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DIY six input tube preamp


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The time has come to update my mostly DIY 2.1 system with the capability to have multiple inputs for multiple sources and a tube buffer/linestage. My current "pre" consists of a passive 48 step khozmo mk2 attenuator. It is essentially an attenuator in its own box. It has no active components, no power supply nothing. The only function is serves aside from volume adjustment is to split the signal between the amp for the speakers and amp for the sub. This a great sounding attenuator, however, it is limited to one input. That is how I made it, because for the most part, I have only used a cd player as a source in this system.

 

I now have the desire to be flexible with my input capabilities without having to unplug and plug new components in to do so. This of course could be accomplished by simply adding a rotary switch of some kind to my existing pre, but what’s the fun in that. I have decided to go all out and build a pre with all the features and expandability that will last me until I the bug bites again. Because I have been very happy with the khozmo unit I decided to go that route again. Khozmo has added several new products to their lineup since I purchased my mk2. After a few emails to Arek from khozmo I decided to go with the new 64 step relay based attenuator and the six channel input selector plus the power supply. Both of these units run off 5vdc so the attenuator can be powered straight from the input board. The input channels can be switched using a supplied rotary switch that matches the attenuator, or by momentary switches. Both the attenuator and the source board come with individual digital displays. Another great feature is the source board also contains a relay to trigger an AC circuit. Best of all, this also includes a remote that will control all of this, especially volume, without me having to get up!

 

The khozmo suite should provide a solid platform to build a preamp around. This could essentially be all I would need to build a pre, however I am curious to see what active circuits can do the sound in my room. I know imaging is not important to some people, however I feel it is a very important aspect of the listening experience. I am hoping that by adding an active circuit i will achieve an even better soundstage and increased depth. To do this, I have decided to try the tubes for hifi VTA SP-14 linestage as the active circuit. I have read many great reviews and there seems to be excellent support available from Roy. This of course will add gain to my system that is not necessarily needed to drive my amp from the cd player, however I want to experiment with the cumulative effect of adding more tubes will have on the sound.

 

The best part about my build is if i decide the active circuit is not right I can bypass, change or eliminate it. The khozmo platform can stay in place, and I can easily drop in a new circuit, or simply a longer run of wire.

 

Housing all of this circuitry is going to require a large enclosure. I have the perfect enclosure just sitting on a shelf. It is the first DIY audio project I built. It is a dual mono integrated amp built using peter daniel PCBs. It was a great build that taught me a lot and I enjoyed it for a long time. I have built a few more amps since then an just moved on to different designs. So it just sits. The front panel though has always been my favorite and I have wanted to use it again for something so this is perfect.

 

The thought of having an enclosure all ready to go is a great time saver, hurray! Well, not so much. I decided to completely rebuild the enclosure. Being that this was my first project I made compromises based on the tools I had at the time and my ability to work with these materials. Of course being that this is going to be my best work yet, aesthetically speaking, that simply wouldn’t do. Functionally it would have been fine. I did however add almost 2 inches to the depth of the enclosure which I will need, so maybe it was not all aesthetics.

 

This is what I started with.

 

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Thank you wdecho and hardway. I actually do not have access to a machine shop, I wish I did though. All of the aluminum panels were cut with a jig saw and then the edges were filed flat. Then increasingly finer grit sandpaper was used to get the edges smooth. All of the holes were cut using holesaws in a drill press. Once cut (which takes a while on thick aluminum) I wrap sandpaper around the hole saw adding layers as I go. Eventually the holes end up perfectly smooth. I used a router to get the round edge on the corners of the front panel. That is the beauty of aluminum it works in similar ways to hardwood.

 

The wood sides originally were walnut, but I was lucky recently and acquired some nice rough sawn hardwood of several varieties so I had to make new panels. I cut some nice cherry and came up with two nice sister boards. This is where I spent some serious money on this project (not really). I had to plane these nice boards but all I have is a hand planer. It was time for a thickness planer. It is amazing how much time a planer saves. Anyways, the cherry should get more dramatic the longer it sits in the sun.

 

I did manage to make some progress today. I finished up the signal wiring for each channel and tied up a few more loose ends. 

 

I need to start on the SP-14 board soon. Maybe tomorrow.

 

Here is a pic of the wiring and my workspace. Please note: my desk is never this clean except for right now. That is why I took a picture of it.

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Over the last few days I have been able to get most of the PCB populated, so, it is time to make a final decision on the internal layout of the enclosure. As large as this enclosure is, the SP-14 PCB is huge, and space is still somewhat limited. I have attached a few examples of my options. The standard layout does not leave much room for the toroids, so I am exploring some other orientations. I am also trying to keep all of the A/C current localized to one side of the enclosure, and as far away from the PCB as possible. To do this, the best use of space calls for the transformers to be oriented vertically, however I am worried about magnetic interference from the core interacting with the circuit. I need to do more reading on the topic to understand if this is really an issue. Some shielding between the PCB and the transformers may be all that is necessary. I will also have to build mounts that can support the weight of the transformers being mounted on one side to avoid the dreaded shorted turn.

 

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  • 3 weeks later...

I made the decision to mount the transformers vertically. I built a bracket that would that would easily support the weight of both of the transformers without any movement. I also decided to utilize some shielding. I ordered the transformer covers from Antek that fit these toroids. I have used them in all my previous amp builds. They are a little heavy, but they fit perfectly. This configuration allows more space for the PCB.

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1 hour ago, Schu said:

I absolutely love that stepped attenuation... I'd love to hear this someday.

You should. I was a big fan of the mkII attenuators. I used one for several years. They are miles ahead of any alps or tkd I used. However, this new one is again a giant step forward. I was able to listen to the attenuator alone for a few weeks during the testing phase of this build. The imaging and clarity was noticeably improved. This attenuator appears to be invisible in the signal chain. 

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Did you happen to say where you purchased the board from?  I know where some of the rest of your parts came from (PM) but not sure if you have mentioned board source? 

Very nice work.  If you ever tire of your build ... I'd offer you a very fine Alberta Steak Dinner with some also very fine Italian Amarone wine!  And some money!

 

 

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12 hours ago, MC39693 said:

Did you happen to say where you purchased the board from?  I know where some of the rest of your parts came from (PM) but not sure if you have mentioned board source? 

Very nice work.  If you ever tire of your build ... I'd offer you a very fine Alberta Steak Dinner with some also very fine Italian Amarone wine!  And some money!

 

 

Mmmm. Steak and a dry red. Always a great offer. The tube board came from Roy Mottram at tubes 4 hifi. He is a very helpful guy, and the board is very nice. If for some reason this build does not suit my system I will offer you first dibs. 

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The way to a man's heart and maybe his amplifier is always through Prairie raised world class steak, smoked on an open flame and fine Quintarelli Amarone.  Maybe even some Hawaiian rum as a chaser. 

 

I can't imagine your build not suiting your system since you have total control and can change items out, plus your work looks awesome for someone without a machine shop.  I'll have to see if my electrician son-in-law wants a challenge to match up to your work ... esthetics not a chance, but if I buy parts and lay them out ... he'll rise to the challenge.

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  • 9 months later...

So... after what feels like forever I have finally completed this build. I had to make many changes along the way, but I am happy with the final product. After A great deal of listening I decided the SP-14 circuit was not the best for my system. Too much gain. So I sent it on it’s way and went a different route. In the spirit of keeping this system all tube I opted for a ACF -2 octal from glassware. This is a buffer. There are many options for tubes, but I decided to use 6sn7. I was originally splitting my source signal to an amp for my speakers and sub. This sounded good, but now it sounds much better. The bass is cleaner and my speakers have an increase in detail. I am very happy with the sonic results. I would definitely recommend this project to anyone running multiple amps or driving long cables. 

 

The best best part is I can toggle between sources again without having to change cables. That was getting annoying.

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5 hours ago, No.4 said:

Inside.

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I think the construction and wiring job is outstanding. And you did the wiring quite right. Your signal leads are short and distanced to minimize and crosstalk. But your DC power and DC control signals are neatly bundled as these have no crosstalk issues.

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