Preamp section for active monitors?

Got my the YSM1p's in today, hooked them up.

First off, they sound pretty nice. Not as nice as the type of home audio setup I am used to, but a drastic improvement over my previous Klipsch computer speakers.

These monitors have a "trim" knob on them that adjusts gain from -6db to +9db. Since they are capable of boosting gain, not just cutting, they must actually have a full powered preamp section complete with op-amps, not just an attenuation pot.

Also, the -6db setting is nowhere near tame enough for casual listening. These things can put out some sound when they're in close proximity to the listener.

Further, adjusting levels through soundcard controls is a no-go. Applications frequently reset the volume to maximum, and disabling application control of the audio drivers results in less than ideal results from Cubase. Also, changing levels while some application is currently playing material doesn't actually change anything, which is a pain if winamp or something similar is playing back something from the mixing clinic, and it's too loud. You have to stop the stream, shut down winamp, change the level, and restart, making sure to disable application control, and remembering to reenable it before launching Cubase.

Solution? I'll be looking for a simple stereo preamp, passive if possible, but it doesn't really matter... a couple of op-amps isn't going to degrade the sound enough to notice - no worse than a mixer does.
 
I think the mixer setup I found is pretty clean; I believe I'm bypassing all the mixer preamps and working with only a line level signal. It has an advantage in being
c h e e p . :cool:
 
I'll look into a cheap behringer mixer. It may be less expensive than a decent stereo preamp.

But, a preamp would be a little less bulky to have around my computer workstation. I don't have anything setup for a flat mixer style component, but something like a preamp that sits on a shelf and has a gain adjustment on the front would be very practical.
 
Not bad for $80. I'll put it on the "short list."

What I would really love is a single stereo passive pot in a box. Two inputs, two outputs, no power required, very simple circuitry.
 
Bah, after some reading/researching, it seems that a passive pot actively buffered is the ideal solution for such a preamp.

Maybe I'll just make one myself. Probably would be of better quality than anything I could buy for ten times the price.
 
Lucid posted an image of the mixer I ended up getting - a little Eurorack for $60. I use it as a patch bay. But why wouldn't an audio buddy work just as well for what you want? Active, but clean.
 
I'll keep those in mind. I've promised myself I wouldn't buy any more gear until after Christmas. Hell, I've spent $1500 and haven't laid down the first track yet. :)

I found an interesting article from the Stereophile archives on making a really sweet stereo passive preamp, so I'll look into that too.
 
A passive preamp would just be a stereo pot (or pair of monos) to attenuate the line level input from maximum (straight through, no attenuation) to minimum. No gain is possible, but in my case at least I sure as hell don't need any gain. This would be the same as using a passive fader from a mixer.

Wheras active preamps (using op-amp circuits) treat the source as a voltage source only, passive preamps (incorrectly called preamps btw, since they don't amplify anything... more accurately called passive attenuators, but so much for popular verbage) treat it as both a voltage and current source.

The drawback is that the input and output impedence is rigidly linked. If the input impedence is too low, the source may not be capable of providing the current draw the circuit is expecting. Similarly, if the output impedence is too high there could be capacitive roll-off in speaker cables (line level) that are too long (and, for a 100k-ohm pot the speaker cables have to be very short to retain high frequency detail). Mixers will have this problem as well. It could be the case that the small Behringer mixer is causing some cloudiness in the sound, especially in the high frequencies, that shouldn't be there.

Typical potentiometer ratings for "passive preamps" are in the 20k-ohm range for a good balance. A better solution is using an active buffer, which is essentially a very high quality very high grade op-amp, AFAIK. Military grade buffers intended for video signaling have incredible slew rates, near infinite input impedences (in the trillions of ohms range), and output impedences of just a few ohms. Very nice. And, much more transparent than the IC full op-amp designs found in most powered preamplifiers.
 
bigus dickus thanks for the explanation. that is where my confusion came in; the fact that passive preamps are called preamps, because like you said, they don't amplify anything.

please explain the mixer cloudiness to me again... i'm not quite sure i understand. cheers...
 
Source devices (soundcards, CD players, etc.) typically aren't capable of outputting much current. They are designed to control signal level based on voltage output alone. An active/powered preamp buffers the source from the power amplifier, using its own beefier output stages to drive the downstream cables.

Problem (1) with passive preamps: as the volume pot decreases in impedence, the current drawn from the source (soundcard, CD player) increases. This can result in a lack of low end definition and dynamic range, as typical sources (especially op-amps) aren't designed to supply much current. Possible solution: use a higher impedence volume pot (100k-ohms).




The output impedence of a preamp (passive or active) coupled with the shunt capacitance of the downstream cables forms a first order RC filter which rolls off high frequency response at 6db/octave. Typical powered preamps have output impedences low enough to put the roll off frequencies well out of the audible range.

Problem (2) with passive preamps: since the output impedence is directly related to the the volume pot impedence, it is typically very high. This causes an RC filter well into the audible range that can muffle high frequency response. Possible solution: use a lower impedence volume pot (5~10k-ohm).




You can see that in purely passive preamplifiers problems (1) and (2) are related, as is their solution, since output and input impedence are directly coupled. The best compromise is around a 20k-ohm pot, which most quality equipment is capable of driving, and when used with relatively short cables won't cause much roll-off in the high frequencies.

A better solution is to use a buffer, which is precisely what powered preamplifiers do. Unfortunately, all but the most expensive preamps use underwhelming quality components... typically just cheap op-amp circuits with mediocre slew rate. Military grade video buffers have out-of-this-world specs, and can be used to make a "passive buffered" preamp (that is really a powered preamp... love the mixed terminology) that is much much simpler than commercial designs, and much more transparent as well. About the only drawback is they are just a bit noisier than an IC op-amp, but that would only be noticeable on the most sensitive amplifiers and speakers at very high volume levels.

You take the "essence" of what makes a powered preamp powered instead of passive, remove all extraneous pieces, and then use mil-spec components that were designed for applications with orders of magnitude greater bandwidth demands. It's simple, and not-so-expensive (compared to $1000+ preamps from Bryston, Krell, etc. that aren't of any better quality, and possibly even lower), but getting a hold of the mil-spec parts seems less than easy. I'm looking into it. Should be around $100 for all components, if they can be purchased.

I'm not sure what the specs on the Behringer or Audiobuddy mixers are, but if you are bypassing the op-amps and just using the faders as a passive pot attenuator, then you are likely not getting either the dynamic range and low end response, or undamped high end response that you should be (or both, in the case that the pot is fairly low impedence but fairly long cables are being used between mixer and amplifier stages).

Hope that answered your question.
 
thanks BD! how can i find out if my behringer mixer is clouding the sound, short of opening it up....?
 
I guess the only way is an A/B comparison with a high quality preamp. Short of that, I don't know of any other way. There probably aren't any "measurements" that could be done other than some checks of input and output impedences that would let you know which end you might experience problems with.

Remember though, this is talking about preamps in the snobby world of "golden eared" audiophiles. So the loss of high frequency detail or dynamic range or low end clarity might be very, very subtle. My suggestion would be to use as short of a connection from mixer to active monitors as possible, and not worry about it. ;)
 
the XLR cables (balanced) running from my mixer to the monitors are about 3m long. thanks for the info man...
 
Built it yourself! If you are a little bit handy there are tons of schematics on the net for good quality preamps. And they cost almost nothing. Plus, you can build them in anywhere you want!
 
Yeah, I've been looking into building one myself. Haven't really decided on whether to go full passive or a buffered design.

I think the active buffer is clearly superior simply because it is more practical, since you don't have to worry about source current supply or impedence, or downstream cable length (some passive designs I've seen suggest keeping output cables to the amplifiers - the speakers in the case of active monitors - to a foot or less... wow).

But, a pure passive design is much simpler, and cheaper.

Atari, if you have any good links for schematics, that would be great. I've run across several, but I'm trying to understand a little better the differences between them, and what changes are best suited for particular applications.
 
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