Lundahl Transformers

Tibeliusgatan 7, 761 50 Norrtälje

Phone: +46 176 139 30

Fax: +46 176 139 35

November 2016

I have bought some LL1673/10H from you, but the measured inductance (with my inductance meter) is only 6H. What is wrong?

November 1st, 2016|

Our power supply chokes are designed to work at 100/120Hz and with voltages around 100V. Most induction meters work at a few volts or less, at around 1kHz. This is not the correct operating point for a power supply choke, and the reading on the induction meter is usually around half the specified value of [...]

November 2015

Is there a CAD library for Lundahl Transformers products?

November 19th, 2015|

EAGLE CAD Lundahl library (created by Bob Starr of RTZ Audio): CADSoft EAGLE PCB Design Software is a CAD software available for a number of different platforms. A limited function version is available free of charge for personal use. Please see for downloads.

October 2015

What could be the possible disadvantage to use LL1540 (line input transformer) as output transformer?

October 23rd, 2015|

An audio line is a cable transferring an audio signal from the output of one device to the input of another. Normally the input impedance is fairly high, >1k for mic preamps and >10k for line inputs. If the impedances in both ends of an audio line are high, any noise current picked up by [...]

June 2015

LL1660s Data Sheet Confusion. How come an LL1660S/10mA is used at 18mA?

June 22nd, 2015|

I guess you are right. It _IS_ confusing. If we assume a certain DC current, there are two ways to reach a desired core operating point: 1. With a given coil, adjust the core air-gap or 2. With a given core air-gap, adjust the number of turns on the coil The type number LL1660S/18mA is [...]

May 2015

Problems with LL1660 freq. response.

May 28th, 2015|

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 [...]

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?

May 28th, 2015|

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 [...]

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?

May 28th, 2015|

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 [...]

What is the difference between LL1570XL & LL1581XL?

May 28th, 2015|

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).

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?

May 28th, 2015|

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.

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] ?

May 28th, 2015|

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.