DIY500 mkII Component Values

DIY500 mkII Component Values

To turn your DIY500 mkII Minimal Kit into a complete mic preapmp, you'll need to purchase the rest of the components. This means choosing an op amp and transformer combination, purchasing these from their respective vendors, and then purchasing about $5 USD worth of resistors and capacitors from a parts vendor such as Mouser. The descriptions below list these component values and explains how to determine them.

Component designations (e.g. "R2") refer to the DIY500 mkII schematic, and are printed on the PCB surface.

- N/A means leave that component out altogether
- Short means short circuit, or 0 ohms. Use zero ohm resistors or straight wire, such as clipped resistor legs.
- All resistors are 1/4W %1 metal film
- Resistor values are written in ohms, e.g. 6k8 = 6.8k = 6800 ohms

D1, D2, D3 = DO-41 size rectifier diodes (1N4003 is a good value to use) in series with the power supply voltages to safeguard against polarity reversal, e.g. if momentarily hooking up the power supply backwards or improperly installing a module while the power is on. If you don't think this is going to happen, instal shorts in place of D1, D2, and D3 for 1dB greater headroom.

CONN3, CONN4 = N/A under normal circumstances. These are used during experimentation with different op amps and circuit configurations for temporarily shorting out the I/O coupling capacitors, C4 and C9. R9 and R15 can be stuffed as shorts for permanent shorting across C4 and C9. If in doubt, leave them out.

CONN5 = For connecting/disconnecting the output transformers secondary loading resistor, R23. Normally 1k2 ohms, this resistor guarantees that colorful-sounding steel output transformers are terminated with a minimum load for purposes of saturation and no ringing. Normally this connection can be made permanently by shorting across position 2 of CONN 5. N/A (leave out altogether) if using a transformer with some nickel in it for less color and greater headroom.

CONN6 = If you're using an OSA (Old School Audio) lunchbox/rack, then install CONN 6 (3 position 0.1" pin headers, e.g. Molex C-Grid type) and make sure the mating connector (2 position shunt connector, e.g. Sullins 9001) is in position 2. If you don't want to deal with the header/connector, then solder a short across pos 2 of CONN6 PCB pads. If you're not using an OSA lunchbox/rack then N/A, forget about CONN6.

C1, C13, C14 = 0.1uF 50V or 100V, polyester film capacitor, 5mm radial. These fim caps bypass their accompanying electrolytic caps at high frequencies for better filtering of power supply EMI/RFI noise and better transient response.

C7, C10 = Normally N/A. Would be 0.1uF same as above, but should be left out under normal circumstances.

C2 = 10uF or greater (ideally 68uF) 63V radial electrolytic capacitor. Provides decoupling and slight ramping of the phantom power supply when front panel switch is depressed (the top most of three switches with an optional LED to the left of it).

C3 = Normally N/A. When required, forms a zobel network (in other words tuned termination, a filter) with R7 to tame high frequency ringing of the input transformer response. Rarely required. If in doubt, leave it out.

C4 = Normally N/A. Usually between 1uF and 10uF (4.7uF is an excellent default) 16V or greater. 5mm radial film capacitor (C4) or bipolar/non-polar radial electrolytic (C4ALT). Provides input coupling (and DC blocking) for op amps with significant DC leakage at the + input. It is primarily just the Purple Audio op amps that require C4, but it never hurts to install this capacitor if you also install R11 to drain it.

C5 = Normally N/A. 220pF 50V or 100V required for the JLM Audio op amps. Almost never necessary otherwise. Usually C0G/NP0 ceramic capacitor, axial (C5) or 5mm radial (C5ALT), but mica and any type of film will also work.

C8 = 100pF by default. Some op amps like it lower (e.g. 68pF for JLM or Purple Audio, 91pF for John Hardy 990C, 33pF or 22pF for the Forssell Tech and Millennia Media op amps), but very rarely must C8 be any higher than 100pF. Most any op amp will function with 100pF. Same voltage and package requirements as C5 (see above).

C6, C9 = 470uF or greater 16V bipolar/non-polar radial electrolytic capacitors. Must not exceed 1" (25.4mm) tall or 16mm diameter. If R13 is 68 ohms or greater, then you can probably get away with 330uF (or even 220uF with R13 = 100ohms) before low frequency response starts to roll off at high gains. C9, however, should always be 470uF or greater (1000uF if you can find a part that fits). Some op amps demonstrate no DC offset unless they fail, in which case you can leave out C9, but we don't recommend it.

C11, C12 = 100uF 25V radial electrolytic capacitor. Provides decoupling of the +/-16V power supplies.

C15 = Normally N/A. When required, forms a zobel network (in other words tuned termination, a filter) with R22 to tame high frequency ringing of the output transformer response. We have yet to come across a situation where this is required, so leave it out.

LED1 = Standard 3mm diameter leaded LED in whatever color you please. Indicates phantom power is ON (when SW1, top switch, is depressed). Optional, so you can leave it out too if you please.

OLI = Normally N/A. Jensen and John Hardy make an Output Load Isolator (a 39ohm resistor wrapped in a 4mH inductor to make life easier for the op amp when it is driving heavily reactive loads (e.g. an output transformer, followed by a long cable). Designed specifically for the 990C, but will work with other op amps. In our experience most op amps work just as well with out it, performance is still perfectly acceptable even when they do work better with the OLI, and in some rare cases the OLI causes problems. For this reason, we suggest leaving it out, always. Install R14 instead.

R1 = 100 ohms, as indicated on the PCB silk screen. In series with the 48V phantom power supply, R1 creates a decoupling network along with C1||C2 to help isolate and quiet your preamps portion of the phantom power supply.

R2, R3 = 6k8 ohms. Used for applying the phantom power. This is an international standard, so don't mess with it! Some people say these resistors should be matched to 0.1%, but we've found no measurable improvement in performance over standard %1 matched.

R4, R5 = Normally 680 ohms. Used to create the series portion of the PAD (input attenuation) function (the center switch of the three). 680 ohms yields a -20dB pad when R6 is 150 ohms, or 18dB when R6 is 200 ohms.

R6 = Normally 150 ohms. Creates the shunt portion of the PAD function, as mentioned above Occasionally, transformer manufacturers will specify 200 ohms or 160 ohms, but 150 always works.

R7 = Normally N/A. When required, forms a zobel network (in other words tuned termination, a filter) with C3 to tame high frequency ringing of the input transformer response. Rarely required. If in doubt, leave it out.

R8 = Provides secondary loading of the input transformer to reflect proper input imedance to the mic and tame any rise in high frequency response. Value varies depending on input transformer. 100k is the closest thing to a universal default. If in doubt, leave it out or calculate. Multiply 150 ohms by 10, and then by the turns ratio of your input transformer, squared. For example, I'm using a 1:5 ratio input transformer. 150 x 10 = 1500 ohms (also written as 1k5, or 1.5k), x 5 x 5 = 37500, or 36k which is the closest standard resistor value. In many cases it doesn't hurt to go above or below this value.

R9 = Normally short (0 ohms). When C4 is N/A (almost always, unless using Purple Audio op amps) install a short (ZERO ohms) in place of R9 to short across where C4 would be.

R10 = Normally short (0 ohms). In series with the +input of the op amp, some op amps (namely Purple Audio and the API stuff) benefit from 100ohms instead. When in doubt, short.

R11 = If C4 is N/A, then so is R11, BUT if C4 is present, or when in doubt, it never hurts to install 1M (that's 1,000,000 ohms) as R11.

R12 = Always 10k, unless specifically noted in a particular circuit configuration. When in doubt, 10k.

R13 = Determines max gain, and should be limited depending on how much gain the op amp has available. 20 ohms for John Hardy 990C, Seventh Circle SC99, and Forssell Tech op amps; 27 ohms for Avedis 1122, Seventh Circle SC10, and Purple Audio KDJ 4; 51 ohms for Purple Audio KDJ 3; 68 ohms for all op amps from JLM Audio, Millennia Media and Sound Skulptor; and 100 ohms for everything else. When in doubt, 100 ohms.

R14 = Normally 51 ohms. Some op amps function better with 39, 47, or 68 ohms, but 51 is a good default for acceptable performance and short circuit protection from (almost) all op amps.

R15 = Normally N/A. In the rare instance that C9 is N/A, then install R15 as a short (0 ohms) to short across where C9 would be.

R16 = Always 100k.

R17, R18 = Normally 22 ohms. In series with the +/-16V power supply rails to form decoupling networks along with C11/C12, and also to limit current draw in the event of op amp short circuit failure (burn up like fuses). If you're using an op amp that runs very hot (draws lots of current all the time) then these resistors might get too hot and turn brown during long term use. In this case it's probably best to raise their size to 1/2W or even 1W. Normally, however 1/4W will be fine. If you see these resistor quickly burn up and emit smoke, then you have a problem. Check for accidental short circuits to ground near the op amps output and power supply connections.

R19 = Normally N/A. If no output transformer is present (unbalanced output), then install as a short (0 ohms).

R20 = Normally N/A. If no output transformer is present (unbalanced output), then install the same value as R14 (e.g. 51 ohms).

R21 = Normally N/A if your output transformer has 8 wires. If your output transformer has 4 wires, then install as a short (0 ohms).

R22 = Normally N/A. When required, forms a zobel network (in other words tuned termination, a filter) with C15 to tame high frequency ringing of the output transformer response. We have yet to come across a situation where this is required, so leave it out.

R23 = Normally N/A. Default value is 1k2 ohms when using an all steel output transformer, to terminate secondary. CONN 5 must be in position 2 (or installed as permanent short across position 2) to activate this termination.

R24 = Normally 10k. Increase value to decrease the brightness of optional phantom power indicator LED.

T1 = Input transformer. You've already chosen it if you're reading this list. Please see the input transformer table for more information.

T2 = Output transformer. You've already chosen it (or chosen not to use one) if you're reading this list. Please see the old output transformer table for more information.

A1 = Discrete Op Amp. You've already chosen it if you're reading this list. Please see the op amp table for more information.