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Old 4th Jan 2019, 2:59 pm   #21
GrimJosef
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Default Re: Puzzling audio circuitry

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Originally Posted by Radio Wrangler View Post
... What it shares with the cascode is the passage of HT current through two triodes in series ...
I've seen the general class of circuits with two valves in series described as 'totem pole'. I once came across a modern hifi amp with three triodes in series ! It had the look of a valve amp designed by someone who'd been trained only on solid-state devices. The HT voltage was pretty scary.

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Old 4th Jan 2019, 4:55 pm   #22
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Default Re: Puzzling audio circuitry

If you drew the “Ac Equivalent” circuit would this clarify the discussion on the circuit? I remember doing this in technical college many years ago.
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Old 4th Jan 2019, 5:24 pm   #23
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Default Re: Puzzling audio circuitry

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In the cascode the upper valve greatly reduces the voltage gain of the lower valve; in the SRPP the upper valve raises somewhat the voltage gain of the lower valve. In some senses you could say that the SRPP is the opposite of a cascode!
My understanding of cascode was that it was designed primary for RF circuits to circumvent the Miller effect. The main purpose was to pin the plate voltage at a fixed potential, thereby eliminating the effect of the Miller capacitance at the plate-grid circuit of the lower valve. This feature of course is not a great requirement for audio circuitry, so if a modified arrangement allows the plate voltage of the lower valve to move around with signal voltage, it is not a worry. In effect cascode behaves more or less exactly like a screen grid pentode.

Although, looking into the cathode of the upper valve is effectively a short circuit for signal voltages, the actual voltage amplification of cascode, the voltage gain A, primarily depends on the gm and the load resistance in the the anode of the upper valve and is A= -gm.R, provided u>>1 and and (u+2)rp>>R. (where rp is the plate resistance and u the amplification factor).So I don't agree that in cascode " the upper valve greatly reduces the voltage gain of the lower valve" because it is used with the load resistance, but I do agree the voltage gain of the SRPP will be higher because of the higher impedance to signal voltage in the plate circuit created by the upper tube and the point from where the signal is extracted. So it is more of a Totem pole arrangement than cascode.

Last edited by Argus25; 4th Jan 2019 at 5:28 pm. Reason: typo
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Old 4th Jan 2019, 10:27 pm   #24
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Default Re: Puzzling audio circuitry

There was a fairly detailed article on shunt-regulated amplifiers in Wireless Engineer 1951 May, p.132ff. It is available at: https://www.americanradiohistory.com...er-1951-05.pdf.

Re the origins of the cascode, I think it was really quite simple. The question to be addressed was, in situations where, with wideband radar IF strips, noise considerations made it desirable to use two successive triode stages at the input (a single stage having insufficient gain to minimize the noise contribution of a following pentode stage), what was the best way, out the nine possible combinations, to arrange this triode pair. The answer turned out to be a grounded cathode stage followed by a grounded grid stage.


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Old 4th Jan 2019, 10:48 pm   #25
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Default Re: Puzzling audio circuitry

I think the main issue was that the favorable attributes of the triode, especially noise consideration as noted above, could not be harnessed in RF circuitry because of the lack of isolation between the input and output due to the grid-plate Miller capacitance.

Clamping the triode plate to a fixed potential (although looking into the cathode of a cathode follower has about a 300 Ohm impedance with signal valves) allowing the plate current of the lower valve to simply pass via the upper valve, solved this problem and made the arrangement in that respect as good as a screen grid pentode, but with the noise benefits of the Triode.

It means that if required you can have tuned circuits of the same frequency in the grid & plate circuits of cascode, and they don't exchange energy with each other and don't normally require neutralization.
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Old 5th Jan 2019, 2:19 pm   #26
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Default Re: Puzzling audio circuitry

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Originally Posted by Argus25
So I don't agree that in cascode " the upper valve greatly reduces the voltage gain of the lower valve"
The voltage gain of the lower valve in a cascode is typically around unity, because its anode load is the cathode of the upper valve.

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but I do agree the voltage gain of the SRPP will be higher because of the higher impedance to signal voltage in the plate circuit created by the upper tube and the point from where the signal is extracted.
The voltage gain of an SRPP can be higher or lower than the voltage gain of a cascode. This is because the SRPP gain is approximately mu, while the cascode voltage gain is approximately transconductance x anode load.

What I said was that the voltage gain of the lower valve in an SRPP is higher than the lower valve in a cascode. The ratio between the two is approximately mu.

The cascode is grounded cathode followed by grounded grid. The SRPP is grounded cathode followed by cathode follower, with bootstrapping. Hence they are quite different circuits. The problem is that when used in series form and drawn in the usual way they can look superficially similar at first glance.
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Old 5th Jan 2019, 11:32 pm   #27
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The voltage gain of the lower valve in a cascode is typically around unity, because its anode load is the cathode of the upper valve.
However, there is no voltage gain from the upper valve, as its cathode and grid are at fixed potentials. And you say there is no voltage gain from the lower valve, then you are saying the circuit has no voltage gain which is not correct. There is voltage gain, provided from the lower valve's gm, because, despite the fact that the plate voltage of the lower valve is fixed, the plate current passes via the upper valve and the load resistance there. And as noted the gain is -gmR, where R is the load resistance. And that gm is the lower valve's gm.
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Old 6th Jan 2019, 12:07 am   #28
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Default Re: Puzzling audio circuitry

You seem to have got the two circuits slightly crossed, Hugo.

In the normal cascode, the lower triode is a common-cathode amplifier, but its anode load is the input impedance of the common grid stage above it. This impedance is roughly the same as the lower triode's cathode impedance, so the gain from grid to anode for the lower triode is -1 roughly. The lower triode is giving no significant voltage gain. It still gives plenty of current gain and power gain. The upper valve of the cascode is a plain grounded grid triode. It's anode is provided with a load resistor and the modulated DC current in the totem pole (controlled by the lower valve) can develop an appreciable voltage swing. The top valve exhibits appreciable voltage gain, unity current gain and therefore appreciable power gain.

The benefit comes from the small swing of the lower anode. There is much less Miller effect than there would be with a single triode trying to do the job of the cascode.

In the original post circuit, the lower triode is a common cathode stage but its anode is loaded with a constant-current bias generator in the shape of the upper triode and its cathode bias resistor. This provides quiescent current for the lower stage, but it simulates a very high impedance source, and so the lower triode produces the absolute maximum voltage gain it can manage. Miller effect is rampant. As a neat trick the drive to the next stage comes off the top triode cathode and the top triode does double duty as both the constant current bias source and as a cathode follower buffering the output. In this circuit the bottom triode has large voltage gain the upper stage has unity voltage gain. Quite the reverse of the cascode. Far from minimising Miller, this circuit has it on steroids. Also, getting the DC bias running voltage of the lower triode anode is very tricky. Tricky enough to scare designers away.

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Old 6th Jan 2019, 1:12 am   #29
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The lower triode is giving no significant voltage gain. It still gives plenty of current gain and power gain.
David,

It may just be semantics, please correct this if it is wrong:

A valve in itself is not a voltage amplifier, its a transconductance device. For it to be defined as having any voltage gain or voltage amplification, it requires that the plate current variations be converted to a voltage (in a load resistance or impedance).

Then considering the lower valve in the Cascode configuration, the fact that if the dynamic plate voltage measured with changes in grid voltage is negligible, does not mean that the lower valve has no overall voltage gain in Cascode (considering the circuit as a whole), it just means at that point in the circuit, on the anode of the lower valve, where the voltage has been stabilized by the upper valve, no significant voltage variations are seen, say with the scope.

But the lower valve is still doing its usual job as a transconductance amplifier, and the voltage variations that are seen in the load resistor (in the anode of the upper valve) are due to plate current variations generated by the lower valve. The output signal from the cascode configuration is always taken from the load resistor (or some load impedance) of the upper valve, not the plate of the lower valve of course.

It seems somewhat meaningless to specify a voltage gain across two points in a circuit, where one of the points, the plate of the lower valve, has had its potential stabilized. It makes more sense to specify the voltage gain from the input to output of the complete circuit , and the contributions from each valve in it, lower valve -gmR, upper valve 1.

Do you agree with this ?

Last edited by Argus25; 6th Jan 2019 at 1:24 am. Reason: add remark
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Old 6th Jan 2019, 2:33 am   #30
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Default Re: Puzzling audio circuitry

The lower valve is indeed operating as a transconductance machine. In a normal single-valve amplifier stage the voltage gain is set by the transconductance (modified by any undecoupled cathode resistor) multiplied by the resistive load applied to the anode (accounting for the valve's own Ra)

The grid swings with the input, the anode swings with the output so it's fair to say that voltage gain can be seen at the valve. The amount of that gain is a function of not just the valve but also of the cathode and anode impedances it is embedded in.

It is the movement of the anode voltage versus the movement of the grid voltage which forms the Miller effect, and so this gain, seen at the three electrodes of a single valve which is the right gain to use in considering Miller-related.

Mu is the gain a valve will naturally give (at low frequencies) unless the valve is constrained by external impedances.

In the cascode the lower valve is heavily constrained indeed. Its anode thinks it's looking into a cathode follower backwards and that has a Z pretty similar to the valve's own reciprocal-Gm. -1 gain anode swing/grid swing still leaves some Miller effect, and in calculation of the pole frequency, Zs, Cga and this gain are needed. We might disagree over nomenclature, but it's a necessary parameter whatever we call it.

Done with transistors we can make the top stage Zin much lower and really push the pole up that last octave.

Like any circuit, it can be viewed in several ways. Each is right and each illuminates different aspects.

David
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Old 6th Jan 2019, 3:45 am   #31
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Default Re: Puzzling audio circuitry

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Like any circuit, it can be viewed in several ways. Each is right and each illuminates different aspects.

David
Yes. I guess I was just struggling with the notion that a voltage gain figure for the lower valve on its own (its dynamic plate to dynamic grid voltage) could even be specified or have any meaning in the Cascode configuration, since the plate was held to a near fixed potential by the upper valve. Of course in transistor Cascode circuits the collector of the lower transistor has almost perfect clamping to a fixed potential and better than in the valve case.
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Old 6th Jan 2019, 1:45 pm   #32
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Originally Posted by Argus25
However, there is no voltage gain from the upper valve, as its cathode and grid are at fixed potentials.
In a cascode almost all the voltage gain comes from the upper valve. It cathode is not fixed at all, as it is connected to the lower anode.

Quote:
There is voltage gain, provided from the lower valve's gm, because, despite the fact that the plate voltage of the lower valve is fixed, the plate current passes via the upper valve and the load resistance there.
The anode of the lower valve is not fixed, although it cannot move very much. It moves just enough to drive the upper valve.

Quote:
A valve in itself is not a voltage amplifier, its a transconductance device.
It is both. You use whichever model is a better fit to the actual situation you put the valve in. Put a low resistance in the anode and it acts like a transconductance device. Put a very high resistance in the anode and it acts like a voltage amplifier, although with significant output resistance.

Quote:
Then considering the lower valve in the Cascode configuration, the fact that if the dynamic plate voltage measured with changes in grid voltage is negligible,
It is not negligible. It is significant. It is what drives the upper valve.

Quote:
no significant voltage variations are seen, say with the scope.
Voltage variations here would be easily seen with a scope.

Quote:
It seems somewhat meaningless to specify a voltage gain across two points in a circuit, where one of the points, the plate of the lower valve, has had its potential stabilized.
It hasn't had its potential stabilised. The cathode impedance of the upper valve is small, but not zero. It is roughly 1/gm + Rload/mu, so as the anode resistance is increased and overall cascode gain is increased you see the voltage at the 'join' increasing too.

The voltage gain of the lower valve in a cascode is approximately 1 + Rload/ra. In a typical circuit this could be in the region of 2. So yes you can specify it and calculate it. You can think of the upper valve as either simply passing on the current sent from the lower valve as a transconductance device, or amplifying the voltage at the 'join'. The first analysis gives gm x Rload as the answer. The second analysis gives mu + Rload x gm - but this ignores the degeneration caused by the anode impedance of the lower valve, which can be significant. Doing a full calculation either way will give the same answer, as it must.
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Old 6th Jan 2019, 4:44 pm   #33
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Default Re: Puzzling audio circuitry

It's pointless to argue whether the 'puzzling audio circuit' is a cascode or not, until you define 'cascode.'

But, the cascode in my book, is the name someone gave to a circuit consisting of a common-cathode stage feeding directly into a common-grid (or earthed-grid) stage. And googling seems to confirm that.

That being so, the 'puzzling audio circuit' is no way a cascode, because although the lower valve is definitely common-cathode, the upper one is no way common-grid. In fact, it's earthed anode, because the anode is at the same AC potential as the input to the stage - in fact, at the same AC potential as the input-return and the output-return.

Agree with Dave RW, the 'puzzling audio circuit' has Miller effect in abundance- and the input will look highly capacitive! Whereas a true cascode, the input looks much less - Cgk + (2 x Cak).

Last edited by kalee20; 6th Jan 2019 at 4:48 pm. Reason: Last sentence added
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Old 6th Jan 2019, 6:55 pm   #34
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Default Re: Puzzling audio circuitry

In my post #3, I only mentioned Cascode as it looked similar but soon released it didn’t follow the normal circuit configuration for Cascode.
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Old 6th Jan 2019, 10:02 pm   #35
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It's pointless to argue whether the 'puzzling audio circuit' is a cascode or not, until you define 'cascode.'

But, the cascode in my book, is the name someone gave to a circuit consisting of a common-cathode stage feeding directly into a common-grid (or earthed-grid) stage. And googling seems to confirm that.
I think that the cascode definition was covered upthread, but to reiterate and perhaps elaborate, the cascode circuit was developed and the name chosen by Wallman & co. in 1944. The grounded-cathode followed by grounded grid combination, out of the nine possible for a triode pair, was found to be the best for use as the first stage of low-noise, wideband radar receiver IF amplifiers operating at around 30 MHz. Whilst developed for wideband applications, it was noted that the cascode could also be used for narrow-band amplifiers and was adaptable for use in low-pass amplifiers.

The Wallman circuit was what would later be referred to as a shunt cascode circuit, with AC signal coupling between the two triodes. The series-cascode appears to have been developed by RCA in 1951, for VHF TV receiver RF amplifier applications. Initially RCA differentiated it from the shunt cascode by referring to it as a "driven grounded-grid” configuration, but that distinction was soon lost and the cascode name was applied to both types. That had happened by 1953, if not earlier.

Given the specificity involved in the original definition, it would seem inappropriate to use the cascode name for any of the other eight triode pair possibilities. But meanings do drift over time, and it is not difficult to see that at in the absence of knowledge about the original meaning, a casual observer might see the key feature of a series-cascode circuit as being the DC-series connection of the two triodes, rather than the other circuit details, and so infer that any such combination fitted the cascode description.

The use of the cascode circuit in audio applications was the subject of a recent thread, https://www.vintage-radio.net/forum/...55#post1097055.

The SRPP circuit appears to predate the cascode, apparently having been developed by Marconi in 1940 for use as a video transmitter modulator driver. I donÂ’t think it was named as SRPP at the time. It was referred to as a shunt-regulated amplifier fairly early on, but whether that name was used by Marconi back in 1940 is unknown to me.


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Old 6th Jan 2019, 11:17 pm   #36
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Quote:
Originally Posted by Argus25
However, there is no voltage gain from the upper valve, as its cathode and grid are at fixed potentials.
In a cascode almost all the voltage gain comes from the upper valve. It cathode is not fixed at all, as it is connected to the lower anode.
If one regards the upper valve as a voltage follower, its cathode attempting to follow the grid voltage and the grid is at signal ground, if it were perfect in that application the plate voltage of the lower valve would not move at all. But since it has a source resistance, looking into the cathode of the upper device (valve), of around a few hundred Ohms typical, I agree it will move. In transistor cascode though, the collector voltage of the lower device is pretty well rock solid voltage wise, so I may have been thinking more of that than the valve case.

Also, because the two devices are in series across the power supply, any dynamic changes in the plate or cathode current, in either device should be the same and solely due to the gm of the lower valve or device. I cannot see how the upper device, valve or transistor, is able to increase the dynamic changes of the lower device's plate (or collector current) beyond that provided by the lower device's gm. Therefore, in terms of contributing to the circuit's overall gain, the upper device can only contribute unity voltage gain in my view. I guess to sort that out, the gm of the upper device (or say the transistors hfe) could be doubled in a test circuit to see if the overall voltage gain changed. Have you tried that in a circuit simulator ?
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Old 7th Jan 2019, 1:37 am   #37
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Default Re: Puzzling audio circuitry

In a qualitative sense, given that mutual conductance is usually expressed in mA/V, and that the anode voltage swing of the lower valve is thought to be quite small, does that not imply a fairly large gain from the upper valve (a small voltage swing into a large mA swing), and a fairly small gain from the lower? If one doubled the gm of the upper valve, then given that the mean current through both valves does not change, would that mean that an even lower voltage swing (half as much?) would be required at the lower valve anode to produce the same output swing into anode load of V2. In turn the lower valve would then be operating at half the gain.

Another way of looking at this is, since the current through both valves is the same, to neglect for the time being the non-vacuum interface between the two and to see the grid of the upper valve as an earthed screen (and one that does not partition the electron flow) between the lower valve grid and the upper valve anode. Now we have a pentode-like situation where the output swing is determined by the combination of the mutual conductance of the lower valve and the load resistance of the upper valve.

From either of these simple and simplistic analyses, it would appear that the gain is primarily determined by the mutual conductance of the lower valve, with the mutual conductance of the upper valve determining the gain distribution between the two valves but having little effect on the overall gain.

No doubt such a simple approach fails in detail, but it does seem to help create a mental picture as to what is going on in a macro-sense.


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Old 7th Jan 2019, 1:51 am   #38
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Another way of looking at this is, since the current through both valves is the same, to neglect for the time being the non-vacuum interface between the two and to see the grid of the upper valve as an earthed screen (and one that does not partition the electron flow) between the lower valve grid and the upper valve anode. Now we have a pentode-like situation where the output swing is determined by the combination of the mutual conductance of the lower valve and the load resistance of the upper valve.


Cheers,
I agree with this entirely and for the valve case it is a pretty good way to look at it, as it is very much like a screen grid pentode but partitioned into two vacuum spaces (if separate triodes) with two sources of electron emission with plate currents that are equal.
In the same way you can inject a signal into the screen grid of a pentode you can also do this with the grid of the upper valve in cascode which can make a handy "port" for some applications.
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Old 7th Jan 2019, 2:31 am   #39
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In the same way you can inject a signal into the screen grid of a pentode you can also do this with the grid of the upper valve in cascode which can make a handy "port" for some applications.
In this case there might be a difference between the pentode and the cascode, though. By modulating the screen grid of a pentode (or for that matter its suppressor grid), the cathode current is hardly changed, but the anode current is, given that a variable amount is then diverted to the screen and leaves the valve by that route.

In the case of a cascode, modulating the upper valve grid cannot provide a variable alternative path for the electrons, as there is nowhere else for them to go. And I think it would need a fairly big voltage swing at the upper valve grid to produce (by cathode follower action) enough swing at the lower valve anode to have a material effect on the current through both valves.

I think that you could get there with a “triple” though, essentially a cathode coupled pair sitting on top of a third triode, with signal into the lower unit grid, and oscillator (or agc) into one of the upper unit grids, with the output taken from the anode of the other. In this case varying the signal or bias on the affected upper grid would vary the division of the current from the lower valve between the two upper valves.

Of course, transistor and fet cascodes are different in that respect, and the upper unit can be used to modulate the current flow, for such as oscillator injection, agc bias application.


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Old 7th Jan 2019, 2:17 pm   #40
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Quote:
Originally Posted by Argus25
Also, because the two devices are in series across the power supply, any dynamic changes in the plate or cathode current, in either device should be the same and solely due to the gm of the lower valve or device. I cannot see how the upper device, valve or transistor, is able to increase the dynamic changes of the lower device's plate (or collector current) beyond that provided by the lower device's gm. Therefore, in terms of contributing to the circuit's overall gain, the upper device can only contribute unity voltage gain in my view.
I am not sure whether you are talking here about the cascode or SRPP. In the case of the cascode the upper valve provides most of the voltage gain, as I showed above. In the case of SRPP the upper valve provides a small voltage loss.

In the case of the SRPP the upper valve adds current gain, although this is only significant when a low resistance load is applied to the circuit. The optimum load impedance occurs when the upper and lower valves each contribute about the same amount of signal current to the load i.e. the circuit acts a bit like push-pull.

It took me a while to grasp how SRPP works. I suggest you start by looking at the grounded cathode with bootstrapped load; this is almost equivalent to an SRPP/mu-follower so when you understand one you will understand the other.
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