PPIMV idea - has this been done ?

LPV

Member
Messages
700
I had a LARMAR in my amp for a while and it did work ok to trim back the overall volume, once the amp was down to really usable levels I found the lack of NFB made it change far too much. I guess some people are ok with that sound and feel but it just ruined the character of the amp for me.

I experimented with very very low NFB resistors and still theres not enough. So heres my thought: has anyone ever amplified the NFB with a MOSFET to get more stability at low volume or is that just crazy talk?
 

HotBluePlates

Member
Messages
10,637
... So heres my thought: has anyone ever amplified the NFB with a MOSFET to get more stability at low volume or is that just crazy talk?
Crazy talk. "More Feedback" is like saying, "I want my amp to burst into oscillation & sing without any input."

That's slightly exaggerated, but only slightly. The feedback loop from speaker to phase inverter has phase shift (due to the output transformer and any coupling caps), so there is a limit on how much feedback can be applied before "negative" is shifted to "positive" and the amp howls.

I had a LARMAR in my amp ... once the amp was down to really usable levels I found the lack of NFB made it change far too much. ...
So let's look at the real problem: You have a post phase inverter master volume and low master settings result in too-little feedback (I'm guessing you wanted tighter control of the speaker here). If you really want to solve this, you need to move your volume/power control method outside of the feedback loop. Or you need to have stages before the phase inverter that generate the distortion you like in the amount you need.

The negative feedback tightens control over speaker movement and reduces the distortion among the elements in the loop, including the phase inverter & output tubes. Once you drive the phase inverter and/or output tubes hard enough that you hear them distorting in the speaker output, the "loop gain" has been overwhelmed and there's no negative feedback happening. Speaker movement becomes less damped, and the transition from clean to dirty (for the output tubes & phase inverter) is more abrupt than if there were no negative feedback.

If the audible problem you're trying to fix is loss of that corrective negative feedback, your master volume is in the wrong place. Being after the phase inverter and inside the feedback loop implies that any loss of feedback you're hearing can only happen once the phase inverter is distorting, but that also means the loop has stopped having any effect. Adding more feedback (as you seem to have tried) doesn't work and might make the amp unstable.

The solution seems to be moving the master volume control outside the loop (pre phase inverter master volume, or attenuation between amp & speaker), maybe in conjunction with the existing LarMar. It depends on what amount of distortion you need and whether you can dial-down the signal level ahead of the phase inverter/loop while retaining the sound you want.
 

davidespinosa

Silver Supporting Member
Messages
4,645
On that subject...

London Power master volume kits:
http://www.londonpower.com/amp-mods/london-power-master-volume
http://www.londonpower.com/amp-mods/splitter-limit-master-volume
Circuits described in this book:
http://www.londonpower.com/audio-design-books/TUT4
Table of contents:
http://www.londonpower.com/books/tut4cont.html

Chapter 5: MASTERFUL CONTROL
MASTER OF YOUR DOMAIN
Tone Generator
Master Volume
Transparent Amplifier
Transparent Speaker
System Hurdles
TRADITIONAL MASTER VOLUMES
Conventional MV
Improved Conventional MV
Bootstrapped MV
Improved Bootstrapped MV
Bootstrapped MV for Preamp Channel
POST-PI-MASTER-VOLUMES
Simple Post-PI-MV
Post-PI-MV-DON’T!
Isolated Dual-Section Post-PI-MV
Isolated Single-Section Post-PI-MV
Concertina Uniqueness
See-Saw Possibilities
MODERN MASTER VOLUMES
Impedance Modulation
Current-limited Splitter
TRANSPARENT POWER TRANSFER
Power Amp Requirements
Speaker Limits
Using Conventional Guitar Equipment
Direct Sound
 




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