[Wodgy]

Those of you who read the DIY forum might remember an old thread about an 8-watt tube integrated amp for speakers that only costs $140. Here's the thread if you don't remember it:
http://www4.head-fi.org/forums/showt...threadid=17155

Anyway, I was flipping through the November 2002 issue of AudioXPress today, and it turns out there's a full review (6 pages!) of this amp, including measurements. Overall, the reviewer said very good things about it, especially:

"The K-12M's sound coupled with its modest price tag blew me away. I've heard amplifiers I liked better with our system but none for under $800. If you'd told me before I heard the K-12M that a $140 amplifier could sound this good, I wouldn't have believed you."

and, in comparison with a low-cost solid state amp rated at 75 watts (nearly 10 times the rated power of this tube amp!):

"...the K-12M's bass response was tighter, deeper, and better defined."

I just thought I'd pass this along, in case any of you are interested in a low cost tube amp for speakers. I've said really good things about this amp before, but it's nice that a professional reviewer had good things to say as well. You can buy this amp preassembled if you don't feel like building it yourself, or if you're not handy with a soldering iron. Personally, I can't think of a better inexpensive speaker amp for someone on a budget (e.g. students). If you really like it, you can tweak it to your heart's content, of course! Anyway, I'd encourage anyone considering this amp to order the November 2002 back issue of AudioXPress and read the full review.

 

[aerius]

I have this amp too and I'm using it as a headphone amp for my Senn 580's. I'm lucky in that there's a bunch of electronics supply stores in my area so I upgraded all the resistors and capacitors. I put 1% metal film resistors everywhere and swapped all the generic poly caps to some nice WIMA ones. I also put in sockets for the feedback resistors so I could play with the negative feedback and gain. The amp drives my headphones through a homemade version of one of those Grado speaker output adaptor thinga-majigs.

I have to say that I'm quite happy with it, it's got a nice balance and good details too. I thought it was a little weak in the bass but I was mistaken. The bass is good and strong, it's just that I've gotten use to the boomy bloated bass from the badly placed speakers in my house. Everything else is good too, nice details and probably plays cymbals better than anything else I've heard in a long while. The only downside was I couldn't get rid of the transformer hum so I had to play with the resistor values in my adaptor-majig to drop the hum to a negligible level. I'll probably be picking up a better power transformer soon to see if that helps with the hum issue.

 

[mikeg]

This amp may good for driving an AKG K1000 headphone.

 

[elnero]

aerius,

I assume this doesn't come with a headphone output. Was it hard to modify to add one?

 

[elrod-tom]

In the same review, they also mentioned that the tube sockets should be replaced with ceramic. On the review model, the stock tube sockets became scorched and even melted slightly.

I'm curious if there is anyone out there that can confirm that this is or is not a problem.

 

[aerius]

Quote:

Originally posted by elrod-tom In the same review, they also mentioned that the tube sockets should be replaced with ceramic. On the review model, the stock tube sockets became scorched and even melted slightly. I'm curious if there is anyone out there that can confirm that this is or is not a problem.

I think it is a problem, it's noted on the manufacturer's website that the sockets can get scorched and discoloured, and to leave the bottom of the tubes about 1/8" above the sockets to help prevent this. I did this on my amp and the top of the sockets still turned yellow, but there's been no melting so far even when I left it on constantly for a couple days.

elnero: No, it didn't come with a headphone output, but I didn't have any problems making one. I got myself a headphone socket and some resistors and wires and hooked it up to the speaker outputs on the amp. It's a series/parallel hookup that drops the output to the 'phones a lot to keep them from getting blown out and to get rid of an annoying transformer hum. The hum's barely noticeable with speakers but my Senn 580's are far more sensitive it was quite distracting with them. The diagram's been posted on Head-fi a bunch of times but I can't seem to find it right now.

Other notes of interest. When I powered up the amp for the first time after building it, it sounded like a cheap AM radio playing through a blown out speaker. My 1st thought was "what the hell did I do wrong?", followed by "what the hell was Wodgy raving about in his review?". I then remembered Hirsch's comments about how some tubes can sound absolutely awful until they're broken in, so I just let the amp play for a few hours. When I got back to them they were amazing, and I was up listening 'till the wee hours of the morning.

 

[elrod-tom]

Thanks aerius for clearing all that up. It sure sounds like a nice value, and a nice intro to DIY tube.

I wonder if it could power the AKG K1000? Maybe I'll give it a whirl, after I (ugh) rebuild the Fisher and do the Pete Millet tube amp. Oh...and buy the K1000, of course.

That might be sometime in July, at the rate I'm going. Maybe by then they will have switched out the tube sockets to ceramic.

 

[Joe Bloggs]

You can find a link to the impeder and the Grado headphone adapter schematic (further down the posts) in my sig.