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Reducing Power Output on Marshall 3520

Started by Dino Boreanaz, March 16, 2022, 12:36:03 AM

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phatt

My advice would be to cut the input cable to power amp as per TassieV's idea and mount a 50k~100k pot in the back panel and that becomes the power amp Master volume. Looks like enough room for a pot mount next to the Socket PCB in the back panel.
Phil.

Dino Boreanaz

I very much appreciate everyone's input on this as I'm quite new to the world of modifications.  I have a couple of questions for those with more experience that myself (which is just about everyone!):

As my end goal is to make the volume controls less sensitive at very low settings, is there something preferable about adding resistance (either fixed or a pot) before the power amp versus changing the volume pots from linear taper to log taper?

I have found volume settings that I am quite happy with, but I would really like to have finer control in this area and it seems like log taper pots would accomplish this without adding components or making permanent modifications.

If I may ask another question to further my understanding of this circuit:  It seems like the two volume pots (VR10 and VR11) are not located between their op-amp's input pins and output pins, and so should not affect the gain of these op-amp stages.  Is it correct to say that these two volume pots control the signal passed to the power amp as voltage dividers rather than by altering the gain (and therefore clipping behaviour) of the op-amps?

Thank you again for all your input and insight.

phatt

Quote from: Dino Boreanaz on March 20, 2022, 11:27:49 PM
I very much appreciate everyone's input on this as I'm quite new to the world of modifications.  I have a couple of questions for those with more experience that myself (which is just about everyone!):

As my end goal is to make the volume controls less sensitive at very low settings, is there something preferable about adding resistance (either fixed or a pot) before the power amp versus changing the volume pots from linear taper to log taper?

I have found volume settings that I am quite happy with, but I would really like to have finer control in this area and it seems like log taper pots would accomplish this without adding components or making permanent modifications.

If I may ask another question to further my understanding of this circuit:  It seems like the two volume pots (VR10 and VR11) are not located between their op-amp's input pins and output pins, and so should not affect the gain of these op-amp stages.  Is it correct to say that these two volume pots control the signal passed to the power amp as voltage dividers rather than by altering the gain (and therefore clipping behaviour) of the op-amps?

Thank you again for all your input and insight.
Yes A log pot may well alter how early the volume jumps but I think a larger value pot (Log or Lin) would give a more progressive rise in volume. 5k would make it jump up in volume with only little rotation, as you have noted.

I have a hunch 50k pots might resolve the issue far better,   then log or lin would not matter much.
That may resolve this better than working on adding an extra master pot.

Regards to VR10 & VR11.
Yes they are just voltage dividers and do not effect the gain.
Phil.

Dino Boreanaz

Hey Phil,

Thank you so much!  My understanding is improving slowly, but steadily.  If you'll indulge a few more, somewhat rambling, lengthy questions ... if not, I totally understand and thank you again for everything you've shared.

I tried modelling a few different potentiometer values (4.7k, 47k, and 470k) to gain additional understanding and found that the higher value pots produce the same range of sweep when the upper and lower limits are treated as a percentage of pot value.  For example, the total gain range is the same for the 4.7k pot swept from 5 ohms to 4.7k ohms as compared to the 470k pot swept from 500 ohms to 470k ohms.  This is as I would have expected since the voltage divider should operate the same as long as the proportions remain the same regardless of the absolute values.  Am i correct here?

One thing I did not expect is that the higher pot values seemed to have a slightly stronger low-bass response (from about 40 Hz to 200 Hz) as compared to the lower pot values.  See the attached "Overlay 1" where the blue traces are 4.7k pots, the green are 47k pots, and the red are 470k pots.  The plots are essentially identical above 400 Hz, but show that the higher value pots provide more bass extension at corresponding settings.  Is it correct to assume that this is due to the higher resistance to ground of the high-value pots is allowing more low frequency content to pass through rather than bleed off to ground?

Lastly, I tried modelling the same three pot values swept from an initial resistance value of 5 ohms up to their respective maximums (5 ohms to 4.7k, 5 ohms to 47k, and 5 ohms to 470k).  In these plots I did see that the higher pot values indeed provide greater range at the quiet end of their sweep, as shown in the "Overlay 2" attachment.  This again is as I would have expected, but my question is which sweep is more representative of what a real potentiometer would produce?  In other words is the lowest attainable resistance of a potentiometer some particular value regardless of the pot's maximum value or is it a percentage of the pot's maximum value?  Or (as I fear) does it vary from pot to pot and manufacturer to manufacturer meaning that this has all been an exercise with no practical value?!

Thank you again!

phatt


A larger value pot gives you more room to move over the lowest 10% of the rotation,, especially if using an Audio log taper pot.

The plots of log and linear responses are resistance vs, pot position.
The issue is that there are many different log Curves,, even reverse log. :loco

Ideally the closer you get to 10~15% of total resistance at half rotation point the better it will be for what you want in your application.

Google; *Log pot curves*,, That may help you get your head around it.
And also learn about how we all hear sound;
https://ozvalveamps.org/pots.htm

It may well be cheaper and less hassle to just mount a power amp Master level pot on the back panel. 50k pot should work fine.
Phil.

Tassieviking

Looking at the circuit again, I would definitely go for the Log pot first, that would give you more control over the volume at lower level.
Another option is to put a resistor in series with the pot, but then you cant turn it up to full any more.
I presume it is a 24mm Alpha Taiwan potentiometer you have in there (RV24AF-22 model) since it is a Marshall.

The very last option if it were my amp, I would get a 16mm pot (log A4k7) with a push pull switch on it, and wire that in.
Wire it so if you pull the switch out you put a resistor in series with the pot, when pushed in it is like it is now.
Being 16mm you would have to run short wires to the PCB as the pot won't reach the PCB.
If the pot is too close just turn it sideways so the terminals point sideways.
I don't know if the capacitor C31 will be in the way for this.
There are no stupid questions.
There are only stupid mistakes.

Dino Boreanaz

Thanks Tassie ... very much appreciate your input!

I have decided to go the route of trying the log pots for the treble and bass volume controls.  You are correct, the pots are the 24mm Alphas.  I've ordered the three different values that were available in log taper (4.7k stock value, along with 22k, and 100k).  Based on the results of the circuit modelling, it seems there's nothing to loose by trying the 100k pots first.

I love the idea of the push-pull, but I'm going to keep this simple.  I'll be very happy if I can just get some finer control at low volume levels.

Dino Boreanaz

Well it's been quite a while since I first posted this topic and I just wanted to provide an update for the sake of completeness.

I did try a few different values of audio taper pots to replace the stock linear pots and I found that they made the volume controls slightly less sensitive at the low end of their rotation as compared to the stock linear pots.  It was not as dramatic a change as I had hoped it would be, so I also tried using a reverse audio taper pot and this did not work well at all (being more sensitive at the low end of the rotation ... as it should).

Since the different values did not have any noticeable affect on the behaviour of the volume controls, I stuck with the stock value of 4.7k but used the audio taper pots in both the treble volume and bass volume controls.

I should also note that I am running this amp into a 16 ohm load rather than the rated 4 ohm load.  So, according to Marshall's senior service engineer, the amp should be putting out approximately 75 watts.  Even at this power level, I find that I have both volume controls set at (or slightly below) 1 out of 10 on the dial which still makes it quite sensitive to small changes.  But I have found a balance that I'm happy with and have no plans to make further changes at this point.  I will post a new topic if I change my mind and decide to start tweaking things again.

Thanks again to everyone who contributed their time, knowledge, and ideas.

Tassieviking

I can think of one more thing you can try, increase the taper of the pot.
place a resistor between pin 1 and pin 2 of the pot and the audio taper curve will increase.
The taper effect will be more pronounced at low volumes, you can buy audio pots with different taper curves, but the easiest way is just add the resistor.
You can read more here :http://www.geofex.com/article_folders/potsecrets/potscret.htm
It would give you even more control at low volumes.
There are no stupid questions.
There are only stupid mistakes.

Dino Boreanaz

Great now I've got more work to do ... just when I thought I was done tinkering!  This actually seems like a fantastic option to make the taper more gradual at the low end.  I will have to try this and I'll reply with my results when I do.