What would be the correct output cap for this OTL Headphone Amp?

[thaumaturge]

I am going to try your suggestion, Doc. I have few old tubular capacitors of 1n, 1n5 and 2n7. I will start with 1n.

ooh, too much. Each one would represent about a 10Kohm load at 20Khz (Xl=1/(2pi FHz Cfarrad)) Might cause some noticable high freq rolloff. I'd go lower like 100pF -or less- for RF decoupling. You want the cap to short the FM (88-108MHz) to ground, but not the audio.

Doc

 

[pingfloid]

Thank you Doc.

I do not have a lower cap at this moment, but right now I have managed a good result with a 300 Ohm grid stopper and a 100n orange cap at V+.

Regards, and happy new year!!!

 

[Kay Pirinha]

BTW: Thermal camera in hand, in only five minutes I have measured 180 degC at the 100Ohm/10W power supply resistors, and 165 degC at the 1k/25W cathode resistors Time to start replacing components.

Hi pingfloid,

the cathode resistor R6 dissipates about 10 watts, so there's no need to replace them as they're rated at 25 watts. As for the B+ filtering resistors, we should have some more details: It is a stereophonic amp with a common supply, right? As each channel draws about 100mA, power dissipation is (0.2^2 * 100) = 4 watts - also sufficiently low enough for a component rated at 10 watts. Finally, temperature of resistors surely won't affect the frequency response of your amplifier !

Best regards and a happy new year!

 

[pingfloid]

Hi Kay,

Yes, it is a stereo amp with common supply. If the power dissipation is 10 W, and the resistors are rated 25 W, I don't understand why they got so hot. In 20 or 30 minutes the chasis of the amplifier is around 60 ~ 100 degC. Really dangerous if you touch it while adjusting the volume potentiometer.

I have already ordered a couple of this Vishay Thik Film Resistor rated 50 Watt (2.5 W at free air) and small Heat Sink from Mouser.

I don't know what else can I do in order to reduce the heat. Tomorrow I will check the voltage through the power Supply resistors in order to calculate the current and power.

 

[pingfloid]

At the Vishay Technical Doc there are the instructions to calculate the heatsink, but I have just chosen one that I hope that will fit inside the limited space of the case.

 

[Dimitris AR]

Err.. I have to correct myself: there is a reduction in the volume.

I am going to try your suggestion, Doc. I have few old tubular capacitors of 1n, 1n5 and 2n7. I will start with 1n.

Its hard to belive that 1Kohm grid stopper resistor make any reduction in the volume , however you can add a very small cap ( about 10 - 47pF ) between grid and cathode of the EF95 .

 

[Cookj]

Yet another Chinese room heater, at those case temperatures the Elkos will boil.

The designers are trying to dissipate ~28Watts inside a virtually closed box!

The Heat sinks you have chosen are useless as they have a thermal RTh of approximately 15Deg/Watt, this equates to 150DegC above ambient (Free Air)

I knocked up this circuit on a breadboard for a Christmas project and with 75mmx50mm Heat sinks, the 1K resistors run at approximately 50DegC, which is still a bit to hot but I don't have any others to try.

I used RED LED Bias on the 6sh1's and got similar voltage measurements (10% rule). No problems with Rf breakthrough. Input 1k Gristopper and 47K input.

Cheers

John