[myinitialsaredac]

Okay all, I am having a Millett Max built and was curious as to what differences the mosfet output stage has over a bjt diamond buffer stage.
Specifics on the one I am having built are currently:
Can't Miss MAX Build #1
CA2: Black Gate NX, 1000uf 25V
CA7: Black Gate NX, 680uf 35V
CA8: Vitamin Q's, 0.22uf 100V
CA9: blank (Wimas elsewhere)
2SC2238/2SA968 BJT Diamond Buffer

AE6 tubes.
It will be driving my k701s.

I have the option to do a mosfet output for around 50$ so my question is should I and what differences does it have?

Thanks,
Dave

 

[FallenAngel]

Why would it cost a penny more for MOSFETs? Same price on parts and not any more complicated to build.

Generally MOSFETs are somewhat warmer, can be biased MUCH deeper in Class-A (although BJTs are pushed pretty far in the MAX anyway).

Personal preference I guess. I also heard that 2SC2238/2SA968 is quite bright/detailed, are you sure you don't want something warmer for K701? Guess the BlackGate caps will make up for warmth.

 

[tomb]

Quote:

Originally Posted by FallenAngel /img/forum/go_quote.gif Why would it cost a penny more for MOSFETs? Same price on parts and not any more complicated to build. Generally MOSFETs are somewhat warmer, can be biased MUCH deeper in Class-A (although BJTs are pushed pretty far in the MAX anyway). Personal preference I guess. I also heard that 2SC2238/2SA968 is quite bright/detailed, are you sure you don't want something warmer for K701? Guess the BlackGate caps will make up for warmth.

I'm sure he's thinking of total price, but you are absolutely correct that it shouldn't be $50.

When you add a quad of matched JFETs, which are not used on the BJT version, that ups the price by $12.40 - that's the essential difference in price between a MOSFET-MAX and a BJT MAX. Two pair of tested MOSFETs are $7, but that's only about $1 more than two pair of BJT's. Those are the very same prices that Amb charges, btw - if you total up the separate prices plus testing/matching.

As for the 2SC2238/2SA968's, "detailed", yes, but IMHO, they are not "bright." Remember that the MAX uses no feedback but has huge coupling capacitors up front. The more detail, all the better, because everything has to pass through those capacitors which cloud the signal somewhat - Black Gates do that less, of course. Bottom line, bass and warmth is not an issue in the MAX, so more detail in the buffers is greatly desired.

Having said all of that, I still haven't sampled a MOSFET-MAX, so I can't tell you the differences. Cetoole has built both and there are several others who have, too. Perhaps they'll see this thread and add their comments.

 

[myinitialsaredac]

Thanks for the input thus far.
The price difference is part labor and part that he would need to order the parts because AFAIK he has the bjt parts there already. The other part is also the heatsinks, he mentioned putting in larger heatsinks with the mosfet output.

I am a big detail, soundstage, clarity kinda listener, bass is necessary but it just has to be detailed and deep not thumping or overpowering, I dislike shrill highs but they need to be there.

Keep the opinions coming ^^

Dave

 

[cotdt]

so what is the difference in sonics between BJT's and MOSFETs? i've always been curious about this. if there is no sound difference then i would rather use BJTs for their much lower capacitance and ability to be parallel'ed till kingdom come for more current output and deeper overall biasing. BJTs should also be more linear in Class B mode (when used with speakers).

And why not use the BD138/139 over the Toshiba 2SC2238/2SA968? Are Toshibas really better sounding transistors?

 

[myinitialsaredac]

I am no where near a DIY person so I am honestly not sure to either question. My decision for the 2sc2238/2sa968 was purely that it was listed under the cant miss max build#1 and I figured I couldnt go wrong with that.

Dave

 

[TzeYang]

cotdt,

due to the lack of negative feedback, those transistors might make a difference to your ears.

 

[cotdt]

Quote:

Originally Posted by TzeYang /img/forum/go_quote.gif cotdt, due to the lack of negative feedback, those transistors might make a difference to your ears.

but what is the sonic difference, can you describe? maybe i should try swapping transistors in my CKKIII and see for myself...

 

[tomb]

Quote:

Originally Posted by cotdt /img/forum/go_quote.gif so what is the difference in sonics between BJT's and MOSFETs? i've always been curious about this. if there is no sound difference then i would rather use BJTs for their much lower capacitance and ability to be parallel'ed till kingdom come for more current output and deeper overall biasing. BJTs should also be more linear in Class B mode (when used with speakers). And why not use the BD138/139 over the Toshiba 2SC2238/2SA968? Are Toshibas really better sounding transistors?

Steinchen's reviews on BJT's have been available for several years:
discrete diamond buffer - parts list (scroll down to the bottom of the page).

I agree with his assessment on all of these that I've tried and sell - about half of them - and reproduced these reviews on Beezar.com and the MAX websites at DIYForums.org.

You will note that the BD139/140 is one of the worst with "slim sound, a little on the bright side", whereas the Toshiba 2SC2238/2SA968 is perhaps the best, with "neutral and balanced sound, tight bass. detailed and clear highs without being bright, easy to listen to excellent choice." Moreover, the good reputation for the Toshiba 2SC2238/2SA968 goes beyond our community and is a very rare and sought-after transistor with rampant fakes on the market. Generally speaking, many of the Toshiba BJT's are great sounding transistors and are highly sought-after. The 2SC3422/2SA1359 pair is my personal favorite, while the 2SC3421/2SA1358's are used on the Dynahi.

BTW, BD138 goes with the BD137 - not a bad pair.

Keep in mind that all of these reviews and sound characteristics are pretty much based on zero feedback designs. Add feedback, and the differences in these sound characteristics are much reduced.

 

[cotdt]

Ah thanks tomb. I've been listening to BD139/140 all the time (used them in the Dynahi, in the CKKIII, etc.), I wonder what I've been missing.

 

[millwood]

I am not sure if there is a sonic difference between a quality mosfet vs. a quality bjt.

mosfets in general are easier to drive for low power applications but can be a pain in high power high frequency applications. bjts are very difficult to drive for high current applications. mosfets are in theory not prone to 2ndary breakdown, as bjts are. however, none of that is a consieratiion for headphone applications.

because of their higher capacitance (the gate of a mosfet is essentially a semicon capacitor), mosfets will have worse performance than bjts - but if your ears can hear it, people would be buying your ears not AP.

I use mosfets almost exclusively as output device because they are rugged and I have tons of them. but so far, I have never experienced any unique sonic signature for mosfets.

 

[cotdt]

Quote:

Originally Posted by millwood /img/forum/go_quote.gif I have never experienced any unique sonic signature for mosfets.

The commercial headphone amp I am using now uses the Hitachi 2SK214/2SJ77 output MOSFETs driven by the Hitachi 2SD669A and it has SOFT BASS with low impedance headphones. I am trying to figure out what is the problem, but it is probably the output MOSFETs at fault since when the amp is used as a preamp to drive a speaker amp, the bass tightens up considerably.

The commercial amp otherwise has a similar topology to the Dynahi, and the Dynahi doesn't suffer from soft bass using BJT output. Well, my Dynahi has 4 pairs of output transistors instead of just 1 so it cheats a little. I can't blame the soft bass issue on MOSFETs in general though, since the Beta22 has very tight bass. Maybe Hitachi MOSFETs just have loose bass in general, I don't know.

Other than the bass though, you're right about all MOSFETs sounding the same. This little commercial MOSFET amp sounds just like the Beta22 in the midrange and treble despite the different VAS topology.

 

[digger945]

^^^You use 2SC3421/2SA1358 in your Dynahi output?

 

[cotdt]

Quote:

Originally Posted by digger945 /img/forum/go_quote.gif ^^^You use 2SC3421/2SA1358 in your Dynahi output?

Talking to me? I eventually went with the MJE15030/MJE15031 for much higher current output for speaker use.

 

[digger945]

Looks good, I have everything I need now, thanks to you.
Hope it sounds good, DynaFet will prolly be next anyways.
Thank you for the pic, that really gets me worked up LOL.