Re: Re: Re: Re: LME49830 on higher rails than output devices?
Yes PSRR is a better term. I was referring to common mode signals on the power supply rails so, in this case, they are the same thing.
I said the regulator "in effect" introduces another pole into the amplifier. It obviously doesn't directly introduce a real pole, but the net effect, in my experience, is similar.
The output impedance of the regulator rises at higher frequencies (just like any amplified feedback loop). And the open loop bandwidth of the Borbely amp is quite high. So those two things can interact to create high frequency stability problems well above the audio range. Amps with Mosfet outputs and high open loop gain and bandwidth can be very sensitive to even small things creating serious stability problems. National in their own App Notes have examples of this.
I agree, in theory, it should be possible to design the regulator and amp such that it should work OK. Obviously Borbely thought he did just that. But with the stability of Mosfet output amplifiers often being as much "magic" (some would say "art") as science, I think the regulators just complicate things and are not likely to offer any better performance when used with the LM4702 or LME498xx.
I long ago retired the Borbely amp (although I do still have it). It uses the original Hitachi metal TO3 output devices as used in the Hafler DH-200/220/500 amps. It's possible the particular grounding scheme, etc. of my amp was making it more sensitive to the regulators. But it was clear it was unstable with the regulators and very stable without them.
panson_hk said:
Yes PSRR is a better term. I was referring to common mode signals on the power supply rails so, in this case, they are the same thing.
I said the regulator "in effect" introduces another pole into the amplifier. It obviously doesn't directly introduce a real pole, but the net effect, in my experience, is similar.
The output impedance of the regulator rises at higher frequencies (just like any amplified feedback loop). And the open loop bandwidth of the Borbely amp is quite high. So those two things can interact to create high frequency stability problems well above the audio range. Amps with Mosfet outputs and high open loop gain and bandwidth can be very sensitive to even small things creating serious stability problems. National in their own App Notes have examples of this.
I agree, in theory, it should be possible to design the regulator and amp such that it should work OK. Obviously Borbely thought he did just that. But with the stability of Mosfet output amplifiers often being as much "magic" (some would say "art") as science, I think the regulators just complicate things and are not likely to offer any better performance when used with the LM4702 or LME498xx.
I long ago retired the Borbely amp (although I do still have it). It uses the original Hitachi metal TO3 output devices as used in the Hafler DH-200/220/500 amps. It's possible the particular grounding scheme, etc. of my amp was making it more sensitive to the regulators. But it was clear it was unstable with the regulators and very stable without them.
the output gain will fall to somewhere in the range 10 to 20 at very high currents.bobodioulasso said:
If the driver is feeding a 2pair then it will see currents of the order of 10% to 20% of the peak transient output current. The driver must be able to pass that in the short term when high Vce is present.
The driver gain will also have fallen at these very high output currents.
Maybe in the range 20 to 30. That in turn determines the peak transient current that the pre-driver must supply. guess about 0.2% to 1% of the peak output transient current.
These requirements demand different devices.
A small output device (100W to 150W) could double up as a driver, but certainly not as a pre-driver.
it depends on the design. all the devices are interlinked.
You need to go through the amp output stage going from the worst case speaker current back through each stage checking the graphs of gain vs current and finding the base current, Then use that to determine the previous stage all the way back to the VAS.
Once you have candidates, enter the data into an SOAR spreadsheet with operating temperature and find what DC currents are permissible, what long term AC currents and short term AC currents for each device.
You need to go through the amp output stage going from the worst case speaker current back through each stage checking the graphs of gain vs current and finding the base current, Then use that to determine the previous stage all the way back to the VAS.
Once you have candidates, enter the data into an SOAR spreadsheet with operating temperature and find what DC currents are permissible, what long term AC currents and short term AC currents for each device.
2sc 5174 as pre ?
http://www.datasheetcatalog.com/datasheets_pdf/2/S/C/5/2SC5174.shtml
2sc3263 as drivers ?
http://pdf1.alldatasheet.com/datasheet-pdf/view/37962/SANKEN/2SC3263.html
these driving four pairs of 3264 in the RB1090 Rotel (96v rails)
http://mesnotices.fr/manuel-notice-mode-emploi/ROTEL/RB-1090-3-_E
http://www.datasheetcatalog.com/datasheets_pdf/2/S/C/5/2SC5174.shtml
2sc3263 as drivers ?
http://pdf1.alldatasheet.com/datasheet-pdf/view/37962/SANKEN/2SC3263.html
these driving four pairs of 3264 in the RB1090 Rotel (96v rails)
http://mesnotices.fr/manuel-notice-mode-emploi/ROTEL/RB-1090-3-_E
Bobodioulasso, AndrewT brings up some very valid points about SOA, drive currents, etc. In a very high power amplifier such as you're proposing things get more difficult on many levels.
As your output transistors can only handle 75 volt rails, have you considered MOSFETs and switching to the LME49830? I suspect your transformers (for 90 volt rails) are worth more than your Sanken output transistors? Or, put another way, it might be cheaper to buy new output transistors and the LME49830's than new 1.2 KVA transformers?
MOSFETs tend to have a much more usable SOA at high voltages, and driving them is just a matter of having enough drive to get the gate capacitance related slew rate to an acceptable level (and I'm not sure I personally would even worry about a full power bandwidth to or beyond 20 khz with the power levels you're talking about).
Although I would still be cautious about 90 volt rails with the LME498xx parts. A 15% AC voltage surge (which are not that uncommon) might wipe out the IC and take output devices and possibly your speakers with it. It might be possible to decouple the rails to the LME498xx with resistors and use some sort of over voltage protection on those rails.
And the most simple definition of Class A is simply NONE of the transistors ever turn off in normal operation. In a typical push-pull Class A stage if you try to draw enough current it will fall back to Class B. But if you can design for the maximum real world current, it will always stay in Class A in normal operation. For driver stages feeding bipolar transistors, calculating the actual real-world current required can be challenging as there are many variables to consider. MOSFETs make things much more simple
As your output transistors can only handle 75 volt rails, have you considered MOSFETs and switching to the LME49830? I suspect your transformers (for 90 volt rails) are worth more than your Sanken output transistors? Or, put another way, it might be cheaper to buy new output transistors and the LME49830's than new 1.2 KVA transformers?
MOSFETs tend to have a much more usable SOA at high voltages, and driving them is just a matter of having enough drive to get the gate capacitance related slew rate to an acceptable level (and I'm not sure I personally would even worry about a full power bandwidth to or beyond 20 khz with the power levels you're talking about).
Although I would still be cautious about 90 volt rails with the LME498xx parts. A 15% AC voltage surge (which are not that uncommon) might wipe out the IC and take output devices and possibly your speakers with it. It might be possible to decouple the rails to the LME498xx with resistors and use some sort of over voltage protection on those rails.
And the most simple definition of Class A is simply NONE of the transistors ever turn off in normal operation. In a typical push-pull Class A stage if you try to draw enough current it will fall back to Class B. But if you can design for the maximum real world current, it will always stay in Class A in normal operation. For driver stages feeding bipolar transistors, calculating the actual real-world current required can be challenging as there are many variables to consider. MOSFETs make things much more simple
I looked at the Rotel 1090 schematic and apparently your output devices will work with your 90 volt rails. I was going by the earlier post advising 75 volt max.
Why don't you just copy the Rotel output stage design? That solves lots of problems, including drive currents, bias tracking (the Vbe multiplier Q617) and, SOA protection. Getting stable bias with good thermal tracking from a DIY design can be challenging.
SOA protection/current limiting in high power amps can also be tricky to design and test. When testing you're usually pushing the SOA of the output devices near the limit. So if your design is wrong, or the protection circuit causes instability during limiting, you can blow up the amp. Rotel has sorted all those issues out for you already. So you can use their protection circuit (Q753/755) if you want and be fairly confident it will work well.
While I don't have personal experience with the 1090, I do have experience with several of their smaller power amps. And I've been impressed. The designers seem to know what they're doing and their conventional (non Class D) amps are generally very well respected.
bobodioulasso said:
MCM (in the USA) has 3 of the 4. They're only missing the 2SA1932. You might also be able to get those from Rotel and the rest from MCM:
http://mcmelectronics.com/product/DISTRIBUTED-BY-MCM-SANKEN-/2SC3263
MCM has the 2SA1837 and 2SC4793 which are high voltage complimentary audio drivers similar to the 2SA1932 and 2SC5174 pair used in the Rotel but higher power. Check the datasheets closely, but I believe they might be good subsitutes for the Rotel output stage.
bobodioulasso said:
hi,
i mean that you do not have to use mica insulators when mounting on heatsinks....
the stocks that i have on hand were "pulls" from discarded boards from an export company we bought surplus, they were production over runs....these babies drive the output trannies directly...
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