Building for Tweakability

Virtually every hole in the board is on 0.100" spacing relative to all the related holes. Also, each resistor’s holes are 0.300" apart. This means you can use socket strips and header pin strips in the board. If you did this for everything where it makes sense, you would in effect have a PPA breadboard for maximum tweakability. :) There are fewer places to do this in PPAv2 than there was in the previous version, but the parts most likely to be tweaked are still tweakable. You should at least think about adding connectors or sockets in these places:

    • Feedback loop resistors — Use SIP socket strips to allow easy swapping of resistors. The most common change to a PPA’s components after the fact is to change the gain. If you’re not sure up front what gain you want, adding sockets here is a good idea.

    • Power input — V+, B+ and V- are set up to allow you to put a Molex KK style connector on the board in one of two positions. This would let you disconnect the power jack from the board, or the battery board from the PPA board, depending on how you have the power flow set up.

    • Power switch — I’m not sure what connector would fit here, but probably something could be worked out. A 2-pin Molex KK can be made to work for just one of S1+ or S1-, but a small foursquare connector would work better, if you can find one.

    • Signal input — Like with the power input, you could put a Molex KK connector here. There’s more space here, so almost any connector with 100 mil pin spacing would work here, actually. Between this and the previous tweaks, you could completely separate the amp board from the rear panel, without desoldering anything.

Tweaking the Buffer Bias Point

The best place to learn more about this is in the forums, because it comes down to collective experience with particular configurations.

Biasing the Op-Amps into Class A

“Class A” refers to configuring an amplifier so that its output devices remain turned on all the time. This reduces thermal variation and eliminates crossover distortion, which makes the amp sound better. You can bias the op-amps in the PPA by adding the cascode JFETs and the source resistor.

Before I get into the PPA-specific details, you should read my article Biasing Op-Amps into Class A. Q1 and Q2 in the article are the same as Q1 and Q2 in the PPA, and Rs corresponds to R9 in the PPA.

To set the bias point once the parts are installed, you simply power the amp up and measure the DC voltage drop across R10. If R10 is 1 kΩ, then each volt of drop across R10 equals one milliamp through the cascode, so it’s easy to trim R9 to get a particular op-amp bias current. 1 mA is a good starting value, but you should try other values to see if you can hear an improvment in the sound at different bias levels.

Adding Crossfeed

There are three holes near the rear edge of the circuit board. These are for mounting version 1.1 or 1.2 of my modified Linkwitz crossfeed PCBs on the board. You only use two of the holes, the middle one and one outside one, depending on where you want the PCB to be positioned over the amp board. See the previous link for more information.

Adding Bass-Boost

In cooperation with the op-amp, C7 and a parallel resistance in the PPA form a type of “shelving filter,” raising the output level of low frequencies. In other words, it boosts the bass. The advantage of this type of filter over other more common tone adjustment circuits is that it is completely bypassable and it works in conjunction with the op-amp circuit which had to be there anyway so it only adds two components.

For the moment, we’re only going to think about fixed bass boost adjustment, using R7 as the resistance in parallel with C7. In PPAv2, we added the option of using a pot instead of or in addition to R7, allowing for adjustable bass boost. We’ll leave discussion of that off until later. For the remainder of this section, any time we say R7, we mean either R7, or the pot, or both in parallel.

R7 raises the level of the highest bass boost:

R7 effect on bass boost

C7 changes the point where the bass boost begins:

C7 effect on bass boost

The larger the value of C7, the lower the frequency where boosting starts.

Notice how finely spaced the lines are on the graph. This is a log-log scale graph, though, so in reality the jumps in corner frequency between two close capacitor values get larger as the capacitor gets larger. Therefore, if you want the bass boost to extend clear up into the low midrange, you must use a line of capacitors with fine-grained steps between values or else suffer from large jumps in the point where the boosting begins. The Panasonic ECQP line will work well here, for instance; they go up to 0.12 µF without requiring lead bending. If you want to choose a different type, you should use caps with polypropylene film-and-foil construction for best sound quality. Metalized polypropylene will also be fine, but I’d only do it if I wanted a really low amount of boost, and so needed a high value for C7.

The value of R4 also has a small effect on the behavior of the bass boost circuit:

R4 effect on bass boost

I include this graph only to show that you can vary R4 without worrying about what it will do to the bass boost behavior.

It’s important to realize that this circuit doesn’t roll off at DC, so any DC offset on your amp’s output will also be boosted when you engage the bass boost. It is important that you minimize DC offsets in the amp before adding bass-boost. A 20 dB boost (10×) can transform a harmless 10 mV DC offset into a headphone-frying 100 mV offset!

If you want to explore these issues in more detail before you turn on the soldering iron, I suggest you download a copy of the Micro-Cap 9 demo. Here is a circuit file for you to start with. (This is the circuit file I used when generating the above graphs, so it should be set up properly already.)

You can get some of the information you would get from the simulator without as much work by using the bass boost calculators.

If you’re curious as to how this circuit works, look at R7 and C7 as two separate paths through the feedback loop. C7’s impedance ranges from infinite at DC down to 0 Ω at high frequencies. Since 0 Ω in parallel with any R7 is always 0 Ω, at high frequencies the amp gain follows the standard formula, given as [1] below. (Yes, the gain formula is different in the Jung multiloop configuration, but this approximation suffices for our purposes here.) At low frequencies, C7’s impedance is very high, so R7’s resistance dominates the total impedance; it simply adds to R4’s resistance, giving formula [2]. Between these two levels where the impedance of C7 is near R7’s resistance, you have the slope you see in the graphs above. Formula [3] is derived from these two, telling you the amount of bass boost relative to the amp’s normal gain.

  (R4 / R3) + 1                 [1]
  ((R4 + R7) / R3) + 1          [2]
  (R7 / (R3 + R4)) + 1          [3]

Adjusting the Bass-Boost

The strength of the bass boost feature is controlled by either R7 or the bass boost adjustment pot, or both in parallel. Whether you use R7 or the pot simply depends on whether you want fixed bass boost, or adjustable bass boost.

If you know you want the option of bass boost but don’t want to use a pot and don’t want to figure out which R7 value to use by experimentation, a good default is the same value you used for R4. This gives 6 dB of boost, enough to be audible, but not so much as to be excessive.

The next step up in the complexity vs. cost curve is to wire a pot in place of R7 temporarily to figure out which fixed R7 value to use. Just listen to different kinds of music, noticing where you tend to keep it adjusted. If you find yourself needing a wide range of values, you’re best off using adjustable boost instead of fixed boost. If instead you find that the bass is fine without boosting most of the time but some music benefits from a little kick in the bottom end, just measure the setting of the pot to select your R7 value.

If the cost of the pot isn’t an issue and you don’t want to experiment, I’d recommend just adding the pot and leaving R7 out. It’s simple, effective, and flexible enough to cover all common cases.

It’s possible to use both the pot and R7 the same time. The purpose would be to lower the maximum bass boost if it’s too great with the pot alone. The more you lower the amp’s gain from the default, the greater the likelihood that you’ll find the bass boost adjustment too twitchy, with too small an adjustment giving too much effect, and the maximum boost being too high.

The easiest way to figure out which R7 to use with the adjustment pot is to start out like above, with the temporary pot option: leave R7 out initially, then listen to a lot of music, noticing the highest value you use for the boost adjustment. When you’re satisfied that you’ve covered all the cases, set the pot at the maximum useful setting you found with these experiements. Then, measure the pot’s resistance using the R7 holes for your DMM probes. Let’s say you’re using the recommended 50 kΩ ALPS pot and that in your experimentation you found that you never set it higher than 32 kΩ. We need to pick an R7 that in parallel with 50 kΩ gives 32 kΩ. After some playing with my parallel resistance calculator, you’ll find that a 91 kΩ R7 does the trick here. You’ll need to do your own testing and measurements to find an R7 that suits your situation.

Tweaking the Isolation JFETs

The standard Q3 configuration works well, but there’s some room for tweaking here.

As I mentioned elsewhere, you can get up to about 30 dB of isolation without really thinking about it. You can get better isolation by picking your Q3s such that the current draw of the circuitry below them is a high percentage of their IDSS. Because the IDSS of JFETs varies so wildly, you must measure your JFETs’ IDSSes and use similar JFETs in all of the positions to get the best performance.

There are two consequences of doing this tweak. One, it makes rolling op-amps more difficult: you may have to change the Q3s at the same time to make the new op-amp work. Two, the DC voltage drop across a JFET increases the closer you get to its IDSS. This could be a problem for you if it drops too much voltage for your application, or it could be a way for you to use low-voltage op-amps while still using a high supply voltage for the buffers.

You may have noticed that you can jumper across all of the Q3 positions to get a single set of power rails for everything. It’s my opinion that if you’re trying so hard to save money that the cost of a few JFETs matters, you would probably be better off building an amp with an inherently less expensive design like the PIMETA.

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