Aikido LV: Wall wart ripple & rc filter - diyAudio
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Old 20th April 2017, 12:35 AM   #1
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Default Aikido LV: Wall wart ripple & rc filter

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:

Quote:
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 1kF 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.
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Old 20th April 2017, 01:41 AM   #2
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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
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Old 20th April 2017, 02:24 AM   #3
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Quote:
Originally Posted by jameshillj View Post
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.
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Old 20th April 2017, 02:27 AM   #4
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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

Last edited by Steve Morley; 20th April 2017 at 02:29 AM. Reason: forgot a word
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Old 20th April 2017, 04:12 AM   #5
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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.
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Old 20th April 2017, 11:24 AM   #6
ratbagp is offline ratbagp  United States
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Without commenting on the noise issues, you can buy 48 volt units for $4.50.

48 VDC 0.38A POWER SUPPLY | All Electronics Corp.

ray
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Old 20th April 2017, 12:01 PM   #7
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Default Marginal current rating

This one would power a string of 12BH7s where the filament current is 0.3 amp but not ECC99s that require 0.4 amps. Just sayin'.

Cheers, Steve
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Old 20th April 2017, 12:14 PM   #8
ratbagp is offline ratbagp  United States
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My understanding is that the 48 vdc is for the B+, not for the heaters.

ray
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Old 20th April 2017, 12:17 PM   #9
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The B+ and the heaters are meant to be in parallel off the same power supply. But your point is well taken - there are some cheap options for 48v off the shelf power supplies.
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Old 20th April 2017, 12:18 PM   #10
ratbagp is offline ratbagp  United States
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My apologies, I see Broskie uses it for the heaters as well.

ray
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