[Bruce B]

Hello All,
I have built a battery eliminator using the most popular AES K101a. Using plans and all the AES parts  ordered from their parts list, except for the Q2 transistor, obtained from eBay.  Not wanting to have the circuitry on a board, I instead built this project on perf boards and placed it inside an antique jewelry box.  This power supply seems to work well and hold up to its voltage duties and regulation.  The problem is, it provides a pretty good 60 cycle hum.  In studying their only DC filter tank, a 50 UF electrolytic and no inductors so its not surprising to me to hear hum in the B+
 I noticed that many collectors have built this exact battery eliminator and it is a much discussed subject in the forums.  I have searched about the hum though and come up with nothing.  So I am wondering how other people's projects worked in regard to a DC ripple and if anyone has ever modified it from the original design to eliminate hum?
  I am planning to provide a more sophisticated filter structure, like 2 filter condensers and a choke.  If anyone has any ideas I would like to hear.
Bruce B

[morzh]

Bruce


Could you post the schematic if this power supply?

[Bruce B]

   

[morzh]

Caps of 1,000 uF 250V are about $7 in Mouser.
C1 could be increased.
A NTC inrush limiter could be employed to limit the current at turn-on.

[RodB]

So, I just built a 90 volt 40ma battery eliminator for a farm radio and had to use 1000 mfd for the input filter cap to keep the ripple to acceptable levels. It's just a pi filter, no pass transistor.

[morzh]

They are trying to use a regulator. Essentially it is an LDO schematic. LDOs are supposed to greatly reduce ripple, so they counted on a possibility of using a smaller cap.
However, typically LDO does use some smaller value capacitor at the output.
It could be tried first.

[Arran]

This is interesting, they actually designed the B+ side with some form of voltage regulation, I think? Most of the B+ supplies I have seen, vintage and home brew models, are lucky to have a inductive choke between the input and output filter caps, often just a 1-1.5 K resistor, yet they do not produce much hum if you run a 1940s battery set from one. I always thought that the C- and A+ supplies were much more important for filtering and voltage regulation if you wanted to keep hum out of the circuit, but AES went cheap relying on a single filter cap and a lone Zener diode. I can't figure out why they made the B+ supply so much more elaborate, if you look at the power supply in something like a three way portable like a Zenith T.O, it's just a pi filter arrangement, with maybe a 100 uf cap ahead of the filament string. However Mike/Morzh is the right one to ask, power supply design is how he pays his bills.
Regards
Arran

[Bruce B]

These thoughts are are all quite interesting.  I wanted to mention that I haven't tried the C- + supply yet.  I am new to Battery radios.  Having work on many AC sets of the 30s, I recently decided to step back into the 20s and have been using an ARBEIII for my first 2 battery radios.  I decided to build the AES supply so I could give life to a second radio set when it is off my bench, I like listening to them while working.  I have only used the AES  on my 3rd battery radio, a Freshman Masterpiece 6F5 and it doesn't require a C- + supply.  The B+ seems to regulate well , at no load,  93 volts, no load, and with 5  01As and 5 volts on the filaments, the B+ drops to 91.5 volts  at 14 MA.  Not bad.
I'm hoping another AESBE builder can chip-in and explain how much ripple they have?  I look at the design of the B+ supply and because I don't see much thought given to the unregulated path for DC leveling, I start to wonder if they are depending on that pass transistor is ripple tamiing and it's not working some how?  I might stick a scope into the circuitry and see what I can see, but for now I will wait to see what the expectations should be.  I don't want to chase down something that isn't wrong.  If these supplies just hum, then I will just fix it with a PI design of my own.  The hum is not extremely loud, but loud enough to be annoying, it would send you in to replace the filter caps if it was an AC set. Keep the thoughts coming.?
Bruce B

[RodB]

Mike, I did find an LDO to give me the 90 volts and I got real excited when I discovered it but then I discovered it's only good for 20ma, half what I needed.

Arran, I had to use several hundred micrfarads to smooth the A supply. These were 1.4 volt filaments and any ripple was passed on to the speaker. I was going to use an LM317 to regulate it but ended up with the brute force method.

[morzh]

Rod

Yes, your standard TO-220 LDO is good for 0.7 - 1.5W dissipation (more with a heatsink) and a small current with a large dropout (which in this specific case is...what...55V) will get you that.

Now, they simply took a typical voltage egulator circuit and implemented it using discretes. Using a good enoughg transistor with some heatsink will give you a good regulator.
Also, you could simply take that LDO and couple it with a larger transistor, beefing it up.

But the output cap, a small one that will take care of small ripple and stability, is still needed.
Large cap should not be used as it could create a short but large dissipation and destroy the transistor (an integrated LDO is typically thermally-protected). At small currents 10-20uF would be enough.


Also, it is a good idea, especially with the bi-polar transistor as the power element, to put a reverse diode across its E/C (it is often done with LDOs): this protects it if the output capacitance discharged slower than input ones and a reverse voltage develops across it. Of course if tehre is no output capacitor, this is not a problem.

[RodB]

Well, I like to keep it simple and didn't want to mess with perf board. Plus, the owner of this radio is a relative of a relative.

[David]

Something to look at have one and like it. David       

[David]

The power supply was on Phil's Old Radios web site I did add a isolation transformer. David