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Old 14th May 2010, 07:04 PM   #131
qusp is offline qusp  Australia
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I already have some caddock 50W in both 500R and 1K can I use these or am I better to get some 400 and 200? cant get the exact values you have from elfa or digikey. are any of the other really good fets freely available?
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Old 14th May 2010, 07:07 PM   #132
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I had actually wondered about the 100k R's, I thought perhaps they might be signifying your pot or something
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Old 14th May 2010, 07:11 PM   #133
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scrap the last question, I found FQA32N20 and IRFP3415 at digikey. so do all resistors have to be rated 5w or more? the main R's I have in caddock, but wondered if I might get away with a 5w for the others
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Old 14th May 2010, 08:15 PM   #134
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Thanks Owen.

Are you still planning boards?

Best

Bob
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Old 14th May 2010, 09:57 PM   #135
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qusp:

Since you already have the 500R and 1k resistors, I would just use those. Note that you'll get twice the gain (4VRMS @0dBFS with the Buffalo II). You'll have half the bias current, half the power dissipation, and as a result performance will suffer by a tiny amount.

The only resistors that need to dissipate power are the 200R and 400R resistors. Everything else can be 1/4 watt or smaller. The trimpots used to set the gate voltage will need to be upped to 100K which is what I used.

The 100K resistors from the output to ground will be needed to prevent the leakage in the caps from building up a voltage on your outputs. Make sure to leave them in.

I'm running the circuit without any filter caps at the moment, but I should get around to calculating/testing the correct values in the next few days.


flocchini:
I'm currently revising the boards to allow for heatsinks on the resistors and mosfets, so I should be done sometime nest week.

Cheers,
Owen
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Old 14th May 2010, 11:51 PM   #136
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so once youve got those values all sorted out I wont need the 100k to ground? isnt that messing with my output impedance? so with your PCB, you are still allowing both through-hole and SMD versions of most things?

I thought as much about the rating resistors, but I was thinking of using 1w to be safe anyway because in the 3 different versions of this circuit I have seen anywhere from 1/4-2W specified on all but the power resistors. so will you have spots for the SMD version (DPAK) of the caddock (MP 725) power resistors? this build will be passed on to a friend and i'll probably grab yours. so you are running without output caps? is it possible to do this all the time if the circuit is well tuned? I would love that.
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Old 15th May 2010, 05:37 AM   #137
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qusp:

Both the output caps and the 100k resistors that follow them are going to be needed no matter what. Even though the differential between those two points would be 0 volts in the perfect circuit, there's still going to be 25 volts between each node and ground, so unless you use a transformer, you'll need to use caps to get rid of that. I explained the need for the 100k resistors above, so unless you always have the circuit connected to a preamp or amp, I would strongly suggest keeping the 100k resistors.

As for the layout, I'll provide both footprints where it is possible. The mosfets will only be through-hole, and I'm leaning towards PTH Caddock resistors for the 200 and 400 ohm parts mounted on heatsinks, but I'll allow for SMD here too for those who want to build the 18 volt version.

I've attached below what I consider to be the bare minimum anyone would want to go with this circuit. There are no filter caps to induce a rolloff, which means you'll get flat output well past 100kHz. Some find this bad, I personally can't tell the difference with or without the caps. The two 100k resistors are actually just there to represent a 100k pot. This arrangement provides less fine adjustment than the other way, but it omits 2 resistors. It also allows you to crank up the gate voltage, which means you can blow your mosfets if you don't start them at a low value. This circuit is exactly what was used to make "The Beast" and it works very well.

Whatever you do, ALWAYS ADJUST THE CIRCUIT WITH THE DAC DISCONNECTED and then readjust once you've connected it. If you're using higher rail voltage, you could end up with a high or low voltage at the source of the fets before you adjust it.

Cheers,
Owen
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Old 15th May 2010, 10:01 AM   #138
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Quote:
Originally Posted by opc View Post
Hi Guys,

Ive got some graphs to post finally, so feel free to take a look below. I also measured several more fets, so here's a re-cap:

FQA46N15: 0.355 S
IXTQ96N15: 0.276 S
FQA19N20C: 0.397 S
ACD101NDD: 0.216 S
FQA28N15: 0.340 S
IRF1324: 0.297 S
IRF540: 0.357 S
IRF740: 0.258 S
IRF640: 0.256 S
IRF510: 0.054 S
IRFP4310Z: 0.374 S
IRF1310: 0.356 S
IRF644: 0.281 S
FQA32N20: 0.419 S
ST75NF20: 0.199 S
ST40NF20: 0.189 S
IRFP3415: 0.395 S

What you can take away from this is that you shouldn't use the IRF510, and that you have a plethora of other choices that are good. Looks like the FQA32N20 is the best of this group at 50mA and 10V DS.

As for the graphs, I used the same circuit, and adjusted the rails to various levels, taking measurements at each step. The voltages were 18V, 25V, 30V, 35V, 40V, 45V, and 50V.

As you can see, things just keep getting better the higher the voltage gets.


Bob:

The schematic in post #61 is the non-buffered version, and now that I look back, you should omit the 100k resistor across the two outputs before the cap. I thought it would help with start-up transients, but that turned out to not be a problem.

That's the schematic used in the measurements below, minus two filter caps which need to be adjusted.

qusp:

The IRF540 should be fine for the higher voltage version, just make sure you've got a heatsink. At 50V, the top resistor dissipates about 3.5W, the bottom resistors about 6.5W and the FET will dissipate about 3W.

Cheers,
Owen
thx for your hard work
which Is the device measured on graphs
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Old 15th May 2010, 11:52 AM   #139
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Hi Samoloko,

The device in the graphs is the FQA32N20. It's readily available, and has the highest transconductance at 50mA.

At some point I'll re-do the TC testing at 130mA and 25V DS, but that will have to wait for another day. I'm sure the results will be different, but I'm not sure by how much.

Cheers,
Owen
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Old 15th May 2010, 02:12 PM   #140
qusp is offline qusp  Australia
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Quote:
Originally Posted by opc View Post
qusp:

Both the output caps and the 100k resistors that follow them are going to be needed no matter what. Even though the differential between those two points would be 0 volts in the perfect circuit, there's still going to be 25 volts between each node and ground, so unless you use a transformer, you'll need to use caps to get rid of that.
right, now i'm with you. for some reason I wasnt thinking about a failure condition, hmm well actually this will be connected directly to a balanced headamp in the same case and the output will be switched by a relay, so perhaps I can put the 100k after the relay so its not in circuit when not needed. and at the moment I am running transformers on the output, so I guess I could omit the caps too if I chose to continue using them



Quote:
I explained the need for the 100k resistors above, so unless you always have the circuit connected to a preamp or amp, I would strongly suggest keeping the 100k resistors.
got it

Quote:
As for the layout, I'll provide both footprints where it is possible. The mosfets will only be through-hole, and I'm leaning towards PTH Caddock resistors for the 200 and 400 ohm parts mounted on heatsinks, but I'll allow for SMD here too for those who want to build the 18 volt version.
cool, I just picked some of the 200/400 SMD caddocks in a preemptive strike

Quote:
I've attached below what I consider to be the bare minimum anyone would want to go with this circuit. There are no filter caps to induce a rolloff, which means you'll get flat output well past 100kHz. Some find this bad, I personally can't tell the difference with or without the caps. The two 100k resistors are actually just there to represent a 100k pot.
cool i'll have a look at that. right, so they are representing a pot, I mentioned that before, but had thought you were talking about the other set of 100K

Quote:
This arrangement provides less fine adjustment than the other way, but it omits 2 resistors. It also allows you to crank up the gate voltage, which means you can blow your mosfets if you don't start them at a low value. This circuit is exactly what was used to make "The Beast" and it works very well.
i'll give it a try, just bought one of those prototyping PCBs like you had in your pic. ELFA has some great ones that have DSUB and IDC connection points at the card edge, so i'll likely make up an umbilical for sensing and power using a HV one I bought, and another that has HV contacts and coaxial mixed in the same DSUB. They are military connectors from amphenol, i'll end up using them in the final build too to keep things neat, if I go with an external power supply and just keep regs local.

the case is quite a decent size, but its getting pretty packed. will get some pics up next week some time. because im testing lots of different things and nothing is permanent as yet, I equipped everything with pin headers and using molex, so the below is no problem, but I will for sure print it out and put it above the bench



Quote:
Whatever you do, ALWAYS ADJUST THE CIRCUIT WITH THE DAC DISCONNECTED and then readjust once you've connected it. If you're using higher rail voltage, you could end up with a high or low voltage at the source of the fets before you adjust it.

Cheers,
Owen

thanks for your patience mate, I really appreciate it, I know I can get trying some times. cant wait to crank this up and team it with the new headamps (99% done), everything is coming together, will have the ackodac here next week as well, the buff will be going portable
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