Members Carati Posted October 4, 2006 Members Share Posted October 4, 2006 I've been going through the 'national semiconductor' site looking at various power amp chip possibilities, but dont know what some of the specs mean... Like the 'input impedance' or 'gain'ratings ??? Or, what specs determines how much input voltage (from the pickups) is required to drive and overdrive the chip??? Are there any sites that can teach me about chip specs? Link to comment Share on other sites More sharing options...
Members Sir H C Posted October 4, 2006 Members Share Posted October 4, 2006 Input impedance - if the input was just a resistor to ground, what value it would be. Some of these power amp chips have a relatively low input impedance and so this can affect the way you drive the amplifier. Gain ratings - often unlike a general purpose op-amp they will be set up for a specific gain, or specific gain range as the amplifiers are harder to keep stable with the loads they see. The spec for input to overdrive, this is tricky, it is based on the supply voltage and the gain setting. For instance, if you have a gain of 20 for the amplifier, and a +-12 volt rail, you can look at the "Output Dropout Voltage" (lets say 2 volts) and subtract twice that from the sum of the rails (so 20 volt swing is possible), you can see that 1 volt peak to peak in will give the 20 volts peak-to-peak out, your point of clipping. Link to comment Share on other sites More sharing options...
Members dan-o-guitar Posted October 5, 2006 Members Share Posted October 5, 2006 Originally posted by Sir H C Input impedance - if the input was just a resistor to ground, what value it would be. Some of these power amp chips have a relatively low input impedance and so this can affect the way you drive the amplifier. Gain ratings - often unlike a general purpose op-amp they will be set up for a specific gain, or specific gain range as the amplifiers are harder to keep stable with the loads they see. The spec for input to overdrive, this is tricky, it is based on the supply voltage and the gain setting. For instance, if you have a gain of 20 for the amplifier, and a +-12 volt rail, you can look at the "Output Dropout Voltage" (lets say 2 volts) and subtract twice that from the sum of the rails (so 20 volt swing is possible), you can see that 1 volt peak to peak in will give the 20 volts peak-to-peak out, your point of clipping. Holy crap. You had me up until the last paragraph. Math? Actual subtraction? Crap, I've got a 7th grade math level Is there an ideal input impedance for a pedal or amp input given a typical guitar signal (i.e. a good balance between single-coil and humbuckers)? Link to comment Share on other sites More sharing options...
Members Carati Posted October 5, 2006 Author Members Share Posted October 5, 2006 Originally posted by Sir H C Some of these power amp chips have a relatively low input impedance and so this can affect the way you drive the amplifier.Ok, but what does the phrase 'input impedance' mean...and how does a chip's input impedance determine the input voltage required to drive and overdrive the chip? Link to comment Share on other sites More sharing options...
Members Carati Posted October 5, 2006 Author Members Share Posted October 5, 2006 Originally posted by Sir H C Gain ratings - often unlike a general purpose op-amp they will be set up for a specific gain. Right, the one I'm interested in has a fixed internal gain of 50, but I have no idea what this determines in practical terms. Does this mean the input signal is multiplied by a factor of 50 into the speaker? Link to comment Share on other sites More sharing options...
Members CliffC8488 Posted October 5, 2006 Members Share Posted October 5, 2006 No offense, but, if you don't know what input impedance is then you may want to read up on some basic electronics. There are many good books out there and you could get a handle on things with just a couple weeks worth of reading. That said, input impedance is how much resistance the input "looks like". It gives an indication of how much the device will load down the device driving it. The gain is how much the input signal is multiplied by. If a device has a gain of 50 and you apply one volt to the input you'll get 50 volts at the output (assuming the output can go that high). CC Link to comment Share on other sites More sharing options...
Members Carati Posted October 5, 2006 Author Members Share Posted October 5, 2006 Yes, I have a good book on basic electronics (How to Read Schematics), but it aint about guitars and 'input impedance' isnt in the index. So by your definition 'input impedance' is the chip's resistance to the voltage coming from the pickups? As if it were a common resistor? This chip's resistance is 150k. Does this value determine how much voltage is required to drive and overdrive the chip? Link to comment Share on other sites More sharing options...
Members spentron Posted October 5, 2006 Members Share Posted October 5, 2006 High impedance is considered 50K source impedance. Microphones usually produce best response with a matched load and if the loading circuit actually uses the current rather than just providing a dummy load (resistor) this produces "maximum power transfer" useful for picking up quiet sounds (violin at 50' etc.). Many old effects such as Big Muff used 47K. But most tube amps are 1M resistor in parallel with a tube grid (almost insignificant) which allows guitar pickups to act almost unloaded and produce better high end... except the controls in the guitar still load down the PUs considerably, effectively 250K or less, often 125K (in the high end the tone control's capacitor is essentially a short). The only real reason for making the load higher than 500K is that guitar loads itself down so much that it's at the threshold of too much already. Link to comment Share on other sites More sharing options...
Members Sir H C Posted October 5, 2006 Members Share Posted October 5, 2006 Originally posted by Carati Ok, but what does the phrase 'input impedance' mean...and how does a chip's input impedance determine the input voltage required to drive and overdrive the chip? that is my first sentence, it is the equivalent resistance of the input. If you looked at the amp as a black box, what would it look like if you put an ohmmeter across the input. If you have a high gain system (such as the gain of 50 one you speak of) it can not have too high an input impedance if it were a simple amplifier (assume 1meg for the feedback resistor, then you have 20k input impedance), so often there are extra buffers or other very weird circuits that allow the input impedance to be higher but still keep the gain where they want it. Link to comment Share on other sites More sharing options...
Members Sir H C Posted October 5, 2006 Members Share Posted October 5, 2006 Originally posted by Carati Right, the one I'm interested in has a fixed internal gain of 50, but I have no idea what this determines in practical terms. Does this mean the input signal is multiplied by a factor of 50 into the speaker? Yes, voltage wise. Current wise there is a much larger gain. Link to comment Share on other sites More sharing options...
Members Sir H C Posted October 5, 2006 Members Share Posted October 5, 2006 Originally posted by dan-o-guitar Holy crap. You had me up until the last paragraph. Math? Actual subtraction? Crap, I've got a 7th grade math level Is there an ideal input impedance for a pedal or amp input given a typical guitar signal (i.e. a good balance between single-coil and humbuckers)? On a typical fender, the two inputs are either 68k or 1 meg (IIRC), so right off the bat the high and low input have very wide variance in the input impedance.The higher the input impedance, the more the cable stray capacitance can reduce the high end. For pedals typical in is 1 meg if it goes to an op-amp or FET input. Don't even try to calculate input impedance for a fuzzface circuit as it is all over the place as the fuzz knob is spun. On the math front, we can get really ugly and add caps and inductors and non-linear components and start playing with the numbers. Or do this all for some class D amplifiers. Link to comment Share on other sites More sharing options...
Members Carati Posted October 5, 2006 Author Members Share Posted October 5, 2006 Originally posted by Sir H C The spec for input to overdrive, this is tricky, it is based on the supply voltage and the gain setting. For instance, if you have a gain of 20 for the amplifier, and a +-12 volt rail, you can look at the "Output Dropout Voltage" (lets say 2 volts) and subtract twice that from the sum of the rails (so 20 volt swing is possible), you can see that 1 volt peak to peak in will give the 20 volts peak-to-peak out, your point of clipping. Ok, supposing the chip's input impedance is 150K, has a fixed gain of 50, and has an 18v power supply...is it possible to tell from these specs how much input voltage from the guitar is required to overdrive the chip? If not, what specs are required? Link to comment Share on other sites More sharing options...
Members Sir H C Posted October 5, 2006 Members Share Posted October 5, 2006 Single ended 18 volt supply or +-18? With single 18, I would knock off 3 volts for transistors, then you have 15 volt swing. Divide by 50, you get .3 volts. So .3v peak-to-peak should be the max input signal. Link to comment Share on other sites More sharing options...
Members Carati Posted October 5, 2006 Author Members Share Posted October 5, 2006 Originally posted by Sir H C Single ended 18 volt supply or +-18? With single 18, I would knock off 3 volts for transistors, then you have 15 volt swing. Divide by 50, you get .3 volts. So .3v peak-to-peak should be the max input signal. Thanks! Thats what I want to know about. I dont know what you mean by a +-18 supply though? All the components share a common ground in this hypothetical circuit. Does the 'max input signal' here mean the threshold of clipping...anything above 0.3v will overdrive the chip? Your formula might explain why the LM386 is often used for distortion boxes when its gain is set at 200. 8volts divided by 200 is only 0.04volts. Link to comment Share on other sites More sharing options...
Members Sir H C Posted October 5, 2006 Members Share Posted October 5, 2006 Give or take a little that should be the edge of clipping. Remember 3dB is a 50% increase in signal so the error is small to the ear as to the exact level. Link to comment Share on other sites More sharing options...
Members Carati Posted October 5, 2006 Author Members Share Posted October 5, 2006 I appreciate your help, thanks! Even if you read the best books, there are always questions that come up. Link to comment Share on other sites More sharing options...
Members Sir H C Posted October 5, 2006 Members Share Posted October 5, 2006 Having worked in the analog semiconductor industry for about 10 years I have learned most of the spec concepts and tricks. Link to comment Share on other sites More sharing options...
Members Carati Posted October 14, 2006 Author Members Share Posted October 14, 2006 Originally posted by Sir H C Single ended 18 volt supply or +-18? With single 18, I would knock off 3 volts for transistors, then you have 15 volt swing. Divide by 50, you get .3 volts. So .3v peak-to-peak should be the max input signal. Well I finally got it built and tested today.Lo and behold it works! What the hell...you mean I didn't make a single mistake...no short circuits or bad joints? Nope, I could hardly believe it! The sound was pretty much as I expected in that the guitar by itself did not overdrive the chip with a 12volt 300mA power supply...I had to use my graphic EQ pedal in boost mode to overdrive it. So instead of trying to use 18volts Im going to try and find a supply with 9volts at 1.0 amp. I dont think a 9volt battery with its tiny 200mA current is going to give good tone. I'll post some pics as soon as I can find a good enclosure for it. Link to comment Share on other sites More sharing options...
Members Sir H C Posted October 14, 2006 Members Share Posted October 14, 2006 Congrats on the successful build. I would go thrift shopping, often you can find the AC adaptors for dirt cheap. Link to comment Share on other sites More sharing options...
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