Mosfet 250 audiophile & audiophile pre

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Raj

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Mosfet 250 audiophile & audiophile pre
« on: 11 Jan 2003, 01:27 pm »
Hi,

I recieved my mosfet 250 just a few weeks ago. It's built and now up and running. After many irritating phonecalls to David over the Christmas period I finally completed the amp, I was almost a complete novice with very little experience, and I don't really understand electronics. This was my first really quality component, and the upgrade from my marantz(pm6010ki), was quite amazing! I'd pretty much gone all out with my order, I'd already built the audiophile pre-amp with the buffer amp(audiophile), and had ordered 2 active psu's and also two 500w toroids to run the power amps. I'd also recieved my amp with the larger heatsinks and after a while turned the quiescent current upto 250ma. This in itself was another step forwards in sound quality, this smoothed out the sound. I had heard from David that one of his builders runs the amp at 600ma, so I decided to see if I could do the same. As a result I am now experimenting with fan cooling, which has enabled me to raise the bias upto 500ma, and another leap in performance! The new sound threw me back at first, it was so liquid smooth (I thought to myself this must be what the tubes guys are on about), I wasn't quite sure if I liked what I heard, but this was it, the sound was natural, not that brash edgy and short term grabbing sound you get from 'HI-FI', but much more akin to what the musicians had played, I just wasn't used to it. Further listening convinced me, as a consequence I'm using a very 'dirty' diy fan job to cool the heatsinks cause I just don't want to go back to a lower current setting! The central soundstage has opened up, I'm now getting seperation in 3D and height. There's more space between singers, drums and instruments. Before I had sounds at the left and right speaker and a central sound, now the soundstage has spread, also finer details and notes that I really had not heard before on my cd's were realised. You can almost say that the difference in turning the quiescent current from 250ma upto 500ma, is the same type of difference you get when you first hear the mosfet 250 comparing it to your old sytem. The voices of singers are much more life-like and at times you get the musician in the room effect. Definatley worth the investment in extra heatsinks or fans! If youve got one of these amps and are still running it at a lower rate I thouroughly reccommend you make this your next upgrade! Once I get the fan and heatsink setup completed I should be able to run at 600ma, which I think will be quite enough, the amp sounds good enough at 500ma anyway. In the future I hope to add the cascode power supplies to run the opamps seperatley. David can this be done??  I don't think I would have got into the true audiophile cirlcle in another way unless I was prepared to spend around £5000 - £10,000 on commercial components, when I read reviews of the top class components, I can see that my setup which was bought for much less, is of the same or better specs in some cases!


By the way I don't obtain special benefits from white noise to make these statements, these are my own sentiments!

Thanks
Raj

davidw

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Mosfet 250 audiophile & audiophile pre
« Reply #1 on: 16 Jan 2003, 10:38 pm »
I didn't pay Raj to write this - honest! I've hooked my amps up to a spectrum analyzer and examined the effect of increasing the quiescent current. At the recommended 100-150mA for use with partner friendly heatsinks the distortion products are second harmonic with some third at about half the amplitude. As you wind up the quiescent current the third harmonic component gradually disappears until at about 500mA its almost gone completely. Getting rid of the dissonant 3rd harmonic obviously improves the sound quality. Notice that winding up the quiescent current doesn't usually have the same beneficial effect with bjt output devices. Bjt output stages have an optimum quiescent current of about 20-50mA and deviations from this will worsen the performance ( unless of course you go to full class A ).

Marbles

Mosfet 250 audiophile & audiophile pre
« Reply #2 on: 16 Jan 2003, 11:14 pm »
Quote from: davidw
I didn't pay Raj to write this - honest! I've hooked my amps up to a spectrum analyzer and examined the effect of increasing the quiescent current. At the recommended 100-150mA for use with partner friendly heatsinks the distortion products are second harmonic with some third at about half the amplitude. As you wind up the quiescent current the third harmonic component gradually disappears until at about 500mA its almost gone completely. Getting rid of the dissonant 3rd harmonic obviously improves the sound quality. Notice that winding up the quiescent current doesn't usually have the same beneficial effect with bjt output devices. Bjt output stages have an optimum quiescent current of about 20-50mA and deviations from this will worsen the performance ( unless of course you go to full class A ).


David, did you want to get in on this conversation about what makes an amp sound good?

http://www.audiocircle.com/index.php?topic=360.msg4674#4674&highlight=#4674

WerTicus

Mosfet 250 audiophile & audiophile pre
« Reply #3 on: 18 Nov 2003, 06:44 am »
Thats really interesting about the volage settings... would that be the case with other amplifiers too? :)