-------------------------------------------------------------------------------- INTRODUCTION -------------------------------------------------------------------------------- These two DOAs are based on the Melcor 1731 opamp. These are "character" amps, so the focus was on maintaining "spectral footprint" while cleaning up the performance a bit - and keeping slew rate somewhat symmetrical to maintain some of the "slewing goodness". You will have to do a "homework", though - you will need to "tune" the output stage to your actual devices. It's described below. Also, both circuits are somewhat streamlined, easing it up for the DIYer. One of the main considerations was to keep the component cost below 2 EURO. In other words: If you want clean, go and buy yourself a stash of metal-can LME47910's. They work as advertised. SPECIAL MEMO: You will need to "tune" the output stage to your actual devices. -------------------------------------------------------------------------------- CONSIDERATIONS -------------------------------------------------------------------------------- PERFBOARD LAYOUT: Before you embark on a PCB journey, you will probably need to build a prototype to tune-up the output stage to your actual devices and also perform some "reality checks". A "canonical" perfboard layout is enclosed. COMPONENT VALUES: Much care was taken to unify and simplify component values, so builders can buy f.e. 10 pieces of THIS value and not 2 pieces of THIS, 5 pieces of THAT and 3 pieces of the OTHER value. Also, some "odd" values can be combined with paralleling one of THAT values. HEATSINKING THE OUTPUT DEVICES: The output stages are designed to operate in Class-A, with quiescent current cca. 25mA or more. There is no temperature compensation circuitry included (which would interfere with "purity" of design) and that means there is always danger of thermal runaway and thermal breakdown. In other words: Use a heatsink. This is Class-A. Alternative: Both circuits use JLH-esque "modulated current sources" at output, so they can be pulled back into Class-AB operation if quiescent current is set at cca. 15mA or less, but that is up to the individual builder. Note: The MelChiOr 1337 circuit ties both output devices collectors together, making it very easy to mount them on a heatsink without any insulation. The heatsink will act as an antenna in such case, so check out for any instability or oscillations to be sure everything is OK. If you plan to use two-sided PCBs, you can also use area pours on the top layer instead of a separate metal heatsink. Be generous with copper. -------------------------------------------------------------------------------- TUNING -------------------------------------------------------------------------------- OUTPUT BIAS CURRENT: The two 12R resistors at output transistors determine the Class-A bias current, which should be set at approx 25mA (20-30mA). Suggested output devices are BD139-16 and BD140-16 (i.e. "16" suffix). The useful value for the two resistors will probably be in range from 10-22 Ohm (both equal value), depending on the actual batch of the output transistors. Lower resistor values will result in larger Class-A bias current. Once you find the optimal resistor value, it will usually work for the rest of the batch. Note: You can use other output devices (f.e. SMD builders), as long as they have Hfe in range 200-250. Perhaps try Zetex FZT849/949 pair. "VOICING": The two 220pF capacitors can be increased to cca. 470pF to alter the "transient response", i.e. you can use 330pF, 470pF etc. (both same value). You will have to decide if this has any merit, though. -------------------------------------------------------------------------------- SIMULATED SPECS -------------------------------------------------------------------------------- Power supply: +/-15V, min.60mA @ 21dBu output, depending on Class-A bias current Supply current at no load: 30-40mA typically, depending on Class-A bias current Output swing: cca. +/-13Vpp, depending on actual devices Output load: 600 Ohm Slew rate: +/-2.3V/us Unity-gain bandwidth: - MelChiOr 1337: 6.8MHz - MeJLHior 1337: 3.3MHz -------------------------------------------------------------------------------- REALITY CHECK -------------------------------------------------------------------------------- When you are done with the basic tuning, you can try the following: Set up the DOA in a basic preamp (etc.) configuration. Load it with nominal 600 Ohm load and set it up for full 21dBu output. Check out your heatsinks, power supply and connections. Now, with the full-swing output, SHORT the output load. Like - bypass it with a piece of wire. Leave it running and observe. If your DOA survives THAT, it will serve you well for years to come! Congrats! Your particular DOA is now a member of SPECIAL OPERATIONAL UNITS! WELL DONE! -------------------------------------------------------------------------------- EULA -------------------------------------------------------------------------------- Both MelChiOr 1337 and MeJLHior 1337 circuits are offered completely free for DIY (do-it-yourself) use. You can build as many as you wish, without restrictions in this regard. You can also freely publish modifications (or derivatives) of these circuits, PCB layouts, tips and tricks of usage, provided they stay free for DIY use and of non-commercial nature. RESTRICTIONS: You ARE allowed to use these circuits for commercial work (i.e. in a recording session), but you are NOT ALLOWED to sell for profit: schematics, DIY kits, PCBs and devices that use these circuits (or their derivatives) embedded, like microphone preamps, equalizers and similar recording or mixing equipment. EXCEPTIONS: Members of Prodigy-Pro forum are granted some exceptions from the non-commercial usage rule outlined above: 1). Said members can design and sell PCBs (based on or derived from the perfboard layout provided by author or designed in entirety by themselves) for both circuits in question to other members of said forum, provided they ship to the author 5 pieces of each unique design, free of charge and at their own expense. 2). Said members can sell DIY KITS of both circuits in question (or their derivatives) to other members of said forum, provided they ship to the author 3 pieces of each unique kit, free of charge and at their own expense. Note: each KIT must be in advance fully tested (a testing perfboard layout is supplied by the author); every vendor must assure that the KIT offered works out-of-the-box flawlessly. Also, each vendor must offer full technical support for their offerings. 3). Said members can sell pre-assembled units of both circuits in questions (or their derivatives), provided they ship to the author 3 pieces of each unique embodiment of devices in question, free of charge and at their own expense. Note: each pre-assembled unit must be in advance fully tested and every vendor must assure that devices offered work out-of-the-box flawlessly. Also, each vendor must offer full technical support for their offerings. No "embedded" commercial offering of both circuits in question is allowed. I.e., you are NOT ALLOWED to sell any preamps, equalizers or other mixing/studio equipment or similar devices that would use circuits in question (or their derivatives) as a part of the design. A note to potential PCB/KIT/UNIT vendors: you will likely pay more for postage.