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PostPosted: 12 Aug 2020, 11:40 
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Joined: 25 Apr 2020, 13:45
Posts: 4
Bruce

I just completed this build. Hooked it up to my rotel RB-850 amp connected to 15” studio lab speakers and they never sounded better. One issue I am struggling with is hum. Which increases with volume and stops immediately when I turn preamp off.

I used a star ground scheme. 1 for chassis (earth) one for each channel power and 1 for each signal path. I am using 6n1p’s with 220R’s.

I scoped the power supplies. 0 ripple on 215. I get a small 0.150mv ripple on the 6 volts as a perfect sign wave

I scored the signal path and I get a funky wave same on both channels. It is on the input and gets amplified through the tubes. I assume this is the hum. It is really small. I have to set my scope to lowest settings to see it.

I tried moving my ground points. I poked all my solder connections with a chop stick. Tried grounding pin 9 on tubes moved wires around including the mains. I am running out of options

As this is my first tube project I am not sure what right looks like and struggling a bit

I am thinking of adding a 2.2k cap on 6v for more filtering.

Any thoughts on things I can try?

Btw. I am really impressed by the preamp. Thank you for sharing your design. Once I get this working going to tackle a kt88 amplifier.

Dave


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PostPosted: 12 Aug 2020, 11:40 
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Joined: 24 Oct 2010, 07:05
Posts: 312
I have one more idea. I reread your explanations and the ForeWatt "environment". ForeWatt without the cathode cap has gain around 15dB. It is possible, that in bypass mode there is no amplification of the input noise and the noise is just under the FFT-gear floor (by the way - what do you use to measure FFT?). With gain you may see the amplified input/environment or some other noise.
But I believe in the tubes noise more.


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PostPosted: 12 Aug 2020, 12:47 
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Joined: 04 Jan 2020, 08:22
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Ho, even at full turn on the volume, its quiet using the by-pas path, but already at 5 deg turn on volume knob in active, the hiss is there, almost same level (slightly higher only) at full turn in active mode, and with / without the Cathode cap gives the same hiss noise level.

Br Svante


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PostPosted: 13 Aug 2020, 10:41 
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Joined: 04 Jun 2008, 20:59
Posts: 4256
Location: Arizona, USA
Hi, The bypass modes is just that, straight through. Only the selector switch and volume control in line. Does it hiss on active with the volume control at zero? Or all the time?

Good listening
Bruce

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PostPosted: 13 Aug 2020, 10:56 
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Joined: 04 Jan 2020, 08:22
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Hi

Al the time, but at a lower level when volyme turned to 0-level, then almost constant regardless of volume position.
I also have tone control & balance that are connected in the passive line (and of course active), but they don’t ad any hiss noise, since its quiet in passive.
Br Svante


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PostPosted: 14 Aug 2020, 03:18 
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Joined: 09 Aug 2018, 03:15
Posts: 21
Location: Forsbacka Sweden
Is this Preamp suitable as a Buffer-stage for a DAC? I dont want gain just a suitable tube stage and better impedance match. Can you set it up for lower gain or no gain?

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Tube interested and open for new projects always and too often...


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PostPosted: 14 Aug 2020, 09:16 
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Joined: 24 Oct 2010, 07:05
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To answer this question one should know what problem are you going to solve? What output impedance do you like to have? What amplitude? Why tube?


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PostPosted: 14 Aug 2020, 14:44 
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Joined: 25 Apr 2020, 13:45
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Hi bruce..
I chose your design for my first tube amp and I am very pleased with the result, it drives my vintage 15" spreakers throgh my Rotel RB-850 better than I have ever heard them... I am having one issue and I am seeking your advice...

As this is my first tube project I am not sure what right looks like.. I am having terrible hum that increases with volume.. I am using 6n1P's with 220R's and I have detected the following:

1. High Voltage side has 0 ripple, it is perfect and adjusted to 215V
2. Low voltage is 5.99V with 0.152V of AC ripple - not sure if this is expected or acceptable as I know these tubes can run off DC & AC
3. I am getting a malformed signwave across pin 8 and signal ground on BOTH tubes, with pin2 disconnected the wave is 100MV P2P and about 8ms ..

I am assuming this is the hum.. I am using a star ground and have sperate grounds for chasis(earth), high voltage (L/R), low volatage, Signal path (L/R) .. I have added the 1uf on pin 6 B+ and have 1M R on my output through 1K resistors..

Question, should I see so much ripple on the 6V? maybe bad cap? or should I add a 2.2K to elimitate it.. I am using the M317 to regulate the power


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PostPosted: 14 Aug 2020, 15:35 
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Joined: 13 May 2012, 17:47
Posts: 4
Hello Bruce. Sorry for arriving late to this party, and thanks for sharing your design.
I wanted to know if this preamp would work ok with solid state amplifiers (because of the input impedance of those).
Shall the 2.2k resistors be changed to other values for that purpose?

Thanks!


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PostPosted: 16 Aug 2020, 15:38 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, You can not really adjust the gain it is either X7 or X11 depending on whether or not you use the cathode bypass capacitor. Z out is in the 2K range for 12AU7 tubes and their siblings. I suppose you could recompute circuit values for use with lower mu tubes and get less gain. However I have not found any such tubes that sound as good.

Good listening
Bruce

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