NewYorkDave
Well-known member
[quote author="jhaible"]So I figure the LA2A designer - certainly not stupid - might have had low output impedance as a design goal more than high output voltage swing?
This is just a guess; maybe low output impedance was important for good low frequency response in connection with the transformer?[/quote]
I would tend to think that low output impedance and high output swing are important in this application. The output transformer is 5:1, right? If we take it as a given that the unit should be able to deliver at least +24dBM (20dB headroom over operating level, which is pretty standard) without clipping, it needs to swing 35V peak-to-peak across the load, and 175V pk-pk across the output transformer primary. That's a pk-pk current of 12mA (or a peak current of 6mA), assuming the transformer losses are negligible. The stock output stage idles at 4mA; and due to the high resistances in series with it, will start limiting before it can swing 175V pk-pk across the output xfmr. Of course, this "running out of steam" at high output levels could very well be part of the signature sound of the LA2A. I've never owned one or had one in my possession, nor have I built a clone, so I can only do a dry analysis from looking at schematics.
As I was Googling around just now, I found that Jensen engineers have had a similar thought, that the output stage of the LA2A could be "improved" by increasing its output capability. Their mod is published here. Notice that they also replace the output transformer with one of their own models having a 4.3:1 ratio. (Naturally, they want to sell transformers!). It's interesting to me that they settled on a 680-ohm plate resistor, just as I did.
If you read the patent, it appears that the intention of the inventor was to deliver maximum power into a difficult load (e.g., the capacitive load presented by a CRT). The patent is here.
I have no doubt that the designer of the LA2A knew what he was doing... But "second-guessing" and looking for ways to improve circuits are what keep this hobby interesting. Otherwise, we're just a bunch of clone builders, which to me is not terribly interesting at all. Sometimes the fun comes from finding that your "improvement" sucks and then the reasons for certain choices made by the original designer become crystal-clear. In those moments, I feel almost as if the designer is looking at me from the next world and grinning just a little bit :wink:
This is just a guess; maybe low output impedance was important for good low frequency response in connection with the transformer?[/quote]
I would tend to think that low output impedance and high output swing are important in this application. The output transformer is 5:1, right? If we take it as a given that the unit should be able to deliver at least +24dBM (20dB headroom over operating level, which is pretty standard) without clipping, it needs to swing 35V peak-to-peak across the load, and 175V pk-pk across the output transformer primary. That's a pk-pk current of 12mA (or a peak current of 6mA), assuming the transformer losses are negligible. The stock output stage idles at 4mA; and due to the high resistances in series with it, will start limiting before it can swing 175V pk-pk across the output xfmr. Of course, this "running out of steam" at high output levels could very well be part of the signature sound of the LA2A. I've never owned one or had one in my possession, nor have I built a clone, so I can only do a dry analysis from looking at schematics.
As I was Googling around just now, I found that Jensen engineers have had a similar thought, that the output stage of the LA2A could be "improved" by increasing its output capability. Their mod is published here. Notice that they also replace the output transformer with one of their own models having a 4.3:1 ratio. (Naturally, they want to sell transformers!). It's interesting to me that they settled on a 680-ohm plate resistor, just as I did.
Does this make any sense? Maybe the whole WCF thing is aiming for minimum output impedance rather than maximum output level?
If you read the patent, it appears that the intention of the inventor was to deliver maximum power into a difficult load (e.g., the capacitive load presented by a CRT). The patent is here.
I have no doubt that the designer of the LA2A knew what he was doing... But "second-guessing" and looking for ways to improve circuits are what keep this hobby interesting. Otherwise, we're just a bunch of clone builders, which to me is not terribly interesting at all. Sometimes the fun comes from finding that your "improvement" sucks and then the reasons for certain choices made by the original designer become crystal-clear. In those moments, I feel almost as if the designer is looking at me from the next world and grinning just a little bit :wink: