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Old 3rd November 2006, 04:30 PM   #1
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Default Aikido PSU - inetgrated or standalone???

I know that a number of folk have built the Aikido preamp. I will be building mine in the next month or two. However, I have a question:

What is best a standalone PSU, or one built into the same chassis as the preamp?


Morgan Jones suggests that peamps benefit from standalone PSU's, as they're more sensitive to hum. However, I would consequently lack shelf space for both as separate units.

Bruce Anderson brought his Aikido over to my place last weekend, it sounds great. His PSU is standalone, but he also uses separate chokes and filter caps for each input, so his preamp chassis is already stuffed with large motor-run caps (His PSU only contains transformers and bridge rectifiers). As I am using the Broskie stereo octal board, I will only need half the number of caps and chokes.

Another question:

If I build the PSU into the preamp chassis, is it better to top-mount the transformer and chokes? I was thinking that the aluminum cover would help shield these from the actual preamp components. Of course, enclosed chokes cost and weigh a lot more than "naked" chokes, although the latter would certainly need to be mounted inside the chassis.

Everything will come together in time......

Charlie
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Old 3rd November 2006, 07:32 PM   #2
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Hi Charlie,

I have always felt that seperate power supplies were overkill for the most part. The exception being when size and weight became a real issue on hugh high power amplifiers. It also allows one to be a little less careful with placement of componets.

If you use a large enough chassis and adhere to good layout, I feel there should be an excellent chance of a successful final project. Thoughtful layout and wiring techniques are important to assure a satisfactory outcome. Things like keeping the input circuits as far away from the (properly orientated) power transformer as possible, and the use of a ground buss with a single point chassis ground are necessary. This is usually at the input, the output or the center and can be found by experimentation. But you probably know this already.

I prefer to mount the transformers on top of the chassis because it's more traditional and practical. But rear mounted on the apron of the chassis is not unheard of with small transformers.

I believe aluminum provides only minimal electromagnetic shielding from transformers. So again distance is important. If you use AC for the filaments, twisting the wires and keeping them tucked close to the chassis helps a lot. I like DC filaments on all but drivers and outputs.

Since you're using the circuit board, most all of the critical wiring is done for you, and you have little other choice except where and how to mount it. (above or below chassis) Just don't mount ir near the transformer.

Btw, isn't that Aikido circuit supposed to cancel out power supply noise? (just being a wisenheimer) And fancy silver wire and silver solder is optional.

Victor
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Old 3rd November 2006, 08:17 PM   #3
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Victor,

Thank-you for the reply. I feel encouraged. I have been reading the Morgan Jones book on building tube amps. He goes into great detail on routing of wires, etc. When you say Ground Bus near input/output, I assume that you mean for the signal input and output. My initial plan for the signal ground is to make some kind of bus (possibly shielded) and not connect it at all to the chassis - in other words, the input and output RCAs will be isolated from the chassis. I suppose that they will be grounded to the chassis via the star-earth on the circuit board, which I may float or connect to the chassis. I'll have to play around with this.

Of course, by mounting the chokes to the top cover, the preamp will look great, and I may get some extra shielding. However, this will add to the weight and expense.

Morgan Jones does recommend aluminum over steel, and the aluminum chasses are usually cheaper. However, painting aluminum is a real pain, so maybe an Al chassis sides with black steel top plate. My plan is to then surround the sides with wood.

I expect that AC heaters will be fine, the Aikido is supposed to be extremely quiet as far as noise is concerned. Maybe I should simply go with an all steel chassis and forget about a wood surround/facia.

Thanks,
Charlie
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Old 3rd November 2006, 08:52 PM   #4
jayme is offline jayme  United States
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My aikido is an all-in-one unit, with integrated PSU. I've got a mains transformer, 3 large chokes, 4 large motor run caps, and a tube rectifier. The PSU section takes up two-thirds of the Steel case.

My heaters are AC, with the wires nicely tucked into the corners of the chassis.

I use 2" ceramic mounts to raise the circuit board sufficiently off the base of the chassis that the 6SN7 tubes can stick out the top. Same with the rectifier. The mains and the chokes are all mounted to the bottom of the steel case. and are fully covered by the top.

I have no hum and no noise. Quiet as can be.

And, frankly, it looks awesome! Kinda like a deHavilland.

Oh...and one recommendation: Mount your selector and volume pot at the back of the chassis, and run shaft extensions to the front knobs. If you get the 2" spacers and mount the circuit board high, you can even run the shafts directly under the circuit board.

The only hangup left for me is how I am going to drill out the 3" hole in the top for the biggest motor run.
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Old 3rd November 2006, 09:29 PM   #5
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When I spoke of a ground buss I had temporally forgotten you're using circuit boards, and was thinking more in the way of tube sockets mounted on a chassis. Your boards will probably have a common ground path on them that will need to connect to the chassis at some point.

This is to say that the supply minus and the signal low will be connected together and to chassis at one point. Where that point is will be determined by ear, but is general in one of the three places I mentioned. I don't think you'd want a floating ground not touching chassis. At least I've never done it that way.

I see no reason not to use aluminum. It's much easier to work with then steel. Whether you use a folded box chassis or a flat plate is up to you. Steel is a pain unless you have a good drill press and other cutting tools plus experience working with them.

I generally don't paint my chassis. Just polish them. But I have otherwise painted aluminum many times. A good sanding (power or by hand) with fine wet/dry cloth and cleaning with alcohol just before painting works for me. Proper preparation and cleaning is the key.

Victor
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Old 3rd November 2006, 09:35 PM   #6
EC8010 is offline EC8010  United Kingdom
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Quote:
Originally posted by jayme
The only hangup left for me is how I am going to drill out the 3" hole in the top for the biggest motor run.
Does it have to poke all the way through? Could you put the clamp above the chassis and just cut whatever holes are necessary for the connections? 3" chassis punches are available but they're eye-wateringly expensive. Cut the hole by hand with a fretsaw or with a power jigsaw using a really fine blade (lots of TPI).
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Old 4th November 2006, 09:16 PM   #7
zacster is offline zacster  United States
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My power supply is separate from the chassis in my Aikido, but mine is the 24v version where both filament and B+ run off the same DC power. I bought a pre-built regulated supply from International Power (through Mouser) that came on its own open chassis so I left it off the main chassis.

You will like this pre-amp.
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Old 4th November 2006, 11:29 PM   #8
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Hi Charlie,

I have built a number of Aikido line stages. Some with dual mono supplies, some with power supplies as part of the main chassis and some with power supplies that are separate from the main unit. All work fine but I enjoy the dual power supply Aikido the best because of the added separation between channels.

With respect to the cabinet I like to use brass or copper as a top plate that sits on a wooden base. The copper or brass cost about the same as a aluminum chassis and I think it looks better. I still need to replace the plastic volume control knob but the picture gives you an idea. The VC and rca jacks are insulated above the chassis and grounded to a central point.
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