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newbie here.

I have a Hi-Fi system and I wish to connect its output to a line input in another system while still being able to hear through the Hi-Fi´s original speakers.

If I use the Headphone jack, the speakers will be muted, but my sistem has an extra output meant for "surround" speakers. It specifies that this output is for 16 ohm speakers.

My question is: If I wanted to use this output as a "line level", would it sufice to use resistors in series to rise the impedance to around 47KOhm? or is there a more efficient way of converting this low impedance output to line level?

I should add that I do not wish to tap the audio output from the integrated circuit as this is for the moment beyond my ability as I am a complete noob in electronics (but I do not discard the possibility of doing this in the future).

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  • \$\begingroup\$ Line In is typically 1V while your Hifi might be est. 40V +/-50% so a voltage divider so a 100k trim pot is best with 1 side grounded.. otherwise 47k series to 47k input only reduces 1/2 but if 10k input then 1/6th. Start with 10:1 then increase. \$\endgroup\$ – Sunnyskyguy EE75 Jan 12 at 0:05
  • \$\begingroup\$ Key point made by @SunnyskyguyEE75 that needs to be emphasized: "your Hifi might be est. 40V +/-50%". This means that a direct connection from any speaker output to a line-in input would likely cause permanent electrical damage to the line-in. So make sure you configure your divider or trim pot correctly to reduce the voltage. And just to be safe, start with the volume low and turn it up slowly. \$\endgroup\$ – mkeith Jan 12 at 2:37
  • \$\begingroup\$ The secondary amp if 10:1 with 4V max volume +/-50% into 10 kOhms typ line impedance, should or may not damage it yet will be very distorted, so 40:1 ratio may be better series:shunt R and using RCA coax. If there is any DC on output, a series Cap of 0.1uF into 47k series is good down to 40Hz, 0.2uF down to 20Hz so 48:1 is 47k:1k , 47k:10k is 57:10 ratio and a 100k pot can go 1:1 to 300:1 \$\endgroup\$ – Sunnyskyguy EE75 Jan 12 at 2:46
  • \$\begingroup\$ @SunnyskyguyEE75, 10:1 is not what I am worried about. OP professes to be a complete noob, so I am just concerned about whether he understands how to implement the divider correctly. Your advice is good. I just wanted to emphasize that a direct connection (due to wiring error or whatever) between speaker out and line in may cause permanent damage. \$\endgroup\$ – mkeith Jan 12 at 7:02
  • \$\begingroup\$ I understand. I'd like to add, there may also be hum if the impedance is too high between systems powered by different sources with low leakage currents. so the attenuator at source to lower the impedance may be needed if this happens like 5k:100 Ohms \$\endgroup\$ – Sunnyskyguy EE75 Jan 12 at 7:20
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It specifies that this output is for 16 ohm speakers. ... If I wanted to use this output as a "line level", would it sufice to use resistors in series to rise the impedance to around 47KOhm?

In modern audio systems (after the vacuum-tube-only era), impedances are not matched, but bridged. That is, outputs are made very low-impedance compared to inputs. (This approach has a number of advantages in simplicity, compatibility, and frequency response; its cost is electrical signal reflections that are insignificant for audio signals carried over short distances.) Therefore, you do not need to do anything for the impedance.

However, some speaker amplifier outputs may have their “−” side not connected to ground, but driven to negative voltages. If this is the case for your amplifier, then connecting it to a line-level input would short out the amplifier if the two devices have a common ground (and possibly produce odd signal leakage effects even if they don't).

The device you would need to solve this problem is an audio isolation transformer — a transformer that has a frequency response suitable for audio signals, and (as a starting assumption) a 1:1 turns ratio. I don't have any reliable recommendation for how to tell if you would need one, though a continuity test between the “−” output terminal and the line input ground would be a reasonable start.

Finally, you may find that the voltage range of the output is unsuitable, either too high or too low, so that the output is either too quiet or loud and distorted. In that case, you would need to either use a transformer with a different ratio, or (if it is too high and you do not otherwise need a transformer) a voltage divider, preferably placed at the input of the line-level device to minimize noise pickup (voltage dividers are always a tradeoff between being high-impedance and wasting power). A series resistor can also do the job but the value you need will depend on the input impedance of the line-level device.

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  • \$\begingroup\$ I should have mentioned that my Hi-Fi is not at all a “modern” device. It is. 1991 Sony LBT-D105. I don’t know if that is modern enough to fit your description! \$\endgroup\$ – Javier González Jan 12 at 1:58
  • \$\begingroup\$ @JavierGonzález I could have been more specific — to the best of my knowledge, impedance matching between amplifier and speaker was used in (some) vacuum tube equipment. You're fine. \$\endgroup\$ – Kevin Reid Jan 12 at 2:12
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With the type of output you mentioned it is possible it is a 'bridged' output, that is both wires per each channel are an amplifier output, instead of an output and ground per channel. The 16 ohm minimum load spec kind of gives that away. A conventional output has a amplifier output to drive 4 to 8 ohm loads and a common ground return.

A tap off of this output with series resistors of a 100 ohms or so drives your headphone jack. This type of speaker output can be divided 10 to 1 with a 100K and a 10K resistor.

If the speaker output is bridged (many car stereos are), the only safe interface is a audio grade transformer. Some have a 10K impedance side and a 600 ohm side. Note that to avoid overloading the transformer you may need a 47K resistor in series with the 10K input.

The 47K may have to change according to results. You want so called full volume when your volume control is turned about 3/4 of its mechanical range. If the sound is blasting or distorted by 1/4 turn then increase the 47K value to 100K or even 200K.

Stereo stores and car stereo shops sell RCA type reducers that cut sound level by 3dB to 12dB using resistor dividers, however they will not work with bridged outputs.

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