What is Motorboating and Why does it occur in a vari mu limiter?

Hey all,

I know theres alot of info on here about the motorboating issue some federal AM864's have when being modded or when they are not calibrated properly with the current control.  I have built a handful of Am864 circuits and only have been able to work around motorboating issue when dialing back the current control, leaving stock attack and release times and changing the cap in the SC to let less low frequencies through.  I would just like to know why all these changes make or break the opporation in a Vari-mu federal.

I look forward to learning from you all.

Thanks for your time!

Motor boating is caused when the power supply impedance at the various gain stages is not low enough. That is, insufficient filtering. This means the stages can interact, and if the net gain is positive, the circuit oscillates at low frequency. You can eliminate it by power supply design, using regulators or more filtering. It can also occur with negative feedback if the phase at low frequencies through the feedback network goes positive.

A real 864 is less prone to motor boating because it's band-limited.  The PSU is designed for that band-limiting. 

These basic vari-mu designs (without interstage transformer isolating the vari-mu's) can rarely be optimized for full-frequency use, as they tend to get too sensitive to control-voltage-breakthrough.

Often this is a side effect of second stage (in this case 6SN7 amplifier) going into saturation because of the huge DC shift occurring on its grids when vari-mu stage changes gain and thus anode DC-offset...

The trick is to keep control- and audio signal chains restricted to separate frequency ranges. So either low cut on audio, or only slow gain changes..

Jakob E.

I did a lowcut on my sidechain with the modded faster settings and a series resistor for the threshold.
So my minimum TH is I think -10dBu.
But I can't remember more right now.

Rompstomp89 said:

... the motorboating issue,,,

Click to expand...

Somebody's gotta say it, motorboating is not always a bad thing. ;D ;D ;D

Anarchists...!

Power supply impedance staying low down to deep bass is the usual trick in straight amplifiers. This can include the amplifiers in limiters.

But there are more subtle problems in limiters, as Jakob says. In simulation, with "perfect" power supply and perfect balance, half-wave rectifiers are very prone to moto-boat. I believe Barry Blesser outlined the stability criteria for limiters.

maybe put a 670 regulated circuit in there,

Just change value of C4 (the 3x12uF HT reservoir), or parallel with something reasonably sized rated the same working voltage: If motorboat frequency changes significantly with this, then you have "real" PSU motorboating that can be fixed in supply.

If changing HT reservoir size (and thus high-voltage low-freq impedance) does not change boating frequency significantly, your motorboating is rooted in the direction of limiter instability, which is a much more complex problem..

Jakob E.

Wow this is great information.

Any chance we can touch more on these stability criteria  regarding the sc rectifier ?

Or discuss in more detail these limiter instabilities assuming a good power supply????

Note that this has been discussed fully in many old threads.  Worth finding them; they have comments from many professional designers who no longer participate here.  This is one of the problems with the whole "warning! this thread has not been posted to in 120 days!  consider starting a new thread!" nonsense that pops up. 

Definitely a lot to try to understand.

Looking through some of lolo-m's  posts and the Fairchild threads  is interesting if not mind numbing.....

PRR, Bcarso, Larrchild, NewYorkDave, pstamler, CJ, plenty of others I'm forgetting at the moment.  Apologies....

You asked what motorboating is.  Everyone says it's instability, which is true.  Nobody said it is called motorboating because it sounds like a small boat engine chugging away at 2 or 3 Hz, which is something you may not know.

spent a year on a motorboating problem,  turned out to be capsule thump in an AT5031 condenser mic, p/n might be wrong,