IRS Beta Servo Open Loop Gain Setting

Nikkoholic

Active Member
Lots of threads about servo's but I'd like to post my findings today and see if anyone can chime on if this is normal or if you've had a similar experience.

A little history:
I've had my Beta's nearly a year. Like the sound but found the bass to be a bit overwehlming, and I like bass. I had to cut the bass on the servo about -6 to -9 db to get it to my liking.

So shared this with a few AKr's and recently another AKr experienced a similar situation on his newly acquired Gamma's. In the end when set up properly with the servo cables in place his "large" bass went away and he then he actually had to boost the bass to get it to his liking. So he called and challenged me to look over my system for proper connections. Well I was certain all was connected right. However he mentioned that when he had the servo "on" and pulled out the servo cable; the servo green light went off and then would reset just like when you turn it on. Mine didn't do that. I pull my servo cables in and out and no effect on the servo. Of course this is just with the servo on; not the entire system.

So checked my servo cables (which I think were remade due to lost cables from the prior owner). Pulled back the black cover and sure enough the 2 wires were indeed connected to the wrong pins on the 4 pin DIN connector. I verified this by pulling out the DIN female on the speaker and took the Servo top off and looked at the DIN connector inside and my cables were wired wrong. So even when my servo cables were connected; they really weren't.

So I fixed those this morning; after verifying it a million times, because I couldn't believe my eyes.

So went to hook everything back up and was anxious to hear the "real" IRS Betas. And got a Low Frequency feedback (line outs on my amp were down so brought them up slowly; no damaging sounds). So I checked, double checked and triple... all was right. Still oscillating / feedback. I thought this was the 60hz blast everyone describes. So I though maybe I have an inverting amp as infinity described (although it would be ullikely). So I switched to the "inverted phase" outputs on the servo. Then I got the classic 60Hz blast. Scary sound; but only for a split second. So I was convinced all external connections were correct.

So I spent the day tracing wires and making sure all was "in phase". Pulled all woofers out verified all wiring, verified accelarometer wiring.

Read some more in the Infinity literature and it Infinity mentions in the manual that even when everything is hooked up properly (which it was) you can experience a low frequency oscillation between 5 and 20 Hz. It sounds like turntable feedback to me. However this occurs even with all equipment off and just the servo and LF amp on. I have line output controls on my amp so I was able to "ease" into the range where the feedback started to experiment as to the causes. They state some amps have higher outputs and thus the Open Loop Feedback switch on the back of the servo can be switch to help eliminate this. So I lowered it to -6db and problem disappeared with no inputs on and now only showed up while music was playing (very dangerous) since it is unpredictable. So I switched it to -12db and that seems to have solved the problem. I played some songs with strong deep bass and all was fine. Bass does sound a little better with the servo working properly (assuming it is) however I now have to run my servo LF gain at +4 (on the front panel) to get good balance between the LF and HF towers.

This does make sense though, since previously I had to reduce the LF gain on the front of the servo by 6-9db and now running at +4-5 so about a 10-14db difference. Not sure why the -12db Open Gain Loop on the back reacts / eliminates the feedback vs. on the front controls.

Sorry for the lengthy comment. But I wanted to share the details in hopes of finding some other IRS owners that may have experienced similar Low Freq Oscillation / Feedback.

I know some other Beta owners who run much larger amps on the LF towers, I never understood that because my bass was too strong with both HF and LF amps the same output. Now I likely need more power on the low end or need to cut the line level controls on the HF tower to get a better balance.

SOOOO... can some IRS people chime in with their Open Loop Gain settings on the back. And is it there to eliminate the 5-20 Hz Oscillation that Infinity describes in the manual.

Thanks for reading and listening.
 
Keith,

VERY interesting. Glad you checked the pins and got that straightened out. Hopefully, you'll be happier with the Betas.

I have my Open Gain Loop set to -12 dB, necessary with my amp (SAE A1001) to eliminate the very sound you were describing.

David
 
Keith,

VERY interesting. Glad you checked the pins and got that straightened out. Hopefully, you'll be happier with the Betas.

I have my Open Gain Loop set to -12 dB, necessary with my amp (SAE A1001) to eliminate the very sound you were describing.

David

David,
Thank you for checking and thanks for reading the post, I probably should have condensed it, several people probably fell asleep on their computers reading it.

Any idea why some amps require this? My input sensitivity is 1000mV or 1V.
 
David,
Any idea why some amps require this? My input sensitivity is 1000mV or 1V.

No, I really don't. Infinity talks about differences in gain between amplifier designs requiring it. I don't have enough electronics knowledge to understand exactly what Infinity means by that, though. I know I ran into problems with my RS-1s when trying to use SAE P500s for the bass towers, and the only way they would work was to crank down the front panel gain controls on the amps. (The RS-1s don't have a gain control on the crossover).

Anyway, glad you got that straightened out.

David
 
I have the same experience. Some amps have different gain, or "amplification factor", which should not be confused with sensitivity. Gain is how much the input signal is amplified, where sensitivity is an input level which produces a certain output level. It is possible for 2 amps to have the same sensitivity, while having vastly different gains, or amplification factors or ratios.
The switch is to compensate for amps with higher gain.
This is only my understanding and experience. I could be completely wrong, and would love to understand this correctly if I am wrong.
D
 
I have the same experience and I use the -12dB setting. I use two
Krell KSA300S to power my Betas.

As I understand it this is what causes the servo loop oscillation if the
amplifier has too much gain:

If the amplification in the servo loop has a larger amplification
factor (gain) than the dampening in the servo loop then the following
will happen:

Hum, thermal noise etc from the acceleration sensor and cables will be
amplified and sent to the speaker, where the accelerometer will pick it
up and return it's findings to to the SCU.

The SCU, which has no input signal, will try to correct the noise signal,
the faulty correction is sent to the speakers, when it is returned to the SCU
it tries a larger correction signal etc.

Just my 2 cents...

/Ingvar
 
Ingvar,
Thanks for the explanation; that makes sense. Now I suppose I need a larger amp. Seems my LF output is way down.

Is the "amplification factor (gain)" independent of output Watts?
Is this -12db the same type of factor as cutting power by a 1/2 four times? (-3db = Watts/2 four times) So my 330 w/ch becomes 20 watts?
330/2=165 (-3db)
165/2=82.5 (-3db)
82.5/2=41.25 (-3db)
41.25/2=20.625 (-3db) for a total of -12db

Don't know this stuff too well and not sure if this relates to the Open Loop Gain control.
 
Keith,
I read your post with interest, and Mike shared your email as well. I've never known Mike to make a mistake (seriously; I think it's his anal German side coming out), but I certainly believe it happened. I own the Gammas, and truth be known, I always preferred my Gammas to his Betas. Because of the bass. I had always assumed it was due to his long flat walls, floor and corner loading issues. Apparently that was not the case. Which actually makes sense now that I think about it, because I always thought the Gammas sounded better at his house as well (but I could never tell him that since he loved his Betas and they were his babies).

I've owned two different servo units and ended up selling the spare to an A-goner. Both units I had/have, I need to use in the -12db position. The servo I sold was the first in four that he had purchased that he was able to run in the -6db position (it had the lowest noise of the four). I can't remember what amps he was running.

When I had Adcom 565 monos, my front gain dial was around -6 (about 9 o-clock). When I switched to the Bryston 4BST, the only change was increasing the front gain dial to -2 or -1 (around 11 o-clock).

I didn't ever use or need the resistive interconnects that Mike built for his use with the Bryston. Apparently the issues were not with gain/input matching between the Bryston and Lamm after all.

There have been threads about internal adjustments/calibrations of the servo box. I never read them in full as my system has always worked as it should, and I know well enough to leave things alone. But the threads are out there. It may be time to dial things back in. I think all you need is a 'scope to watch the signal IIRC. Glad you figured it out. I would love to hear them with correctly servo'd bass.
 
Feel free to drive to St. Louis, it is only about 10 1/2 hours with agressive driving and minimal stops.
Mikes mistake was an easy one to make. The diagram / scehmatic is simple enough but confusing in that it could be interpreted either way, depending on what you consider "front", "connector side", "rear", etc.

Thanks for your comments.
 
You're not losing watts, just reducing gain, or amplification factor. If the knob position at +4 bothers you, turn out the lights. What sounds right is probably right. Enjoy!
I hope mine sounds right when I put it together.
D
 
Very interesting results people.
My gain is set to normal on the back and is at zero or even +1 at times on the front,using Adcom 565 mono's for the bottom end of Gamma's.
Again,this is a phenomenum that Infinity should have addressed and explained more in detail for future users as speakers have swaped hands over the years and valuable info and contacts have been lost.
To my findings,the Infinity schematic for the SC which is the same as for Beta's and Gamma's explains some things but not all.Have a look at the part where it says refer to drawing two ( 2 ),well where is drawing two.I think that,that is where all the info about the accelerometers and support circuitry lay,which is not available.Correct me if I am wrong as I would love to see that second part of the schematic.
The threads that people are refering to with adjustment of the SC are a basic procedure where the SC is hooked up to a scope and is fed a signal of a certain freq and you simply adjust the pots until you recieve the same freq coming out.This might be a quick fix for certain components slightly drifting from their value as they age but I cant see it being a fix for the problem at hand.
 
I would love to have the factory process for tuning or adjusting the servo crossover unit for my Gammas. I did a search of the forums and didn't find anything, but that doesn't mean it's not there. I have a gut feeling that each set of Betas/Gammas responds a little differently, and fine tuning the SC to that particular setup would be ideal. Of course, I could be wrong....
Of course, I still need to put my Gammas back together and buy some amps, but I'm enjoying my DQ10s more than ever right now..
D
 
I think that jtucker had tuned his by a method to what I descibed in my previous reply.I think that he will be more than happy to show/tell you the proceedure that he used and it seemed to have worked fine for him.
 
Very interesting results people.
My gain is set to normal on the back and is at zero or even +1 at times on the front,using Adcom 565 mono's for the bottom end of Gamma's.
Again,this is a phenomenum that Infinity should have addressed and explained more in detail for future users as speakers have swaped hands over the years and valuable info and contacts have been lost.
To my findings,the Infinity schematic for the SC which is the same as for Beta's and Gamma's explains some things but not all.Have a look at the part where it says refer to drawing two ( 2 ),well where is drawing two.I think that,that is where all the info about the accelerometers and support circuitry lay,which is not available.Correct me if I am wrong as I would love to see that second part of the schematic.
The threads that people are refering to with adjustment of the SC are a basic procedure where the SC is hooked up to a scope and is fed a signal of a certain freq and you simply adjust the pots until you recieve the same freq coming out.This might be a quick fix for certain components slightly drifting from their value as they age but I cant see it being a fix for the problem at hand.

Checkout the SM for the Epsilons. They have some schematics for accelerometer preamp including revision updates. It could be similar to the Gamma's ;)

regards
 
Just to follow up on a few items. Since I got the servo cables and everything working and now know that it is "normal/common" to have the open loop gain set to -12db I've finally had a chance to do some listening.

The bass is much better and more controlled. I have taken my equalizer out of the system and just run it flat. I do occassionally put it in to slightly boost the highest freq., but it really isn't necessary anymore.

Some of the songs with the most challenging bass are deep and steady, not so muddy and floppy. I have run my amps up to 0db on the meters and the LF seems steady and under control. I like it.

Also my son asked about the "tap" test tonight (that I could never figure out (because it wasn't working)), which I forgot about and had not tried since the repair. Yes it did work and it was very obvious.

I liked my Betas for the last year (since I purchased them), but I didn't get what all the hype was about. Now with the Servo operating.... I understand.

Thanks for all your comments on this post, especially those who chimed in with their open loop settings; that is very comforting.

Now I'm going to enjoy my new speakers for a while.
 
PS
Albikes, thanks especially for remembering about my problem and picking up the phone and talking to me about it. Without that info or conversation, these Betas likely would have never seen proper operation. I can't thank you enough! This forum is full of guys like you, but you saved a very expensive pair of speakers and brought them to life.
Sincerely. Keith (nikkoholic)
 
Nikko,just out of curiousity "how did the Beta's even work if the servo cables were not even connected to the accelerometer by being connected to the wrong pins on the DIN plug?"
I am not game to try my Gamma's like that,but I was always under the impression that if no contact was made betwwen the accelerometer and the servo controller you would not have any sort of control,saying the least.
When my connection on the back of the Gamma's ( DIN ) was not terminated properly,the drivers floated and carried on like they were with "no control".
 
Nikko,just out of curiousity "how did the Beta's even work if the servo cables were not even connected to the accelerometer by being connected to the wrong pins on the DIN plug?"
I am not game to try my Gamma's like that,but I was always under the impression that if no contact was made betwwen the accelerometer and the servo controller you would not have any sort of control,saying the least.
When my connection on the back of the Gamma's ( DIN ) was not terminated properly,the drivers floated and carried on like they were with "no control".


They worked just like standard woofers without a servo. The controls on the front of the servo still worked, Hi pass, Lo pass, Low Freq gain, (not sure about countor). However the servo portion just didn't work. I'm not techincally inclined to the workings of the servo, but No signal coming back to the servo no correction was being made.

Thus if the control woofer was doing something it wasn't supposed to be doing, then the servo simply was not correcting it; no feedback loop. The most obvious was the tap test, it simply did not work. Now with all hooked up right, with only the servo controller on and the LF amp, if you push ever so slightly on the control woofer the servo sees movement and thus sends a corrective signal to the amp to offset the movement, thus all woofers are essentially resisting the movement created on the tap test.

The result of the servo seeing the signal from the accelerometer resulted in poor LF quality and uncontrolled bass. Which was my complaint with the Beta's (prior to fixing this). The bass was overwhelming and poor quality.

The simple discover (which I think was covered in my original post) came from this test; with just the servo controller on I (reluctantly) pulled out the servo cable from one channel. When doing this the servo unit did not react, it no goes into a a "restart" mode, the green light goes out and in 15 seconds or so it resets. Since the cables were not corrected properly it was as if they were not plugged in; thus the servo would not react when the cables were unplugged.

I hope this answers your question.
 
Yes it does,to a point.
I didnt think that the servo controller could be used as sort of a EQ when the control wires were not terminating causing a break in the feedback loop.
I thought that the results of this would have been more than just over welming bass.
This is weird.
More tests are in need here to discover some more.
 
Back
Top Bottom