Riddle Me This, AudioKarma.. Sub Quandry

[IamSpartacus]

I'm in the process of setting up a new sound room and I've run into a problem I've never had before....

I just purchased a Yamaha A-S701 amp after 50 years of using an older Yamaha CA-810

I'll be switching between some vintage DCM Time Frame speakers (8ohm) and a pair of Magnapan LRS+ speakers (4ohm)... and until I can afford the subs I do want... (2) polk powered subs.

In the past and with an older CA-810 Yamaha amp, I ran the two powered subs directly to the amp.. then ran speakers wires off the 8 ohm subs to the 8ohm DCM speakers.. which drops the ohmage ('m told) to 4 ohms.. It's worked just fine... Even when I was using (4) tower speakers and the same two subs... All was well... No idea what that did to the ohmage..

Looking trough the new Yamaha manual, it says to NEVER connect subs this way.... Instead... this is what it says.. (pasted below) ..

I'll add... My end game plan is to connect ALL my speakers to one of those Monster Speaker Junction boxes and just pick and choose which speakers I want to listen to with or without the subs... Right now I'm just wondering if I should stop connecting the subs like I have for decades and follow the advice in the new manual and buy some kind of "splitter" and use the dedicated sub port?

Opinions? I'd hate to blow a new amp with simple ignorance... Somebody educate me...

How to Connect Subwoofers Safely

• Active (Powered) Subwoofers: If your two subwoofers have their own built-in amplifiers, you should not wire them to the speaker terminals. Instead, use the dedicated line-level Subwoofer Out RCA jack on the back of the Yamaha A-S701 Integrated Amplifier (using a Y-splitter if needed for two subs). This completely bypasses the amplifier's speaker load impedance.

 

[peterp1]

In the past and with an older CA-810 Yamaha amp, I ran the two powered subs directly to the amp.. then ran speakers wires off the 8 ohm subs to the 8ohm DCM speakers.. which drops the ohmage ('m told) to 4 ohms.. It's worked just fine... Even when I was using (4) tower speakers and the same two subs... All was well... No idea what that did to the ohmage..

I don't think the above statements about impedance are correct -- the speaker level inputs on active subs intentionally have very high impedance -- it doesn't really matter what impedance the internal sub driver is because the main stereo amp (Yamaha) never sees that load. Passive subs would behave as described, but not active/powered subs. The high-level inputs on active subs only need to read the main amp's signal waveform to replicate it with the sub's internal amp -- they don't need or want any of the amp's power, and they also want to be transparent impedance-wise to the main speakers -- so they virtually always design active sub high-level inputs to have high impedance.

In your 2-speaker DCM scenario, it would still be 8-ohms because the sub high-level inputs and outputs are essentially tied together internally, so the sub is just passing the amp signals through to the main speakers, and the high-impedance of the sub connection means it doesn't affect overall impedance. In the 4-speaker scenario, assuming all 8 ohm speakers, with each pair wired in parallel, you'd then be at 4 ohms. In some cases, subs can have a high-pass filter between the high-level inputs and outputs to offload the main amp and speakers from trying to reproduce the deepest lowest frequencies now being handled by the sub, but input impedance is always still high.

Looking trough the new Yamaha manual, it says to NEVER connect subs this way.... Instead... this is what it says.. (pasted below) ..

I'll add... My end game plan is to connect ALL my speakers to one of those Monster Speaker Junction boxes and just pick and choose which speakers I want to listen to with or without the subs... Right now I'm just wondering if I should stop connecting the subs like I have for decades and follow the advice in the new manual and buy some kind of "splitter" and use the dedicated sub port?

Opinions? I'd hate to blow a new amp with simple ignorance... Somebody educate me...

How to Connect Subwoofers Safely

• Active (Powered) Subwoofers: If your two subwoofers have their own built-in amplifiers, you should not wire them to the speaker terminals. Instead, use the dedicated line-level Subwoofer Out RCA jack on the back of the Yamaha A-S701 Integrated Amplifier (using a Y-splitter if needed for two subs). This completely bypasses the amplifier's speaker load impedance.

I was baffled why Yamaha has this recommendation given the high impedance on active sub inputs, so I asked AI. The response cut & pasted below below seems to make sense, though it says it really applies to the higher end models than the 701 that have a "floating balanced power" amplifier (where it claims using speaker-level to drive a sub could destroy the amp ).

I wouldn't take AI's word on this, or what models have which topology and do more investigation to be certain -- but apparently there is potential for significant risk for some of their amps .

Cut & Paste of AI Response Below:

The reason Yamaha warns against or discourages high-level (speaker level) subwoofer connections is not because of the high impedance or power draw—you are entirely correct that the sub's high input impedance makes it virtually invisible to the amplifier. [1]
Instead, the warning is driven by grounding topology conflicts and risk of short circuits. [1]

1. The Floating Ground / Balanced Amplifier Trap
While the Yamaha A-S701 uses a traditional common-ground Class AB design, Yamaha uses a "Floating Balanced Power Amplifier" topology in its higher-end sister models (such as the A-S1200, A-S2200, and A-S3200). [1, 2]

• • In a floating/balanced amp, the negative (-) speaker terminal is not grounded. It is actively driven with a negative-phase signal (bridged/BTL). [1, 2]
  • Many active subwoofers tie their left and right negative speaker-level inputs together to a common internal ground. [1]
  • If you connect a balanced amp to a common-ground subwoofer using standard high-level cables, you instantly short the amplifier’s active negative terminals together or to ground, which can destroy the amplifier's output stage. [1, 2]

To keep documentation simple and foolproof across their entire modern integrated amplifier lineup, Yamaha issues a blanket recommendation: always use the dedicated line-level Subwoofer Out (RCA).