Thank you Paul, I was hoping for a post from you.
These are Christer's models for said devices. I renamed them to give him credit and so I know what models I'm using. Sorry, I forgot to include them.
Attached is the simulation file with the models included in the netlist. Currently, it is set up to simulate with 10k source impedance, so to remove the source impedance just set parameter Rser (the directive line I added) to zero.
TOINO:
So, I'm guessing that the voltage sensing is meant to detect and cancel the effects of the base current of the output transistors in order to linearize the output stage?
- keantoken
mfc said:
These are Christer's models for said devices. I renamed them to give him credit and so I know what models I'm using. Sorry, I forgot to include them.
Attached is the simulation file with the models included in the netlist. Currently, it is set up to simulate with 10k source impedance, so to remove the source impedance just set parameter Rser (the directive line I added) to zero.
TOINO:
So, I'm guessing that the voltage sensing is meant to detect and cancel the effects of the base current of the output transistors in order to linearize the output stage?
- keantoken
Keantoken,
I like your `Allison-Keane' variation to the standard Allison circuit, which overcomes the need to use large emitter resistors. However quite a bit of LTspice simulation leads be to believe that your design, and a very similar variation that I came up with, will both suffer from two problems:
- Temperature changes in the Allison bias transistors effect the output bias currents considerably and this is made worse by the small emitter resistors of the output transistors. Your design would probably benefit in using one of the THAT packages that contain 2 NPN and 2 PNP transistors. My variation was even worse.
- The transition from Class A to Class B is very abrupt, and hence will almost certainly produce high order harmonics.
Inspired by your `Allison-Keane' design I tried to come up with a design that would allow small emitter resistors but give a gradual transition into Class B.
The attached design uses large resistors (R3, R4) to set the bias current, and then bypasses these with diodes to allow high currents without the large voltage drop across R3, R4. The diodes cause a very gradual transition into Class B, with gradual turn off, but not `Non Switching Class B style (but almost). Note that Q3,Q4 will need to be thermally bonded to D1,D2.
So far this design appears to be promising, and I hope to build and test it some time next month (my day job is getting in the way at the moment). I fear that I may have swapped one set of problems for another, namely diode switching harmonics destroying the THD figures.
I have some questions that maybe someone can answer ...
- Has anyone tried putting diodes across emitter resistors like this before, and if so, do they generate unacceptable switching harmonics?
- Is a MUR820 the best choice for the diode in this application?
Paul Bysouth, March 2009.
ps - Sorry about the delay in posting this, however my Internet router power supply decided to break just before I pressed the submit key, and as it was midnight I went to bed. I've spent this morning fixing it. I hope my cut and paste above works.
pb.
I like your `Allison-Keane' variation to the standard Allison circuit, which overcomes the need to use large emitter resistors. However quite a bit of LTspice simulation leads be to believe that your design, and a very similar variation that I came up with, will both suffer from two problems:
- Temperature changes in the Allison bias transistors effect the output bias currents considerably and this is made worse by the small emitter resistors of the output transistors. Your design would probably benefit in using one of the THAT packages that contain 2 NPN and 2 PNP transistors. My variation was even worse.
- The transition from Class A to Class B is very abrupt, and hence will almost certainly produce high order harmonics.
Inspired by your `Allison-Keane' design I tried to come up with a design that would allow small emitter resistors but give a gradual transition into Class B.
The attached design uses large resistors (R3, R4) to set the bias current, and then bypasses these with diodes to allow high currents without the large voltage drop across R3, R4. The diodes cause a very gradual transition into Class B, with gradual turn off, but not `Non Switching Class B style (but almost). Note that Q3,Q4 will need to be thermally bonded to D1,D2.
So far this design appears to be promising, and I hope to build and test it some time next month (my day job is getting in the way at the moment). I fear that I may have swapped one set of problems for another, namely diode switching harmonics destroying the THD figures.
I have some questions that maybe someone can answer ...
- Has anyone tried putting diodes across emitter resistors like this before, and if so, do they generate unacceptable switching harmonics?
- Is a MUR820 the best choice for the diode in this application?
Paul Bysouth, March 2009.
ps - Sorry about the delay in posting this, however my Internet router power supply decided to break just before I pressed the submit key, and as it was midnight I went to bed. I've spent this morning fixing it. I hope my cut and paste above works.
pb.
Attachments
PaulBysouth said:
Can you attach the simulation file and any models you used that may be hard to find?
All that I know about the diodes is that if they have high Cj, they may cause stability problems in the Allison. Also, Distortion may benefit if you decrease R9 and R10 and increase C9 and C11, as these resistors cause a miniscule (probably insignificant in most cases) amount of distortion. Also, with some carefully placed caps you may be able to quiet any switching harmonics.
What about these THAT packages? I Googled and found nothing.
About temperature stability, the bias wanders about 47mA per 10 degrees Celsius, which is 4.7mA per degree Celsius (according to simulation). About the only thing this affects is clipping point, which won't matter much if it is being operated within its limits.
There wouldn't be any bias wander if the Allison was thermally insulated. I'll bet this could be done without a fan but I have no experience here.
- keantoken
h_a said:
The ultrasonic distortion products can intermodulate with audible fundamentals to produce IMD in the audible band. So I believe you need to measure distortion products at high fundamentals, or measure IMD. Either one provides valuable information and is certainly not of academic interest only.
THAT package
keantoken,
The 'THAT Series' transistors are monolithic array of pnp/npn transistors. These are new in the market, catered for tightly matched applications.
http://www.thatcorp.com/300-series_Matched_Transistor_Array_ICs.html
-Atiq
keantoken,
The 'THAT Series' transistors are monolithic array of pnp/npn transistors. These are new in the market, catered for tightly matched applications.
http://www.thatcorp.com/300-series_Matched_Transistor_Array_ICs.html
-Atiq
Attachments
keantoken,
I`m a little bit disappointed. Time rolls by but nothing convincing so far. I`ve been hoping that you would come up with something really great. Take all the necessary precautions in order to limit the damage and losses this experiment will inevitably bring about and get going. The guru says, achievement is born of sacrifice (manifesting infinite wisdom).
I`m a little bit disappointed. Time rolls by but nothing convincing so far. I`ve been hoping that you would come up with something really great. Take all the necessary precautions in order to limit the damage and losses this experiment will inevitably bring about and get going. The guru says, achievement is born of sacrifice (manifesting infinite wisdom).
Lumba Ogir said:
Of course! Infinite wisdom, my friend. I think we must all be patient. I'm a fast learner but I'm 15 years old, I've only been at electronics seriously for 4 years, and even then mostly on the simulator (which I think has actually been quite an advantage save for a few quite outstanding issues).
If no one so far has noticed, I'm pretty much on a quest to get as low distortion as possible but also to have a nice stereo amp as a souvenir. Class A should probably make this easier, and I believe this output stage is a good start. I do worry about what effects interference might have as a result of the high input impedance of my enhanced version. I will have to start point-to point soldering things together as currents get too high for a breadboard.
My immediate struggle has to do with the Cdom cap. It works fairly well but tends to destroy 20KHz THD if you use one large enough to get the recommended -100 degree at 0db OLG phase plot.
My aim at the end is to get this together with a Rush cascode at the input, to give some nice 2nd harmonics in places where distortion is unavoidable. In simulation I've noted that the Rush Cascode also tends to be more stable than the common LTP, which in the end means that we can have higher OLG and lower THD. What THD there will be will mainly be 2nd harmonic, supposedly pleasant sounding. Unfortunately, this is theoretical (the part about me getting it to work). So I cannot make guarantees.
If you are interested, this is what I am currently playing with in the simulator. It simulates unimaginably well at full output power if you can just get it stable (EDIT: the 100n cap across the speaker is overkill, maybe I can be safe without it)! This is also an excellent example of how a large Cdom can magnify LTP nonlinearity at high frequencies. I have tried a zillion different ways to get better stability without loading the LTP down at high frequencies. The only time I have had success with this, is when I was simulating with the Rush cascode. But I've done enough greasework to experience the difference between simulation and reality. In general, my circuits were more stable in real life than in the simulator. If I want to use the simulator exclusively with real life, I will have to ensure that they are comparable.
I will next try simulating with the Rush cascode instead of the normal LTP and see if I have more success. But until I get a decent power supply I can't start experimenting with the MJL devices.
- keantoken
Attachments
Lumba Ogir said:
Yes, I am very independent but I assure you that I consider everything I hear and try to look at things without bias. Maybe it's folly but it's my learning experience and I've come to understand that's not a bad thing. Even if you don't think you will be listened to, I'm sure there is someone else who can benefit from what you have to offer.
So if you have something interesting of yours to post, by all means... I will not be the only one to benefit. If you do, I will see it and do my best to understand. But my own learning method comes in to play here and I may not act in a predictable manner...
I'm trying to breadboard a 24V version of the enhanced Allison. If I get something done, I'll post. Until then, this thread if free for anyone who has ideas or suggestions relating to the Allison.
I do thank everyone for their suggestions and comments, whether or not I decide to follow through with advice. I always enjoy mental stimulation.
- keantoken
atiq19 said:
These are very good indeed but the cost is not.
You get what you pay for I suppose. Although they have high ft and very low noise, the current gain is only about 80-90. The ones I have measure about 86. This and the fact that Vce is only 36V is why I used them in a constant power differential. Excellent results, btw. Try to keep Ic max to a few milliamps or so. Also there is no need for emitter degeneration. After all, that is what you pay for, eh? I'm not sure I could have gotten these waveforms from this ( TOP) ugly looking thing without using them as the amplifying devices. (Output is actually a bridged output so the top photos show each output. Bottom ones is what the load sees.) If they just weren't so darned expensive, I would use them for just about everything possibe. Hey, they still don't compare in price when buying a duel matched J-fet....if you can find them.
Hi
Using SMD's, cheap vero-board is a good option. The SMD metal film resistors are very cheap and are good quality. I used 603 size for most of this circuit but 1206 works good two. If your circuit is symmetrical, SOT-23 transistors are handy because if you mount half of them upside down, the circuit layout is symmetrical. It is very easy to manipulate as well. If you want to disconect a pin to test a component, just simply break the solder bridge. If you screw up, it's easy to correct. Of course you have to be aware of the voltage and power limitations. If you have a little bit of soldering skills you can route (sculpt) complex circuits quite effectively......for proto-typing. Between sculpting the bottom side and wire routing of the top, just about any circuit can be built this way for relatively cheap. While it seems like a lot of solder is used, I re-use the solder from the old PCB's and projects I've made like this to build the new ones. For example1 and example2, this is the VAS PCB of the previous circuit. Since I have figured out this method, I use it extensively for new circuits to test because bread boards suck, especially when they have been used a lot. Now if each contact inside the breadboard were made of gold plated steel, it would probably work really well. But, then you couldn't afford it and neither could I.
Using SMD's, cheap vero-board is a good option. The SMD metal film resistors are very cheap and are good quality. I used 603 size for most of this circuit but 1206 works good two. If your circuit is symmetrical, SOT-23 transistors are handy because if you mount half of them upside down, the circuit layout is symmetrical.
It is very easy to manipulate as well. If you want to disconect a pin to test a component, just simply break the solder bridge. If you screw up, it's easy to correct. Of course you have to be aware of the voltage and power limitations. If you have a little bit of soldering skills you can route (sculpt) complex circuits quite effectively......for proto-typing. Between sculpting the bottom side and wire routing of the top, just about any circuit can be built this way for relatively cheap. While it seems like a lot of solder is used, I re-use the solder from the old PCB's and projects I've made like this to build the new ones. For example1 and example2, this is the VAS PCB of the previous circuit. Since I have figured out this method, I use it extensively for new circuits to test because bread boards suck, especially when they have been used a lot. Now if each contact inside the breadboard were made of gold plated steel, it would probably work really well. But, then you couldn't afford it and neither could I.
Has anyone taken a look at the output circuit of the STAX DA-100?
I was looking at it earlier, and it looked a lot like an Allison setup... there's a link to it in an earlier post that I don't have at my fingertips at the moment on this forum that links to another site with the pdf of the schematic. An interesting amp. A search should reveal...
Now, I am wondering, are we saying (as does the original article segment that was posted) that the Allison circuit only controls the DC bias or that it functions at audio frequencies and actually performs some sort of feedback function, like Hawksford or Cordell's implementation?
Note that in the STAX amp there is a large cap associated with the two transistors, leading me to guesstimate that it works only at LF for bias control.
Has anyone investigated the HF response & circuit effect of the two transistors in these simulations so far?
I don't know these answers.
_-_-bear
PS. keantoken, did you say ur 15 yrs old?
I was looking at it earlier, and it looked a lot like an Allison setup... there's a link to it in an earlier post that I don't have at my fingertips at the moment on this forum that links to another site with the pdf of the schematic. An interesting amp. A search should reveal...
Now, I am wondering, are we saying (as does the original article segment that was posted) that the Allison circuit only controls the DC bias or that it functions at audio frequencies and actually performs some sort of feedback function, like Hawksford or Cordell's implementation?
Note that in the STAX amp there is a large cap associated with the two transistors, leading me to guesstimate that it works only at LF for bias control.
Has anyone investigated the HF response & circuit effect of the two transistors in these simulations so far?
I don't know these answers.
_-_-bear
PS. keantoken, did you say ur 15 yrs old?
bear said:
The Allison (in my schematics) simultaneously controls bias current AND signal transfer, but it appears here that they are being used strictly for bias control, as the caps isolate any AC.
In the case of the STAX amp, it has no audio function. But in my examples, it acts as a buffer with feedback to the output CFP's. The feedback is taken from the bias resistors.
I will simulate and post the frequency response plots. What do you mean "two transistors"?
Yes, that is my age.
Everything I know about electronics I learned from DIYAudio (true story). So in short, thanks!
- keantoken
Here is the frequency response of all the circuits. Vout1 is the trace from the standard Allison, and seems to be the most healthy one. Vout4 is my enhanced version, which has a strange spike that I didn't notice at first. Some compensation is probably healthy.
- keantoken
- keantoken
Attachments
the two transistors refers to the "Allison" transistors.
I don't think that Allison's original intent was to provide feedback for anything but the bias.
So, the question is then, does the technique of not limiting the the HF response so that it works only at near DC provide a reduction of distortion for the output stage, and if so, how much?
Should be easy enough to simulate by throwing in the cap and taking it out of the original simple circuit?
And, is it then just a variation of Hawksford or not??
keantoken, nice going! Have you built any circuits into real world units or are you just doing the simulations so far??
Btw, you ought to be able to get into a top notch engineering school without any trouble... fyi. Email me if you need any insights into how to do that...
_-_-bear
I don't think that Allison's original intent was to provide feedback for anything but the bias.
So, the question is then, does the technique of not limiting the the HF response so that it works only at near DC provide a reduction of distortion for the output stage, and if so, how much?
Should be easy enough to simulate by throwing in the cap and taking it out of the original simple circuit?
And, is it then just a variation of Hawksford or not??
keantoken, nice going! Have you built any circuits into real world units or are you just doing the simulations so far??
Btw, you ought to be able to get into a top notch engineering school without any trouble... fyi. Email me if you need any insights into how to do that...
_-_-bear
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