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Old 6th January 2014, 09:25 PM   #1
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Default Subwoofer Channel on Tube Amp

Hi Folks,

I'm building my first tube amp and have chosen the attached schematic for my left and right channels (changes include a different power supply and a James output transformer). I've bought the parts, and will be prototyping soon.

I would love to have a subwoofer channel for this amplifier however. It would be a very modern concept in an amplifier using very antique parts. Do any of you have advice for doing this? The way I see it is that there are multiple options:

1) Filter at the input of the amplifier. Use a separate amplifier circuit for the bass frequencies (would need to buy parts for an entire separate channel).

2) Filter just before the output transformers. Send mids and highs to left and right output transformers, while sending the lows of one channel to a third output transformer dedicated for the subwoofer (would need to buy a single extra output transformer).

2) Filter after the output transformers. Send mids and highs to left and right speakers, while sending the lows of one channel to the subwoofer. Would need to worry about matching but no need for extra parts

What do you all think? I'm wondering about tradeoffs for each and if I am approaching this in the right way.
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Old 6th January 2014, 09:50 PM   #2
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Why go through all that unnecessary trouble. There are many makes of stand alone subwoofers with built in amplifiers that will do all the filtering and mixing for you. Yea, they are solid state but so what. You're only talking low frequency bass and SS is actually better for that. Many of these subs will accept left and right channel signals from either speaker or preamp. If you go this route, be certain to get a sub with a phase switch on the rear which makes them a whole lot easier to set up.

If you really want to use individual componets, I always liked the Dahlquist DQ-LP1 filter.
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Last edited by HollowState; 6th January 2014 at 09:59 PM.
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Old 6th January 2014, 09:52 PM   #3
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In my opinion you would be better off using a 150 to 200 watt solid state amp for the sub. My reasoning for this is:

1. If you take signal before opt and add another opt you will have serious plate load issues.
2. If you take signal after the opt you will have aforementioned impedance mismatch which will also skewtthe reflected load on the plate.
3. In order to make enough watts and have the looow freq. Bandwidth you will need large and expensive iron.

I run my tubes through full range speakers and use a 150 watt solid stater made by TOA to handle the sub after an active line stage crossover. I think it sounds great. IMO there is no advantage to a tube amp powered sub other than more glowing glass and heat.

Just my 2 cents.
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Old 6th January 2014, 10:30 PM   #4
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Thank you guys for your advice. It sounds like my best bet would be to build my tube amp for my left/right mid/high channels and then use a high wattage powered sub.
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Old 7th January 2014, 04:11 AM   #5
chrish is offline chrish  Australia
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I have two tube systems that have solid state driven woofers. For both, I used an active crossover to split the signal before the tube amp. Both are using this crossover from Elliot Sound Products Linkwitz-Riley Electronic Crossover

Both also use sealed subs with a linkwitz transform circuit Linkwitz Transform Subwoofer Equaliser

For my Tubelab SSE amp, I built a copy of the subwoofer that Linkwitz designed for his Pluto system, crossed over at 100 Hz Pluto + subwoofer

The sub was cheap and easy to build and has very good performance. I got lazy and purchased a cheap remote controlled pre-amp board from eBay with an aluminium chassis. In the same box I put a simple power supply, the pre-amp, the crossover board and the Linkwitz transform board. I put a simple chip amp in the woofer box to power the woofer, along with a signal detector circuit (also found on the Ellioit Sound Products site) to automatically turn the sub on when it detected a signal.
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