[wroman214]

  I would like to eliminate the wall wart for my Soundsmith phono preamp and try something different.  I plan on using a low value toroidal for low EMI radiation but what I do not know is what the amount of voltage drop to expect with a choke.  If I understand correctly if a choke is used directly after the diode bridge instead of a cap the voltage will drop more than if a cap is used directly after the bridge.  I need to know what voltage transformer to start with if I want to arrive at 24 volt output.

For practical purposes I am looking at chokes on Parts Express:  Enclosed Filter Chokes and Open Bracket Filter Chokes.  I see the chokes are rated for milliamps, should I pick a choke rated for the mA of the draw of the preamp?

[JohnR]

This is the best way to work it out:

http://www.duncanamps.com/psud2/index.html

[wroman214]

Thanks John.  Looking for answers using low voltage drop diodes and L/C/L/C  choke filtering.

[cheap-Jack]

Hi.

  Looking for answers using low voltage drop diodes and L/C/L/C  choke filtering.

Why NEED to pull up the big guns for a Class A phono-preamp?

L/C filter topology is designated for high fluctuating current loads of Class AB or Class B power amps.
For small steady current load of Class A preamps, C/L/C is good enough!

Don't need any L/C filters, my friend. Unless you want to go the 'extra miles'!

FYI, my 1-tube 1-stage stereo phonostage operates at 440VDC. I design/built a sand diode rectifier + 600VDC C/L/C/R filter bank for it, feeding a chip HV regulator.

Full volume with TT spinning, no hum detectable!

c-J

[wroman214]

  Jack, appreciate the reply.  The choke filter simply gives me to option of doing away with electrolytic caps and using a couple of Obligatto 30uF oil caps.   That said  I will probably build THIS unregulated supply first because the work was done and can order the boards from chipamps.com  :   

http://chipamp.com/images/ps-rev3-sch.gif

 The choke filtered supply is a curiosity since it is very rarely done and is said to sound much different.

[cheap-Jack]

Hi.

  Jack, appreciate the reply.  The choke filter simply gives me to option of doing away with electrolytic caps and using a couple of Obligatto 30uF oil caps.    That said  I will probably build THIS unregulated supply first because the work was done and can order the boards from chipamps.com  :   

http://chipamp.com/images/ps-rev3-sch.gif

 The choke filtered supply is a curiosity since it is very rarely done and is said to sound much different.

OK. You're one of those willing to go the "extra miles". Good show!

Yes, for TUBE amps, always go for oil cap as first cap after the rectifier even for C/L filters.

I did so in my super-rebuilt 50-year Dynaco PAS-2 phono-preamp:- 
The tube rectifier feeds a C/L/C/R/C filter bank. I installed a 10uF350VAC oil cap as first cap & the 2nd & 3rd caps both are 120uF330V  hi-speed 'lytic caps. The filter bank then feeds a chip regulator to provide 228VDC O/P HV for the phonostage tubes.

For 24V LV SS phono-preamp, I doubt using oil caps will be so noticeable.

c-J

[wroman214]

So back to intent of the original Question,  will choosing to use an inductor first significantly drop the voltage? 

[cheap-Jack]

Hi.

So back to intent of the original Question,  will choosing to use an inductor first significantly drop the voltage?

I need to know is it a full wave or half wave rectification, DC resistance of the I/P filter choke, current loading of yr amp before I can help you further.

c-J

[Quiet Earth]

So back to intent of the original Question,  will choosing to use an inductor first significantly drop the voltage?

Hi wroman,

I will let the more qualified posters answer your question, but I think the DC resistance of the filter choke will be responsible for the majority of your voltage drop. I guess that means you need to pick a choke. The chokes from Parts Express range anywhere form the upper 50s (in DC ohms) to over 400 ohms. Like c-j says,,, if you pick a choke and explain how much current the preamp draws then you can calculate.

Also, are you going to have a regulated or unregulated 24 volts? Just curious....

[JohnR]

Yes, using an inductor first will significantly drop the voltage. Use the PSUD I linked to...

[wroman214]

 
http://chipamp.com/images/ps-rev3-sch.gif

  This power supply uses a 18 volt transformer but after rectification and caps puts out 25 volts.  Is the voltage gain the result of the rectification or the caps and filters that follow or both. 
  The preamp draws around 80ma.   I would choose to use a choke rated at 100ma 10H and 262 ohms resistance.  Hammond indicates a 10% reduction in voltage using a choke. 
  I did not download the calculator because it was centered around tube rectification.

 

[JohnR]

  I did not download the calculator because it was centered around tube rectification.

It has a number of solid state diodes... from the page:

"Key features

   Rectifier types: Solid state, Vacuum tube types ...
"

You really ought to try it, you can plot the voltage and currents anywhere in the power supply, and this will answer your questions.

[srb]

This power supply uses a 18 volt transformer but after rectification and caps puts out 25 volts.  Is the voltage gain the result of the rectification or the caps and filters that follow or both.

If the rectifier consists of diodes without additional regulation components, the output is primarily the result of the value of the AC peak voltage rectified to DC, or 18V X 1.414 = 25.45V.
 
Steve

[wroman214]

  SRB,    Thanks for the info, exactly what I wanted.

[cheap-Jack]

Hi.

If the rectifier consists of diodes without additional regulation components, the output is primarily the result of the value of the AC peak voltage rectified to DC, or 18V X 1.414 = 25.45V.

  SRB,    Thanks for the info, exactly what I wanted.

This 25.45V is only theoretically true with an I/P cap, neglecting voltage drop across the power iron secondary winding resistance, rectifier diode bias (0.7V) & the DC resistance of the choke.
So the net O/P voltage at the O/P of the filter choke will be much less than 25.45V on full load.

But now you want to use choke I/P instead of cap I/P. There will not be any I/P cap to boost up the O/P voltage from the rectifier diode as show above.

(1) O/P DC voltage of a half wave one diode rectifier fed from 18VAC = V/3.1416= 5.7295V max = 5.7295Vxsq.root 2= 8.10V (no I/P filter cap)

(2) O/P DC voltage of a full-wave 2-diode rectifier fed from 18VAc = 2V/3.1416=11.459Vmax = 11.459Vxsq.root 2 = 13.179V. (no I/P filter cap).

You can see using a choke I/P choke is no good as the 18VAC voltage is way too low to deliver 24VDC O/P.
Since yr 10H choke gets a large DCR of 262R, the voltage drop across the choke with 80mA load will be:
0.08x262=20.9V alone  !!!!!!!!!

I'd strongly suggest you to give up yr ideal of using choke I/P filter for 25VLV unless you want get a higher  voltage power transformer & a lower H (hence lower DCR) choke. The choke you mentioned is for 300V or so HV filter use, not for 24VDC LV application as per the engineering manager of Hammond (whom I know for many years).

FYI, I made the filter chokes used in my tube phono-preamps by using the primary winding of the small power irons scrapped from redundant wallwarts, modified to be used as DC filter chokes. Much much lower DCR. They work.

c-J

[wroman214]

  C-J,
I really appreciate your effort to explain the electrical engineering behind why my idea will not work.  I will stick to my plan to do a regulated power supply with snubber.  Much cheaper anyway.  Thanks all for the input. 
Walt

[wroman214]

 John I will download the application and play around.  Spent some time looking at Paul Hynes Design web sight and he has some unusual takes on using regulator chips in power supplies.