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  • The DIY 2.1 amplifier. Should I build it? Would you buy it?

    OK guys,

    This thread is evolving out of my thread listed here. I wanted to separate this thread since the project took a decided turn away from a flexible base board to a design that focuses simply on the 2.1 amplifier project.

    I took the time today to generate a first cut of schematics (see attached) and a proposed BOM for the 2.1 amplifier. The idea behind this is to be able to buy TWO of those TDA7492 boards, mount them on top of a "base board", fit them into the larger SURE case sold here at PE, and voila - a 2.1 amplifier in a reasonable case. The base board will provide the crossovers and volume controls, but nothing like BSC or EQ. I'm assuming that the EQ and BSC are handled in the speaker design. This choice reduces the costs and complexity of the board significantly.

    When assembling the board, you'll have your choice of op-amps, crossover frequencies, and so on. The board is designed to provide LR4 crossover slopes, and the schematics show values for 80Hz. The frequency is adjustable by changing the resistor or capacitor values around the sallen-key filters.

    There are 5 op-amps required, and the op amps can be anything that supports about 20mA or greater of short-circuit current. The current requirement comes from the fact that one op-amp is used to supply the midrail bias voltage. I'd recommend an NE5532 or better. The TL052/72 parts won't have enough current and the noise performance won't be that great. An LM4562 would be awesome, but adds about $10 in cost to the board and probably won't buy you that much. The TDA7492 board will probably be a more limiting factor than the op-amp. R1 is the the master volume control that will be on the front of the amp. R12 is an adjustable gain for the subwoofer. This will protrude from the back of the amp and allow for easy adjustment of the subwoofer volume.

    I'm choosing a 2.5mm by 5.5mm DC jack to accommodate a wide range of power supplies. The jack will support 10A, which is far more than the normal DC jack, but it's required for a design like this.

    A few challenges exist with the design:
    1. The back panel of the amplifier will need to be drilled out for 2 extra binding posts and the extra subwoofer volume control.
    2. The volume knob is a funky size and hard to source. There's some options on eBay and one at Mouser, but not much.
    3. The hole for the binding posts is an odd size. One might have to drill out that hole larger depending on the binding posts chosen.
    3. Power supplies for this aren't cheap. Also, there's different voltage recommendations for 8-ohm or 4-ohm. Overall, I'd recommend using a 150W laptop supply at 19V. An Asus compatible power supply will work and costs around $22 shipped from Asia. This provides a good in-between for 8 and 4-ohm loads. Technically, 150W isn't going to actually allow the chip to get to full power, but I'm willing to bet that it has a more than enough headroom and volume for a desktop design or even a bedroom design.

    I did manage to make all the components through-hole, so hopefully anyone can solder it.

    Now for the nitty-gritty. Buying all the required components from Digi-key, the amp boards, case, and binding posts from PE, the powersupply from eBay, and the PCB from me the total cost would come out to just about $130. That's a complete project! You'd be responsible for assembling, but it should be very easy. There are several ways to save money such as buying several parts from eBay (amp boards are $7 cheaper each, pots are $2 cheaper, op-amps can drop $3 or more), but there are inherent risks with this option.

    With this in mind, I need to know just how many PCB's you'd be interested in buying. Basically, if we get anywhere north of about 20, I'll try and complete the project.

    Amp Base Board.pdf

    Thanks!
    DARPA Jr - 2015 InDIYana Winner - RS180-8 + RS100P-8 + ND25FA
    The Aria's - RS150-4 (or RS150-8) + XT25SC90
    The Mariposa's - TEBM65C20F-8 + ND16FA
    The Canzonetta's - RS100P-8 and ND16FA
    AudioSource AMP-100 Mods OR Pyle PAMP1000 Mods

  • #2
    Re: The DIY 2.1 amplifier. Should I build it? Would you buy it?

    I didn't mention it above, but I could really use some support reviewing the schematics from you op-amp guys. The sallen-key filters on a single supply rail provide a challenge, and I may have missed something.
    DARPA Jr - 2015 InDIYana Winner - RS180-8 + RS100P-8 + ND25FA
    The Aria's - RS150-4 (or RS150-8) + XT25SC90
    The Mariposa's - TEBM65C20F-8 + ND16FA
    The Canzonetta's - RS100P-8 and ND16FA
    AudioSource AMP-100 Mods OR Pyle PAMP1000 Mods

    Comment


    • #3
      Re: The DIY 2.1 amplifier. Should I build it? Would you buy it?

      Tyger, what would be the approximate cost of the PCB?
      What would be the output VRMS?
      If I were to just run the PCB, what are the PSU requirements?

      With that asked, I would be interested in getting 2-3 PCBs
      My Statement monitors
      My Piccolatas
      My LM3886 amp

      Comment


      • #4
        Re: The DIY 2.1 amplifier. Should I build it? Would you buy it?

        Please disregard if this was covered before, but is there an ability to have an auto-on function? This isn't critical since I could rig something up using the speaker bar power output from my monitor, just thought it would be a nice to have.

        Comment


        • #5
          Re: The DIY 2.1 amplifier. Should I build it? Would you buy it?

          Am I incorrect or should your input & output grounds be connected to VGND to keep the output from being at 1/2 VCC. Or should the outputs be capacitor coupled?
          9 out of 10 British housewives can't tell the difference between Whizzo Butter and a dead crab.

          http://www.youtube.com/watch?v=19CvEO3Riy0, http://www.youtube.com/watch?v=xs1aUws0Lrs
          www.rane.com/pdf/pi14data.pdf, http://www.realoldiesradio.com/docs/EV/EV_13-5.pdf, http://www.technicalaudio.com/pdf/Je...e_informer.pdf

          Comment


          • #6
            Re: The DIY 2.1 amplifier. Should I build it? Would you buy it?

            The outputs are at 1/2 of VCC and the outputs should be cap coupled unless you float the supply with respect to the actual ground and connect the virtual ground of the crossover directly to actual signal ground of the system. I don't know about using an op amp as the regulator and hanging a 220 uF cap on the output without a pass transistor. VCC had better be specified as well. Most op amps would probable like to see 10 volts or more for VCC. Get of a pot and put it at the output of U4a. If you use JFets input op amps you can make the caps in the low pass filter smaller values and the resistors large values. A 10k pot is going to have a maximin output impedance of 5k at the mid point setting. Assuming you do use the negative supply as the ground reference, you are putting half of the noise on VCC into circuit with your filtering cap bypassing scheme at audio frequency range. While may reduce turn on thumps, it really degrades the signal to noise of the circuit. I would work of the virtual ground circuit a bit more.

            Comment


            • #7
              Re: The DIY 2.1 amplifier. Should I build it? Would you buy it?

              Total price puts this in the maybe category. If the x-o is remote for the mains, I’d only need an x-o for the sub. Maybe this could reduce the cost.

              Not that I know what I’m saying, but can you use a master volume and then just a trim pot for the sub? I guess if you had really efficient mains you could consider a trim pot for the mains. I want the sub to track with the main volume.
              John H

              Synergy Horn, SLS-85, BMR-3L, Mini-TL, BR-2, Titan OB, B452, Udique, Vultus, Latus1, Seriatim, Aperivox,Pencil Tower

              Comment


              • #8
                Re: The DIY 2.1 amplifier. Should I build it? Would you buy it?

                The sub will track with volume with this circuit

                Comment


                • #9
                  Re: The DIY 2.1 amplifier. Should I build it? Would you buy it?

                  Sounds pretty awesome $130 isn't impulse purchase money for me so I can't commit to buying one right now. Seems like a really solid idea tho. If you end up following through on this would people have the flexibility of getting it without the case? The case is very pretty but it sounds like there's a lot of fiddly extra stuff that needs to be done that might not make it a goood value for everyone at $20.

                  Comment


                  • #10
                    Re: The DIY 2.1 amplifier. Should I build it? Would you buy it?

                    Originally posted by evilskillit View Post
                    Sounds pretty awesome $130 isn't impulse purchase money for me so I can't commit to buying one right now. Seems like a really solid idea tho. If you end up following through on this would people have the flexibility of getting it without the case? The case is very pretty but it sounds like there's a lot of fiddly extra stuff that needs to be done that might not make it a goood value for everyone at $20.
                    I may have misread the OP but I think the $130 is total for all the parts + board, not the board. If so I'd be interested in one, maybe two, I can always pick up the case and fiddly stuff later.
                    Failure is not an option. It comes bundled with Windows.

                    We are passionate about great sound at whatever level we can afford, so don't let the audio atheists reduce the experience to a set of numbers and squiggly lines. - jbruner

                    Comment


                    • #11
                      Re: The DIY 2.1 amplifier. Should I build it? Would you buy it?

                      Originally posted by Silver1omo View Post
                      Tyger, what would be the approximate cost of the PCB?
                      What would be the output VRMS?
                      If I were to just run the PCB, what are the PSU requirements?

                      With that asked, I would be interested in getting 2-3 PCBs
                      Cost of the PCB depends on how many ordered. However, I think about $10-15 each. It could go as low as $5 if we get upwards of 50 total.

                      Output power is dependent on several things - the power supply used and the thermal capabilities of the design. Remember that the amplifier design is already BTL, so summing to mono only increases the amount of current the amplifier can provide to the speaker, and not the amount of power out of the amplifier. What this means is that the amplifier can drive a lower resistance load, but it can't drive a mono speaker any harder than a stereo load if the power supply is limited. Thermal is another concern, but I can't comment on that until I get the design built. I don't expect it to be an issue....

                      With the power supply I was calling out (19V) and an 8-ohm load, the system would provide about 20W per channel before distortion starts to become audible. Into a 4-ohm load, it would provide about 30W per channel. The mono output would be the same wattage, but the mono allows you to drive down to a 2-ohm load. At that load, you would probably get about 70W. Note that's out of the mono channel only. The stereo channel can only support 4 ohm loads minimum.

                      Since the mono channel is parallel-BTL and can support the extra current, you should be able to run the whole thing off of 24V supply. With that voltage, 8-ohm loads would produce about 30W and 4-ohm loads would be about 50W-60W. A 2-ohm load at 24V might cause some thermal issues, but it should produce close to 100W. Note that you should NOT exceed 22V if running 4-ohm loads on the stereo amplifier.

                      So, PSU requirements are dependent on your loads:
                      • If 8-ohms on the stereo amp and 4-ohms on the mono amp, then 24V should work well. A 2-ohm sub would likely work at 24V, but that's getting close to the limits.
                      • If 4-ohm loads on the stereo amp, then I'd probably stick to 19V maximum.
                      • Power requirements for the PSU also vary for the loads and voltage. Let me know what you are planning, and I can help you figure it out. Generally, though - for the two scenarios above, I would want 120W or 150W minimum, respectively.
                      DARPA Jr - 2015 InDIYana Winner - RS180-8 + RS100P-8 + ND25FA
                      The Aria's - RS150-4 (or RS150-8) + XT25SC90
                      The Mariposa's - TEBM65C20F-8 + ND16FA
                      The Canzonetta's - RS100P-8 and ND16FA
                      AudioSource AMP-100 Mods OR Pyle PAMP1000 Mods

                      Comment


                      • #12
                        Re: The DIY 2.1 amplifier. Should I build it? Would you buy it?

                        Originally posted by superspeeder View Post
                        Please disregard if this was covered before, but is there an ability to have an auto-on function? This isn't critical since I could rig something up using the speaker bar power output from my monitor, just thought it would be a nice to have.
                        I hadn't thought about it, but there should be room to build in an option for that (you could choose between a switch or auto-on). Let me think about that....

                        FYI - the class D should take very little current when not playing audio, so it could probably always stay on without much concern.
                        DARPA Jr - 2015 InDIYana Winner - RS180-8 + RS100P-8 + ND25FA
                        The Aria's - RS150-4 (or RS150-8) + XT25SC90
                        The Mariposa's - TEBM65C20F-8 + ND16FA
                        The Canzonetta's - RS100P-8 and ND16FA
                        AudioSource AMP-100 Mods OR Pyle PAMP1000 Mods

                        Comment


                        • #13
                          Re: The DIY 2.1 amplifier. Should I build it? Would you buy it?

                          Originally posted by fdieck View Post
                          The outputs are at 1/2 of VCC and the outputs should be cap coupled unless you float the supply with respect to the actual ground and connect the virtual ground of the crossover directly to actual signal ground of the system.
                          You are correct - but the amp board has series caps built in. I didn't want to have caps in series, so I left them off of the base board.

                          Originally posted by fdieck View Post
                          I don't know about using an op amp as the regulator and hanging a 220 uF cap on the output without a pass transistor.
                          I'm not sure about your main concern here - can you explain more? The circuit I designed in has been used in several applications in the past, but I'm up for learning new tricks.

                          Originally posted by fdieck View Post
                          VCC had better be specified as well. Most op amps would probable like to see 10 volts or more for VCC.
                          Yes - 12V minimum, up to 26V max (the limit of the amp board)

                          Originally posted by fdieck View Post
                          Get of a pot and put it at the output of U4a.
                          I don't understand this, can you help explain more?

                          Originally posted by fdieck View Post
                          If you use JFets input op amps you can make the caps in the low pass filter smaller values and the resistors large values.
                          Agreed, but I was trying to accomodate Jfets and bipolars. I personally don't like the sound Jfets compared to bipolars. Call me different, if you wish.

                          Originally posted by fdieck View Post
                          I would work of the virtual ground circuit a bit more.
                          I'm open to any and all suggestions.
                          DARPA Jr - 2015 InDIYana Winner - RS180-8 + RS100P-8 + ND25FA
                          The Aria's - RS150-4 (or RS150-8) + XT25SC90
                          The Mariposa's - TEBM65C20F-8 + ND16FA
                          The Canzonetta's - RS100P-8 and ND16FA
                          AudioSource AMP-100 Mods OR Pyle PAMP1000 Mods

                          Comment


                          • #14
                            Re: The DIY 2.1 amplifier. Should I build it? Would you buy it?

                            Originally posted by jhollander View Post
                            Total price puts this in the maybe category. If the x-o is remote for the mains, I’d only need an x-o for the sub. Maybe this could reduce the cost.
                            The way the board is designed, you could populate any or none of the crossover components. It's flexible.

                            Originally posted by jhollander View Post
                            Not that I know what I’m saying, but can you use a master volume and then just a trim pot for the sub? I guess if you had really efficient mains you could consider a trim pot for the mains. I want the sub to track with the main volume.
                            The board is designed to do exactly what you want. There's a volume control that will affect the sub and the mains, and there is a separate trim for the sub.
                            DARPA Jr - 2015 InDIYana Winner - RS180-8 + RS100P-8 + ND25FA
                            The Aria's - RS150-4 (or RS150-8) + XT25SC90
                            The Mariposa's - TEBM65C20F-8 + ND16FA
                            The Canzonetta's - RS100P-8 and ND16FA
                            AudioSource AMP-100 Mods OR Pyle PAMP1000 Mods

                            Comment


                            • #15
                              Re: The DIY 2.1 amplifier. Should I build it? Would you buy it?

                              Originally posted by evilskillit View Post
                              Sounds pretty awesome $130 isn't impulse purchase money for me so I can't commit to buying one right now. Seems like a really solid idea tho. If you end up following through on this would people have the flexibility of getting it without the case? The case is very pretty but it sounds like there's a lot of fiddly extra stuff that needs to be done that might not make it a goood value for everyone at $20.
                              Basically, I would just be selling the board only, and at cost to me. I'm not looking to make money - only friends ;). The PCB would be about $5-$15 depending on the total quantity the community commits to. You could put it in a case or in anything else you want. It will be fairly big, though, as the case it will be designed for is slightly larger than 4"x6".
                              DARPA Jr - 2015 InDIYana Winner - RS180-8 + RS100P-8 + ND25FA
                              The Aria's - RS150-4 (or RS150-8) + XT25SC90
                              The Mariposa's - TEBM65C20F-8 + ND16FA
                              The Canzonetta's - RS100P-8 and ND16FA
                              AudioSource AMP-100 Mods OR Pyle PAMP1000 Mods

                              Comment

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