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ferdinand the bull
11/26/2001 6:00 PM
Where does the low end go?
I'm building my first homebrew project--for the fourth time. It keeps getting better, and looking
better, and I keep learning a great deal. (It's a 15W SE parallel 6V6)
One area I'm stumped on now is the bass response. How can I boost it??? Which caps are suspect
when the bass is little or nil?
These are my cap values: filter, 40/20/20; coupling, .022; cathode 25
I've been doing a side-by-side comparison with a SF Princeton Reverb, (using the same speaker)
and the homebrew just doesn't have much on the bottom. All the voltage measurements I took
seem OK.
Any suggestions?
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kg
11/26/2001 11:58 PM
Re: Where does the low end go?
often when we experience a deficit in one area, it is because the response in another is too strong.
it's all relative. if you roll off the HF you should perceive an increase in LF. you may have to add
more gain to compensate.
ken
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ferdinand the bull
11/27/2001 1:25 AM
Re: Where does the low end go?
Now when you say "roll off the HF...perceive an increase in LF", that does not mean the LF is
boosted right? You're talking perception, yes?
I guess I could simplify my aim.
If I were to start playing with the value of caps to modify the bass response, which caps are most
likely to respond best to having their values raised--and, what might be an appropriate range of
values to test with, for instance, coupling caps (.022uf -- ???uf). I ask this because I don't have a
bucket full of different caps to swap with.
I hope I make sense. Thanks
(also, if you wouldn't mind filling me in on how to cut HF and how to boost gain, in thirty words or
less, I'd be very happy to add that to my notes.) always milking
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... 12/9/2001
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kg
11/27/2001 8:20 PM
buncha shit about frequency response
Now when you say "roll off the HF...perceive an increase in LF", that does not mean the
LF is boosted right? You're talking perception, yes?
no, strictly speaking, there is no "additional" LF signal present.
in order to actively boost frequencies, to actually get more out than you put in, you need to have
some sort of active gain stage. you could use this stage in one of two ways:
a) actually amplifying only the signal frequencies you're interested in boosting.
b) attenuating all signal frequencies you're NOT interested in boosting and then applying gain to
what's left.
in reality both methods are very closely related to one another.
see, the problem with just raising all the coupling cap values is that the cap just determines the
frequency at which the -6db/oct rolloff BEGINS. in an amp that has a nice breakup, with a nice
touch sensitivity and responsiveness, doubling a coupling cap's value can cause too much LF to
pass through to subsequent stages, and wreak havoc on your dynamic response. lowering the 3db
point does NOTHING to those frequencies which were already fairly flat in terms of response... it
only makes the passband of flat response extend a little lower.
likewise, just placing shunt caps here and there to ground will roll off highs (look at most boogies),
but it will also decrease the HF amplification of the amp. it will probably lose some of the life it
once had, the sparkle and sheen (listen to most boogies).
HF response is also VERY closely related to overdrive characteristics... by accentuating HF you are
essentially accentuating all of the THD generated by the transmission of signal. in other words,
more HF means more distortion, literally. conversely, lowering the HF gain will DECREASE the
amount of THD present at the output.
the harmonics and overtones are what lives in the HF region--the fundamentals live in the LF
region. one might think that an increase in LF response would consequently decrease THD, since
the fundamental would be accentuated, and that would be correct. obviously it works in reverse
too--decreasing LF gain will increase the effective THD, since the fundamental would be attenuated
while the harmonics are not.
in all cases one must remember that frequency shaping of any sort MUST favor some frequencies
at the expense of attenuation of others. only via the addition of an active gain stage can this
limitation be overcome.
generally some thump can be had by pumping up the filter cap values, but that only works so far...
again, you're not actually providing an active BOOST of LF so much as you're minimizing the
attenuation that's already occuring by extending the -3db point a little bit lower in frequency. you
might look at it like you're DECREASING the frequency shaping that was previously occuring.
so to answer your question, it ain't easy to cut HF AND boost gain without somehow coming up
with a LOT more voltage gain... cutting the HF will throw some gain out the window, so unless
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... 12/9/2001
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you've got some untapped voltage gain (like in an unbypassed cathode resistor, or a heavy voltaedividing attenuator somewhere) you can take advantage of, you're probably looking at adding
another gain stage.
sorry it's longer than 30 words!
ken
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... 12/9/2001