Adding a subwoofer, how to calculate impedance?

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goofytwoshoes

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Adding a subwoofer, how to calculate impedance?
« on: 17 Apr 2019, 01:22 pm »
I'm considering a sub for my stereo system, most likely a Martin Logan 600X.  ML says the wireless adapter has an input impedance of 5.5K ohms, and the sub wired direct from my Tortuga  LDRv3.25K with RCAs has an impedance of 8.3K ohms.  Amps are Pass Labs Aleph 2 monoblocks (10K ohms input impedance), and Nelson Pass F5T and F5 amps, both having 100K ohm impedance.  How do I figure out impedance using each of these amps? 

tortugaranger

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Re: Adding a subwoofer, how to calculate impedance?
« Reply #1 on: 17 Apr 2019, 02:00 pm »
I'm considering a sub for my stereo system, most likely a Martin Logan 600X.  ML says the wireless adapter has an input impedance of 5.5K ohms, and the sub wired direct from my Tortuga  LDRv3.25K with RCAs has an impedance of 8.3K ohms.  Amps are Pass Labs Aleph 2 monoblocks (10K ohms input impedance), and Nelson Pass F5T and F5 amps, both having 100K ohm impedance.  How do I figure out impedance using each of these amps?

Connecting a typical passive preamp to any amp creates 2 impedances in parallel. Mathematically the effective impedance is Z = (Z1 x Z2)/(Z1 + Z2). Thus if both the preamp and amp have say 20k input impedance the effective impedance (seen by your source component) would be 20*20/(20+20) = 10k thus effectively reducing the impedance by 50%.  The amp/preamp with the lowest impedance is main influencer. For example with a 20k preamp and a 100k amp you'd get 20*100/(20+100)=16.7k which is better than 10k but nowhere near the high 100k of the amp. Fortunately our preamps have adjustable impedance thus you could set the preamp to 50k with your 100k amp and get effectively 50*100/(150) = 33.3k.

The challenge is when you connect to something with very low input impedance like the 5.5k subs  you're considering. That's a ridiculously (and needlessly) low impedance for any amp in my view. Let's say you go with your preamp at 50k with the 100k amps for 33.3k effective. Then you connect that also to the 5.5k subs you end up with 5.5*33.3/(5.5+33.3)= 4.7k effective.   Using the 50:1 guideline (not a "rule" just a guideline) you'd want to be using a source with a low output impedance of around 100 ohms. But if you your source is say closer to 1000 ohms output impedance I think you'd likely have disappointing flabby sound with poor bass and dynamics. Much depends on how robust the output stage is within you source component - something very difficult to discern from specs alone.

lokie

Re: Adding a subwoofer, how to calculate impedance?
« Reply #2 on: 17 Apr 2019, 04:06 pm »
If you can pull it off, Infinite Baffle subs are very good value.

goofytwoshoes

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Re: Adding a subwoofer, how to calculate impedance?
« Reply #3 on: 10 Jul 2019, 08:52 pm »
Didn't want to buy a $1,000 subwoofer setup, only to discover that I must also add a buffer, so I have been putting off any purchase.  Then a few days ago, I picked up a ML Dynamo 300 sub for cheap, mostly to see how it's low input impedance (10K ohms, same as my Aleph 2 amps) would work with the amps/Martin Logan electrostatic speakers.  Much to my surprise, the Dynamo sub and Aleph amps work very well with the Tortuga - I don't detect any negative change with the sub in the system.  To make sure, I turned on the sub, but set the volume to "0" and had a listen.  Then I disconnected the sub and had a listen - no change, so the sub's low impedance is not affecting the SQ of the Aleph/Martin Logan setup.  With the sub turned on, there's not much difference in the bottom end, but that was expected as the sub has an 8" woofer, as do my speakers. 

 I will probably buy the Dynamo 800X, which has a slightly lower input impedance (8.3K ohms), but I think it will be compatible, too, with the Aleph amps.

tortugaranger

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Re: Adding a subwoofer, how to calculate impedance?
« Reply #4 on: 10 Jul 2019, 09:02 pm »
Didn't want to buy a $1,000 subwoofer setup, only to discover that I must also add a buffer, so I have been putting off any purchase.  Then a few days ago, I picked up a ML Dynamo 300 sub for cheap, mostly to see how it's low input impedance (10K ohms, same as my Aleph 2 amps) would work with the amps/Martin Logan electrostatic speakers.  Much to my surprise, the Dynamo sub and Aleph amps work very well with the Tortuga - I don't detect any negative change with the sub in the system.  To make sure, I turned on the sub, but set the volume to "0" and had a listen.  Then I disconnected the sub and had a listen - no change, so the sub's low impedance is not affecting the SQ of the Aleph/Martin Logan setup.  With the sub turned on, there's not much difference in the bottom end, but that was expected as the sub has an 8" woofer, as do my speakers. 

 I will probably buy the Dynamo 800X, which has a slightly lower input impedance (8.3K ohms), but I think it will be compatible, too, with the Aleph amps.


That strongly suggests your source can handle the lower impedance load without any noticeable drop off in performance. The proof is in the pudding as it were.

goofytwoshoes

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Re: Adding a subwoofer, how to calculate impedance?
« Reply #5 on: 14 Nov 2019, 10:28 pm »
I ended up getting a Dynamo 800X sub, with wireless kit and Perfect Bass Kit.  First tried it with the First Watt F5 amp.  The Martin Logan wireless transmitter has an input impedance of 5.5k ohms.  This proved to be too low with the F5 - I lost too much volume no matter what impedance value was chosen on the Tortuga.  Found a good deal on an iFi iTube Buffer/Preamp, and connected it between the Tortuga and the wireless transmitter (in buffer-only mode).  The iFi's input impedance in buffer mode is 1M ohms. No more issues with low volume!  After enjoying this for a month, I substituted Pass Labs Aleph 2 monoblocks for the F5.  Even with their low input impedance (10K ohms), they worked well with the buffer.

The 800X sub (10" woofer) fills in the bass nicely, although the Source electrostats have pretty decent bass (for my taste).  As a senior, I am very aware that my living arrangements could go all to hell (in an instant) if I suffer a serious health issue.  If I no longer have a room large enough for the floor standing speakers, I will need good stand-mount monitors.  To that end, I also have a pair of ML Motion 4 speakers (borrowed) and have listened to them with the sub for a few weeks so far.  This is where the sub really completes the system.  The Motion 4 speakers roll off below 70 Hz, and the sub does an excellent job of filling in the low bass.  This setup sounds amazingly close in SQ to the Source electrostats, and I could easily live with it if need be.

As a subsidiary of Paradigm/Anthem, Martin Logan's latest line of subwoofers has benefitted from the Anthem ARC Genesis room control software.  In conjunction with the Perfect Bass Kit, setup of the 800X is far easier and far quicker than previous ML subs.  Also, their Subwoofer Control  App (installed on my android tablet) is the perfect companion - I can change settings from my listening position which allows instant comparison.  My son says that any quality home theatre receiver has similar bass management features.  However, it sure is nice to have the Anthem/ML two-channel setup option.