DIY 250W combo .

[JohnH89]

A quick backstory behind this project . I use a Markbass 801 for rehersals with small acoustic folk band . I love it for its sound and form factor , a lot easier than schlepping a head , cab , cables etc . For outside performances that need more volume , I have been using a homemade 112 cab and TC electronics BQ250 . I though I would combine both ideas and made myself a bigger , louder combo , like the Markbass on steroids .
The Celestion BN12 - 300S 4 ohm speaker was robbed from the cabinet , and a new cabinet made . 16in cube made from 9mm ply that was laying around , laminated in parts for strength but still remarkably lightweight . Rear ported and the TC BQ250 built in . Fitted with a DI XLR socket with a ground lift wired in .
The cab was designed to be resonant at 41Hz and plugged it in this afternoon and was blown away by the volume and tone .

17.2kg - 38lb .
Heavier than I thought but still lighter than the original cab that was all timber .

[naxos10]

Great project and end result, nice one.

[chyc]

This looks lovely. Congratulations.

 

I've always had half an eye on the BN12-300S. It is a crazy light woofer that doesn't go deep, which is fine by me as I aggresively use a HPF anyway.

[JohnH89]

1 hour ago, chyc said:

This looks lovely. Congratulations.

 

I've always had half an eye on the BN12-300S. It is a crazy light woofer that doesn't go deep, which is fine by me as I aggresively use a HPF anyway.

It does deep very well in a well designed cabinet .

[LarsTimm]

Looks great 👍.
How closed is the compartment with the amp?

I could be worried that there is a risk for the amp to overheat, if there is not free air around it.

Might not be a problem, just my immediate thought…

[JohnH89]

Amp is mounted to the back board via 4 bolts and a ply pressure plate . The sides of the case are free for air to move . On the BQ250 , the fan and vent are mounted on the side of the case . With the amount of air that moves with the 12in driver , I would be very surprised if I have an overheating problem . 

[Downunderwonder]

22 minutes ago, Wally Gogg said:

air that moves with the 12in driver

Don't kid yourself. The thing to do is run it on its side so the ports act as an efficient thermal chimney. Cold in at bottom and warm out at the top.

[JohnH89]

We'll see . I am not expecting any problems but can always add forced ducting if necessary . I have driven the amp flat out for 4 hours without the fan coming on for more than a few minutes when it was sat on top of the cab . I like the idea of it on its side as a simple solution if there is a problem .

Edited November 7, 2022 by Wally Gogg

[Downunderwonder]

The air movement in the cab is very little as it sits in the photo. Any driver induced air moving out and being exchanged as it moves back into the ports is the coldest. Inside the cab it's snug as a bug in a rug. There's a little space heater gradually getting warmer and warmer at the top.

 

You can easily test it by giving it the next workout on its side and feeling the draught exiting the top. All that air changing is what keeps the inside temp liveable for the amp.

[Phil Starr]

First of all this is a great project, nice choice of driver too.

 

I also wondered about heat dissipation, mainly because I've been planning a similar combo. ClassD is highly efficient so there won't be a lot of heat from the amp components but the rest of the power goes to the speaker which is also producing heat into the cab. It's a myth that the air moves in and out of the vents. The air in the vents acts as a plug of air that moves backwards and forwards at around resonance and not at all at higher frequencies. the cab air is mainly cooling through the walls.

 

On the plus side your amp will only be operating at a few watts for most of the time, you don't have to lose 250W or anything like that so overheating probably isn't very likely. The temperature in the cab is likely to be considerably in excess of room temperature though and the heat loss depends upon the temperature gradient. The amp designers will have made calculations based upon a duty cycle (what % of the time the amp is producing power) and designed the heatsinking to keep the temp of the power consuming components down to a set level when the room temperature reaches a certain maximum. They will have left a considerable leeway but in your case the room temperature is actually the inside of your cab and the free flow of air is restricted. Your problem is that you don't know what assumptions they have made. I think you'll be fine at normal UK temperatures though.

[JohnH89]

I understand your comments and concerns but am very aware of how it all works . I really don't foresee a problem but will give it a hard run at rehersal on Thursday and let you all know how it works out . I have used the head a lot in the past and even when driven hard for a long period , it barely gets warm . At approx 95% efficiency , at full power constantly of 250W (which it is nowhere near with normal playing ) it would have to dissipate a lowly 12.5W . If it turns out that there is a problem , the amp will protect itself and cut out . If it does I will design a forced cooling duct system for it . 

I'll keep you posted .

[Downunderwonder]

5 minutes ago, Wally Gogg said:

it barely gets warm

That's thanks to fresh air being pumped through the amp by its fan

 

6 minutes ago, Wally Gogg said:

in the past

But now it only pumps air previously warmed by the last verse / tune / set very well insulated by the furry enclosure, a recipe for overheating.

[JohnH89]

Thanks for your concern . We'll try it out first . Two small ports at the top of the rear panel , ducted to the fan / outlet will take care of that if necessary . I'll have a virtual bet with you to see if I can get it to overheat at rehersal this week . I'll try hard 🤪

[Phil Starr]

12 minutes ago, Wally Gogg said:

Thanks for your concern . We'll try it out first . Two small ports at the top of the rear panel , ducted to the fan / outlet will take care of that if necessary . I'll have a virtual bet with you to see if I can get it to overheat at rehersal this week . I'll try hard 🤪

Ha ha, I won't be taking that bet

 

I genuinely think you'll be good but little amps do get quite hot, My Warwick Gnome runs pleasantly warm but this summer at an outdoor gig it was 37 in the shade and the amp did get too hot to rest your hand on it. The BQ250 is a bigger amp than the Gnome though, the heatsinking in the Gnome is a diminutive lump of Aluminium and it really needs its fan. 

 

Have Fun

[JohnH89]

As pointed out , the case and heatsinking on the BQ250 is a lot larger than the Warwick . I have also come across the following from TC electronics

 

Which is a 750 combo using this head

Also vented from the side and a lot less airflow than the BQ250 .

My money is still on the little BQ having adequate cooling .

[agedhorse]

The amount of heat in the speaker being discussed is insignificant in the big picture

[Downunderwonder]

1 hour ago, agedhorse said:

The amount of heat in the speaker being discussed is insignificant in the big picture

Please explain. Ime micro amps all get nicely warm to hot enough to be properly glad it's got a fan.

 

My gut instinct is the inside of a 112 is claustrophobic enough that it would get extra warm in there with that amount of heat being shed continuously and the main heat loss being out through the walls of the cabinet.

[JohnH89]

1 hour ago, Downunderwonder said:

Please explain. Ime micro amps all get nicely warm to hot enough to be properly glad it's got a fan.

The BQ250 is a lot bigger than the BAM200/Warwick gnome/TE Elf with better heatsinking and considerably more venting and bigger fan . 

Edited November 8, 2022 by Wally Gogg
adding pic

[Downunderwonder]

49 minutes ago, Wally Gogg said:

The BQ250 is a lot bigger than the BAM200/Warwick gnome/TE Elf with better heatsinking and considerably more venting and bigger fan . 

Either I am missing something or you don't have any appreciation of the thermodynamics at play here.

 

All amps sink heat into the air one way or another. Heat from transistors goes to finned heat sinks or direct to air. The air may be fan assisted or just convection.

 

In this case it is the air inside of the cabinet, with no ventilation! Thus the air gradually gets warmer and warmer with each pass through the amp. All of the aml heatsinks and fans in the world don't change the heat transport to the outside air.

 

Agedhorse seems to be saying the cab exterior surface can radiate enough heat that the air inside won't ever get uncomfortably hot?

 

I think if I ran a hairdryer inside a cabinet it would take quite a long time for the surface to get a little bit warm by which time it would be hell hot in there and likely blow up the hairdryer first. Quid pro quo a cabinet is a good thermal insulator.

 

Down at the other end of the curve the amp heats much less than a hairdryer so conceivably it could warm up the cab just enough that it radiated the same amount but I still reckon that it get would too warm inside for the good of the amp.

 

So all in all it's a bit of  mystery.

 

Added to that I don't understand you not wanting to do the first test with the amp sideways, feeling for a warm draught out of the top port, with a chance to decide discretion is the better plan.

[JohnH89]

Because , my learned friend , I am , maybe optimistically , confident that the original design , all things considered , has a far better than even chance of being absolutely fine . 

If you can wait until thursday late evening or more likely Friday morning GMT , then we will know for sure . 

I think you may have misunderstood #agedhorse and his very well informed opinions but I will leave that between the two of you . He has quite a background in design and development of bass amplification .

Be assured that I will post the results of a test and will willingly admit if my assumptions/calculations prove inaccurate .

[agedhorse]

3 hours ago, Downunderwonder said:

Please explain. Ime micro amps all get nicely warm to hot enough to be properly glad it's got a fan.

 

My gut instinct is the inside of a 112 is claustrophobic enough that it would get extra warm in there with that amount of heat being shed continuously and the main heat loss being out through the walls of the cabinet.

There's not much heat involved, the average dissipation is less than 50 watts worst case.

 

1 hour ago, Downunderwonder said:

Either I am missing something or you don't have any appreciation of the thermodynamics at play here.

 

All amps sink heat into the air one way or another. Heat from transistors goes to finned heat sinks or direct to air. The air may be fan assisted or just convection.

 

In this case it is the air inside of the cabinet, with no ventilation! Thus the air gradually gets warmer and warmer with each pass through the amp. All of the aml heatsinks and fans in the world don't change the heat transport to the outside air.

 

Agedhorse seems to be saying the cab exterior surface can radiate enough heat that the air inside won't ever get uncomfortably hot?

 

I think if I ran a hairdryer inside a cabinet it would take quite a long time for the surface to get a little bit warm by which time it would be hell hot in there and likely blow up the hairdryer first. Quid pro quo a cabinet is a good thermal insulator.

 

Down at the other end of the curve the amp heats much less than a hairdryer so conceivably it could warm up the cab just enough that it radiated the same amount but I still reckon that it get would too warm inside for the good of the amp.

 

So all in all it's a bit of  mystery.

 

Added to that I don't understand you not wanting to do the first test with the amp sideways, feeling for a warm draught out of the top port, with a chance to decide discretion is the better plan.

Not going to be a problem at <50 watts of dissipation, it would take DAYS to raise the temperature 10 degrees C above ambient.

Edited November 9, 2022 by agedhorse

[Downunderwonder]

1 hour ago, agedhorse said:

Not going to be a problem at <50 watts of dissipation, it would take DAYS to raise the temperature 10 degrees C above ambient.

My back of envelope calc says it would take 12 seconds for 50 w to raise 50l of air by 10 degrees C. So either my maths is stuffed or you're saying a cabinet is a really pish poor insulator.

 

Specific heat of air 1kJ/kgC

Density of air 1.2kg/m³

50l = 0.05m³

50W is 50 Joule per second.

10deg x1kJ/kgC x0.05m³ x 1.2kg/m³ = 0.6kJ unless I am mistaken. 600J isn't a while lot of energy. 12 sec worth of 50w.

 

BZZZZ, WHAT THE HECK IS WRONG HERE!!!!!???

 

5w would be 2 minutes. 20 minutes of 5w would be cooking except the cab surface gets warm and radiates. But how warm must it get inside for it to do that???

 

 

[Matt P]

@Downunderwonder your maths looks right to me (although it is nearly 20 years since my thermodynamics lectures at uni)

 

the TC Electronic combo that is shown above might have a side mounted vent/fan but it also has an air gap all round the head in that enclosure so there is some ventilation for the amp (and i'm not going to mention the actual rating of that particular amp being much lower than the quoted value)

 

I think this little 250w head is going to get rather warm and possibly shut down (assuming it has a thermal cut-out)

As there is an insulating layer of wood between the amp casing and the outside world it will be difficult to keep track of how hot the amp itself is getting. 

 

Matt

[JohnH89]

The BQ250 has a thermal cutout . Let me try a real world scenario later this week and I promise I will get back to you . To please #downunderwonder I will even turn it on its side and check the temperature of the upper port . 

[Downunderwonder]

4 hours ago, Matt P said:

air gap all round the head in that enclosure

???? Looks pretty tight to the cab to me, as it should be or it could wheeze.

 

If the maths is right 12 sec is so much less than DAYS that there must be some major disconnect between what OP and Agedhorse are seeing and what I am seeing.