RE : audio distribution

HI,
i wanna built a hifi sytem for my pc.
finished a LM3886 power amp and want to include a headphone amp and one or two lineouts.
what´s the simplest and best way to distribute the line-in to for example three stereo outs?
so far i´ve only seen an application in the TL074 datasheet.
is this recommended for a good hifi system?
should i maybe exchange the TL074 with a better opamp?
thanks in anticipation
 

Jamie F

Member
2001-12-15 12:33 am
UK
If I understand what you are trying to do correctly, a simple distribution amplifier circuit, i.e. an op-amp buffer to distribute the input to the three driver stages, then an op-amp buffer to drive each cable.

The input buffer should have its input impedance set for compatibility with whatever the input source you are using (usually 10k). The output drivers should have a small resistor in series due to cable capacitances (75R-100R).

Look at earlier threads for advice and experiences with better opamps, but then you are using a PC.....
 

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
> what´s the simplest and best way to distribute the line-in to for example three stereo outs?

That TL074 datasheet is selling op-amps. Unless you expect "many" of your outputs to be shorted, you do NOT need individual drivers for each output. Even if you expect shorts and odd loads, you really do not need multiple drivers.

My first repair task at a radio station, {mumble} years ago, was a Distribution Amp, 24 outputs, each one supposed to be loaded in 600Ω but could be connected to any dang thing (accidents happen). No matter what, "good" outputs "must not" be affected by shorted outputs. It had just one boosted op-amp to drive all the loads.

You build a low impedance amplifier, then put a bunch of resistors on the output:
[IMGDEAD]http://music-club.rutgers.edu/headfonz/DistAmp.jpg[/IMGDEAD]

The buffer amp needs to have an output impedance MUCH lower than the build-out resistors. Ordinary op-amp chips have Z(out) under 1Ω over most of the audio band.

The value of the build-out resistors should be much higher than the buffer output impedance but much lower than the load impedance (or equal to the system impedance for older matched systems, but these are obsolete). If you are fussy about levels, make the build-out resistors 10 times smaller than the load impedances for 1dB loss, which is usually negligible.

However they should be larger than the amplifier's minimum load impedance. If you used 10Ω resistors, and some idiot plugged in a shorted cable, the poor TL074 would distort like mad trying to drive a 10 ohm load. However, a speaker amp would be fine with that.

In my work, I assume hi-fi type loads are 10KΩ and that less than 1dB loss is nice, and I sometimes drive long lines, so I use 470Ω. But in general hi-fi work, 1K or even 2.2K is fine. The TLO74 will comfortably drive one shorted line behind a 470Ω build-out resistor, and maybe two shorts at 2VRMS without distress. I have a dozen outputs, and a professional better than me says "assume 1/3rd of lines may get shorted". So my distribution amp has to drive 470/4= 120 ohms, and needs to be somewhat stronger than a single TLO7x chip. (I also have several shortable 150Ω runs so my buffer is beefy.)

For stereo, you only need 2 op-amps, so a TL072 is plenty for any reasonable number of output, if you do not have to be very concerned about multiple shorted loads.
 
Thanks PRR for the insight and your experience with such circuits.

Cool the thread´s still alive.
So far I actually just tapped off the OUT from my soundcard to feed the active speakers and headphone amp.
Connecting the hifi to the PC I have to replug a little but it doesn´t matter so much and works quite nicely.

Still the question is in my head as I´ll plan some preamp soon.
Maybe something like this active pot which won´t need any buffer at all methinks.

As the Bride of Zen will be another option for the preamp I´d probably have to take the output impedance a bit more into account as it ranges from 0 to 1.5kOhms depending on setup.

Talking about buffers...
What about a simple mosfet follower.
Any drawbacks with that kind of circuit?
You could use it as headphone amplifier (like the Szekeres) and tap off all the line-outs you need from that.
Two flies with one clap?
 

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
> this active pot which won´t need any buffer at all methinks.

Well, except that it IS a buffer, actually two buffers around a pot.

Sound card outputs are usually under 100 ohm source impedance. As long as you don't have shorted wires, or don't mind trouble-shooting when sound goes dead, you can stack lots of 10K or 22K loads on it, simply parallel ("Y-cords"). Ten 10K loads in parallel is 1K, and a sound card can drive 1K fine.

In for-pay audio, we have to be defensive. A radio station has to be on the air 24/7, even when some idiot helper like me is trying to get a tap of the main signal. Recording live concerts, there is always some kid who comes running up the same instant that the conductor goes on-stage, and wants to tap his beat-up MiniDisc player into the recording bus. And I myself have plugged outputs into outputs and wondered what was wrong.... So a "good" distribution amp has to be short-proof, and even isolate stray signals fed in the output so they do not leak into other outputs.

> For PC, as declared by joensd in the first post, this might be considered as sufficient solution.

In general, one buffer with many build-out IS the best way, assuming you can find a beefy enough buffer. In broadcast the standard now is 60Ω build-out resistors, so if you might have a dozen shorted outputs (it is amazing what can happen in a large broadcast operation {at least this was true in analog days}) then your buffer looks a lot like a loudspeaker amp.

You may argue that a 074 is only good enough for PC music, and I won't debate that too much. Of the cheapo chips, I do like the LM837, if only because the output is beefier than the 074s. There are tons of better op-amps if that's what you want.