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Old 11th December 2007, 05:31 AM   #1
diyAudio Member
 
Join Date: Nov 2006
Smile Reverb Project (Ideas Welcome)

Dear friends,
I just finished a new amp using the LM3886.

Now I want to add a reverb to it justa after my preamp,
and want to make it in the simplest way possible, so a
wondering if using the reverb section of marsh@ll 8020
can bring me some good results.

I have doubts about the red section of schematic, since
the M5201 Ci is a opamp with 2 inputs and only 1 output
I don't know if it's important to drive the reverb or just
is used for the channel selection.

I made a pcb from the green section in a hurry, so I know
that it haves a lot of errors, image is just to give an idea
about the project.

Please take a look, any information will help me a lot,
any idea is welcome. and...If someone haves this amp,
whats the reverb tank used (code)?

Project Image:
http://img6.pictiger.com/c6b/13381072.gif

Thanks a lot, sorry about bad english language.

Blindsjc
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Old 11th December 2007, 09:21 PM   #2
teemuk is offline teemuk  Finland
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Location: Suomi, Finland
I’m lost: Your link doesn’t work and the section depicting the reverb circuit in Marshall 8020 schematic (http://www.drtube.com/schematics/marshall/8020.pdf) has nothing to do with channel switching.

It is true that the signal path to the reverb circuit is split from the output of a switching input opamp (IC3). However, this type of IC has a shared output pin and even if it wouldn’t have the outputs of the related gain stages would be connected together anyways. More than that, you really don’t care what happens “to the left” from C28, as long as this point of the circuit is fed with a signal that has proper amplitude and as long as the source resistance of the stage feeding this point is moderately low.

The last clause is filled when you precede the reverb stage with any ordinary opamp. I believe that even a discrete stage is enough (after all, the load impedance is about 200K). The first clause (proper amplitude) depends on the reverb tank’s properties so I will not comment on that. (I have no idea what tank this specific amp uses anyway).

Basically, the signal amplitude should be high enough so that the gain of IC4b reverb driver is sufficient to drive the tank properly. If you feed this circuit with a signal that has a too high amplitude you overdrive the reverb driver (not good). If the amplitude is too low the reverb tank won't be driven properly and it will not sound as "full". Anyway, when you figure out the tank's specs you get some idea about proper signal level.

Hopefully you have realized that reverb circuits are designed for specific reverb tanks. For example, in this case the gain of the reverb driver stage is specifically defined by the tank’s impedance divided with the value of R36 + 1. (It’s a basic non-inverting opamp stage). So, unless you can acquire the exact tank you may need to tune this stage to make it work correctly. So, Yes: You can get good results using this circuit but a lot of that depends on picking a proper reverb tank and tweaking the driver stage to work properly with it. The schematic doesn’t seem to indicate any model number (it could be some cheap Belton tank that is impossible to acquire anyway) but since just a single opamp is used as the driver it is clear that tanks with very low input impedance are out of the question. The lowest load that typical opamps can drive is about 600 ohms; perhaps that helps a bit.
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Old 11th December 2007, 09:58 PM   #3
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Join Date: Nov 2006
The link is working here, don't know why i doesn't
work for you but... Thanks a lot for your attention friend,
it helps a lot.

After think a lot about it and some google I realized that
a Craig Anderton Stage Center Reverb could be much more
secure to build, It haves all what I need. I will use your post
info to make some experiments with this project. Thanks a lot
again.

And, I want to say thank you one more time for your info
about the variable impedance output for the LM3886 amp
at ssguitar.com. This is my second amp project, and I can
say that is really the best sounding solid state that I ever
heard. My girl (girl ears are always sharp) loved it too.

Thanks Friend
Blindsjc
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Old 13th December 2007, 03:07 AM   #4
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Join Date: Nov 2006
Hi friends,
Just finished my Stage center reverb and tested it.
A Marshall Valvestate V80 is here to repair, and the
reverb tank is exactly what I needed to SCR.

It sounds fine, you can go from nothing to a hard
reverb effect. Some amps I used lately haves too much
subtle reverb effects.

Some posts about this effect at diystompboxes.com
talk about some sound darkness with the TL074 opamp
and it's true, sometimes the sound appears really dead
and without details. Tried a TL082 and the result was
even worst, too much noise. Finally with LM837 it comes
to life, perfect. I hope to test other quad opamps in near
future. Great effect to build.

I plan to use it at my diy amp, so the dwell control
is a dead control walking (dear master Teemuk, time
for new perfect words). Dwell controle acts like a
weird volume control, just found a sweet place and
leave it there. I will be searching for info about the
best way to remove this control and use the circuit
at the amp with little changes about the preamp tone
excluding the reverb effect.

Thanks a lot
Blindsjc
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Old 13th December 2007, 01:36 PM   #5
teemuk is offline teemuk  Finland
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Join Date: Nov 2004
Location: Suomi, Finland
Quote:
Originally posted by blindsjc
Dwell controle acts like a weird volume control
Well, that is because the “dwell” control is basically just a volume control. If you examine the circuit you will notice that R2 and R1 connect the “virtual ground” inverting input of the IC1c. In conjunction with C6 and series resistance from the “swell” control potentiometer a high pass filter is formed. Now, when you increase the volume the more bass frequencies get cut.

At the furthest extreme the wiper of the dwell control is grounded and you get no output signal. When moving slightly towards the other extreme of the dial you have nearly 50 kilo-ohms in series with R2 and R1, thus the RC filter will have a very low –3dB point and not much low frequency content will be cut away. However, at this setting the potentiometer also attenuates quite a lot. Now, when the wiper is completely in the other extreme the resistance to ground is the 50 kilo-ohms in parallel with the series resistances of R2 and R1 (remember the virtual ground at the inverting input). At this point the 50 kilo-ohms doesn’t weight much and the high pass filter cuts off a lot of low frequency content. (IMO, this doesn’t seem like a very smart control). Naturally, you can modify the effect of the RC filter by tweaking the value of C6. For example, make it a decade higher (2.2uF) and the “dwell” control turns into an ordinary reverb level control with nearly flat frequency response.

By the way, I used this schematic as a reference:
http://www.generalguitargadgets.com/..._reverb_sc.gif

If you have a “sweet spot” (with just the right amount of attenuation and LF cut) and you wish to replicate it then the best practice is likely just measuring the two resistances that the potentiometer has from its wiper lug to the two other lugs. Then replace the potentiometer with two suitable resistors. If you do not wish to make any compromises between attenuation and low cut you can replace the potentiometer with a suitable resistor divider attenuator and then tweak C6 to get just the right amount of low cut.

There’s also a low pass filter formed by the “click suppressor” capacitor C2 and all resistors involved. The –3dB point of this filter is about 7 kHz so I don’t know how much it will really effect the reverb signal. It could have some effect when you reverberate some distorted tones. Anyway, if the signal seems too dark just decrease the value of C2 and see if this helps.

Which, brings us to the dark sound you mention the circuit has with some opamps: Now, a spring reverb transducer is essentially just an inductor and an inductor exhibits higher resistance at higher frequencies. Consequently, when you drive an inductive load with an ordinary voltage amplifier (such as the one in this particular circuit) the amplifier will feed less current to the load at higher frequencies. However, the driving force of a transducer is current, not voltage, thus you loose some of the “brightness”. When you compare this circuit to the reverb circuit of the Marshall 8020 you’ll see the difference. If I remember right, Rod Elliott has an article (or was it a project) about spring reverb circuits at his site in which he explains this stuff in higher detail. Aside different noise levels I don’t know what explains the differences between various opamps, though.


Also, you have likely figured out that from the point “X” you can run two (shielded) wires to a remote foot pedal where you house a “toggle” switch. Pretty much every schematic depicting some older guitar amp that had a spring reverb shows a perfect example of this arrangement.
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Old 13th December 2007, 08:35 PM   #6
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Join Date: Nov 2006
Hi Teemuk,
I just finished some tests, I studied with attentions
your advices and used at the circuit. With a smaller
C2 the sound really improved, used a 1nF and got a
little more noise but good highs, I will change it some
more times to get the sound I need.

Changes at dwell control (2.2uF cap) brings me the
full tone that comes from my preamp, but it starts to
overload the reverb circuit at low volume, lower that
my preamp alone.

Finally what I understand is:
The circuit appears to be designed for a much lower
input signal than my preamp (ROG Tonemender) with
a 25x gain set with a 120K resistor in place of gain pot.

Setting the filters to get full details comming from the
preamp starts to overload the reverb circuit too soon,
when the preamp volume is set to a under reverb overload
the final output is much lower than my preamp alone.

With a too low volume (under overload) volume set at
the reverb circuit I have just a soft reverb effect, the
volume control affects the reverb driving.

For my too limited knowledge, a solution can be some
of these:

1) Keep the smaller C2.

2) Change the recover section to
get some more output volume.

or test the Elliott project34 like you said and see if it
works better for what I need. I will keep testing some
new values today.

Thanks a lot
Blindsjc
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Old 14th December 2007, 04:13 AM   #7
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Lot of work this week,
Just finished Elliott's reverb driver and
it's very good. The effect is a lot more subtle
than stage center reverb but since this new
driver is not made for exactly my reverb pan
some tweaks are needed.

Used 2 batteries to supply, one for the LM386
and 5532 and one for the negative rail of 5532.
Like noted at the Elliott's website this is something
below the really necessary, but it's working.

The NE5532 is one of my favorite opamps and
I use it at my Tonemender preamp, very detailed
and bright, a lot better than TL074 used at stage
center reverb. This new project follow these two
characteristics. Great.

I will made some tests with a higher gain setting
at Lm386, let's see how much I can amplify this
gain to get more effect before reverb saturation.
I don't know if it's the best way to do it, but it's
a lot of fun to work with these projects.

Thanks
Blindsjc
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Old 2nd July 2009, 09:22 AM   #8
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Join Date: Jun 2009
Default reverb project

want to join this thread with a question. Wanting to take a brass 1 1/8 inch tube (O.D.) 2 inches long , a decent (neodymium baseed) headphone speaker (1 1/8) , a 1/4 inch (O.D.) microphone condensor element, 4 lightweight coil springs (from cheap ink pens), and one 50k linear pot to add reverb to a Marshall MS2 battery powered 2 watt amp. I know the speaker (driver) must be hot and also the Mic (recieving transducer) and that they must be tied back into original signal to produce reverb effect. But how should I wire the pot (please speak in the redneck, I dont know enough about technical crap, but I understand the general idea language), so that I can control the amount of reverb. Correct me if I am wrong but his is my idea of how it should go: main signal out (to speaker) spliced into two leads( 1 main 1 sub). the main being wired directly into speaker (front end of reverb tank), the sub wired directly into driving speaker(headphone). As I see it there must also be hot and ground leading from speaker to mic(on the outside of tank), with siganl from mic being fed into potentiometer to control amount of reverb. From the pot the signal is then fed back into the main to mix with "pure" signal to power the amp's speaker. What I am lost on is the pot wiring (how to ,where to) etc...
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Old 3rd July 2009, 02:53 AM   #9
Trout is offline Trout  United States
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Location: Midwest Madman
Ideas Welcome?

Ok here goes,

How about a really simple/low cost approach.

The Belton Digi-Log reverb module.
PDF Data Sheet
These modules sell for less than the average reverb tank at around $12-$18.00 depending on source.

This module comes in 3 different delays, short, medium, and long.
There is an application circuit in the data sheet though I would like to see it adapted to a tube amp rather than a effects pedal.

And speaking pedals,

There is a boutique pedal builder already using these modules in a pedal called the Lee Jackson "Mr Springy" pedal.
These pedals sell for around $199.00us

Here is a crude demo of the pedal on youtube.
http://www.youtube.com/watch?v=TK6PTtzKnbA
or
http://www.youtube.com/watch?v=DFxwukLdED8
or
http://www.youtube.com/watch?v=MqujWmPbSe0

Seems like a great project.
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Old 3rd July 2009, 01:54 PM   #10
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Join Date: Nov 2006
Hi friends,
The post was from 2007, so a lot of things
happened since...rs. I used a digi-log verb
too, it works fine, really great device.
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