Dual Impedance switching with LUNDAHL Tube Output Transformers.

Electron Engine ™
Printed Circuit Boards by Emissionlabs

First published: 05/23/2019 11:17. Update: Last Update:

Output Boards, for LL1620, LL1623, LL1627, LL1679, LL9202

We have copyright on this design.

Introduction

The technical background items are mostly moved to the "Notes" all at the bottom of this page. Yet this small piece of "tech talk" I want to put up front here. Every transformer is always a compromise between dimension, complexity, specification, and price. The ideal transformer is very small, easy to use, works from 5 to 100kHz, and cost almost nothing. The price indicates for a large part, what you can expect from it. There have always been cheap ones and expensive ones, and that will never change.

Tapped windings of not?

A tapped transformer is a very much compromised product by definition. We get this question so often, why don't Lundahl make those, as it seems so nice. Well, yes indeed, that is only what it seems. Please read the notes about this, explaining the difference, and why Lundahl does not want to build such.

How these PCBs are used.

First of all, there is not need any more, to understand these "Alt" variations in the data sheet. So do not ask yourself, or us, "how to make Alt-C...". Just look in the table, under the desired input- and output impedance, there you will find the transformer of your need, choose the PCB, and you are already finished.

These printed circuit boards will allow to change between two output impedances with a switch, or get one single impedance by soldering three small wire pieces at the position of the switch. The Impedance switching can be done on the transformer board itself, or by a small remote board, that we supply with it. Alternatively you can use a switch of yourself, but it has to be a very low impedance switch, such as 10Ampere type, or more.

The loudspeaker can be connected directly to the transformer board, or to the small external board. Since the external switch board is probably going to be located near the loudspeaker terminals, the speaker can be connected to the switch board with short wires. But there are also speaker connections on the transformer board.

These boards work on all Versions, SE, PP, CFB, PPZ.

SE	Single Ended
PP	Push-Pull
CFB	cathode feedback for PP amplifiers
PPZ	Parafeed. Combines the best of SE with PP

EE16 Transformer Board + EE17 Switch Board + Switch. Order nr: 310-022-29

Low-Medium impedance board

EE18 Transformer Board + EE19 Switch Board + Switch. Order nr: 310-024-89

Medium-High impedance board

EE32 Transformer Board + EE17 Switch Board + Switch. Order nr. 310-032-00

Board for LL9202 only.

This is a Lundahl Approved Product

"Hello Jac, It took some time but now I have deciphered your EE16 + EE17 boards. Very pretty! And yes, you can write that this board set is approved by me".

Per Lundahl, by Email, Oct 7th, 2022

Part #1. Switching the OUTPUT Impedance.

This is the normal way to use it. You can choose a fixed Output impedance, OR if you like, change between two output impedances.

These Boards allows following TWO Output Impedances:

Line	Trans-former	Input Impedance	Output Power SE	Output Power PP	Set the switch to 'Low' Position	Set the switch to 'High' Position	Board + Switch Combination
1	LL1627	650Ω	50W	100W*	4Ω	8Ω	EE18-EE19
2	LL1627	1k2	25W	100W*	4Ω	8Ω	EE16-EE17
3	LL1627	1k2	25W	100W*	8Ω	16Ω	EE18-EE19
4	LL1623	1k6	50W	100W*	4Ω	8Ω	EE18-EE19
5	LL1627	2k3	13W	62W	8Ω	16Ω	EE16-EE17
6	LL1623	3k	25W	100W*	4Ω	8Ω	EE16-EE17
7	LL1623	3k	25W	100W*	8Ω	16Ω	EE18-EE19
8	LL1620	3k3	50W	100W*	4Ω	8Ω	EE18-EE19
9	LL1679	4k5	20W	100W*	4Ω	8Ω	EE32-EE17
10	LL1623	5k6	13W	62W	8Ω	16Ω	EE16-EE17
11	LL1620	6k	25W	100W*	4Ω	8Ω	EE16-EE17
12	LL1620	6k	25W	100W*	8Ω	16Ω	EE18-EE19
13	LL9202	6k5	50W	100W*	4Ω	8Ω	EE18-EE19
14	LL1679	9k7	9W	45W	8Ω	16Ω	EE32-EE17
15	LL9202	11k	25W	100W*	4Ω	8Ω	EE16-EE17
16	LL9202	11k	25W	100W*	8Ω	16Ω	EE18-EE19
17	LL1620	11k5	13W	62W	8Ω	16Ω	EE16-EE17
18	LL9202	23k	13W	62W	8Ω	16Ω	EE16-EE17
* Output power is limited at 100 Watt at 4Ω. Switch may only be used with the amplifier off.

Part #2. Switching the OUTPUT Impedance. OPTIONAL use.

Though this is not the normal way to use it, we still encourage you to try it. You loose nothing, but you may achieve something interesting. Explained below here:

A difficult part of the amplifier design, is find the right primary impedance. So just assume, you will always use 8Ohms speaker, and the primary needs 3k3. So no need for switching? Well... think about this: The damping factor of the bass chassis, is roughly doubled when the primary of transformer is increased to 6k. You may like that sound a lot more, but you lose some volume. This means when you really need the volume, stay with 3k3. Or when you want to have better quality lower volume, use 6k. In addition, this makes you loose 3dB hum and noise. (Yes!). So why not make it switchable? Look in the table below, and it is line 34, for this example. In this example, this requires board EE18, with switching board EE19 attached to it.

The intention of the table below, is that you don't need to calculate this, and don't need to understand the way of achieving this. Just use the right PCB and you are done. (If you insist on a head-ache,read the notes on it).

Line	Trans-former	Speaker	Set the switch to 'Low' Position	Set the switch to 'High' Position	Board + Switch Combination
4 Ohms use
19	LL1627	4Ω	650Ω	325Ω	EE18-EE19
20	LL1627	4Ω	1k2	650Ω	EE16-EE17
21	LL1623	4Ω	1k6	800Ω	EE18-EE19
22	LL1623	4Ω	3k	1k6	EE16-EE17
23	LL1620	4Ω	3k3	1k6	EE18-EE19
24	LL1620	4Ω	6k	3k3	EE16-EE17
25	LL1679	4Ω	4k5	2k6	EE32-EE17
26	LL9202	4Ω	6k5	3k3	EE18-EE19
27	LL9202	4Ω	11k	5k5	EE16-EE17
8 Ohms use
28	LL1627	8Ω	1k2	650Ω	EE18-EE19
29	LL1627	8Ω	2k4	1k2	EE16-EE17
30	LL1623	8Ω	5k6	3k	EE16-EE17
31	LL1623	8Ω	3k	1k6	EE18-EE19
32	LL1627	8Ω	2k3	1k2	EE16-EE17
33	LL1627	8Ω	1k2	650Ω	EE18-EE19
34	LL1620	8Ω	6k	3k3	EE18-EE19
35	LL1620	8Ω	11k5	6k	EE16-EE17
36	LL1679	8Ω	9k7	4k5	EE32-EE17
37	LL9202	8Ω	11k	6k5	EE18-EE19
38	LL9202	8Ω	23k	11k	EE16-EE17
39
16 Ohms use
40	LL1627	16Ω	2k3	1k2	EE18-EE19
41	LL1627	16Ω	4k6	2k3	EE16-EE17
42	LL1620	16Ω	11k5	6k	EE18-EE19
43	LL1620	16Ω	23k	11k	EE16-EE17
44	LL1623	16Ω	5k6	3k	EE18-EE19
45	LL1623	16Ω	11k2	5k6	EE16-EE17
46	LL1679	16Ω	19k	9k7	EE32-EE17
47	LL9202	16Ω	23k	11k	EE18-EE19

Part #3. How to use (or not use) the switching board.

SELECT ONE OF THESE WAYS TO SWITCH IMPEDANCE:

1) Transformer Board, use wire pieces.

This is Output Board used the simplest way, just two input wires and two output wires.
Still the impedance may be changed later. RIght now, the board is set for the "High" impedance.
The switch board is not connected, but may be connected later, if the wire pieces are removed.

2) No external switch board is used.

This is when you want to switch impedance, but do not want an external switch for this.
The switch can be mounted directly on the transformer board.
In this case, the external switch board can not be used any more, because you can not remove the switch any more from the EE16 board. So think about this before choosing this option.

3) External switch board is used.

There may be no switch or wire pieces mounted at the switch position of the transformer board.
The switch board is connected, the switch can be mounted on both sides. Just chose the most convenient way for yourself.
The speaker itself may be connected to the switch board, or if you choose, also to the transformer board.
Connecting the speaker to switch board, makes a lot of sense, since the external switch is somewhere near the speaker posts anyway. That means, speaker wires are shortest that way.
The Switch board itself needs only a 6mm hole in the chassis, and the small PCB is attached via the switch itself. Wires and / or the switch, can be attached from either side, whatever you like better.
The blue connector block is just a standard type, but not supplied. It is not needed to use such a connector block, the speaker wires can o just be soldered into the connections * and Gnd

EE32 board for LL1679. Order Number: 310-032-00

The special lay out of the LL1679 transformer required a dedicated board.

This board can be used stand alone, or with the optional switch board EE17.
If used with the switch board EE17, the speaker can be connected to the switch board, or to the EE32 board.
On the EE32 Board the speaker can be soldered with wires, using the connections * and Gnd. Or, use a PCB Screw Terminal (not supplied with the board).
This board is tin-plated.
Low impedance: Bridges E + J +O.
High Impedance: Bridges B + D + N.
Use with External Impedance Switch Board EE17: Connect EE17 to the terminals 1..6 and leave all Solder Bridges open.

DIMENSIONS for all Boards:

Switch Board: 40x 43mm.
Transformer Board: 92 x 66mm

SALES: We have copyright on this design.

Dual Output Impedance Solution

With Lundahl output transformers, people do not like to wire those by hand. No matter how good you can solder, it looks often not nice in the end. Another issue is wiring errors. Some errors will cause a malfunction, which you will notice. This does not mean yet, you found the error. Other wiring errors can be unspecific, and the transformer still works, but performance is somehow below expectation. This is very hard to find out. I can tell, because I had that myself.

So, with PCB's the wiring is less, and mistakes are not possible. Most of all it enables to switch between two output impedances. Please note, it may seem, you know the ideal impedance already, but I always recommend my customers to try out the next lower impedance too. Though this stays often theory, because people avoid the re-wiring of course. With the switched solution, it is not theory any more.

A very useful application is also when the speaker system is extremely high efficiency, like it can easily happen with the AVANTGARDE Systems, or similar horn drivers. Read also this article. The amplifier has in fact too much output power, and the remaining hum will become too loud. In that case, switching to the next lower impedance will reduce the hum by 50%. Just try it out, and you will see!

Mounting of the transformer board.

Do NOT rotate the board, upside down by mistake. Though it seems to work, transformer windings become connected not ideal. This may give less good frequency range. So there is this white U-Shape marking on the board. When this is oriented the same as the transformer, the board is mounted correct.
This little board of 4 x 4 cm only, is mounted just by the switch itself. To optimize the wiring inside the amplifier mechanically, the switch on the EE17 board may be mounted from either side.
Lundahl Housings: The transformer board will fit nicely inside those LUNDAHL covers

Notes

It is not necessary to read all of this. But it does reflect a little bit the things I learned and found out while using those transformers. Perhaps you find some parts of this text useful.

Speaker Output transformers are always inter sectioned. So there is a primary layer, a secondary layer, a primary layer, etc. This will improve the high frequency performance. The disadvantage of this is, any isolation failure between primary and secondary, would lead to the DC high voltage on the output circuit. To prevent this, the "cold" side of the speaker output (the "Black" terminal) must always be connected to the chassis, and the chassis to safety ground. This is the connection NOT marked with a star (*) symbol. The "hot" side is marked with a star (*) symbol, and is connected to the "Red" speaker terminal. The switch should be grounded as well, so mounted at a metal part of the chassis.

For safety, the speaker connector (BLACK TERMINAL) must always be grounded to the chassis.

Tapped windings or not?

Tapped windings, are easier to use, we all know that. But for constructing a good transformer, that is not the best. Reason is simply, for most configurations, a part of the winding stays unused. This unused winding is in the way. It obstructs the magnetic path for the used windings, and it prevents good functioning of the "layering". So where is the unused winding, the layering can not do it's good work. Also this forces the remaining part of the winding to be made with thinner wire, because space inside the transformer is limited. Such transformers are simply not really the best, and you won't find them with Lundahl. We are aware DIY require such tapped transformers, but that is because the reasoning behind it, is not really known, or not understood at all. The result of a tapped transformer is ALWAYS, these work as specified if both primary and secondary are used at maximum impedance. But however, the further down you tap them, the more frequency roll off they will give, and the higher the power loss due to copper resistance and no good use of the magnetic path. Needless to say, this problem is carefully hidden in their data sheets, so we just talk about it here :) . If the problem is not hidden, they must give frequency roll off data, and power loss data, depending on the tap configuration. If that is missing, well this is why. For SE transformers, the loss of Inductance is considerable, simply because not all windings are used at a tap. This problem is hidden by specifying the transformers at 30Hz low frequency, and using 3dB for that. Meaning, you loose 50% of the signal at 30Hz. Judge for yourself if that is no problem, but please read data sheets carefully when it comes to roll off, and power loss. So transformer efficiency is something they should not "forget" to mention. It is not fun to pay 1000's of Euro for a top class project, and then loose 10% of the output power in the transformer.

So by tapping, efficiency, inductance, and frequency roll off, it all suffers. This is why Lundahl company is not doing so. The lower impedance is created by putting the output winding layers in parallel, making copper resistance low, which is something needed, if working at 4 Ohms only. The same windings are put in series to get 16 Ohms. This will raise copper resistance, but at 16Ohms, that would give the same efficiency still. At 8 Ohms it is a combination of serial and parallel.

The above explains the advantage, and it is why specs of Lundahl transformers are better. The disadvantage is having to wire this by hand. First, you need to do this, and second you can make a mistake with it. The idea to make a switchable circuit for this, was actually mine, but I got stuck with the complexity. After bringing this up to Per Lundahl, he came up with a very clever circuit for this. It works with Jumpers, connected quite difficult. Yet by rearranging the schematic, I found a way to replace these jumpers by a 3-pole switch. Technically it works the same.

Moreover, but that is too difficult to explain in detail, tapping increases the susceptibility of the transformer to external magnetic fields, such as radiated by the mains transformer, and audible hum can be caused by this.

Output Impedance vs input Impedance - Notes for table #2.

Sometimes people think a transformer has a defined input impedance. This is not so! If you take a transformer, wipe off the product number and give it to an expert, he will not be able to tell you the input impedance. That is because input impedance depends on the load impedance you connect to the output. When you say, ok it is 8 Ohms. Then he can tell you. When you say it is 4 Ohms, he will give you a totally different number. On top of that, most speaker have not the impedance as written on it. If they write 8Ohms on it, it can be 8Ohms indeed, but it may as well be 6 or 12 Ohms. Frankly, it's a big mess. Even so, loudspeaker impedance is not constant over the frequency range, and varies wildly (upwards) in the low frequency range. Read more about it here....

Damping Factor. The next good thing, which happens, the amplifier has such a thing as damping factor with the bass loudspeaker, and this gets improved. The less it is damped, the louder the whole speaker will be, but the more muddy the bass sound will be also. This is always a compromise between what the amplifier can do, and what it should do. Damping factor is defined as REAL load (for instance 8 Ohms) divided by REAL output impedance (for instance 1,6Ohms, and then damping factor is 5. When connected the EE16 board with a 3k transformer to an 8 Ohms speaker, and then switch the board to 4 Ohms, this DOUBLES the damping factor. You can try it, and it will improve the bass quality (faster bass), and reduce the amplifier microphonics and hum. This will be on the cost of maximum volume, so if maximum volume is more than you need anyway, this may be a sound you even prefer.

The required output impedance

This should fit to your speakers. Just not always 8 Ohms speakers work best at an 8Ohms Output. Most of the time yes, but just not always. Frankly many speaker builders compromise on the output impedance, to be able to use the chassis they want, or put some chassis in series or parallel. There is really very much compromising done here. So an "8 Ohms" speaker is really only 8 Ohms in theory. Add to this the impedance in the low end is seldom as specified. A typical issue is also when the amplifier is too loud for the speakers. This will make hum and noise louder too, and you may not like that. In such a case, switching the output to 4 Ohms, will not compromise sound quality, but it will reduce noise by half. So I always recommend, to find out by your own hearing tests, what sounds best. And no, that is not always connecting the speaker to the specified output impedance. Here comes the point, with a hand-wired configurations. Suppose you have 16 Ohms speakers, and you wire the transformer for 16 Ohms, and it sounds good. What will you do? You will say you are finished. Nobody is motivated to take it apart, only to try another impedance, which is supposedly wrong anyway. This is however such a nice feature, to be able to try this out, and it would be a pity not to do so.

Transformer pin arrangement of these PCBs.

Sometimes, users want to verify if these PCBs have the same connections as the Lundahl data sheets write. A first test seems to confirm, we made PCB errors, and we are asked to comment on these findings. We build these PCBs since a few years now, and of there were errors, we would know it of course. The explanation is another. These transformers are symmetric constructed for a good part, and this allows us to inter change specific coils. We have been doing so to improve the PCB lay out. Meaning, resulting tracks are shorter, and the lay out becomes better quality. It is not possible to swap random coils, but as long as inter sectioning is kept intact, a swap is possible. Just as an example: for LL1620, 'Alt-D' you will see in the data sheet pin 11 and 12 are connected. This puts those two coils in series, and the resulting coil is found on pin 19 and 20. But you may as well connect 19 to 20, and then the resulting coil is found on 11 and 12, which gives exactly the same signal and same sectioning. If that resulting coil, for best PCB lay out, would be needed close to 11 and 12, then we swapped the coils, of course. This is why some of the connections are as in the data sheet, and some are not.