Question about system design with the K-12

[irishbk]

Hi all. Forgive the possibly elementary nature of this post. I'm embarking on my first foray into the world of DIY tubes. I've spent my entire week reading these forums and have learned a lot. Of course, I still have many many questions.

I was looking at purchasing a K-12/K-502 amplifier kit. It seems like a good beginner project, and one of the few within my meager budget. But I also need a pre-amp, specifically for a turntable. I'd love to do the Bottlehead Seduction, but I'm trying to keep my costs down. I know that's a "budget kit" relatively, but my lack of funds just require me to go cheap as possible for now.

S-5 also sells the 12AX7 pre-amp tube kits. But I don't think this is designed specifically as a phono pre-amp. Would this be a poor choice? Another option I was considering was the Hagerman Bugle . I know, I know... it's SS (gasp) and I'll be bastardizing the tube sound. But as a temporary solution, until funds allow me to replace it, will that be a decent/workable pairing?

I would also, ideally, be able to incorporate some sort of stand-alone, multiple-input switching block into the system between pre-amp and amp. So I can plug in CD, Tuner, etc.

Any thoughts/insights you might have would be most appreciated.

 

[Sherman]

About source switching- why not build it in to the K-12? Put the K-12 in a chassis with your RCA inputs on it and add a rotary switch to select the input. That's what I did with my K-12.

The phono-pre is a more difficult matter. You need to have a phono preamp that was designed as a phono stage. A regular preamp won't have sufficient gain and won't have RIAA correction. If your budget is tight going solid-state and then building a tube pre when funds permit is a good idea. Take a look at the Art DJ preamp. It has adjustable gain, selectable input capacitance and actually sounds quite good. They are usually available for about $50 US.

 

[Jeb-D.]

About source switching- why not build it in to the K-12? Put the K-12 in a chassis with your RCA inputs on it and add a rotary switch to select the input. That's what I did with my K-12.

That's what I did too. + added a phono stage of my own design in with it.

The Phono stage out of the enclosure.

 

[irishbk]

Sherman,

Thanks very much for the response and recommendations. The Art DJ Pre is an interesting option. I was hoping to build my own, but for cost and ease... maybe I should be happy with just building the K-12 for now.

Question on building the switch into the K-12. This would be great. Did you find wiring schematics somewhere to do so, or do you have any tips on achieving this? I'm not comfortable designing something myself yet. Thanks again!

-bk

 

[Sherman]

Sherman,

...

Question on building the switch into the K-12. This would be great. Did you find wiring schematics somewhere to do so, or do you have any tips on achieving this? I'm not comfortable designing something myself yet. Thanks again!

-bk

I've attached a quick diagram on the basics. I've only drawn 2 inputs but you should get the idea. The round thingy is the back of the rotary switch.

One thing to note is you need to make sure which pairs of contacts on your rotary switch are switched together for the left and right channels.

If you go this route don't solder the RCA jacks that come with the kit to the PCB. Instead solder the leads from the rotary switch and the grounds from the new jacks to those holes in the PCB where the original jacks would have mounted.

HTH,

 

[Sherman]

That's what I did too. + added a phono stage of my own design in with it.

I like that enclosure!

 

[irishbk]

Thanks a lot, Sherman. That drawing is very helpful. Seems like something even a noob like myself can manage.

 

[irishbk]

That's what I did too. + added a phono stage of my own design in with it.

The Phono stage out of the enclosure.

Very nice, Jeb. Thanks for the pics.

 

[Sherman]

Thanks a lot, Sherman. That drawing is very helpful. Seems like something even a noob like myself can manage.

If you look at Jeb's amp in the photo you'll see he has simply used a DPDT toggle switch as a source selector. A very good solution if you only need two sources.

 

[Jeb-D.]

Thanks for the compliments. I got the chassis for $18 from surplus sales of Nebraska, it's half way down the page.
http://www.surplussales.com/Cab-RacksHardware/encl_2.html

The walnut veneer is really nice, the pics don't do it justice. The panels were machined from www.frontpanelexpress.com

 

[irishbk]

Out of curiosity, what kind of speakers do you guys have going with this? I'm looking for some basic speakers for comfortable listening in my living room. Nothing too loud or fancy. I've found very little info on speakers that go well with the K-12. Either DIY or already made. I considered making some DelSol's, but those drivers seem to be out of stock. Thanks!

 

[Sherman]

The ideal speaker choice depends on so many factors, size of listening room, typical listening volume, type of music, personal taste and so on. I've used DIY speakers as well as Dynaco A25s and Advent /1 with mine and thought they all sounded good.

The DIY speakers were moderately efficient at 93dB, the Advents and Dynacos are probably between 87 and 89dB. ([email protected]) If you aren't trying for ear-bleeding SPL that level of efficiency is probably OK, especially with most acoustic music. (Orchestral music, while acoustic has incredible dynamic range if recorded correctly and presents IMO more of a challenge to systems than heavy metal.)

 

[Jeb-D.]

Out of curiosity, what kind of speakers do you guys have going with this? I'm looking for some basic speakers for comfortable listening in my living room. Nothing too loud or fancy. I've found very little info on speakers that go well with the K-12. Either DIY or already made. I considered making some DelSol's, but those drivers seem to be out of stock. Thanks!

I'm running some BR speakers bassed on the Fostex FE127E. My woodworking skills aren't very good, so I bought a built pair off ebay for $330. To get good sound with the speakers I had to add some correctional circuitry which only cost like $20. Now the combo does very well given it's price point.

The K-12/K-502 also does good with my vintage JBL L100's when running the stock cross-overs. When I run upgraded crossovers the speaker becomes too much for this amp to handle.

If your planning on building your own, I'd do this
http://www.quarter-wave.com/Project02/Project02.html
The FE167E will work as a replacement for the FE-164(this model was discontinues). However you will have to use different correction circuitry because the K-12/K-502 has an output impedance of about 5.5ohms(that's what mine measured). If you go this route I can help you determine the circuitry needed. If those are too tall for you, Bob Brines has a folded version design. Bob will also sell you the flats if you want to build it yourself but don't want to cut the wood.

 

[irishbk]

Thanks, Jeb and Sherman. Very helpful!

 

[Jeb-D.]

I noticed a mistake in my above post. My speakers are bassed on the FE167E not the FE127E.

 

[irishbk]

A couple of other questions, specifically in regards to the power supply. The Bugle pre-amp kit is designed using two 9v batteries as it's power source. My assumption is that there's no clean way to "steal" power from the K-12 kit. Am I wrong? Just trying to look for ways to not open up the chassis to replace batteries (or build separate power supply).

Additionally -- and I think this is a simpler matter -- if I wanted to add a power on indicator lamp into the amplifier... can I just wire that in series between the 12VAC input to the heaters? Or is there a better way to do it? Thanks.

 

[Jeb-D.]

what do you need an indicator for when the tubes light up . If you really want to I'd suggest a small lamp that's meant for 120V and wire it after the switch on the primary side of the transformer. It's best not to add load to the transformer if you don't have to.

 

[Sherman]

...

Additionally -- and I think this is a simpler matter -- if I wanted to add a power on indicator lamp into the amplifier... can I just wire that in series between the 12VAC input to the heaters? Or is there a better way to do it? Thanks.

You would put it in parallel with the heaters but I agree with Jeb. The power transfomer is running near capacity in this kit and I wouldn't add to the load.

Consider using an illuminated on/off switch. They have three contacts on them. Two are for switching the AC hot and the third is for the AC neutral so the light can be illuminated. Easy, fast and no added stuff in the tube circuit.

 

[irishbk]

Thanks, guys. That's what I thought as far as the power supply load. And you make a good point with regards to not really needing a power indicator light. Like in life, I tend to make things unnecessarily compliclated.