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 Post subject: Noisy fx loop fixed
PostPosted: Sat Oct 31, 2020 6:55 pm 
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Joined: Sun Feb 10, 2019 2:46 pm
Posts: 9
Covid lock down made it possible to investigate one item that slightly bothered me with this great amp.
My kit came with a granger v2 active fx loop. I have seen that now a fx with a mini tube is provided instead. Perhaps it does not have the senitivites that I experienced with my fx loop.

The initial problem was that the fx distorted the signal at low master volume setting and had a bad signal to noise ratio. When you activated the fx the noise floor was increased. I wanted to find out why and if this could be fixed. According to the instructions from Trinity this fx should be installed after the master volume just before the phase inverter. When I set the MV on zero and played the guitar some ugly distortion was produced at low level and was audible in the speaker only when the fx was engaged (with a patch cable so no external influence from a pedal). With fx off the amp was dead silent with mv on zero as it should be. It turns out the source of this low volume distrotion is coming via the B+ power supply to the fx. The preamp and distrotion stages are still active drawing power from B+ even if the MV is turned down. This causes a small signal on top of B+ that the fx pickes up and sends with its signal output to the phase inverter (there is no volume pot between fx and pi in the original design). The first instinct may then be to filter B+ better at the fx.
Instead I took another route and moved the 1M ohm master volume pot so it sits after the fx just in front of the phase inverter. This means the fx always has a large input signal. That worked great to remove the ugly distortion at low mv level and the bad signal to noise level was also fixed (in fact the distortion from B+ and the fx noise floor are still there but now drowns out in the larger fx signal output level, so these are impossible to hear). I switch on the fx and now there is no audible added noise at all !
Now another problem occurred... when the fx is engaged it added a small amount of distortion that was noticeable to the clean setting of the amp... I figured this could be because the original design of this amp has the signal from V1B going straight into the PI (only a coupling cap inbetween, but no voltage divider other than the 1M mv pot that I had now moved to the output of the fx). Now if a fx is installed here it may get overloaded with high signal from V1B. I suspected this because if I turn down the gain control (pot between V1A and V1B) this distortion issue dissapeared... However you want the possibilty to set the gain controll high to affect the tone of V1B, so playing with low gain when fx is engaged is not a solution.
I then tested by reducing the signal input to the fx by inserting a 33% voltage divider at the input of the fx board. I used a 300k and 150k resistor. That worked great ! Now the fx made no additional distortion at the clean channel at any gain setting. However now the distortion channel got a reduced output obviously which could be an issue, so to compensate for the new additional signal reduction I changed the ratio output pot VR8 of the distortion section from 100k to 250k and bridged the 150k R24 series resistor. This way the output stage V2B still sees the same impedance (250k) but more signal is taken out from the distortion section.

Result: no additional noise floor from fx. No background distortion coming the backway from B+ via fx causing audible distortion at low volume setting. No added distortion from the fx at hi gain setting. Same output from the distortion channel as before. Now I cannot hear any change in the sound at all when I engage the fx regardless of volume, distortion or gain settings !

Perhaps this mod may have changed the clean sound or response of the amp with the additional 33% voltage divider in the signal chain before fx and PI. This I cannot tell as no A/B comparison of this can be made, hower I don't notice a difference. The amp sounds great now also with fx engaged in all situations.

Full volume can still be achived with the voltage divider in the signal chain. The last 25% of the MV setting makes no difference to the output volume.


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