Hello everybody,
I'm now working on another power amplifier and I want it to have balanced inputs along with unbalanced ones. I did several balanced input preamps and amps with the "classic" one op-amp and three op-amps designs but I was looking for a higher performance solution (if there is one). After reading several articles and the chapters dedicated to this subject from D. Self's books, I decided to try and build the schematic below. From several sources on the net, this seems to be a very low noise solution. I do not need level control but I would implement a gain switch to be able to switch from high to low gain and I will add an unbalanced input as well (from the IN+ and GND connections).
If any of you have experience with these and maybe built something like this, I would like to hear your input. I'm open to other ideas as well. The only real downside I see on the schematic below, is that it uses 4x opamps per channel. For me, this is not a real concern as the LM4562 opamp that I use extensively is not that expensive and its performance is very good (ignore the LME49720 MPN from the schematic). From D. Self's book, there is a lower noise solution that uses 6x LM4562 but the noise difference from the schematic below is not that big (2dBu) and the one below is more tan enough for what I need.
I'm now working on another power amplifier and I want it to have balanced inputs along with unbalanced ones. I did several balanced input preamps and amps with the "classic" one op-amp and three op-amps designs but I was looking for a higher performance solution (if there is one). After reading several articles and the chapters dedicated to this subject from D. Self's books, I decided to try and build the schematic below. From several sources on the net, this seems to be a very low noise solution. I do not need level control but I would implement a gain switch to be able to switch from high to low gain and I will add an unbalanced input as well (from the IN+ and GND connections).
If any of you have experience with these and maybe built something like this, I would like to hear your input. I'm open to other ideas as well. The only real downside I see on the schematic below, is that it uses 4x opamps per channel. For me, this is not a real concern as the LM4562 opamp that I use extensively is not that expensive and its performance is very good (ignore the LME49720 MPN from the schematic). From D. Self's book, there is a lower noise solution that uses 6x LM4562 but the noise difference from the schematic below is not that big (2dBu) and the one below is more tan enough for what I need.
After some further reading and simulations, I came to the conclusion that there is a simpler way to do all the things I need with the circuit below. This is taken from the preamp I built a while ago (based on the D. Self precision preamplifier) and it is the input section of the said pre-amp. In the pre-amp, I use a pot to set the balance between the L and R channels. For the purpose of the balanced to unbalanced interface, the pot will be replaced by a relay and two resistors. The relay will switch between the two gain settings and the resistors will allow me to exactly set the two gain settings. Without the resistors, the gain can be adjusted from -3.3dB to +6.6dB. This is more than enough. I will most probably have a 0db and a +6dB option, 0 being the default. The performance is ok as well. Not as good as the schematic above, but still good. The noise will be around -106dBu (a bit below 4uVrms) for max gain and the CMRR is around -50dB. The noise performance is really good and the CMRR is acceptable. I'm not 100% happy with the CMRR but it will have to do. In the field of Hi-Fi, we don't really need that high CMRR as we would need in the world of professional audio (microphones, mixing consoles, long wires, etc.)
I will build the first schematic as well and I want to measure both. The only big downside of the first one is that if I want to be able to set the gain of the stage, I have to use several relays to do so. The schematic below is a lot more flexible in this regard. It also only uses two double op-amps. I can even try and use the almighty OPA1612 🙂 .
I will build the first schematic as well and I want to measure both. The only big downside of the first one is that if I want to be able to set the gain of the stage, I have to use several relays to do so. The schematic below is a lot more flexible in this regard. It also only uses two double op-amps. I can even try and use the almighty OPA1612 🙂 .
The schematic you've shared appears to be a differential amplifier with balanced inputs, which is a common design for low-noise applications. The LM4562 op-amp is known for its high performance, so it should be a good choice for this application.
Using 4x op-amps per channel may not be a concern for you in terms of cost and performance, but it could be a concern for power consumption and space constraints. You may want to consider these factors in your design.
Adding a gain switch is a good idea, as it allows you to adjust the gain for different input levels. Adding an unbalanced input is also a good idea, as it increases the versatility of the amplifier.
Overall, it seems like you have a solid design for a low-noise balanced input power amplifier.
Using 4x op-amps per channel may not be a concern for you in terms of cost and performance, but it could be a concern for power consumption and space constraints. You may want to consider these factors in your design.
Adding a gain switch is a good idea, as it allows you to adjust the gain for different input levels. Adding an unbalanced input is also a good idea, as it increases the versatility of the amplifier.
Overall, it seems like you have a solid design for a low-noise balanced input power amplifier.
