No Preamp?

[MikeSt]

I'm using my PC as a source and wondering if I can just hook it to an amp without a preamp? The pc software has volume and eq so I don't see why I'd really need a preamp. I could just run the rca from the pc directly into the amp.

The reason I want to do it is 1) to save money and 2) I've heard stereos sound better without a preamp. The only thing I'm concerned with is if the volume would be hard to control, I've never tried this before.

I was thinking about doing this with a B&K ST-140 and my KG4's.

Also, are there amps (without preamps) that recieve digital input like from a digital coaxial or S/PDIF?

[russ69]

I never had much luck running without a preamp. The preamp is there to provide a better load into most amps. People do it all the time but I've never heard a tip top system without a preamp.

Thanx, Russ

[jwc]

I have no preamp and love it

jc

[Don Richard]

Preamp? What's that?

[PrestonTom]

There are two issues and you need to consider both.

1. Attenuation by your computer. If this is done "digitally" by eliminating bits, then you will unfortunately be decreasing the signal-to-noise ratio. With the newer sigma delta modulations schemes, the penalty is not as severe nowadays, but it is still a problem. I would not advise doing it this way, especailly if much attenuation is required.

2. "Passive pre-amp". This is one of the great misnomers, so I put it in quotes. Typically this simple means that a potentiometer is inserted between the computer (source) and the amplifier (load). However, this is where the fun begins. The source is typically a low impedance (Z output) and load is typically a high impedance (Z input). This does two things. It will maximize the transfer of voltage (but not the transfer of power). It will also help minimize the amount of power or current that the load (amp) is trying to "draw" from the source (the source, perhaps a CD player, is typically not designed to provide much power or current). Now for the fun. When you insert the potentiometer (simply a variable voltage divider). It looks like a load (remember output Z) to the source and it looks like a source (remember input Z) to the amplifier. Now, how do you simultaneously create a large impedance mismatch between the source and the pot and at the same time create a large impedance mismatch between the pot and the amp? This is where it gets tricky, since it is difficult to do both at the same time.

Digression: the value of the pot (say 10k Ohm) will be the Z input as seen by the source. However the the amp will "see" the pot as a source with an output Z of 1/4 that value, or 2.5k Ohm (approximately - this is the worst case and I left out some small effects).

There is yet another headache. The cable between the pot and the amp now acts like a low pass filter (the resistance and the cable capacitance). If the cable is long the capacitance will also increase. You need to do the calculation (Corner freq = 1 / (2 * pi * R * C)) to see if this will be a problem. Incidentally, fancy cable is not always low capacitance. This is why short cables are required.

Yet another headache: this will only work with "line level" sources. Your computer CD out is okay but phono is not for instance.

So I have presented a number of problems that you need to design around when you choose the potentiometer and amplifier. This is why "passive pre-amps" don't always work out very well. However, sometimes they can sound quite good.

Folks will use words like "synergy" or sucking the life out of the music" etc, I hope this helps explains part of the problem.

An alternative design for a "passive pre-amp" is done using transformers. Those will be out of your price range.

Now let me be positive. On e-bay you can pick up a Hafler 101 pre-amp (a "real" pre-amp) for less than $100. They sound fine.

Good Luck,

-Tom

[Daddy Dee]

Mike,

Try it out. You may be pleased with the results. I've done this before and it's worked OK.

If it's not satisfactory, you know you tried to manage without the Preamp.