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 NEW  Bruce Heran outlines the details and construction of his simple DIY 6DJ8 (ECC88) Tube Hi-Fi Headphone Amplifier Project.

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 Post subject: DIY amp with sub out?
PostPosted: 24 Jun 2014, 10:40 

Joined: 24 Jun 2014, 10:12
Posts: 1
Greetings all, this is my first post and I'm looking for some help because I'm feeling a bit overwhelmed! Here's what I'm trying to accomplish:

I am needing to build (or buy actually, but I like the thought of building one) an amp that can take audio in from a computer's headphone jack (so 3.5mm plug) and then feed that out to two speakers (a pair of Dayton Audio 60W max / 30w RMS 8ohm 90hz-20kh 91db sensitivity) *and* have an output I can send to the RCA input(s) (Line In) of a Polk 10" powered subwoofer.

The purpose of these is to drive the audio on a homemade pinball machine (that's the reason for the powered sub--subwoofers mounted into the bottom of pinball cabinets just don't sound that great). I purchased one of those cheap Lepai amps and it sounded like crap and the subwoofer deal (connected via speaker wire) sounded terrible.

So I don't think I need "world class" sound (and I'd like to keep the thing small and inexpensive) but I have recently gotten into electronics and I find that I really enjoy building and soldering together kits. So I thought I might come here and possibly find a kit, or some instructions/tutorials, on building something like this? or is there some other cheap option I can buy that might suit me better for this task?

I'm sorry this is such a noob post but I'm really just starting to wade into this area! Thanks in advance.

PostPosted: 24 Jun 2014, 20:51 
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Joined: 04 Jun 2008, 20:59
Posts: 2394
Location: Arizona, USA
Hi A worthy endeavor. A few questions. Does the powered sub have a low pass filter so that it will not respond to higher frequencies? If so it may be possible to modify the Lepai amp. If you tapped off the volume control (provided it is used as a voltage divider and not as a control element for gain) with a resistor from the adjustable terminal (the middle one) you could obtain a signal that would vary with the main volume setting to feed the sub. I would probably try something like a 4.7-10K 1/2 watt resistor from the volume control tap and run the ground return from the ground end of the control. Use shielded wire to connect to the sub. If the volume control is used to alter the gain of the amp and not the signal level then this will not work.

I am sort of surprised that using the fairly typical connection to the speaker terminals is not working satisfactorily. For non-critical applications this should be fine. Have you tried reverse phase on the sub. It may be that there is a phasing problem making it sound poorly.

Another alternative is to put a volume control in front of the both the Lepai and the sub. You could then set the Lepai on max and adjust the sub to match - or vice versa depending on which one has more gain. I believe I would use a 100K or perhaps 50K stereo volume control and use 4.7-10K fixed resistors to the sub and Lepai inputs to provide some isolation.

Good listening

Some of my DIY Tube Amplifier Projects:

PostPosted: 26 Jun 2014, 21:21 

Joined: 09 Oct 2012, 19:43
Posts: 145
Location: Vancouver Canada
The Lepai amps usually do not come with their own power supply. Therefore i assume you bought this separately. If so an under rated pwr-sply could cause horrible sound from those amps. Usually a buzz and enough distortion to curle your toe nails, due to filter capacitors not being able to do their job and maintain a good DC filtered supply of 12V to the amp. Also try another source instead of the comp with the amp. I know that is what you plan but, I have used the same amp for a friend to have similar system and the amp gave nothing but problems only when connected to comp though. Finally i used isolating transformers in the line between comp and amp. That cleared up the prob.
I agree with Bruce about feeding the sub from the same location as the main amp. For the same reason as above, the output of the Lepai amps is not a good choice for signal tap. On the other hand if the Lepai was sending out crap and sub was connected at output of Lepai sub will def sound like crap to.
Solve one prob at a time. Those Lepai amps can give more than enough vol and good sound for what you are doing. Start with amp only and try a diff source like CD or DVD and see what happens.

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