Definitive Technology (Def Tech) BP7006/BP7004/BP7002 - BP8954MOD

[MrGadget]

I'm about to give this mod a shot, when wiring it all back up there were 2 additional leads from the speaker RCA inputs that connected to the OEM amp. I'm assuming this is what you meant by losing the ability to power through just the speaker line inputs? So my configuration would have to be speaker line inputs plus LFE?

Yes, that was covered early-on in this thread.

I almost took that route myself (replacing the amps in the BP7006 units). But I got mired in all the options - everything from a 'boxed' external amp (like the Fosi), to the many "amp boards" available on eBay and Amazon. I wanted to retain speaker level inputs, so that limited my options. If I built it myself (using an "amp board"), I could retain speaker level inputs by adding a "Line Output Converter" (capable of about 40 watts of input power) for about $10-20 each on eBay, Amazon, or Parts Express. I also wanted the ability to adjust the volume level and the cutoff frequency. Some of the cheap / low power Amp Boards included those features, but the better, more-powerful amp boards did not ... so I'd need a separate subwoofer preamp board (available on Amazon for under $10, though I didn't find too many options to choose from). Powering the amp board was another big question. Some include their own power supply on the board (e.g. ICEPower), but they're big and expensive (over $100 each) and they still needed a separate preamp and LOC. Cheaper amp boards required a separate power supply, and I thought I'd reuse the original toroidal transformer from the Def Tech amps - which produces both 24VAC and 49VAC. But not all Amp Boards accept AC voltage, so I thought I might build my own bridge rectifier / filter cap circuit using a PCB kit easily found on eBay specifically for that purpose (another $10 or so). Of course, I'd need to mount everything on the original subwoofer amp backing plate, not knowing what kind of hurdles I might hit trying to mount everything. Still, it all sounded doable, but the cost would still probably be $100 per speaker ... AND a LOT of work!

In my case, I was able to repair my original DT amps (hope they stay fixed). But if I replaced them, I think I'd buy standalone / boxed / external amp(s) like the $49 Fosi subwoofer amp (and one LOC) per speaker. I have a Fosi amp (small stereo amp with subwoofer output) which powered the DT subwoofer driver fairly well (moderate listening levels). And I could adjust the cutoff frequency and relative volume. And an external amp could be redeployed on another subwoofer in the future. Anyway, these were the many thoughts that went into this project. Hope it helps.

 

[MrGadget]

Glad this thread is still active. I inherited0 FOUR BP7006 units and one BP7002 unit. With some help from another forum (AVForums), I replaced the SMT caps in the ±15V power supply section, which eliminated the infamous hum. But one of the units exhibited additional / sporadic humming and scratchy noises. I nearly gave up, until the suggestion about "tin whiskers" convinced me to clean both PCBs better. I removed lots of old glue from around the U4 integrated circuit, and cleaned the entire DS300 Amp Board with a toothbrush and alcohol (after removing the heat sink). I used a toothpick to clean between the contacts of the integrated circuit, which were gummed up with old glue (which some have suggested might become conductive as it ages). Tried to be gentle since some of the PCB traces are very small and delicate. Sure enough, the noises stopped (fingers crossed).

Unfortunately, I'm having additional troubles with two of the units. On one, the bass level potentiometer raises/lowers the bass level only slightly. Even if I short circuit the pot to zero-resistance, the bass level is lower than the other speakers. Maybe there's a faulty Op Amp somewhere in the amplification pipeline. Or maybe "tin whiskers" or old/hardened (electrically-conductive?) glue are shorting something. I'll try cleaning its PCBs. On the other unit, the bass level jumps dramatically when the pot is turned slightly above minimum. I'll try some tuner cleaner on the pot. More to come on these problems.

Thanks to everyone for their thoughts and comments. I was on the verge of replacing the amps (the main topic of this thread), but the warning about losing the equalization boost (@35Hz) convinced me to keep trying to fix the original amps. Also, the cost of replacing the amps was easily over $100 per speaker since each required an amp board, a pre-amp (with volume & crossover adjustments), a Line Output Converter (to accept speaker-level inputs), and some harnesses, mounting hardware, and lots of work.

Even if I get these units all repaired, they're too "boom-y" for my taste. The bass isn't as tight and deep as my home-made subwoofer. Oddly, these Def Techs don't have a pot to adjust the crossover frequency. Maybe Datrumole is right about the crossover frequency being high to blend with the upper speakers ... higher than normal for a subwoofer (and hence someone's comment that these aren't really subwoofers, but rather just woofers). So, once the repairs are complete, maybe I can sell them to a local buyer (they're too big to ship), else maybe I'll donate them to my local Habitat-for-Humanity "ReStore". The wife gives me funny looks when she envisions me cluttering the house with four more tower speakers!

Reply to myself ... I replaced the potentiometers which fixed the level control problems. Now I have four functioning DT BP7006 speakers that are too boom-y for my taste! I wonder if the boom-y-ness is intrinsic? ... or if it's being caused by the equalizer circuitry not functioning properly?? Given all the other problems with these amp units, I wouldn't be too surprised.

 

[bmfkai]

I went ahead and gave this mod a shot since I got the speakers for free but I still seem to be getting a slight buzzing from both. It's nowhere near as loud as the stock amps and certainly usable. I have to be right up next to the speakers to hear it (when not in use). Wondering if anyone else has the same issue?

 

[Stu Pidasso]

I had the same issue with one of the amplifiers. It had a non-stop 120hz hum, which hints right away the problem is after the full wave rectifier and near the voltage regulator. Replace the 10uf 16v caps on the +/-15 volt supply. It's stupid that they're 16v for starters. They're surface mount so you'll have to have the ability to rework surface mount components. Additionally, don't forget that the mark on the SMD cap is on the positive side NOT the negative one. I ended up gluing two 10uf 50v caps to the board because of size issues and jumped two Kynar leads from the pads to the new caps.

 

[bmfkai]

My boards already have 10uf 50v caps

 

[MrGadget]

My boards already have 10uf 50v caps

But are those caps any good? I sprayed mine with freeze spray and the hum disappeared. Granted, I'm talking about the the SMT caps on my original DT amplifier, not on a replacement amplifier. Still, a cap could be bad.

P.S. I wish I understood the DT schematics as well as Stu Pidasso does ... I don't understand the parameters like Q=.784; D=1.2755; and A=4.771 DB. I understand the boost at 35Hz, and that's largely why I wanted to repair the amp rather than replace it.

P.P. S. Two of my four units aren't going into standby. I didn't notice that until just recently. Now I have a new mystery to solve! But at least the schematic from Stu should help narrow it down. Thanks again for that!

 

[bmfkai]

But are those caps any good?...

Ah, yeah I was asking about the TDA8954 board but it certainly wouldn't hurt for me to check the caps

 

[Stu Pidasso]

Ah, yeah I was asking about the TDA8954 board but it certainly wouldn't hurt for me to check the caps

If you're driving the TDA board with the output from the preamp section in the stock amplifier, (which of course you would be, because why would you skip the filters which make the box sound as the designers intended) the hum is most likely emanating from bad caps on the +/- 15v line. If you're not driving the TDA board with the output from the stock preamp, well I can't help you.

 

[Stu Pidasso]

But are those caps any good? I sprayed mine with freeze spray and the hum disappeared. Granted, I'm talking about the the SMT caps on my original DT amplifier, not on a replacement amplifier. Still, a cap could be bad.

P.S. I wish I understood the DT schematics as well as Stu Pidasso does ... I don't understand the parameters like Q=.784; D=1.2755; and A=4.771 DB. I understand the boost at 35Hz, and that's largely why I wanted to repair the amp rather than replace it.

P.P. S. Two of my four units aren't going into standby. I didn't notice that until just recently. Now I have a new mystery to solve! But at least the schematic from Stu should help narrow it down. Thanks again for that!

The Q refers to quality - or an easy way to think of it is the width of a cut or boost - D is damping and refers to what happens filter roll-off, A is amplitude in DB.

 

[MrGadget]

The Q refers to quality - or an easy way to think of it is the width of a cut or boost - D is damping and refers to what happens filter roll-off, A is amplitude in DB.

Hah! I googled it for a better understanding ... but the first hit about Analog Filters was WAY beyond my skillset. (I started electrical engineering in college, but didn't have the stamina for THAT MUCH math!). Still hoping to fix the "standby" circuitry that is preventing two of my BP700x amps from "going to sleep". I'm looking at the third page of the schematic you uploaded (labeled as Page 10). I suppose my first suspects will be capacitors. Another suspect might be the LM324 IC whose pins are coated in 20-year-old glue (which someone suggested might have become slightly conductive over the years). The schematic shows an orphan "x" coming off pin 1 of a BAV99 (dual in-series diode right in the middle of the diagram). I'm confused where that goes?

 

[branman12]

@Stu Pidasso Do you have an update on your amp replacement? Definitely want to hear if those amps worked for you!

 

[Dutchmaster]

I repaired the hum from two BP 7006s, switching out the capacitors. The issue I am still having is that the subwoofers do not turn on when I power the receiver. I have to unplug and then plug them back in manually, and they turn on and work fine. They will then shut off around 30 minutes later, and I must repeat the process. I am thinking about removing the LFE cables, and I am using a Wiim Plus Pro to stream music with RCA cables connected to my AVR. Any suggestions would be appreciated before I go ahead and buy a new subwoofer because it drives me nuts.

 

[Goodfellas27]

Any word on a new DefTech Mythos?