Thanks for the help! While I seem to have my head around the series thing I'm still a little confused on the power supply filtering. Is there a reason that the Hot Harmonics schem shows the power to the IC via the filter while the power for the input/preamp stage is directly from +9v? Possibly to isolate the 2 sections? If I were to add the typical 100R/100uf filter to the power supply would this just be redudndant as far as the IC is concerned? If I add this should I remove the 200R/22uf? Should I just move the power for the input/preamp stage to the same place the IC gets it's power? It's a hot day here, should I just open an ice cold beer (it is kind of early though
) and quit thinking too hard?
back from the game--germany won handily and munich is full of celebration. it's a nice time for germany. they have hosted a very successful world cup and gone farther in the competition than most of them expected. it's nice to share it with them. even though that means i won't be getting any sleep for a while yet.
my understanding (and i am not the most qualified to speak on this), the transistor might as well get the filtered power, too. you can see in a previous post that i also noticed this difference between the IC and the transistor in mark hammer's forty-niner schem and moved the power supply for the transistor. no one has objected, so at least it's not a bad thing to do.
i picked this approach up from posts by a number of folks. here's one by martymart
. if you read the thread you will see that the concept was still sinking in for me.
martymart has said at least twice in my presence
that he always
sticks a 100uF cap and a 100nF in parallel from the power supply to ground before the circuit. also, a diode for polarity protection. the caps put a low pass filter between the power supply and everything else.
also, if you look at lots of circuits (like a tubescreamer) you will see (at least) the 100uF cap sitting there. and another cap, maybe a 47uF, from the bias voltage to ground. the small resistor isn't always there. so the input buffer in a standard tube screamer gets cleaner power. same thing in the bsiab 2.
one filter seems to be sufficient. i have never seen two. so if you are going to filter the supply the the transistor, you might as well just move the connection after the 200R/22uF filter like i did with the forty-niner. and you might change the filter to 100R/100uF instead of the current values. the difference is how quickly these two filters roll off as the frequency goes up.
the thread i mentioned above also contains a formula that is often used for comparing two filters like this: the "cut-off frequency" is F=1/(2*pi*R*C) where R is the value of the resistor in ohms and C is the value of the capacitor in farads. R.G. explains that the "cut-off frequency is defined as the frequency where the signal being filtered is at half power, 6DBv down or 3db power down" and that "a single RC filter response slopes down at -6db / octave." so in both cases we are dealing with a single RC filter and we can take their roll-off shapes to be the same. the respective cut-off frequencies are about 36 Hz for the 200R/22uF and 16 Hz for the 100R/100uF. Both are well-below the 100 Hz where guitar frequencies apparently start but with only 3db attenuation. If we go another octave with each that would mean 9db attenuation at 72Hz for the first filter and at 32Hz for the second. clearly the second filter is killing much more of the stuff you typcially don't want.
so you might decide to just replace the original hot harmonics power supply filter with a 100/100uF filter.
all that said, it also appears that it doesn't matter to a lot of folks. we have been happily building the hot harmonics for a while with no complaints (that i can find) about the noise from the power supply. and there are plenty of circuits with no power supply filtering that have probably also had many satisfied users. like the triangle big muff pi that some of us were working on last week. FWIW, i like martymart's approach. it's easy insurance. and you don't have to build it onto your circuit board--it can be built separately which is kind of cool because it gives you more flexibility for cramming everything into that 1590B.
whew! maybe that was more than you wanted. but the cars were honking pretty steadily until a moment ago. now that they have settle down, i'm going to call it quits ... but i would still like to hear about that 22uF cap that sits between ground the unused inverters of the 4049. you don't always see that either. probably more insurance.