Re: guitar amp
- From: "Lost'n Found" <omakke1@xxxxxxxxxxx>
- Date: Tue, 8 Aug 2006 10:34:16 -0400
Very clear and helpful, thanks
"Bob Eld" <nsmontassoc@xxxxxxxxx> wrote in message
news:ZZ0Cg.5646$uo6.1943@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
"Lost'n Found" <omakke1@xxxxxxxxxxx> wrote in message
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Hello. I want to ask about few things that have been bothering mea
recently
lot.
1) I've seem some solid state guitar schematics. My question is, why
don't
they ever use op-amps in solid state amplifiers?
Op-Amps are sometimes used, however they are not easy to incorporate into
a
power design because they have too much open loop gain and insufficient
voltage swing. It's easier to design a circuit using discrete components
that provides the required voltage drive and the right amount of open loop
gain than it is to use what is pre-canned in an op-amp. This comes about
because all gain stages have a natural "pole" or break point in their
response characteristics each with up to 90 degrees of phase shift. For
stability reasons, it is problematic to cascade more than two of these
within one amplifier feedback loop structure. Most op-amps already have
these two stages but more are required to get the necessary voltage range
for a power amp. This makes the design messy with at least three poles in
the response. So, the simple answer is to not use op-amps, design exactly
what you need. In most amplifiers only two voltage gain stages are within
the feedback loop.
2) I've seen some tube schematics tubes are used, and on the gird of the
input stage tube, there is a resistor (about 100k-500k) that goes from
the
grid straight to the ground. My question is, if they want high input
impedance, why would they put a resistor and not just connect the input
to
the grid resulting in very high input impedance?
It is necessary to have a DC path from the grid to ground to insure that
the
contact potential of the tube is "leaked" off to set the required bias for
the tube. If this is not done, the grid will aquire an increasingly
negative
charge because of the electrons wizzing by that will eventually cut the
tube
off and no signal will pass. It is possible to have a DC path that
includes
the source, but, since the source is unknown and undefined, it is not done
this way. Usually the source is AC coupled with a capacitor and a known
resistor provides the DC path.
3) Assume there is a capacitor before the input stage, and we want to
have
lots of negative feedback. Does it matter were we feed the signal back to
increase the bandwidth of the amplifier?
Yes it matters how the feedback path is created. See number one above.
Feedback must never accumulate more than 180 degrees of phase shift at any
frequency while the loop gain is above unity or your amplifier will be an
oscillator.
Bob
Thank you for your response!
.
- References:
- guitar amp
- From: Lost'n Found
- Re: guitar amp
- From: Bob Eld
- guitar amp
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