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

[fdieck]

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

I've seen a virtual ground driver circuit drawn both ways (with the feedback before and after the series resistor). I think the reason for having the feedback after the series resistor is that the op-amp will compensate for any voltage droop caused by current draw through the series resistor. Which do you think is better? I will yield to your expertise.




Ok - now I understand you. I thought I had designed the circuit to avoid any interactions between the pot and the filter. On the master volume pot (R1), U1A and U1B form stereo buffers to prevent any filter interaction (U2/U3). On the subwoofer volume pot (R2), U4B is a buffer designed to prevent any filter interaction. In case anyone is wondering, the series resistors (R13/R14) are there prevent crosstalk between the left and right channels when the pot is at maximum volume.



I chose to keep the solution as a dual pot because that increases purchasing power and helps reduce the cost. For example, the person who builds the design can easily source two of the same pot form eBay, etc. to help save money. A single pot + a dual pot usually doesn't save any money as versus two dual pots. A lesser reason is that it prevented me from having to create two separate symbols in the DesignSpark software....

I do have to ask, though - why "no ground plane"??? I've been doing audio layouts in PC's and consumer electronics for 15 years, and I'll admit that careful attention must be paid to the grounding, but I certainly wouldn't call out a blanket "no ground plane" rule. My experience has shown that planes are preferred to shield grounding. Star grounding can be beneficial, but it can be complicated to get right. Basically, a star ground can yield better performance than a plane, but it's so much easier to screw up and yields worse performance when it's bad. Do you have some specific concerns here? The design could be done as a one layer board, especially if I were to use jumpers. The pricing for a two layer is cheap enough to not worry about trying to make it a one layer.

I am not calling for a blanket ban on ground planes. Simple analog circuits don't usually need them since they just add stray capacitance if used for the sake of using them. The serious audio designers I know like tin plated copper and no solder mask with fairly thin signal traces.
Star grounding is a good idea and makes you think about you grounding and keeping the sensitive signal returns from sharing ground impedances with dirty power supply and decoupling return currents. Mixed signal grounding and decoupling is almost an art and there have been countless numbers of articles and books written on the subject. I would be surprised if I have read less than fifty articles of the subject.

I worked on a telecom line card that had four different grounding subgroups for digital, analog, lightning protection, and electrostatic discharge. Capacitive and electromagnetic coupling between all these grounds had to be considered. Keeping the ground bounce from resisting the micro or ASICs was a task I would not wish on anyone. I had the best layout guy
I will ever meet using Mentor Graphics software on a Sun workstation. Both of us nearly lost our minds and it took over a year out of my life. The board was only 2 layers in smaller in area than a shoe box. When someone comes out and says that competently designed audio products should sound the same, the word competently covers a lot of ground (no pun intended.

[tyger23]

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

The 221 ohm resistor for the VCC/2 op amp should be outside the feedback loop and could be as small as 50 ohms without any problems I think. The smaller the value the less signal voltage will appear your active ground.

I've seen a virtual ground driver circuit drawn both ways (with the feedback before and after the series resistor). I think the reason for having the feedback after the series resistor is that the op-amp will compensate for any voltage droop caused by current draw through the series resistor. Which do you think is better? I will yield to your expertise.

The point about the pot is that if the pot is driven by a low impedance the impedance looking at the wiper will have a maximum impedance of half the pots value. If the circuit after the pot has an impedance say 20 or more times the impedance at the wiper, different settings of the pot should not impact the frequency response of the filter.

Ok - now I understand you. I thought I had designed the circuit to avoid any interactions between the pot and the filter. On the master volume pot (R1), U1A and U1B form stereo buffers to prevent any filter interaction (U2/U3). On the subwoofer volume pot (R2), U4B is a buffer designed to prevent any filter interaction. In case anyone is wondering, the series resistors (R13/R14) are there prevent crosstalk between the left and right channels when the pot is at maximum volume.

By putting the pot after the mono summing op amp you could loss a pot since the level you are adjusting is mono. Probably a 5k pot min and 20 or more times that impedance means jfet input op amps would be advisable. You have fixed all the real issues and put me down for a couple of boards. PS don't use a ground plane, and don't be afraid the use jumpers to keep
It at a 2 layer board.

I chose to keep the solution as a dual pot because that increases purchasing power and helps reduce the cost. For example, the person who builds the design can easily source two of the same pot form eBay, etc. to help save money. A single pot + a dual pot usually doesn't save any money as versus two dual pots. A lesser reason is that it prevented me from having to create two separate symbols in the DesignSpark software....

I do have to ask, though - why "no ground plane"??? I've been doing audio layouts in PC's and consumer electronics for 15 years, and I'll admit that careful attention must be paid to the grounding, but I certainly wouldn't call out a blanket "no ground plane" rule. My experience has shown that planes are preferred to shield grounding. Star grounding can be beneficial, but it can be complicated to get right. Basically, a star ground can yield better performance than a plane, but it's so much easier to screw up and yields worse performance when it's bad. Do you have some specific concerns here? The design could be done as a one layer board, especially if I were to use jumpers. The pricing for a two layer is cheap enough to not worry about trying to make it a one layer.

[fdieck]

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

%$#@! Double post.

The 221 ohm resistor for the VCC/2 op amp should be outside the feedback loop and could be as small as 50 ohms without any problems I think. The smaller the value the less signal voltage will appear your active ground. The point about the pot is that if the pot is driven by a low impedance the impedance looking at the wiper will have a maximum impedance of half the pots value. If the circuit after the pot has an impedance say 20 or more times the impedance at the wiper, different settings of the pot should not impact the frequency response of the filter.
By putting the pot after the mono summing op amp you could loss a pot since the level you are adjusting is mono. Probably a 5k pot min and 20 or more times that impedance means jfet input op amps would be advisable. You have fixed all the real issues and put me down for a couple of boards. PS don't use a ground plane, and don't be afraid the use jumpers to keep
It at a 2 layer board.

[tyger23]

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

%$#@! Double post.

[tyger23]

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

All,

I did a fair amount of work on the layout today. It looks like all of the components will fit even with the larger TPA3116 boards in place. Again - asking for help reviewing the schematics and placement.

Thanks!
Ty

[tyger23]

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

OK - Here's another schematic. The only changes here are:

1. Remove the large value capacitor (C18) that was between VCC and the mid-point on the input to U4a. This will reduce the opportunity for VCC noise to enter the mid-rail voltage.
2. Remove the large capacitors on the output of U4a. Since I'm using U4a as a current source, there shouldn't be a need for those capacitors. They'll only cause instability issues.
3. Add in a 220-ohm resistor before the feedback on U4a. This is to help with stability issues on any op-amps that are chosen by the end user. The value could vary to help with stability.
4. Changed C19 from 10uF to a 0.1uF bypass cap. With a 100K series resistor, the 10uF would have taken forever to charge up, and it wasn't really necessary anyways.

I still am concerned about Fred's comment on the maximum output impedance of the 10K pot... I'm just not sure I understand the concern. Fred, if you're still out there, please comment (or anyone else that understands the concern).

Again - this is a group effort. I'm certainly not professing to know everything, and I hope that we can all work together to generate a design we can all be proud of.

Neil, I looked at the TLE2426, and if I didn't have a spare op-amp in the design, I would use it. However, since I have a spare op amp, I thought I would use the op-amp instead of throwing another $2 at the design. In theory, a decent op-amp should perform just as well as the TLE2426 for this purpose.

Amp Base Board.pdf

Thanks!
Ty

[jhollander]

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

not to hi-jack the thread but... has anyone considered making a small sub and possibly primary speakers to go with this? I know I could build my own but i don't really have the time or tools, besides I have always been way better with electronics than wood ;_

Go over to the Forum, speaker project gallery and pick out what you like. The project showcase also has some detailed designs.

[neildavis]

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

Also, I meant to add a low value resistor on the op amp output (220 ohm or so) to help with stability and I forgot. I'll get that done, unless you guys think that the pass transistor would be a better solution that's worth the money.

You might want to consider a rail-splitter like the TLE2426IP. Mouser sells them for less than $2.

[Asbot]

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

not to hi-jack the thread but... has anyone considered making a small sub and possibly primary speakers to go with this? I know I could build my own but i don't really have the time or tools, besides I have always been way better with electronics than wood ;_


Thanks

[tyger23]

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

As Fred mentioned, C18 will let 1/2 of the power supply noise into your virtual ground, can you get away without it? The pass transistor comment was likely to buffer the op-amp from that large capacitive load (C20), which the op-amp may not much appreciate. Had you found C20/C21 cut down on noise?

I'll get rid of C18. You guys are correct - it's likely to cause more noise than help. I've seen that type circuit used in the "cmoy" amps of various internet fame, but that doesn't make it a good design ;).

Also, I meant to add a low value resistor on the op amp output (220 ohm or so) to help with stability and I forgot. I'll get that done, unless you guys think that the pass transistor would be a better solution that's worth the money. This type circuit would certainly take the current load off of the op-amp, allowing a wider choice of low-cost options:


C20/21 are much less about noise and far more about maintaining the voltage with current draw. The bypass caps on each op-amp are more about the noise.

[Asbot]

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

I will commit to a least one.

Thanks

[relder]

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

I'm open to any and all suggestions.

As Fred mentioned, C18 will let 1/2 of the power supply noise into your virtual ground, can you get away without it? The pass transistor comment was likely to buffer the op-amp from that large capacitive load (C20), which the op-amp may not much appreciate. Had you found C20/C21 cut down on noise?

[tyger23]

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

Here are some scaled pictures of how the boards should lay out. Hopefully this gives a better image of what I've been designing. The POT on the right side is the volume pot, while the one on the back is the subwoofer trimmer. FWIW - there's not room to lay out components UNDER the amp boards without the use of spacers, and then there's not much anyways. The maximum height from the top of the PCB to the top of the amp case is 33mm. The amp boards are about 25mm tall, which only leaves 8mm. DIP package op-amps are >5mm high and the film caps I was allowing for are about 7mm tall...

First the 7492 amp board from PE:



There's lots of room left here. However, Here's the shot of the TPA3116 board from YJ:



I'm going to try and lay out this board so that the YJ TPA3116 boards can be used without needing spacers or surface mount components, but there's very little room left for the op amps and easy-to-solder caps. I'll see if I can fit it, but there definitely won't be room to populate the extra bulk caps I provided on the supply rail.

Questions? Suggestions?

[jhollander]

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

I'll do 2 at $10, and shipping costs. Under $10 I'll do a few more.

[Silver1omo]

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

My questions were oriented to running the XO board "alone" and feed pre-existing amps (my chipamp for mains and a sure ta7492 for sub, for example).