Lundahl Transformers FAQ, Audio transformer and tube amplifier transformer

F.A.Q. November 2010

The primary impedance of most of your transformers are not listed in the data sheets. Why?
The primary impedance of a transfomer in an application, as seen by the source, is the primary no load impedance in parallel with the transformed load.
The "primary no load impedance" for a gapped tube amp transformer is the primary inductance.
The "transformed load" is the secondary load as seen from the primary winding (Load * (turns ratio)*(turns ratio)).
Thus, the transformer by itself does not have an impedance that can be used without a context.
Can I use an LL1671/30mA in a PP configuration?
Yes, you can use our SE transformers in PP applications. The drawback is that due to the DC-accepting core airgap in the SE transformer, transformer inductance is less than for corresponding PP version.
Can I use any of your transformers for converting consumer level -10dB unbalanced signals to professional +4dB balanced signals?
In order to lift signal level from -10 to +4 you need a stepup transformer with turns ratio 1:5, such as LL1935 used 1:5 The consumer equipment output impedance should be reasonable low, and pro audio input impedance must be high, as the transformed output impedance will be 25 times higher than the original source impedance. The transformer should be placed close to the balanced input. If the source impedance is much higher than 600 ohms, the band width will be limited.
Why not parallel LL9206 high impedance windings?
I use the LL9206 as a MC-stepup with turnratio 1:10, and I'm wondering why you don't recommend termination "D". A lower copper resistance should give a better S/N-ratio, or do I miss something?
In a step-up transformer the internal capacitance in high impedance windings are always high, due to the many turns of thin wire. In addition, the signal level across a high impedance winding is high (relatively) due to the step-up. When putting high impedance windings in parallel, the result is a high capacitive load on the primary, and dropping HF response.
This is the reason why we generally recommend NOT to parallel high impedance windings.
I have some old Lundahl transformers and I can not find a data sheet for it. What is it and how can I use it?
Please check this list.
Would it be possible to use the 1620PP in UL mode by connecting the screens to the junctions of terminals [1,6] and [2,5] ? It appears that one should obtain a 50% tap in this manner.
Yes, you can use the LL1620 in UL mode if you connect the screens as you describe.
I am building a mic preamp based on the 990 amp block. The 990 likes to see 600 ohms.I have heard the 1538xl and was wondering if that will work or if you have any other suggestions?
For such a low input impednace would suggesth the LL1570 used 1:2. The LL1538 are ideally used with amplifiers with around 5k optimum source impedance.
What is the difference between LL1570XL & LL1581XL?
The internal structures of the LL1581XL and LL1570XL are quite different. The LL1581XL is a splitting transformer with very high immunity to external noise and noise crosstalk. When used as a splitting transformer, the LL1570XL may pick up electromagnetic hum (from e.g. motrs or power supplies).
One of the LL1674 seems to be off-spec, the secondary static resistance should be about 605 ohm and 3 ot the transformers do indeed measure 600-608 ohm. The fourth one measures 560 and 600 ohm for the two seconday resistances, this would lead to an off-balance surely? Gain matching and CMR would not be so good.
Difference in copper resistance is due to difference in wire gauge. It is possible that we used wire from different runs or from different vendors.
However, the number of turns should be correct. (I say "should be" as shit do happens, even at Lundahls. But if the number of turns was not correct, the difference in copper resistance should have been around 100 ohms for the LL1674 (representing two full layers).)
Which problem will arise?
The difference in DC resistance of the transformer should be compared to the impedance of the load. If the load impedance is 20k + 20k, (reasonable or even low load for the LL1674), the 40 ohms difference of the transformer is in the magnitude of 1/1000 of the load impedance. In my opinion, this is ignorable.
Besides, is the core degaussed after measuring DC resistance with a DC current? This is probably a bigger problem.
I was measuring the Inductance of the primary coil of the LL1649. It measures around 2-3 H with the primary coil in series. In general what is the optimum required H value for the primary.
Why do you want to measure the inductance of a mains transformer? The interesting parameter for a mains transformer is the no-load current. For our mains transformers LL1648 -- 51 the no load current is less than 100mA, indicating a no load impedance above 2kohms at 50 Hz. (corresponding to 6H) The no-load current should be compared to the full-load current, which for a 250VA transformer is around 1A. A 1:10 factor between no-load current and full load current is in my opinion quite acceptable.

Problems with LL1660 freq. response.

> > I've built an amp with 2A3 and LL1660. I like the sound of it,
> > but was a little frustrated when I measured the frequency response
> > today. Around 9.5KHz I get a climbing response
> > that doesn't go away until I've reached the end of the bandwidth of the
> > amp. With a few measurements, I located the problem to be the 1660. It's
> > wired in the ALT-T hookup, to allow 20mA current through the 6H30.
> > 
> > Is this expected? If not (and also if so, I suppose :-), what can I do
> > about this deviation?
> 
> Hi Per,
> 
> Seems I already got this explained to me by an audioasylum inmate, it's
> a impedance problem. So I've got something to work with now, sorry for
> wasting your time.

In case you want to know, 24K resistance from the grid of the 2A3 to
ground made the frequency response curve perfectly flat. I can't wait to
listen to it and see how this changed the sonics of the amp.

BTW, thanks a lot for making quality products for tube amps. I already
absolutely love the sound of my new amp.
-- 
Jens Axboe

Per Lundahl,
Lundahl Transformers AB
Tibeliusgatan 7
SE-761 50 Norrtälje
SWEDEN
Phone: +46 - 176 139 30
Fax: +46 - 176 139 35
Email: