I'd be worried about safety. The chassis connection must be such that it cannot fail. Otherwise, you might loose connection to safety ground.
On my build I used the Heavy Duty version of the 4 pole speakon connector. The ones with the metal casing and 40-50amp rating. I then had + - common and chassis ground wires. They are twist lock and rated for 300v . They make a very secure connection and are reasonably priced. I have the cable coming out of the power supply with the plug on the end of it and the socket connector mounted on the back of the amplifier chassis. I have CLC filtering with the CL in the ps chassis and then a DIY store ps board with another 120kuf in the amplifier chassis. Pictures here at post 4763
F5 Turbo Builders Thread
F5 Turbo Builders Thread
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Hi there,
Very nice build!! Just curious what is the power output of your F5T V2 and what is the bias current you have set. Also would like to have your comment on the heat dissipation capability of your chassis related to the heat generated by the output MOSFETS. Thanks!
Regards,
Hi Lo_Tse,
Thanks! I followed the original V2 as in the F5T article, using 24-0-24 transformer, resulting in +-32V rails, and that gives 50W into 8 ohms. The bias current was set at around 290mV. The chasis has quite good heat dissipation capability. Following the rule of thumb by Papa, i can touch the heatsinks for 10 seconds, so the temperature should be around 50-52 degress celcius
Best regards
Liu
Hi Lo_Tse,
Thanks! I followed the original V2 as in the F5T article, using 24-0-24 transformer, resulting in +-32V rails, and that gives 50W into 8 ohms. The bias current was set at around 290mV. The chasis has quite good heat dissipation capability. Following the rule of thumb by Papa, i can touch the heatsinks for 10 seconds, so the temperature should be around 50-52 degress celcius
Best regards
Liu
Hi, Liu
Thank you for the quick response! I am planning to build a pair of F5T V3 mono blocks (at ~100 W or less output). Just wondering whether using a chassis like yours for one channel is OK. Each side of the heatsink will still serve 4 output mosfets. However, since the supply voltage will be higher (~ +-42V), power dissipation for the mosfets will be higher.
Regards,
Hi,
I am no expert at heat dissipation calculations, so I would just give a little more info on my build, and maybe other fellow members can chip in as well. The chassis I used has heat sinks (per side) measuring 140mm height, 380mm depth, fins with 32mm length. This puts it roughly between the 4u and 5u chassis of the diyaudio stores. Also, the room temperature here in Malaysia is around 28 degrees, so with +-32V rails and 290mV bias, the temp rise is around 24 degrees. If you have a cooler room, it should give you some extra room to accommodate more heat with the same heat sinks.
I am no expert at heat dissipation calculations, so I would just give a little more info on my build, and maybe other fellow members can chip in as well. The chassis I used has heat sinks (per side) measuring 140mm height, 380mm depth, fins with 32mm length. This puts it roughly between the 4u and 5u chassis of the diyaudio stores. Also, the room temperature here in Malaysia is around 28 degrees, so with +-32V rails and 290mV bias, the temp rise is around 24 degrees. If you have a cooler room, it should give you some extra room to accommodate more heat with the same heat sinks.
I'm glad to see this discussion, my build is a v3 balanced using four MOSFETS each side. I was thinking of using heat sinks 320mm long X 250mm tall with 2" fins and a base plate 8mm thick. With 48v rails is this enough? When I look for heat sinks, dimensions are given, thermal characteristics are not.
No one can say for sure. Heatsink capacity depends a lot on number of fins, spacing, thickness, etc, etc,....
Some heatsink vendors will tell you the degrees C rise per watt you are looking to dissipate. This is best.
Then, you will know the expected temperature rise and you will know how big a heatsink to buy.
If you have them already, make sure you can measure temperature. Then this will be the limiting factor.
I do not know why people want to use high voltage transformers. I really don't get it.
What does high voltage give you? You need to cascode, no biggie, but is there something *better* about high voltage?
Some heatsink vendors will tell you the degrees C rise per watt you are looking to dissipate. This is best.
Then, you will know the expected temperature rise and you will know how big a heatsink to buy.
If you have them already, make sure you can measure temperature. Then this will be the limiting factor.
I do not know why people want to use high voltage transformers. I really don't get it.
What does high voltage give you? You need to cascode, no biggie, but is there something *better* about high voltage?
Power.
How many volts can you swing into the speakers without having the current collapse. These amplifiers have good current capability, so ultimate power will be determined by the rail voltage. Power in Watts is Volts RMS squared into the load in ohms;
32v rails is 64v pk-pk
64*.3535 = 22.6vrms
22.6^2 = 511
511/8ohm = 63W
48v is 96v pk-pk
96 * .3535 = 33.9vrms
33.9^2 = 1149
1149/8 = 143W
Now in reality the PSU won't make the full voltage, it will be a few short, and the amp circuit won't swing 100% to the rail voltage, so subtract maybe 3-4 volts from this, but you get the idea.
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My F5T3s are running with 46v rails at 255mv bias. This gives about 65w class A at 8 ohms and somewhere over 100w classAB. My monoblock chassis have heatsinks 400mm deep and 160mm high with a base 10mm thick and 40mm fins (50mm total) In my room, which runs between 19 and 22c the heatsinks are in the 44-48c range.
There are lots of examples of builds in this thread with different DIY store and EBay chassis. In many cases the builders have measured and reported heatsink temperature.
Some things you can do to maximize the efficiency of your heatsinks are, using tall feet on the amp to encourage convection airflow up through the heatsink fins. Leaving plenty of space around and above the amp for air circulation. Removing the anodizing and polishing the area under the output devices for better heat transfer, torquing the device mounting bolts to a consistent and appropriate torque. Note: the max. Torque recommended is 10 in/lb. Making sure you have lots of ventilation from the bottom of the chassis up past the inside of the heatsinks and out the top. On my chassis the top covers were ventilated but the bottoms were not. I drilled 2 rows of 3/8” holes in the bottom, just inside of the heatsinks, to promote convection. This gives you a column of cool air rising past the outputs and helps keep the interior of the chassis cool which is good for component lifespan. Adjusting your bias to a conservative level where your output devices are not running so hot that long term reliability is in question. In my case I could have gone further than 65w class A but I felt that was enough given my room size and speaker efficiency.
You could also add a fan or fans under the heatsinks to help lower temperatures. Zen Mod came up with a design named The Babysitter that is one way to do this.
In the end, if you already have the chassis, you will need to balance class A power output against operating temperature and reliability. That is a judgement call only you can make.
There are lots of examples of builds in this thread with different DIY store and EBay chassis. In many cases the builders have measured and reported heatsink temperature.
Some things you can do to maximize the efficiency of your heatsinks are, using tall feet on the amp to encourage convection airflow up through the heatsink fins. Leaving plenty of space around and above the amp for air circulation. Removing the anodizing and polishing the area under the output devices for better heat transfer, torquing the device mounting bolts to a consistent and appropriate torque. Note: the max. Torque recommended is 10 in/lb. Making sure you have lots of ventilation from the bottom of the chassis up past the inside of the heatsinks and out the top. On my chassis the top covers were ventilated but the bottoms were not. I drilled 2 rows of 3/8” holes in the bottom, just inside of the heatsinks, to promote convection. This gives you a column of cool air rising past the outputs and helps keep the interior of the chassis cool which is good for component lifespan. Adjusting your bias to a conservative level where your output devices are not running so hot that long term reliability is in question. In my case I could have gone further than 65w class A but I felt that was enough given my room size and speaker efficiency.
You could also add a fan or fans under the heatsinks to help lower temperatures. Zen Mod came up with a design named The Babysitter that is one way to do this.
In the end, if you already have the chassis, you will need to balance class A power output against operating temperature and reliability. That is a judgement call only you can make.
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I'm running the same voltage as Bfpca (46v), under full bias load of 0.7a / pair (350mv) x4 pr. No load with 35vac secondarys is around 51v. My line runs a little high at 126vac. If your no load is 48v then you can probably expect around 43v under full bias.
I'm using 2x store 5U Deluxe chassis' as mono blocks (one channel per chassis) and temps run around 48-49C in a 20C (68F) room. They can get up to 55C or so in the summer months with central air running. I'm also using the store Keratherm pads. Very acceptable although the room will get pretty toasty if I don't have a fan circulating the air out of the room.
I previously had a 2 pair stereo V3 at 34v in a single, same chassis, which ran about the same temps at full bias.
I wouldn't recommend anything smaller for a full on F5T V3.
TJ
I'm using 2x store 5U Deluxe chassis' as mono blocks (one channel per chassis) and temps run around 48-49C in a 20C (68F) room. They can get up to 55C or so in the summer months with central air running. I'm also using the store Keratherm pads. Very acceptable although the room will get pretty toasty if I don't have a fan circulating the air out of the room.
I previously had a 2 pair stereo V3 at 34v in a single, same chassis, which ran about the same temps at full bias.
I wouldn't recommend anything smaller for a full on F5T V3.
TJ
Bfpca - Your heatsinks have 64,000 square mm of surface area, the heatsinks I'm looking at would have 75,000 square mm of surface area, with the base 2 mm thinner and the same size fins. Rough guess is they would be close to the same capabilities for cooling. Thanks for the build tips.
Zen Mod - I'm building this amp the same way I've built race motors over the years, my 944 turbo with 20 lbs of boost gives 450 bhp on paper. I run it at 10 - 12 lbs boost because with 20 lbs, Crower connecting rods stay attached for about 30 seconds. I always over build and under use so I only have to build $7K race motors once a year.
With the F5t, I have no intention of finding peak output, the intent is to have the parts last longer with low stress. Considering the time and effort I've put into this project (I'm only half way done), I'm going to baby this amp.
I just want to build a 100w class A amp, once in my life because.....
Zen Mod - I'm building this amp the same way I've built race motors over the years, my 944 turbo with 20 lbs of boost gives 450 bhp on paper. I run it at 10 - 12 lbs boost because with 20 lbs, Crower connecting rods stay attached for about 30 seconds. I always over build and under use so I only have to build $7K race motors once a year.
With the F5t, I have no intention of finding peak output, the intent is to have the parts last longer with low stress. Considering the time and effort I've put into this project (I'm only half way done), I'm going to baby this amp.
I just want to build a 100w class A amp, once in my life because.....
If you mean 6 output pairs/channel it could certainly be done but you would need the heatsink size to do it. I believe Nelson Pass writes about using more outputs and ways of getting more power at the end of the F5 turbo article.
The easiest way to accommodate balanced inputs on an F5 is with an input transformer. If you are referring to building a fully balanced F5 then you would not want 65 volt dc rails from 48ac transformer windings unless you were looking to get somewhere in the neighbourhood of 8-900 watts into 8 ohms. If you mean 48volt cente tapped or 2- 24volt windings then that would work for a balanced amp yielding 32volt rails and possible balanced output of about 200w 8 ohms.
A balanced F5 is a hard build. Search for a thread on the F5X by EUVL for a look at a very well thought out approach.