Passive Loop, Impedance, and Tone Questions

[wizard333]

This is a question about a specific amp, but its really a question about the subject in general, this amp is just an excellent example of the passive loop "drop your signal 20db before the loop then use a recovery stage to get it back" phenomena I see people complain about with passive loops.........


Looking at the schem for a Marshall 2205:



There is R29, a 10k resistor to ground at the send of the loop.

This resistor is problematic, in that its effect on the tone is to slam a whole lot of signal to ground and make the whole amp sound dull as heck. High end goes bye bye in a big way. How does Marshall deal with this? Take a look at R32/C24, which is a resistor serving as essentially a fixed master volume with bright cap.

This brightens things back up, but it leaves a brittle edge on the overall tone of the amp. Slamming most of your signal to ground and then using a bright cap is not the same as just not slamming your signal to ground in the first place, tonally.........

Now, you can eliminate R29 and C24, which takes off the brittle edge and makes the amp sound much more full (and a WHOLE LOT hotter, too hot given the gain of V4a) but thats not ideal either because it screws up the loop.

My questions given the above are:

a) What affect does that 10k R29 have on the output impedance of the loop?

b) If I want to eliminate C24 and raise R29 so its not bleeding so much signal to ground and dulling things up, whats a reasonable value? 220k? Other than raising the value of the loop send a whole lot, what is that doing to loop output impedance?

c) If you raise the value of R29 way up, you get a hotter signal out of the loop, and you are hitting V4a so hard that any but minimal channel volume settings will clip the V4a and everything after it, so whats the best way to drop your signal back down before it hits V4a without, again, messing up the tone? Drop R34 from 1M grid leak to ..? Something else? Note no grid stopper at V4a as well.

[wizard333]

Anyone?

Also thought about making that loop a parallel loop instead of series.

On those amps I've moved the loop after V4a before to get a hotter signal; the signal from the stock loop with the channel volumes way up is lower than instrument level, not really friendly to either pedals or rack units.

[Jazz P Bass]

Marshall FX loop

Do you really want to put a pedal in the FX loop?
Most times it is good for ater preamp pedals.
Volume , delay.
That circuit is designed pretty goog. If you plug your guitar into the FX in, the 100mv signal from the pickups will drive the amp just about to full power.
The 120pf cap is there to help cut down on sending shrill . The 10K is an input pad.
Yeah, the side effect is it changes the rest of the amp.
But, the other stages take this into account.
I really don't think the engineers at Marshall simply tacked it on.
Keep in mind when the amp was made (1980's) & the type of music that was played at the time.

[wizard333]

Yes I've been playing them since they were made, so I'm aware of their strengths and shortcomings.

The engineers at Marshall did some things I dont like, which is why I've modded these over the years. The clean channel is downright awful, which is why I changed it to a Fender clean channel. Why does Marshall continue to insist on using that Bax stack on the clean channel? People have complained about their clean channels since they started the channel switching thing, you'd think they'd take the hint........

It needs a solo boost, which is why I added one almost 20 years ago, etc.

The engineers at Marshall continue to do things I don't like, and other people dont like, which is why I still mod their amps to this day!

The 120pf is there to compensate for the horrible tonal effects of a 10k resistor to ground, it compensates for dulling up the tone by making the tone brittle.....a compromise at best. A common complaint on these is that, unless you keep the treble control very low, they get thin and brittle really quick as the treble control goes up; part of that is due to the interaction of the RC network I mentioned above with the tone stack.

Anyone more familiar with these passive loops want to chime in? These amps are a bit of a pain in the rear to work on being PCB and having to pull it completely apart every time you make a change, so I'd like to avoid the 'guess and make incremental changes' scenario as much as I can.

[Jazz P Bass]

Marshall JCM 2205

I have one of these on the bench.
Came in cause it was buzzing.
I did notice that the heater wires where laid flat to the chassis.
I was under the impression that it is best to put them up in the air,
Twisted uniformally for sure.
Is this how Marshall assembled them?
Another topic:
In testing this amp, I found a bleed over from the clean channel when on the dirty channel.
Makes for some interesting sounds.
Use it as a one channel & crank up the clean channel & master volume while controlling the dirty channel.
Cool!
Another topic:
The clean channel is not & never will be anything like a Fender.
Fender has there tone control issues.
Marshall most certainly.
I have never understood what "sound" Marshall was after.
I will say they have been consistent.

[wizard333]

Came in cause it was buzzing.
I did notice that the heater wires where laid flat to the chassis.
I was under the impression that it is best to put them up in the air,
Twisted uniformally for sure.
Is this how Marshall assembled them?

Yes. And they do buzz and hum, do a search on this forum for other things I've done to them to limit that.

In testing this amp, I found a bleed over from the clean channel when on the dirty channel.

Also part of the things they did wrong on this amp, and its built in. Bypass the .22 cap from the clean channel to the channel switching chip with a 1uf. That helps.

The clean channel is not & never will be anything like a Fender.

Not unless you do some serious surgery to it, which I have on mine. Its not only "like" a Fender now, it is a Fender clean channel.

But all of that is sort of off topic.

[Merlinb]

I was under the impression that it is best to put them up in the air,

Against the chassis is best because it suppresses the electric field around the heater wires. Hum often arises from improper wiring of the sockets though, creating hum loops. And the twisting must be good right up tot the socket.

[Merlinb]

a) What affect does that 10k R29 have on the output impedance of the loop?

It prevents the output impedance from exceeding 10k. Increasing its value will raise the output impedance.

Dumping signal to ground does not affect treble response; you need some capacitance to do that.

[wizard333]

Aurally, that is whats happening though.

The effect becomes very apparent when that 120pf resistor is removed without changing the 10k resistor. Dull dull dull. Removing both the 120pf and 10k resistor fixes that, but then the loop signal is very high.

Not sure what the output impedance would be with that resistor just gone.......

I assume the high end loss effect is due to similar loading issues as occur when you turn your guitar volume way down without a treble cap across the pickup and output legs of the pot........

In any case, its there and rather a strong effect, whatever the root cause may be.

[Merlinb]

The effect becomes very apparent when that 120pf resistor is removed without changing the 10k resistor. Dull dull dull. Removing both the 120pf and 10k resistor fixes that, but then the loop signal is very high.

Here is a response plot of the circuit from the output of the tone stack to the grid of the recovery stage. As you can see, without the bright cap the response is flat within 1dB across the whole audio range- there is no loss of high end. The cap actually adds a massive treble boost, which is what you obviously prefer to hear. Removing both the cap and the 10k increases the gain enormously, but the actual high-end loss is worse! (although it is still flat below 10k, which is where the guitar exists, so tonally it is essentially the same response as with the 10k and no cap).
I think the simple answer to your problem would be, remove the cap and turn up the treble control!

[wizard333]

What progam did you use to generate that?

from the output of the tone stack to the grid of the recovery stage

The problem there is, you dont listen to the amp from the output of the tone stack to the grid of the recovery stage, you listen to the whole thing.

You're trying to tell me "Don't trust your lying ears, trust this program!". I know what I'm hearing, and there is definitely a major high end loss, not just a gain loss, with the 10k in there.

The cap actually adds a massive treble boost, which is what you obviously prefer to hear.

Obviously not, since its what I'm trying to eliminate. The cap acts like a bright switch to compensate for the dull lows, I've always hated the nature of high end brought in by bright caps, this is no exception. If present and not on a switch, they are the first thing I remove on any amp I own.

Pulling the cap leaves the amp dull as a turd. Pulling the 10k brightens things back up, not nearly as much as having the cap in there, but more like "what you'd expect to hear on an amp without a bright switch engaged" which is exactly what I'm after. It also adds too much gain (talking volume here, not "overdrive".

To be clear: I don't like the cap, it makes things brittle. I dont like the 10k, it makes things dull. Pulling both makes the tone just fine, but you end up with too much gain after V4A, and the impedance of the loop does??

Thats what I'm after, not "well just leave it in" when I dont like it in, not "well my program says its fine, you must be hearing things" which I'm not, but:

What happens to loop impedance with that pulled or changed to a much higher value,

A suggestion on the best way to drop the gain before or from V4a so the signal doesn't hit the power section like a sledge hammer with the channel volumes set to anything above about 2.5.

[Merlinb]

What progam did you use to generate that?

I use Tina 7 Analog, Digital, Symbolic, RF, VHDL, MCU Circuit Simulation & PCB Design

The problem there is, you dont listen to the amp from the output of the tone stack to the grid of the recovery stage, you listen to the whole thing.
...I've always hated the nature of high end brought in by bright caps

Yes, but it's the loop you are complaining about. If the amp is voiced normally before and after the loop, and the loop has a flat response, then it doesn't affect the freq' response of the whole amp. In this case the loop has some built in treble boost. I'm not sure how you can blame brittleness on any and every bright cap though, they don't do anything but increase the treble. The boosted treble might cause more overdrive in following stages of course, but then, how else can you get the same voicing? You still need to boost the treble somehow!

I know what I'm hearing, and there is definitely a major high end loss, not just a gain loss, with the 10k in there.

But is is not really a loss, it is a lack of treble boost that you are getting, when the cap is removed. But because you are accustomed to the brighter sound you are thinking of that as the "normal" response, and the truly flat response as the "lossy" response.

But what you are asking for is a way to eliminate the cap and just drop the gain level, which would still leave you with the same flat frequency response that you don't like. In other words, you are asking the impossible. You can't get rid of the cap and retain the treble boost without, well, adding the cap back again!

Pulling both makes the tone just fine, but you end up with too much gain after V4A, and the impedance of the loop does??

Pulling the 10k raises the output impedance to approximately 120k ohms. The 10k (or any other value you like) appears in parallel with this, lowering the output impedance.

A suggestion on the best way to drop the gain before or from V4a so the signal doesn't hit the power section like a sledge hammer with the channel volumes set to anything above about 2.5.

How about a potential divider right after v4a?

[pdf64]

Looking at the schematic, the master vol VR10 is wired as a variable resistor, with just 100k R22 between the treble wiper and the MV. Therefore the action of the tone controls will be very dependant on the MV setting.
If you are using the MV to compensate the levels with R29 in and out of circuit, that might explain why the tone is changing so much.

[wizard333]

Yes, but it's the loop you are complaining about

No not really. Its the RC network that I'm complaining about, my question is how to get rid of the bad effects of that RC and keep the loop usable.

But is is not really a loss, it is a lack of treble boost that you are getting, when the cap is removed. But because you are accustomed to the brighter sound you are thinking of that as the "normal" response, and the truly flat response as the "lossy" response.

But what you are asking for is a way to eliminate the cap and just drop the gain level, which would still leave you with the same flat frequency response that you don't like. In other words, you are asking the impossible. You can't get rid of the cap and retain the treble boost without, well, adding the cap back again!

Actually I'm talking about changing or elminating the resistor and the cap.



First of all, you seem to still be confused in some way that I like the cap and the "treble boost" it creates. I've tried to make it as clear as I can that i do not, I simply cannot make that point any more clear than I already have. I don't like the cap, I dont like the 'treble boost' it creates.

The sound I like is NOT the sound of that cap. I do not like that cap. The sound I like is with both the cap and the 10k removed. The problem then becomes 1) too much signal level 2) a usable output impedance on that loop.

Second, this is semantics. You're talking about "flat on a scope", I'm talking about relative levels of high end. I don't really care about flat on a scope to be honest, flat on a scope is not any sort of magic bullet for holy grail tone, usually quite the oposite. What I care about is relative levels of high end:

a) There is to much brittle high end with the cap in the circuit, because again, to be clear, I do not like the high end boost of the cap.

b) Removing the cap but leaving the 10k resistor to ground leaves it too dull.

c) Ditching the 10k resistor, and that cap that I dont like, or going to a very high value resistor, makes the tone just fine. It just may make the impedance of the loop (120k seems an awfully high output impedance to be usable with may effects) too high and leaves too much signal going into/coming out of V4a.

I'm not sure how you can blame brittleness on any and every bright cap though, they don't do anything but increase the treble

Well, they pass high end around a resistor that blocks the rest of the frequency range. The implementation I've seen when they are used ends up with a mix of frequencies I find objectionable; its like an icepick in the ear. If I wanted the sound of an icepick in the ear, I'd play through my home stereo and call it a day!

Pulling the 10k raises the output impedance to approximately 120k ohms. The 10k (or any other value you like) appears in parallel with this, lowering the output impedance.

Excellent! Thank You! Thats what I'm looking for. I guess the problem then becomes, given the low input impedance of some effects units, how to strike a compromise there. Too bad I dont have another triode to work with, a cathode follower driving that loop at low impedance would solve a lot of issues. But I don't have a spare so oh well.

And for the record, I don't like the treble boost provided by that cap.

[wizard333]

Looking at the schematic, the master vol VR10 is wired as a variable resistor, with just 100k R22 between the treble wiper and the MV. Therefore the action of the tone controls will be very dependant on the MV setting.
If you are using the MV to compensate the levels with R29 in and out of circuit, that might explain why the tone is changing so much.

Well actually, that schematic is not accurate exactly for this amp. Its been modded significantly, one of those mods is removing the clean channel from the master volume.

The MV is right after the output of the tone stack, the 100k shown in there is a mixing resistor and its now AFTER the master volume, and its a standard 220k mixing resistor.

The Level of the MV for the gain channel doesn't affect much tone wise.

The clean channel is the same as the clean channel on a blackface bandmaster, the output of which comes in before the verb and bypasses the master volume, which only works on the gain channel.

And before some wise guys posts, no the mods do not affect the tone of the gain channel much if at all, that brittle high end thing has always been an issue on these amps.

If you look at the gain staging on the amp, you'll see why I did that. The channel volume and gain controls are really both just gain stage controls, the true channel volume is the MV which really needs to be on the gain channel only since, in stock form, the clean channel with distort badly if you use a lot of gain on the gain channel and drop the MV to compensate, then try to match the clean channel to that volume. That mod keeps the clean channel clean and the gain channel controllable, volume wise.