Hi,
I'm a newbie (with electronics). For many reasons indicated in this thread (clean design/low noise/class A/....) I've decided to build a pair myself. I've used standard components with a 24V (meanwell) supply and Hammond 159ZC choke. A voltage rea1.25V across the 0.7ohm coil should give me about 1.8A bias current. I don't manage to get a stable voltage reading. It keeps "drifting". When I dial in the potentiometer the voltage across the coil seem to respond extremely slow in either direction. Is this normal?
Any advice on this matter would be much appreciated!
I'm a newbie (with electronics). For many reasons indicated in this thread (clean design/low noise/class A/....) I've decided to build a pair myself. I've used standard components with a 24V (meanwell) supply and Hammond 159ZC choke. A voltage rea1.25V across the 0.7ohm coil should give me about 1.8A bias current. I don't manage to get a stable voltage reading. It keeps "drifting". When I dial in the potentiometer the voltage across the coil seem to respond extremely slow in either direction. Is this normal?
Any advice on this matter would be much appreciated!
- The drift can have the origin on the bias supply: the SIT could require a stable temperature; so after some time readjust the bias for the current you want.
- And when the choke warms up the resistance increases so the voltage across it varies too. If then you dial in less drain current then the temp drops, res drops etc. That is an unstable feedback loop to you as operator! Luckily they do not manage thermo-nuclear plants like that. 🤯
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Yep, as temps of your output mosfets and coils change, so too will the bias current. Let it fully warm up (no less than one full hour) and make your adjustments then. Then forget about it. You'll drive yourself crazy trying to manage temps and bias levels otherwise. Been there, done that.
Let it warm. Set it and forget it.
Let it warm. Set it and forget it.
Hi,
Many thanks for the inputs. Meanwhile I've populated a new pcb and all worked out fine! The voltage across the choke kept increasing to a certain extend until everything reached a stable temperature. I was surprised how sensitive the setup is to temperature. If I just blow (single puff) on the FET,
I immediately see the effect on the voltage across the choke!!!
Don't worry, I have no ambition to cool the FET my self
. I'm using a massive heatsink for the cooling of the PSU and the 2 FET's. If needed I'll add some fans. Next up is to repopulate the first board the caused me some headache.... should run ok now.
Many thanks!
Michel
Many thanks for the inputs. Meanwhile I've populated a new pcb and all worked out fine! The voltage across the choke kept increasing to a certain extend until everything reached a stable temperature. I was surprised how sensitive the setup is to temperature. If I just blow (single puff) on the FET,
Don't worry, I have no ambition to cool the FET my self
. I'm using a massive heatsink for the cooling of the PSU and the 2 FET's. If needed I'll add some fans. Next up is to repopulate the first board the caused me some headache.... should run ok now.Many thanks!
Michel
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Glad to hear it's working out now! As usual, images reveal a great deal of insight. With four digits of measurement to the right of the decimal place on your HP meter, you'll notice a change every time someone in France turns on a light switch . Setting bias to the tenth of a volt will be more than sufficient.
. Setting bias to the tenth of a volt will be more than sufficient.
Back in post #2,995 I shared a link for a choke I found. I believed, given the size of the choke, it was somewhere between a 193T and a 195T5. It's not. When I initially built my monoblocks, I set the bias at 1.8A. Shortly after, I felt I made a mistake with the Toshiba FET I used, so I started trying different FETs. Somewhere along the line I started to set the bias at 2A as well. And somewhere along the line I lost my Mofo mojo. Tried different speakers, DACs, preamps, stuck a MiniDSP in the mix to offload the lowest frequencies. Even after swapping in the recommended IRFP250's, there was no improvement. Well, I didn't secure one of the IRFP250's well enough to the heatsink and it burnt out, so I decided to shelve this project as it wasn't sounding good to me any more. Given this is my first foray into Class A, I wondered if I fooled myself in the beginning and whether I should keep going down this road(I'm collecting parts for an F3 clone). A few months passed and decided to give the Mofos another shot. So I ordered more FETs and got the dead monoblock running again. No mojo Ok, maybe I need more current. Up the bias to 2.25A 
No mojo... wait a minute, have things gotten even worse? Back the bias down to 1.8A. AHA! Mofo mojo back in full effect Moral of the story, don't waste your time with those chokes. At best, they're on par with the 193T.
Ok, maybe I need more current. Up the bias to 2.25A No mojo... wait a minute, have things gotten even worse? Back the bias down to 1.8A. AHA! Mofo mojo back in full effect Moral of the story, don't waste your time with those chokes. At best, they're on par with the 193T.
The question I have, is how we can measure the saturation point of unknown chokes. (I have one myself, big, but unknown) Yes, order a well-known choke. That's the remedy for lack of mojo and abundance of lofi...
Let it flow again (cash that is). I intermittedly have good and choked (figuratively) sound. I'm getting bald from not understanding and pulling my hair out.
Let it flow again (cash that is). I intermittedly have good and choked (figuratively) sound. I'm getting bald from not understanding and pulling my hair out.
I bought and used a pair of these chokes as well with IRFP240's and old 19V HP laptop bricks for power. They worked fine for me, though I never did systematically compare them to MOTs or other chokes. sorry to hear they didn't work for you.
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It was a hack
I didn't have access to a drill press at the time so I didn't attempt to drill & tap these rather expensive hunks of aluminum@ranshdow - It's not that they didn't work. They just didn't sound good with 2+ amps idle current. They sound fine at 1.8A
In other news, the donor chassis for my next source follower build came today. Last month, I ordered a 4U Deluxe chassis for my upcoming F3 build. Two days later I see this on the 'Bay. Seller was letting it go cheap because it has a few dead channels. After being in limbo somewhere between Alabama and here(South Carolina) for a few weeks, it has finally arrived. It's a Sonamp 1250. Almost as big as a 5U Deluxe. But didn't cost me quite as much. I haven't decided if I'll go for a Big Mofo or something else. Whatever it is, I think these heatsinks can handle it
In other news, the donor chassis for my next source follower build came today. Last month, I ordered a 4U Deluxe chassis for my upcoming F3 build. Two days later I see this on the 'Bay. Seller was letting it go cheap because it has a few dead channels. After being in limbo somewhere between Alabama and here(South Carolina) for a few weeks, it has finally arrived. It's a Sonamp 1250. Almost as big as a 5U Deluxe. But didn't cost me quite as much. I haven't decided if I'll go for a Big Mofo or something else. Whatever it is, I think these heatsinks can handle it
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Hi there!
A new Mofo is born. Two in fact (monoblocks)...
Nothing fancy, there are even cheap caps in place. Cannot yet evaluate it properly, since there is no preamp yet, so it it connected directly to Dac output.
The choke was some surplus part, no idea what was it original function. It has 96mH, a thick wire, and just 0.6 ohm dcr. But, I am not sure if it has an air gap...
Running on 19v smps...
A new Mofo is born. Two in fact (monoblocks)...
Nothing fancy, there are even cheap caps in place. Cannot yet evaluate it properly, since there is no preamp yet, so it it connected directly to Dac output.
The choke was some surplus part, no idea what was it original function. It has 96mH, a thick wire, and just 0.6 ohm dcr. But, I am not sure if it has an air gap...
Running on 19v smps...
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