Tube Preamps and Polarity

[juno]

I was wondering why some tube preamps reverse polarity and some don't.
Is it just a design choice? And if so, why do some designers do this and others do not?
Are there benefits that would result in one or the other?

 

[thorpej]

Gain stages typically invert the signal as a side-effect of how they work. So whether or not your pre-amp inverts the output vis a vis the input is basically just a function of how many gain stages it has.

 

[Palustris]

It is true that there is an absolute phase. That is to say that the waveform emitted by a musical instrument has a phase and that it is best that the reproducing equipment adhere to that phase. In reality, the phase of a signal at the speakers is a random thing once it has been reproduced by the entire signal chain and there is a 50/50 chance the phase is correct at the speakers, probably having been inverted dozens of times between the original waveform and the sound emanating from the speakers. And again, if there are multiple instruments and multiple microphones, the chances are that some will be in phase with each other and some will be out of phase. In other words there is mixed phase among the instruments so there is NO actual absolute phase to the recording. This does not mean that reproduced music will not sound better in one phase or the other, it just means that is impossible to predict which phase will emerge from your speakers.

 

[gadget73]

Possible inverting preamps were meant to pair with inverting amps, producing an overall-correct phasing.

or it just wasn't something that was given a lot of thought.

 

[Dave451]

Is this the purpose of the phase inverter/phase reverse stage/switch in the Sherwood S8000 IV? I've been wondering exactly what this circuit is for and how it affects what the amplifier is doing. The PI stage is right after the pre-amp and before the tone control amps in the circuit.

 

[whoaru99]

Disregard.

 

[larryderouin]

You're asking about 2 maybe 3 different circuits.
The polarity reverse switch is strictly for the speakers. Only one of them is connected to the switch. If you have the schematicfor a unit that has one, you'll find it btwn the speaker on one side and the output transformer terminals. All it does is swap polarity of the speaker connection from pos to neg so you can match the other speaker. It's easier to flip the switch than swapping speaker wires. The Sherwood doesn't appear to have a polarity switch for the speakers.

Very basically The phase inverter part of the Driver circuit is used to invert the phase of one tube of the output pair so you have a push(positive) and a pull(negative) pair. One tube is setup to work the upper 1/2 of the sine wave and the other is setup to work the lower 1/2 of the sine wave. They do have to overlap just slightly to do a hand off, but other than that they have to be identically opposite or all you'll get is 1/2 of the sound. And it'll sound like crap. Single ended amps I don't believe have a phase inverter as one tube is doing all the work.

 

[gadget73]

What he said. Often phase reverse controls went along with the AM/FM simulcast systems. Something about the phasing not matching with that sometimes, and being able to reverse the phase with a switch was a convenient thing to have. It only existed on one side. Once in a while you'd see it switch phase upstream of the amp stage though. My S-7000 changes the feed from the tone amp into the power amp from the plate to the cathode to make that happen. My S-5500II and Fisher TA-600 flip the speaker leads in the more usual fashion.

The "stereo reverse" position swaps left and right speakers, if thats the control you mean.

 

[Dave451]

Hmmm, still puzzled. Here is the circuit. It's a true phase inverter stage and the "phase rev" switch switches this stage, not the speakers. Definitely not the phase inverter for the power amplifier stage, but a free standing circuit early in the signal path, just after the line inputs or the pre-amp (if it's in use). The S8000 IV is a late-model tube FM multiplex receiver, so it likely didn't have anything to do with AM-FM simulcast stereo. It's also a pretty complicated addition to the signal path, so I have been scratching my head about it since beginning work on this receiver.
Dave
PS apologies to the OP, but this is probably the kind of circuit you're talking about in your pre-amp. I think I have the same question you do.

 

[willyrover]

Hmmm, still puzzled. Here is the circuit. It's a true phase inverter stage and the "phase rev" switch switches this stage, not the speakers. Definitely not the phase inverter for the power amplifier stage, but a free standing circuit early in the signal path, just after the line inputs or the pre-amp (if it's in use).

This might explain it...

The S-5000 could also be used as a mono power amp, giving 72 continuous watts. This was done by connecting the two speaker leads to the same figure (4.8 or 16) at each output channel at the rear. By using the "phase" switch at the front both amps would drive the same speaker - with double power.

So the phase switch could correct a speaker with the wires reversed, but it would also allow you to bridge the amplifier to mono operation. (It also seems to affect the REC OUT which is strange.)

 

[gadget73]

The 8000-IV apparently does it exactly like the 7000 does, changes from a non-inverting cathode follower to an inverting anode follower.

 

[kward]

...this is probably the kind of circuit you're talking about in your pre-amp.

Unlikely, unless it's some sort of esoteric circuit in that preamp. The common cathode gain stage, as is commonly used in preamps, inverts polarity as it amplifies. One signal in, one signal out, but phase inverted with respect to input. Its Just the nature of how it works. (Transistors behave he same, by the way.) Technically speaking the reason for the polarity inversion is because the "load line" for the stage has a negative slope.

The phase inverter stage of a push pull power amp creates two identical signals with one being phase inverted. One signal in, two identical signals out, other than one of the outputs is phase inverted. You only need this for a push pull output stage.

 

[juno]

I just wondered because, for example, the
Quicksilver linestage reverses polarity. Mine, which is made by MAGI, does not. They both use two tubes.I have noticed some others where the specs say polarity 180 degrees/reverses polarity. None have a polarity switch that I know of. I mean of the ones I am talking about. I just wondered why some do and some don't.
Carry on. This is an interesting discussion!

 

[triode17]

Is this the purpose of the phase inverter/phase reverse stage/switch in the Sherwood S8000 IV? I've been wondering exactly what this circuit is for and how it affects what the amplifier is doing. The PI stage is right after the pre-amp and before the tone control amps in the circuit.

I also have an 8000 and the Phase rev switch is to reverse the polarity of just one of the speaker outputs.
Let's say you have neatly wired the system, pushed the stereo unit against the wall and then realized that the speakers are mis-polarized. Flip the switch and everything is OK again. It doesn't change the Absolute Phase.

 

[triode17]

This might explain it...

The S-5000 could also be used as a mono power amp, giving 72 continuous watts. This was done by connecting the two speaker leads to the same figure (4.8 or 16) at each output channel at the rear. By using the "phase" switch at the front both amps would drive the same speaker - with double power.

So the phase switch could correct a speaker with the wires reversed, but it would also allow you to bridge the amplifier to mono operation. (It also seems to affect the REC OUT which is strange.)

BTW, if the amps are bridged, the (theoretical ) power becomes 4x one channel or 36x4=144w. but most amps. can't produce that much power unless they have a very robust power supply.

 

[triode17]

I did an Absolute polarity test a few months ago. Using a dual polarity reversal switch, I would switch back and forth between positive polarity (on both channels) to negative polarity. There was very little difference in sound. I did hear more treble when in-polarity and a smoother , more mellow tone when out-of-polarity. Overall though, I didn't feel the difference was important either way.

 

[gadget73]

Let's say you have neatly wired the system, pushed the stereo unit against the wall and then realized that the speakers are mis-polarized. Flip the switch and everything is OK again. It doesn't change the Absolute Phase.

well, it does but only on one channel. More importantly it changes the phase of one channel relative to the other. If both channels have an inverted absolute phase, they are still in-phase with each other and it won't sound obviously funny.

Dunno, some amps invert, some don't. My Pilot SA-260 does. The Pilot preamp that was most commonly paired with that amp does not though. Overall you'd end up with a phase inversion from input to output. I guess if I really wanted to fix that I could simply add in an inverting zero-gain op-amp to work as a buffer but frankly I don't care all that much. If I were to add the buffer it would be to use that preamp with something that it wouldn't drive natively, and to be dead honest with you I've got better preamps that will drive lower impedance anyway.

 

[triode17]

well, it does but only on one channel. More importantly it changes the phase of one channel relative to the other. If both channels have an inverted absolute phase, they are still in-phase with each other and it won't sound obviously funny.

Dunno, some amps invert, some don't. My Pilot SA-260 does. The Pilot preamp that was most commonly paired with that amp does not though. Overall you'd end up with a phase inversion from input to output. I guess if I really wanted to fix that I could simply add in an inverting zero-gain op-amp to work as a buffer but frankly I don't care all that much. If I were to add the buffer it would be to use that preamp with something that it wouldn't drive natively, and to be dead honest with you I've got better preamps that will drive lower impedance anyway.

Gadget - Something I forgot to add was that when running in Bridge mode, the speaker has to connect from Plus to Plus speaker outs.

 

[gadget73]

I might have to try it on my S-7000 just for yucks. The S-5500II does not have the same sort of phase inversion system, it just flops the speaker leads around.

+ to + is about what I expected. 4 ohm to 4 ohm for an 8 ohm speaker, and probably the mode selector would have to be set to "channel 1" or whatever so the signal fed into both amps. Throw the invert switch and it should end up as a fully bridged mono amp with double the power output.

 

[BinaryMike]

So the phase switch could correct a speaker with the wires reversed, but it would also allow you to bridge the amplifier to mono operation. (It also seems to affect the REC OUT which is strange.)

It's not so strange if you consider the AM/FM stereocast issue, so I think that clinches gadget's argument quite nicely.