Sennheiser HD 650 Question

[nuhor68]

Dear Osprey,

Many thanks for your answers and explanations.

I am thinking about headroom and need an advice if I am doing it correctly. So, I made an operational point analysis using ECC99 plate characteristics, which are similar to 6BL7. I used a load line based on:

E = 300V
Zp = 20k
Zs = 600
RH = 300 (HD 650)
RL = RH x (Zp/Zs) = 10k

I chose Io = 15.5mA based on my transformer rated current and Ug = 4V. Then Uo = 146V (see PDF attached)

Let’s assume that input signal have average level 200mv RMS. From the PDF we can see that grid voltage can change from -0 to -8V (peak values) or from 0 to -5.6V RMS or with amplitude of 2.8V RMS. So,

20log(2.8/0.2) = 23dB

Does it mean that the amplifier has headroom of 23dB?

Thank you,

Igor

[Osprey]

Excellent choice of the ECC99 tube; it should work perfectly for this application. Your load lines look correct and all your computations seem OK. The only thing I would caution you about is the RL. This can only be considered an approximation. In reality it could look very different to the tube.

So now it is time to breadboard the amp. Build it up with the components you have chosen, but use a 1k ohm linear pot for the cathode resistor. You should have about 300V on the plate. Adjust the pot for your chosen current of 15.5mA which is correct for you OP trans. This should give about -4V on the grid. You can go up to 20mA as you know for the plate current. Once you have determined the currect cathode resistance measure the pot and put in a fixed resistor of the nearest value. You will need a volume control pot too. 50k or 100k will work fine.

With both channels working on the breadboard you should be able to listen to the amp to see what you think about the sound.

[nuhor68]

Again thank you very much for your comments.

The only thing I would caution you about is the RL. This can only be considered an approximation. In reality it could look very different to the tube.

Is it because in reality the OPT represents a reactive load?

Thank you,

Igor

[Osprey]

Is it because in reality the OPT represents a reactive load?

No its because Zs and RH are approximations.

[nuhor68]

Many thanks.

Your load lines look correct and all your computations seem OK.

I would like to make sure that my consideration of the headroom is correct. Please let me know.

Thank you,

Igor

[Osprey]

You should have massive headroom, but you won't know the actual numbers until you build the unit and measure the voltages.