Local feedback in the Dumblelator

[Normster]

I've looked at three other tube buffered loop schematics and none of them include local feedback on the return side of the loop like Dumble does. Has anyone built a Dumblelator without LNF? Is it really worth the extra complexity?

[glasman]

I've looked at three other tube buffered loop schematics and none of them include local feedback on the return side of the loop like Dumble does. Has anyone built a Dumblelator without LNF? Is it really worth the extra complexity?

I have experimented with the Dlator feedback and I find I like the sound better with the feedback included. It helps to control the gain (reduces the gain of the recovery amp) as well flattens the response somewhat.

Gary

[Bob-I]

I've had the opposite experience from Gary. The LNFB killed the tone of the D-Lator. My 4x6V6 amp has a D-lator in it with no LNFB and I run the return at about 50% for 0 gain.

That said I've gone to a parallel loop design using a straight feedback amp. The CF drives both sides of the loop, one side into the drive pot, the other into a 100k/.033uF. The mixer is a feedback amp with no ground reference. It's just about 0 gain and 0 coloration but mixes the 2 signals without interaction. It's important that the effect unit doesnt' allow ANY dry signal through or the phase relationship will be screwed.

[Normster]

I was really tempted to put in a switch just to hear the difference, but I guess I'll just leave it as is for now. (w/LNFB) This is the first time I've put a D'Lator in a build so I'm still a newb.

[Tonegeek]

My built in dlator very, very slightly cuts some highs. Has anyone else had this experience and if so, is there a fix? I think I tried a 180pf bleeder across one of the pots to good effect, but then discarded it as not stable enough since the sound changes as the pot is dialed up. Now Iam thinking I want that high end back. Maybe I just need to tweak the LNFB.

[glasman]

My built in dlator very, very slightly cuts some highs. Has anyone else had this experience and if so, is there a fix? I think I tried a 180pf bleeder across one of the pots to good effect, but then discarded it as not stable enough since the sound changes as the pot is dialed up. Now Iam thinking I want that high end back. Maybe I just need to tweak the LNFB.

The high end loss probably has more to do with the miller effect of the recovery amp. You can experiment with different values for the 220K (lower) feeding the recovery amp. This should help improve the top end. Be careful not to go to low, otherwise you may experience grid blocking.

Gary

[Fischerman]

I noticed it too. I bumped up the 500pF across that 220K to 560pF but that's really affecting the mids more than the treble. I like Gary's remedy but my guess is that it will only take you so far...and not quite all the way.

I actually don't mind the hi-cut when using the OD channel...I prefer it. I just tried to make it reasonably transparent and then just tweek the amp based on how it sounds with the loop (the loop is always in the path in my amp...intentionally). I considered increasing the Clean Master bright cap a bit but I don't think I need it (I'm using a 47pF right now).

Oh yea, I have the .22uF/22M LNFB on my Return stage.

[glasman]

FWIW, I have also used the bright cap on the return control with good results.

I have spent HOURS AND HOURS spice modelling the recovery amp to see how it can be improved and in general biggest bang for the buck is the grid resistor . The problem with fooling around with the cap is that becomes a high pass filter (with the resistor) and that starts screwing with the phase in the 2 to 4 kHz range.

Gary

[Tonegeek]

My built in dlator very, very slightly cuts some highs. Has anyone else had this experience and if so, is there a fix? I think I tried a 180pf bleeder across one of the pots to good effect, but then discarded it as not stable enough since the sound changes as the pot is dialed up. Now Iam thinking I want that high end back. Maybe I just need to tweak the LNFB.

The high end loss probably has more to do with the miller effect of the recovery amp. You can experiment with different values for the 220K (lower) feeding the recovery amp. This should help improve the top end. Be careful not to go to low, otherwise you may experience grid blocking.

Gary

That seems reasonable and i will give it a try. My loop is always on and lately I have been wanting a tiny bit more crispness overall out of the amp equal to what the dlator is taking away. Thanks