Phase Inverter for Guitar Amp
in [Design]
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From: Phil Allison on 5 Jul 2010 01:43 "Bitrex" > The FB signal is referenced to ground through the 100 ohm resistor in the > feedback loop, so there shouldn't be a problem. ** Nonsense. The drawing has an error. ..... Phil
From: Bitrex on 5 Jul 2010 03:03
George Jefferson wrote: > > > "Bitrex" <bitrex(a)de.lete.earthlink.net> wrote in message > news:F5OdnWqxW99tyKzRnZ2dnUVZ_h6dnZ2d(a)earthlink.com... >> George Jefferson wrote: >>> http://www.schematicheaven.com/boogieamps/boogie_lonestar.pdf >>> >>> I'm trying to understand how the phase inverter works in the amp >>> above(3rd page before the power amp). >>> >>> I seee it is basically a normal common emitter. >> >> Looks more like it's a "cathode coupled" phase splitter/differential >> amplifier with a resistor acting as a pseudo constant current source. > > Thanks, it makes a lot more sense seeing it as a difference amplifier > and the common node is a virtual ground for it. The only thing that > still doesn't make much sense is the FB into the volage divider. If the > 15k resistor was grounded then it would make sense. > > I guess though that cap will look like a short for audio frequencies and > if the FB has no DC then the 15k resisitor is effectively grounded. > I'm not entirely sure why they used that arrangement for the feedback, instead of putting the 15k resistor to ground and taking the feedback directly to the grid of the bottom triode and making a straight differential amplifier. It would have more gain than the cathode coupled arrangement, and equal-valued anode resistors could be used. > > Seems like some hocus pocus going on though ;/ How can you be sure the > feedback would never drift due to changes in the transformer and/or > resistors? Also for low frequencies it would have an effect on the > overall bias? I guess not much to really matter though? > > What I don't understand is why not just use something as simple as > > http://en.wikipedia.org/wiki/Phase_splitter > > or > > http://www.audioxpress.com/magsdirx/ax/addenda/media/bennett2671.pdf > It seems the designers are taking the feedback from the output stage to the phase splitter instead of the input stage because they want to keep the output operating as linearly as possible, and yet not interfere with the "good" distortion for guitar created by overdriving the input stage. In that case the first type of phase splitter is out, because it has no gain. Because of the impedance transformation by the transformer the power output stage will probably only have a voltage gain of about 2, which isn't much for the feedback to work with! In addition that design also has the disadvantage that the output impedance from the cathode and anode are wildly different. The circuits on the audioXpress site may be superior to both designs, but involve pentodes and cascodes and stuff that the amp builders aren't going to get into for reasons of cost and availability of parts. One common dual triode for the phase splitter is all the bean counters have allowed. :( |