ARC Ref 3 Main Output Impedance Question

I need some advice on how to hook my ARC Ref 3 pre up to my self-powered Paradigm Signature Servo subwoofer. As further explained below, ARC recommends that I use a crossover/connector tdevice hat has a higher input impedance than the one I currently use and is set up for balanced operation.

The Servo's input impedance is 25KOhms in SE mode and 20KOhms in balanced mode. I use the Servo for low frequencies (10hz to 35hz) in mono mode. To do this, I sum the L/R outputs of the Ref 3 by using a Paradigm X-30 crossover unit. The X-30 has an input impedance of 20KOhms in SE mode. There is no provision for balanced interconnect.

My Ref 3 is connected to the rest of the system as follows. Main 1 outputs are connected directly to my ARC VS-115 in balanced mode. The VS-115 has an input impedance of 300KOhms in balanced mode. Main 2 L/R channels are connected to the X-30 unit in SE mode. The X-30 sums the channels into mono and connects to the Servo is SE mode.

Based on a call with ARC, I was advised that my current subwoofer set-up is compromising the sound quality of the Ref 3 because (a) the X-30 input impedance of 20KOhm is the bare bones minimum that is recommended and (b) I am running one Main output in balanced mode and the other Main output in SE mode. Further, I was told that the current set up is making the Ref 3 work harder than it should.

ARC recommend that I use a crossover/connector device that operates in balanced mode and has a higher impedance than 20KOhm. If I recall correctly, something north of 40-50KOhm would be much better.

Any suggestions would be appreciated. Thanks
I had the same issue with my ARC Ref 3 and JL Audio
subs. You should be able to resolve this by inserting
an impedance buffer between the preamp and sub. Call
Tom Tutay from Transition Audio Design in Florida.
He can make a solid state buffer to your needs.
(850) 244-3041 Good Luck.
Normally if you have a balanced circuit, 20K should be no problem. Balanced circuits **should** be able to drive 600 ohms- so 20K should be like a walk in the park. But many high end audio products don't support the balanced standard, and ARC is one of them.

So here is something you can do. Jensen makes a transformer that can perform the operation of going balanced to single-ended, and the transformer I have in mind is optimized for subwoofer operation, so it won't compromise the bass.Their part number is JT-112P-2HPC. you need two for stereo. If you set it up right, it would load the preamp at about 47K (or 39K, which ever you want). This would allow you to run balanced or single-ended. It would be a very simple matter to add a summing network at their outputs, bypassing the need for your summing crossover.

Tom Tutay is also an excellent resource.

FWIW, (disclaimer: manufacturer) we make a preamp that does support 600 ohm balanced operation so you could bypass the need for the summing amplifier- it can drive both the 20K and 300K input impedances in parallel in the balanced mode.
Vmk & Atmasphere:

Thank you very much for the suggestions. I called Tom Tutay and discussed the problem with him. Tom will build a custom connector box having stereo SE and balanced inputs and a mono SE and balanced output. The inputs will have a 300KOhm impedance, which should match well with the ARC Ref 3 Main 2 output. As stated in the OP, the Main 1 balanced output is also loaded at 300KOhms into the VS-115 balanced input, so the Ref 3 Main 1 and Main 2 balanced outputs will see the same load.

The price was very reasonable, especially since the device will be custom built to my spec. Tom was very knowledgeable and seems like a real gentleman. I'll report back when I get the unit and try it out. Tom said that it will take a couple of weeks to build.

Thanks again for the help!
Bifwynne, Tom is a great guy. I have had quite a few nice conversations with him...
Hifigeek, agreed 100%. I should get the connector device in about 7 to 10 days. Tom is loading the balanced input at 330KOhms, which is close to the input load of my ARC VS-115, which is 300KOhms balanced. Does that seem good to you?

BTW, as crazy as this may sound, Tom does not use a PC, does not advertise on the net, nor does he subscribe to A'gon or other similar forums. So, he relies entirely on hand out ads and word of mouth.

I look forward to receiving the connector so I can objectively review it. Tom comes with such high recommendations, I can only assume I will be 120% satisfied. So . . . after I get the device and review it, I would like to help Tom out by posting something a little more conspicuous than a forum thread. I'll figure something out.

BTW btw, another A'goner just raised a very similar question to the one on this OP. It goes to show you that this issue does arise from time to time.

Be well and regards.
hmmm.... never looked at this. I have a REF2 connected SE to a Velodyne. Am I working the pre to hard?
No_money, I don't know. Call Cal at ARC. He'll want to know how you hooked up the Ref 2 to your amp and sub, i.e., balanced and/or SE, and what the impedance loads of each application are.

In my case, I had a double problem: (1) the imput impedance of my pre was too low; and (2) the Ref 3 was asymetrically loaded, i.e., Main 1 to the amp was balanced and Main 2 to the sub was SE. Not a good set up. Cal explained that as a result, I was prematurely aging the tubes AND potentially denigrating sonic performance.

If Cal thinks you need a buffer device to increase the impedance loading of your Ref 2 and sum the channels into mono for the sub, call Tom Tutay at Transition Audio Design. His number is listed above. The other posters above say he is solid. He's currently building an impedance/channel summing custom buffer for me.
He has enough business so I'm sure more would just be added pressure....
Post removed 
Bob, by active or passive, I assume you mean powered or not. FWIW, it plugs into a 120 AC socket. Tom mentioned that it has a 1:1 op amp in it -- whatever that means. I don't know if it uses a transformer.

Hifigeek, 2 other A'gon posters have recently asked similar questions about impedance and asymetrical loading. At least in the case of ARC gear, being a retail owner of an ARC pre, phono pre, CDP and amp, I believe it's a fair question to raise with Calvin. Even more so if the inquiry is from a technical novice, especially since not correcting the problem can prematurely wear out the tubes in the pre. BTW and FWIW, Calvin asked for Tom's contact information. I can only assume this question may come up from time to time.

When you set up an op amp you can choose the amount of gain you want. Since it's acting as a buffer, no gain is required and therefore, the op amp is set to 'unity' or 1:1 gain. Meaning the signal you put in is the same as you get coming out.
Hifigeek, that makes perfect sense. As stated above, I expect that there will be two benefits to the device. One, longer tube life (b) because the pre will be loaded at a higher impedance level AND (b) because the load will NOT be asymetrical, i.e., balanced on Main 1 and SE on Main 2. Two, hopefully, some sonic improvement.

I'll report back after I set the device up and run it for a while. Thanks to all for the great advice.
Post removed 
Bob, why poo-poo'ed? If pure resistive, why not just stick a resister in series with the load?
Bob, as there is no such thing as a straight wire with gain, an active device added to the signal path will have some effect on the sound. However the effects should be minimal as the buffer used is being set to unity. I don't know which buffer Tom is using but he's very good at what he does. I'm sure the benefit of impedance matching is worth the minimal effects. Has anyone thought of doing a tube buffer stage?? Interesting. It would certainly be more expensive because buffering is needed for the inverting as well as the non-inverting input, but might be worth it. Maybe I need to make one if enough people would be interested
Hifigeek1, we've built tube buffer versions of our preamp. They have a direct-coupled output. IME what you run into with tube buffers is voltage loss that is more noticeable than with semiconductors. As long as your sources have enough voltage that is not a problem.

I'm not buying the idea that the asymmetrical loading of the ARC would have any effect on tube life. Most tube preamps can be connected to a dead short without the tube life being affected...

Now the asymmetrical loading issue is another matter, but the way I have seen ARC preamps designed, that should not be a big problem if one half is loaded differently from the other.
Atmasphere, do you think asymetrical loading would affect sonic quality? Not being an electronics techie, I of course have no view. All I can say is that ARC did not think it was a good way to load the Ref3.
Someone needs to do a test to settle this debate.I'm wondering if distortion products go up?
Do you think asymetrical loading would affect sonic quality?
Pending Ralph's response, my expectation would be that it could degrade the common mode noise rejection that would otherwise be provided by the balanced interface, at least a little. That would occur for two reasons. One is the imbalance that would be introduced in the impedance between each of the signals in the balanced pair and ground. The other is that the single-ended connection could introduce noise onto the balanced connection that is not common mode (i.e., that is not equal on the two signals in the balanced pair).

The higher the output impedance of the preamp, the greater I would expect both of those effects to be.
I'm wondering if distortion products go up?
I don't think that would occur as a result of the imbalance per se, but it might occur to some extent if the preamp has difficulties dealing with the lower impedance on the signal that is driving the two loads.

-- Al
Certainly an interesting discussion. However, in my case, now that I have been alerted to the issue, the problem is moot because Tom is making a custom buffer device with high impedance balanced inputs that will sum the channels for my sub. I am a little curious if I will detect any sonic improvements, albeit lower and tighter bass response, lower noise or cleaner output (i.e., less distortion products).

FWIW, just wondering out loud for a second, I wonder if the issue of asymetrical/low impedance loading is just an ARC issue or cuts across the board for all pre/line stages, regardless of type (i.e., SS or tube). If this is more of a ubiquitous issue, then Hifigeek's suggestion about an objective test might benefit other A'gon members, IMHO.
I wonder if the issue of asymetrical/low impedance loading is just an ARC issue or cuts across the board for all pre/line stages, regardless of type (i.e., SS or tube).
The two effects that I described above, related to degradation of common mode noise rejection, would assume significance that is roughly proportional to preamp output impedance, as I indicated. So I would expect them to be pretty much insignificant for most solid state preamps, but potentially significant for many tube preamps (that have relatively high output impedance), as well as being dependent on the noise environment, cable lengths, susceptibility of the particular components to ground-loop induced noise, and the amplifier input impedances.

Ralph's preamps, btw, although tube-based, have uncommonly low output impedance, and I would expect them to be essentially immune to these effects.

-- Al
Well Al, . . . if one of you techies think this could potentially affect a significant swath of owners of tube pre/ equipment, other than the Atmasphere OTL type of course, perhaps someone with the equipment and know-how will run some tests as Hifigeek mentions above to see whether this issue is just hype or real.

As I already stated, the point is moot for me now, but it might be useful for other A'gon members. However, as soon as I hook up my impedance buffer with the balanced inputs, I'll A/B my set up to see if there are any noticeable sound improvements and report back. Right now, I'm still getting used to the exchange of my old SED 6550 tubes for new Tung Sol KT-120s, which as I said in another post are quite impressive. In any case, it may be that the only improvement I realize will be longer tube life rather than significant sound improvement. We'll see.

Well I can tell you that I have looked at driving a balanced input of an ARC amp with a single ended input while looking at it on my scope. This was done with a 250W Dale non-inductive resistor loading the amps 8 ohm tap. This was on an amp that only had balanced inputs. The results were not pretty. It does tend to shift the DC operating points a bit and the amp would not clip at rated output. In fact you give up about half the amps output by doing this because there's no inverted input to drive that section of the amp. In my shop since I do not have a balanced output signal generator, I use one of ARC's IC based balanced converters. Signal goes in single ended and comes out balanced. Pretty nifty as I didn't want to spend the time to build one myself. How that relates to this issue is hard to tell although I would tend to think, at least with ARC products, it's best to present the balanced input of the amp with the same impedance on both inverting and non-inverting inputs.
Bifwynne, the issue lies with the input of your amplifier. If it is differential with a high Common Mode Rejection Ratio, you will be fine even if the loading of the preamp is unequal (the amp will do fine even if one input has 1/4 the input voltage of the other input). But if the amplifier is balanced but not differential, the CMRR will be comparatively rather poor, and then it will make an audible difference.

Being the 21st century and all, I really would not expect too many amps out there that are balanced but not differential, but maybe my expectations are not that realistic :)
Thanks Ralph: I believe the amp, as you suspect, is balanced a AND differential, whatever that means. I clipped an excerpt from the ARCDB website, which describes the amp:

"Drawing upon the successful REF110 input stage design, the VS115 uses a FET input with two 6H30 triodes in each channel for gain and as a cathode follower-driver for the 6550C-equipped output stage. This allows both balanced and single-ended connections in a true differential topology yielding a much wider power bandwidth (110kHz in contrast to 40kHz for the VS110)."

So does that mean that the asymetrical loading of the Ref3's main outputs with a combined load that may have lower impedance than recommended will NOT affect the sonic quality of the power amp's output? I think both Tom Tutay and ARC thought there could be some rolloff on the low frequency side of the spectrum in such cases. I don't know at what frequency that would occur, but here again, this could be a big whoop too because I can adjust bass loudness.

Bottom line here: does all of this mean the issue which is the subject of the OP does NOT impact sonics, but perhpas just tube life??
Bifwynne, what this means is that the preamp will drive a solid state amp the same way it would as if the tube amp was not even there.

ARC is pretty careful to make sure their preamps will drive the 10K input impedance of a solid state amp. So if its OK playing bass into that right now, then adding the tube amp will be of no consequence as its input impedance is about 10 times higher. If you work out the math, in engineering terms the higher impedance can be ignored.

Here's the math: 10K times 100K divided by 10K + 100K.

That works out to 9.09K ohms as opposed to 10K. I think you'll be OK :)
Thanks Ralph, as stated in the OP, "[b]ased on a call with ARC, I was advised that my current subwoofer set-up is compromising the sound quality of the Ref 3 because (a) the X-30 input impedance of 20KOhm is the bare bones minimum that is recommended and (b) I am running one Main output in balanced mode and the other Main output in SE mode. . . . ARC recommend that I use a crossover/connector device that operates in balanced mode and has a higher impedance than 20KOhm."

Using your formula, the combined impedance is 20K x 300K, divided by 20K + 300K, or 18.75K, which is below the minimum recomended threshold. Further, ARC commented that running the pre with "one Main output in balanced mode and the other Main output in SE mode" is not an optimal way to load load the outputs.

Thanks again for your comments.
Bifwynne, sorry, seems I have been shooting my mount off; upon looking at the specs of the preamp, I have to agree. It did not occur to me that the preamp was not designed to drive 10K as a minimum.
No problem Ralph. In fact, the combined impedance may even be a little lower. I read a review on the VS-115 the other day which reported that when the VS-115 was bench tested, its input impedance was only 266K Ohms -- not the 300K Ohms reported in the ARC literature. By itself, no big deal. But matched with a 20K OHM load on the pre's other output presents a combined load that's slightly lower still than the amount calc'd above.

Personally, I am dubious that I'll hear much difference in sound when I hook up Tom Tutay's impedance buffer device. But if nothing else, maybe the Ref 3 won't have to work as hard with the load currently presented and I'll realize extended tube life. Either way, I'll report back to the group.

P.S. And just as I was ready to hit the submit button, Tom's buffer just. Kinda curious how it works. I'll report back in a couple of days.

BTW, Tom sends his regards and spoke very highly of you.
To All --reporting back on results of using impedance buffer. Ok, I hooked up Tom Tutay's buffer. As a threshold matter, let me say a couple of things about Tom. He is very professional and knowledgeable -- a real gentleman. He turned around my order in lightening time. The shell of the buffer device is made wholly of metal, and is about 9" x 5" x 2". By all appearances from the outside, the device seems to be very sturdy and well made.

Now . . . as to how it works. As you can see from the posts above, the device was designed to: (a) load my Ref 3 symetrically (i.e., Main 1 and Main 2 in balanced mode), (b) sum the left and right channels without shorting the Mains since I am using only one sub, and (c) raise the impedance level of the two Mains to a level that better matches ARC's design specs.

As to tube life, I take it on faith based on conversations with ARC that by reducing the load on my Ref 3's outputs, tube life will be extended. I will have to wait and see.

As to sound quality, I can say with 100% certainty, my rig does NOT sound worse. Does it sound better?? Yeah, I think so. I think the imaging, detail and sound stage are a little bit improved. The highs seem a little smoother and better refined. But quite honestly, I don't think the change, if any, is day and nighgt. It's just subtle, kinda like a tweek, maybe two or three tweeks.

Now the impact on bass is more noticeable. Although I removed one active artifact from the system and added another, I think the X-30 crossover/controller did more than the buffer because the X-30 could adjust loudness, phase, cut-off and summed the channels. By contrast, the buffer has an active unitary op amp that raises the input impedance and just sums the channels. That's it.

When I first inserted the buffer, I initially thought something happened to the bass output from the sub. On Tom's suggestion, I tried using the old hook-up again by running an SE I/C from one of the SE Mains off the Ref 3 directly to the sub (sans X-30). I then ran the Ref 3 in mono mode to sum the channels. Interestingly, the bass seemed equally lame this way too.

Then I realized what was going on. I had set the cutoff frequency on the sub at 35 Hz to blend into my fronts. What I realized is that there simply wasn't very much going on below 35Hz in my source material. By contrast, when I raised the cutoff frequency -- bass galore.

So realizing what was going on, I then critically listened to some bass heavy source material, e.g., Norah Jones CD and Solti conducting Chicago Symph Orchest., Beethoven 9th 1st and 4th movements LP. Tons of bass in these source materials. But when I raised my sub's cutoff to about 50 Hz I had more than enough tight and smooth bass -- NOT boomy.

In summary, Tom Tutay of Transition Audio Design is a great resource for audiophiles. He is not limited to just buffer devices, but can custom design many different types of devices. Keep him in mind. Tom's phone number is: (850) 244-3041.

Thanks guys for the terrific advice.
remember that buffer needs to break in a bit and yes Tom is a great guy...
Thanks again to all for the great advice. I am writing this follow up post because I noticed something today that was quite interesting. On 4/25 I said "[a]s to sound quality, I can say with 100% certainty, my rig does NOT sound worse [because I installed the impedance buffer]. Does it sound better?? Yeah, I think so. I think the imaging, detail and sound stage are a little bit improved. The highs seem a little smoother and better refined. But quite honestly, I don't think the change, if any, is day and night. It's just subtle, kinda like a tweek, maybe two or three tweeks."

Well here's my last update. Since April 25th, I've been listing mostly to vinyl. I haven't used my CDP much at all. Today, I loaded up my new Norah Jones CD and just sat back and closed my eyes to listen and relax. I have a very good auditory memory of the Norah Jones CD because I've played it quite a bit since acquiring it. I think it's pretty well recorded and I just like the music.

After a couple of minutes of listening, I noticed something very different than what I remembered. I was amazed at the significant improvement in stereo imaging in 3D space. I could actually make out the separate voices of Norah Jones singing next to her co-vocalists belting out the tunes. Bass response was phenominal -- great slam, not boomy. And so forth and so on.

I attribute the significant sonic improvements to five possible tweaks. First, as I reported in another OP, I retubed my ARC amp with KT-120 tubes. I retubed the amp a couple of weeks before picking up the impedance buffer. Maybe the KT-120s continued to break in??? Second, perhaps, as Gary (aka Hifigeek) remarked above, the impedance buffer needed to break in some more. Three, I raised the combined output impedance of the loads presented to the Ref 3 from 16-18K Ohms to 157K Ohms. Four, the Ref 3 was presented with a symetrical load - all balanced from the amp (Main 1) and the impedance buffer (Main 2). Five, I removed a major artifact from the sub woofer circuit (i.e., the Paradigm X-30 controller/crossover) and replaced it with a less invasive artifact (Tom Tutay's impedance buffer).

In summary, it appears that one or more of the tweaks listed above effected a very noticeable improvement in my system sonics across the board. My advice to fellow A'goners is to be mindful of how you load your equipement. Also, if your power amp uses 6550 tubes, think about retubing your amp with KT-120 tubes, after first checking with the factory to ensure compatibility.