Oh I see. There is an extra cap and zener there too. I guess Jim's audio was working from an upgraded design. I suppose that's good. I will just build it per the docs I received and hope for the best. I do remember seeing pics of the zener mod. As far as Greg releasing details, he openly released this amp to public domain. He actually encouraged people to mod it to their content. From what I read, folks asked Jim's audio to produce some boards and he is not charging an exorbitant fee for them. I'm actually happy I was able to get some boards to build. No telling when someone will get a group buy together here. I do see on the SKA forum where you have been working with Greg to get your own version going. Good for you.
Blessings, Terry
Blessings, Terry
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I did manage to get my own version up and running and it sounds very good based on initial tests. I decided not to use any of Greg's IP beyond the circuit he published or provided to me on his forum and instead I added an emitter follower-zener buffer to the input stage which has some technical benefits over the simple zener mod. I believe Greg later came up with an alternative to the simple zener mod that he says performs even better.
I also modified the temperature sensor, eliminating two transistors so that overall I'm not using any additional active devices. I also included a source resistor for both MOSFETs as this provides for better current sharing between the matched pairs. I added protection diodes to R14 (common practice from what I understand) and I specified a bipolar cap for the feedback shunt (if using a cap there at all - some don't). I never heard the original so there's no way for to tell if my mods are really any better than anybody else's when it comes to the sonics but I like to put my own stamp on anything that I build - otherwise it makes sense to buy the original directly from Greg. And that's where I have an issue with Jim's Audio - I don't see any innovation or value-add nor am I aware that Greg supported their activities so it looks like an attempt to clone his pcb and undermine his business - nothing illegal, I just don't like it. I never had an intention to make a business out of his amplifier although I have sold a pair of boards to a fellow DIY'er since I had some left over from my own board run.
By the way, it looks as if Jim has the MOSFET protection diodes connected differently than in the original - looks wrong to me - maybe a typo in the schematic.
I also did a ground up pcb design. I don't know that my layout it better than the original, but it's always nice to think that I've done a good job of it. It was my first pcb layout that I had somebody fabricate professionally and I'm very pleased with it. The size is actually smaller than Greg's, it requires no fly-wires for the temperature sensors, has a single easy to use connector and the whole things neatly bolts onto a heatsink. There's a thread about it on this forum 'TGM7' if you want to know any more about it. I have been distracted building a speaker for a friend but I'm slowly putting together the power supply and will build two more pcb's to make a 3 channel amp. Then I can do some proper listening to it.
With the zener mod you have to be carful about the value of the capacitors as there are some competing requirements. Large size means lower impedance to shunt the zener noise, but the charge time can affect turn-on behaviour. I noticed from doing a lot of simulations that Greg really worked to optimize parts values, there are many tradeoffs in this design that he has worked out.
I also modified the temperature sensor, eliminating two transistors so that overall I'm not using any additional active devices. I also included a source resistor for both MOSFETs as this provides for better current sharing between the matched pairs. I added protection diodes to R14 (common practice from what I understand) and I specified a bipolar cap for the feedback shunt (if using a cap there at all - some don't). I never heard the original so there's no way for to tell if my mods are really any better than anybody else's when it comes to the sonics but I like to put my own stamp on anything that I build - otherwise it makes sense to buy the original directly from Greg. And that's where I have an issue with Jim's Audio - I don't see any innovation or value-add nor am I aware that Greg supported their activities so it looks like an attempt to clone his pcb and undermine his business - nothing illegal, I just don't like it. I never had an intention to make a business out of his amplifier although I have sold a pair of boards to a fellow DIY'er since I had some left over from my own board run.
By the way, it looks as if Jim has the MOSFET protection diodes connected differently than in the original - looks wrong to me - maybe a typo in the schematic.
I also did a ground up pcb design. I don't know that my layout it better than the original, but it's always nice to think that I've done a good job of it. It was my first pcb layout that I had somebody fabricate professionally and I'm very pleased with it. The size is actually smaller than Greg's, it requires no fly-wires for the temperature sensors, has a single easy to use connector and the whole things neatly bolts onto a heatsink. There's a thread about it on this forum 'TGM7' if you want to know any more about it. I have been distracted building a speaker for a friend but I'm slowly putting together the power supply and will build two more pcb's to make a 3 channel amp. Then I can do some proper listening to it.
With the zener mod you have to be carful about the value of the capacitors as there are some competing requirements. Large size means lower impedance to shunt the zener noise, but the charge time can affect turn-on behaviour. I noticed from doing a lot of simulations that Greg really worked to optimize parts values, there are many tradeoffs in this design that he has worked out.
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Hi Gareth,
Yes, I followed your conversations with Greg. I have also been watching your thread here. Before I left DIY audio in 2008, I was very interested in what Greg was doing with this design so I am glad to be able to try it now. I don't know enough about electrical design to know what you have changed. Maybe some day if I stay at this long enough. Probably not the best way to learn but I enjoy the building and trouble shooting and I learn a little more each time. I don't have a problem with Jim's audio. I know he has stepped on toes a few times but not sure he is doing anything illegal and he does provide a service for those of us not talented enough to get boards made. The boards he sells are of good quality. These are the second set I've bought. I got the JLH 1969 boards and built them. Anyway, once I finish the two DX amps I am currently building, I will tackle this one.
Blessings, Terry
Yes, I followed your conversations with Greg. I have also been watching your thread here. Before I left DIY audio in 2008, I was very interested in what Greg was doing with this design so I am glad to be able to try it now. I don't know enough about electrical design to know what you have changed. Maybe some day if I stay at this long enough. Probably not the best way to learn but I enjoy the building and trouble shooting and I learn a little more each time. I don't have a problem with Jim's audio. I know he has stepped on toes a few times but not sure he is doing anything illegal and he does provide a service for those of us not talented enough to get boards made. The boards he sells are of good quality. These are the second set I've bought. I got the JLH 1969 boards and built them. Anyway, once I finish the two DX amps I am currently building, I will tackle this one.
Blessings, Terry
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Joined 2009
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If you look at the schematics in post 179 you will see there is a zener diode connected to the gate of the MOSFETs - look at Greg's design. In normal operation they are non-conducting because they are reverse biassed. But if the amplifier asks the MOSFETs to pass a very high current (for example, into a low impedance load) then the gate potential of the MOSFETs move further and further away from their associated power rail, increasing the voltage across the diodes. At some point the zener diodes break-down/avalanche and current flows between the MOSFET gate and power rail and pins the voltage at the gate - preventing the gate voltage swinging further from the rail. This limits the current flow through the MOSFET. The zener diode has a specific breakdown voltage specified by Greg. Its a protection feature to stop the MOSFETs blowing up too quickly into low impedance loads / shorts.
The schematic for the Jim's boards have the diodes connected not between gate and power rail, but between gate and source of the MOSFET. Wired like this it doesn't offer the same current limit as Greg intended because the zener diode no longer sees the voltage drop across the source resistors. The MOSFETs are still protected, but the maximum current allowed to flow will be higher than with Greg's design. Remember, the fuses are slow-blo whereas the zener protection is fast.
With 10A of current, those source resistors will drop 1.5V across them. So Jim's boards will allow the gates to swing 1.5V more than Greg had set the limit to. I guess you could compensate by using zener diodes with lower breakdown voltages than specified by Greg and all would be good.
Terry - I've never built a DX but expect it will be great. Which version are you building ?
The schematic for the Jim's boards have the diodes connected not between gate and power rail, but between gate and source of the MOSFET. Wired like this it doesn't offer the same current limit as Greg intended because the zener diode no longer sees the voltage drop across the source resistors. The MOSFETs are still protected, but the maximum current allowed to flow will be higher than with Greg's design. Remember, the fuses are slow-blo whereas the zener protection is fast.
With 10A of current, those source resistors will drop 1.5V across them. So Jim's boards will allow the gates to swing 1.5V more than Greg had set the limit to. I guess you could compensate by using zener diodes with lower breakdown voltages than specified by Greg and all would be good.
Terry - I've never built a DX but expect it will be great. Which version are you building ?
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I would have to suspect that somewhere along the way, the zener design changed. I don't have access to the owners forum so I can't see where or when it might have happened but I seriously doubt that Jim's audio just made that change on their own. I believe someone who has access gave them the schematic and they just followed it.
Hi Gareth.
I am building a DX Blame MKIII and a Dx Super A. The MKIII is about 90% finished and the Super A has resistors only soldered. Both are awaiting parts. I got interested in those because of a thread about a 250w amp where Kevin expounded on the value of the Blameless amp, So I ordered boards for the Honey Badger and the MKIII. Both were touted as being based on the Blameless. The honey Badger sounds very, very good. My number one choice right now. We shall see how the DX stands up to it. I'm doing the SKA because I always wanted to try one. I'm not too familiar with MOSFET amps. I've only built one and that was my first diy amp, a P101. It is just OK sounding compared to some of the other amps I've built.
Blessings, Terry
Hi Gareth.
I am building a DX Blame MKIII and a Dx Super A. The MKIII is about 90% finished and the Super A has resistors only soldered. Both are awaiting parts. I got interested in those because of a thread about a 250w amp where Kevin expounded on the value of the Blameless amp, So I ordered boards for the Honey Badger and the MKIII. Both were touted as being based on the Blameless. The honey Badger sounds very, very good. My number one choice right now. We shall see how the DX stands up to it. I'm doing the SKA because I always wanted to try one. I'm not too familiar with MOSFET amps. I've only built one and that was my first diy amp, a P101. It is just OK sounding compared to some of the other amps I've built.
Blessings, Terry
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Joined 2009
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I have access to the owners forum and I have the assembly manual provided by Greg. There is no change in the zener arrangement that I can find.
I'm thinking that too...I suspect Jim messed up, that's all
In the original schematics there was no source resistor. These resistors were added latter on to ease power up in very cold climates, like in the nordic european countries.
I need some advice here. What would be better:
1. leave every thing as it is in Jim's version;
2. Short out the source resistors and revert to the original Greg's design;
3. Change zener values (for what values?) and go with Jim's arrangement.
Thank you guys!
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If you already have the pcb's then you may as well use them. But I would not short out the source resistors. They add another level of safety in terms of thermal management of the output devices and slightly reduce the current rush when the power rails are first energized (a soft start circuit ahead of your power transformer would help too).
I would suggest Option 1. You will have a little less protection on over-current but if you don't short the output of the amplifier or drive it hard into low impedance loads you'll never test this feature out.
If you are more confident in your engineering skills and are willing to run a simulation or some bench-top experiments you could do even better than the original design by selecting zener values to give a good level of protection and to have it balanced between the n-MOS and p-MOS devices. Greg prescribes different value zeners for the n-mos and p-mos but they are still not balanced. A starting point would be to select zener diodes with around 1V lower breakdown than specified by Greg.
Ok, here's some answers
"In original GB design, there was no degeneration resistors, and the zener diode values were as such."... "The conclusion was the diodes placing on either end would serve similar protection function. The passage you quoted may be right, but to me, each mosfet can allow 12A of current to pass through without damage. Then why a transient of 10A passing though two mosfets in parallel would cause damage ? Conventionally, the intend of the zener diodes were to prevent overvoltage from the gate side rather than to shunt the gate-source voltage feedback from the output."
So everything should be fine?
Thanks
Do
"In original GB design, there was no degeneration resistors, and the zener diode values were as such."... "The conclusion was the diodes placing on either end would serve similar protection function. The passage you quoted may be right, but to me, each mosfet can allow 12A of current to pass through without damage. Then why a transient of 10A passing though two mosfets in parallel would cause damage ? Conventionally, the intend of the zener diodes were to prevent overvoltage from the gate side rather than to shunt the gate-source voltage feedback from the output."
So everything should be fine?
Thanks
Do
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Joined 2009
Paid Member
The turn on voltage (a.k.a. gate threshold voltage Vgs) is different for n-mos and p-mos devices which means that for roughly the same current flow you need to apply different gate voltages. Hence you need different breakdown voltages for the respective zener diodes to get roughly the same maximum current flow + and -.
Is the value critical - well you can judge that for yourself - how much current flow do you want to allow the FETs to pass before the output fuse blows many 100's of ms later ?
Is the value critical - well you can judge that for yourself - how much current flow do you want to allow the FETs to pass before the output fuse blows many 100's of ms later ?
Thanks Gareth,
I'm just trying to understand how things work. Above you suggested that maybe a lower value for the zeners would be better. That is why I asked how critical it is. Evidently, from the answer that Do recieved, Jim's actually did change it on purpose. I wonder if they did any simulations before they used that value. Maybe not since it is the same as what Greg used.
It it is likely that mine will be hooked up to 4 ohms sometimes since two of the sets of nearfield speakers I have are 4ohms.
Blessings, Terry
I'm just trying to understand how things work. Above you suggested that maybe a lower value for the zeners would be better. That is why I asked how critical it is. Evidently, from the answer that Do recieved, Jim's actually did change it on purpose. I wonder if they did any simulations before they used that value. Maybe not since it is the same as what Greg used.
It it is likely that mine will be hooked up to 4 ohms sometimes since two of the sets of nearfield speakers I have are 4ohms.
Blessings, Terry
Member
Joined 2009
Paid Member
MOSFETs are generally pretty rugged. I asked Greg how safe his design is if faced with a shorted output. He thought the MOSFETs would survive the experience - remembering that he has those zener diodes in place. Based on this I would assume your 4 ohm load is pretty safe, even if the impedance dips. However, I also remember somebody on his forum mentioned that they blew the output fuses with a difficult speaker load - I think they had 4 ohm 'maggies' (whatever that means) and Greg simply suggested replacing the 3A fuses with 5A fuses. So he has a lot of confidence in the amp with such loads.
The trouble with simulations is the accuracy of the FET model. I used Bob Cordell's FET models but I can't vouch for Jim. In my simulation of my TGM7, which uses the zener values and configuration of Greg's design (not Jim's) it looked as if the protection was set at a sensible maximum current - but I can't remember the currents that I got from it right now and I'm typing this from my iMac which doesn't run LTSpice.
I'm not going to provide support for Jim's design as I disagree with what he is doing. People who can't make their own boards can get them from Greg and Greg plus his forum will provide all the technical support they need.
I realize I may be making a nuisance of myself in this thread - the limit of my experience is in the advice I've given in post 191 and I suspect that you are fine building as per Jim's design and you'll enjoy a long life of great sound
The trouble with simulations is the accuracy of the FET model. I used Bob Cordell's FET models but I can't vouch for Jim. In my simulation of my TGM7, which uses the zener values and configuration of Greg's design (not Jim's) it looked as if the protection was set at a sensible maximum current - but I can't remember the currents that I got from it right now and I'm typing this from my iMac which doesn't run LTSpice.
I'm not going to provide support for Jim's design as I disagree with what he is doing. People who can't make their own boards can get them from Greg and Greg plus his forum will provide all the technical support they need.
I realize I may be making a nuisance of myself in this thread - the limit of my experience is in the advice I've given in post 191 and I suspect that you are fine building as per Jim's design and you'll enjoy a long life of great sound
.I realize I may be making a nuisance of myself in this thread - the limit of my experience is in the advice I've given in post 191 and I suspect that you are fine building as per Jim's design and you'll enjoy a long life of great sound
No, you're not. I (for one) really appreciate your tech input here and on 'your' GB150 thread.
BC546, Bc 556---C and B grades substitute with 2N 5401, 5551
To simplify parts collection and matching,
can i substitute both types of BC transistor with 2N 5401 and 5551 as these are easier to come by.
2N5401 - MULTICOMP - TRANSISTOR,PNP,0.6A,150V,TO92 | element14 Malaysia
2N5551 - MULTICOMP - TRANSISTOR,NPN,0.6A,160V,TO92 | element14 Malaysia
i intend to use +/- 50 v dc as power supply.
thanks
kp93300
To simplify parts collection and matching,
can i substitute both types of BC transistor with 2N 5401 and 5551 as these are easier to come by.
2N5401 - MULTICOMP - TRANSISTOR,PNP,0.6A,150V,TO92 | element14 Malaysia
2N5551 - MULTICOMP - TRANSISTOR,NPN,0.6A,160V,TO92 | element14 Malaysia
i intend to use +/- 50 v dc as power supply.
thanks
kp93300
