DIY Audio Projects Forum

Hi, You need the 1M resistor or at least something over say 100K. It allows the charge on the output capacitor a path to ground if there is a bolcking one in the power amp. The preamp can easily drive any SS amp that has over about a 2K ohm input. Optimum is over 10K. The output you indicated is the max it can deliver, using the volume control you can get anything you want . Typically you would adjust it for 1-2 volts to drive most power amps. My power amps need about 2.5 volts for full output.

Good listening
Bruce

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Some of my DIY Tube Amplifier Projects:

• PoddWatt: 5751 / EL84 Push-Pull DIY Tube Amplifier Project
• OddBlocks: KT88 Push-Pull DIY Tube Amplifiers (Mono Blocks)
• ForeWatt: ECC802S (12AU7 / ECC82) DIY Tube Preamplifier Project
• GrooveWatt: Tube (Valve) RIAA Phono Preamplifier Project

You can meansure the voltage approximatley with multimeter on ac mode, then put a test tone at 100HZ and observe the output voltage in RMS

Maybe I'm going to repeat all the answers before my post, but I should clarify several things.
First of all about the stated 20V maximum output: the value is for estimating the overload capacity, not for normal usage. Having the gain of 7 Forewatt should get 20/7=2.9V input to output 20V. I can't say this input voltage level is impossible to get, especially from modern devices, but far before this point you will overload the power amplifier. So the volume control is your help in mitigating the problem.

About the resistor on the output. As you can see the left side of the output capacitor is connected to the voltage near 100V. If the right side is not connected to anything (like when you omit the 1M resistor) any path to the ground (the input side of connected power amplifier will serve the duty) will charge the capacitor and the first pulse of the charging current will pose the 100V voltage to the path. The left-side 100V is not appears instantly, so if the path in the right side has a resistor the voltage on the charging path will also rise gradually. But if the input of the power amplifier has a blocking capacitor the initial voltage will divide between the output capacitor of the Forewatt and the blocking capacitor. The input capacitor maybe not rated for such voltage at all.
Answering to the question: I doubt that buzzing is related to the 1M resistor. Most probably there is a ground issue.

Thank you Bruce and Poty for both answers. Very helpful. I definitely have one or more ground issues I need to resolve...

Best regards,

D