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How Does a "HI-LEVEL IN" Affect Impedance Load?

I'm looking at adding a sub (possibly a used Sunfire MK2) to a 2-Channel integrated tube amp, with no pre-amp/sub outputs. Would it be ill-advised to connect (in parallel) the speaker outputs with the Hi-Level In, on the sub? What effect. if any, does this have on the impedance load? The currently-used (custom built) speakers run around 6-ohm, and the amplifier (Jolida) does have both 4, and 8-ohm terminals, for whatever that information is worth.  

Thanks!

-Brian
brian_stabinger
erik_squires
15,911 posts
 
Sub speaker level inputs are usually in the range of hundreds to tens of thousands of Ohms.

So, doing the normal math for parallel resistance (which is close enough to impedance for us to illustrate) :

R = 1 / (1/8 + 1/10000)

where 8 is the speaker impedance, and 10,000 is the sub impedance, gives us 7.99 Ohms, and a difference of around 00.1%.

It’s insignificant to a power amp.
millercarbon's avatar
millercarbon
12,531 posts
 
Right. No effect. This paper explains exactly what is going on. If you can find the circuit diagram for your sub you will find this same circuit inside the sub. The values of the resistors may be different to accommodate a wider range of amplifiers, and there will be a trim pot to adjust level, but its really the same basic circuit inside all of them.
https://www.epanorama.net/circuits/speaker_to_line.html
brian_stabinger OP
19 posts
 
Thank you all so much for the detailed information. It's hard to believe that in all these years, a guys has never used speaker-level inputs on a sub. The more you know... 
bkeske's avatar
bkeske
5,596 posts
 
That is how my two subs are hook-up, in stereo. I like it vs LFE.
brian_stabinger OP
19 posts
 
Hmm. This certain local seller does have two Sunfire MK2's, for sale. I like the stereo option! 
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