Built in HP Filter?

[Alex2507]

Nevermind. Please delete.

 

[Alex2507]

Well since nobody would delete this thread I may as well use it as I'm having some on going issue here. Here's the link to the JBL MPC200 amp I am using for the subs I just built. The link has links to a Spec Sheet and a Manual.

At first I thought the amp automatically high passed the output but then I realized it was the 'T' model which is for those fixed voltage/transformer speakers that I know nothing about ... hence the thread title. Since I have the non-T MPC200 that doesn't apply to this amp. That just explains the thread and request for deletion.

Built-in second-order 50 Hz
(MPC200T) and third-order 45 Hz (MPC300T and 600T) Butterworth high-pass filtering
helps prevent saturation in speaker transformers by sharply reducing the amount of ultra-low
frequency energy the amplifier delivers.

One thing I am kind of curious about is from the Spec Sheet. Why would they give the response from 45 Hz up? What in all likely hood happens below 45 Hz?

4 ohm Load/Channel2, 45-20k Hz, 0.1% THD: 225 W

Onto my current issues. In the interest of using space efficiently I turned the amp on it's side only to have it cut out. I went to the manual and found this:

Thermal Protection—Temperature sensors on the channel heatsinks and on the power
transformers are part of the thermal management circuitry. The sensors govern whether the
dual-speed fan runs at low or high speed. Below 55°C the fan runs at low speed; above it runs
at high speed. Above 85°C, the channel mutes for thermal protection.

I figured heat dissipation was hampered by the vertical placement so I laid it flat again. I then noticed the amp still cut out just not as frequently. I had installed a 'quiet fan' and am now thinking that the original fan might be what is needed for this amp to drive subs. When I used this amp to drive my mains it was never problematic except that it threw off a lot of heat and when a movie was over I could hear the fan operating in it's high speed mode.

I'm fixing to open the amp back up and put the original fan back in. Pic's will follow. I'll take a good look at the caps for swelling and am considering popping loose the output transistors to clean off their mounting surfaces and remount them to the heat sink with Arctic Silver if they are screwed onto the heat sink.

That's kind of a maybe depending on what I find when I go back in to replace the fan. I'm kind of bummed out about going with the louder fan. I've actually never heard that fan because I did the mod before I ever hooked up the amp.

I'm going deep ...

 

[Alex2507]

Here's the basic layout. You can see that the heat sink is horizontal and the vents opposite the transformer are partially blocked forcing more air to exit around the transformer. I guess thermal dynamics would muck up the unit's ability to dissipate heat if it was up ended with the transformer being at the bottom.

Here's all the electrolytic caps I could see. They look fine to me.

I believe the output transistors live behind that shield and I don't feel I need to be digging into that ... but my curiosity runs deep. What could be back there?

I'm going out for breakfast right now but expect to find this completely derailed by the time I return.

 

[TLS Guy]

Well since nobody would delete this thread I may as well use it as I'm having some on going issue here. Here's the link to the JBL MPC200 amp I am using for the subs I just built. The link has links to a Spec Sheet and a Manual.

At first I thought the amp automatically high passed the output but then I realized it was the 'T' model which is for those fixed voltage/transformer speakers that I know nothing about ... hence the thread title. Since I have the non-T MPC200 that doesn't apply to this amp. That just explains the thread and request for deletion.



One thing I am kind of curious about is from the Spec Sheet. Why would they give the response from 45 Hz up? What in all likely hood happens below 45 Hz?



Onto my current issues. In the interest of using space efficiently I turned the amp on it's side only to have it cut out. I went to the manual and found this:



I figured heat dissipation was hampered by the vertical placement so I laid it flat again. I then noticed the amp still cut out just not as frequently. I had installed a 'quiet fan' and am now thinking that the original fan might be what is needed for this amp to drive subs. When I used this amp to drive my mains it was never problematic except that it threw off a lot of heat and when a movie was over I could hear the fan operating in it's high speed mode.

I'm fixing to open the amp back up and put the original fan back in. Pic's will follow. I'll take a good look at the caps for swelling and am considering popping loose the output transistors to clean off their mounting surfaces and remount them to the heat sink with Arctic Silver if they are screwed onto the heat sink.

That's kind of a maybe depending on what I find when I go back in to replace the fan. I'm kind of bummed out about going with the louder fan. I've actually never heard that fan because I did the mod before I ever hooked up the amp.

I'm going deep ...

From your instruction manual: -

Infrasonic / Subsonic and Saturation Protection—Built-in second-order 50 Hz
(MPC200T) and third-order 45 Hz (MPC300T and 600T) Butterworth high-pass filtering
helps prevent saturation in speaker transformers by sharply reducing the amount of ultra-low
frequency energy the amplifier delivers. However, some lower-quality speaker transformers
have responses that do not extend that low; if you have any of these in your distributed line
system, you should insert a corresponding high-pass filter in the audio path before the
amplifier(s) unless you are also using subwoofers. All models have a subsonic filter to prevent
passing DC or excessive subsonic energy.

So as far as I can tell only the T would have a third order HP filter at 45 Hz. Yours does have an infrasonic filter, though of unspecified frequency and slope. The T has this filter as it is for transformer high voltage speaker distribution systems.

 

[Swerd]

One thing I am kind of curious about is from the Spec Sheet. Why would they give the response from 45 Hz up? What in all likely hood happens below 45 Hz?

4 ohm Load/Channel2, 45-20k Hz, 0.1% THD: 225 W

Rating the power output at 45-20K instead of 20-20K is a way to cite higher power output. But with that high pass filter, I'm not sure they could measure output below 45 Hz? I'm also not certain whether that high pass filter affects your use of this amp for subwoofers. Can you test sound below 45 Hz?

Where do the batteries go?

Onto my current issues. In the interest of using space efficiently I turned the amp on it's side only to have it cut out.

I figured heat dissipation was hampered by the vertical placement so I laid it flat again. I then noticed the amp still cut out just not as frequently. I had installed a 'quiet fan' and am now thinking that the original fan might be what is needed for this amp to drive subs. When I used this amp to drive my mains it was never problematic except that it threw off a lot of heat and when a movie was over I could hear the fan operating in it's high speed mode.

You can see that the heat sink is horizontal and the vents opposite the transformer are partially blocked forcing more air to exit around the transformer. I guess thermal dynamics would muck up the unit's ability to dissipate heat if it was up ended with the transformer being at the bottom.

Before you replace the fan, check to see if that other vent opposite the transformer is blocked intentionally or by accident. Is that rectangle of stuff screwed or rivited to cover the vent? Or was it stuck on and some time later fell into that location to block the vent?

Does that fan pull hot air out, or does it push cool air into the cabinet? In Artic Silver terminology, does it blow or suck? From the photo, it makes more sense to me if the fan sucked hot air out of the cabinet. That would make the opening by the transformer the cool air inlet. But that may not be how it was built.

When you stood the amp on end, I dont understand which end was up and which one was down. Was the opening by the transformer on the top or bottom? Hot air rises on it's own. With a fan that can be ignored, but I think it would work better if the fan and natural convection worked together.

 

[Swerd]

I finally looked at the manual you linked. You did clearly say you have the MPC200 model and not the MPC200T. So you do not have that transformer for constant voltage or the associated 45 Hz high pass filter.

The manual also clearly answers my other question about the fan:

"The 2-speed cooling fans draw air in at the rear and exhausts it through vents in the front panel for better cooling."​

Make sure your fan works in that direction, and that there is no dust build up or other obstruction to air flow any where. Keep the cabinet flat as shown in their diagrams. And I would use their original fan unless you know the air flow rating of any replacement fan is as least as much as the original.

 

[Alex2507]

Yours does have an infrasonic filter, though of unspecified frequency and slope.

Ahhh ... I missed that part.

Can you test sound below 45 Hz?

Ummm ... yeah. The Rives disc shows an F3 of ~35 Hz after a little EQ'ing. My seating/measurement position is in the middle of a broadband lull. If I listen in other spots in the room or outside the room the bass is absolutely ripping in condo dweller terms.

Where do the batteries go?

I was kinda wondering that myself.

Before you replace the fan, check to see if that other vent opposite the transformer is blocked intentionally or by accident. Is that rectangle of stuff screwed or rivited to cover the vent? Or was it stuck on and some time later fell into that location to block the vent?

It looks factory.

Was the opening by the transformer on the top or bottom?

It was like this:

But it doesn't matter since it still turns off laying flat like it's suppose too. I'll just clean off the old fan and re-install it to give it the opportunity to pass.

You did clearly say you have the MPC200 model and not the MPC200T.

and as TLS pointed out:

All models have a subsonic filter to prevent
passing DC or excessive subsonic energy.

My slope in my listening position is 12db/oct from 20-40Hz. I even EQ'ed in a couple of adjustments with results going in the right direction as shown by admittedly rudimentary measuring gear. I'm okay with all that. I just can't have the subs flake out on me whenever the amp gets a little warm. If I have to I'll drag Walter's Carver amp back in here ... I just don't feel like going through the hassle.

Make sure your fan works in that direction, and that there is no dust build up or other obstruction to air flow any where. Keep the cabinet flat as shown in their diagrams. And I would use their original fan unless you know the air flow rating of any replacement fan is as least as much as the original.

Yes to all above. I tried taking a pic of the fan but it wouldn't cooperate. I gotta clean the old fan before I put it back in and I gotta get this back together pushing bass because I couldn't leave well enough alone and am now undoing work I did in an 'upgrade' in yet another upgrade. That is a dizzying waste of time, effort and like 12 bucks for the fan.

Ugh ...

 

[TLS Guy]

Well since nobody would delete this thread I may as well use it as I'm having some on going issue here. Here's the link to the JBL MPC200 amp I am using for the subs I just built. The link has links to a Spec Sheet and a Manual.

At first I thought the amp automatically high passed the output but then I realized it was the 'T' model which is for those fixed voltage/transformer speakers that I know nothing about ... hence the thread title. Since I have the non-T MPC200 that doesn't apply to this amp. That just explains the thread and request for deletion.



One thing I am kind of curious about is from the Spec Sheet. Why would they give the response from 45 Hz up? What in all likely hood happens below 45 Hz?



Onto my current issues. In the interest of using space efficiently I turned the amp on it's side only to have it cut out. I went to the manual and found this:



I figured heat dissipation was hampered by the vertical placement so I laid it flat again. I then noticed the amp still cut out just not as frequently. I had installed a 'quiet fan' and am now thinking that the original fan might be what is needed for this amp to drive subs. When I used this amp to drive my mains it was never problematic except that it threw off a lot of heat and when a movie was over I could hear the fan operating in it's high speed mode.

I'm fixing to open the amp back up and put the original fan back in. Pic's will follow. I'll take a good look at the caps for swelling and am considering popping loose the output transistors to clean off their mounting surfaces and remount them to the heat sink with Arctic Silver if they are screwed onto the heat sink.

That's kind of a maybe depending on what I find when I go back in to replace the fan. I'm kind of bummed out about going with the louder fan. I've actually never heard that fan because I did the mod before I ever hooked up the amp.

I'm going deep ...

Alex, I just noticed that part.

Leave the power transistors alone. You will need new mica washers likely, if you remove them

That Arctic silver is fine, for processors, but not power transistors. Arctic Silver is conductive and will very likely sort and blow your output stages.

Always use the white zinc oxide stuff mounting power transistors.

I'm hope I have caught this in time before you create a disaster.

Your issue is the fan. Obviously the quiet fan does not produce enough air flow.

I still don't understand where all the power goes in members subs. These subs should be pretty efficient I would have thought.

The grand children brought Tron Legacy last weekend, and we played it at good volume. It was vibrating and shaking everyone's "naughty bits."

It was well engineered, with fantastic sound effects, great sense of space created also.

Anyhow, the bass amps did not break a sweat. The amp also handling the BSC was a little warmer. All the other amps were much warmer. You certainly would not have wanted more low end spl. it was there in spades, so I just remain puzzled. I guess my TlLs must just really couple to the room without brute force and ignorance.

 

[Swerd]

Ummm ... yeah. The Rives disc shows an F3 of ~35 Hz after a little EQ'ing.

You clearly can generate sound below 45 Hz. It's always nice to have a direct demonstration that confirms all our theoretical hand waving.

But it doesn't matter since it still turns off laying flat like it's suppose too. I'll just clean off the old fan and re-install it to give it the opportunity to pass.

I couldn't leave well enough alone and am now undoing work I did in an 'upgrade' in yet another upgrade. That is a dizzying waste of time, effort and like 12 bucks for the fan.

If it ain't broke, don't fix it.

We all learn that sooner or later. Some of us take longer than others . But learning that the original designers had good reasons for doing what they did is part of the fun of DIY.

I only worry about those who never seem to get it, despite hands-on experience .

 

[Alex2507]

You clearly can generate sound below 45 Hz. It's always nice to have a direct demonstration that confirms all our theoretical hand waving.

Here's the graph with before and after the EQ'ing. Ignore the squiggly line in the after shot. Connecting the dots is a b!tch. Anyway only my DVD player is EQ'ed. The 300 CD disc changer and turntable don't receive the benefit of my knowing touch.

If it ain't broke, don't fix it.

It ain't gonna happen. That may be good advice for most but people with my particular super powers should definitely be taking stuff apart. There is maintenance in life.

... learning that the original designers had good reasons for doing what they did is part of the fun of DIY.

That Behringer EP4000 likes the fan mod ... this one doesn't.
Ya pays yer money and ya takes yer chances.

I only worry about those who never seem to get it, despite hands-on experience .

I do pretty good for the most part.
Remaining teachable is sometimes hard when you're as smart as me.

Leave the power transistors alone. You will need new mica washers likely, if you remove them.

I think these transistors are meant to have their backs grounded out to the heat sink just like my 'I think it's a processor chip' in my computer. Here's a shot of the clamp removed.

Here's a close up of the transistor having moved away from the heat sink.

That Arctic silver is fine, for processors, but not power transistors. Arctic Silver is conductive and will very likely sort and blow your output stages.

Always use the white zinc oxide stuff mounting power transistors.

I'm hope I have caught this in time before you create a disaster.

Well you have but I already put it back together after cleaning everything nice nice. I can't imagine that those two metal surfaces didn't touch and create a closed circuit but I can't ignore what you are saying either. I want you to change your words and possibly reality to meet my circumstances without discrepancy.

Your issue is the fan. Obviously the quiet fan does not produce enough air flow.

Agreed.

I still don't understand where all the power goes in members subs. These subs should be pretty efficient I would have thought.

I really think maybe the thermal link was due for an overhaul especially since I have been toasting it up a bit with the quiet fan. Are you positive that I need this zinc oxide voodoo?

Here's some more pic's ...
Before and after on the heat sink.

Before and after on the transistors.

Don't anyone have a cow. I'm off to buy banana splits for me and the girl and I might drop by the rat shack to get the white stuff ... I'll have it in hand by shortly and I still haven't even plugged the unit in. Should I have to replace the Arctic Silver for real I will be ready and of course to the girl I will be the giver of the banana slit. Do you have any idea what kind of gratitude that buys one?

 

[TLS Guy]

Here's the graph with before and after the EQ'ing. Ignore the squiggly line in the after shot. Connecting the dots is a b!tch. Anyway only my DVD player is EQ'ed. The 300 CD disc changer and turntable don't receive the benefit of my knowing touch.

It ain't gonna happen. That may be good advice for most but people with my particular super powers should definitely be taking stuff apart. There is maintenance in life.



That Behringer EP4000 likes the fan mod ... this one doesn't.
Ya pays yer money and ya takes yer chances.



I do pretty good for the most part.
Remaining teachable is sometimes hard when you're as smart as me.



I think these transistors are meant to have their backs grounded out to the heat sink just like my 'I think it's a processor chip' in my computer. Here's a shot of the clamp removed.

Here's a close up of the transistor having moved away from the heat sink.

Well you have but I already put it back together after cleaning everything nice nice. I can't imagine that those two metal surfaces didn't touch and create a closed circuit but I can't ignore what you are saying either. I want you to change your words and possibly reality to meet my circumstances without discrepancy.



Agreed.



I really think maybe the thermal link was due for an overhaul especially since I have been toasting it up a bit with the quiet fan. Are you positive that I need this zinc oxide voodoo?

Here's some more pic's ...
Before and after on the heat sink.

Before and after on the transistors.

Don't anyone have a cow. I'm off to buy banana splits for me and the girl and I might drop by the rat shack to get the white stuff ... I'll have it in hand by shortly and I still haven't even plugged the unit in. Should I have to replace the Arctic Silver for real I will be ready and of course to the girl I will be the giver of the banana slit. Do you have any idea what kind of gratitude that buys one?

I can't tell from the pictures.

Is there anything conductive on the devices on the heat sink side?

If there is anything conductive check carefully for thin pieces of mica under the zinc oxide that was there.

Make sure when you solder the power transistors you use clip on heat sinks on the electrodes while you solder. They should have been used when you desoldered it.

You need this piece of voodoo for sure.

 

[Alex2507]

I didn't desolder anything because I'm not that crazy yet so no need for the heat sink. I didn't see any wafer thin film washer thingies in between the transistors and heat sink ... but this banana split is to die for.

I did pick up a silicone base thermal compound with zinc oxide listed in the ingredients but am just dieing to plug this in without further work. The last pic in that last post is of the back of a line of output transistors with Arctic Silver globs on it. In removing the old compound I must have wiped them 5 times and that whole back is a shiny metal conductor under the little blobs of thermal compound.

Tell me this ain't cool lookin'. How could anything be amiss?

The back of the heat sink.

Low noise fan. Made in China.

Stock fan. Made in Japan. I should have known right there, huh?

While I was at it ...

 

[Alex2507]

Well this was a close one ... too close to call really.

I got impatient and plugged it in. It let out the evil wail. I unplugged it and cleaned off the Arctic Silver and replaced it with the Zinc Oxide and plugged it in again. It wailed once again. I took off the cover and stared at it for a while. I then saw that I had plugged this connector in misaligning everything by one pin.

I think that's my voltage input on the left and my output on the right. What ever the sound only came out of the right sub/channel two. I corrected the misalignment and plugged it back in ... sans wail. The ultimate good news is that little hum that the subs had that went up as I cranked the gain is now completely gone. I think the Arctic Silver was fine for this but now that is all used up and the Zinc Oxide is now in place as the good doctor says is right and I got through the whole process without breaking anything. I almost certainly learned nothing but that's how it is when you're not right in the head.

I'm really glad nothing blew up. I'm thinking the quiet fan would handle regular speakers no problem especially now that the thermal compound got re-done but I'm not going to torture the amp by overheating it with these subs. I'm wicked quick at taking this thing apart now.

I happen to own this cd ... TLS ... do ... not ... click.
The CD has a lot more bass than the U-toob thing ... it's sick.

 

[Swerd]

I thought I had seen something in the manual about these amps not having "insulating wafers" between the output transisters and the heat sink. See page 4 of the manual:

High Cooling Capacity Every MPC amplifier features a large diameter two-speed fan and massive extruded aluminum heat sinks. The output devices couple directly to the heat sink. This provides better cooling of the output devices while eliminating problematic insulating wafers that are an integral part of most other amplifier designs. Forced air cooling in a back-to-front direction provides for more effective cooling, preventing problems that otherwise occur from a continual heat build-up inside the equipment rack, which occurs in amplifiers with front-to-back or side-to-side flow schemes. These design factors allow the MPC amplifiers to work in high duty-cycle instances when many other professional amplifiers cannot.​

 

[Alex2507]

See page 4 of the manual:

Oh ... the manual.

I just don't absorb information any more or maybe don't retain it but I did just now notice on page 3 that they do give a 20-20k Hz number into 4 Ohms for this amp. It's a respectable 225 watts per channel.

 

[Alex2507]

In the spirit of putting the cart before the horse I am now doing my home work on these thermal compounds. In this wiki article it says that the silver stuff out does the ceramic stuff.

Metal-based thermal grease contain solid metal particles (usually silver or aluminum). It has a better thermal conductivity[citation needed] (and is more expensive) than ceramic-based grease.

I only mention that because rat shack had both. Then the article goes on to say that the silicone based compounds don't do as well at thermal conductivity as the silver compounds.

The typical thermal conductivities of the silicone compounds are 0.7 to 3 W/(m·K). Silver thermal compounds may have a conductivity of 3 to 8 W/(m·K) or more.

The longevity of thermal compounds is also mentioned.

Some thermal greases have a durability up to at least 8 years.

I would have been fine if I hadn't misaligned that connector when I first put this back together as I had a really nice thin layer of Arctic Silver on it. I could tell what it looked like when I took it apart ... again. Anyway the amp's original fan is quite loud ... loud enough to be heard with music playing at low levels and that's at the fan's low speed at start up. Once the amp is running for a few minutes even without music it goes into high speed and the SPL meter shows like a 10db jump in volume. Moderately loud music play back masks that but I normally listen at low levels.

I'm totally leaning toward going at this one more time with Arctic Silver which I understand to be the better thermal conductor and the quiet fan ... again. I'm thinking about wet sanding the extruded aluminum heat sink to get it as flat as possible. While I think about this I can enjoy the fact that the amp is currently capable of driving these subs and produces absolutely no hum from the drivers. That's a huge improvement over what was going on yesterday. I may just go through the Carver C-500 amp I have in the living room and bring that in here to drive these subs. It runs way cooler.

Thanks for the help guys. I had a lot of fun (and a little worry) doing this. Okay, I'll be honest, I practically sh!t my pants the first time I turned the amp on and it let out that hellatious cry but in hindsight that's the best part of the story. So maybe I did learn a thing or two by going deep but it certainly wasn't to leave well enough alone.

EDIT: Here's an article I found and am reading on these thermal pastes.

 

[TLS Guy]

In the spirit of putting the cart before the horse I am now doing my home work on these thermal compounds. In this wiki article it says that the silver stuff out does the ceramic stuff.



I only mention that because rat shack had both. Then the article goes on to say that the silicone based compounds don't do as well at thermal conductivity as the silver compounds.



The longevity of thermal compounds is also mentioned.



I would have been fine if I hadn't misaligned that connector when I first put this back together as I had a really nice thin layer of Arctic Silver on it. I could tell what it looked like when I took it apart ... again. Anyway the amp's original fan is quite loud ... loud enough to be heard with music playing at low levels and that's at the fan's low speed at start up. Once the amp is running for a few minutes even without music it goes into high speed and the SPL meter shows like a 10db jump in volume. Moderately loud music play back masks that but I normally listen at low levels.

I'm totally leaning toward going at this one more time with Arctic Silver which I understand to be the better thermal conductor and the quiet fan ... again. I'm thinking about wet sanding the extruded aluminum heat sink to get it as flat as possible. While I think about this I can enjoy the fact that the amp is currently capable of driving these subs and produces absolutely no hum from the drivers. That's a huge improvement over what was going on yesterday. I may just go through the Carver C-500 amp I have in the living room and bring that in here to drive these subs. It runs way cooler.

Thanks for the help guys. I had a lot of fun (and a little worry) doing this. Okay, I'll be honest, I practically sh!t my pants the first time I turned the amp on and it let out that hellatious cry but in hindsight that's the best part of the story. So maybe I did learn a thing or two by going deep but it certainly wasn't to leave well enough alone.

EDIT: Here's an article I found and am reading on these thermal pastes.

I think the silver for you will work. The mounting of those transistors is unusual, and I think part of the problem. Generally the collector is exposed and has to be insulated from the heat sink. The pins also often go through the sink. I have tried to use the silver stuff and created shorts, as it is electrically and thermally conductive.

I will be honest and say I don't like the look of that amp, and I suspect it may have a short life. The output devices, even though they are tripled, look really miserable for and amp of that power. So I bet it does take a hurricane to stop it self destructing.

 

[Swerd]

It didn't register with me either until TLS Guy mentioned the mica insulating wafers. Only then did I remember reading something about them in the manual. I had only skimmed over the manual, but while I was looking for sections about the cooling fan, I stumbled over the part I quoted above. Two sets of eyes are better than one when you're working on something you've never done before.

Thanks for those articles about thermal greases. I knew nothing about them.

I wonder if you wet sand the aluminum surface will the thermal grease will work better or worse. Polishing the aluminum may make it work better by making better contact between the sink and the transistor. But polishing the aluminum may make the Artic Silver stuff not adhere as well. Maybe I'm overthinking this.

No where in those articles you linked did I find any mention about where to put the batteries in the Artic Silver applicator. Can you lube the trannies manually without any batteries?

 

[Alex2507]

Two sets of eyes are better than one ...

... always, especially when one of those sets is mine.

But polishing the aluminum may make the Artic Silver stuff not adhere as well.

It's not an adhesive. In all cases the heat source is clamped, screwed or sprung to the heat sink. Thermal compound displaces air which isn't a good thermal conductor. I'm sure once you read the articles you'll walk away with more info than I.

Can you lube the trannies manually without any batteries?

Every tranny deserves a meal, clean sheets and fresh batteries.
That right there is what makes me a gentleman.

I will be honest and say I don't like the look of that amp, and I suspect it may have a short life.

LOL ... you may be right but this is a re-badged QSC amp with the JBL logo. QSC is suppose to be pretty robust ... maybe not this particular line but I've tortured this amp with low CFM airflow at the end of it's thermal compound life span and it's hanging in there. Beyond it's performance for the $125 that I paid for it it has provided me with that much diversionary value not to mention being a great teaching tool. I honestly think with the work done to it now and with what I may yet do to it this amp should be fine for some time to come. Besides ... if it was worth much more I would be really scared of breaking it. As it is I'm not so worried.

The output devices, even though they are tripled, look really miserable for and amp of that power. So I bet it does take a hurricane to stop it self destructing.

Tripled? I was starting to learn something about transistors but then I got side tracked. I bet it was cheaper for the manufacturer to provide a killer 2 stage fan than to provide an excellent heat transfer point. I want to see what I can do to remedy that in hopes of keeping the fan in it's low speed stage during low output. I think I can improve on it's current state.

After I read that article which compared 80 compounds I will make plans for the next tweak with this thing. Maybe I'll just wait for Swerd to read it and ask him what I should do.

 

[Rickster71]

My $0.02
I'm thinking the drivers are dipping below 4-Ohms much more than expected, and overheating the amp. Hope this wasn't mentioned already, I didn't read every post.