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It is currently 31 May 2020, 17:14

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PostPosted: 03 Jan 2020, 21:42 
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Location: Arizona, USA
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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PostPosted: 03 Jan 2020, 22:52 
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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


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PostPosted: 07 Jan 2020, 09:50 
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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.


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PostPosted: 07 Jan 2020, 10:05 
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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.


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PostPosted: 07 Jan 2020, 12:35 
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Location: Ottawa, ON, Canada
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


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PostPosted: 05 Feb 2020, 08:20 
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gofar99 wrote:
Hi, At the given voltages (180-225 B+) IMO there is no best value. There are several issues with ECC81s in this circuit. First the gain is way too much (instead of 7 it is 25). Too much for a line stage. Second the output impedance will be high....about 6.5K. It would be OK for a tube amp input of say 50K or more but might be unsuitable for a lot of solid state gear. All that said if you wish the best values I can determine are for both cathode resistors to be 100 ohms. This gives an idle current of 5.2 ma at 220volts. Output voltage into 50K is about +47/-31. Since you really need only about 1-3 volts it ought to be OK. The non-symmetrical range is likely to cause some harmonic distortion, but I have no way to tell short of building it. My guess would be that 1-3 volts output it should be in the sub 1% range. You can not bypass the lower cathode as the symmetry really gets bad there. I didn't check for higher B+ but it would seem to put the tube in a better range. Let me know what you have available and I re check it.

Good listening
Bruce

Hi Bruce
I modified the cathode resistor to 95 ohm and raised B + to 268 volts. I used ecc81 China, Chelmer, and Mullard cv4024. The best result is with cv4024. Distortion is not audible compared to ecc82. The sound is balanced, best low and high frequencies. Ecc82 has more mid frequencies.
Regards


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PostPosted: 05 Feb 2020, 11:29 
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Hi, good information. The biggest issue I see with this is the gain. You will have more noise feeding the power amps as the volume control is at the input of the preamp to prevent over driving it. The internal noise with about 25 X gain vs 7X could be significant. Since I have found that most gear will actually run just fine with a passive volume control even 7X may be a bit much.

Good listening
Bruce

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PostPosted: 30 Mar 2020, 08:33 
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Location: Italy
hi Bruce, is it possible to add a balanced output after the srpp? i'm trying to use this pre that succesfully drive my amp to "calm" down a very angry IC amp of a friend but it needs balanced in ( bridged amp) thank you

edit:
i was thinking about using a 10k:10k balanced to unbalanced transformer at the output of the forewatt, would it work?


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PostPosted: 30 Mar 2020, 09:55 
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Hi, Yes that is possible. 10K to 10K is probably ok. To further prevent loading of the Forewatt I would likely go to something more like 25K to 25K. The secondary impedance is not really all that critical, but will alter the voltage signal delivered. Since the Forewatt can deliver roughly +/-20Volts signal and still be linear and low distortion that is not usually an issue. You could see what the input of the amp to be driven is and possibly match that on the secondary side. Let us know how it works out. Watch out for how grounds are worked out. It may be that the secondary side ground would work better if not fed back to the Forwatt signal ground and go only to the individual amps to avoid ground loops.

Good listening
Bruce

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PostPosted: 30 Mar 2020, 14:26 
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Kind as always, thank you Bruce, I'll start with some chinesium transformers and if it works fine I might swap to some Lundhal and let you know the results


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