Stepping down speaker outputs to a sub or find a cheap tube amp w/ sub out?

[shadow5606]

So,

I'm building a 2.1 ch setup primarily for vinyl but Bluetooth is a requirement.

Currently, I'm running a pair of Klipsch 600M Bookshelf speakers and a Klipsch SPL120 powered sub.

I use an inexpensive but good sounding tube preamp, and an inexpensive Rockville tube amp with Bluetooth.

Unfortunately, the SPL120 sub only accepts line-level or LFE input and the Rockville amp only has speaker level outs.

I can run out of my tube preamp to both the amp and the sub by splitting the RCA with a y-adapter but then I have to manually adjust the sub volume (does not go up and down w/ the amp volume) and I also don't have the Bluetooth flowing to the sub this way, because it is built into the tube amp.

I want a flow like this:
turntable ----> Tube preamp -----> Rockville tube receiver ------> Speakers/sub

I could do 3 things that are occurring to me.

1.) Replace the tube amp with a solid-state amp that has Bluetooth and a sub out --- This is OK but I reallllllllly like the sound and looks and price of the Rockville that I have. --- I have had no luck finding an inexpensive good looking tube amp like the Rockville with sub outs... I found one but it's only 12W per channel and ugly. Found others that are closer to 1K in price... Open to suggestions.

2.) Replace the Sub I'm using with one that allows speaker level output to pass through and out to the speakers. ---- I got this sub through a friend and I'm not sure if I even have return options... this would get messy to sell it and buy another sub. Probably my least favorite idea.

3.) Add a speaker level to line level converter to flow signal from the speaker outs of the tube amp to both the powered sub and the passive speakers. ---- I like this but want to confirm it will work. This is my 1st choice.

I'm open to other ideas but those are my only 3 that come to mind. Are there other ways I'm not considering?

If I went w/ option 3 and used a converter like this one, or this one, or this one to take the speaker level output from the tube amp to the line-level input on the sub would I run into issues with impedance, wattage or volume matching between the sub and speakers?

Those adapters seem ok on wattage because my amp says...

"25 watts x 2 RMS @ 8 ohm. 35 Watts x 2 RMS @ 4 ohm."

I don't know enough about impedance to know if I'll have a problem... my amp says 4 to 8 ohms... my sub is active with it's own amp so I don't believe that impacts impedance, right?

And volume is the other big question... if I rigged up one of these converters as well as my bookself speakers to the tube amp (both wires out of the same speaker level terminal outs) would I still get volume or amplitude control passing through to the sub? i.e. when I turn the volume up and down on the amp will my bookshelf speakers and sub both go up and down in parity on volume?

Problems with this plan? Better ideas?

 

[DonH50]

Can you solder or crimp connectors to create your own attenuator? That would work for option (3). Be aware that not all tube amps put the (-) terminal at ground so you might have to be careful using a standard attenuator. Wattage through the divider will probably be small.

25 W into 8 ohms is about 14 Vrms so assuming you want 1 Vrms into your sub's line input you need 20log10(14/1) = 23 dB of attenuation, a voltage attenuation ratio of ~14:1. If you use a fixed converter then volume will change with speaker volume. If your divider is close enough and the sub has a gain control you should need anything else, but you'll need to know you sub's input impedance. Say it is 10 k-ohms, then ignoring the amplifier's output impedance, you would add a series resistor of about 130 k-ohms to provide roughly 23.5 dB of attenuation. Actually, you'd probably use a pair of 130 k-ohm resistors to sum the right and left channels into the sub.

Let Rs = resistor in series with the signal, Rin = the sub's input resistance (load), and VR the voltage ratio you need to attenuate by (so 14:1 in this example), then Rs = (VR-1) * Rin. (Check my math.)

HTH - Don

 

[shadow5606]

Hi Don, I can solder but I'm not terribly deep in the math behind this stuff... If I went the route of a home brew attenuator I might trapse through some trial and error.

I wonder, do the speaker to sub converters that I linked to up above essentially do the same thing?

converters like this one, or this one, or this one (links for each embeded)

If I did one of those would I be fine on volume changes (speakers and sub in parity), impedance (active sub won't impact it right?), and wattage, looks like those step down converters handle 50W and my amp flows 25 to 35...

Just wondering if this would do the trick? Simple enough to spend $10 and call it a day if this will work.

 

[DonH50]

Missed the embedded links, sorry. The speaker to line-level converters do the same thing, yes, though the resistive ones I have seen (by no means exhaustive) include a series and shunt resistor. The catch is they are usually single-ended (sometimes with RCA connectors) so you need to ensure a common ground connection is OK. And note even with a common ground you may cause a ground loop. The transformer-based converters may isolate ground, obviating that problem, but transformers may have limited low-frequency bandwidth (depends on the transformer) so you'd have to make sure it goes low enough for you. Those appear to target car radio applications, and those tend to use bridged outputs that float ground, so they will probably work OK. I'd double-check before risking the amp.

Look in the amp's manual or contact the manufacturer to make sure it is OK to use a common ground, and/or the converter's manufacturer to make sure it isolates grounds, then you can try one and see how it works. Or buy one and measure with an ohmmeter to make sure the grounds are isolated.

HTH - Don

 

[shadow5606]

Thank you very much, very helpful!