Total Domination VFET (TDV) Amp (using 2SK2087C)

[codyt]

Yeah, I just used keratherm from store

 

[codyt]

Any build updates? Listening to mine now – sounds great!

 

[e_fortier]

It’s next on my list, can’t wait. Gotta finish my F3 first.

 

[Jerdy]

I received a pair of 2SK180s + TDV boards in the BAF 2022 raffle, so I’m hoping to put a little amp together really soon. I did a quick prototype on the bench and gave it a listen. Excited for what’s to come!

 

[codyt]

Nice! I'll be curious what resistance value you land on in front of the choke. Guessing maybe around 2R?

 

[Jerdy]

With a bench supply of 15V and drain current of 2A, I was seeing about -2.5 to 0-3.0V on the gate of the sample I had. Once I decide on the power supply design (First Watt PSU seems to be the suggestion) and final supply voltage, I will experimentally dial in the resistance.

 

[von Ah]

I’d like to confirm some part specs for my build (CYOF boards):

1. What wattage rating is needed for R1, R2, and the 1k resistor across the speaker terminal?
2. What are recommended values (or part numbers) for Z1?

 

[codyt]

.25W for R1 and R2. 3W for the one across the terminals. And I think 18-20v for the zener.

edited

 

[codyt]

I just used RN55 or RN60 for R1 and R2. The R3 ‘group’ should be around 10W dissipation.

 

[von Ah]

Thanks! Is 1W rating ok for the Zener?

 

[codyt]

yeah, that’s plenty.

 

[codyt]

One quick thing to consider when mounting that 3W-1K across the terminals. You can easily mount one on the back of the board, if you like that idea.

 

[ra7]

Nice build Cody. Where is that chassis from?

 

[codyt]

Nice build Cody. Where is that chassis from?

Thanks! Its an eBay special from China. I really like it. About a 4U/400 and has plenty of dissipation. https://www.ebay.com/itm/223591731160

 

[codyt]

It has a 5U big brother, which I've been meaning to try too: https://www.ebay.com/itm/323862440733

 

[Jerdy]

As of today I got the 2SK180s mounted and wired up to the CYOF boards. Power supply is based off First Watt schems. 24vac fed, nominal 30V out. Static tests using a bench supply to the gates showed about -4.5v for 2A Id @ 30vds. The Hammond 159ZC chokes I have measure about 0.9~1.0 ohm (not super accurate here) so I figured I could parallel two 2.7R (had on hand) and that should give me enough. SITs are pretty decently matched and gave about 1.85A with 1.35R plus the chokes and just shy of 30v on the drain. I need higher wattage resistors long term, so I will likely go a little higher so the bias isn’t as hot. I measured a case temp of about 50C on the SITs when I turned it off. I was able to hook speakers up and get a little listen too, and it sounded alright with no hum.

Anyone know what a reasonable case temp is for these? Datasheet gives max junction temp and nothing else. The chassis as envisioned will see these sinks attached to a large and thick aluminum plate, but I’m not well versed in designing for dissipation on devices like these.

 

[Zen Mod]

regular praxis - with top of heatsink at 55C, no worries

 

[ra7]

Remember, ZM said heatsink. Case temp can be higher. You can do the math and figure out the junction temp from the case temp given the datasheet numbers (junction to case thermal resistance).

 

[Jerdy]

Remember, ZM said heatsink. Case temp can be higher. You can do the math and figure out the junction temp from the case temp given the datasheet numbers (junction to case thermal resistance).

The 2SK180 datasheet doesn't give the thermal resistance figures. I could work backward as far as the case-to-heatsink, but without the junction-to-case I would ultimately be guessing at true junction temp. Once I have the right resistors in, I will do some tests to monitor the thermals over time.

 

[Zen Mod]

just chill

buggers are tough

with 60W of heat you can use cardboard as isolator pad, and they'll endure, until wire is melting-desoldered from source

and then again