Foreplay III Second Pair of Outputs for Subwoofer

AudioDave · 10939

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Offline AudioDave

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on: April 12, 2010, 06:47:37 AM
Hope this isn't to silly of a question.  I would like to try using a non-powered subwoofer in my system by connecting a mono amp to the second pair of outputs on my Foreplay III.  Would I simply connect the left and right outputs together using a splitter and then on to the mono amp,  passive crossover, and subwoofer or is there a better way?
Thanks!
Dave 



Offline Paul Joppa

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Reply #1 on: April 12, 2010, 09:13:12 AM
Connecting them together will reduce separation on all the outputs. The simple way is to replace the 1K resistors (back by the RCAs) with at least 10K, up to 50K resistors. More resistance gives less loss of separation. The high resistance may reduce the signal, depending on the amplifier's input impedance, and it will certainly reduce the high treble due to the capacitance of the interconnect - but that's not something you need to worry about in a bass amp! The high impedance will make the cable more susceptible to picking up hum from electric fields, so choose an interconnect with a good high-coverage shield.

Instead of the speaker-level filter, you could use an active crossover, or probably even a passive line level one, upstream of the mono amp. That would reduce wasted power in the mono amp. Id you do use the passive speaker-level filter, and if the mono amp has negative feedback (i.e. solid state or push-pull pentodes) make sure there is a load at high frequencies to prevent amplifier instability.

Paul Joppa


Offline AudioDave

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Reply #2 on: April 12, 2010, 01:22:55 PM
Thanks Paul,  I appreciate that explanation.  It was a little more involved than I thought.  That's why I like to ask first!
Thanks again.
Dave



Offline AudioDave

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Reply #3 on: April 12, 2010, 03:05:31 PM
After thinking about Paul's suggestions, I am leaning toward an active crossover for my subwoofer.  Do any of you have a good suggestion for either a DIY or assembled active crossover for a subwoofer powered by a mono amp?
Thanks!
Dave



Offline 2wo

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Reply #4 on: April 12, 2010, 04:43:45 PM
How about a plate amp? It has the crossover, amp, and L&R inputs all in one package

John Scanlon


Offline Brillo

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Reply #5 on: January 14, 2015, 08:47:44 AM
The simple way is to replace the 1K resistors (back by the RCAs) with at least 10K, up to 50K resistors. More resistance gives less loss of separation. The high resistance may reduce the signal, depending on the amplifier's input impedance, and it will certainly reduce the high treble due to the capacitance of the interconnect - but that's not something you need to worry about in a bass amp!

Paul (or anyone able to comment) - I've had 10k resistors between my outputs for years, but struggled with too much high frequency coming across to my powered sub.  I swapped in a pair of 43K resistors today.  I don't notice much difference on the sub (a small improvement), but bass on the primary outputs (to Paramounts) seems to be slightly rolled off.  Technically, is that a potential effect of moving from 10k to 43k, or am I just making this stuff up in my head?

Chris
« Last Edit: January 14, 2015, 10:46:57 AM by Brillo »

Chris


Offline Paul Birkeland

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Reply #6 on: January 14, 2015, 12:26:05 PM
Are you using a "Y" cable at the second set of outputs of your Foreplay? 

If you want to roll off the frequency response for your sub amp, this is best done with an active or passive line level crossover, or a speaker level passive crossover.

-PB

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Offline Paul Joppa

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Reply #7 on: January 14, 2015, 01:52:19 PM
The larger resistor will attenuate the input to the sub amp - did you re-balance the subs after the resistor swap?

There could be a small effect on the main outputs, if the sub amp input impedance is very low. I can't see a technical reason to expect that effect to be audible, but stranger things have happened.

To cut more from the subs, leave the 43K resistors but put a cap across the sub-out RCA. Try 0.022uF, that gives about a 170Hz corner. Bigger cap = lower cut-off.

Paul Joppa


Offline Brillo

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Reply #8 on: January 16, 2015, 07:10:02 AM
Are you using a "Y" cable at the second set of outputs of your Foreplay? 

PB - No, not using a Y cable.  The single sub has L/R RCA inputs, so I'm using both L/R outputs (and two cables) from the Foreplay. 

... did you re-balance the subs after the resistor swap?

To cut more from the subs, leave the 43K resistors but put a cap across the sub-out RCA. Try 0.022uF, that gives about a 170Hz corner. Bigger cap = lower cut-off.

PJ - No, haven't rebalanced/adjusted the sub, but I must not have matched the previous volume settings on the Foreplay (or something like that).  Confirmation there's no technical reason for an audible change sets me straight.   I'll try a cap across each RCA - that seems to be the easiest and cheapest first step.  I assume that cap doesn't need to be high quality??? 

Thanks to you both for the input.  Either solution will cure issues I've simply 'lived with' for several years now!

Chris

Chris


Offline Paul Joppa

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Reply #9 on: January 16, 2015, 09:30:46 AM
The cap should only affect the sub output, so it is probably less important than one in the main output circuit. If you happen to have a really high resolution sub, it's possible there could be an audible effect. I suggest you start cheap until you know whether it does anything useful!

Paul Joppa


Offline Brillo

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Reply #10 on: January 16, 2015, 01:39:44 PM
I suggest you start cheap until you know whether it does anything useful!

Yep, that's the plan.  Will do.  Thanks Paul.

Chris


Offline Brillo

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Reply #11 on: January 19, 2015, 01:35:56 PM
To close this out: the 0.022uF cap worked like a charm.  Thanks gents!

Chris