Part Selection Guide

Passive Parts

C1 (film)

To understand the issues involved in deciding whether to add C1 or to put a jumper across this position, read my article Input Capacitors for Headphone Amps.

Many types of capacitors will fit here, up to 0.4" pin pitch. You will probably have to go with metallized polypropylene or polyester to fit a usefully high value cap here.

Optional? Yes, jumper across it.

Largest Part Size: 13mm × 6mm. Lead spacing 10mm.

C2 (electrolytic)

These are the main power reservoir capacitors.

If you just want me to tell you what will work here, use 330μF or 470μF capacitors with voltage ratings higher than that of your power supply. For example, use a 25V capacitor if your power supply is 24V. I recommend the Panasonic FC and Nichicon PW lines; in the US, they're available from DigiKey and Mouser, respectively. If your chosen distributor doesn't carry one of these lines, try to find a cap line that features long life and low ESR.

If you want to choose your own power capacitors, there are two main rules to keep in mind:

Sometimes you must compromise on quality (rule 2) to get a sufficient amount of capacitance (rule 1). For a MINT, the minimum I recommend is 220μF. Because small size is so important with MINT amps, you probably won't be able to get more than 470uF in this spot. Personally, I use 330μF Panasonic FCs here.

If you want to select your own capacitors, you first need to decide on the physical cap size. The diameter needs to be 10mm to fit properly on the board. You'll probably need to limit the height to under 12.5mm if you're going to use the amp in a mint tin. For other case types, you can probably get away with a taller cap. Next, you need to know your power supply voltage. It's best to use caps with a voltage rating that's higher than your power supply's maximum output voltage, but no higher. For instance, if you have a 30V supply, 25V caps could be damaged by the power supply, 35V caps are good, and 50V caps are wasteful. (For more on this topic, read my article Op-Amp Working Voltage Considerations.)

Optional? No.

Largest Part Size: 10mm diameter

R1

R1 interacts with R2 to form a voltage divider. If R1 is much smaller than R2, this effect is negligible, which is the way you'll almost certainly want it. I imagine someone might choose to configure this to divide the voltage down by a significant amount on purpose, but that's not the intent of this layout.

The main purpose of R1 is to help balance the op-amp's input impedances. I'm not sure how to calculate a proper value for this resistor; 1 kΩ or less seems to work well, however.

I recommend that you populate this position.

Optional? Yes, jumper across it.

R2

This is the input impedance resistor. It must be significantly higher than the highest resistance of your volume pot, or else your source will see the amp as a varying load impedance as you rotate the volume knob. This may cause it to have different sound characteristics at different volume levels. You should make R2 10× the value of the pot, if you can get away with it. You can go higher, but it won't improve the performance of the amplifier.

Optional? No.

R3 through R6

These are the feedback resistors. They set the amplifier topology and the gain. Although there are a couple of different reasonable topologies possible, I only recommend Jung multiloop unless you're just playing around. This requires populating all four positions per channel.

The simplest path is to just use the values given on the schematic. Next simplest is to leave R3, R5 and R6 at their default values and adjust R4 to change gain. (Gain calculator.) If you want to fiddle with everything, read about how to optimize the multiloop values in the PIMETA docs.

Optional? For a standard configuration, populate all four positions.

R7

This series resistor decreases ringing on the op-amp's output at high frequencies. (That's the electrical engineer's use of the word ringing, not the bell type of ringing.) It's not clear to me that the BUF634 actually requires this, but the position is available if you need to use it. It's simple enough to put a 100 Ω resistor here and just leave it at that. If you wanted to be frugal, you can probably jumper this position without problems.

Optional? Yes, jumper across it.

R8

If you are getting audible hiss at normal listening volumes with the source disconnected, put a 10 to 100 Ω resistor in R8. (This mainly happens with low-impedance headphones.) Alternately, you can put a similar resistor between the 'O' pads and the output jack. The difference between these two configurations is that the resistor is inside the feedback loop (R8) or outside it. I suggest you jumper R8 and then only change it if you have hiss problems.

Optional? Yes, jumper across it.

R11

This resistor is for modifying the bandwidth of the buffer, which can improve its sound audibly. The downside is that it consumes more current as bandwidth goes up. A practical maximum value for this resistor is about 4.7 kΩ. As the value drops, bandwidth goes up and so does power draw. You could jumper this position for maximum bandwidth, but since MINT amps are generally battery-powered, this probably isn't a good idea. I usually put somewhere between 220 Ω and 1 kΩ here. If you want maximum battery life, leave this position empty.

Note that this resistor position is smaller than the others. You need to use a standard size 1/8W resistor here to get it to fit.

Optional? Yes.

RLED

This is the LED current limiting resistor. Use Ohm's law to figure current given the LED's voltage drop and the power supply voltage. For example, consider a 1.8V LED with a 15V power supply and a 4.7 kΩ RLED:

	I = V/R
	I = (15 - 1.8) / 4700
	I = 13.2 / 4700
	I = 0.0028
	I = 2.8mA

Most LEDs require 1mA to get a minimal amount of brightness. More current gets you more brightness, but of course uses more power, which mainly matters with battery power supplies.

Typical values are 1 kΩ to 10 kΩ. I personally use 2.2 kΩ and 4.7 kΩ most often.

Optional? Yes. If you don't use an LED, do not jumper here. If you use an LED with an integral resistor, jumper across this position.

D1

This is an optional "crowbar" diode. If you put a diode here, it will normally be unused, since it's reverse-biased with respect to proper power supply connection. But if the power supply is connected backwards, this diode will short-circuit the power supply so that your amp circuit's components aren't damaged. If the power supply is a battery, it will make the battery overheat and possibly leak, but that's preferrable to frying your op-amps and buffers.

You can use any old diode here, but standard types tend to be rated for high voltage and relatively low amperage. The ideal part for this application is a Schottky diode — these are typically only good for 20-40V, but higher amperage than standard diodes. Perfect. The standard Schottky for the board is the 1N5820-5822 series. Alternately, the silicon 1N540x series diodes will also work. The main advantage of the latter are that they are available in more places.

Optional? Yes, do not jumper.

D2

This is a CRD for biasing the op-amp into class A. This buys you better sound with most op-amps, at the expense of power drain. Most chips benefit from about 1mA of draw. The most widely available line of CRDs is the 1N5283-1N5314. There are others. The main thing to look for is the DO-35 package style.

Optional? Yes, do not jumper.

Are 1/8W Resistors Sufficient?

1/4W resistors are the most readily available sort and the board will accept standard 1/4W resistors, but it takes extraordinary circumstances to make the circuit put more than 1/8W through one of the resistors.

If you put a dead short across the output of the amplifier and play music through the amp, you could potentially damage an 1/8W R8. If you go up to 1/4W, you'd need to stack output buffers to have a chance at damaging an 1/8W R8. With headphones plugged in, the headphones limit the current the amp can put out, so R8 can only be damaged if you're running the headphones so hard that you're likely to damage the headphones or your ears before R8. If you try to make the amp power speakers directly, you could damage R8; you deserve to smoke some resistors if you try this stunt.

Resistor Sizes

The resistor pads on the MINT board are only 300 mils apart, which limits the size of the resistors you can use. Standard 1/4W metal film and carbon resistors will fit in the board without a problem. If you use Vishay Dale CMF series resistors, use the RN55 series, not the RN60s. These are 1/8W, but as I explained above, 1/8W is sufficient. RN60s will not fit in the board without creative mounting.

Choosing a Power Supply

The MINT is designed to be battery powered. If you're going to use multiple batteries, it's simplest to put them in series for a single voltage supply. You could also put two batteries in parallel for more current and longer run time, but at a lower voltage.

If you want to use a wall power supply, see the corresponding section in the PIMETA docs.

Choosing an Op-Amp

The op-amp (operational amplifier) is the chip that does the actual amplification in the MINT circuit. It has the single biggest effect on sound and power draw of any component, so it behooves you to pick this part carefully.

The most suitable op-amps for the MINT are dual-channel FET-input chips. You can only use SO-8 packages on the board without getting creative. Suggested chips are the AD8620, AD823, OPA2132 and OPA2227.

You can get Analog Devices chips through Newark, RS Components, or direct from Analog's web store. You can get Burr-Brown chips from Digi-Key and RS Components.

For more details about op-amps, see the companion article, Notes on Audio Op-Amps.

Choosing an Enclosure

The MINT board is designed to fit in mint tins along with two 9V batteries or several AAA cells. Cases similar to this size will also work well. If you want to use a significantly larger case, you're probably better off using some of that space for more features, so you wouldn't want to use the MINT.


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