MOSFETS vs BJTs

[lucpes]

@djk: hmm.. I just ordered the kit so I'm already fried My Infinity Reference Standard II speakers behave naughty with 1.5ohms impedance at 40Hz and @6000Hz.... will see what comes out.

@others: 'related' thread:

400W amp project

[traderbam]

Azira,
I thought the idea that FETs were inherently better sounding in audio power amp applications than BJTs died in the early 80s.

[Nelson Pass]

Originally posted by azira
I've now been convinced that, at best, FETS are good for the input stage but BJTs are far better for VAS and especially the output stage. Lower distortion, and less feedback needed to linearize, so more gain.

True if you want to run lower bias currents. If you are running
high bias currents, the Mosfets start getting better. If you want
an amp that measures well and runs cool and doesn't have too
much feedback, BJT's are a good choice. If you want to make
a simple Class A amp that sounds good, Mosfets are a good
choice.

For some reason, unknown to me, I have never liked the result
of any of my efforts to drive Bipolar with Mosfet. Similarly,
I haven't had much enthusiasm for BJT's driving Mosfet outputs.

[Bricolo]

Originally posted by Nelson Pass


True if you want to run lower bias currents. If you are running
high bias currents, the Mosfets start getting better. If you want
an amp that measures well and runs cool and doesn't have too
much feedback, BJT's are a good choice. If you want to make
a simple Class A amp that sounds good, Mosfets are a good
choice.

For some reason, unknown to me, I have never liked the result
of any of my efforts to drive Bipolar with Mosfet. Similarly,
I haven't had much enthusiasm for BJT's driving Mosfet outputs.

Is the high feedback factor important?
Will a SE class A mosfet amp with just a little feedback not be a good amp?

[StratoFan]

Maybe here is a good thread to get a little help. I built a discreet mosfet poweramp with 2SK1058 and 2SJ162. While I testing with sinus or something else (triangle or square) I have no problem, everything working well with +-15V...+-70V power. But when I connected some music to this circuit, the output DC shift to -Vs for a very short moment. Therefore when I want to use to listen music, the DC protection always off the speakers. I saw the result on scope, this is not a problem of protection circuit (which is UPC1237). Somebody can help me why wrong this circuit with music? The DC offset voltage very low (around 0.1-0.3mV) when no input. When something loud in music (for example drum) the offset voltage jumping to the -Vs as I said. Both channels do same. I already changed R49 and R50 to 200 Ohm trimmer pot, but nothing happend.

I think the schematics can help to send me some advice:
http://rentahost.net/stratofan/pdf/S...c_poweramp.PDF

The board:

[Nelson Pass]

Originally posted by Bricolo
Is the high feedback factor important?
Will a SE class A mosfet amp with just a little feedback not be a good amp?

Feedback is like a credit card - very convenient, but the bill
shows up at the end of the month.

Feedback works perfectly in perfect circuits. The less you need
feedback, the better feedback works.

[millwood]

Originally posted by Nelson Pass
Feedback works perfectly in perfect circuits. The less you need
feedback, the better feedback works.

that reminds me of bankers: the less you need money, the more they want to lend money to you. The moment you need money, they are nowhere to be found.

[lucpes]

Originally posted by lucpes
@djk: hmm.. I just ordered the kit so I'm already fried My Infinity Reference Standard II speakers behave naughty with 1.5ohms impedance at 40Hz and @6000Hz.... will see what comes out.

Sorry for replying to myself in this old thread, but I have something to correct. I built the kits from Randy Slone and I must say that the sound is astonishing. Good quality parts, idiot proof instructions (this being my first ever project in DIY), best assistance one can desire. You can't go wrong with these! Unless you try to them using a hammer.