PPI A100 (Art Series) needs repair/advice - diyAudio
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Old 18th February 2010, 06:40 PM   #1
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Default PPI A100 (Art Series) needs repair/advice

Greetings. First post here, and hoping for some assistance.

I've got a PPI A100 amp that I've owned/used for a long time. I've recently pulled it out of storage and have/had it driving a pair of CDT 6x9's. However the amp doesn't seem to have any punch and what little bass there is is extremely muddy. At higher volumes the amp seems to be clipping when the bass hits (the CDT's emit an awful low-frequency pinging). This happens even when the amp gain set to below 1/4.

I've swapped out the PPI amp for another 100w amp, and the CDTs sound as they should. (Same audio input, same power, same ground.)

I want to see if I can repair the PPI before giving up on it, but where do I start? I've cracked the amp case, but don't see anything burned or any leaky caps. I've contacted PPI but they don't service these nor provide schematics.

I've been reading about testing the FETs using a multimeter, etc, but the instructions I'm following all approach the problem as if the amp were dead/shorting/in protection mode, and that doesn't seem to be my case.

For reference, it's possible that the PPI previously saw some abuse in the form of driving a 2ohm 10" sub.

Any pointers on where to begin?
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Old 18th February 2010, 10:50 PM   #2
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Well I've been reading this, and I'm wondering if the clipping & lack of bass is a result of the output transistors having failed, or one of the rails not powering -- does this sound plausible? (The symptoms seem to indicate that the speaker might only be moving for half-phase / in one direction.)

I've found the following transistors on the board, though I'll be honest in that I'm not sure which ones I should check:
  • BDT81 (NPN transistor?)
  • BDT82 (PNP transistor?)
  • 5N05E (Power FET?)
  • LM340T15 (positive regulator?)
  • LM7915CT (negative regulator?)
When checking/testing these, do I first need to remove these from the board? I'm following procedures on the aforementioned site, though perhaps I missed if it mentions this.

Thanks
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Old 19th February 2010, 12:39 AM   #3
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Location: Louisiana
You need to measure the rail voltage on the output transistors when it's being driven hard. The problem is... the bottom cover MUST be in place (screws tightened) when you do this. Otherwise, many of the transistors are at risk of failure. You may have to solder a wire to the center leg of the output transistors (1 on a BDT81 and 1 on a BDT82). Place the black meter probe on one of the non-bridging speaker terminals. Post the voltage at idle (no audio) as well as when it's being driven hard.

At idle
Positive rail:
Negative rail:

When driven hard
Positive rail:
Negative rail:
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Old 19th February 2010, 06:25 PM   #4
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Thanks, I'll take these measurements this weekend. Also appreciate the warning about the backing plate being part of the heatsink; I'll put in some temporary wires to get these measurements safely.

Question; the amp has 2 each of the BDT82 and BDT81 -- do I need rail voltage measurements from all of them? Or will measurements from one BDT82 and one BDT81 be enough?
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Old 19th February 2010, 07:00 PM   #5
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from one BDT82 and one BDT81
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Old 22nd February 2010, 05:30 PM   #6
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At idle
Positive rail: +18.7
Negative rail: -18.7

When driven hard
Positive rail: +16.7
Negative rail:-16.7

Driven 'hard' entailed turning up the volume to the point where the speakers started clipping on the bass notes.
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Old 22nd February 2010, 06:09 PM   #7
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Location: Louisiana
The power supply is holding up well. Are you sure that you're not expecting too much from the amplifier? It's only rated to produce 25w/ch which is only slightly more than a head unit can produce.
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Old 22nd February 2010, 06:41 PM   #8
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Thanks for responding. Yes, I'm certain that something in this amp isn't right; the sound quality is absolutely horrible -- no bass, distorted sound, etc. This amp used to be a gem, which I used in previous installs to drive a bazooka sub in bridged mode (100Wx1 @4Ohm). But now it can't seem to push a set of 6x9's in stereo (25wx2 @4Ohm).

I do recall that at some point the amount of bass from the bazooka seemed to have fallen off, but thought it was my imagination. But I'm even more convinced that there's something amiss with the amp since I'm now having the same symptoms with another set of speakers.

I mentioned earlier that I've swapped the PPI A100 out for another amp with similar stats (25wx2 @4ohms) and the difference in sound quality is nothing short of amazing. Same speakers/power/gnd/inputs, just different amp. Same volume level, same bass adjustments -- no distortion, punchy bass, acceptable listening levels.

Anything else that might be suspect? If not the power supply, could something else in the output circuit be bad? Or could there be something on the input circuit that is distorting the sound before it's amplified?

I've had this sucker for 15+ years, and would love to continue using it. Yeah, it's easy to replace it with another amp, but my Yankee ingenuity won't let it go that easy...
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Old 23rd February 2010, 11:02 PM   #9
jol50 is offline jol50  United States
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Does this one have the 'laydown' outputs? In my PPI some of them were falling off the board and caused a reduced output, I loaded them up with more and all new solder. The legs lay on the board.

What is the offset at idle? Though I don't know how much that tells you other than maybe catching something bad in the input section/etc. (offset is DC voltage at speaker terminals w/no speakers attached, no input, is usually mv)

Last edited by jol50; 23rd February 2010 at 11:09 PM.
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