DIY subwoofer output for receiver

Tube Radio

Tube Radio

Super Member
I have a Fisher 202 Futura Series receiver. I want to add an output to drive a powered sub. My plan is to add a mono output that works like the center channel output that some of the older receivers had. I will make the box external to the receiver so that I do not have to add anything to the receiver itself.

I plan on using a couple resistors and a variable resistor to provide a level control for the sub since IIRC the subwoofer's level control is after a gain stage or two. I will also add a couple 220 uF caps in series with the main speaker outputs for a high pass crossover.

I do have a question. Given I will not be using film caps (large and expensive for 220 uF) what is the next best cap to use? I was gonna use a 220 uF Nichicon audio grade electrolytic cap, but don't know if a polarized cap will work ok for use to pass an AC signal given there will be no DC voltage on the cap.

What value of resistors and variable resistor will be best for this project. I am thinking maybe a 10K variable resistor and 1K series resistors. What do y'all think? for the variable resistor I will use a panel mount screwdriver adjust pot as the variable resistor will not be adjusted very often if at all.
 
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Mono sum output from Sansui 1000A.

Posted below is the "Mono Sum" output from a 1000A receiver.

Just use the 3 resisters attached to the "Center channel out", all are 1/2 Watt. (two 2.5K one 5K resisters)

You could put a 5K pot where the 5K resister is and get your variable output.

Your "box" would have 3 wires and a RCA jack on it, a "common" wire to chassis, a "left channel" wire (one 2.5K resister) to your left speaker hot terminal, a "right channel" wire (the other 2.5K resister) to your right speaker hot terminal, and a RCA jack for signal to the sub.

Your sub may already have a low pass filter built in so you don't need the series caps (220 uF is too high a value for line level service).

Here is the link.

Mark T.:music:
 
The 220 uF caps would be in series with the speakers for the midbass-highs to make a high pass filter for them. I am not sure if the JVC sub still has its low pass filter or not as I had to do some work to it (increased size of one cap to get amp flat to 20 Hz removed a cap to raise crossover frequency somewhat & replaced original 10" sub with 10" Sony Xplod sub) to make the sub usable. I will have to try that circuit and see how it works.
 
I assume it's a powered sub? if so, seems like you're over thinking things..no need for a low pass filter if your sub has an active crossover. And if it is a powered sub, couldn't you just hook it to the tape monitor out? most have either L&R in via RCA connections, or just a mono RCA connection...connect the L&R tape monitor with a Y adapter, to the RCA in of your sub if it's mono only?
 
BinaryMike said:
The external high-pass caps must be non-polar lytics. I believe Mouser stocks some that big.
Click to expand...

I already ordered a couple Nichicon 220 uF audio grade electrolytic caps. I will try those and if it don't work right I will have to get six more of the caps and put two in series paralleled by two in series to get the 220 uF capacitance.
 
Active sub with passive cross-over at speaker outs

Here is my proposition. See attached file.

The equations are correct if the input impedance of the sub-woofer amplifier is many times larger than the variable resistance VR1.

You may enter the impedance of the satellite speaker to find the correct value for the series capacitor C1, according to the High Pass corner frequency that you want.

Then, you need to compute the correct value for the low pass capacitor C2. It make sense to have the same frequency for the High-Pass and the Low-Pass filters.

For example, with this choice of components:

C1 = 200 uF
VR1 = 10k Ohms
R1 = 1k Ohms
Zs = 8 Ohms

High-Pass corner frequency = 100 Hz

C2 = 3.36 uF

All capacitors must be non polar. Large non polar capacitors do exist. You can use oil capacitors like the ones used for motors.

To limit the insertion loss in the mixer, try to keep R1 much smaller than VR1.
 

Attachments

I'll look at that circuit, although I think the powered sub still has some sort of a crossover in the amp.

I could use regular bipolar caps, but I don't want sound quality to suffer so I'll just buy six more Nichicon 220 uF audio grade caps and do a series parallel configuration of the caps. I could connect the caps right at the output transistors where the 2,200 uF output coupling caps are then I wouldn't need the other six caps. That would require a minor modification though. All I would have to do is connect the cap to the connection for the original output caps and then wire the new caps to the A speaker output terminals. That way the B outputs will be full frequency and I can make the A outputs full frequency by setting the speaker switch in A or A+B with B being the switch position for the high pass filter on the A speaker outputs.
 
ecluser said:
Here is my proposition. See attached file.

The equations are correct if the input impedance of the sub-woofer amplifier is many times larger than the variable resistance VR1.

You may enter the impedance of the satellite speaker to find the correct value for the series capacitor C1, according to the High Pass corner frequency that you want.

Then, you need to compute the correct value for the low pass capacitor C2. It make sense to have the same frequency for the High-Pass and the Low-Pass filters.

For example, with this choice of components:

C1 = 200 uF
VR1 = 10k Ohms
R1 = 1k Ohms
Zs = 8 Ohms

High-Pass corner frequency = 100 Hz

C2 = 3.36 uF

All capacitors must be non polar. Large non polar capacitors do exist. You can use oil capacitors like the ones used for motors.

To limit the insertion loss in the mixer, try to keep R1 much smaller than VR1.
Click to expand...
Thank you so much for that! :yes:
I am very math-limited and your equations and explanation tripped some type of neural trigger in the "Understanding Lobe" and I can now make some sense of this and other circuit equations. Ever have one of those moments, wierd?:scratch2:
 
Tube Radio said:
I could use regular bipolar caps, but I don't want sound quality to suffer so I'll just buy six more Nichicon 220 uF audio grade caps and do a series parallel configuration of the caps. I could connect the caps right at the output transistors where the 2,200 uF output coupling caps are then I wouldn't need the other six caps. That would require a minor modification though. All I would have to do is connect the cap to the connection for the original output caps and then wire the new caps to the A speaker output terminals. That way the B outputs will be full frequency and I can make the A outputs full frequency by setting the speaker switch in A or A+B with B being the switch position for the high pass filter on the A speaker outputs.
Click to expand...

Why do you think the series/parallel arrangement of polar lytics is superior to a single non-polar unit?

Your mod idea is clever and very tempting, but you would want fairly close tolerance polar lytics. They might be hard to get, even at 10% tolerance, which is worse than many designers would accept for filter duty.

The right way to do this job is to add a mixed mono output just downstream of the tone amp section, then add a switchable HPF between that point and the power amp input. In this way, you keep high-amplitude low-frequency signals out of the high-frequency power amp entirely, which is how the biggest benefits of bi-amping are achieved. And, you avoid the whole problem of LF distortion caused by lytics used in the high-pass filtering role. It might make more sense to just add pre-out and main-in jacks, so you can use an external electronic crossover, but that might be more difficult to do without bad cosmetic effects.
 
ecluser said:
To limit the insertion loss in the mixer, try to keep R1 much smaller than VR1.
Click to expand...

In fact, since there is some gain in the sub-woofer amplifier (probably more than 20 dB), it is perfectly acceptable to have some insertion loss in the mixer.

We could use for example 100k resistors for R1. Then we would need only 0.2uF for C2 in the above example. A small film cap.

I didn't know that the output of the receiver was cap coupled. If you try the circuit I proposed, you will need bleeder resistors at the outputs to charge the output capacitors. With large values for R1 it will take too much time to charge the output capacitors.
 
That is correct so I most likely will want to connect the caps directly to the output transistors then?
 
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Tube Radio said:
I do want to keep the receiver as original as possible.
Click to expand...

For various reasons, I've added RCA jacks to a few old components by running thin coax through ventilation slots and just letting the jacks dangle on pigtails. Zero cosmetic damage, and easily reversible.
 
Unfortunately the only vent holes are on the too and bottom of the receiver and the holes are not anywhere near big enough to pass an audio cable through.

I am hoping the parts come in today as I want to go ahead and take the receiver to work.
 
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