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May 19, 2022, 08:22:52 AM

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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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Dino Boreanaz

I've got a Marshall 3520 Integrated Bass System 200W head that I really like, but I'm finding that it is too loud for home use.  The volume controls (separate treble and bass volumes) are barely above zero and the slightest movement takes it from silent to way-too-loud.

Can I reduce the output power in half by eliminating two of the four output MOSFETs in order to make the power amp like that of the 100W 3510 model?  I've attached power amp schematics of these two models and encircled the differences between the 3510 and the 3520.

If so, would this be as simple as desoldering one leg of R17 and R22 while leaving everything else in place and undisturbed?

Jazz P Bass

"Can I reduce the output power in half by eliminating two of the four output MOSFETs in order to make the power amp like that of the 100W 3510 model"
Simple answer: NO.
The mosfets are handling the power of the amplifier.
ie: The voltage rails.
Not too sure how to go about it but the key is how hard the preamp is driving the power amp.

phatt

If it has an FX loop just insert a volume pedal in that loop.
That then becomes the master volume for the power amp.

As Mr Jazz Bass said, Don't touch the Amp circuit as it could lead to tears. :-[
Phil.

willpirkle

Agree with Phil - simple attenuator in the FX loop is the least intrusive option.

Dino Boreanaz

Thanks for the replies all!  This is the first amp I've owned with an effects loop ... never even occurred to me to try a volume pedal in the loop.

Dino Boreanaz

I'm thinking about building a simple DIY volume control to put in the effects loop rather than buying a more complex (and expensive) volume pedal.  Is there an ideal potentiometer value if I go with a simple input->pot->output arrangement?  Any drawbacks to such a simple circuit?

Thanks again for talking me down earlier!

Enzo

#6
A pot in a box?  Sure.  After all, that is what a volume pedal is.  The amp has no idea what the control is housed in.  I had a pot/box on my bench for years.  You never know when you might want to turn down a signal.  250k worked well for me. 

You neglected to circle the other difference between the two models..The power transformer.  The lower power unit uses a lower voltage power supply.   And that is where the power comes from in the first place.   You want to convert 200 watts to 100 watts?  Swap out the transformer.   And half power will only be a 3db reduction in volume.  Power isn't volume.

But it occurs to me, this is a solid state amp, you have no plans to overdrive the power amp, so why not just turn down the Master volume.  Or it might be called Post volume.   That is what your FX loop control would be doing.

Dino Boreanaz

#7
Yup, my dumb mistake.  I realized after reading the first few replies that the transformer is in fact different between the 100W and 200W models.

Today I tried the "pot-in-a-box" in the effects loop, but it is quite finicky and also affects the way the boost control responds and hence the tone.  This amp doesn't have a conventional single master volume control.  It has separate treble volume and bass volume with a variable crossover.

I've attached the preamp schematic, and it seems there are three gain stages and their corresponding controls (boost, treble volume, and bass volume) after the effects loop.  Would it be correct to say that the placement of the effects loop within this circuit limits the effectiveness of the "pot-in-a-box" to control the overall output volume and causes it to affect other aspects of the amp's tone?

I also noticed that the treble and bass volume pots (VR10 and VR11) are linear taper rather than audio taper.  Would this tend to make them more sensitive at the lower end of their rotation as compared to audio taper pots?  If so, would it be worthwhile swapping these for audio taper pots?  I suppose this thread has become more about managing the quiet end of the amp's volume range rather than reducing the amp's power output.

Thanks again to everyone for your input.

willpirkle

#8
You could replace VR10/11 with log pots if you want to - I'd probably leave them alone.

To get back to the original issue, The input preamp has a max gain of (47/2.2) + 1 = 22.36 = 27dB

If you want to lower the gain of the preamp which is the first op amp after the input, you have to either lower the 47k feedback resistor R4 or raise the 2.2k shunt resistor R5. To experiment, you could replace the 2.2k resistor with say a 10k tweaker-pot and raise the resistance above 2.2k while playing through it till you get what you want.

If still too much gain, change to a 50k tweaker-pot. When you get the gain where you want it, either leave the tweaker pot in place, or measure it and replace with a fixed value.

With 6.8k, you get ~18dB max gain. Note that raising this resistor value also lowers the minimum gain, when the 22k gain pot is at max resistance. So changing that one resistor will alter the overall gain range as well as the max gain.

Will

Dino Boreanaz

Thanks Will!

I probably should have clarified in my original post, that while I would like to reduce the volume (or have better control over the volume at the quiet end), I DO want the preamp to be pushed into clipping.  I like the tone and overdrive grit that I'm getting with the first gain control at maximum and most of the EQ knobs boosted to about 3 o'clock.  So, while lowering the gain of the first stage would certainly reduce the overall volume, it would also not overdrive as much and that's less desirable to me than the too-sensitive volume controls.

I think I'm going to order the log pots for the volume controls and see how that works out.  I'll post back here if/when I try it.

joecool85

Quote from: Dino Boreanaz on March 17, 2022, 08:18:08 AM
Thanks Will!

I probably should have clarified in my original post, that while I would like to reduce the volume (or have better control over the volume at the quiet end), I DO want the preamp to be pushed into clipping.  I like the tone and overdrive grit that I'm getting with the first gain control at maximum and most of the EQ knobs boosted to about 3 o'clock.  So, while lowering the gain of the first stage would certainly reduce the overall volume, it would also not overdrive as much and that's less desirable to me than the too-sensitive volume controls.

I think I'm going to order the log pots for the volume controls and see how that works out.  I'll post back here if/when I try it.

You could consider using an L-Pad: https://www.parts-express.com/speaker-components/crossover-components/speaker-l-pads

That way you turn that down to half, let's say, and then you can turn up the volume on the amp and make it distort as you wanted without it being ear-bleeding loud.
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

Tassieviking

Instead if a volume pedal you could make a simple patch lead with resistors in it.
Just solder the resistors into one of the plug.
You have to experiment with the resistor values until you like the volume.
Maybe some one here can work out the best resistor value for you.
There are no stupid questions.
There are only stupid mistakes.

Dino Boreanaz

#12
Thanks Tassie, that's a nice solution in a single cable.  If the volume control in the effects loop had worked out, this would have made a very neat and tidy installation.  Unfortunately, since the boost and volume controls are after the effects loop in this circuit, this didn't work out as well as it would have in a circuit where the effects loop is located after all the preamp controls.

Earlier today I remembered that I had modeled this preamp circuit in Micro-Cap so I took a look at it again.  I used the stepping analysis to model the effect of sweeping both volume controls from 0 to 100% in 10% steps using both linear and logarithmic tapers.  I've attached the plots and it seems the linear taper produces a huge jump in the first 10% of the pot's rotation, where the log taper uses over 70% of the pot's rotation to cover the same range.

I don't have much experience using Micro-Cap, so I'd be happy to share the file with anyone who's more familiar with it and could verify whether my analysis is valid or find any mistakes in my use of the AC analysis tools or the model itself.

I've written to an engineer at Marshall to get their input on whether swapping from linear to logarithmic taper pots would make the low end of the volume controls less sensitive to small movements.  I'll post an update if/when I get a response.

Enzo

FX loop is a concept, not necessary to take it literally.  Your FX loop is before some controls, I see.   But I also see the split highs and lows output jacks over on the right.  It appears using the bass alone jack is still full range, until the highs jack is used.  Don't plug into highs, then the bass jack is full range.  Added bonus you still can use the highs and lows controls to balance your tone.    Does not the power amp have an input jack?   Try the volume control there instead of in the official FX loop.    That is after all controls.   

Bass jack out to power amp in jack.

Tassieviking

#14
My fault, I should have looked at the schematic closer.
You want to reduce the signal between the pre-amp to the power amp.
It looks like the place to insert the patch lead is between jS4 on the pre-amp schematic and JS1 on the amplifier schematic.
I think that means the Master Post socket is the pre-amp out. I think Marshall did not install JS1 on the power amp circuit on the back panel.
If you really wanted to you could remove the shielded wire on the power amp PCB and put the resistors there.
Or you could install JS1 on the power amp schematic on the back yourself, but it means drilling a hole.
If you did drill a hole, a switch would be simpler to add / remove the resistors.
A power amp in jack would make it simple.
If you can provide a photo of the bottom of the PCB the rear jacks are on we could check to make sure, just have to look at the tracks.

There are no stupid questions.
There are only stupid mistakes.