Sheldon said:
Definitely. It is a very interesting approach, pared down to the basics. The next step being to optimize it, and determine which enhancements add to its performance, and which detract from it.
No, this time I missed something. Yet again, not for the first time
Yeah. A universal problem. My TODO-list always grows faster than I can process it, regardless of how quickly I work
Susan-Parker said:
These are JFETs, not MOSFETs. If I read Mr. Curl's comments on another forum correctly, JFET followers are significantly less prone to oscillation than MOSFET followers.
If oscillation turns out to be a problem, gate resistors, ferrites and gate-source capacitors are all viable tools to eliminate it.
Won't 1-2 ohms significantly affect the idle current?
I was actually thinking about lowering the voice coil DC resistance (e.g. by paralelling sections) to counteract this problem. 5A at 0,2, for example, would only be 5W on average. I seem to recall this design did well also with low bias currents, so that part shouldn't be a problem.
The inductance is more difficult, I'd imagine. What kind of inductance would one need to get full range operation?
Thanks, that's a brilliant idea.
Come to think of it, wouldn't it be possible to drive the voice coil with the same signal as the "magnet", or would this be yet another case where the inductance cannot be made to reach the desired level?
suiraMB said:
I'm pretty new to all this, but this issue seems central to the difference between this design and a regular pp amp. And not understanding this has made me want to pursue it even more (what issues that speaks to, I'll leave to the observer). You cite class AB operation. Susan cites class A operation. So what's going on here? As the light in my noggin slowly goes from black to dull red (or is that light at all), it occurs to me that the crux of the matter is the fact that the rail voltage actually swings with the drive voltage. Unlike the usual situation in which one rail is fixed positive, the other negative. So it seems like class AB amps in that the power output can exceed the idle dissipation, but like class A in that significant current flows in both devices throughout the ouput cycle. Or?
Sheldon
Sheldon said:
Sorry. I guess it's a case of semantics.
As I understand the design, both transistors are in conduction at all times, which fits some definitions of class A, as neither transistor will cut off at any point, entirely eliminating crossover distortion.
However, I believe the conduction angle (at least in terms of linear operation) is less than 360 degrees, which excludes the class A designation in its strictest sense.
Please correct me if I'm wrong, and also note that I do not mean to imply inferior sonics or design by my statement.
This has been my understanding of the circuit as well.
In the strictest sense, the circuit is not class A, as its conduction angle is not 360 degrees at maximum output.
However, herein lies one of the more elegant aspects of this design, in my opinion:
Unlike a conventional class AB amplifier, Susan's design has a continously variable conduction angle, in that the conduction angle is proportional to the signal, and the conduction angle always exceeds 180 degrees, meaning you get no cross-over distortion.
In fact, I believe you can select the minimum conduction angle by adjusting the bias, though I haven't tried to derive a formula for it.
suiraMB said:
Sorry, I haven't explained myself clearly.
Using the method of paralleling devices but having each device on its own winding is an attractive method of ensuring that each part doesn't current hog as each one has it's own separate load to look after.
However there is a problem with oscillation as each device tries to optimally do its follower 100% degenerative feedback thing. Note that this is completely separate to the oscillation some devices are prone to from having too low a value of gate resistor.
The resistors are to couple adjacent device sources i.e. between the devices, they are not in series with the windings or across the transformer to ground.
Therefor there will be no current flow (of any significance) as all the sources are at the same voltage point (within a few mV anyway).
However what this does is provide a low resistance path between the sources and ensures that they all stay in step with each other since the windings above DC actually have significant impedance.
So in the simplest case of having two pairs of devices, one pair per side, each on its own winding one would use two resistors. One is across the first pair's sources, the other is across the second pair's sources.
(Will draw up a schematic at the weekend.)
Hope this is a little clearer.
Best wishes,
Susan.
Susan-Parker said:
Actually, you've been pretty lucid. I'm sure I've been a lot less clear about things.
Actually, current hogging isn't as much of a problem as it would be with bipolars, or even MOS fets. However, the bias voltage can get a bit high when you have several devices on the same winding, due to the autobias arrangement.
Plus, seperate windings should give better high-frequency performance, according to some pages I read. I could be wrong here, though.
Ah. Yes. I misunderstood.
That makes a lot more sense, yes.
Yes. Perfectly clear.
Although I'm no good with transformers, so if you're feeling awfully bored, you could explain the details of the hows and whys of this kind of resonance.
suiraMB said:
In chasing up materials to do this, I come across two possibilities with wildly different pricing. Indium Corportion has a variety of solders for electronics that they list on their web store. For a 1 meter, .75mm piece, the prices are from $180-350. Ouch.
The other type is a brand used by mostly by hobbyists and jewelry makers. The brand is Tix. It seems to sell for about $15 for 20 8cm pieces. The only information I could find seems to indicate that it comprises indium, lead, and tin. It has a melting point of 135 degrees C. Unless someone knows some reason why this wouldn't work, I'll give this type a try.
Sheldon
Sheldon said:
Indeed it is somewhat expensive. You don't need very much per device, though.
Well, it's cutting it fine, but it should work, provided you have a thermometer that is reasonably accurate in that range. The Lovoltechs can handle 150 degrees storage temperature, which leaves you a headroom of 15 degrees over the melting point of the solder. Remember that the device will be at this temperature for some time, since you'll need to heat the entire heatsink in order for this to work. You'll also want to start low on the bias, I think, since you don't know the thermal conductivity of this solder.
Since this topology generally employs a sub-360-degree conduction angle, you may want to incorporate some advice from another forum member (again, I forget who). Basically, solder the devices to a large copper bar, then solder that to the heatsink. While this may slightly reduce constant dissipation, it will improve transient thermal performance, as the mass (and higher heat conductivity) of the copper bar will absorb larger heat transients.
Whether this trick works for you is, I guess, dependent on the bias setting you choose. Of course, a 12dB fan would also help a lot, possibly enabling you to go as high as 30W idle dissipation (operating temperature of 80 degrees).
Thanks suiraMB,
No, I don't need much. The problem is that no quantity less than $180 worth is offered.
I have a good multimeter with a thermocouple, so temp. monitoring should be no problem. On the spec sheet, lead soldering for 10 seconds at 260 degrees C is stated, so I would guess that short exposure to the junction of somewhat higher than 150 degrees would not be problematic. In any event, cooking a couple of transistors is a whole lot more cost effective than a couple hundred bucks worth of solder.
I would guess also that the thermal conductivity of the solder, assuming a thin bond, has to be better than thermal grease, or thermal grease and mica. My plan was to solder to a copper plate about 3cm square, then mount that on a thick aluminum sink. I'm looking at 10 watts maximum per device.
Sheldon
No, I don't need much. The problem is that no quantity less than $180 worth is offered.
I have a good multimeter with a thermocouple, so temp. monitoring should be no problem. On the spec sheet, lead soldering for 10 seconds at 260 degrees C is stated, so I would guess that short exposure to the junction of somewhat higher than 150 degrees would not be problematic. In any event, cooking a couple of transistors is a whole lot more cost effective than a couple hundred bucks worth of solder.
I would guess also that the thermal conductivity of the solder, assuming a thin bond, has to be better than thermal grease, or thermal grease and mica. My plan was to solder to a copper plate about 3cm square, then mount that on a thick aluminum sink. I'm looking at 10 watts maximum per device.
Sheldon
Cooking the transistors will indeed be more cost effective. 
The problem with the lead soldering temperature is that you will not be able to limit the exposure to 10 seconds without forced cooling, which may affect the solder.
Though, as you said, it will probably take more than a slightly raised junction temperature to destroy the device. I'd try scoping it before and after, though, to check that you're hearing the performance of an uncooked device
Thermal conductivity should indeed be significantly better with a metal-metal bond. Keeping it thin can be accomplished with a vice while the solder is still in its liquid state.
Obviously, you'll have to make sure you don't end up shorting anything; I can't recall whether there's a live backplate on the thing.
I like the copper plate idea in that regard. Giving the device a larger effective area to dissipate from, and then isolating this area from the heatsink, neatly avoids some problems.
The problem with the lead soldering temperature is that you will not be able to limit the exposure to 10 seconds without forced cooling, which may affect the solder.
Though, as you said, it will probably take more than a slightly raised junction temperature to destroy the device. I'd try scoping it before and after, though, to check that you're hearing the performance of an uncooked device
Thermal conductivity should indeed be significantly better with a metal-metal bond. Keeping it thin can be accomplished with a vice while the solder is still in its liquid state.
Obviously, you'll have to make sure you don't end up shorting anything; I can't recall whether there's a live backplate on the thing.
I like the copper plate idea in that regard. Giving the device a larger effective area to dissipate from, and then isolating this area from the heatsink, neatly avoids some problems.
Have you looked into the PC solution for CPU heatsink adhesive http://www.arcticsilver.com/as5.htm - don't know if the thermal conductivity is enough but it's good to > 180 degrees C.
Modern CPUs get damn hot so I imagine this along with maybe some water cooling would be cheaper than the Indium solder & no cooked JFets.
JOhn
Modern CPUs get damn hot so I imagine this along with maybe some water cooling would be cheaper than the Indium solder & no cooked JFets.
JOhn
If I try the Tix solder, I'll play with some proxy parts first to get a process down.
It's not clear from the data sheet whether the backplate is live or not, but I could ask in the GB thread. I would think that a live backplate would be better for thermal transfer.
Jkeny, the arcticsilver stuff looks interesting, but without a relative value it's hard to tell how it compares to a solder bond. More interesting maybe is the statement they make about this vis a vis conductive greases. Makes me wonder if the conductive greases wouldn't be even better.
Sheldon
It's not clear from the data sheet whether the backplate is live or not, but I could ask in the GB thread. I would think that a live backplate would be better for thermal transfer.
Jkeny, the arcticsilver stuff looks interesting, but without a relative value it's hard to tell how it compares to a solder bond. More interesting maybe is the statement they make about this vis a vis conductive greases. Makes me wonder if the conductive greases wouldn't be even better.
Sheldon
The Arctic Silver 5 actually looks like a fairly good candidate for heat sinking, and it's not particularly expensive either.
Modern CPUs don't necessarily get all that hot, since they are heavily heatsinked, but they do generate a lot of heat. In fact, we'll soon have to start using liquid sodium or liquid potassium for cooling, as the heat per surface area is getting close to what you find in a fission reactor.
It would be very interesting to compare this compound to the solder based approaches.
Modern CPUs don't necessarily get all that hot, since they are heavily heatsinked, but they do generate a lot of heat. In fact, we'll soon have to start using liquid sodium or liquid potassium for cooling, as the heat per surface area is getting close to what you find in a fission reactor.
It would be very interesting to compare this compound to the solder based approaches.
Here are some measures of thermal grease effectiveness http://www.overclockers.com/articles662/
Don't know how this compares to solder based approaches but some measures given in the article suggest that
- there's not much difference between greases (in the overall scheme of the cooling problem)
- Greases are about 1/20 to 1/40 as effective as pure metal
However, this is assuming a perfect contact with metal - this is not the case - so we need greases to fill in microscopic uneveness of contact surfaces. How does this fact sit with the idea of a solder interface? Do we not still need some microscopic thermally conductive filler between surfaces?
Don't know how this compares to solder based approaches but some measures given in the article suggest that
- there's not much difference between greases (in the overall scheme of the cooling problem)
- Greases are about 1/20 to 1/40 as effective as pure metal
However, this is assuming a perfect contact with metal - this is not the case - so we need greases to fill in microscopic uneveness of contact surfaces. How does this fact sit with the idea of a solder interface? Do we not still need some microscopic thermally conductive filler between surfaces?
As Sheldon said, the solder interface should work, because you keep it liquid while using a vice to press the device to the sink, so it should let the air out. The pressure on the vice will make the solder layer as thin as possible. I believe the indium solder can properly wet the surfaces. And the cleaning/fluxing should eliminate other impurities that could impede heat transfer.
Of course, the thermal conductivity of indium (81.8W/m/K) is lower than that of the heatsink, but I assume some of the other components of the solder have higher conductivity.
One approach that would be excellent with these devices, is to get a copper block with a water-maze and peltier element (something like this)
to enhance the cooling. And, of course, use solder to attach the device(s).
The theoretical minimum thermal resistance in this way is about 2.0K/W, due to the junction-case thermal resistance. Add a single peltier element, and you can lower the temperature by 20 deg C, which means you can get close to 45W continous dissipation.
Of course, the thermal conductivity of indium (81.8W/m/K) is lower than that of the heatsink, but I assume some of the other components of the solder have higher conductivity.
One approach that would be excellent with these devices, is to get a copper block with a water-maze and peltier element (something like this)
to enhance the cooling. And, of course, use solder to attach the device(s).
The theoretical minimum thermal resistance in this way is about 2.0K/W, due to the junction-case thermal resistance. Add a single peltier element, and you can lower the temperature by 20 deg C, which means you can get close to 45W continous dissipation.
Hey, John.
The rough and quick translation for the first one, which I assume is the one you're looking for, is as follows:
Also, NOK 500 is roughly USD 75 or EUR 60 or GBP 40, while NOK 1190 is roughly USD 180 or EUR 150 or GBP 100. Bear in mind that these prices probably include VAT, which is 25% in Norway.
Note also that I'm not endorsing this product, just giving an example of the approach.
With a copper router, three copper blocks, a peltier element, two or four O-rings, two nozzles, some screws and silicone, you could probably make something like this quite easily. And there are several companies that will supply something like this.
EDIT: As to the indium soldering bit, I know someone suggested it here, and I believe he succeeded in it. Either way, it shouldn't be all that hard. Indium itself has a melting point low enough that semiconductors that can take a 175degC junction temperature can be attached with the pure metal itself. The solders are compounds, so they melt at even lower temperatures (there are indium compounds that are liquid at room temperature).
EDIT2: Here is a picture of how you might do something like that, minus the peltier element. A pin matrix is probably simpler to make, though.
The rough and quick translation for the first one, which I assume is the one you're looking for, is as follows:
Also, NOK 500 is roughly USD 75 or EUR 60 or GBP 40, while NOK 1190 is roughly USD 180 or EUR 150 or GBP 100. Bear in mind that these prices probably include VAT, which is 25% in Norway.
Note also that I'm not endorsing this product, just giving an example of the approach.
With a copper router, three copper blocks, a peltier element, two or four O-rings, two nozzles, some screws and silicone, you could probably make something like this quite easily. And there are several companies that will supply something like this.
EDIT: As to the indium soldering bit, I know someone suggested it here, and I believe he succeeded in it. Either way, it shouldn't be all that hard. Indium itself has a melting point low enough that semiconductors that can take a 175degC junction temperature can be attached with the pure metal itself. The solders are compounds, so they melt at even lower temperatures (there are indium compounds that are liquid at room temperature).
EDIT2: Here is a picture of how you might do something like that, minus the peltier element. A pin matrix is probably simpler to make, though.