NOSValves Posted February 25, 2004 Author Share Posted February 25, 2004 Shawn, You have not proved anything to me you may think you have but you haven't. I never said that AC coupling did not change the wave form all I ever said is that it changed it the same at the input and the output and the difference between the two results is what is important. I NEVER ARGUED THAT DC COUPLING DOES A CLEANER JOB !!! You have proved a point that I never argued or said was wrong. I agreed in the post with the AC coupling pictures that DC is a better way to do it !! What more do I have to do for you ???????????????????????????????????????????? Craig Quote Link to comment Share on other sites More sharing options...
NOSValves Posted February 25, 2004 Author Share Posted February 25, 2004 gerbache, I have a question that I'm curious about. If I read your post correctly there is a DC component to the square wave that a function generator produces ? How could any tube amp with coupling caps designed to block all DC in the signal path passing to the next stage ever pass this part of the wave form ? Craig Quote Link to comment Share on other sites More sharing options...
Painful Reality Posted February 25, 2004 Share Posted February 25, 2004 Capacitors explained Quote Link to comment Share on other sites More sharing options...
3dzapper Posted February 25, 2004 Share Posted February 25, 2004 Craig, If you ever want to see an amp with BAD square wave response put a Dynaco SCA35 on the scope. The only way to get a semblance of a square wave as the frequency is changed is to fiddle with the tone controls and filter. Not to mention all the induced hum in the preamp section. Looks like I'm going to take this one apart too and build another DIY ST35 out of the trannys. Rick Quote Link to comment Share on other sites More sharing options...
NOSValves Posted February 25, 2004 Author Share Posted February 25, 2004 why did you post that Jeff ??? Quote Link to comment Share on other sites More sharing options...
Painful Reality Posted February 25, 2004 Share Posted February 25, 2004 Simply because in this link you have the answer for your last question to Gerbache. Quote Link to comment Share on other sites More sharing options...
maxg Posted February 25, 2004 Share Posted February 25, 2004 Painful, Nice link - thanks - I learned something today. Cheers Quote Link to comment Share on other sites More sharing options...
Painful Reality Posted February 25, 2004 Share Posted February 25, 2004 Thanks Max, Glad it could help someone! Quote Link to comment Share on other sites More sharing options...
NOSValves Posted February 25, 2004 Author Share Posted February 25, 2004 I really don't see where it answered my question except to say that the amp can't pass the DC portion of the signal if one is present. Craig Quote Link to comment Share on other sites More sharing options...
maxg Posted February 25, 2004 Share Posted February 25, 2004 Well, If I understood what it said on capacitors (which is as improbable a way to start a sentence as I have ever written), capacitors will allow DC current to pass up to the point that they charge up and then they will stop it until they are discharged again. So, depending on the frequency of the charge/discharge cycle, it seems DC can leak(?) through.... Sprinkle the above with liberal amounts of Joules, coulombs, farards, ohms etc in the above, as appropriate (because I am buggered if I can). Quote Link to comment Share on other sites More sharing options...
Painful Reality Posted February 25, 2004 Share Posted February 25, 2004 Max, you're one scope away of becoming NOSMax! Craig, Gerbache wrote: "For instance, in a perfect square wave, the rise and drop between high and low points is a perfect vertical line, which means in terms of frequency that it's happening extremely fast. Also, between transitions, a square wave holds a constant, DC value, even if it's only briefly before it drops back down to the low value." Ain't this what you were refering too when you asked: "I have a question that I'm curious about. If I read your post correctly there is a DC component to the square wave that a function generator produces ? How could any tube amp with coupling caps designed to block all DC in the signal path passing to the next stage ever pass this part of the wave form ?" If it was, the link I gave provided the answer as why a cap can (or cannot depending of the frequency and capacitance value) let pass what could be seen as a "short alternating burst of DC" making for a square wave. Quote Link to comment Share on other sites More sharing options...
NOSValves Posted February 25, 2004 Author Share Posted February 25, 2004 Unless I'm mistaken DC is what charges a coupling cap and does not rise and fall as music is played or a waveform is run though the amp so my question still stands. Craig Quote Link to comment Share on other sites More sharing options...
sfogg Posted February 25, 2004 Share Posted February 25, 2004 "Unless I'm mistaken DC is what charges a coupling cap and does not rise and fall as music is played or a waveform is run though the amp so my question still stands." ... look at the pictures of square waves on your scope... Shawn Quote Link to comment Share on other sites More sharing options...
gerbache Posted February 25, 2004 Share Posted February 25, 2004 If I had to place a guess, even on the high frequency square waves, there is a little bit of a drop across the flat part of the square wave. It won't fall nearly as much, though, because the pulses are staying at a DC value for only a very brief amount of time. Are you familiar with time constants? For a reactive circuit, like a capacitor, the time constant is related to how quickly it charges up when a DC voltage is applied. On the low frequency square waves, it it maintaining a DC value for long enough in relation to the time constant that you are able to easily see the drop in voltage. At a higher frequency, since the waveform is only DC for a short amount of time, it hasn't dropped enough to be perceptible. In truth, it is still blocking the DC portion, it's just that it takes a certain amount of time to drop, and the waveform is not maintaining the value long enough. I hope this clears it up some. I'm trying to avoid using too much actual math, so I'm not sure how well it's translating. In essence, it is still filtering, just not fast enough for us to be able to see it on the scope. Quote Link to comment Share on other sites More sharing options...
Painful Reality Posted February 25, 2004 Share Posted February 25, 2004 Craig, If you have a pure DC signal with a AC signal riding over it the cap will block the DC portion. There is no question about it. But do you agree that a low frequency square wave frequency can be "seen as DC" by a low value cap? The cap will see the half wavelength as a DC signal. The point here is at low frequencies cap may see the signal as being DC. Quote Link to comment Share on other sites More sharing options...
gerbache Posted February 25, 2004 Share Posted February 25, 2004 Ok, I have another way of thinking about a square wave, that might be a little clearer than bringing Fourier analysis into this. We can almost think of a square wave as being two different components, an extremely fast change from either positive to negative or negative to positive, and a brief DC period. So in a sense, even when we're inputting a square wave, which is alternating between values, and hence AC by definition, in between each switch, the signal still looks the same as a DC input. So basically, during the time that the signal is flat, the amplifier's response is very similar to what we'd see if we suddenly turned on a DC source into a capacitor. Then, when the input switches to negative, the capacitor suddenly has to charge itself in the opposite polarity, but the signal still looks like a DC signal with a negative value. While the signal is changing from plus to minus, it is an extremely fast change, which can be represented as high frequency. Even though it looks like it instantly changes, if you look at this on a short enough time division on the scope, it will actually look curved, similar to the way a capacitor charges, only much faster. This is because no circuits we can build truly have infinite frequency response, so there is always some amount of filtration on the rise time. Quote Link to comment Share on other sites More sharing options...
NOSValves Posted February 25, 2004 Author Share Posted February 25, 2004 I'm sorry but you guys are completely losing me ! I need Mark Deneen in here fast I do not see how any DC would get by a properly working capacitor especially when you facvtor in what charges the capacitor has nothing to do with the waveform and I also do not see how or why there would be any DC component to a wave form. Craig Quote Link to comment Share on other sites More sharing options...
Guy Landau Posted February 25, 2004 Share Posted February 25, 2004 I just lost you guys, on that one. How the hell did we get all this scope talk? This boring thread should have been long gone. Quote Link to comment Share on other sites More sharing options...
Painful Reality Posted February 25, 2004 Share Posted February 25, 2004 square wave, time contant and fun with the scope Quote Link to comment Share on other sites More sharing options...
NOSValves Posted February 25, 2004 Author Share Posted February 25, 2004 ---------------- On 2/25/2004 4:46:35 PM Guy Landau wrote: I just lost you guys, on that one. How the hell did we get all this scope talk? This boring thread should have been long gone. ---------------- Boring to those with little minds Quote Link to comment Share on other sites More sharing options...
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