General properties:
Driving circuitry for line output transformers should have low output impedance. In order to reach the very best performance in terms of low distortion and low line output impedance, we recommend that our output transformers are used with
mixed feedback drive circuitry
for reduction of output impedance and for best THD performance.
In the book "OP AMP APPLICATIONS" (Analog Devices, edited by Walt Jung)
the use of (op amps and) audio transformers in line input and output applications are discussed in depth.
(Book review)
In particular the mixed feedback drive circuits for line output transformers are analyzed in great detail. I strongly recommend the book for anyone who is designing transformer line output applications.
The book is available on-line at www.analog.com/library/analogDialogue/archives/39-05/op_amp_applications_handbook.html
and the mixed feedback cuircitry is discussed at
Chapter 6 (3.2Mb), page 6.71 and forward.
Sectioning of transformers are discussed in a separate paper.
Three-section output transformers with internal electrostatic shields.
Can be used with balanced or unbalanced drive circuits.
| Type | Turns Ratio | Size (LxWxH mm) | Max Prim. Level @ 30 Hz | Drive type |
|---|---|---|---|---|
| LL1517 | 1+1,ct : 1+1 | 47x34x19 | +24 dBU | Universal output. |
| LL1585 | 1+1 : 1+1 | 47x34x21 | +30 dBU | High level universal output transformer. |
Three section output transformers without internal electrostatic shileds.
To minimize capacitive coupling, the secondary windings are placed between the cold ends of
the primaries.
For balanced and unbalanced drive.
| Type | Turns Ratio | Size (LxWxH mm) | Max Prim. Level @ 30 Hz | Drive type |
|---|---|---|---|---|
| LL1539 | 2 : 1+1 | 47x34x21 | +27 dBU | For balanced drive |
| LL1555 | 1+1+1+1 : 1+1 | 33x26x20 | +27 dBU | For balanced drive. Small size. |
| LL5402 | 2 : 1+1 | 43x28x21 | +18 dBU | For unbalanced drive |
Five section output transformers.
The high sectioning results in low leakage inductance and thus a very high frequency range.
| Type | Turns Ratio | Size (LxWxH mm) | Max Prim. Level @ 30 Hz | Drive type |
|---|---|---|---|---|
| LL1524 | 1+1 : 1+1 | 48x34x22 | +24 dBU | Balanced drive |
| LL7401 | 1+1 : 1+1 | 47x34x17 | +20 dBU | Balanced drive |
Special type output transformers.
| Type | Turns Ratio | Size (LxWxH mm) | Max Secondary Level @ 30 Hz | Drive type | Comment |
|---|---|---|---|---|---|
| LL1560 | 2+2 : 1+1+1+1 | 47x34x23 | +20 dBU (each sec.) | Balanced drive | Four independent outputs active split |
| LL2811 | 1+1 : 1+1 | 31x26x23 | +24 dBU | Balanced drive | Four section windings. Small footprint. Fits three in a row across a Euroboard |
| LL1582 | 1+1 : 1+1 | 31x26x23 | +24 dBU | Balanced or unbalanced drive | Pin compatible with LL2811, but with internal electrostatic shields. |
| LL2734 | 1+1 : 1.4+1.4 + 0.3+0.3 | 66 x 39 x 53 | +25 dBU | Single end class A, 100mA DC | For solid state SE output circuits, based on LO1166A |
Per Lundahl, email: web_master/AT/lundahl/DOT/se
Lundahl Transformers AB
Tibeliusgatan 7
SE-761 50 Norrtälje
SWEDEN
Phone: +46 - 176 139 30
Fax: +46 - 176 139 35
Email: office/AT/lundahl/DOT/se