I'm hoping to get some clarification on the phase of single ended tube amplifiers with multiple stages.
I've seen a few different variations of this design, the middle section I believe is a MU Follower:
The way I understand tubes, is when a signal is taken from the anode side, the phase is inverted.
If that is the case why do I see this design come up a few times, isn't the output phase inverted when compared to the original input.
Thanks
I've seen a few different variations of this design, the middle section I believe is a MU Follower:
The way I understand tubes, is when a signal is taken from the anode side, the phase is inverted.
If that is the case why do I see this design come up a few times, isn't the output phase inverted when compared to the original input.
Thanks
The way this works, if you increase the grid voltage, the tube draws more current, thus more drop across R4, thus anode potential drops. That's inverse phase. In the input stage, the anode signal is the inverse of the input signal. Same with the other stages. That's what is shown, so what exactly is the issue?
Jan
Jan
Thanks for the reply, are you saying the output will be in-phase, and not out of phase as I've shown (assuming the correct resistors etc are used).
I will build a prototype to check things like voltages, phase, gain etc just wanted to do a sanity check first.
I've not built it yet, just planning the design at the moment. This will be a full build and I will provide the full schematics for free once I have everything correct. This will be more than a basic SE Amp.
I will build a prototype to check things like voltages, phase, gain etc just wanted to do a sanity check first.
I've not built it yet, just planning the design at the moment. This will be a full build and I will provide the full schematics for free once I have everything correct. This will be more than a basic SE Amp.
No. The output is in reverse phae to the input. Look at the drawing, check input phase. Phase is not absolute it is in reference to another signal. Here 1st stage output is shown in reverse phase to input
Jan
Jan
So what I wrote is unclear? Anything there that's confusing? Do you see the phase reverse between input and output?
Edit: maybe the confusion is that the input starts with a negative going wave, which is unusual. The first stage anode output output starts with a postive going wave so it is correctly inverting phase.
Edit2: if you built it, how are you going to check it? You have a dual phase triggered scope?
Jan
Edit: maybe the confusion is that the input starts with a negative going wave, which is unusual. The first stage anode output output starts with a postive going wave so it is correctly inverting phase.
Edit2: if you built it, how are you going to check it? You have a dual phase triggered scope?
Jan
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The middle stage is an SRPP, not a mu follower. BTW if you want the output be in phase with the input, just swap the secondary terminals of the output transformer.
@Ratti3
Are you talking about the phase for individual gain stages, ie. each tube, or the whole circuit?
In the end, a transformer can correct the phase by shifting the phase output (usually planned in advance and taken into consideration during manufacturing) and if possible reconnecting the 4, 8 and 16 Ohm wires in correct phase and order, but to be exact, your transformer symbol is missing those phase dots, it's often left out and assumed "everyone knows it", see attached picture.
https://www.electronics-tutorials.ws/resources/transformer-symbols.html
Are you talking about the phase for individual gain stages, ie. each tube, or the whole circuit?
In the end, a transformer can correct the phase by shifting the phase output (usually planned in advance and taken into consideration during manufacturing) and if possible reconnecting the 4, 8 and 16 Ohm wires in correct phase and order, but to be exact, your transformer symbol is missing those phase dots, it's often left out and assumed "everyone knows it", see attached picture.
https://www.electronics-tutorials.ws/resources/transformer-symbols.html
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Thank you, I knew I wasn't going crazy with what I was thinking. I'm probably not explaining myself very well.
The transformers I'll be using will be the Hammond 1628SEA 5Kohm, I'll do some testing to verify all this.
1628SEA
The whole circuit
As is shown in the diagram, the whole circuit is inverting, and you normally don't want that.
Reversing output xformer secondary connection would correct that, yes.
But it's very unlikely you'll hear a difference, unless you know what is playing at any time ;-) .
Jan
Reversing output xformer secondary connection would correct that, yes.
But it's very unlikely you'll hear a difference, unless you know what is playing at any time ;-) .
Jan
It all makes sense now, as mentioned in one of the previous replies, a lot of these schematics assume the phase will be corrected (via transformer or input signal).
But because a lot of these schematics don't indicate stuff like this made me question if people build stuff and ignore the phase.
I believe the speaker will move in opposite directions with the phase inverted compared to the original studio recording. This may or may not be ok, of course it would be best to get the output phase correct in the first place.
Thanks all 🙂
But because a lot of these schematics don't indicate stuff like this made me question if people build stuff and ignore the phase.
I believe the speaker will move in opposite directions with the phase inverted compared to the original studio recording. This may or may not be ok, of course it would be best to get the output phase correct in the first place.
Thanks all 🙂
When people build stuff like this, pre-designed, they assume the phase shifts are already handled. In this circuit the phase at the output is irrelevent. But if you had a GNFB return to the front it would matter. With no FB there's nothing to create a problem as long as both channels are the same. Or if you don't have any external signal processing.
This is only true at one particular frequency well into the region where the capacitor is rolling off the frequency response. At 1 KHz there should be no phase shift across a coupling cap in a broadband audio amp.
All OPT's are not created equal. Unless the dots are explicitly shown in the OPT specs they can be inverting or non inverting. In a push pull amp this can usually be corrected by swapping the plate and UL wires with the wires to the other tube, but a SE transformer with a UL tap can only be used one way, and a non UL OPT may perform better when wired according to the published diagram due to stray capacitance effects.
Capacitors Vs. Resistors
Capacitors do not behave the same as resistors. Whereas resistors allow a flow of electrons through them directly proportional to the voltage drop, capacitors oppose changes in voltage by drawing or supplying current as they charge or discharge to the new voltage level.
The flow of electrons “through” a capacitor is directly proportional to the rate of change of voltage across the capacitor. This opposition to voltage change is another form of reactance, but one that is precisely opposite to the kind exhibited by inductors.
Therefore, the instantaneous current is zero whenever the instantaneous voltage is at a peak (zero change, or level slope, on the voltage sine wave), and the instantaneous current is at a peak wherever the instantaneous voltage is at maximum change (the points of steepest slope on the voltage wave, where it crosses the zero line).
This results in a voltage wave that is -90° out of phase with the current wave.
Which is why Williamson amps are so sensitive to any OPT high frequency shifting. Because there are 3 coupling stages there is an odd number of 90 degree shifts that take the output closer to positive FB and any additional shift gets you closer to oscillation. Mullard reduced the shift by removing 1 coupling stage.
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Consider the circuit shown in post #1 of this thread. Assume a solid state power supply with instantaneous turn on for simplicity.
At power on the B+ instantly jumps to its unloaded value and C3 charges from zero volts toward the B+ value through R4 and R5. During this charging time the current "through" the cap will lead the voltage by about 90 degrees on average. There will be some discharging of C3 as the tubes warm up but an equilibrium will be eventually reached where the plate voltage on the EF86 stabilizes and the grid of the 12AX7 stays at about zero volts. From this time no charging or discharging of the capacitor takes place at a frequency well above the LF rolloff point. Connecting a scope probe on one side of C3 and a second probe on the other side should reveal identical waveforms except for the DC offset voltage on the plate side. There should be no phase shift at any audio frequency well above the low frequency point of the RC network formed by C3 and R5 plus the Rp of the EF86. As the frequency of the input signal is reduced there will be a phase shift across the capacitor that increased with a reduction in frequency.
At power on the B+ instantly jumps to its unloaded value and C3 charges from zero volts toward the B+ value through R4 and R5. During this charging time the current "through" the cap will lead the voltage by about 90 degrees on average. There will be some discharging of C3 as the tubes warm up but an equilibrium will be eventually reached where the plate voltage on the EF86 stabilizes and the grid of the 12AX7 stays at about zero volts. From this time no charging or discharging of the capacitor takes place at a frequency well above the LF rolloff point. Connecting a scope probe on one side of C3 and a second probe on the other side should reveal identical waveforms except for the DC offset voltage on the plate side. There should be no phase shift at any audio frequency well above the low frequency point of the RC network formed by C3 and R5 plus the Rp of the EF86. As the frequency of the input signal is reduced there will be a phase shift across the capacitor that increased with a reduction in frequency.
A. If the output stage did not use the UL Tap, then you can swap the plate and B+ leads to make the input phase and output phase the same.
That will work for Beam Power/Pentode operation, and for Triode Wired operation.
B. If the amplifier uses global negative feedback from the output transformer secondary, then you can not use the methods of # 1. above.
You would create positive feedback, and it would oscillate (a different amplifier than the schematic in your Post # 1.)
Your Post # 1 schematic does not have global negative feedback, so "A". phase reversal method above is OK.
C. Many do not care about the absolute phase of amplifier input to output.
But, you must be sure that all amplifiers in the system are in the same phase, or that speakers are fed the same phase (some may have to have their + and - terminals swapped as needed, to make the "sum" of each amplifier + speaker set phases the same.
I hope that clears it up.
That will work for Beam Power/Pentode operation, and for Triode Wired operation.
B. If the amplifier uses global negative feedback from the output transformer secondary, then you can not use the methods of # 1. above.
You would create positive feedback, and it would oscillate (a different amplifier than the schematic in your Post # 1.)
Your Post # 1 schematic does not have global negative feedback, so "A". phase reversal method above is OK.
C. Many do not care about the absolute phase of amplifier input to output.
But, you must be sure that all amplifiers in the system are in the same phase, or that speakers are fed the same phase (some may have to have their + and - terminals swapped as needed, to make the "sum" of each amplifier + speaker set phases the same.
I hope that clears it up.
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