Simple Mosfet SE amp

[acorn]

Hi Pre

Yes I would second that about the Mosfet amps, I have built a few complemetry output pair amps and they sound very good to me a much closer sound to valve amps than other type of transistors , but you do need a good power supply at least 10,000 uF each side of the smoothing resistor as they take a lot of current, if you get the power supply right the bass will sound very good indeed and you also need to by pass the 10,000uF with a 10uF and 1uF solid cap.



Acorn

[pre65]

I have managed to make the effort and finish one of the monoblocks !

http://i111.photobucket.com/albums/n...-fetamp001.jpg

http://i111.photobucket.com/albums/n...-fetamp002.jpg

It powered up OK and sounds "not bad" in mono !

Just got to get the enthusiasm to start on the other one now.

[acorn]

Nice work Philip, were do you get the energy from ?

Acorn

[JerryT]

Originally Posted by acorn

Hi Pre

Yes I would second that about the Mosfet amps, I have built a few complemetry output pair amps and they sound very good to me a much closer sound to valve amps than other type of transistors , but you do need a good power supply at least 10,000 uF each side of the smoothing resistor as they take a lot of current, if you get the power supply right the bass will sound very good indeed and you also need to by pass the 10,000uF with a 10uF and 1uF solid cap.



Acorn

I'm feeling quite nostalgic - approx. 27 years ago I built one of John Linsley Hood's Mosfet amps. Loved it and modified it for 12 years before the circuit board called a halt to proceedings and young children removed the time amd funds for suitable replacement. This was the amp that highlighted to me the impact that component choice could have on sound - particularly how capacitors and resistors could affect the sound quality.

Key findings that may be useful to you:

MosFETs need good thermal control - can take significant temperature changes and function, but function and sound good aren't the same thing! In general marginal heat sinking and poor airflow will give poor sound and marginal enjoyment.

Often sound quality is quite sensitive to standing current.

Agree with Acorn about PSU importance. Just as finicky as a valve amp to quality and quantity of capacitance, rectification and transformer. Can get 'astounding' results with mildly underrated mains tranny and carefully chosen C values to get PRAT. How to make your CD sound like it is being played on a Sondek!

MosFETs could be troubled by high kHz parasitical oscillations usually caused by poor circuit board design/ layout. Unlikely to be a problem in your single device amp but worth looking for with a scope if the sound seems 'mushy' - or if cats, mice, rats and bats are driven from the building when using speakers with extended high frequency capability. A ferrite bead on power wires or a little wire wound R usually provided a quick cure.

Oh and 20 years ago they could fail dramatically - none of the quiet death throws of valves when mistreated, or the smoke, smell and little molten puddle of silicon devices. A power FET that had been seriously abused would explode, catch alight and fling little incendiary bullets in all directions.

Enjoy - let us know how you get on.


Jerry

[pre65]

Originally Posted by JerryT

Agree with Acorn about PSU importance. Just as finicky as a valve amp to quality and quantity of capacitance, rectification and transformer. Can get 'astounding' results with mildly underrated mains tranny and carefully chosen C values to get PRAT. How to make your CD sound like it is being played on a Sondek!

Enjoy - let us know how you get on.

Jerry

Interesting about the "mildly underrated mains transformer" comment.

I have always gone with the Russ Abbott (sorry-Andrews ) school of thought where the transformer is as large as practically possible !

Is there a technical reason why this is ?

Second monoblock is now up and running and sounds good so far,not that much different from the valve sound I'm used to.

[JerryT]

Hi

I think the explanation is about parallel L,R and C circuits - otherwise known as 'tank circuits'. By tuning the values of L,R and C you get the resonant frequency of the tank to be say 40 Hz. Then when the current drawn has a 40Hz component the circuit gets excited and increases the size of the 40 Hz ripple until the R value damps it down, giving a pronounced 'bounce' to notes around 40 Hz but not to those lower.

I'm sure the more erudite amongst us can make a better explanation, but two 80's 'sound giants' K. Ishiwata and Tom 'GTE' Evans both featured in a number of articles where they explained that they'd chosen a smaller mains transformer for better sound. Tom Evans was responsible for the Pioneer A 400 GTE that was ' a giant killer'. Interestestingly a number of other highly regarded amps eg Nytech, some of the Rotels and the early NAD amps used similar techniques - not least because they were cheap. Valve guitar amps do this still. It is, I think, the reason for the characteristic 'bounce' in Fender amps, and the boogie factor in the 'Mesa Boogie' range.

I understand the attraction of the Ross Abbott approach, because it pushes the resonant frequency right down - but sometimes it also increases the damping - leading to a sound that is called 'very controlled' or even 'leaden'. The cheapskate version reduces the damping and raises the frequency to give some 'sympathetic resonance'.

Best I can do. Help (including corrections) anyone?

Jerry

[pre65]

I thought you needed a bit of "reserve" for climaxes (musical ones ) .

[JerryT]

Originally Posted by pre65

I thought you needed a bit of "reserve" for climaxes (musical ones ) .

Ah. The English method. Reserve and a stiff upper...



The rhythmic bounce is supposed to be more pleasurable and therefore any climax may not need the same emphasis to be satisfying. When the journey is fun, reaching the destination does not need to be the only reward etc...

Sorry, couldn't resist!


Seriously I didn't say the approach was the right one, merely that you could do it and a number of 'revered' people had done so to significant acclaim. Remember we are talking better enjoyment of Dire Straits here, not something a tad more challenging such as Mahler's 3rd Symphony, as the desired outcome!

Enjoy!

Jerry

Hi
I seem to remember an article by Stan Curtiss where he said that under some circumstances it is better to have smaller power supply caps so that the power supply is always being charged,he said you have always got the national grid behind the supply and, it made for a bouncier sound.Dont know if this makes sense.I have just got a home brew Nelson Pass up and running which is SE mosfet and first impressions are quite favourable compared to my gm70 se amp.

[Ali Tait]

Which Pass amp have you built Dave?