F5 power amplifier

[sekess]

Originally Posted by tinitus

Thermal switch?
I suppose that would be the famous thermistors, TH1/TH2 in original schematic
Tho some have mentioned they may not be effective with laterals(Toshiba)

Regarding Xed balanced F5
I suppose it will see half the load impedance, as usual with balanced amps
Which may half its classA output, bias depending
Or am I missing something

Hey Tinitus,
No, not thermistors. I'm talking about a thermal switch that is wired into the primary side of the transformer that goes open if the temp gets above a certain level -- I think 75deg C in this case. It's for protection in case the amp starts overheating.

Thanks,
Steve

[CBRworm]

Originally Posted by sekess

1. Are any of you guys using a thermal switch to shut power off if the temp get's too high?

2. If yes, where are you guys mounting them -- free standing or on the heatsink?

Thanks,
Steve

I was considering mounting an actual thermal switch to each heatsink that would open above 70C. If either one opened it would disconnect power from the amplifier. I had not worked out the exact details yet as to whether it would be directly wired, or if it would trigger a latching relay. I prefer to keep AC voltage away from my fingers, I also prefer not to need a small secondary power source for protection duties.

I chose 70c ish because my heatsinks typically run in the mid 50's. My logic was that the only condition the heatsink would get that hot is if airflow was impeded or too much power was being dissapated indicating a fault.

[sekess]

Hi CBRworm,
Yea, I installed one on each heatsink on an Aleph J that I built and they worked out good. But, I was just wondering if they are really necessary. I hate to have to tap more holes in my heatsinks just for those guys.

Thanks,
Steve

[CBRworm]

I don't know if they are necessary. Ideally they will never be called to action. In my case I have the heatsinks on the inside and a slow, quiet fan moving air over them. If the fan was to stop for some reason, or if the amp was set on shag carpet instead of a hard surface - I could see it getting too hot.

I would say it is not necessary.

[EUVL]

Firstly on matching JFETs for the balanced version. If you were to use different source resistor values for the JFETs as I did, then you should match the Id of the JFET with the corresponding source resistor in place (i.e. not at Idss).

Zobel at the output would not work, as it only loads the amp at HF and not DC, which is what we want in this case.

On thermal drift, you can improve drift stability by coupling all JFETs together to a common thermal ground (a piece of aluminium or even better a heatsink). You may of course use a pair of my Dual JFET heatsinks (see GB forum), but that might limit your PCB layout freedom.

Toshiba Dual JFET Heatsink

As to thermal switch, I do use them on my reference amp, and I have them mounted with thermal grease on the main heatsinks.

Since smyslow has apparently built the X-version with floating X, maybe he can also share his experiences.


Patrick

[gaborbela]

Originally Posted by EUVL

Firstly on matching JFETs for the balanced version. If you were to use different source resistor values for the JFETs as I did, then you should match the Id of the JFET with the corresponding source resistor in place (i.e. not at Idss).

Zobel at the output would not work, as it only loads the amp at HF and not DC, which is what we want in this case.

Patrick

Thank You Patrick
I was ready to order the parts then I sow your post , so I canceled the order .
I will match the JETs Idss and I will use the same value source resistor for all 4 place ! Do I use 10R or less is better?
What is the purpose you use different feedback resistor value ?
I see Smyslow use 4x100R .
Yes I realized the Zobel wouldn't work in here .
I just look up some AX schematic . So I will buy Caddock MP930 resistors .Probably with a small TO220 heat sink .
Thank you one more time .
Greets

Jeno

[EUVL]

If you use 4x 10R for source resistors, then you should also use identical values for the feedback resistor, as you want the same close loop gain, I presume.

Patrick

[tinitus]

Thermal protection
Holton sells a speaker protection module with optional thermal protection
Not sure how thats supposed to work

Wonder if thermal protection could be incorporated into a softstart module

[sekess]

Hey Tinitus,

Yes you can incorporate thermal protection in the soft start-etc. module. I have seen that approach taken. but, you still need a thermal switch to sense the temperature.

Steve

[jackinnj]

Originally Posted by tinitus

Thermal protection
Holton sells a speaker protection module with optional thermal protection
Not sure how thats supposed to work

Wonder if thermal protection could be incorporated into a softstart module

You can cobble together the NP Power Supply on the F5 service manual webpage, then incorporate Troy Huebner's ideas from Nat Semi Application Note AN-1849 "An Audio Amplifier Power Supply Design". Although I don't like bridge rectifiers!

http://www.national.com/an/AN/AN-1849.pdf

Anyone heard from Troy recently?