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Aikido LV: Wall wart ripple & rc filter

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I'm pretty (extremely) new at this, so if this is a dumb question feel free to say so. I've looked around on the site and haven't found much that deals with this question though...

As a first preamp build, I'm doing Broskie's Aikido LV. The ability to use an external power supply sold me. In his write-up on the preamp he says using a 24v or 48v wall wart is possible, with this extra info:

The key feature is the small 150mVpk-pk of ripple, as some switcher power supplies put out closer to 1Vpk-pk of ripple. I would prefer 1mV of noise. Fortunately, the ripple frequency is supersonic, say 40kHz to 80kHz, which allows us to use a simple RC filter to scrub the ripple from the DC voltage. The LV Aikido PCB holds pads for such an RC filter. I used a 1-ohm resistor (R17) and a 1kµF 63V capacitor (C2) to post filter the incoming DC. Alternatively, an LC filter could be put in place by substituting the resistor (R17) for a small, low-DCR hash choke.
(Aikido Cathode Follower & More Triadtron)

Trouble is, this model wall wart is no longer available. The ones you can still buy have 240mv ripple at 48v (e.g., GST90A48-P1M: Mean Well : 48 Volt 1.87 Amp 90 Watt Regulated Switching Table Top Power Supply : Power Supplies & Wall Adapters). I've scoured parts express, jameco, mouser, etc. and this is the best I've been able to find for wall warts. The 150mv that Broskie quotes is 0.3125%, and 240mv is a good bit more at 0.5%.

My question to anyone who can answer, or just point me in the direction of relevant resources is: do I need to adjust the specs on the incoming RC filter to account for this more ripply power supply?

I could probably build a power supply but that sort of defeats the purpose of starting with the LV in the first place. I figured it'd be a good place to cut my teeth with less danger than a higher voltage project.

For what it's worth, I'm re-taking some math classes in my adulthood with the intent of learning the diff-eq necessary to understand all this better but I'm starting from a position of extreme ignorance. Still having a great time with it though!

Any help is much appreciated. :)
 
Is there any reason you're restricting yourself to a wallwart power supply? There's a whole lot of other 24 & 48 volt supplies around that'll do the job - some are adjustable so you could add a regulator after the supply to get quite good results

With all the Aikido ccts, better power supplies result in better sound - for some strange reason, the idea of the 'noise cancelling' arrangement of the output pair seems to indicate to some people that the Aikido design will somehow magically tolerate poor quality power supplies - it doesn't. Spend a bit of effort &/or some $s on a good power supply.

... my 2 cents
 
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Is there any reason you're restricting yourself to a wallwart power supply? There's a whole lot of other 24 & 48 volt supplies around that'll do the job - some are adjustable so you could add a regulator after the supply to get quite good results

With all the Aikido ccts, better power supplies result in better sound - for some strange reason, the idea of the 'noise cancelling' arrangement of the output pair seems to indicate to some people that the Aikido design will somehow magically tolerate poor quality power supplies - it doesn't. Spend a bit of effort &/or some $s on a good power supply.

... my 2 cents

Broskies documentation is what gives that impression . Now that I know better i will swap out my wallwart at some point.
 
James is correct, the better the supply the supply the better it will sound. It's all down to budget of course. A 48 volt linear supply will set you back probably 50 bucks. I take it you are building the 48 volt version for use with 12BH7 or ECC99 tubes. I've built a couple of the 24 volt ones but then I bought a bunch of 6GM8s years ago when they were cheap. All of my supplies were linear (non-switching) ones.

At any rate I'd say give it a shot with the supply from Jameco you mentioned and the stock LV part values. If you want to try a choke instead of resistor R17 maybe a Hammond 155B at about $16 would be a good place to start. You could always go bigger (=heavier) but these will cost more.

If you upgrade to a linear supply later on you can still use the choke. I take it you've already bought the Broskie kit. If you haven't his 12Vac kit with 6DJ8s works really well and all you need to add is a 12volt transformer, preferably a small torroid ~$20.

Good luck, Steve
 
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Hi folks - thank you very much for the replies! This is really useful information. I think i'm going to go with the recommendation to try with the available external supplies, and adjust if desired. That makes sense, and would be good for getting an intuitive idea of what different configurations can do, which I definitely don't have yet.

Onward and upward.
 
Being interested in building a 12vac aikido, can I ask you about the safety level of this kind of power supply?. You are operating with a 12vac supply but then this low voltage is raised to more dangerous levels so there must be potential harm. Any suggestions? If I understand correctly the 12VAC input us floating, not attached to chassis.
 
The 12 volts AC is only converted up to about 100 volts DC so it's not crazy dangerous. Though yes, one side does not sit at ground. 12 volt wall warts are cheap but my preference is a transformer in the same chassis as the PCB. This way the there is a ground path form the third pin on the AC cord to the chassis. A small 12 volt torroidal trans is only about $20 US so not too high a penalty for safety.

Link to 12Vac article on Tubecad: Aikido 12Vac

Link to 12Vac build article: Battle of the Cheap Line Stages – Part 1 | Wall of Sound | Audio and Music Reviews There are 3 more parts to this series on this site detailing the build if you are interested.


Cheers, Steve
 
I used an InternationalPower.com +/-24vdc supply IHBB24-1.2 for my very early LV Aikido. They aren't as cheap as those above but they are good with low ripple. I use it for the heaters in series, as B+ and the negative goes to the Creek phono pre card that I pulled out from my Creek amp when it died. It required +/- 25vdc. I think I bought the 1.2amp version, that was plenty.

When I built mine I was able to buy a quad of Bugle Boy 6GM8 tubes for $20 from Antique Electronic Supply, actually less because he gave me a discount because he could barely give them away. It wasn't that long ago either.
 
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Jordi,

Wall warts are used because they are primarily less expensive than a transformer that you would build in. As far as safety goes when you use a w w there is only 12V AC inside the amp chassis instead of 240V AC (I see you are in Europe). If you can "get over" the fact that a built in transformer means that you will have full AC line voltage in the chassis (240 volts in Europe) I still like the fact that the chassis can be connected solidly to ground.

A good toroid will typically have better regulation than the transformer in a wart. Regulation in this case means the % difference between full load and no load voltage. Which ever transformer you use it should have about 2 amps output at 12V AC. A little more is OK but don't for instance use a transformer that will output 4 amps as the voltage output when used with the current draw in an Aikido 12Vac might be quite a quite a bit over 12 volts. If the AC voltage from the transformer runs too high you could stress some of the capacitors it the power supply. Use a transformer, wall wart or other that is rated for between 1.8 to 2.5 amps at 12 volts.

I hope I haven't made this too complicated.

To answer your earlier question yes, the 12 volts AC does float above ground but this is not a problem with the voltages encountered in the Aikido 12Vac.

Cheers, Steve
 
But if you use a 12VAC wall wart there is no earth connection to chassis, as this connection is floating. The amplifier is isolated from 240V mains but also from earth grounding. Then the 12VAC are not dangerous but... can the 100-150VDC developed by the voltage multiplier be a potential danger? Any safety tips for this kind of power supply?
 
But if you use a 12VAC wall wart there is no earth connection to chassis, as this connection is floating. The amplifier is isolated from 240V mains but also from earth grounding. Then the 12VAC are not dangerous but... can the 100-150VDC developed by the voltage multiplier be a potential danger? Any safety tips for this kind of power supply?

I'm very interested in this question too, thanks for adding it!

I've been reading up but don't have the confidence in my understanding to hazard a guess. I'm also working on a power supply for a boozhound phono pre, so there's a lot of learning I need to do about grounding and safety.
 
If your power amp chassis is connected to ground then your line stage will be grounded when you connect the two. Sure, the ~100 volts is "dangerous" but as tube amps go it's a pretty low voltage. Just don't go sticking your fingers in carelessly and you should be OK.

If you build an Aikido 12Vac make sure the power supply diodes and filter caps are oriented correctly and you'll save a lot of grief. See link.

http://3wqmcw1jc4oh3afanm29507a.wpe...2016/12/Line-Stages-Part-4-board-assembly.pdf

It is usually best if the circuit ground is not connected directly the chassis. I've found that a 10 ohm 2 watt resistor in parallel with a 0.01 to 0.1 film cap works well. Broskie shows a few grounding schemes in his manuals. See picture for example.

Cheers, Steve
 

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Thank you for your advice Steve, you're very kind! The R+C connection is a good idea, I've seen it in a opamp phono preamp schematic I built some time ago. So in this kind of preamp the only connection to chassis would be through this one, right? Only for sonic purposes or will it make the circuit more safe?
 
Jordi,
The chassis should be connected to the circuit ground in some way or other for sonic reasons, shielding primarily and for safety too. As long as your power amp chassis is connected to the 3rd pin electrical ground and the electrical ground is connected to the circuit ground you should be OK. The interconnects between amp and line stage will connect the line stage to ground.

You aren't guaranteed 100% grounding of the line stage this way. You still might have too connect the line stage chassis to the 3rd pin ground (or the power amp chassis). The best way to determine this is experimentally.

But build the 12Vac aikido. With a set of Genalex reissue (Russian) E88CC tubes and output cap upgade to ~$15 Mundorf or what ever you like for like for caps it's a real sweet little line stage. I know several people who have built and/or owned one and they all really like them.

Cheers, Steve
 
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