EDIT: Just realized the original poster put this in the "portable amp" section... Yes, it's a CMoy but isn't portable... Oh well. If a moderator would move this to the desktop amp section, that'd be nice. Thanks.
Impressions of a custom CMoy 12AU7 amplifier
OVERVIEW
Hopefully someone thinking about a budget DIY tube amp can benefit from this write-up. Fred's 12AU7 CMoy amp [http://fredsamplifiers.com.au/] is a great kit for the budget DIYer/music lover. I am thoroughly happy with the music coming from my modded AKG K140s/CMoy tube amp pairing. The online instructions are straightforward, and Howard of
Fred's Amps was patient, quick-to-respond, and helpful with all my noob questions. The ship time from Thornlie, WA, Australia to Seattle, WA, USA was just over a week.
For the record: my modded AKG K140 consists of a Mogami/sleeved re-cable and crude circumaural earpad attachments. The circumaural earpads make a huge difference in soundstage and midrange clarity, just by giving a bit of space between my ears and the drivers -- not to mention comfort. Bass is a tad weaker but this to me is well worth the trade.
THE BUILD
Since the amp design is so simple, it's easy and not horribly expensive to use some nice custom parts and personalize the amp any which-way you like. For my build, I used an old Fossil watch tin case that is about 3.5" cubed, and has an aesthetic which pleases me. I also used an Alps blue velvet 10K pot, Elna Silmic II 470uF 50V (output) caps, 1uF 630V (DC blocking input) film caps, Texas Instruments LME49720 op-amp w/ a heatsink, Connex teflon silver-plated tube socket, gold knurled volume knob, and brass spikes/feet. For the power supply, I'm using a Pyramid PS3KX. Since I'm using wires to connect some of the parts to the PCB, Howard recommended I plait/twist them to help with possible interference/noise. Luckily, all is well as the amp is dead quiet.
THE MUSIC
I've been wanting to simplify my music set-up for some time now -- I just wasn't happy with my much more expensive set-up. I fell a bit too much for hi-fi snake oil and much to my dismay -- and thin wallet -- discovered hi-fi is not the same as musical enjoyment, BLEH. So I had to make an adjustment. Mostly because a lot of music I like isn't well recorded, I wanted a setup that didn't slap me in the face with details and clinical precision.
6moons' Music Lovers Series Leben/Harbeth review cuts straight to the marrow of the bone in this regard: "The equipment must serve the music, not the music the equipment as is often the case in audiophile circles." I do however like nice quality parts for durability and ease of mind, I just try not to fly off the handle when I'm selecting. This isn't to say the CMoy tube amp lacks detail, contrarily, the big change here was my AKG K140s over my AKG K612s. My K140s breathe new life in terms of musicality into my music. My K612s are analytical and the treble is often peaky, but aside from that I love the open, mid-centric AKG house sound.
What really makes me happy is, as I said before, the amp is dead quiet -- even when turning the Alps volume knob all the way up. Another welcome nicety concerning the volume is the music through my K140s doesn't get too loud as I turn up the volume, I can really dial in the exact loudness I like, with extra power to spare afterward. The JRC4556 op-amp drives my phones well too but the TI LME49720 is a bit more refined and smooth to my ears, less mechanical sounding. The overall defining sound characteristics of this amp, I'd say, are: slightly warm, powerful, clear and unrestrained. There's an open clarity to the sound absent from the Altoids Cmoy and quite close to the Bottlehead Crack, though the Crack sounds more spacious and, to be fair, is clearly the better amp. I think the Pyramid PS3KX has a big role in this and is really controlling the power well, I had a cheap 12V wallwart power supply and the amp hummed over the music. You clearly need a quality power supply for this amp for it to even function properly.
With my Bottlehead Crack, the Mullard 12AU7 square-getter [date code: k61] was my favorite (warm and just sounded natural and right). With my CMoy tube amp, I really like the Brimar 12AU7 with the large halo-getter (which has a similar warm sound). The deciding factor for me was when listening to Ray Charles'
Ev'ry Time We Say Goodbye. With the Mullard, Ray's voice sounds a tiny bit thin and restrained, giving the impression he's forcing it. I kept clearing my throat as I was vicariously imagining I'd project his voice for him. With the Brimar, there's more meat in the tone, more weight, and he sounds effortless and live; his voice searing through the air. With the Crack, the Mullard was just as weighty as the Brimar, go figure. It was maddening because the Mullard was way more expensive, I wanted it to be the best but I also needed to be honest for my enjoyment's sake. However, this open, gutsy midrange really won me over, and is wonderful with all vocals and acoustic recordings, which is mostly what I listen to. It's actually hard now to focus on the sound instead of the music, which is nice for a change. Poorly recorded tracks of Chuck Berry, Johnny Cash and Kurt Cobain were never as easy to listen to and as enjoyable as they are now.
CONCLUSION
For $40, you can't really go wrong. I obviously realize my build is more than the base kit, but what I mean to say is a
base price of $40 is very reasonable, then put down a couple ten spots to get some nice output caps and a quality power supply and you can get that crucial headroom with admirable finesse and musicality to help you get more into the music. Cheers to any future builders. Take care and I hope you have happy and meditative winter solstice festivities.
