Not sure I understand your reasoning with regards to the absolute (common mode) offset of the amplifier in standby. The output will be at the negative rail voltage with the little or no bias. Upping the bias to the desired standby current value will cause the output to float at around -4V or so. Increasing the bias further will not get entirely to zero and is not what we want anyway. INMHO the bias voltage must be offset towards +ve (to make the output devices connected to +ve supply conduct more than the -ve ones) to get the output to zero. Not that this has to be done - the speaker will not see the absolute or common mode voltage anyway.
I agree with you about the therminstor and series resistor but also note that some folks don't like the idea of a series resistor at all - presumably because it requires seperate AC bypassing.
Ian.
I agree with you about the therminstor and series resistor but also note that some folks don't like the idea of a series resistor at all - presumably because it requires seperate AC bypassing.
Ian.
Wooooooooooooooooowwwwww!
6moons review of the XA30.5 is published!!!
Have a look at www.6moons.com
Very nice pictures!!!
I love this wonderfull amps!!!
Mr. Pass, your amplifiers are not only the best sounding they are the most beautiful!
My favourites are the two smallest case sizes!...like the heatsinks!
Hi Ian - will answer you later or tomorrow - also have some questions to you about the applied "bootstrapping" network in your schematic.
Best regards
Dirk
6moons review of the XA30.5 is published!!!
Have a look at www.6moons.com
Very nice pictures!!!
I love this wonderfull amps!!!
Mr. Pass, your amplifiers are not only the best sounding they are the most beautiful!
My favourites are the two smallest case sizes!...like the heatsinks!
Hi Ian - will answer you later or tomorrow - also have some questions to you about the applied "bootstrapping" network in your schematic.
Best regards
Dirk
Another nice review! Congrats, Nelson.
What a nice view of the UGS-boards
http://www.6moons.com/audioreviews/passlabs/xa305.html
PS. Where can I order the Pass Labs T-shirt?
What a nice view of the UGS-boards
http://www.6moons.com/audioreviews/passlabs/xa305.html
PS. Where can I order the Pass Labs T-shirt?
steenoe said:Another nice review! Congrats, Nelson.
What a nice view of the UGS-boards
http://www.6moons.com/audioreviews/passlabs/xa305.html
PS. Where can I order the Pass Labs T-shirt?
that's certainly UGS bstrd
am I feeling post - F4 stampedo ?
DiyAs are worse than Longhorns ...........
I hope Mr. Pass isn't upset that I quote his manual, but I thought this was hysterical...
"This is a conservatively built industrial design, not a frail tube circuit run on the brink of catastrophic failure. If it breaks, we will simply get it fixed, so sleep well."
Source is the last line of this:
http://www.passlabs.com/pdf/product manuals/xa30.5_om.pdf
His manual reminds me of the old Audio Control manuals... Very good technically but a great sense of humor.
"This is a conservatively built industrial design, not a frail tube circuit run on the brink of catastrophic failure. If it breaks, we will simply get it fixed, so sleep well."
Source is the last line of this:
http://www.passlabs.com/pdf/product manuals/xa30.5_om.pdf
His manual reminds me of the old Audio Control manuals... Very good technically but a great sense of humor.
Hi Ian,
OK today I have found a bit more time to answer...
I think there is some misunderstanding between us about the offset.
Leaving this problem aside first.... but think you are right that there is a resistor or something which makes the N-ch more conducting than the p-ch in stand by.
I'm wondering a bit about the bootstrapping network in your circuit.
First, I don't really understand the princip in which ways this can be used (only that this technics feeds back a portion of AC current)
Second, what extra swing will you reach with it? You are using 40V front end voltage which will be more than enough to get 23V peak (absolute) from the output (2V left over for the MOSFET's drain-source).
Perhaps you could help me to understand?
Greatings
Dirk
OK today I have found a bit more time to answer...
I think there is some misunderstanding between us about the offset.
Leaving this problem aside first.... but think you are right that there is a resistor or something which makes the N-ch more conducting than the p-ch in stand by.
I'm wondering a bit about the bootstrapping network in your circuit.
First, I don't really understand the princip in which ways this can be used (only that this technics feeds back a portion of AC current)
Second, what extra swing will you reach with it? You are using 40V front end voltage which will be more than enough to get 23V peak (absolute) from the output (2V left over for the MOSFET's drain-source).
Perhaps you could help me to understand?
Greatings
Dirk
Let me start with some caveats. I don’t pretend to be any kind of expert on the subject of bootstrapping and I applied the technique shown in my schematic retrospectively, i.e. the power supplies and other details were calculated assuming no bootstrap. I also suspect that it is unlikely to be the implementation that Nelson uses, mainly because it provides only a small benefit in terms of additional signal swing.
The principle is fairly simple. The two 10k resistors (R15 and R19 in the schematic if we take the LHS) connected to the power supplies load the output of the FE which sees 5k, i.e. the two in parallel. This load requires current from the FE in the presence of a signal. More current from the FE causes more voltage drop across the FE source resistors (R13 and R16). The current required by the 5k load at full signal (say 23v) is 4.6ma which is equivalent to a voltage drop across R16 of roughly 1.5v (this being in addition to the DC value).
We bootstrap the two 10k resistors by isolating them somewhat from the supply by an additional pair of resistors (R14 and R17) and connecting via capacitors to the output. The effect of this is to superimpose the output signal on the power supply side of the 10k resistors. Given that the drive signal from the FE is extremely similar to the output signal provided by the bootstrap (just a little larger in magnitude), the effective resistance seen by the FE is increased substantially, i.e. it is driving into a load with much the same signal at its opposite end. In effect what we have done is separate the DC load from the AC.
Given that the FE is now operating into a much higher effective AC impedance, the current required from it is lower and hence we gain something just short of the 1.5v mentioned above. A side effect is also to increase the AC open loop gain of the amplifier due to less loading by the 10k resistors. I’m not sure whether this is a good thing or not in this design.
The F4 uses an additional trick, which is to modulate the FE power supply also by extending the bootstrap. This can provide still more voltage swing but I can’t see how to apply this to a bridge amplifier like the AX.5.
Ian.
The principle is fairly simple. The two 10k resistors (R15 and R19 in the schematic if we take the LHS) connected to the power supplies load the output of the FE which sees 5k, i.e. the two in parallel. This load requires current from the FE in the presence of a signal. More current from the FE causes more voltage drop across the FE source resistors (R13 and R16). The current required by the 5k load at full signal (say 23v) is 4.6ma which is equivalent to a voltage drop across R16 of roughly 1.5v (this being in addition to the DC value).
We bootstrap the two 10k resistors by isolating them somewhat from the supply by an additional pair of resistors (R14 and R17) and connecting via capacitors to the output. The effect of this is to superimpose the output signal on the power supply side of the 10k resistors. Given that the drive signal from the FE is extremely similar to the output signal provided by the bootstrap (just a little larger in magnitude), the effective resistance seen by the FE is increased substantially, i.e. it is driving into a load with much the same signal at its opposite end. In effect what we have done is separate the DC load from the AC.
Given that the FE is now operating into a much higher effective AC impedance, the current required from it is lower and hence we gain something just short of the 1.5v mentioned above. A side effect is also to increase the AC open loop gain of the amplifier due to less loading by the 10k resistors. I’m not sure whether this is a good thing or not in this design.
The F4 uses an additional trick, which is to modulate the FE power supply also by extending the bootstrap. This can provide still more voltage swing but I can’t see how to apply this to a bridge amplifier like the AX.5.
Ian.
Many thanks Ian,
really good and precise answer!
Think I've got it....
Often I have problems to think in AC... :-(
DC calculations are considerably easier
Perhaps you could help me again with the following:
Mr. Pass said that the caps are for bypassing the bias. It seems to me a bit that I was on the right way with the first bias bypassing variante of the AX-PP-SE.
But what will happen with the AC signal when the bypassing is done on this way, isn't it a short?
(Ich stehe auf dem Schlauch....Mist)
Whant to skin the cat!!!!!!!!!!!!!!!!!!!
(Hope this slogan has the right meaning for what I suppose)
Dirk
really good and precise answer!
Think I've got it....
Often I have problems to think in AC... :-(
DC calculations are considerably easier
Perhaps you could help me again with the following:
Mr. Pass said that the caps are for bypassing the bias. It seems to me a bit that I was on the right way with the first bias bypassing variante of the AX-PP-SE.
But what will happen with the AC signal when the bypassing is done on this way, isn't it a short?
(Ich stehe auf dem Schlauch....Mist)
Whant to skin the cat!!!!!!!!!!!!!!!!!!!
(Hope this slogan has the right meaning for what I suppose)
Dirk
noisefree said:
I agree. The bias generator is there to set the DC bias for the output stage. Ideally it should have zero impedance to AC. The TL431 will provide a pretty low AC impedance anyway but adding a series resistor will increase this and hence bypassing at AC with a capacitor is usually the way to go.
I see from the XA30.5 thread that Nelson doesn't use bootstrapping in any of his shipping product so we need to look elsewhere for techniques that can provide additional voltage swing.
Not sure about "skinning the cat"
Ian.
Ian.
Hi,
....that's why I have asked Mr. Pass for the purpose of the caps....
I would not be surprised when there is nothing really new implemented and I keep my mind: the only secret is a tricky bias circuit.
Mr. Pass said that his resistors are choosen for double duty. Perhaps it is possible that the gate - supply resistors have two jobs:
One duty is to source current to the VBC in stand by mode and the other is to get more voltage swing. I try to understand the behave of such a resistor in parallel to a device working in common source mode.
There has to be a way to get more swing (and perhaps more music swing, too )
....am waiting for my boards......hoping to get them next week.....
My hands are burning!
Dirk
....that's why I have asked Mr. Pass for the purpose of the caps....
I would not be surprised when there is nothing really new implemented and I keep my mind: the only secret is a tricky bias circuit.
Mr. Pass said that his resistors are choosen for double duty. Perhaps it is possible that the gate - supply resistors have two jobs:
One duty is to source current to the VBC in stand by mode and the other is to get more voltage swing. I try to understand the behave of such a resistor in parallel to a device working in common source mode.
There has to be a way to get more swing (and perhaps more music swing, too
)....am waiting for my boards......hoping to get them next week.....
My hands are burning!
Dirk
noisefree said:Hi,
....that's why I have asked Mr. Pass for the purpose of the caps....
I would not be surprised when there is nothing really new implemented and I keep my mind: the only secret is a tricky bias circuit.
Mr. Pass said that his resistors are choosen for double duty. Perhaps it is possible that the gate - supply resistors have two jobs:
One duty is to source current to the VBC in stand by mode and the other is to get more voltage swing. I try to understand the behave of such a resistor in parallel to a device working in common source mode.
There has to be a way to get more swing (and perhaps more music swing, too
And Nelson was kind enough to answer
I too noticed the "double duty" phrase but this could mean any number of things. My guess would be for standby and for loading the FE for better DC stability.
The "tricky bias circuit" is the interesting bit. To get as much as a 20% increase in power, this has to be doing something significant. I don't really see why the bias circuit should restrict the power in the first place, the "swing" is limited by the need to swing the gate above the PS voltage by ~Vgs of the output devices. I'm obviously missing something...
Ian.
Ian Macmillan said:
Based on the description that followed, I would say you understand
all there is to know. One item, and that is where I have seen
arguments about the detrimental effects of "regenerated
distortion" which would refer to the fact that the bootstrap
voltage contains the output signal and its distortion, thus
reinforcing that distortion at the output of the VAS. I don't
think this is a real problem in intelligently designed circuits,
but you will want to use sufficient resistance in the circuit
to avoid it.
Ian Macmillan said:
It's really pretty simple. By driving the center point of the bias
voltage, the front end can effectively swing about 4 more volts
on each peak. The resistors from each supply function to provide
current to the bias voltage generator and load the front
end with a known resistance.
On a circuit like this, that 4 extra volts can be the difference
between 100 watts and 144 watts.
Also, keep in mind that the output devices want some extra
volts left over between Drain and Source on peaks, something
on the order of 5 volts or so.
Yes, very, very likeable and noble from our master giving so many hints!
Many thanks!!!!!!
OK, all VBCs were wrong.....
New try......
Perhaps I am wrong again, but think there has to be a voltage divider in parallel to the voltage reference where each of these two resistors is bypassed with an elcap. In the center of this bypassed divider the front end (neg and pos output connected together) will drive it. Without the divider there is no trimming possibility for differential and absolute offset.
When the series resistor idea should work in this new approach, than there could be switchable resistors in parallel or in series to the known 10k/3W, to allow sourceing two different currents through the bias stage.
Regards
Dirk
Many thanks!!!!!!
OK, all VBCs were wrong.....
New try......
Perhaps I am wrong again, but think there has to be a voltage divider in parallel to the voltage reference where each of these two resistors is bypassed with an elcap. In the center of this bypassed divider the front end (neg and pos output connected together) will drive it. Without the divider there is no trimming possibility for differential and absolute offset.
When the series resistor idea should work in this new approach, than there could be switchable resistors in parallel or in series to the known 10k/3W, to allow sourceing two different currents through the bias stage.
Regards
Dirk
gl said:
Nelson also said that the bias circuit is not the same as the F4. Of course it depends on whether you interpret this as just a little different or something more radical. I would guess it is a derivation of the F4 circuit but I doubt that it uses AC coupling of FE to output stage as otherwise the feedback cannot correct for any DC releative offset.
Ian.
noisefree said:
I feel you may be on the right track with the bypassed voltage divider (which would also place the caps across the bias circuit to answer Graame's comment). I'm less convinced by the suggestion of needing to add additional trimming. Come to think of it, do we know that the FE is shutdown in standby or was this an assumption? I'm thinking that it may make more sense to lower the bias by shunting the bias control resistor and leaving the FE powered on to deal with relative DC offset. I suppose one might also have to short the input or similar to ensure the amplifier wouldn't pass any signal in this condition.
Ian.