your discovery about the mosfet output stage gives the slewmaster output boards, even greater versatility, i hope that you can come up with full documentation for easy reference for us newbies...😉
What constitutes 'Full Documentation'? A detailed build guide with pictures / illustrations, BOM and anything else in particular? If you wanted the perfect build documentation what would you want / need to see in it?
Seriously, give me a guideline. Some things are almost second nature, so it is easy to forget what someone else might appreciate as far as a little guidance goes.
Seriously, give me a guideline. Some things are almost second nature, so it is easy to forget what someone else might appreciate as far as a little guidance goes.
something like a before and after picture of the board with the mosfets as output, a small write up how to do it, a bom, all in one place.....
I thought a BOM would be nice when I started but I quickly realized it would be tough to include all the possibilities for components. There's likely 50 different output transistor possibilities alone.
Ok, it might now be time to get a proper document together for both the EF3 and MOSFET options. It will take me a bit of time, but I think I can do that for the community.
There are lots of possibilities, but I think putting together something for those uncomfortable with parts selection might be good. A list of alternatives, or at least the desirable specifications would be useful.
One problem I see with repurposing the BJT SlewMaster boards for MOSFET outputs is there there are no provisions for gate zobel networks. No-one here has reported stability problems without them, but everyone seems to be using fairly high gate stoppers of ~220R or more.
With the zobels in place it should be possible to reduce the stopper value to as little as 47R. Some people talk about better bass definition with the smaller gate stoppers.
With the zobels in place it should be possible to reduce the stopper value to as little as 47R. Some people talk about better bass definition with the smaller gate stoppers.
Yes, those obsolete LED part numbers threw me off. A guide to adjusting input stages and bias settings would make life easier too. Many hours of reading in this thread to figure it out.
perhaps a mention of in-circuit forward drops and current helps
since led's are everywhere and they are not so hard to screen
for voltages and currents...and they are very cheap...
Gate zobels a could be (like the protection zeners) added under board if they are desired for the use of smaller gate stoppers. I have no personal experience if there is a difference in sound using one technique or the other.
No one has tried smaller stoppers as far as I'm aware, say 100R, to check local stability.
I mentioned that earlier but you guys convinced me it really does belong here. It's a pretty simple modification and I'm sure if OS were here he would be all for it. He could have just as easily started with the MOSFET OPS.The whole purpose of the OSP was just a base to test out the various topologies. I think he just considered the EF3 to be a good platform. I already see guys looking to continue tweaking the fet OPS so that will give us more stuff to work on. I'm happy.
Blessings, Terry
This kind of help for us with less technical prowess is appreciated more then I can express. A list of safe devices (not likely to oscillate) would be great.
Evan
Jason and Terry thanks for all the hard work you guys are doing. Terry I would have a bunch of these things built if I could just stop bleeding money into other expenses. I really need to spend some money on getting a good set of power supplies built for all the different amplifier modules that I already have. I need to decide between transformer types first of all. Toroidal s just take up a bunch of space that I don't know that I will be able to get away with.
Build Guide
Jason,
Responding to your build guide request for comment. I like your suggestion of targeting specific parts of the board and highlighting what would make a good choice for certain key components. Not by calling out the specific component but by describing the characteristic you are looking for in a Capacitor, Resistor or BJT in a certain position.
Good example is the small exchange you head with Keantoken on the use of FILM or LITYC caps for bypassing the BJT's and how you need just the right impedance in the cap for a FILM to be stable.
Build techniques for tuning critical cap values...I'm not sure if measuring techniques as to how to observe instability at a certain frequency would also be very helpful...however that might be asking to much from a single guide.
Last I'd say if the build can be broken down into sections with test checkpoints along the way would be extremely helpful. Not sure if this is possible but it would allow guys to build a smaller section of the project at a time and verify their work as they went.
I did a BOM a while back, didn't receive a comment on it. However having folks look at a BOM for an individual is less useful than building a generalized guide so folks can feel confident with a BOM they put together.
Jason,
Responding to your build guide request for comment. I like your suggestion of targeting specific parts of the board and highlighting what would make a good choice for certain key components. Not by calling out the specific component but by describing the characteristic you are looking for in a Capacitor, Resistor or BJT in a certain position.
Good example is the small exchange you head with Keantoken on the use of FILM or LITYC caps for bypassing the BJT's and how you need just the right impedance in the cap for a FILM to be stable.
Build techniques for tuning critical cap values...I'm not sure if measuring techniques as to how to observe instability at a certain frequency would also be very helpful...however that might be asking to much from a single guide.
Last I'd say if the build can be broken down into sections with test checkpoints along the way would be extremely helpful. Not sure if this is possible but it would allow guys to build a smaller section of the project at a time and verify their work as they went.
I did a BOM a while back, didn't receive a comment on it. However having folks look at a BOM for an individual is less useful than building a generalized guide so folks can feel confident with a BOM they put together.
Hi Kindhornman, this is a good approach. There's one thing to keep in mind in this case - couple of additional diodes (Baker clamp) will be required to protect the output section from potential damage at high swing (when the input section swing may exceed the output section rails).
Jason, in your doc it would be worth mentioning, as an option, I think.
Cheers,
Valery
Valery, can you give an example of implementing the Baker clamps we would want to consider installing when using boosted rails on the SlewMaster OPS schematic?
If I remember correctly somewhere in the thread OS was talking about using boosted rails on the Slewmaster but I am not sure if that was specifically referenced to a mosfet design or if that was for one of the earlier designs such as the Gonad The purpose of the boosted rails was to lower noise in the input section if I remember correctly. I don't quite understand how that works but I will have to go back and reread Self and Cordell and then I will get a better idea of what is going on. You guy's are just so damned smart!
Hi Jason and All,
I did some simulations and realized that baker clamp is not so easy in this case. The problem is that in our particular case it makes sense to use it not between the VAS and power section's emitter follower (EF), but between the emitter follower and MOSFETs (this is where we want more swing - at MOSFET gates). But in this case, we need to add current limiting resistors between EF and baker clamp (and MOSFET gates). Those resistors save EFs while clipping, but may slightly reduce the quality in normal operation conditions (especially with the higher resistors' values in case of higher rails difference).
So, my recommendation - if somebody wants to use higher rails for the front-end (plus EF) - simply make those rails 6-7 volts (but not more than 7) higher than the OPS rails. This will prevent the output from touching the rails (leaving 0.5 - 1.5 V headroom), so baker clamp is not required - and then we have no problem with EF over-current. VAS clips nicely and OPS never clips in this case.
Cheers,
Valery