I do this for a living too, although my professional products are for space and aircraft applications, not consumer electronics. Working in that environment has meant rigorous worst case analysis on every design. I tend to carry that over in my audio products. For whatever reason, it seems audio designers fall into a line of thinking that if 3 mA is good, 6 mA must be better. But better why? You certainly don’t need it do drive the next stage. The voltage swing is miniscule and the small miller compensation capacitors require next to nothing to drive even well beyond audio frequencies.
To anyone that might build this variation, I encourage you to use the parts I specified. If you do, you will be assured of duplicating the results I achieved. Additionally, if you use the tubeshunter kit, these parts will fit on that board. There is no room on the board for a bias pot.
If you wish to experiment with these values, have fun. That’s what it’s all about.
To anyone that might build this variation, I encourage you to use the parts I specified. If you do, you will be assured of duplicating the results I achieved. Additionally, if you use the tubeshunter kit, these parts will fit on that board. There is no room on the board for a bias pot.
If you wish to experiment with these values, have fun. That’s what it’s all about.
It seems to me that you 1/2'd the slew rate and increased the input stage distortion 4 times, but I could be mistaken. Please don't second guess the original designer.
This is like some guy buying a Porsche 911 and modifying it seriously by changing the fuel mixture, or timing and saying that they have 'improved' the performance, without offering proof. This same person designs engines for a tractor factory. (nothing personal implied)
This is like some guy buying a Porsche 911 and modifying it seriously by changing the fuel mixture, or timing and saying that they have 'improved' the performance, without offering proof. This same person designs engines for a tractor factory. (nothing personal implied)
I mean nothing personal to you Mr. Curl. Not sure the same applies. I assure you, I hardly fit the tractor engine analogy.
I have not demeaned your original design. My application was different and my parts selection reflect that. Your design has proven itself by standing the test of time.
Ultimately the slew rate will be determined by the load capacitance, not the front end stage current (unless the output is unloaded). The front end voltage swing is so tiny (only a few millivolts) and the load capacitance so small that 3 mA is still well in excess of 30V/uS worst case.
The input stage distortion on my version will run these parts in a very linear part of their operating curve. The lower values may, or may not and likely not as linear as 3 mA will. Again, because the swing is so tiny, the distortion difference is unmeasurable. I didn't concentrate on that, it was setting the bipolar output current that concerned me. I have previously stated my reasoning on that.
My suggestions for improvement were aimed at the tubeshunter kit. I think they are worth considering for anyone interesting in building this kit. I didn't build an original JC-2 nor do I have one for reference. I have auditioned Mark Levinson products in the past, but that is as close as I got. I did build my version of the tubeshunter kit. It performs flawlessly in my intended application and the bias measurements were exactly as calculated. The second channel performance matched the first within a few millivolts. Distortion was well below .01% at audio frequencies on both channels. I'm not sure what aditional proof one need supply.
I have not demeaned your original design. My application was different and my parts selection reflect that. Your design has proven itself by standing the test of time.
Ultimately the slew rate will be determined by the load capacitance, not the front end stage current (unless the output is unloaded). The front end voltage swing is so tiny (only a few millivolts) and the load capacitance so small that 3 mA is still well in excess of 30V/uS worst case.
The input stage distortion on my version will run these parts in a very linear part of their operating curve. The lower values may, or may not and likely not as linear as 3 mA will. Again, because the swing is so tiny, the distortion difference is unmeasurable. I didn't concentrate on that, it was setting the bipolar output current that concerned me. I have previously stated my reasoning on that.
My suggestions for improvement were aimed at the tubeshunter kit. I think they are worth considering for anyone interesting in building this kit. I didn't build an original JC-2 nor do I have one for reference. I have auditioned Mark Levinson products in the past, but that is as close as I got. I did build my version of the tubeshunter kit. It performs flawlessly in my intended application and the bias measurements were exactly as calculated. The second channel performance matched the first within a few millivolts. Distortion was well below .01% at audio frequencies on both channels. I'm not sure what aditional proof one need supply.
The JC-2 line amp design was first designed 35 years ago for professional amplications. Originally it used larger To-5 output devices, with heatsinks, and operated at about 50ma. Later, when it was adopted to the Levinson JC-2 we experimented with smaller TO-92 parts, such as the 2N4401/4403. However, we ALWAYS used thermal epoxy around to parts to dissipate the heat.
I doubt that anyone lately is using heatsinks on the TO-92 devices, so they are deliberately 'starving' the output devices, in order to keep them from overheating.
The 'changes' brought out here, will only compromise the design, because you will have increased the input distortion, lowered the slew rate, and the lower open loop bandwidth (very important in the original design).
Please don't try to 'improve' the design until you understand it well.
By the way, the original design had a slew rate of 100V/us.
I doubt that anyone lately is using heatsinks on the TO-92 devices, so they are deliberately 'starving' the output devices, in order to keep them from overheating.
The 'changes' brought out here, will only compromise the design, because you will have increased the input distortion, lowered the slew rate, and the lower open loop bandwidth (very important in the original design).
Please don't try to 'improve' the design until you understand it well.
By the way, the original design had a slew rate of 100V/us.
Ouch.
30 milliamps is hardly starved operation.
I do understand the design, it's elegant, but not that complicated.
I have provided sound engineering rationale for every decision I made. My circuit has a higher open loop gain than your original. Your input stage may have had a tad over unity gain, but that's it. With a normal distribution of parts, it would have likely been closer to a gain of .75 (at best). The increase in the drain resitor in my approach increases the input stage gain to 7.5 or more.
It is debatable if your design with the new SK and SJ devices is higher, since the operating parameters you suggest are undefinded. With some parts it will be slightly higher, with others it will be slightly lower. As for what I suggest, the decreased current (and therefore gain) in the bipolars is compenstated by the increased gain provided by the input stage. So much for open loop gain.
Again, the output load sets the slew rate. Connect a 1 kHz squarewave to your amplifier circuit and adjust output for 5 V p-p. Use an oscilloscope to observe the output waveform first unloaded and then with a 1000 pF capacitor. Assuming yours remains stable, and it should, you'll see the change in slew rate. Crank on your cherished pot all you like and it won't change the output slew.
Do you really think I don't understand how it works?
30 milliamps is hardly starved operation.
I do understand the design, it's elegant, but not that complicated.
I have provided sound engineering rationale for every decision I made. My circuit has a higher open loop gain than your original. Your input stage may have had a tad over unity gain, but that's it. With a normal distribution of parts, it would have likely been closer to a gain of .75 (at best). The increase in the drain resitor in my approach increases the input stage gain to 7.5 or more.
It is debatable if your design with the new SK and SJ devices is higher, since the operating parameters you suggest are undefinded. With some parts it will be slightly higher, with others it will be slightly lower. As for what I suggest, the decreased current (and therefore gain) in the bipolars is compenstated by the increased gain provided by the input stage. So much for open loop gain.
Again, the output load sets the slew rate. Connect a 1 kHz squarewave to your amplifier circuit and adjust output for 5 V p-p. Use an oscilloscope to observe the output waveform first unloaded and then with a 1000 pF capacitor. Assuming yours remains stable, and it should, you'll see the change in slew rate. Crank on your cherished pot all you like and it won't change the output slew.
Do you really think I don't understand how it works?
I admire your capabilites as a designer Mr. Curl, but even you can't change physics.
The input stage current must charge the effective capacitance of the second stage which consists of the input capacitance plus the compensation capacitor times the gain of the second stage (basic miller effect). So reducing the bias current does affect the slew rate. Interetingly, your original circuit likely had a slew rate equal to mine since the idss of those original devices was so wide. An idss of 7 mA was the specified minimum.
You are correct that the feedback compensation capacitor rolls off the high frequencies, so it does have an effect. I did not use it in my circuit. I don't like to roll off the response in that manner. Compensation in that manner can destabilise an otherwise stable amplifier as it increases the gain at high frequency where there is likely to be a second order roll off in place.
Again, the purpose of either circuit is to amplify signals and not be a bench curiosity. The second stage gain is set by the feedback value and further reduced by the load. In the case of a cable, the capacitance presented will likely determine the ultimate slew rate. Your circuit is just as susceptable. The higher bias current will keep it a little higher, but it will still show up.
The input stage current must charge the effective capacitance of the second stage which consists of the input capacitance plus the compensation capacitor times the gain of the second stage (basic miller effect). So reducing the bias current does affect the slew rate. Interetingly, your original circuit likely had a slew rate equal to mine since the idss of those original devices was so wide. An idss of 7 mA was the specified minimum.
You are correct that the feedback compensation capacitor rolls off the high frequencies, so it does have an effect. I did not use it in my circuit. I don't like to roll off the response in that manner. Compensation in that manner can destabilise an otherwise stable amplifier as it increases the gain at high frequency where there is likely to be a second order roll off in place.
Again, the purpose of either circuit is to amplify signals and not be a bench curiosity. The second stage gain is set by the feedback value and further reduced by the load. In the case of a cable, the capacitance presented will likely determine the ultimate slew rate. Your circuit is just as susceptable. The higher bias current will keep it a little higher, but it will still show up.
The gain increases as the bias current increases, no arguement there.
The values I gave will give repeatable results for the BL suffix devices, and I posted this for anyone considering building the tubeshunter board. Using other values for bias resistors and drain resistors will work, I have never argued that. I even encouraged experimentation.
If you want to measure the devices and optimize the bias for each set, that will probably lead to a very minor improvement in the circuit performance. I doubt it could be measured or heard.
If you want to build the kit as is, I said it would work, but it really won't be optimized without at least matching the devices.
I haven't heard a JC-2 preamp for a long time so I can't comment on it's sound. I've listened to what I built and it is fine for my needs.
Mr. Curl, I have appreciated your comments on this thread.
The values I gave will give repeatable results for the BL suffix devices, and I posted this for anyone considering building the tubeshunter board. Using other values for bias resistors and drain resistors will work, I have never argued that. I even encouraged experimentation.
If you want to measure the devices and optimize the bias for each set, that will probably lead to a very minor improvement in the circuit performance. I doubt it could be measured or heard.
If you want to build the kit as is, I said it would work, but it really won't be optimized without at least matching the devices.
I haven't heard a JC-2 preamp for a long time so I can't comment on it's sound. I've listened to what I built and it is fine for my needs.
Mr. Curl, I have appreciated your comments on this thread.
We covered JFETs to some degree while I was engineering school. Most of my experience with them came a few years later when I used them in very low noise signal processing applications. I have several engineering texts, but I think I learned the most from a Siliconix application note in one of their data books.
I doubt the book is still around (I have mine) but the article is online and may be found here: http://www.vishay.com/docs/70595/70595.pdf
It is a very comprehensive yet concise article on device biasing.
Enjoy.
For those of you that want to skip ahead, they recommend biasing devices at 60% of their idss. But there are exceptions of course.
I doubt the book is still around (I have mine) but the article is online and may be found here: http://www.vishay.com/docs/70595/70595.pdf
It is a very comprehensive yet concise article on device biasing.
Enjoy.
For those of you that want to skip ahead, they recommend biasing devices at 60% of their idss. But there are exceptions of course.
I met Ed Oxner at a Siliconix seminar back when they were promoting power MOSFETs. He was a nice guy and even gave me a couple samples of one of their 400V TO-3 devices. I'm sure he would not remember me. I hope he is in good health.
I don't know how many, or if any of the parts mentioned in the article are still available, but it is still a good read for how JFETs operate, high gm or not. The Toshiba devices have the similiar curves, just different values on the axis.
I don't know how many, or if any of the parts mentioned in the article are still available, but it is still a good read for how JFETs operate, high gm or not. The Toshiba devices have the similiar curves, just different values on the axis.