Manufacturer of  Ultra High Quality Audio Transformers
		Introduction (the Part number below, is the same as in the price list
		Tube Power Supply Chokes)	Part14..20		Tube Output
Part 2		Filament Chokes / High Current Chokes	Part 21		Tube Mains transformers with DC cancellation
Part 3		Plate (anode)  Chokes	Part 22		Mains Cleaning transformers with DC cancellation
Part 4		Grid Choke	Part 23		Various transformers, not mentioned here
Part 5		Interstage
Part 6		Signal Input
Part 7		Signal Output
Part 9		Moving Coil
Part 10		Guitar Input
Part 11		Microphone	Tech Info		Product Storage
Part 12		For Digital Audio. Finest Quality.	Tech Info		Amorph Cores
Part 13		Interconnect Solutions	Tech Info		DATASHEET DOWNLOAD

Choosing a Lundahl Power Supply Choke

All Lundahl products are based on their double coil (symmetrical) C-Core technology. Also the chokes. This means you have always two coils available. These you can wire in three ways:

NOTE: for For small power supplies with very low hum requirements, such as in pre-amps, you can use plate chokes like LL1667/25 as power supply chokes, they are qualified for this use as well. In the table (below) you see some of the plate chokes listed. You will get extremely high inductance that way. This allow you to work with less big capacitors, and in that way reduce capacitor charge peaks which are responsible for hum radiation. The relatively high DC resistance will however give not much voltage drop, since you work are low current. Even the DC resistance is good, since it is an additional stabilizer element, and also it is helpfully at start up when capacitors are still empty. Then again with higher current versions such as 50mA and more, you will see the DC resistance gets lower. All together the below table shows a very harmonic family of products.

 In the double coil configuration, one coil is in the positive lead to the second capacitor, and one on the ground lead to the second capacitor.  This will give lowest field radiation coming from the choke.  Such good results can not be achieved with lower cost E-Core chokes, and the Lundahl double coil chokes are a major step into the direction of a really hum-free amplifier.   The data sheets gives a good diagram of how  to connect a double coil choke, using it's full features and get best noise rejection from it. (check for instance here, all at the bottom of the last page). Of course, it is still possible to use them as a single coil also. 

Overview of types

1. The BIG misunderstanding from people that made low voltage power supplies before. Here, using BIG capacitors,  like 10.000...50.000uF will improve the result, because with Low Voltage designs,  the energy storage is only in the primary capacitor.    Not so with tube rectifiers!  These are high impedance, and  the energy storage is mainly in the choke.  With tube rectification,  you need to take the a small primary capacitor. (see also the next note).   There  is a maximum value for that in the tube data sheet.  It is for instance 4uF for the 274A. The maximum is the highest value, and you should feel free to take a lower value also.  The  higher the  value, the larger the charge current through the primary capacitor.  The wires will radiate an AC magnetic field, and this gives hum into the pre-amp. (and you'll never find where this hum is coming from. The path is through air....)
2. Some applications leave the primary capacitor completely away, and use only the energy storage in the choke.  This will improve the efficiency of the power supply, and gives more current.  This is a  method, not recommended with low cost chokes,  because it will  make the plate choke hum (mechanically). With the LUNDAHL chokes this is possible, and no hum will result from it.
3. Push pull amplifiers are very insensitive to power supply hum.  They can work completely without hum, even with 30Volt AC on the DC voltage, but.... only as long as the rest of the construction is well done.
4. SE amplifiers are very sensitive, but  if there are any hum problems it is normally not the power supply.  Improving the power supply first capacitors can will help,  because it reduces all AC voltages in the whole amplifier. Still you are curing the problem at the wrong place, and never get a full cure because of that.  More often it is bad wiring, or some other mistakes somewhere. The recommended capacitor values in the circuit diagrams are normally more than enough.
5. With Mercury rectifiers  people often say they "hum", or sound in some way "unquiet".  That is true,  but it comes from bad designs. Good designs do not hum at all.  These rectifiers have an "ignition" before they get in the conductive state. Because of that,  they have a lower internal impedance, and can work with surprisingly large primary capacitors.  But... this is the industry method, and not what we want for HiFi.  Again.... what these rectifiers show in their data sheets are maximum capacitor values for industry applications, and  no "must use"  values for HiFi!   Take the same low primary capacitor value as for non-mercury rectifiers, and you will have no hum.
6. Here is the final conclusion:  If you want to oversize the power supply,  the best effect comes from:

a) The CHOKE
b) The SECONDARY Capacitor
c) there is no c;)

---

Subcribe or unsubscribe our Mailing List
 Copyright © JAC Music. All Rights Reserved.