Hi, I have made a simple impedance matching circuit for changing high Z audio signals to low Z (DI box):
http://i985.photobucket.com/albums/ae333/MrShhh_2009/DRV134DIbox.jpg
It uses an OPA134 unity gain buffer stage driving a DRV134 balanced line driver.
All op-amp power supplies are bypassed with 4.7uF tantalum and 100nF ceramic caps and the power supply is bypassed with 100uF electrolytic & 100nF ceramic caps where it enters the pcb (not shown).
When I input various sine wave test signals at -6dB (0.388 Vrms), from a Zo=50ohm source, the signals at the DRV134 outputs (Vo+ and Vo-) are way out of balance at frequencies below 10k. They go out again above 15k but the low end is worse:
.........(mV rms)
Hz.....Vo+...Vo-
250....404....430
1k......366....430
5k......379....398
10k....391....391
15k....391....391
17.5k..404...398
20k.....391...385The figures were collected with the outputs unloaded. When I connect the outputs to my active monitors (Zin = 20kohm) the above figures get way, way worse. It doesn't matter what input signal strength I use, if I double it to 0dB for instance, the imbalance doubles.
Is this normal behavior for the DRV134?
The worst case quoted signal balance ratio for the DRV134 is 35dB @ 1kHz using the test rig on page 10 of the following datsheet:
http://www.datasheetarchive.com/dow...om/pdf-datasheets/Datasheets-7/DSA-125103.pdf
I have tried to the following mods with zero change in the imbalance:
1. Direct couple the OPA134 to the DRV134 (remove C2)
2. Remove the 10uF non-polar caps at the sense/output pins
3. Use bigger caps at the sense/output pins
4. Change the OPA134 to NE5534, TL071 etc.
Is there a way to improve the situation? (Other than sending the output of the DRV134 to an audio transformer to re-balance it).
Cheers, Mark
http://i985.photobucket.com/albums/ae333/MrShhh_2009/DRV134DIbox.jpg
It uses an OPA134 unity gain buffer stage driving a DRV134 balanced line driver.
All op-amp power supplies are bypassed with 4.7uF tantalum and 100nF ceramic caps and the power supply is bypassed with 100uF electrolytic & 100nF ceramic caps where it enters the pcb (not shown).
When I input various sine wave test signals at -6dB (0.388 Vrms), from a Zo=50ohm source, the signals at the DRV134 outputs (Vo+ and Vo-) are way out of balance at frequencies below 10k. They go out again above 15k but the low end is worse:
250....404....430
1k......366....430
5k......379....398
10k....391....391
15k....391....391
17.5k..404...398
20k.....391...385
Is this normal behavior for the DRV134?
The worst case quoted signal balance ratio for the DRV134 is 35dB @ 1kHz using the test rig on page 10 of the following datsheet:
http://www.datasheetarchive.com/dow...om/pdf-datasheets/Datasheets-7/DSA-125103.pdf
I have tried to the following mods with zero change in the imbalance:
1. Direct couple the OPA134 to the DRV134 (remove C2)
2. Remove the 10uF non-polar caps at the sense/output pins
3. Use bigger caps at the sense/output pins
4. Change the OPA134 to NE5534, TL071 etc.
Is there a way to improve the situation? (Other than sending the output of the DRV134 to an audio transformer to re-balance it).
Cheers, Mark