PA and disco that often overdrive their amps to try and produce excessive SPL at the expense of terrible distortion.
Now back to your gain problem. It has much to do with the input signal voltage and the amplifier gain.
Can you identify the gain setting resistors in the ESP amp and confirm their values?
Please also confirm the average voltage level coming from your source. Not the maximum level. eg CD has a maximum level of about 2Vac to 2.2Vac, but average level will be between 200mV and 600mV.
We can then advise.
I should have 37V into 8r. At least that's what I'm thinking when I do the math. I don't have immediate access to a scope and yes those were true RMS measurements on an Extech DMM.
Here is the schematic on Rod's site. Since it's on the regular site and not on the secure site, I hope he doesn't mind.
I'm not sure which are the gain resistors, but gain is set with VR1, so I'm assuming R7 and R8
Gain is set with R4 and R9. VR1 does not control gain, it controls bias, careless adjustment will destroy your amplifier.
VR1 does not set gain! It sets the bias current through the output stage. R7+R8 along with C5 form a bootstrap, which acts as a current sink.
The gain is set by R9 and R4, standard negative feedback as you'd have with any opamp circuit.
Hi Bushroot, are you finished that amp , or not .
If you finished this amp, put on forum part list if you can.
Doing that is against Rod Elliot's wishes, so....
Just buy the PCB if you want to know, they aren't that expensive.
Yeah, buy the boards. Rod is a great guy and he's been a lot of help building this.
I guess I was wrong about the nomenclature. Which is why I put the schematic up. The amps are both adjusted correctly according to Rod's specs (bias). Again, this is all very new to me and this is my first project. All the part values are correct. I've taken it apart to verify that and I also did so when the boards were complete.
Oh and R4 is 1k and R9 is 22k.
the ESP gain is = 22k / 1k +1 = 23times. = +27.2db.
What is your speaker sensitivity?
Let's do an example to show where I am going.
Assume you have 87db speakers.
Add 87 + 27.2 = 114.2db.
Input 100mVac into your amplifier. Out comes 88.2db SPL.
But where is the correlation between 88.2db and 114.2db?
Let's do another calculation.
To produce 1W into 4ohm speakers you need 2Vac from the amplifier.
100mVac divided by 2.00Vac, 0.1 /2 = 0.05 = -26db
Now we have some numbers to work with.
Input a 100mVac (-26db relative to 1W into 4ohm) into amplifier with 27.2db gain then into 87db speakers gives an output of (-26+27.2+87)= 88.2db SPL.
For each doubling of input voltage you will gain 6db until you overload either the speakers or the amplfier.
So using our 114.2 db SPL per equivalent watt input
200mV is -20db >= 94.2db
1600mV>=112.2db, we are now close to overloading the amp. and the speakers are probably producing gross distortion.
Give us a bit more info:- source output voltage and speaker sensitivity and we can advise.
Maybe you can follow my explanation and try the numbers for yourself.
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