by Eldon Pawliw
I started this project after reading an article in Stereophile magazine called “Aunt Corey’s Buffered Passive Preamplifier” by Cory Greenberg (Stereophile, November 1991, pg. 91). My version can be both a great headphone driver and preamp. The 4 ohm output of the buffers will drive headphones without reaching their maximum output current. The idea with buffers is that with unity gain you should be minimizing any distortion added to your signal. And trust me, the idea works.
I’m going to assume that you’ve got some experience building circuits. If you haven’t, you may want to be sure you have access to someone willing to help you out with this stuff. The schematic may look simple, and it is. But if you’ve never done this stuff before, things can get really confusing really quick.
The headphone driver schematic is a drawing of a one channel (figure 1). R2 is a DC offset pot. Use it to set the output offset to 0.0V. You’ll need to build the whole thing x2, and that includes the power supply. This design needs separate power supplies for each channel. I only used one transformer, but past there, everything is in pairs.
If this project has a weak link, it’s the power supply (figure 2). It is kind of weak, but considering the load its running, it’ll work just fine. But for you maniacs out there, if you can find a schematic for a good solid power supply, I’m quite sure it’d only do you good to use it.
I built my circuit on a simple breadboard with everything hardwired. PC boards have pretty feeble conductors, and are also pretty costly to have cut. Make sure you use star grounding for everything, i.e., connect ALL of your ground wires to the same point. Yes it does matter. If you start sending signals to ground through your chassis, you’re going to get nothing but trouble. Whether or not you ground your chassis is your decision. I left mine floating, but Mr. Greenberg claims that chassis grounding can eliminate some noise problems.
I’m a strong believer in the quality of all of the components in your system, and I always try to buy brand name semiconductors and passive components. If you’re on a budget, you can scrimp a bit on the power supply, but bear in mind that in a circuit like this, the supply is a pretty big factor. However, I believe that the signal path is still the most important circuit. You’ll want to buy the best wire and connectors your budget will allow.
My personal choice was Kimber PBJ interconnect wire. You can get it single strand, in many pretty colors, and not too many fins per foot. My RCA connectors are Rhodium plated Cardas connects. The headphone jack should be a Neutrik 1/4″ locking, chassis-mounted jack. Not bad for the dough.
And don’t forget, your R1 pot is probably the most vital component, so splurge and enjoy. What impedance you get depends on your source. You want to match it to the output impedance of the source. If you can’t find this out, you’re going to be safe with anything between 200 – 500 kohms. WARNING: I’d really advise AGAINST Alps pots. I’ve already gone through 2 of their top of the line “Black Beauties”. The connecting pins always seem to come loose and cause alot of distortion. Considering the price, and lack of consumer support, I know there are better deals out there. I’ve been told that Bourns builds a pretty good pot for the money, and if you want to go insane, ($200 Am.) go for a Penny and Guiles. If you’re going to build this as a preamp, use an Electroswitch four position rotary switch. Once again, not cheap, but definitely worth it.
Now for the piece de resistance, the center of it all, the buffers. Mr. Greenberg has found an excellent selection, although a little on the pricy side. (Mine were $35 Cdn a piece for one model below the milspecs.) Analog Devices’s BUF03-J series are extremely fast, very clean buffers, with an fc in the MHz range, and I think the slew factor was in the range of 12V/usec. Darned impressive pieces they are and only $25 a piece.
As far as other buffers go, you don’t need to stick with the Motorola unit. In fact, any buffer you choose will work – it’s just a matter of how well. Things I take into account when choosing a buffer are:
- Output impedance – The lower the better (4 ohms on the Motorolas)
- Slew Factor – The faster the better.
- Bandwidth – I wouldn’t go with anything below 1 MHz
- S/N – The higher the better.
Where do I get all this stuff? There are a number of high end audio parts suppliers around, but they aren’t usually that easy to find. I’d suggest you look for “Electronic parts” or something similar in the yellow pages, and head over to the closest parts barn. These guys should be able to set you up with the majority of your parts, including the Motorola buffers. Be forewarned though, alot of these guys figure that audio is audio, no matter what the source is, and all the parts are going to sound the same. Have faith.
The quality of all the parts does matter when all is said and done. Always use metal film resistors with the tightest tolerance you can get. There is one small exception to the quality rule. When you pick a higher class of buffers, remember you are only paying for the amount of testing done on each unit. The military grade buffs come off the same line as the cheapest consumer grades. The milspecs, however, will be much better matched to each other, and will more closely approximate ideal buffers.
Oops, guess I’m a little off topic. Anyways, your local guys can get you the majority of this stuff, but for the more specialized parts, you’ll have to go to a distributor. To date, I haven’t been able to find a good distributor. If anyone knows of one, please do tell.
A word on Diodes
It is generally thought that the only important stat for diodes is their ability to pass current. Not true. A good diode sports such attributes as current handling, recovery time and ring suppression. Generally, the bigger the current handling, the better the diode for the other traits, but not always. I used Motorola MUR 4100E diodes throughout. They are a 4A ultrafast recovery diode. To counter any ringing effects, run them in parallel with 0.01 uF ceramic or tantalum caps .
Well, I guess that’s about it. I really recommend this project simply because for the money I spent, I couldn’t have bought a better headphone driver/preamp. Anyways, if you do decide to go ahead with this, send me a letter, even just to say you’re trying it. I’m curious to see what kind of success everyone else will have with it. And if you want a hand getting stuff, or just want building advice, I’m not the best, but what do you want for free?
5/1/99: Corrected orientation of diode D6 in power supply (figure 2).
12/29/99: The BUF-03 buffer opamp used in this project is actually made by Analog Devices, not Motorola. The BUF-03 has been superseded by the BUF-04, which is pin-compatible with the BUF-03. Thanks to Walt Jung for pointing out the mistake.
2/18/01: Corrected transformer specification and orientation of diode bridge in power supply.
c. 1998, Eldon Pawliw.
From the original preamp article by Eldon Pawliw (currently not available online).