Warning: We usually write long responses. Read only the bold portions for a summary.
Q: There are several other 500 Series mic pres on the market already. What makes this one different?
A: It is a given that every microphone preamp, no matter how excellent, cannot achieve theoretical transparency and therefore will impart some of its own sonic character on the signal (and many recording engineers exploit this feature to help give each track an enhanced quaity and/or distinct position in the mix). Indeed, other 500 Series mic preamps are no exception. They're fixed designs, each with a particular sonic signature. The designers have worked hard to create something they think most users will deem awesome, but maybe you've tried all of these deisgns and still don't have what you need for a given application.
In contrast the DIY500, while relying on the same elegant and time-tested topology as most of its competitors, is entirely user-configurable. With a Minimal Kit and some guidance the builder can choose from over 20 discrete op amps, more than 50 input transformers, and at least 12 different output transformers to achieve something even closer to his or her ideal sonic color (or lack thereof!).
The pairing of high-quality audio transformers with a discrete transistor op amp is what made the sound of some 1960s and 70s mixing consoles so coveted. The process of selecting and mating these devices is how most other manufacturers of 500 Series mic preamps are coming up with their designs. They might change a single transformer, keeping everything else the same, and market this as a new module. Why should you have to pay upwards of $500 all over again to obtain this simple timbre variation? With the Eisen Audio DIY500 you can install the alternative transformer yourself, save money, have fun, and learn something along the way.
Why should you be stuck with the combination we like best? Pick your own parts for a nearly limitless variety of tone!
Q: Does the DI go through the input transformer?
A: No, because a microphone input transformer is designed to be driven by a microphone, which has a very low output impedance of around 200 Ω. The input transformer presents an impedance just high enough to bridge the microphone signal, but not nearly high enough to bridge or even match the output impedance of most instrument sources. For example, a passive bass guitar pickup could have an output impedance of 1 MΩ (read: 1,000,000 ohms). Connecting it to a microphone input would cause a severe impedance mismatch, resulting in a loss of signal level, detail, and bandwidth. Correcting this mismatch is precisely why direct boxes were invented; to inject unbalanced, high-Z line level into balanced microphone inputs.
Because, however, most op amps have a non-inverting input impedance of 1 MΩ or greater, it is okay to send an unbalanced instrument output, or any line-level signal, straight in. Plugging into the DIY500 1/4" front panel connector bypasses the input transformer and does just this. The technique is nothing fancy or new, and is shared by some of our competitors.
Q: Well, what does that sound like?
A: Whereas sending the DI signal through the mic input transformer would sound muffled, sending it straight into the op amp just sounds rather plain (depending on which op amp and output transformer you use, of course).
To be quite honest, the 1/4" input was intended to serve more as a means of securing the front panel than as a full-fledged DI. While you can form your own opinion, we think it is functional, but nothing special. Eisen Audio recommends using your favorite direct box in tandem with a DIY500 mic preamp for optimal performance. OR if you want to, it is possible to use a direct input transformer instead of a mic input transformer and make your DIY500 a dedicated instrument preamplifier.
Q: Do I need a scope? What other tools are required to build a module?
No, you do not need an oscilloscope, but a digital multimeter (DMM) is recommended for testing continutiy (e.g. to make sure your power rails aren't shorted to ground or something like that). Of course, if you're careful to put all of the components in correctly and make good solder joints then there's no need for any test equipment. Just plug it in and enjoy! The following tools are required...
- Fine-tipped, temperate-controlled soldering iron for PCB work
- Your preferred type of small-diameter, rosin-core solder (0.025” or 0.031” recommended)
- Component lead cutters/shears (wire cutters)
- 20 to 24 gage wire stripper (for output transformer leads)
- #1 phillips screw driver
- 0.05” hex wrench/allen wrench (for control knob set screws)
- 1/2” and 7/16” nut drivers or wrenches, or adjustable wrench (for securing pot and 1/4" jack to front panel)
Q: How can I tell what each transformer or op amp is going to sound like? Is there a resource to help me figure it out?
A: While compiling this type of subjective information is much harder than listing specs, we are slowly making progress. Such a resource will be available in due course. In the mean time, here are a few guidelines to get you started, based on my findings thus far. This is a work in progress, and by no means a definitive guide.
Transformers:
- In general, a transformer with a lower turns ratio (e.g. 1:1 or 1:2) is going to sound more transparent. Granted, the higher the turns ratio, the more nonlinear it gets. However, an excellent deisgn will overcome this characteristic.
- Core size is also a determining factor. To generalize, the bigger the transformer, the more level it can handle, and the less likely you are to overload it. Apply too much level to any transformer and you saturate the core. This saturation, usually due to low frequencies, can produce pleasing distortion.
- Physically tiny transformers saturate very easily and tend to be the most colored. They usually cost less too. In many cases if the core is small enough, e.g. Beyerdynamic (you've heard them in the 3M M56 and Ampex MM1100), it will start to roll off low frequencies and push the midrange forward. Although I've noticed most Lundahls are an exception to this rule.
- Core material is the other easily identifiable factor in regards to color. Nickel (aka mumetal) and Steel (usually M6), or a comibnation of the two, are most common. Nickel, used almost exclusively in microphone input transformers, is cleaner. It passes high frequencies very easily, but can't handle as much bass per equal volume and distorts more abruptly than steel. Steel is very much the opposite. It has a more color/less detail, can handle lots of bass, overloads more gradually and at greater levels, but causes more loss - especially of high frequencies.
To summarize, when in search of color look for smaller and less expensive input transformers with a higher turns ratio, and/or steel output transformers, also with a turns ratio greater than 1:1 if possible. Beware of exceptions.
Discrete Op Amps:
- While there have been many, it is best to discuss only the few we can easily obtain. The John Hardy 990C is least expensive and probably the most transparent, although it has a hyped low end to my ears. I find that it's a good default, mating well with almost any transformer, letting you hear more of the transformer, and it can drive very heavy loads. The only downside is that it clips quite abruptly at about +22dBu output, and I think this clipping sounds pretty unpleasant.
- I have less experience with the Purple Audio KDJ4, but sofar it seems to be slightly less transparent than the 990C, arguably in a good way, while sharing the load driving capabilities. The low freq response is more relaxed. $65 instead of $50.
- As far as I know, the Seventh Circle Audio SC25 is supposed to be a 2520 replacement. Therefore, it should have that unmistakable aggressive midrange character. $75. Let's stop there.
- Aside from the JLM Hybrid op amp, which is self explanatory, all the other choices are JFET op amps designed by Fred Forssell. They each have a distinctive character, which I can't describe well at the time of writing, and share a common flaw (at least that's what I'd call it)... trouble driving heavy loads (e.g. 600ohms or less), especially through an output transformer. Best to run these guys unbalanced out when in doubt. $65 and up.