View Full Version : Split sub output
02-21-2008, 05:49 PM
I recently set up a system using two home buit subs (modified JBL 128H). The output comes from an Adcom GTP-600; it has one sub output. It sends the signal to an Adcom GFA-545II power amp were the signal is split to feed both inputs. I am sure I have weakened the signal by splitting it and I am not getting as much out of the subs as possible. Can I boost the signal going to the power amp? Are there any line level preamps avaliable for this?
02-22-2008, 08:30 AM
What controls the volume of your subs?
How many watts/ channel (RMS, both running) on the GFA-545?
02-22-2008, 08:54 AM
The GTP-600 controls the volume of the subs and the GFA-545 is rated at 100 at 100 watts into 8 ohms.
02-22-2008, 04:07 PM
if you split the RCA in two you have NOT reduced each signal.
if you don't believe me you can replace your Y splitter with a straight wire going to one of the subs and see that there is no difference.
when you split a signal in parallel it does not get weaker until the impedance of your load becomes lower than the output is comfortable with. if your load is in the tens of kilo-ohms range (as line-level loads usually are) you have nothing to worry about.
paralleling speaker loads is another story because you start with a load of only a few ohms and and if you keep making it smaller (by connecting things in parallel) pretty soon you're out of the comfort range of your amp.
02-22-2008, 06:55 PM
Does your distortion led come on?
02-27-2008, 03:35 AM
02-27-2008, 03:37 AM
I will take ouit the Y to check
>I recently set up a system using two home buit subs (modified
>JBL 128H). The output comes from an Adcom GTP-600; it has one
>sub output. It sends the signal to an Adcom GFA-545II power
>amp were the signal is split to feed both inputs. I am sure I
>have weakened the signal by splitting it and I am not getting
>as much out of the subs as possible. Can I boost the signal
>going to the power amp? Are there any line level preamps
>avaliable for this?
First, double check your wiring to make sure the woofers are operating in like phase. If one is wired out of phase, output will be significantly reduced toward lower frequencies where the radiation is summing with correlated phase, the interference nearly cancelling at very low frequencies (overtones and distortion harmonics will remain if the two sources are not co-located in the room).
At a listening position equidistant to the two similar sources:
Where the two sources sum with uncorrelated phase, the sum is +3_dB relative to the output of one operating by itself. Where the two sum with correlated phase (located within 1/4 wavelength of each other), the sum is +6_dB relative to the output of one operating by itself. So if the subs are not co-located, then you should try placing them together for +3_dB in the upper end of their operating range, which is also the more audible portion of that range as hearing sensitivity decreases steeply with decreasing frequency in the double digit frequency range.
If that doesn't do it for you, then you need a step up input transformer or a preamplifier to boost the signal going into the amplifier. A good input transformer will likely provide far more common mode noise rejection than an inexpensive preamp, but the preamp will buffer a weak upstream source. So which is the better solution depends on which is the bigger problem.
The Jensen JT-112P-2HPC is made specifically for use at low frequencies. Wire the primaries in series for 1.1:1 voltage boost, or in parallel for 2.2:1 boost. Excursion and SPL are proportional to voltage when everything is operating within the linear range. It should be connected to the inputs of the power amplifier with _very_ short interconnects to provide best common mode noise rejection.
The price is listed at $70 direct from Jensen:
For ~$80, you can get a Behringer DSP110, or for ~$140 a Behringer FBQ2496, both of which can provide some boost and also a little parametric EQ, if placed in between the GTP-600 and the wye connector.
>...Excursion and SPL are
>proportional to voltage when everything is operating within
>the linear range...
Apparently I was typing faster than I was thinking on that.
SPL is not proportional to voltage. SPL is proportional to power. But excursion is proportional to voltage, so doubling voltage will double excursion, which increases SPL +6_dB.
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