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Firstwatt F2 Build Log

post #1 of 48
Thread Starter 
Recently Nelson Pass released the schematic for the Firstwatt F2. It's a current amplifier that can be used to power single driver speakers (eg, single driver speakers, AKG K1000 and possibly other headphones).

This is a build log just for the sake of documentation, and also to assist anyone who's interested in building their own f2. Please post any comments or questions along the way.

Introduction

For the longest time I'd been building myself up to purchase a nice pair of SET monoblocks- I had my eyes on a pair of 300bs amps. Although this idea brought me great excitement, it also gave me some cause for concern. All 300b amp reviews seem to talk about going through great collections of tubes to find the brand which suits the best. 300b tubes are seriously expensive, so this was starting to sound a little bit painful. I'm a poor student.

While reading the diyaudio.com “full range” speaker forums I read lots of people using the firstwatt and saying it was great, but I never put much thought into it due to my SET dream. Somewhere along the line, however, I started to read about the firstwatt and how apparently amazing it was with my kind of speaker. Even better is that I heard many people claim that the F2 sounds just like a great SET. Could this be my dream amp? Eying off my SET monoblock budget, a DIY F2 seems easily affordable in comparison. And as luck would have it, just a few days after I started to seriously consider the F2, Nelson Pass released the schematic. It's a sign.

The F1 and F2 are current-amplifiers. This means that they wont work with passive crossover networks. K1000 people enjoy them, and so do people who use speakers without crossovers (fullrange driver speakers usually). I'm the latter.

Disclaimer

The Firstwatt F2 is a commercial design, and the following build log is about a clone. Although this would usually be outside the headfi rules (i think) and possibly unethical, it's important to note that Nelson Pass (the guy who designs and sells the firstwatt) only builds and sells 100 models of the Firstwatt amplifiers and then publicly released the build plans to encourage DIYers to clone it. This clone is thus done with full permission of the designer.

Secondly, if you decide to take inspiration from this build-log, and build your own F2, I take absolutely no responsibility to any harm done to yourself or your possessions in the process.

Day -2

Here is the schematic for the F2:

This is just for the amp circuit, and as the power supply uses the one from the firstwatt f1 service manual. This is covered in the next post

The first step in the build process is to find and order parts. Aside from the normal build parts from the schematic, it's very important to heatsink the F2 properly. The F2 runs in class A and puts off a constant 35w of heat per channel (over 2 MOSFETs). A quick lot of calculation indicated that, in order to properly heatsink the MOSFETs, a heatsink with a thermal resistance of 0.3C/W OR LOWER is required per channel. With this in mind, I decided to buy the MF30-2F-151.5 from:
http://www.conradheatsinks.com/produ...le_f.html#MF30

As a bonus, the heatsink is large enough to act as the side of the case, Since the rest of the case will probably be screwed onto the heatsinks, that should provide some additional heat dissipation I hope.

The following is my parts list for those who want to save the trouble of finding their own parts. I give zero guarantee that the parts are right, and take no responsibility for if you order the parts and find yourself with only 80% of an amp.

http://tinyurl.com/2zwbqn

Two film caps need to be chosen by the builder because i messed up when i did the buy and bought electrolyics. the two fim caps are marked on the list

The document is a little silly because I just hacked it together out of the real list that i used. The “real list” contained lots of extra parts incase I blow things up, was all in Australian dollars and was for two people.

As of the time that I'm doing this, the f2 schematic that Nelson has released is only for the amplifying circuit. For this reason, I'm using the power supply schematic from the F1 service manual. I believe that the F2 and F1 use the same power supply anyway.

Day -1

Time to order the parts. I got in contact with Conrad Heatsinks and they informed me that four MF30-2F-151.5 will cost $52.05 each (including gst. 10%ish which americans wouldn't have to pay), plus $12.50 for 2 working day delivery, for a total of $220.70. This is in AUD and is to buy the heatsinks for two amps (me and a friend). AUD$110 per amp. USD$86.

Next up, digikey. The price of both of our amps came to $397 usd, but take into account that this is buying lots of extra parts just to be careful (living in australia, I dont want to have to wait/pay for postage from america if one of my mosfets overheat or one of my caps or diodes explode.. plus i'm always happy to have spare parts around for future projects). This comes to $199 USD per amp. AUD$250. Oh, and this doesn't include the transformer and some various small parts that I'd rather buy locally.

Post to Australia was 46USD or so via global express mail (5-10 working days).

The transformer cost around $70AUD locally. Transformers should be 300VA or more to stop it from choking up, apparently

here is the transformer for aussies:
http://jaycar.com.au/productView.asp...Max=&SUBCATID=
post #2 of 48
Thread Starter 
Day 1 - Power supply

The parts finally arrived after some big mess up with the postal company. Everything seems in order from digikey.

Today me and a friend got started on the power supply. We approximately followed the layout provided on page 9 of the F1 service manual (here), but since it uses a custom PCB and we're using stripboard we had to wire it a little differently.

We laid it out like this:


And wired it up like this:


Or, in drawing form:

It's important to note that in the photos the right channel output wires are inverted (ie red goes to GND, not white like it should be. this was an accident)

All the capacitors are of the same value so their respective positioning isn't important (ie c8 can go where c3 is because they're the same value). The same goes for the resistors too, except except R9 and R10.

We raised all the parts off the board about 1cm for heat dissipation, but I dont think its really needed because the resistors don't get hot.

The other power supply parts are all just wired point-to-point at the moment.

and it works! yay! The easiest (but the most potentially explosive) part is now over... The psu puts out a reading of around 25 volts DC. Nelson's schematic says "24v", but 25v makes sense considering that it's turning 18V AC into DC.

The bridge rectifiers get HOT. I dunno if they get too hot, but I'd probably recommend putting them somewhere that they can be heatsinked somehow (i'll screw mine to my heatsinks, or to the chassis). I've run them floating in free air for several hours in a hot Australian summer without them overheating though.

And here it all is, doing it's thing (not really since the output leads aren't plugged into anything)



Day 2 - Left Channel

Next up is the amp circuit.

Here is the shematic:


We did the layout like this:



It was done this way for simplicity and also to have the mosfets a good distance apart for heat sinking.

Put together it looks like this


(note that the "positive" of the speaker goes to the GND of the amp circuit. for some reason.)

... and the bottom



The bottom of the mosfets are conductive and are not GND, so you need to put a non conductive element between it and the heatsink. we used silicone pads

The heatsinks seem perfectly capable of handling the heat, so far.

To bias the amp, measure between the GND and the opposite side of the output capacitor (ie the side of the C2 which doesn't go to the speakers). The correct bias is half of the input voltage (eg if your input voltage is 24V DC, you should be measuring 12V DC here). If not, turn the trimpot (P1) until it's correct. This should probably be done after it's warmed up.

Be careful that your DC inputs are the correct way. Originally ours were wired in reverse (see the power supply), and the blue resistor bank heated up like crazy and smoked. eep.

Similarly, ensure that your source inputs are correct. Ours was ALSO the wrong way and it resulted in massive distortion, amongst other things.

I initially had weird ground noise problems, but that was likely caused by a dodgy computer monitor connected to the same outlet.. i hope. I'll provide an update if anything changes there.

The mosfets are actually capable of holding the board to the heatsinks through the strength of their legs. Nonetheless, we decided to bolt the circuit boards to the heatsink, which can be seen in the photo. To do this you simply have to drill a hole through the heatsink flange and the circuit boards. Be careful not to snap the boards. This will also have to be done to screw the MOSFETS to the heatsink flange. It's possible to use thermal epoxy to connect the mosfets, but I prefer to bolt it since it's somewhat reversable, unlike epoxy which is a glue.

Also, as can be seen in the photo we had to use 1 and a half boards. Although all the parts could probably be crammed onto one board, I felt better about having them spread over two as it allows the MOSFETS to be further apart.

The heatsink that I used has two flanges. We used the bottom flange on the advice of someone at diyaudio.com. I suppose this is because heat rises.

I don't have too much in the way of impressions so far since there's only one channel. 5w seems plenty to power my speakers (96db + horn), even without a preamp

All of the 1ohm resistors get pretty hot, so if i was building it again I'd consider raising them off the board a bit
post #3 of 48
Thread Starter 
Day 3 - Right channel

The right channel is very much similar to the left channel, with just a few (important) changes. "Day 2 - left channel" of my journey actually took two days (although they were reasonably short days), whereas day 3 only took one short day since it was largely just copying the left channel, and we now had our methods down (in day 2 we had to draw up the layout etc.)

The right channel is largely just the left channel but mirrored, however some of the wiring needs to be changed, for the mosfets and the ztx550.

Here is our slightly adjusted layout for the right channel:



You can see that i just mirror'd the left channel with the following changes:
1) the mosfets have been rewired. for the irfp240 we just went over the board since the wires have to cross
2) the ztx550 wiring can stay the same, but you have to put it in backwards compared to the other channel

Here is an image of the wiring that was used to jump over the board for irfp240:


And by comparing these two photos you can see how the ztx550 has its orientation chanegd on the two channels. Look at how the curved side of the transistor faces the resistor on one image, and faces away from the resistor on the other image:

vs


Important Note

One small issue that we came across is that the legs to the mosfets were too big to fit through the stripboard. Our solution for this can kinda be seen in the "wire jumper" image above. Basically we used a dremel tool with a bit similar to the middle one on the image below to increase the size of the holes. try not to remove the solder pads while doing this though, because it'll make it a pain.



Also, the resistor bank R7-R12 gets really hot! If i was to do the build again I'd space them above the board for better air flow around them
post #4 of 48
Quote:
A quick lot of calculation indicated that, in order to properly heatsink the MOSFETs, a heatsink with a thermal resistance of 0.3C/W OR LOWER is required per channel. With this in mind, I decided to buy the MF30-2F-151.5 from:
http://www.conradheatsinks.com/produ...le_f.html#MF30
holy crap, those are the same heatsinks i used for my F1 which puts out >3x the heat! well it still works, so, i guess they sink enough. i do have a fan pointed at the thing bec i'm quite paranoid about the heat output.

you wont regret those conrad sinks. excellent build quality and performance.
post #5 of 48
Thread Starter 
Quote:
Originally Posted by choariwap View Post
holy crap, those are the same heatsinks i used for my F1 which puts out >3x the heat! well it still works, so, i guess they sink enough. i do have a fan pointed at the thing bec i'm quite paranoid about the heat output.

you wont regret those conrad sinks. excellent build quality and performance.
F1 has 100w input, 10w output (per channel).. 90w heat

F2 has 75w input, 5w output (per channel).. 70w heat

So although the F1 has far more mosfets than the F2, it doesn't generate a whole lot more heat. The fact that the heat is spread over more mosfets may actually be better for it (ie 90w of heat over 5 mosfets vs 70w of heat over 2 mosfets), but maybe not! I'm not pro enough to know.

Nonetheless, it's reassuring that your f1 is surviving with the same heatsink that I'm using (although the fan part doesn't fill me with confidence!). How hot does it get without the fan? The black part of my heatsink gets fairly warm but I'm easily able to hold it for as long as i want without feeling like I'm going to get a burn. The flange gets a bit hotter, but I still dont feel like i'm gonna burn by holding my finger on it. This is after it being on all day in Australian summer heat.

Unfortunately I have no effective way of measuring the temperature.. however just the feel of the heatsink fills me with confidence that it will be fine. I bought lots of excess mosfets anyway
post #6 of 48
Quote:
Originally Posted by hugz View Post
F1 has 100w input, 10w output (per channel).. 90w heat

F2 has 75w input, 5w output (per channel).. 70w heat

So although the F1 has far more mosfets than the F2, it doesn't generate a whole lot more heat. The fact that the heat is spread over more mosfets may actually be better for it (ie 90w of heat over 5 mosfets vs 70w of heat over 2 mosfets), but maybe not! I'm not pro enough to know.

Nonetheless, it's reassuring that your f1 is surviving with the same heatsink that I'm using (although the fan part doesn't fill me with confidence!). How hot does it get without the fan? The black part of my heatsink gets fairly warm but I'm easily able to hold it for as long as i want without feeling like I'm going to get a burn. The flange gets a bit hotter, but I still dont feel like i'm gonna burn by holding my finger on it. This is after it being on all day in Australian summer heat.

Unfortunately I have no effective way of measuring the temperature.. however just the feel of the heatsink fills me with confidence that it will be fine. I bought lots of excess mosfets anyway
I'm personally running even smaller heatsinks (11" x 5" x 1.5") in a tiny case on my F1 clone, and it reached about 65'C before I adjusted the bias down. Now it runs at a peak of 62'C and no problems here. All of my caps are rated 105'C for longevity, and mosfets can run much hotter still. I've got about 500 hours on mine and it's running great.

When I say 62'C I mean hot enough that it hurts to pick it up. Can't carry it for longer than a few seconds.

Nice work, btw.

I know it's a little late to suggest it, but I would have gone with 1/2" or even 3/4" (which I did) stock for the flange. The mating point on that bar is pretty small.

Did you use silicon/silver nitrite grease between the flange and the heatsink?

*edit* er... oh, were those flanges included stock on the heat sinks?
post #7 of 48
Thread Starter 
Quote:
Originally Posted by nightfire View Post
I'm personally running even smaller heatsinks (11" x 5" x 1.5") in a tiny case on my F1 clone, and it reached about 65'C before I adjusted the bias down. Now it runs at a peak of 62'C and no problems here. All of my caps are rated 105'C for longevity, and mosfets can run much hotter still. I've got about 500 hours on mine and it's running great.

When I say 62'C I mean hot enough that it hurts to pick it up. Can't carry it for longer than a few seconds.

Nice work, btw.
Thanks

Quote:
Originally Posted by nightfire View Post
I know it's a little late to suggest it, but I would have gone with 1/2" or even 3/4" (which I did) stock for the flange. The mating point on that bar is pretty small.
What do you mean by this? 1/2" - 3/4" tall or what? The flanges are 6mm tall (~1/4") and come out 35mm (~1.4").

I'm not sure if they could be improved by being thicker, but I think the thermal conductivity rating of 0.25 includes the flanges.

I'm not sure what you mean when you say "the mating point [...] is pretty small", however 100% of the mosfet is on the flange.

Quote:
Originally Posted by nightfire View Post
Did you use silicon/silver nitrite grease between the flange and the heatsink?
I am using silicone pads on the heatsinks, the flange side of the mosfet is "drain", and thus we wanted to isolate the mosfet from the metal heatsink. I dont actually know what "drain" does, but I dont think it's ground, because the drain pin of the mosfet doesn't go to ground in the amp circuit.

The case will be earthed so I dont want to connect drain to earth if it's not ground (heatsinks probably aren't very conductive but it's very easy to add a silicone pad so I wont take the risk)

Do silicone pads help or hurt heat transfer? I'm guessing they hurt it, but at least they'd give more consistent contact. If heat even becomes an issue I'll consider thermal grease instead

Quote:
Originally Posted by nightfire View Post
*edit* er... oh, were those flanges included stock on the heat sinks?
Yup the flanges are a part of the heatsinks

Thanks for your comments on suggestions, especially coming from a firstwatt builder

edit:

Here's an action shot



I'll do the writeup for the 2nd channel later tonight, and some listening impressions. for now, it's time for me to enjoy some tunes
post #8 of 48
oh ooops, i messed up my computations i thought the F2 did 35w per channel. without the fan, with the AC on (my normal ambient room temp is 30 deg C, so i dont think its a fair comparison) it gets too hot to hold for 10 seconds straight.
on the other hand, my entire chassis is like a secondary heatsink, there are 4 4mm thick aluminum panels for the top bottom front and back, thermally coupled with the sinks, so that adds a fair bit of dissipation as well.

here's how i hooked my sinks up:
http://www.head-fi.org/forums/showpo...&postcount=879

for the F2, i think it should be ok at around 25 deg C ambient, on an open area, like top of the shelf.

i dont have any way to measure the temp myself, i'll get a thermal sensor one of these days

@nightfire, those flanges are machined with the sinks, its all one huge hunk of aluminum. you really get your money's worth with these.
post #9 of 48
Quote:
Originally Posted by hugz View Post
What do you mean by this? 1/2" - 3/4" tall or what? The flanges are 6mm tall (~1/4") and come out 35mm (~1.4").
Yeah I meant taller in your case (the 1/4" figure). The problem is where the flange mates with the heatsink may not transfer heat as quickly as it should... which would explain why the flange itself gets hotter than the heat sink.

It's not a big deal as yours is cool enough to touch, but in my case I used 1/4" stock and the bar was about 10'C hotter than the heat sink.

Quote:
I'm not sure if they could be improved by being thicker, but I think the thermal conductivity rating of 0.25 includes the flanges.

I'm not sure what you mean when you say "the mating point [...] is pretty small", however 100% of the mosfet is on the flange.
Well that's its rating, but in order to transfer heat from one object to another (assuming it's not welded, but even then), one object has to be hotter than the other. How much hotter depends on its surface area, mating quality, etc. So in your case maybe the flange is 5'C hotter than the heat sink. You could get that down to 1'C with a 3/4" bar. The "T" piece at the bottom does increase the surface area, but not as well, because the heat still has to transfer from the main portion of the bar out through the T flaps.

The other thing a large heavy bar does is distribute heat better for better thermal stability. Because of the higher heat capacity, it heats and cools more slowly, and is less prone to heat localization. This can make a big difference when you've got mosfets in a balanced config.

Quote:
I am using silicone pads on the heatsinks, the flange side of the mosfet is "drain", and thus we wanted to isolate the mosfet from the metal heatsink. I dont actually know what "drain" does, but I dont think it's ground, because the drain pin of the mosfet doesn't go to ground in the amp circuit.
I meant the actual mating between the flange and the heat sink. When just bolted, mine were running about 3'C hotter than the heat sinks. When I coated them with thermal compound, it went down to 1'C. In retrospect it probably wasn't worth it, but if the mating surface is small (ie my config before), it might be.

It looks like you used Silpads to isolate the drains from the flange.. Same here. Those bastards are so expensive.

Quote:
The case will be earthed so I dont want to connect drain to earth if it's not ground (heatsinks probably aren't very conductive but it's very easy to add a silicone pad so I wont take the risk)

Do silicone pads help or hurt heat transfer? I'm guessing they hurt it, but at least they'd give more consistent contact. If heat even becomes an issue I'll consider thermal grease instead
It depends. The ultimate for heat transfer is a quality silver compound with metal on metal, but you're right - no good in our case. Silpads are very good, and may even be better than bare metal on bare metal if the surfaces are crap.

I wouldn't worry about heat. Mosfets are tough (check the data sheet; I recall the internals rated to survive upwards of 200'C). Caps are the biggest thing to worry about if the case isn't well vented.

Quote:
Yup the flanges are a part of the heatsinks
Ah. Then disregard everything I've said.

Quote:
Thanks for your comments on suggestions, especially coming from a firstwatt builder
I'm really curious to hear your impressions.
post #10 of 48
Quote:
Originally Posted by choariwap View Post
@nightfire, those flanges are machined with the sinks, its all one huge hunk of aluminum. you really get your money's worth with these.
I see.

That would have saved me some effort.

Man your F1 looks like it should be in an M1 abrams. 4mm panelling? If I had to take something to war, that would be it.
post #11 of 48
Thread Starter 
Quote:
Originally Posted by choariwap View Post
I really dig the look of your front panel. Do you have any photos of it with the top on? I'm struggling with whether to use silver metal (easy) or paint it black so that the colour of the case matches the heatsinks (less easy).

I also really like the bolts on the front panel. do they go through the heatsink flanges?

So far I'm having two little problems with the F2:
1) when my computer monitor is plugged into power and plugged into my laptop (music source) then I get a low frequency hum. Is it the world's longest ground loop? (monitor to laptop, through usb to my usb/spdif, through coax to my dac, to my f2). When I remove the earth of my F2 it goes away, which also suggests ground loop.
a) if the problem is with the monitor then I dont care since it's crap anyway, but if the problem is in the F2 then i do care
b) can this be fixed (within the f2) easily? is there any way to protect it from ground loops?
2) I'm getting some noise from it. It's early days yet, but so far I can say that it was noisier last night than it was during the day.. and this morning it's quieter again. So I suspect it may by line noise since the power at my house is terrible, and power changes between day and night. How to fix?

edit:

and here's another "action photo". Sorry about the lighting, but it was taken at night with my not-too-impressive camera. the angle is dumb too because I had to put it on a flat surface (a chair) to stop it from blurring in night mode.

This is my F2 powering my really-not-complete speakers. They uh, still need a lot of cleaning up

post #12 of 48
Quote:
Originally Posted by hugz View Post
I really dig the look of your front panel. Do you have any photos of it with the top on? I'm struggling with whether to use silver metal (easy) or paint it black so that the colour of the case matches the heatsinks (less easy).

I also really like the bolts on the front panel. do they go through the heatsink flanges?

So far I'm having two little problems with the F2:
1) when my computer monitor is plugged into power and plugged into my laptop (music source) then I get a low frequency hum. Is it the world's longest ground loop? (monitor to laptop, through usb to my usb/spdif, through coax to my dac, to my f2). When I remove the earth of my F2 it goes away, which also suggests ground loop.
a) if the problem is with the monitor then I dont care since it's crap anyway, but if the problem is in the F2 then i do care
b) can this be fixed (within the f2) easily? is there any way to protect it from ground loops?
2) I'm getting some noise from it. It's early days yet, but so far I can say that it was noisier last night than it was during the day.. and this morning it's quieter again. So I suspect it may by line noise since the power at my house is terrible, and power changes between day and night. How to fix?

edit:

and here's another "action photo". Sorry about the lighting, but it was taken at night with my not-too-impressive camera. the angle is dumb too because I had to put it on a flat surface (a chair) to stop it from blurring in night mode.

This is my F2 powering my really-not-complete speakers. They uh, still need a lot of cleaning up

Don't know about the ground loop, but I don't think there's anything you can do to prevent it really. Having multiple paths to ground will always create a loop antenna. I don't know much about the F2, but are the - inputs grounded, or is the amp balanced? You could try grounding or ungrounding the - (to the chassis).

Otherwise it might be worth investing in an isolating power conditioner. Even if it doesn't help it's nice to know your equipment is protected from surges, spikes, harmonics, DC, etc.

What do you mean by noise? ie. static? clicks/pops?

If you're not doing it already, you might want to carefully monitor the DC offset while you've got it hooked up, because at least on the F1 static can be caused by a shifting imbalance between the mosfets which creates a DC offset. This was a problem I had until I dumped some extra capacitance on the rail, close to the mosfets.
post #13 of 48
Thread Starter 
Quote:
Originally Posted by nightfire View Post
Don't know about the ground loop, but I don't think there's anything you can do to prevent it really. Having multiple paths to ground will always create a loop antenna. I don't know much about the F2, but are the - inputs grounded, or is the amp balanced? You could try grounding or ungrounding the - (to the chassis).
F2 is single ended (not balanced). I'll try ungrounding the input sometime. If there's no good solution then I will just be hopeful that the monitor is the only device which causes problems. It's a really dodgy half broken monitor, so I wouldn't be surprised. Maybe I'll add a switch that disconnects the earth on command

Quote:
Originally Posted by nightfire View Post
What do you mean by noise? ie. static? clicks/pops?
It's like a buzz with a static crackle through it. Not a hum

Quote:
Originally Posted by nightfire View Post
If you're not doing it already, you might want to carefully monitor the DC offset while you've got it hooked up, because at least on the F1 static can be caused by a shifting imbalance between the mosfets which creates a DC offset. This was a problem I had until I dumped some extra capacitance on the rail, close to the mosfets.
How do I do this? I asked on diyaudio about matching mosfets for the f2 and was told it doesn't matter. From what I can tell the DC offset is affected by how the difference mosfets get along with each other? (how closely matched they are).

The F2 only has two mosfets per channel, and they are a different type (irfp240 and irfp9240), so I dont think there's anything to match since there's only one of each
post #14 of 48
Quote:
Originally Posted by hugz View Post
I really dig the look of your front panel. Do you have any photos of it with the top on?
here you go:


some closeup detail: note the thickness of the panel


Quote:
Originally Posted by hugz View Post
I'm struggling with whether to use silver metal (easy) or paint it black so that the colour of the case matches the heatsinks (less easy).
i like the two tone look of mine. also saves you the hassle of painting everything. mine is actually bare aluminum since we couldnt find anyone to anodize it. the little scratches gives it a rugged look and gives it character

Quote:
Originally Posted by hugz View Post
I also really like the bolts on the front panel. do they go through the heatsink flanges?
the whole thing is supported by an aluminum skeleton frame. the panels and sinks are bolted to the frame, you can see signs of the frame in the pics with the top off. the panels are not bolted to the sinks. it took a lot of work to build the chassis, most of the build time actually went into it.


Quote:
Originally Posted by nightfire
Man your F1 looks like it should be in an M1 abrams. 4mm panelling? If I had to take something to war, that would be it.
well, you would definitely need a tank to move the thing it was a struggle to this thing up a flight of stairs, whew!
post #15 of 48
Quote:
Originally Posted by hugz View Post
F2 is single ended (not balanced). I'll try ungrounding the input sometime. If there's no good solution then I will just be hopeful that the monitor is the only device which causes problems. It's a really dodgy half broken monitor, so I wouldn't be surprised. Maybe I'll add a switch that disconnects the earth on command



It's like a buzz with a static crackle through it. Not a hum

How do I do this? I asked on diyaudio about matching mosfets for the f2 and was told it doesn't matter. From what I can tell the DC offset is affected by how the difference mosfets get along with each other? (how closely matched they are).

The F2 only has two mosfets per channel, and they are a different type (irfp240 and irfp9240), so I dont think there's anything to match since there's only one of each
Hmm... so it uses a blocking capacitor I guess then? I couldn't find the schematic but from the F2 service manual it appears the IRFP9240 would be the current source, so yeah, no matching needed.

If you're using a blocking cap then DC offset is impossible and not something you have to worry about.

Other things you could check:

If you used flux, make sure you cleaned the joints around everything well. Flux is conductive so if you missed a spot with flux remover (particularly around input caps, resistors, etc) that can create noise.

There could be an oscillation at play but I doubt it would have that effect. You may want to beef up or shorten your power wires from the capacitor bank.

Try bypassing the input cap with a wire and make sure it's not the cause.

If you're using zener diodes to dissipate static on the inputs, try disconnecting them.

Also try an alternate power supply if you have one.

*edit* The DC offset that can arise in the F1 is due mainly to a mismatch between the two gain and two current source mosfets. It happens due to manufacturing differences (which is why they must be tested/matched), and thermal differences. In your case, you don't have to worry because 1) you're almost certainly using a DC blocking cap which won't pass the offset and 2) the IRFP240 and IRFP9240 aren't balancing each other - one's providing a constant current (with the help of, I'm guessing, a ZTX550), while the other is providing the gain.
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