Lquam says he has been ordering parts from polida. I haven’t ordered from this source. The pictures match the parts I have received from Newark.
From Lhquam:
My source for FETs was the EBay seller Polida Electronics Ltd. For $14.99 + $5.00 shipping you can order 5 pairs of FQA28N15+FQA36P15.
My source for FETs was the EBay seller Polida Electronics Ltd. For $14.99 + $5.00 shipping you can order 5 pairs of FQA28N15+FQA36P15.
from octopart FQA28N15 is still available and we can buy them with more peace of mind :
Hopefully this isn't inappropriate, but I just received 100 FQ28N15 ONSEMI from Newark, 10 are spoken for and I will keep a couple dozen +/-. If anyone is interested, before shipping they are $1.90/ea plus shipping, let me know if interested in private conversation.
I'm hoping to get some input on my F5m project, which in time I hope to offer as a kit. Block diagram is below and full schematic is attached.
PCB layout is next. I'll start a new thread when I've got a prototype to share, or if this ends up being distracting from the main topic.
- Two 24V, 2.5A SMPS are used on each channel for the supply rails (monoblocks).
- There's enough board space for filtering, so I've added Mark Johnson's AmyAlice SMSP filter to V+ and V-
- Switches rated for this DC voltage and current are hard to find and get expensive quickly, so I've opted to use MOSFETs to do the heavy lifting and the physical button will be switching 9V created by an always on DC/DC converter.
- All the protection modes of the power supplies result in that supply turning off it's output, so I added a basic undervoltage monitor (using zener diodes) that will switch off both supplies (via optocouplers) if either drops out.
- Will this be sufficient protection for the amp and speaker? Is a solid state output relay a better solution?
- A fan keeps the heatsink cool, so I need a way to turn the amp off in case of fan failure. I'm using the fan's tachometer output to drive a pair of charge pumps to turn on the supply rail power switches. As a bonus, since the charge pumps are AC coupled, they can be referenced to the source of each FET. This allows N-channel parts to be used on both rails, which are cheaper and have lower on resistance than the available P-ch options.
PCB layout is next. I'll start a new thread when I've got a prototype to share, or if this ends up being distracting from the main topic.
Attachments
I am building two stereo amps based on the F5M circuit. I'm planning to use these to try biamping my speakers (open baffle planar magnetic line arrays). I decided to design my own PCBs to add a few extra features.
In addition to Papa Pass's elegant circuit, I have added a line input transformer for balanced inputs which is relay-switched with the RCA input, a DC servo circuit, a speaker protection circuit with delayed turn-on, and additional local decoupling on the power supply rails.
I wanted the protection circuit and DC servo since I'll be connecting directly to the tweeters with no inline blocking cap so I wanted to make sure DC offset remains very low.
The amp is built using the SFP soft-start into an Antek 400VA 20V transformer which feeds a SLB power supply. I designed another PCB which includes an additional 60,000uF of capacitance for each channel, separately connected to the SLB through some 0R1 resistors.
I've currently got the bias set to around 1.3A (23V rails) and the heat sink temp next to the MOSFETs is around 52C.
In addition to Papa Pass's elegant circuit, I have added a line input transformer for balanced inputs which is relay-switched with the RCA input, a DC servo circuit, a speaker protection circuit with delayed turn-on, and additional local decoupling on the power supply rails.
I wanted the protection circuit and DC servo since I'll be connecting directly to the tweeters with no inline blocking cap so I wanted to make sure DC offset remains very low.
The amp is built using the SFP soft-start into an Antek 400VA 20V transformer which feeds a SLB power supply. I designed another PCB which includes an additional 60,000uF of capacitance for each channel, separately connected to the SLB through some 0R1 resistors.
I've currently got the bias set to around 1.3A (23V rails) and the heat sink temp next to the MOSFETs is around 52C.
@Vunce ,
I'm using RC filters (500R, 100uF) from the main rails into +/- 12V regulators (LT3045, LT3094) for power to a LT1007 op amp. The amp output is filtered through a 1M into 2.2uF cap. I have diodes across the op amp inputs to limit the maximum input. The output of the op amp connects to the jfet sources through a 1K resistor. I have a jumper on this connection so that I can set the bias before the servo circuit is connected.
I'm using RC filters (500R, 100uF) from the main rails into +/- 12V regulators (LT3045, LT3094) for power to a LT1007 op amp. The amp output is filtered through a 1M into 2.2uF cap. I have diodes across the op amp inputs to limit the maximum input. The output of the op amp connects to the jfet sources through a 1K resistor. I have a jumper on this connection so that I can set the bias before the servo circuit is connected.
My F5m case is speeding here now from Italy, along with black knobs for Iron Pre. In mean time took out original F5, first First Watt I built, years ago.
Also used original first build Iron Pre, and dug out old Pearl 2 because it seems To work well with analog front end at hand. Since main turntable is in other room, I used second table.
Second turntable occurred as I modified my AR the turntable. It has the full house of Merrill mods including speed control. This basically replaced everything in and on the turntable except wood piece and metal top plate. As mods replaced original parts, put in box.
On day on eBay I found guy who was making/selling diy wood boxes that duplicated my turntables wood. Bought from Merrill delrin arm board, extended posts and springs, and his replacement motor and wall wart power supply. Also found on Vinyl Nirvana iirc, new metal top plate for AR tables.
Arm board cut for SME arm, and mounted as new Infinity Black Widow tone arm, and Shure V15 vxmr cartridge I bought few years ago for close out, and handful of replacement stylus.
Works well with Pearl 2, output on cartridge only 3 mV so not overloading Pearl 2 too much.
This was one fun setup! I forgot all the pluses of this old cartridge! Giant sound stage, powerful bass. Not nearly as transparent and refined as main table with moving coil setup, but quite powerful sound that hit fun category on head!
Once listen to this for few days, then will put in Pearl 3 dialed in for this cartridge. That will allow some comparison. Also, once finished can compare new Fm5 to old version. Pearl 2 was rocking this set up, fun factor very high! Turntable built from cast offs from old one.
Russellc
Also used original first build Iron Pre, and dug out old Pearl 2 because it seems To work well with analog front end at hand. Since main turntable is in other room, I used second table.
Second turntable occurred as I modified my AR the turntable. It has the full house of Merrill mods including speed control. This basically replaced everything in and on the turntable except wood piece and metal top plate. As mods replaced original parts, put in box.
On day on eBay I found guy who was making/selling diy wood boxes that duplicated my turntables wood. Bought from Merrill delrin arm board, extended posts and springs, and his replacement motor and wall wart power supply. Also found on Vinyl Nirvana iirc, new metal top plate for AR tables.
Arm board cut for SME arm, and mounted as new Infinity Black Widow tone arm, and Shure V15 vxmr cartridge I bought few years ago for close out, and handful of replacement stylus.
Works well with Pearl 2, output on cartridge only 3 mV so not overloading Pearl 2 too much.
This was one fun setup! I forgot all the pluses of this old cartridge! Giant sound stage, powerful bass. Not nearly as transparent and refined as main table with moving coil setup, but quite powerful sound that hit fun category on head!
Once listen to this for few days, then will put in Pearl 3 dialed in for this cartridge. That will allow some comparison. Also, once finished can compare new Fm5 to old version. Pearl 2 was rocking this set up, fun factor very high! Turntable built from cast offs from old one.
Russellc
That was an enjoyable recounting of your journey with a classic turntable. It is a well spoken reminder of the way we achieve our audio nirvana is much of the fun itself. 
I also started with an AR turntable that I bought in the Boston area when I was living there in the early 80's. It came with a Linn Basik arm and Astatic cartridge. I was considering the Merrill mods for a while but ended on a different path. A VPI HW 19 Jr became transmogrified into an HW 19 mk II with old school lead lined acrylic platter, acrylic & stainless arm board and Linn Ittok arm. The cartridge will be changed from a Rega Exact to a Hana SL as I introduce my Pearl 3 into the system.
And the Pass F5m that I built a while back has been the instrument of choice for judging the upgrades to convert a Rega P3-24 into something more P6-like. My collection of vinyl has generally only been played about three times since my practice was to record albums onto cassette tape. Clean vinyl played on a good 'table is just plain fun.
I also started with an AR turntable that I bought in the Boston area when I was living there in the early 80's. It came with a Linn Basik arm and Astatic cartridge. I was considering the Merrill mods for a while but ended on a different path. A VPI HW 19 Jr became transmogrified into an HW 19 mk II with old school lead lined acrylic platter, acrylic & stainless arm board and Linn Ittok arm. The cartridge will be changed from a Rega Exact to a Hana SL as I introduce my Pearl 3 into the system.
And the Pass F5m that I built a while back has been the instrument of choice for judging the upgrades to convert a Rega P3-24 into something more P6-like. My collection of vinyl has generally only been played about three times since my practice was to record albums onto cassette tape. Clean vinyl played on a good 'table is just plain fun.
I thought it interesting that all the parts I removed from my main turntable were re cooked back into a turntable, once I stumbled on the guy making the AR plinth/ base part. Why not? It makes a good bedroom turntable, where it resides now.
The AR was one of the best examples of an affordable spring suspension turntable back when the Linn was the main and much more expensive alternative. Perfect for a second system. I was thinking of setting up the AR in my office, but needed to install a Rega wall mount shelf to make that happen.
The Sorbothane puck supports in the VPI Jr were a step back, but the acrylic platter was an improvement, so kind of a wash overall relative to the AR. The VPI really needed the spring suspension upgrade, and the Ittok arm handles a wide range of cartridges. I could build a second VPI Jr from remaining parts if I wanted to, but that’s not an interesting option now that I have the Rega as a second table in my main system.
The Sorbothane puck supports in the VPI Jr were a step back, but the acrylic platter was an improvement, so kind of a wash overall relative to the AR. The VPI really needed the spring suspension upgrade, and the Ittok arm handles a wide range of cartridges. I could build a second VPI Jr from remaining parts if I wanted to, but that’s not an interesting option now that I have the Rega as a second table in my main system.
The first AR turntable was built in 1961, while the Linn LP12 was introduced in 1972, and was based on the AR.
