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Old 1st March 2007, 08:36 PM   #1
pjpoes is offline pjpoes  United States
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Default DIY High to Low level converter

I'm pretty sure these are easy to make, but wanted to know if anyone could give me some advice on how to do it for best sound, and also what values to use. I want to build a device that will replicate the many, for instance, car high level to low level signal devices, which are normally a set of resistors. I'm not really sure what else, if anything, goes into them. However my complication is that this is to go from the speaker level outputs of a tube SET integrated amp into a subwoofer, and I would like to have the the SET amp see a nice even load, but I dont know what load is best, and how much resistance I need to turn all that current and voltage into 2 volts and a few ma of current. I was also originally considering building a simple lowpass crossover into it, but now I dont see a real advantage to that, since the sub has a built in crossover in the amp. If I'm wrong, let me know, and why.

An alternative method might be to hook a set of RCA's onto the volume control, allowing it to act as a passive preamp, but I was worried that because it has to drive a 15-20ft long pair of RCA's, it might not work so well. I know passive preamps are picky about the loads they are matched with, and I'm not sure what type of load a sub amp presents. I also considered, since I believe the amp is essentially a preamp at the 12au7-second tube in the chain, 4th triode (Correct?) that I could somehow tap in at that point and have an active preamp driving it as well. However, I was worried about how that might affect the amp.

The final option is to bypass the volume control, which I already added a switch to do that, and use it like a power amp, then use a tube preamp to control volume and feed the sub as well. However, thats an extra expense I don't really want I don't think.
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Old 1st March 2007, 08:51 PM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
taking the speaker outlet signal and converting it to drive a line level is not the best way, but for a sub it might be adequate.

What impedance can the tube amp feed? I am guessing 8ohm.

Adding a 10k resitor in parallel to the speaker will make almost no difference to the load the tube output sees.

Now add a resistor ladder in parallel to the speaker to give the attenuation you need.
The attenuation will depend on the relative sensitivities of the sub and mains and on the gain in the sub.

-20db (=1/10) may be about right but it could vary a lot from this guesstimate.
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Old 1st March 2007, 09:28 PM   #3
pjpoes is offline pjpoes  United States
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I thought that would be the case, if it was in that range, but wanted to be sure. To make sure I understand you correctly, by ladder, you mean, start with a 10k ohm resistor, and add further resistance from there until I have what I want for the reduction.
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Old 1st March 2007, 10:45 PM   #4
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Try this

I used it on a home made sub amp and it worked fine. No noticable loss of channel separation.
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Old 1st March 2007, 11:10 PM   #5
pjpoes is offline pjpoes  United States
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Thank you thats exactly what I was looking for. I actually have a 22K and 1k set of Caddock resistors that I have no use for. They are great sounding in low level signals, so should be pretty decent. Thanks.
-Matt
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