Fully Integrated HiFi Studio Monitor

Studio Monitor and PCB

Have you ever wanted to build a high quality audio crossover and amplifier? [Rouslan] has put a lot of thought into making his dual amplifier studio monitor both high quality and simple to build.

With a concise schematic, a meaningful block diagram, and simulation results to boot, his well-written post has everything you need to build self-powered bi-amped speakers based on the LM4766 from Texas Instruments. It is great to see simulations which verify the functionality of the circuit, this can go a long way when working with complicated analog filters and audio circuitry. For those of you who do not have access to PSPICE (an expensive professional simulation tool), [Rouslan] uses LTspice from Linear Technology. TINA-TI from Texas Instruments is another great free alternative.

Additionally, [Rouslan] goes over the typical issues one has with a bi-amplifier studio monitor, such as phase misalignment and turn-on pop, and then provides a solution. Note that his project is powered by 20VAC, which requires an external transformer to convert the 120VAC in the wall to 20VAC. Be careful with high voltages! In the future, adding a high quality voltage regulator will most likely increase the performance.

His post finishes up with a very clean circuit board, which he ordered from OSH Park. With such a complete design, there is nothing keeping you from building your own. Go out and put that old speaker sitting in your basement to good use!

If you don’t have an old speaker sitting around, check out these very cool DIY speakers.

22 thoughts on “Fully Integrated HiFi Studio Monitor

  1. 1. He forgot the Zobel networks.

    2. Audio amplifiers don’t benefit from regulated power supplies – see Doug Self’s book.

      1. not in his series on what makes a good amplifer, but it still stand that a good power amplifer doesn’t gain much if anything from a regulated supply, it already has excellent power supply rejection ratio

    1. I thought that he whole point of active monitors is that the crossover is done at the signal level and not after amplification. Thus, removing the inductors also removes any phase misalignment that they cause.

      1. inductors doesn’t cause phase misalignment per se, filters have phase shift whether they contain inductors or not, active filters doesn’t change that.

        There is still big advantages in doing the filtering at line level and using multiple amplifier
        but phase shift isn’t it

        1. Digital filters can easily be made to have linear phase response if that is what you want. A linear phase filter only indroduces a time delay, but no frequency-dependent delay…

      2. It is probably easier to *match* component absolute tolerances as well as tempco for RC than L/C. Non-air core inductors would have non-linear characteristics.

        You can use much smaller values (also ratings) and the smaller physical size allow for higher order filters if you do the crossover before the power amplification.

  2. Might want to watch out for ceramic caps in the audio and/or feedback path.

    Certain classes (2 & 3) of ceramic caps are known to have piezoelectric effects. Probably last thing you want inside a speaker as it can pick up vibrations . See here: http://product.tdk.com/capacitor/mlcc/en/faq/faq00031.html

    If you must use ceramic caps for the audio path, stick with C0G/NPO in Class I and you’ll get low temperature coefficient, low dielectric loss and stable over voltage. There are also film caps for higher values and they are available in SMT form.

    Not as much of an issue for decoupling.

    Note PSRR. That tells you how sensitive of the analog part of the power supply with respect to frequency.

  3. The zoble network is required for amplifier stability. Saying that loudspeakers with short leads are inductive to quite high frequencies and therefore this network can (dependent on amplifier) be as simple as a resistor and capacitor in series from the output to ground. This is the network I use on my similar chip amplifiers with no problems.

  4. I love making active monitors. They let you implement LR4 filters, which have a roll-off of 24dB, and sum to 0dB at the crossover point. Unlike typical butterworth filters, that have a 3dB bump at the crossover point. You can invert the phase of the tweeter to attenuate it 3dB and flatten it out, but now your phase response is messed up. That and a steeper rolloff will help your tweeters mechanical power handling (you can cross lower). <3 fully active monitors.

  5. I’m not at all fond of using the IEC C30-plug on the back for less-that mains voltage – that will end in a bang sooner or later when someone plugs the wrong cord into it…

    1. The IEC plug is for 110V AC. The transformer is inside the speaker, mounted close to one of the bottom corners (to break the otherwise perfectly symmetric insides).

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