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DIY Class A amps

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Every now and then I get curious about building a Class A solid state amp. I have a couple of large heatsinks. I'm sure quite a few of us have been tempted. But who has actually built a Class A amp and can describe the build, where they sourced the PCB and other components, how the build went, and what the final sound was like? Could be a Nelson Pass amp kit, or a Chinese clone, an original JLH, or whatever. 

So let's have your experiences of DIY Class A builds. 

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I have made a few that I "made up" my self, but none recently (although I have been feeling a little itch of late). My current feeling, like you notice with your 2 stage amplifier, the gain stage sets the flavour. My most recent imaginary (designed but not built) circuit included a valve gain stage, and a solid state out put. Out of all of my 4 so far, this brings the best sound to my ear. 

Happily the Pass has recently blessed the p and n channel lateral MOSFETs, which are available from Exicon last time I checked and which are used in several respected circuits, like the Bakoon iirc.

Edited by dave

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Built a Wonfor SECA from kit, that sounds excellent, polished, hefty, tonally spot on. Its currently in the system, love it. Of course its not perfect but what amp is?

I've got a DIYAudio MoFo built but not tested yet. Thats a weird one, single stage, single ended pure class A, but the 'active' element is a choke! Good for high output pre amps apparently, that's why I've built it.

Lots of fun, my class A sand amps have been a doddle compared to valve builds.

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I'm nearly finished building a Pass F5 with boards /parts mostly from diyaudio. I've run into a couple of issues with the biasing and the diyaudio forum has been a great source of help, so if you need any hand-holding (like me) I would recommend going down this route.

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2 hours ago, Jazid said:

Lots of fun, my class A sand amps have been a doddle compared to valve builds.

I already knew you were significantly younger than me, but that's universal proof :D

Give me a valve, some volts, the specs for the valve and a few components and I'll give you a working amplifier in a couple of hours, yet I can't glue four diodes together without getting at least one the wrong way round. I tend to buy ready-made boards for anything solid state, because it usually ends in chaos otherwise. I've tried a few assorted class A with those strange small components with three wires, but every one has sounded vaguely awful. At least, every one that has made a noise, rather than an unpleasant burning smell.

One day I'll have another go I suppose...

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I like the idea of a hybrid. The Pass Zen is just a single device, isn't it? As opposed to the Aleph 3 and Aleph J and their clones. I like the idea of a DHT driver. I have plenty of those. Can you guys recommend the latest flavour single device amp? The Zen is a bit old, though I see there are revisions. 

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2 hours ago, Jazid said:

Built a Wonfor SECA from kit, that sounds excellent, polished, hefty, tonally spot on. Its currently in the system, love it. Of course its not perfect but what amp is?

I've got a DIYAudio MoFo built but not tested yet. Thats a weird one, single stage, single ended pure class A, but the 'active' element is a choke! Good for high output pre amps apparently, that's why I've built it.

Lots of fun, my class A sand amps have been a doddle compared to valve builds.

The mofo is as good as what you drive it with... 

I used a single output valve per channel to drive it.. it's really good, but will pick holes in the rest of your system ?

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Ive seen a few designs that Ive been tempted with which use a driver valve with a Mosfet follower o/p. Very simple.

Edited by zeta4

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The MoFo looks perfect for me - I have several chokes which are 280mH, 2.7A, 3.4R. And a couple of big heat sinks and some more biggish ones I could link up. I haven't yet looked at how you size the heatsink, of if you do it with weight. And I'd use a DHT driver. I have plenty of parts for a linear power supply, maybe using one of the above chokes. They do suggest a 19v switching PSU, don't know how that compares. 

Any other MoFo experiences? 

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I have a pair of Mofos all assembled but haven't powered them up as yet as can't find the power supply bricks I purchased they have quiet a high Amp rating if I remember correctly and a bit more spendy than your regular pc power brick. I know they are in the house somewhere but damned if I can find them so they will have to wait until they turn up.

Also built a couple of Nelson Pass Amp Camp Amps a few years ago using just the PCB and eBay pre used heatsinks. 

I used a Bottlehead Quickie which is a  simple inexpensive battery power DHT tube pre amp.  Think the ACA and pre worked out at under £200 all up and sounded really quiet nice indeed.  If memory serves correctly they are meant to use 19v supply but depending on your heat sinks you can run them at 24V with a increased bias setting which for a better word hot rodd them by driving down the distortion noise measurements but I might be talking bollox as it was a few years ago when I built mine. 

At that time the ACA  PCB were only available from the DIY audio store but they can be found now on eBay for only a few pound.shipped from China.

Edited by JamieMcC

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Steve Guttenberg recently posted this video 

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Ive got a JHL Class A I bought from china for £180 which sounds as good as they say. No worth building my own as it was so cheap and very good quality. Only thing against it is the large electrolytic cap on the o/p but some of the Nelson Pass amps also use caps on the o/p.  I do like the MoJo +dht  though with the choke which will do away with the cap. 

Oops. Thinking of another design. Of course the mofo has a coupling cap as well but still looks good to me.

Edited by zeta4

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