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DIY Mosfet Hybrid amp, Donor amp suggestions?


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Hi All,

I'm considering making a DIY hybrid amp (tube preamp, mosfet power amp), "inspired by" a mesa walkabout/m-pulse, probably with the addition of a tube compressor, and maybe with two different switchable preamp circuits/channels.

I've been looking at parts for the poweramp, and it looks like the mosfets, heatsinks and transformer are going to be a lot more expensive than I'd anticipated (the mosfets alone are £11 each and I'll need at least 8!)

So it occurred to me that there must be some suitable donor amps (old PA amps, other hybrid heads with a similar power stage etc) out there, which I can pick up cheap and modify...

I'm looking for something with a decent sized power section (say 300-600w), ideally with BUZ901/906 Mosfets or similar in the power amp - any suggestions?

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The original series Ashdown ABMs used those trannies I believe... Might be able to find one of those cheap enough.

Or you could try and find a B&O icepower module if class d was of any interest to you.

Edited by Bigwan
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[quote name='Bigwan' timestamp='1381137602' post='2234671']
The original series Ashdown ABMs used those trannies I believe... Might be able to find one of those cheap enough.

Or you could try and find a B&O icepower module if class d was of any interest to you.
[/quote]

Thanks, the Ashdown is a good suggestion, particularly if I can pick up a cheap faulty spares/repair one somewhere.

I did consider class D (in particular these [url="http://www.hypex.nl/product/ucd-amplifier-modules/ucd700hg.html"]Hypex[/url] modules), but I'm going for a more classic/vintage vibe, which based on many reviews seems to be more readily achievable with more traditional mosfet (or tube, obviously) power amps.

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[quote name='Bigwan' timestamp='1381137602' post='2234671']
The original series Ashdown ABMs used those trannies I believe... Might be able to find one of those cheap enough.

Or you could try and find a B&O icepower module if class d was of any interest to you.
[/quote]

Yes, and the Trace Elliots too. An alternative to the BUZ901 is the Exicon ECF10N20 and it's complementary device, but is probably not much cheaper. I recently fitted some BUZzes in my TE AH200 and are great, though.

How about the suggestion of a class D module?

EDIT: 8 Devices? That's a chunk of current :)

Cheers

Geoff

Edited by RandomBass
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Try out these guys: http://www.bkelec.com/Modules/Amplifiers.htm - I had one of their 450W modules years ago - it was bullet proof. My first 'big' amp was something I put together myself using one of these!

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[quote name='RandomBass' timestamp='1381140462' post='2234752']
Yes, and the Trace Elliots too. An alternative to the BUZ901 is the Exicon ECF10N20 and it's complementary device, but is probably not much cheaper. I recently fitted some BUZzes in my TE AH200 and are great, though.
[/quote]

Thanks for the suggestions, will look into TE, and the Exicon device!

[quote name='RandomBass' timestamp='1381140462' post='2234752']
How about the suggestion of a class D module?
[/quote]

Yeah as mentioned in my reply above, I have considered using a Hypex class D module, but tbh I'm drawn towards a more traditional mosfet design, because I can understand how they work, and fix it if it breaks, whereas I don't really understand how class-D amps work, and the component density and surface-mount stuff on the Hypex modules pretty much means if it fails I'll have to replace it.

[quote name='RandomBass' timestamp='1381140462' post='2234752']
EDIT: 8 Devices? That's a chunk of current :)
[/quote]

Yeah, well based on the schematic's I've looked at so far, the walkabout uses 6 mosfets and the m-pulse 600 uses 12, so I think I'd want 8 at a minimum, since the walkabout only delivers 165w RMS into 8ohms. Obviously I'll start with one pair if I build from scratch to avoid a huge fire if I wire them up wrong :D

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[quote name='dood' timestamp='1381140501' post='2234753']
Try out these guys: [url="http://www.bkelec.com/Modules/Amplifiers.htm"]http://www.bkelec.co.../Amplifiers.htm[/url] - I had one of their 450W modules years ago - it was bullet proof. My first 'big' amp was something I put together myself using one of these!
[/quote]

Interesting, that's the sort of module I've been looking for, but only found lower power or class D modules so far, thanks!

I may shoot them a mail or give them a call, as there's not much detail on their website but it definitely does look promising!

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[quote name='JapanAxe' timestamp='1381146226' post='2234874']
Why not just buy a cheap used Ashdown head and use the whole power amp section and transformer?
[/quote]

Well that's basically what has already been suggested above, and I am considering it. The main thing putting me off atm is the ebay definition of "cheap" in this case appears to be around £300, which doesn't make sense to me as a donor amp, particularly when you can get something like a Hartke LH500 for less than than new.

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[quote name='bertbass' timestamp='1381163394' post='2235209']
I've got some of those BK modules sitting around doing nothing, they were used with our PA for awhile. Really nice amps. PM me if you're interested.
[/quote]

Ooh, PM incoming! :)

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[quote name='LukeFRC' timestamp='1381179072' post='2235573']
what schematic/type are you building the preamp from?
[/quote]

Not decided yet, but I've got boxes of tube amp bits sitting around, as well as several guitar amps in various states of modification, so initially I'll probably raid the spare parts and build a fairly simple 12AX7 based preamp, then add features when I've got a solid basic tone.

I started out looking at the mesa walkabout schematic, and it's essentially just a Fender style preamp, into a solid-state active EQ, then a solid-state compressor, a tube driver, and a mosfet power stage. So I might emulate that, or just go more all-tube and copy something like an SVT preamp with an LA2-a compressor.

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Just a thought, had you considered a Baxandall type eq, rather than the FMV stack? I built one a couple of years back, and found it more versatile. I had to apply a little voltage gain before and after the EQ to accomodate the inherent (but small) losses. Mine wasn't a valve pre though.

Cheers

Geoff

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  • 2 weeks later...

Thanks to all for the suggestions - this is now sorted, I've got some BK modules (thanks bertbass!), and have made a start on the preamp.

Link to build thread:

[url="http://basschat.co.uk/topic/220279-diy-tubemosfet-hybrid-amp/"]http://basschat.co.uk/topic/220279-diy-tubemosfet-hybrid-amp/[/url]

I'm still mulling options for the preamp design, but I will definitely consider the [color=#282828][font=helvetica, arial, sans-serif]Baxandall eq idea - I'll post schematics to the build thread, so feel free to post suggestions over there :)[/font][/color]

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