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Old 14th November 2012, 03:06 PM   #561
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Originally Posted by Loudthud View Post
Along with the higher transconductance comes higher input capacitance. I tried IRFP460s and the results were poor. It could be that R6 needs to be lower, I didn't try that. I tried running three Q4s in parallel with R9 at 220 ohms and the results didn't improve much. Q1 becomes very non-linear as the Drain voltage dips below the Gate voltage. Visible distortion at 1kHz. At that high a rail voltage, you will need gate protection zeners. I'm going the other way. Next I'm going to try IRF120s. Suggest you search for my posts in this thread.
I went through the thread again and found all your useful posts. I missed the post where someone mentioned using BF862 instead of K170. It appears that the stock version is pretty much optimized. Ofcourse, the bias can be doubled for double the pleasure. So for the present IRFP240 it is. Your results with IRF120s would be interesting. I have some unused IRF620s.

One question - IIRC JLH had mentioned about lowering distortion by using a Darlington BJT for the lower output device. Any comments?
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Old 14th November 2012, 03:59 PM   #562
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In the case of the ACA, it is Q2, the upper Mosfet used as a CS.
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Old 15th November 2012, 12:45 PM   #563
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Ok got this finished at last. Couldn't trace the mains hum I was getting on one channel but after discovering it went away when the two mono blocs touched the way forward was obvious
Apart from the kit and sinks from the store I spent £10 on aluminium angle from a hardware store and £10 for two pc power supplies on the bay uk.

The amp sounds great....great power, bass and massive detail. Probably loses just a little smoothness in the mids compared to my tube/valve gear but not by much. Amazing amp for the build cost. The amp is dead silent between tracks and the sinks get warm but still ok to touch after 30 mins. I'm driving 15 ohm speakers with it....anyone know if it needs optimising for this load compared to 8 ohm? I got some spare boards and will probably have a go at a version with a "proper" power supply at some point. Anyone got any thoughts on a power supply?
Mike
Mike, have you found the way to optimize the design for 15ohm load?
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Old 15th November 2012, 03:35 PM   #564
waam68 is offline waam68  United Kingdom
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Hi Commstech
I'm afraid not yet. To be fair the stock amp sounds very good indeed with my vintage 15ohm 3 way Wharfedales (no problem driving the 15" woofers) so I wouldn't let it put you off building it if you have 15 ohm speakers.

That said if anyone more clued up would like to chime in that would also be cool.
Mike
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Old 15th November 2012, 04:30 PM   #565
6sigma is offline 6sigma  United States
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The kits & bundles appear to be sold out. Is this an item that is likely to be restocked soon?
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Old 16th November 2012, 08:16 PM   #566
zany is offline zany  Australia
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Variac said in about a month.
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Old 17th November 2012, 01:02 AM   #567
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Originally Posted by JRKO View Post
Just waived goodbye to Macfixer after fun afternoon!!

Amps go together so easily! Really a great initiation! I completed mine up to the stage where I would mount the board to the heat sink and left it there as I have yet to finalise my chassis arrangements.

We completed one of Macfixers and let it warm up as per the build notes. Then a first listen via a pair of Minimus 7's to check all was well. Sound was big & full.

so onwards to my Emkens absolutely massive, dense, fluid, sharp as a knife sound. Played a few tracks I'm familiar with and the hairs on the back of my head nearly lifted me off the floor!! This was a straight comparison to my amp that is soon to be replaced.

Cant wait to finish my amps and get it up and running. Will post pix once I've finished mine and chassis is built.

Only issue is a thump at turn off - all in all not bad for an afternoons work
first off thankyou James for walking me through my first build,it was a bit tricky a first but the music kicked in and it all flowed great. I had to leave quick in eeded to plug those amps in at home............................now its friday and it late. What a week not a lot of macfixing but got the system sounding alright.

Firstly thankyou Nelson and all at DiyAudio....it sure do sing sweet ad lots of grunt. the family frendly system is firestone spitfire Flacs with burr brown upgrade through bi-amp quad 306es into B&W 603, a very friendly system.

All week if been tinkering with the ACA from a heybrook TT2 with a cardridge given to me so i can go through my vinyl and it been a bitter sweet time of extreme highs and lows......no one likes it.....but cant you see the boxes are to small etc etc i have FE206e shouting so phase plugs in gunck and felt applied still not sounding right, questioning do i want to invest in the ply to build the BIBs. Got desperate took the tweeter out of some old linn speakers wired up an alps took another output from the pre and buggered about with te volume to just colour the bottom a bit.......Ok sounds more presentable.


So tonight Ive gone an plugged in the dac and played some Flac.......joy joy happy happy joy joy.

God i love that amp and my cartridge is ****!

Attached the most Flugly room youve seen in a while it was clear on Monday

So Thanks to everyone so cool!

Clive maccfixer
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Old 17th November 2012, 01:06 AM   #568
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you see one of the ACA red LEDs just by the righthand speaker if you ignore the rest of the crap!
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Old 17th November 2012, 01:09 AM   #569
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That amp sure loves the gratefull dead
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Old 17th November 2012, 03:15 AM   #570
Variac is offline Variac  United States
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Well it's from Northern California!
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