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Post subject: Noise cancellation for Blues Deluxe & Blues Jr
Posted: Sat Mar 09, 2019 9:43 pm
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So I have been studying noise to make a quieter amp. I find I'm retracing Leo Fender's footsteps and finding fruit in the modern Deluxes.
Let's start with the three types of noise.
Johnson noise accompanies any resistive element. It's high frequency hiss and increases with resistance. Its the sound of electrons turning into heat.
Shot noise is a sizzling that is present in cathodes of tubes and the emitters of transistors. Its the sound of electrons leaving a quiet pool and tumbling down the rapids.
1/F noise is also generated in active devices. It is strongest at zero Hz and drops off quickly. It is the slow surf noise. I refer to it as fractal noise as it is found everywhere in nature.
Johnson noise is controlled by limitations to high frequency response to reduce hiss, but the sizzle and surf noise can be dealt with by using a differential amp. Both shot noise and 1/F noise are coherent and can be phase reversed for effective noise cancellation.
So I'm reading about connecting two FETs to a differential
OP amp to cancel noise. They use a common source and their noise is fed in opposite phases where it cancels out. I recognized the circuit as the same as Mackie and Behringer mike preamp circuits. These are some of the quietest preamps around.
Then I saw the post about the 68 Custom Deluxe Reverb that was so noisy. After examining the schematic, I realized that Leo had built the same type of differential noise cancelling circuit into all the BF/SF dual channel amps.
The extra stage to run the reverb also adds phase reversal. So shot noise and fractal noise get cancelled out when they are mixed back together at the phase inverter input.
Two actions come to mind.
On twin channel Fenders, matching the gains of V1, V2, and V4 should improve the noise cancellation for a quieter amp. If the noise is equally amplified it should cancel better.
On modern Fender Blues Deluxe and Blues Jr, there is no such cancellation going on. But there is an unused part of a preamp tube. I have previously pressed it into service to make a mike channel for a Blues Deluxe. I'm pretty sure it could be set up as a dummy stage to pass cathode noise to a place it can be applied differentially to quiet the amp. I haven't worked it out in detail yet.
Unfortunately, I don't have ready access to amps since I retired, but I'm going to pursue this idea and see if its workable.


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Post subject: Re: Noise cancellation for Blues Deluxe & Blues Jr
Posted: Sun Mar 10, 2019 12:01 am
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Joined: Wed Oct 29, 2014 1:50 am
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Location: Arizona USA
Wow, Tim, I knew you you were good but this is way over my head. I think I understand what you are saying but we're in a different league. Good luck and, from what I've read over the years, if anyone can do it, it's you. 8)

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Post subject: Re: Noise cancellation for Blues Deluxe & Blues Jr
Posted: Sun Mar 10, 2019 2:48 am
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Joined: Fri Feb 05, 2010 4:31 am
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Tim you are great !


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