[aceinc]

One of the problems we sometimes have when using stereo subwoofers with our speakers is many newer subwoofers do not have speaker level input anymore. So we end up running an RCA cable from the amp (either with Y or Sub out) to the plate amp on the sub. This is kind of messy.

I like my subs right next to the speakers so I looked for a speaker level to line level output "converter" and there are plenty. However they all seem to be stereo. This means placing them closer to the amp and running a long RCA, which is what I am trying to avoid.
So I worked with AI, as I have no circuit design skills and we came up with the circuit design below. I was hoping there are folks with these skills who could verify that AI isn't lying to me.

My thought is to build two little boxes (one for each speaker) with short wires on one side to attach to the speaker terminals, and Female RCA jack on the other to connect a cable from the box to the plate amp.

[I.Greyhound Fan]

Why not look for a wireless adapter?

[aceinc]

Why not look for a wireless adapter?

The last time I used wireless was in a HT system. It introduced a 20+ms delay which I was able to compensate for (mostly) using the delay parameters in the sound processor. My current use case is a 2 channel, mostly vintage, system.

[aceinc]

Values are ok for a 60W amp.
R1 must be 1 watt.
R2 must be 1/2 watt.
Need to know amp’s voltage output to calculate accurately.
A simple 1/1 audio transformer for isolation is essential, and you will need a ground lift option.
Wire in a trim pot after the transformer to fine-tune level.

The input is from a 200 watt amplifier. The plate amplifier will accept up to 1.4v, AI tells me this will work up to 40v input with no more than 1.51v output. The plate amplifier has gain & DSP built in. The current amp I am using to feed this have Autoformers (transformers?) on the output stage. Not sure if this obviates the need for transformers in this circuit.

Both the Stereo amp and the plate amps are two wire, will this still require a ground lift option?

[nlitworld]

I ran a line level converter a while back. I set my R2 for 1K and my R1 was something like 20K. You can easily play with your R1 value to get gain level matched with your main speakers. The main thing will be cutting enough to match at multiple volume points as otherwise you'll constantly be dicking with the volume levels on sub amps. These are really easy to build into a tiny little project box on Amazon and a couple RCA connectors and speaker binding posts. Have R1 in line with the + speaker cable, then just have R2 across the connections on your RCA plug.



Here was my proof of concept. I built mine with I believe 1/4w or 1/2w resistors but I'm never driving more than 2 or 3w in my small room. I rebuilt this in a proper little box and hid it under my speaker stand with some double sided tape.

Keep us posted how it turns out!

[aceinc]

I ran a line level converter a while back. I set my R2 for 1K and my R1 was something like 20K. Really easy with a tiny little project box on Amazon and a couple RCA connectors. Have R1 in line with the + speaker cable, then just have R2 across the connections on your RCA plug.



Here was my proof of concept. I rebuilt it in a proper little box and hid it under my speaker stand with some double sided tape.

Thanks for the boost in confidence.

I've had some naysayers on a different forum say the sound wouldn't be acceptable, due to the double amplification issue. No comments about the design or the resistor values. What was your experience regarding the sound?

As far as the design and values of the resistors, I used two different AI engines they both came up with the same layout but different values on the resistors, so I kept prompting each with the values from the other until they agreed on the values I showed in the circuit diagram. They both said 1/4 watt was enough, I doubled that to 1/2 watt for safety. What did you use for power handling?

[nlitworld]

I think I used 1/4w for mine, but bumping to 1/2w is not a bad idea. As for sound quality, you wouldn't exactly want to run this circuit for main speaker signal, but for subs it's no big deal. You could try carbon or metal film resistors. Might be low enough frequency to not notice a difference though. As to values for R1 & R2, it's all about the ratio, not necessarily the end values. I chose 1K just for ease of math to dial things in. All in all, it's a cheap deal to DIY and try for a bit.

[richidoo]

After you figure out the correct resistor values and like how it sounds with inexpensive composite or metal oxide resistors, please invest in some Mills wirewound non-inductive resistors to replace the prototype resistors. Mills are not terribly expensive, they are 12W, and they do sound MUCH better than oxide. The Mills pass so much more information than oxide that I often need to use half ohm larger size to achieve the same musical balance as with oxides.

[DaveC113]

A simple voltage divider is fine for an A/B single ended amp, but if your amp is balanced the black binding post is NOT ground. Also, many class D amps do not have their black/ground binding post at earth ground, this can vary from it floating above ground to it also being balanced but for both balanced and class D you should use chassis ground, I think REL may even suggest not using the ground at all, but then IDK how you implement a voltage divider... I think you can safely use chassis ground as long as the amp has a real ground connection and isn't double insulated.

Amp damage due to improper understanding of the negative binding post in different amps is such a big problem that REL now seems to recommend using a chassis ground for all amps.

This is a relatively simple speaker level to line level conversion because you don't need to sum both channels to mono, in that case you need to use a summing amplifier, in this case 2 resistors are all you need as I assume the sub has level controls.