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Old 22nd February 2013, 05:24 PM   #1
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Default Low wattage power amp for preamp.

Hello, I've been lurking for a good while, and I think this might be my first actual post here.

Me and my dad have been working on a DIY Mesa Mark IIc+ pre-amp which is going really well.

I'm looking for suggestions for a low wattage power amp to match that won't cost too much. Generally it would be played through something like a Mesa 2:90, but I'm going for something home friendly somewhere in the 2-5watt range. For testing purposes we ran it through a Fender Champ which actually didn't sound half bad, but I'm looking to spend less than $100 if that is at all possible.

Kits are preferred, but if it can get the cost way down, I'd be willing to source parts.

Any Ideas?
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Old 23rd February 2013, 12:41 AM   #2
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Check out the Amp Camp Amp in the Pass Labs forum. It's a class A design with only four transistors and runs off of a laptop power supply. You'll need a moderate sized heat sink since it burns about 20W. Output is 5W. I like the sound with the feedback disconnected. It sounds alot like a tube amp with lots of low order even harmonic distortion. Look for my posts in the Sticky thread. Kits are available but currently out of stock. It's easy to wire up on proto board.
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Old 23rd February 2013, 01:29 AM   #3
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Do you think that would really sound good with a high gain guitar pre amp? As far as I understand, the power section of a guitar rig colors the sound to a degree. Having any kind of solid state distortion would make all of that nice preamplifier distortion pointless. And a transparent amp would also result in an undesirable sound. Or am I wrong about this?

Last edited by unklejman; 23rd February 2013 at 01:41 AM.
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Old 23rd February 2013, 02:08 AM   #4
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You won't be disappointed. The only thing you have to watch is applying too much voltage to the input. A 12V zener diode on Q1's gate to ground is all you need for protection. Leave R11 in the circuit to limit current and don't install R12. Use a MPF102 for Q4, the 2SK170 will be hard to find.

What sounds bad about solid state power amps is the high damping factor (>20). Most solid state guitar amps these days use a current feedback technique to raise the output impedance. They don't sound that bad. The Amp Camp Amp doesn't need it because it has a damping factor of about 2 without feedback. That's lower than most amps, even some tube amps.
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Old 23rd February 2013, 01:15 PM   #5
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A lot of people do not run their amps to full output (tube) and get their distortion from their preamp or pedals. As Loudthud says much of the difference between conventional transistor amps and guitar amps is the dampening factor. The low dampening factor makes the speaker more lively and delivers different amounts of power to the speaker depending on the impedance just like a tube amp does.
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Old 24th February 2013, 12:08 AM   #6
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Well I'll put that on the list. The preamplifier has an AC coupled cathode follower and is supposed to have a low impedence output. Would that modification still be needed?

I'm also open to any tube based solutions if any one has any other ideas.

I've thought about the stand alone AX84 5 watt power amp. They have a single ended and push pull version. Not sure what the total cost for parts will be though.
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Old 24th February 2013, 03:01 AM   #7
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A push pull ecc82 stage should be ok, not too loud when pushed, can pull off some cleans, you only need 300v or so, look up the AX84 firefly. if you want more output gor for a 12BH7, i would think a push-pull output would sound closer to the higher power amp. though a chip amp may do the job TDA2030 with a +/-12v supply,
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Old 25th February 2013, 03:43 AM   #8
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Quote:
Originally Posted by unklejman View Post
Well I'll put that on the list. The preamplifier has an AC coupled cathode follower and is supposed to have a low impedence output. Would that modification still be needed?

I'm also open to any tube based solutions if any one has any other ideas.

I've thought about the stand alone AX84 5 watt power amp. They have a single ended and push pull version. Not sure what the total cost for parts will be though.
Something like the schematic below but some kind of Master Volume control between the preamp and power amp is suggested.
Attached Images
File Type: gif ACA1-G.GIF (7.4 KB, 186 views)
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Old 27th February 2013, 06:39 PM   #9
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I'll keep that in mind. In fact I might just build it myself just for the experience. My dad is the one builds amps ect... I have interest in it, but no experience besides assisting him in about 70% of the preamp he built for me.

I'm thinking now, just for simplicity's sake I might just get him to build me a Champ clone, since I may have a bit extra to spend than I thought and you can easily get kits. I'm curious though, does any one know how much the preamp section of the Champ affects the sound coming from my Mark IIc+ preamp? And is there an easy way to bypass it if it's detrimental?
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Old 28th February 2013, 03:03 AM   #10
shanx is offline shanx  Canada
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Quote:
I'm curious though, does any one know how much the preamp section of the Champ affects the sound coming from my Mark IIc+ preamp? And is there an easy way to bypass it if it's detrimental?
Based on your first post, you probably have an impression of how it sounded (not bad). If you want to bypass the champ preamp and tone stack, just need to run your MarkIIc signal in past the champ preamp section.
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