MAC 4100 DC offset and Bias adjustment.

[Skydogcable]

Does the MAC 4100 have a DC offset trim? My left channel mesures around 20mv and the right 40mv. I have the service manual but I do not see any mention of the adjustment. I'm also curious about any alternative methods for setting bias. I read somewhere about an alternative method for other models but nothing 4100 specific. Thanks in advance for any help.

 

[nabeelsayegh]

Old post...but valid question with no response. Is there an alternative method to set the bias. Not a fan of what the SM wants you to do plus it is not as accurate as taking measurements off of the power transistors/resistors themselves. Anyone have any feedback on this?

 

[c_dk]

Sidney Coderman, VP of engineering was adament about doing it as per the service manual.......since I did not feel "man" enough to carry his slip stick case who was I to argue with him.

 

[clinic-audio]

Hello

There is no offset adjustment in MAC4100
Regarding bias : yes , there is !
Just follow instructions in service manual
I don't see anything complicate

 

[c_dk]

Well it is obvious that McIntosh engineering did not hold as important the "what I read on the Internet about bias and DC offset", at least back in the 70's with their 2nd and 3rd generation of solid state units.

I suppose a debate could be had as to why they took this position with these units.... They did add these adjustments and new measurement procedures with the 4th generation designs such as the Mc2255.

If you spent one minute talking with Sidney it was obvious you were out of your league and I actually did keep a slide rule in my desk drawer when he would put on his professor hat and cause me to feel like I was back in the classroom trying to keep up.

 

[kevzep]

Old post...but valid question with no response. Is there an alternative method to set the bias. Not a fan of what the SM wants you to do plus it is not as accurate as taking measurements off of the power transistors/resistors themselves. Anyone have any feedback on this?

Why, whats wrong with the manual?
I have never had issues following the instructions in the service manuals and following Mc's procedures...

 

[nabeelsayegh]

Thanks for the feedback....but to clarify....nothing is wrong with the manual. I am absolutely sure McIntosh has reasons for doing measurements like this...and I am absolutely certain that they had reasons for changing it is later models. This is my personal opinion only, but taking a measurement of current from the AC feed cannot be as exact as taking a measurement directly from an emitter resistor or something much closer to the amplifier section itself. Plus, you have no way to determine if both channels are set equally. Perhaps it me just nitpicking...but I though that over time, there was a more accurate method that was developed to DC bias these guys. I used the manual method and the system is working fine.

 

[kevzep]

Thanks for the feedback....but to clarify....nothing is wrong with the manual. I am absolutely sure McIntosh has reasons for doing measurements like this...and I am absolutely certain that they had reasons for changing it is later models. This is my personal opinion only, but taking a measurement of current from the AC feed cannot be as exact as taking a measurement directly from an emitter resistor or something much closer to the amplifier section itself. Plus, you have no way to determine if both channels are set equally. Perhaps it me just nitpicking...but I though that over time, there was a more accurate method that was developed to DC bias these guys. I used the manual method and the system is working fine.

The monitoring of the AC line draw is actually a lot more accurate than you might think, and is most certainly accurate enough to set the quiescent current on a class AB amplifier.
Set your bias, get it right, then put the lid on the amplifier and watch it climb 4 or 5 mV, does this change the performance of the amplifier? No, of course not. The working margin for quiescent current is a lot wider than you might think. If it climbed 50mV then you might have a problem, but as I say when that lid goes on and you slide it in the case, the offset and bias wont be the same as what you set it at. But it doesn't matter...it will be within the working margin.

People make way too much out of it than is necessary....

 

[kevzep]

Plus, you have no way to determine if both channels are set equally

C. Repeat step B for each channel independently.

 

[c_dk]

I think digital meters with readouts in the .0001 fuel the OCD in many, forgetting or not knowing of the days when reading a needle of a VTVM and the inherent inaccuracy caused by the parralex issue.

 

[kevzep]

I think digital meters with readouts in the .0001 fuel the OCD in many, forgetting or not knowing of the days when reading a needle of a VTVM and the inherent inaccuracy caused by the parralex issue.

Exactly!!

 

[c_dk]

Understand I am not trying to be flippant but as Kev has suggested the stated procedure has worked well the few times I have had to use it over the years.

Curiosity though compelled me to examine the MAC4200, 4300 and MC752 schematics to see what might have changed as this amp design matured over the decades.

Interestingly a quick look through the 4200 and 752 schematic shows no bias procedure, the 4300 shows a different procedure than the 4100 but without actually comparing it to the original who knows how accurate it would be for the earlier unit.

Next time I have a MAC4100 on the bench I will have to check.

 

[nabeelsayegh]

Well, I certainly understand and respect all comments...perhaps my OCD is being too stubborn. I do not know how many other makes/models use this method as their 'official' procedure, but the whole thing caught me off guard as it was the first system I have worked on that wanted you to do it this way.

 

[kevzep]

There are several amplifiers I have serviced and repaired which are like this, there was an Altec Current dump amplifier which was like that, and pretty much ALL MOSFET amplifiers as they don't even have emitter resistors, hows that for your OCD....!!
There are other more accurate ways at looking at Bias too, and I use these where I can, but I don't have time to go into it now...
All you are trying to do is ever so slightly turn the transistors on so it minimises crossover distortion, you do not need to be within 0.00000000000001 of a mV to achieve this....there is a fairly generous operational window, and doing it wit current draw/power consumption gives you more than enough accuracy to achieve an excellent result.
Then, is it happens, the distortion will never make to the output anyway because all these amplifiers use a lot of negative feedback....
Follow the manual, there is no need to get creative, the engineers that designed these amplifiers knew what they were doing, all the fiddling around and getting within that last mV achieves absolutely nothing.
You do know as soon as you put the lid on and slide it back in the shelf the bias goes up a bit, then next time you turn the amp on its different again, change of the seasons and ambient air temperature change the bias....etc..
Just do what they say in the manual and put it back into service...

 

[nabeelsayegh]

Perhaps I did not clarify...but I DID follow the manual and all appears to be fine with the unit. I am aware of what biasing is supposed to achieve and also that there will always be drift due to physical movement, temperature, humidity and many other contributing factors. I am also well aware of what happens when biasing is done wrong or not set correctly. It is my goal to ensure that when I return a unit back to my Customers, that it is in the best shape I can get it. Please don't ding me for looking for other ways of doing things....that journey itself yields substantial value in and of it's own, even if you end up landing right where you started.

 

[kevzep]

Perhaps I did not clarify...but I DID follow the manual and all appears to be fine with the unit. I am aware of what biasing is supposed to achieve and also that there will always be drift due to physical movement, temperature, humidity and many other contributing factors. I am also well aware of what happens when biasing is done wrong or not set correctly. It is my goal to ensure that when I return a unit back to my Customers, that it is in the best shape I can get it. Please don't ding me for looking for other ways of doing things....that journey itself yields substantial value in and of it's own, even if you end up landing right where you started.

Sorry, didn't mean to sound like I was "dinging" you....We obviously work a bit differently, I do not have the time to go down that road, I have to get the job done and get the next one on the bench, I am not addressing you particularly, but other people who may read the thread and decide to get creative.
I often work and recover gear which has been worked on by "experts" and in fact I am doing one right now, so I might have been reacting to that a little.
My apologies.

 

[nabeelsayegh]

No worries at all. The one thing I love about AK and the people here is that there is so much passionate knowledge and experience....we can all get lost in this stuff!

 

[Coast_SLO]

So, what if it starts at 27watts and goes up slowly no matter which you turn? Can go up a 100mW with a slight turn of either amp and continues goes up as turned further, got to 30.5v at 200 of 360 degree turn on the pot - other amp turned it continues to go up. Its going up consistently, not sure how you decide when to stop in that case
Was at ~29.5watts when I decided I should at least set it before closing it up.
Was only in there to clean up oxidation on the plugs and tighten up the RCA to begin with. Everything was functioning well.

 

[riverRatt]

So, what if it starts at 27watts and goes up slowly no matter which you turn? Can go up a 100mW with a slight turn of either amp and continues goes up as turned further, got to 30.5v at 200 of 360 degree turn on the pot - other amp turned it continues to go up. Its going up consistently, not sure how you decide when to stop in that case
Was at ~29.5watts when I decided I should at least set it before closing it up.
Was only in there to clean up oxidation on the plugs and tighten up the RCA to begin with. Everything was functioning well.

I did this adjustment on my 4100, yesterday. I used the setting on my multimeter for measuring AC current. I sort of know what you're saying because at full counterclockwise on the bias pots the current measured about 0.294 amps and as I rotated each one clockwise the current slowly increased up to about 0.298 amps but there was indeed a point during rotation where the current suddenly jumped up to about 0.312 to 0.315 amps. I set mine to just before that point . Be sure your grounds and all connections are good. I, also, let mine warm up for about an hour before making the adjustment.

 

[Coast_SLO]

Nice! I ended up doing exactly what you suggested. I finally figured okay, if 40watt is safe, then I can at least turn this baby until I see a good change or hit 40w.... got to about 32-33w and it started to ramp slightly faster. I ended up leaving it at about 35 watts with the other side set as well. So pretty close to the same spot as yours!
Thanks!