Confused About Subwoofer Connection

I am running a PAS restored with the Tubes Nirvana Z-Mod kits into an ST-70 restored with the Tubes 4 Hifi VTA70 PCB, played through a pair of Royd Merlin speakers.  I have a M&K V8 powered subwoofer that I would like to add to the system to provide some additional low end.  I have reviewed the various posts and threads about adding a subwoofer to a PAS/ST-70 system, both here and on other sites.  I have come away confused.  

Question 1:  Will this setup work, i.e., will I be able to boost the bass without compromising the great mids and highs I currently get through the system?

Question 2:  If the answer to Question 1 is yes, how should I connect the subwoofer?  From what I have read, it appears I have at least the following options:

a)  Use RCA "Y" splitters to split the signal coming out of the PAS.  Run one of the split signals to the subwoofer.  Run the other split signal to the ST-70, then on to the Merlins.

b)  Run the signal from the PAS to the subwoofer; then run a signal out from the subwoofer to the ST-70, and on to the Merlins.

c)  Add a speaker selector box to the system.  Connect the Merlins and the subwoofer to the selector box.  Run the signal from the ST-70 to the selector box.  Use the selector box to play both the Merlins and the subwoofer at the same time.

What are the pros and cons of these options?  Are there other/better options available?  Again, the ultimate goal is to improve the low end without otherwise degrading the sound.

Before I got my big Bozak speakers, I used an EAC Electronic Crossover. It's a little active crossover with adjustable hi / lo pass levels. Both Line and Speaker level capability.

Line Level in from Pre-Amp, Line levels out to 1 or 2 powered sub-woofers and your main stereo amp.

Worked great.   Allows you to adjust the levels of where your sub cuts off and how much boom you want without having to reach behind the sub.

The only 'all-tube' way to do it would be with a center channel subwoofer with a passive crossover to isolate the low frequencies. (I don't know if the VTA board will support a center channel connection, but some Dynacos do.) This is not much different from switching to other speakers that have more bass.

Bi-amp systems generally depend on an electronic crossover after the preamp. I don't know of any of these that use tubes. After the electronic crossover you would have a pair of amps for the high frequency speakers and one or more amps strictly for the low frequency speakers.

Since your subwoofer is powered, it needs a line level input. If you use a Y adapter to get one, you will have a mono signal to the other channels and maybe some impedance problems as well.

Based upon aguaazul's reply, I have been researching the Energy EAC and other outboard crossovers.  It appears the real question is what the outboard crossover unit does to the signal that passes through to the ST-70, i.e., does it merely pass the signal through "as is," or does it process or modify the signal somehow?  From what I have read, it sounds like the Dahlquist DQ-LP1 passes the signal straight through to the amp without any modification.  Of course, those units are rare and expensive.  I am not sure what units like the Energy EAC do to the pass through signal.  Likewise with the Paradigm X-30, which is another unit I have read about.  There is no point in doing this if I am simply going to lose my Dynaco sound.

flip69 wrote:Based upon aguaazul's reply, I have been researching the Energy EAC and other outboard crossovers.  It appears the real question is what the outboard crossover unit does to the signal that passes through to the ST-70, i.e., does it merely pass the signal through "as is," or does it process or modify the signal somehow?  From what I have read, it sounds like the Dahlquist DQ-LP1 passes the signal straight through to the amp without any modification.  Of course, those units are rare and expensive.  I am not sure what units like the Energy EAC do to the pass through signal.  Likewise with the Paradigm X-30, which is another unit I have read about.  There is no point in doing this if I am simply going to lose my Dynaco sound.

The outboard crossover processes the signal before it goes to the ST-70. The ST-70 would be connected to the L and R Outputs on the right in the picture. Your subwoofer would connect to the middle outputs marked SUB. The ST-70 would be used only for high frequencies and your Royd Merlins would actually get less bass than they do now. The electronic crossover doesn't make much sense to use with a powered subwoofer, because the powered subwoofer has its own crossover built in. Electronic crossovers are for using separate amps for high frequencies vs. low frequencies.

OK. It is sounding more and more like there simply might not be an effective way for me to accomplish this given my current set-up. That is not the worst thing in the world. The Merlins sound pretty darn good on their own, especially for their size. This is the system in my workroom. There are space limitations that restrict the use of full-size speakers.

flip69 wrote:OK.  It is sounding more and more like there simply might not be an effective way for me to accomplish this given my current set-up.  That is not the worst thing in the world.  The Merlins sound pretty darn good on their own, especially for their size.  This is the system in my workroom.  There are space limitations that restrict the use of full-size speakers.

Yes, powered subwoofers and electronic crossovers are creatures of the solid state realm. You may not have yet found the speakers that work best with your tube gear - that was a long road for me. I'm using speakers hat were designed more than 50 years ago and have a woofer in each cabinet. I like the amount of bass I get, but it's just old-school regular natural bass - no tone controls on my tube rig.

I would split the preamp output to both the ST70 and the sub using the RCA y-connectors. If you want to save your Merlins from bass duty you can also add high pass filter in-line at the input of your ST70. (I use a 0.03uF cap on my ST120)

You note concern about degrading the signal going to the main speakers, but you didn't ask if there is a way to actually improve it. The answer(s) are, yes, you can set up these powered sub woofers to work with your system, and yes, you can do it without degrading the mains, and yes, you can actually enhance the performance of the mains.

One option that I recommend against is feeding both the ST 70 and the Sub plate amps through the cross over that's built into your sub woofers. In theory, that is an elegant way to go, because you're subs probably let you choose your high pass frequency (for the mains). That should make it easy to dial in your whole system. Easy, yes, but in my case, the cross over in my (Rhythmik) subs sucked the life out of the sound going into the ST 70.

So, instead, I fed the subwoofers (their built in amps) from one pre-amp out put, and fed the ST 70 with the other. If only one pre amp output, then using a y-splitter does the same thing.

This worked well, and it's what I'm back to doing now, but only because my current main speakers play down to about 40 Hz. I said you could improve on the mains performance, and with my last speakers that dropped off below 100Hz, here's what I did.

I constructed a simple 1st order high pass filter (consisting of a single bipolar capacitor in each line) going between the pre amp and the ST 70. This relieves the ST 70 of the burden of trying to deliver low frequencies the mains couldn't really cope with anyway. Unburdened with the heavy lifting of these low frequencies, the ST 70 was expected to have a much easier time dealing with the frequency range it was designated to handle. Supposedly, a livelier sound results. I can't say I really noticed it, but it was probably there.

At the speaker end of the equation, doppler distortion is reduced, because you don't have the speaker cones flapping in and out with those low frequencies they can't really handle anyway.

So two kinds of distortion are being reduced - doppler distortion (at the speaker) and transient intermodulation distortion (iirc) from the amp.

If you have a sound pressure level reading microphone, determine at what frequency your Royds fall off by 3dB. Make that your cross over frequency, and dial in your subwoofers accordingly. Then calculate the capacitor value needed for your mains high pass (capacitor). The formula is:

Fc = 1/2piRC

Where Fc is Cross over Frequency.
R is input impedance of your ST 70 (270,000 ohms, if I recall)
C is the capacitor value.

Sorry, Corndog, I didn't see you'd already answered when I got all excited about responding.

One other thing, to the OP, if you want to get finicky -- the high pass creates a phase shift. If your subwoofers have a phase shift (or delay) dial, you can use this to get your mains and your subs in phase. The problem is, most often the sub woofer is positioned farther away from the listening position than the mains.

To cope with this, reverse the wiring to your main speakers, effectively delaying their signal by 180 degrees. Now the phase from the mains will be "lagging" behind the sub woofers, and you can further delay the signal at the sub woofers to get them in phase. I'd do this by playing a tone at the cross over frequency (one channel at a time) and adjust the phase delay on your sub woofer to maximize volume at the listening position. You'll need a volunteer, or an SPL meter on a tripod that you can read from your position at the sub woofer control panel.

Tube Nube wrote:Sorry, Corndog, I didn't see you'd already answered when I got all excited about responding.

One other thing, to the OP, if you want to get finicky -- the high pass creates a phase shift. If your subwoofers have a phase shift (or delay) dial, you can use this to get your mains and your subs in phase. The problem is, most often the sub woofer is positioned farther away from the listening position than the mains.

To cope with this, reverse the wiring to your main speakers, effectively delaying their signal by 180 degrees. Now the phase from the mains will be "lagging" behind the sub woofers, and you can further delay the signal at the sub woofers to get them in phase. I'd do this by playing a tone at the cross over frequency (one channel at a time) and adjust the phase delay on your sub woofer to maximize volume at the listening position. You'll need a volunteer, or an SPL meter on a tripod that you can read from your position at the sub woofer control panel.

That is an imaginative solution, TN. Contrary to logic and aesthetics, subs should be placed in front of mains. But there are even more vexing problems with bass management, having to do with The Room and, to some degree, the type of music one prefers.
The Room presents a raft of phase cancellations and additions that even the best-informed experts in acoustics can only provide approximate solutions for. And single-subwoofer solutions do not work for those whose tastes run to acoustic music recorded in a real space (as opposed to studio, where the low bass is generally centered - in effect, solving some of the problem in mastering).
In short, a combination of utilizing any analytical equipment you can bring to bear should be augmented by the best tools we have, and the most sophisticated - our ears. Just be prepared to fight the inevitable battles with Those Beings You Love - and your back muscles, as you move that eighty pound beast once more, and your Sig. O's patience flags in "sympathy". Good luck.

Interesting about having the Subs out front! I hadn't hear of that before, but was advised to have mine back against the wall (a pair of them), one of them close to a corner. In addition, I'm using parametric equalization to tame room modes, best I can, with the aid of Room EQ Wizard.

I didn't want to bring REQ Wizard into the discussion too soon, though, fearing that might scare the OP out of trying his mains / sub integration.

Tube Nube wrote:Interesting about having the Subs out front! I hadn't hear of that before, but was advised to have mine back against the wall (a pair of them), one of them close to a corner. In addition, I'm using parametric equalization to tame room modes, best I can, with the aid of Room EQ Wizard.

I didn't want to bring REQ Wizard into the discussion too soon, though, fearing that might scare the OP out of trying his mains / sub integration.

This is where things get very dicey. The sub's response will, of course, be maxed in a corner and against walls, but this increases the possibility of phase differences, room node problems, and the need to adjust you parametric EQ for every source. It can increase Valium consumption greatly. The Holy Grail, IMHO, is to integrate your equipment into your space so that Maximum Pleasure is derived with Minimum Effort. Taint easy. I've been on this for fifteen years. Almost there....

Oh NO! the PEQ settings will differ with a different source?

That's a bother, since I use the digital system with REQ to program the equalizer, but for my serious listening almost exclusively go vinyl.

Tube Nube wrote:Oh NO!  the PEQ settings will differ with a different source?

That's a bother, since I use the digital system with REQ to program the equalizer, but for my serious listening almost exclusively go vinyl.

Of course. Remember this - the art of the scientific method is knowing what question to ask. Read the inappropriate meter and ... only bad things can ensue. So sometimes the most scientific approach is to have a batch o' beer {or equivalent!} and listen to some album you've never listened to before and "turn off your mind, relax and float downstream". And I'm not even a Beetle's fan....

And vinyl has inherent differences from digital sources that make A/B tests meaningless. Most of us old farts are accustomed to the immediacy of vinyl, but we should be aware that a portion of this has to do with vinyl's restricted dynamic range and its inability to reproduce extreme transients especially in the bass freqs.
We all seek a "non-interventionist" approach to sound reproduction, but we get caught up in methods. EQ - based systems cannot begin to solve phase-induced problems. Etc. To bring things home, the VTA products provide "non-interventionalist" solutions that we can base our systems upon - proven circuits upgraded simply and efficiently. The rest is noise, so to speak.

Yessss, hmmmm, while keeping an eye on the world going by your window!

Thank you all very much for your input. I am going to pick up some RCA y splitters this weekend and start experimenting. I will start with no high pass filter, then see what kind of difference the filter makes. Just to make sure I understand correctly, the filter cap is added on the back of the ST70 input RCA, where the signal wire connects to the RCA, i.e., signal wire attaches to one lead on the cap, the other lead attaches to the RCA jack. Is that correct? Also, what kind of cap are we talking about here? Film? Does it matter?

flip69 wrote:Thank you all very much for your input.  I am going to pick up some RCA y splitters this weekend and start experimenting.  I will start with no high pass filter, then see what kind of difference the filter makes.  Just to make sure I understand correctly, the filter cap is added on the back of the ST70 input RCA, where the signal wire connects to the RCA, i.e., signal wire attaches to one lead on the cap, the other lead attaches to the RCA jack.  Is that correct?  Also, what kind of cap are we talking about here?  Film?  Does it matter?

Since I wasn't sure I wanted to do this I got cheap orange drop caps. I connected each lead to the pin of an RCA plug so the cap is in the signal path. I only connected the shields on one channel. It worked better than I expected.

Huh! I would have thought you'd need to conmect both, but come to think of it, those shields are already connected in the preamp to the common ground, arent they?

So Flip, I think you are on your way. Lots of info on the web about setting up your speaker mains and subs this way. When I initially set up my subs, I downloaded test tones and used a SPL mic to measure volume.

When setting the volume on your subs, You want to match the volume of, say, a 120 hz tone (from the mains only) to the volume of a tone playing mostly from the sub woofer. Just don't pick a tone Whose frequency is equal to 565 divided by any one of your room dimensions, because that's where you will have modes (a.k.a. Standing waves that resonate loudly).

Tube Nube wrote:Interesting about having the Subs out front! I hadn't hear of that before, but was advised to have mine back against the wall (a pair of them), one of them close to a corner. In addition, I'm using parametric equalization to tame room modes, best I can, with the aid of Room EQ Wizard.

I didn't want to bring REQ Wizard into the discussion too soon, though, fearing that might scare the OP out of trying his mains / sub integration.

A few years ago I bought a SVS powered sub to run on my two channel set up. I have a Holger Aretha and M 125s driving Zu Audio Omens Defs. I just couldn't get the sound I wanted or expected out of the sub. It sounded like it wasn't suppose to be there.

It so happened I got an email from the president of SVS inquiring on my purchase of their powered sub. I explained I couldn't get the performance I had hoped for in my system. He replied quickly to my email and set up a phone call with the lead engineer. After a lengthy conversation with him about the slight delay in their power amp driving their sub, he asked were my sub was located, which was in between my speakers on the same plane of them. He told me to play a song with a good bass rhythm and slowly move the sub forward. I asked him how much forward, he replied about 6 inches on my set up, but it could be more or less and I would know when I hit the magic spot, because the sub would sound in phase and disappear. He was correct and about 7 inched in front of my main speakers it sounded right.

Just my experience with a powered sub.

TM

yes, high pass filter cap at the input of the ST 70.