RE : audio distribution

[joensd]

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]

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.....

[joensd]

thanks for hints jamie f
think i´ll try that tl074 buffered thingy
still can change the opamps if it´s worth
people say pc´s are noisy (which they are) but the headphone amplifier i built for example sounds brilliant and without noise (unless the cd-rom starts to spin)

[BC108man]

Hi - Did you ever build that "thingy"? How did it work out? Any problems with oscillations? I'm about to try something similar, although I need 10 outputs. Anybody have any advice?

Cheers, BC

[JOE DIRT®]

the tl074 op amps work fine as buffers and there are great circuit examples at different sites as well as from the manufactures data sheets....I like them because they cost peanuts and are easy to work with and you can change anything if you dont like it...good DIY parts

DIRT®

[PMA]

OPA4134 is definitely better than TL074.

[theChris]

BTW, for quads i love the lm837n. cost 25 cents each. i mean you can get like 10 to the big mac... (and mc donalds humor is always funny...)

[BC108man]

Thanks for all the replies. I'm going to build the circuit in the TL074 datasheet - great tip, Dirt.

Cheers, BC

[PRR]

> 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:


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.

[PMA]

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