Pass DIY Addict
Joined 2000
Paid Member
Yep - zener is mandatory. Use a big, fat, 5w one. You might also want to add a 1,000uF cap across your DC input if you are using a power brick PSU. Both will help prevent mosfet-zapping coil kickback in the case where your DC power supply is disconnect instead of disconnect at the AC source.
I learned this one the hard way...
I learned this one the hard way...
Mofo is alive!
Hi guys,
Just finished my mofo last night, after tweaking the bias and making sure all is well, i tried it in my usual setup: a pair of celestion sl6 in a nearfield setup. These speakers are rated at 82db/1w/1m, most inneficient... The sound is glorious. Bass, highs, image, clarity, presence... Never heard anything like it!
For the price its amazing.
One question: I couldn't find a schematic with the zener in place, where does it go?
Thanks
Pete
Hi guys,
Just finished my mofo last night, after tweaking the bias and making sure all is well, i tried it in my usual setup: a pair of celestion sl6 in a nearfield setup. These speakers are rated at 82db/1w/1m, most inneficient... The sound is glorious. Bass, highs, image, clarity, presence... Never heard anything like it!
For the price its amazing.
One question: I couldn't find a schematic with the zener in place, where does it go?
Thanks
Pete
...see if I can edit the first post...
That would be kind.
Here's some Zener filler:
Attachments
I think that will fix your issue.
The Zener was upgraded to "mandatory" some time ago, but I'm afraid that info is somewhat buried in lots of other information. It is especially important when using other MOSFETs (which may have even lower maximum Vgs) or other inductors (which might have a lot more "kick")
Let us know how you fare.
Uh oh.... Do I need to go back and install a zener? I've been running mine without for about 2 years now. IRFP250 and Hammond 159ZC, 60mH coil. Maybe the gentleness of a linear power supply helps alot? Even when I was using a switcher, it had CRC big cap bank after (36mF + 36mF).
Also the habitual grounding of the RCA plug tip before plugging into the input jack saves the gates.
I guess I need to post some pics now that I got this amp built into a proper chassis. Still loving the transparency of this amp Michael.
This is primarily a "turn-off" situation. When DC is removed, the GS voltage can surge higher than the power supply voltage. If that's higher than the max Vgs of your MOSFET, then trouble can ensue.
The Zener limits this voltage, and so you want to choose one in accordance with the MOSFET you're using.
Also, a cap of perhaps 220uf or more will damp this event, but again how much you need depends on the how low the max Vgs of your MOSFET is. In the case of the IRFP250 220uf is fine.
So, the sure fire fix is a Zener, but adding the cap is nice too (and it will smooth the turn-off thump)
Much of this is alleviated by disconnecting the AC side of the power supply, because most of them have some filter caps on the output. But I realize this is maybe inconvenient, so go with the Zener and bonus points for the cap.
The Zener limits this voltage, and so you want to choose one in accordance with the MOSFET you're using.
Also, a cap of perhaps 220uf or more will damp this event, but again how much you need depends on the how low the max Vgs of your MOSFET is. In the case of the IRFP250 220uf is fine.
So, the sure fire fix is a Zener, but adding the cap is nice too (and it will smooth the turn-off thump)
Much of this is alleviated by disconnecting the AC side of the power supply, because most of them have some filter caps on the output. But I realize this is maybe inconvenient, so go with the Zener and bonus points for the cap.
The IRFP250 has a max Vgs of something like 20V, and with some filtering or a lower supply voltage the surge may stay below that. However, it's an easy upgrade and cheaper than a new new MOSFET, so I say go for it.
Ah.... I've been under 20V with my power supply., about 17.2V at the MoFo power inputs.
Turn off is REALLY slow too. Those LEDs keep glowing for a couple minutes after powering off.
Bias at 1.9A
Here's a couple pics. HiFi 2000, 3U x 300 x 360 Dissipante worked for my version.
AnTek 200VA, 15V + 15V transformer.
Repurposed bi-polar, chip amp, cap + rectifier boards that were split bridge rectifiers.
Those got reconnected into a full bridge again,
with the minus side caps reversed and wired over to the plus side.
Little inductors are 4mH for a CLC power supply filter.
Listening right now using my newly built ACP+.... Sweet!
Thanks Pa and adopted son Michael.
Attachments
Pass DIY Addict
Joined 2000
Paid Member
Thanks y'all.
Yeah, part of my problem is I get something tweaked and sounding good on the bread board and end up sitting back and enjoying it for years and years...
It looked like this while I experimented with various FETs and buffers and tranformers for like 2 years.
I even tried some old Motorola T-MOS FETs (which sounded awful by the way).
Then made room in the rack for it while building ZM's Iron Pumkin Preamp
(highly recommended, like ZM said, "Iron Pumpkin rules all Papa Amps".
Attachments
