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Open Loop Vs Closed Loop


 
4/17/1999 9:57 AM
RJH
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Open Loop Vs Closed Loop
Can any of you explain to me the "sonic" differences between an open (no feedback) vs. a closed (varing feedback) power amp design. It seem to me that we are somehow compromising between gain and freq. response. If closed loop is better -- which closed loop design is better? The one with the 5k pot or the one with the 25k pot? If open is better can a closed loop design be made open loop by simply removing the speaker tap? Thanks RJH
 
4/17/1999 10:43 AM
SpeedRacer
I assume that you're talking about the Marshall circuit specifically due to your mention of the "25K vs 5K pot"..  
 
Open loop response is going to be mid-heavy in pretty much any amp. Yes, you have more gain and accordingly it takes a smaller iput signal to drive the amp to full output power. Distortion (normal HD, not clipping) is higher and clipping occurs more "slowly", not as suddenly. (generally speaking of course)The output imp of the circuit will be higher, which will give less dampening of the speaker (less tight control of its motion).  
 
Yes, we are compromising betw gain & freq response, but also output power vs HD.. and the dynamics ussue attached to dampening. It will all hinge on how much -fb we need to use to achieve our particular goal. The only _really_ popular amps that run open loop are the AC30 and the 5E3 Deluxe in my own exp.  
The AC30 deals with the dampening issue by using multiple output tubes (parallel tubes = lower imp) feeding a high imp (16-ohm) load. This minimizes the turns ratio of the output tranny, which if my memory is working this morning is good for this sort of thing..  
 
Many folks like the closed loop due (I think) to it's more defined change over from clean to distorted. The clean is cleaner, and the mean is meaner IMO, and you can get from one to the other simply by backing off on your pick attack. It's harder to do that in an open loop circuit IMO. Witness the huge commerical success of closed loop amps vis open loop. Very few truly excellent open loop amps. Open loop will expose any "deficiencies" in your design & materials much more quickly.. (no -fb to correct for them..) In particular any nasty HD. (upper order odd harmonics.. 5th, 7th etc)  
Which circuit is "better" is a loaded question (and me w/o a vest!) I personally like the 1986/1987/1959 circuit (all the same amount of -fb) 47K to 8-ohm tap for 50W, 100K for 100W. 5K pot for presence without the other resistor across it (the early kind) Yes, it has DC on it and it will always make the "scratchy" noise, but I really do think it sounds better.. why, I have no idea.  
 
And finally, yes, any amp can go "open loop" by simply disconnecting the -fb lead from the speaker jack.
 
4/17/1999 10:53 AM
RJH
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Thanks Speed!!! I've currently got the 25k pot verson with the 100K resistor and the presence pot across the 470K to ground. I'll try your 47K type. Thanks RJH
 
4/19/1999 5:58 AM
GFR
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Remember that the VOX AC30 runs in class A, so it doesn't need NFB to reduce crossover distortion.
 
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