Phase Inverter for Guitar Amp
in [Design]
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From: George Jefferson on 4 Jul 2010 19:24 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. For DC, we simply have the 470 and 15k resistor in series giving the bias for the two tubes which happen to run off the same bias but could have separate resistors. I assume the reason this was done was for accuracy. Things I don't get or I'm unsure of. 1. The 470 resistor. The 330K and 1M resistors seem like gate drain resistors so why not just take them directly to ground? In this case the gate of B effects the gate of A and vice versa. 2. The FB signal. On the secondary of the output transformer there is no ground so the FB seems to be floating and I can't see how it would do anything but potentially add noise. It also only looks to work when the 4Ohm speaker connections are used. 3. The strange network of resistors. The common point between the 4 resistors(looks like a +) seems to complicated things an extra order of magnitude. 4. 120 cap on the anode of A. What the heck is it for? Won't it just allow high frequency noise on the output? Also 100k vs 82k. I was thinking about improving this a bit but since I know so little about it. I was thinking about adding an emitter capacitor for a more stable bias voltage but it doesn't look like it will work because of how the originals ignal and feedback signal are "injected" into the circuit. I'm guessing that most of the "strange stuff" is to help with the problem of crossover distortion in the power amp? Anyone mind clarifying whats going on here? I thought the whole idea of the phase inverter was to create an inverted signal for the pull pull power amp section? Inaccuracies in this could easily create unwanted distortion?
From: Phil Allison on 4 Jul 2010 23:21 "George Jefferson" > 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. > > For DC, we simply have the 470 and 15k resistor in series giving the bias > for the two tubes which happen to run off the same bias but could have > separate resistors. I assume the reason this was done was for accuracy. ** An essential feature of the circuit is that the cathodes of V5A & V5B are linked - that is how signal gets from one to the other half of the tube !!! > Things I don't get or I'm unsure of. > > 1. The 470 resistor. The 330K and 1M resistors seem like gate drain > resistors so why not just take them directly to ground? ** They provide a controlled negative bias to each grid - ie the 1.4 volts dropped across the 470 ohms. > 2. The FB signal. On the secondary of the output transformer there is no > ground ** Drawing error - in fact the bottom end of the secondary goes to chassis. > It also only looks to work when the 4Ohm speaker connections are used. ** Don't be silly. > 3. The strange network of resistors. The common point between the 4 > resistors(looks like a +) seems to complicated things an extra order of > magnitude. ** Fraid that is the heart of the circuit. > 4. 120 cap on the anode of A. What the heck is it for? ** High frequency stability of the output stage under actual conditions of use. > Also 100k vs 82k. ** Compensates for the - error in output level matching inherent in the circuit. > I was thinking about improving this a bit but since I know so little about > it. ** Leave it alone - pal. . > I was thinking about adding an emitter capacitor for a more stable bias > voltage but it doesn't look like it will work because of how the originals > ignal and feedback signal are "injected" into the circuit. ** Go on, try it. > I'm guessing that most of the "strange stuff" is to help with the problem > of crossover distortion in the power amp? ** Guess again. > Anyone mind clarifying whats going on here? ** Doubt you will ever get it. ..... Phil
From: Bitrex on 4 Jul 2010 23:41 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. > > For DC, we simply have the 470 and 15k resistor in series giving the > bias for the two tubes which happen to run off the same bias but could > have separate resistors. I assume the reason this was done was for > accuracy. The 470 ohm resistor does provide for correct bias, but there couldn't be two separate resistors or else the circuit wouldn't work properly. > > > Things I don't get or I'm unsure of. > > 1. The 470 resistor. The 330K and 1M resistors seem like gate drain > resistors so why not just take them directly to ground? In this case the > gate of B effects the gate of A and vice versa. The problem with taking those two resistors directly to ground is that because of the 15k "long tail" resistor the cathodes of the two sections are at positive 47 volts with respect to ground. If the two resistors were grounded it would put the grids at -47 volts wrt the cathodes, putting the tubes well into cutoff. Connecting them to the junction of the 470 ohm resistor and the 15k resistor allows the grids to bias up a few volts negative wrt the cathode, putting the tubes in their linear region of operation. > > 2. The FB signal. On the secondary of the output transformer there is no > ground so the FB seems to be floating and I can't see how it would do > anything but potentially add noise. It also only looks to work when the > 4Ohm speaker connections are used. There'll always be a signal on that leg of the transformer secondary, regardless if the 4 ohm speaker is connected or not. The FB signal is referenced to ground through the 100 ohm resistor in the feedback loop, so there shouldn't be a problem. > > 3. The strange network of resistors. The common point between the 4 > resistors(looks like a +) seems to complicated things an extra order of > magnitude. > Again, it does have to be connected up that way for an AC coupled differential amplifier using this topology to work. :) > 4. 120 cap on the anode of A. What the heck is it for? Won't it just > allow high frequency noise on the output? Also 100k vs 82k. f the bottom triode were at AC ground, then the resistors would definitely have to be unequal since the only signal the bottom triode would then be getting would be through the cathode of the top triode. In that situation, if the resistors were equal then the currents through each tube would be equal, and a current rise in the top tube caused by the input signal would cause an equal current drop in the bottom tube, and there wouldn't be any signal to drive the second tube! The cap is probably to equalize the low frequency response of the two outputs given the differing output impedances. In this case however, where there's also a feedback signal coming in to the bottom tube, I'm not sure that the two resistors have to be unequal for the circuit to work, since now it's acting more like a true differential amplifier. Maybe someone with more tube design experience can comment. > > I was thinking about improving this a bit but since I know so little > about it. I was thinking about adding an emitter capacitor for a more > stable bias voltage but it doesn't look like it will work because of how > the originals ignal and feedback signal are "injected" into the circuit. > > I'm guessing that most of the "strange stuff" is to help with the > problem of crossover distortion in the power amp? > > Anyone mind clarifying whats going on here? I thought the whole idea of > the phase inverter was to create an inverted signal for the pull pull > power amp section? Inaccuracies in this could easily create unwanted > distortion? > > >
From: Bitrex on 4 Jul 2010 23:45 Bitrex wrote: > >> 4. 120 cap on the anode of A. What the heck is it for? Won't it just >> allow high frequency noise on the output? Also 100k vs 82k. > > f the bottom triode were at AC ground, then the resistors would ^^ bottom tridoe's grid > definitely have to be unequal since the only signal the bottom triode > would then be getting would be through the cathode of the top triode. In > that situation, if the resistors were equal then the currents through > each tube would be equal, and a current rise in the top tube caused by > the input signal would cause an equal current drop in the bottom tube, > and there wouldn't be any signal to drive the second tube! The cap is > probably to equalize the low frequency response of the two outputs given > the differing output impedances. > > In this case however, where there's also a feedback signal coming in to > the bottom tube, I'm not sure that the two resistors have to be unequal > for the circuit to work, since now it's acting more like a true > differential amplifier. Maybe someone with more tube design experience > can comment. >
From: George Jefferson on 5 Jul 2010 00:41
"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. 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 These don't have any delay which can ruin a amp from what I hear(can't respond fast enough for the transients). |