Really good information!
Thanks Graeme
I have detected that I told you nonsense about the dissipation of a TO-92 device, sorry....
Have downloaded the TL431 datasheet from ST and it seems that it will be no problem for it to dissipate 200mW. So it can run without a parallel resistor network.
When you can renounce the faster warm up you don't need a VBC which drifts a bit downward to eliminate the rising output current bias.
It should be possible to build the VBC only with the TL431 + trimming network and a resistor of about 22Ohm in series with it (to reduce the GS-voltage in standby about the lower current of the 10k resistors)
My principle question to the experts is the following:
What will happen to the front end when its supply is shut down but the FE-output drains will see the +/- 4V bias voltage? Will that be a problem and destroy the FE Fets?
Regards
Dirk
Thanks Graeme
I have detected that I told you nonsense about the dissipation of a TO-92 device, sorry....
Have downloaded the TL431 datasheet from ST and it seems that it will be no problem for it to dissipate 200mW. So it can run without a parallel resistor network.
When you can renounce the faster warm up you don't need a VBC which drifts a bit downward to eliminate the rising output current bias.
It should be possible to build the VBC only with the TL431 + trimming network and a resistor of about 22Ohm in series with it (to reduce the GS-voltage in standby about the lower current of the 10k resistors)
My principle question to the experts is the following:
What will happen to the front end when its supply is shut down but the FE-output drains will see the +/- 4V bias voltage? Will that be a problem and destroy the FE Fets?
Regards
Dirk
I agree with you about the TL431 although the datasheet doesn't give a derating value for use in hot environments like the inside of a class A amplifier case.
I don't follow the 22R resistor in series with the TL431 and trimming components though. Won't this increase the voltage across the combination? I think you want something like a PTC thermistor in series that will increase the bias voltage as the current through it increases, e.g. when coming out of standby. Perhaps I misunderstand your intentions?
Ian.
I don't follow the 22R resistor in series with the TL431 and trimming components though. Won't this increase the voltage across the combination? I think you want something like a PTC thermistor in series that will increase the bias voltage as the current through it increases, e.g. when coming out of standby. Perhaps I misunderstand your intentions?
Ian.
Very nifty idea with the PTC element.
Sadly I cannot say if the thermistor I can see is a NTC or a PTC type.
I think that the PTC and the series resistor idea could work.
For no misunderstandings short explanation:
Referencevoltage (of TL431) is set to say 7,5V.
- When it is driven by the front end say 30mA are running through it and the 22R resistor (27mA by FE and 3mA by 10k resistors) than the complete voltage bias is set to 8,16V
- When front end is shut down it is only driven by the 3mA - means 66mV voltage drop about the 22R and voltage bias is set to 7,566V
...that's it...
.......my horrible english.......forgive me.......try my best!
Dirk
Sadly I cannot say if the thermistor I can see is a NTC or a PTC type.
I think that the PTC and the series resistor idea could work.
For no misunderstandings short explanation:
Referencevoltage (of TL431) is set to say 7,5V.
- When it is driven by the front end say 30mA are running through it and the 22R resistor (27mA by FE and 3mA by 10k resistors) than the complete voltage bias is set to 8,16V
- When front end is shut down it is only driven by the 3mA - means 66mV voltage drop about the 22R and voltage bias is set to 7,566V
...that's it...
.......my horrible english.......forgive me.......try my best!
Dirk
Hi Graeme,
Have read your transformer reply a bit late!
Have a small hint for you!
I know to which picture you refer with the transformer.
There are approx eight wires but there is not enough space on the supply board for connecting them all. Isn't it possible that the used transformer has seperate winding and they are not used or better: they are connected in series. The sinks which you see are not for regulators but for the rectifier doubble diodes. Also there is no connector on the mainboard for this extra voltage. The two "triple screw contacts" are reserved for other functions. Sure, there can be soldering contacts under the UGS module, but will Mr. Pass do that? ...I don't think so - the installing of the board would be more complex!
And thanks for the Threshold bias idea for more information about the possibilities!
Best Regards
Dirk
Have read your transformer reply a bit late!
Have a small hint for you!
I know to which picture you refer with the transformer.
There are approx eight wires but there is not enough space on the supply board for connecting them all. Isn't it possible that the used transformer has seperate winding and they are not used or better: they are connected in series. The sinks which you see are not for regulators but for the rectifier doubble diodes. Also there is no connector on the mainboard for this extra voltage. The two "triple screw contacts" are reserved for other functions. Sure, there can be soldering contacts under the UGS module, but will Mr. Pass do that? ...I don't think so - the installing of the board would be more complex!
And thanks for the Threshold bias idea for more information about the possibilities!
Best Regards
Dirk
Having thought a little more about your series resistor idea, I think you are on the right track. It makes sense and also suggests what the thermistor is there for - temperature compensation for this resistor. This would require an NTC device in parallel with the series resistor. Moreover, if the values are chosen correctly, it should be possible to arrange for the initial voltage on full power up to be a little greater than steady state (i.e. before the NTC warms up) and hence give you the desired faster warm up of the output stage. Well, it is a theory anyway!
Ian.
Ian.
You are right. In my published bias network I arranged the thermistor network in parallel to the voltage reference series resistor network for taking the half of the consuption. But this is not necessary - the TL431 can endure the whole current. So it will be easier to leave out this parallel network and replace the series resistor with a NTC-resistor network. You said a NTC in parallel with the s.-resistor, perhaps you need one more resistor to bring the NTC in the right working range.
We have to find that out....
(An interesting fact: search the thread "Thermistor question for Nelson.")
A big question mark are the elcaps....?
I'm not sure that there is really bootstrapping applied.
Have done some calculations with the facts from the manuals:
It seems that the specified values there can be reached with the high voltage losses (say 10V or so) of the front end and without some kind of extra swing...
...but you can use elcaps in the bias circuit for fixing some voltages to get more constancy.......just an idea....
Dirk
We have to find that out....
(An interesting fact: search the thread "Thermistor question for Nelson.")
A big question mark are the elcaps....?
I'm not sure that there is really bootstrapping applied.
Have done some calculations with the facts from the manuals:
It seems that the specified values there can be reached with the high voltage losses (say 10V or so) of the front end and without some kind of extra swing...
...but you can use elcaps in the bias circuit for fixing some voltages to get more constancy.......just an idea....
Dirk
I’ve been thinking some more about the single power supply idea for the whole amplifier and I am finding it increasingly less plausible unless some kind of bootstrap technique can increase the voltage swing of the FE significantly. I can see no way to achieve this (but this could just be down to my lack of vision) with a UGS front end. The need for increased voltage on the FE comes from several causes: the voltage drop across the driver FET (common source VAS) source resistor, Vgs for the output devices plus the voltage drop across their source resistors. This amounts to some 9 – 10v steady state and increases with signal. Increasing the PS voltage for the whole amplifier by this amount would lead to (even more) excessive heat dissipation.
Where bootstrapping can help a little, is in increasing the effective AC load seen by the FE. This can be achieved by bootstrapping the 10K resistors between the bias generator and the PS rails. This has the effect of reducing the signal current required from the FE and hence the voltage drop across the source resistor of the driver FETs. Intriguingly I calculate this to be about a 20% saving so perhaps this is could be what Nelson is referring to in the manual? This kind of bootstrap also increases the open loop gain of the amplifier which may or may not be a good thing depending upon your viewpoint.
My current theory is that a voltage doubler is used (no extra transformer windings required) and that some sort of regulator may well be hidden on the part of the PS board that is under the motherboard (the one on which the UGS5 sits). This rather implies that one of the 3 way terminals must supply the power as opposed to being there only for the relay or whatever. Of course I may be way off base here...
Temperature compensation of the bias resistor you mention may require an additional series resistor. It all depends on what characteristic you desire and the value of the resistors and NTC thermistor.
I’ll draw up a schematic when I have a spare moment which should make things clearer.
Ian.
Where bootstrapping can help a little, is in increasing the effective AC load seen by the FE. This can be achieved by bootstrapping the 10K resistors between the bias generator and the PS rails. This has the effect of reducing the signal current required from the FE and hence the voltage drop across the source resistor of the driver FETs. Intriguingly I calculate this to be about a 20% saving so perhaps this is could be what Nelson is referring to in the manual? This kind of bootstrap also increases the open loop gain of the amplifier which may or may not be a good thing depending upon your viewpoint.
My current theory is that a voltage doubler is used (no extra transformer windings required) and that some sort of regulator may well be hidden on the part of the PS board that is under the motherboard (the one on which the UGS5 sits). This rather implies that one of the 3 way terminals must supply the power as opposed to being there only for the relay or whatever. Of course I may be way off base here...
Temperature compensation of the bias resistor you mention may require an additional series resistor. It all depends on what characteristic you desire and the value of the resistors and NTC thermistor.
I’ll draw up a schematic when I have a spare moment which should make things clearer.
Ian.
noisefree said:
As long as it is your interpretation of the circuit, as opposed to a direct copy of the actual circuit, then I don't see it will be a problem based on past experience. If you are worried, why not send an email to Nelson and explicitly ask him? This forum exists to experiment with ideas originating with the master just as long as we are not breaching his copyright or planning anything commercial.
Ian.
Seems I get to go first... Here is my latest interpretation of the AX.5 with bootstrap and bias compensation. Don't get too hung up on the exact values as I have put this together way too quickly and haven't done any real checking. I've also excluded any standby details but this really just amounts to switching off the front end supply.
As usual, comments and opinions are very welcome.
Ian.
As usual, comments and opinions are very welcome.
Ian.
Very nice piece of work Ian.
For myself I would agree with Tazzz' take on the overall topology.
On the detail side of things VR2 needs to be 100 ohms instead of 220. It has twice the current flowing through it as the relative offset pot. I found this out the hard way when I built a similar circuit and had to figure out why I had the abs offset adjustment pot all the way over and still hadn't arrived at zero volts.
I also still believe strongly that the front end has only the JFETs for voltage gain followed by current mirrors to provide level shifting and current gain.
Other than that, it looks like time to start building. Have you found any JFETs following that issue with the GB?
Cheers,
Graeme
For myself I would agree with Tazzz' take on the overall topology.
On the detail side of things VR2 needs to be 100 ohms instead of 220. It has twice the current flowing through it as the relative offset pot. I found this out the hard way when I built a similar circuit and had to figure out why I had the abs offset adjustment pot all the way over and still hadn't arrived at zero volts.
I also still believe strongly that the front end has only the JFETs for voltage gain followed by current mirrors to provide level shifting and current gain.
Other than that, it looks like time to start building. Have you found any JFETs following that issue with the GB?
Cheers,
Graeme
OK, ok.....
sorry I'm very late this evening. Had some promlems to convert the picture and had to install the acrobat destiller first.... :-(
Very, very much possibilities for reducing the size and resolution
Now here is the schematic...
Some parts are not connected. It's only the relay coil and the switching parts for it.
Will test this circuit, but need more time for doing it. Have ordered parts again and found a possibility where I can get the long "photo positiv printed boards" for creating the output stage.
Mr. Pass, many thanks for allowing me to publish it!!!
Two sentences to you:
There is also a great review of the INT-150 in the LP magazin (02/2008), perhaps you will take it in the very long review list on Passlabs.
Second: Is it possible that the specified max. output voltage of the INT-150 in the manual is not correct - looks more like the rail voltage...
Thanks again...
Best regards to all
Dirk
sorry I'm very late this evening. Had some promlems to convert the picture and had to install the acrobat destiller first.... :-(
Very, very much possibilities for reducing the size and resolution
Now here is the schematic...
Some parts are not connected. It's only the relay coil and the switching parts for it.
Will test this circuit, but need more time for doing it. Have ordered parts again and found a possibility where I can get the long "photo positiv printed boards" for creating the output stage.
Mr. Pass, many thanks for allowing me to publish it!!!
Two sentences to you:
There is also a great review of the INT-150 in the LP magazin (02/2008), perhaps you will take it in the very long review list on Passlabs.
Second: Is it possible that the specified max. output voltage of the INT-150 in the manual is not correct - looks more like the rail voltage...
Thanks again...
Best regards to all
Dirk