Jeff Rowland now use the Lundahl transformer in their new amplifiers (Jensen were used in their older amplifiers). There is an article at Jeff Rowland's web site discusses the advantage of using high quality audio transformer in Hi-End audio equipments. The advantages of using input transformer are :
- Ground loops breaking
- Ultra high common mode rejection ratio
- Provide prefect balanced signal output
- Bandwidth limiting (block RFI and DC)
However transformer also has some disadvantage :
- Frequency roll off when used with high impedance source and low impedance load
- Higher THD% at low frequency
I obtained 2 Lundahl 1545a transformers and some measurements are done. I used the E-MU 1212m PCI sound card as a measurement tool because this sound card has low distortion and low noise floor. The 1212m sound card has a output impedance of 560 ohm and a input impedance of 10k ohm. RMAA 5.5 is used for testing.
I ran the following tests :
- Frequency Response
- THD at 50Hz and 1kHz
- 1545a setup as 1:1 and 2:1 turn ratio
- Signal level at 0 dBu, +7 dBu, and +20 dBu
- Source / Load impedance at 560 ohm / 10k and 3k / 5k
Assume the amplifier gain is 20 and speaker is 4 ohm, a 0 dBu signal would generate a 60W output. At 7 dBu, 300W is generated.
Test results -
Frequency response at 7 dBu signal level :
- The white line is the sound card external loopback.
- The green line is the LL1545a (1:1 ratio) with source impedance at 560 ohm and load at 10k ohm.
- The cyan line is the LL1545a (1:1 ratio) with source impedance at 3.2k ohm and load at 5k ohm.
- The purple line is the LL1545a (2:1 ratio) with source impedance at 3.2k ohm and load at 5k ohm.
Below is the frequency response test with different source / load impedance combinations, LL1545a setup as 1:1 turn ratio, signal level at 7dBu.
- White line - Source impedance 2.5k ohm, Load 5k ohm
- Green line - Source impedance 2.5k ohm, Load 10k ohm
- Cyan line - Source impedance 560 ohm, Load 5k ohm
- Purple line - Source impedance 560 ohm, Load 10k ohm
In this test we can see that the source impedance has quite some impact in the frequency response. Conclusion : The LL1545a has a very flat frequency response if matched with low source impedance. High frequency roll off is apparent when matched with high source impedance.
THD measurement - since there are many combinations of test cases, the below table gives a summary of the THD performance :
Below is the 7dBu sound card loopback THD result, you can see that the sound card has low noise floor and low distortion:
Below is the 7dBu LL1545a (1:1 turn ratio) with source impedance at 560 ohm and load at 10k ohm.
The THD result at 1kHz is excellent at 0.0009%, while the THD performance at 50Hz is still acceptable at 0.019%.
As you can see from the THD table, if the LL1545a is setup as 2:1 turn ratio, the THD performance would be degraded.