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jtougas[_3_] jtougas[_3_] is offline
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Default Help modding a circuit: MultiVox MXD-5

Hi folks -

I picked one of these up a few years ago, and while I've always liked
the delay portion of it, the reverb always sounded a little weak. So,
acting on impulse, I pulled out the little 2 or 3 inch long reverb
tank, and riveted in a 9 inch Accutronics from a Carvin amp that had
recently gone up in smoke (not my fault, I never touched the insides
of it! - shame, too, great sounding amp).

So, now the reverb tails off much more nicely, and sounds better, only
it's much more dim than it was before.

So, what do I need to do to the circuit now? Replace some caps? Change
out a potentiometer? Both? Neither?

Look somewhere else? *grin*

Any leads would be appreciated.

Thanks -
--
jtougas

"listen- there's a hell of a good universe next door
let's go" - e.e. cummings
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Scott Dorsey Scott Dorsey is offline
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jtougas wrote:
Hi folks -

I picked one of these up a few years ago, and while I've always liked
the delay portion of it, the reverb always sounded a little weak. So,
acting on impulse, I pulled out the little 2 or 3 inch long reverb
tank, and riveted in a 9 inch Accutronics from a Carvin amp that had
recently gone up in smoke (not my fault, I never touched the insides
of it! - shame, too, great sounding amp).

So, now the reverb tails off much more nicely, and sounds better, only
it's much more dim than it was before.

So, what do I need to do to the circuit now? Replace some caps? Change
out a potentiometer? Both? Neither?


Odds are that the reverb tank you installed has different impedances on
the coils than the one you took out.

Find out what the impedance of the coils are on the original, then call
up Accutronics and order a new one with the same values. Brand new
Accutronics units built to spec are only twenty bucks or so.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."
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Default Help modding a circuit: MultiVox MXD-5

On Monday, January 21, 2008 at 11:49:27 AM UTC-5, jtougas wrote:
Hi folks -

I picked one of these up a few years ago, and while I've always liked
the delay portion of it, the reverb always sounded a little weak. So,
acting on impulse, I pulled out the little 2 or 3 inch long reverb
tank, and riveted in a 9 inch Accutronics from a Carvin amp that had
recently gone up in smoke (not my fault, I never touched the insides
of it! - shame, too, great sounding amp).

So, now the reverb tails off much more nicely, and sounds better, only
it's much more dim than it was before.

So, what do I need to do to the circuit now? Replace some caps? Change
out a potentiometer? Both? Neither?

Look somewhere else? *grin*

Any leads would be appreciated.

Thanks -
--
jtougas

"listen- there's a hell of a good universe next door
let's go" - e.e. cummings


i realize this post is 7 years old but you said that you changed out the spring and i was wondering if you could tell me where the connections were on the PCB? Mine had the spring unit ripped right from the board with very little of the wires remaining. i took a guess and rewired it but it doesn't work so i guess i did it wrong.
thank you
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Scott Dorsey Scott Dorsey is offline
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wrote:

i realize this post is 7 years old but you said that you changed out the sp=
ring and i was wondering if you could tell me where the connections were on=
the PCB? Mine had the spring unit ripped right from the board with very li=
ttle of the wires remaining. i took a guess and rewired it but it doesn't w=
ork so i guess i did it wrong.
thank you


The spring unit only has four wires... two going in, two coming out. If you
put a 2" speaker across the two going on, you should hear the speaker making
some sound. If you put it across the two coming out, when you tap the speaker
you should have sound in the output. So you can use this as a diagnostic to
make sure the electronics are good.

You can also check the resistance across the spring unit with an ohmmeter...
if the values are very high, odds are the spring unit was damaged in the
tearing-out process and should be replaced. Antique Electronics Supply
will carry replacement Accutronics spring delays but you will have to make
sure the impedances are correct since there are a lot of different options.
--scott


--
"C'est un Nagra. C'est suisse, et tres, tres precis."


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On Thursday, July 9, 2015 at 8:47:58 AM UTC-4, Scott Dorsey wrote:
wrote:

i realize this post is 7 years old but you said that you changed out the sp=
ring and i was wondering if you could tell me where the connections were on=
the PCB? Mine had the spring unit ripped right from the board with very li=
ttle of the wires remaining. i took a guess and rewired it but it doesn't w=
ork so i guess i did it wrong.
thank you


The spring unit only has four wires... two going in, two coming out. If you
put a 2" speaker across the two going on, you should hear the speaker making
some sound. If you put it across the two coming out, when you tap the speaker
you should have sound in the output. So you can use this as a diagnostic to
make sure the electronics are good.

You can also check the resistance across the spring unit with an ohmmeter....
if the values are very high, odds are the spring unit was damaged in the
tearing-out process and should be replaced. Antique Electronics Supply
will carry replacement Accutronics spring delays but you will have to make
sure the impedances are correct since there are a lot of different options.
--scott


--
"C'est un Nagra. C'est suisse, et tres, tres precis."


OT:

Not sure why, but I was given (decades ago) a (used) reverb unit for car use. I could see it had a spring coupling two magnetic transducers, one would induce a magnetic field (audio signal) to vibrate the spring and a similar one would change the vibrations back to an audio signal. Never used it, just looked simple.

Jack
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jtougas[_2_] jtougas[_2_] is offline
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Default Help modding a circuit: MultiVox MXD-5

Scott -

Is there any way to determine from the rest of the circuit what values the reverb tank should be?

I seem to have lost my original tank...

- John
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Mike Rivers[_2_] Mike Rivers[_2_] is offline
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On 5/9/2017 4:32 PM, jtougas wrote:

Is there any way to determine from the rest of the circuit what values the reverb tank should be?
I seem to have lost my original tank...


"Lost" as in "I know it's around here somewhere" or as in "The original
one doesn't work any more?" If it's the latter, are you sure the spring
unit is what failed and not one of the bucket brigade chips or other
electronics driving it?

I read something on Gearslutz about that unit and it mentioned a 4 inch
spring unit. I've never heard of one that small, only the big and the
little ones from Accutronics (Hammond), though it might have been a
Japanese-made part.

If you can fit a 9" Accutronics in the box, you might just buy one and
try it. They're only about $20, and if it doesn't work, you could make
something else out of it. Craig Anderton used it in his Electronics
Projects for Musicians book from 1975. I built one of those and it was
actually pretty good. You could get an Ampex AG-440 for about $500 and
use it as a pre-delay.

https://www.amplifiedparts.com/products/reverb_tanks






--

For a good time, call http://mikeriversaudio.wordpress.com
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On 5/9/2017 4:32 PM, jtougas wrote:

I seem to have lost my original tank...


Or, if you can find 5 volts in the box somewhere, you could certainly
fit one of these in the

https://www.amplifiedparts.com/produ...files_and_docs

It certainly can't sound any worse than a 4" spring, though that may be
part of the charm of this unit.


--

For a good time, call http://mikeriversaudio.wordpress.com
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There seem to be two common configurations, one with an input impedance of around 10 ohms that can be driven by the speaker output of an amplifier, and the other with an input impedance of around 2.5 k Ohms, which can be driven by practically anything. They all have a pretty low output impedance so they can drive anything.

I suppose that they differ all over the place regarding sensitivity (volts out for volts in), so if you can't find the original part, you'll just have to cut-and-try. At the risk of it sounding too good, you can probably get one of those digital "springless" replacements that I pointed to in another post to work without too much trouble. There are a few different models that, as far as I can tell, only differ in how they're mounted (PC flat or vertical or screw mounted).


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In article ,
jtougas wrote:
Scott -

Is there any way to determine from the rest of the circuit what values the reverb tank should be?


Probably.
But if it's an Accutronics tank, you have something like five different
choices for input impedance, five different choices for output impedance,
a bunch of choices for reverb time and there are I think four different
models with different numbers of springs.

But, if it's an Accutronics tank, you can just call up Accutronics and they
will know which model you need to order.

If it's a weird Japanese tank, then putting an Accutronics tank in it won't
match the original, but you could likely find something with the right
impedances and with whatever reverb time you happen to like the sound of.

I seem to have lost my original tank...


How did that happen?
And how did this 20-year-old thread get resurrected?
--scott
--
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Phil Allison[_4_] Phil Allison[_4_] is offline
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Mike Rivers wrote:

------------------


There seem to be two common configurations, one with an input impedance
of around 10 ohms that can be driven by the speaker output of an amplifier,
and the other with an input impedance of around 2.5 k Ohms, which can be
driven by practically anything. They all have a pretty low output impedance
so they can drive anything.


** Not quite true.

The input and output coils of reverb tanks are inductors, so the impedance rises with frequency right across the audio band. The numbers quoted in the published data are for 1kHz only PLUS the normal input power level is less than 10 milliwatts.

The 8ohm input types cannot be driven direct from an audio amp, it would massively over power them. Output impedance values vary from a few hundred ohms to a few thousand at 1kHz.


..... Phil

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Phil Allison wrote:
Mike Rivers wrote:

There seem to be two common configurations, one with an input impedance
of around 10 ohms that can be driven by the speaker output of an amplifier,
and the other with an input impedance of around 2.5 k Ohms, which can be
driven by practically anything. They all have a pretty low output impedance
so they can drive anything.


** Not quite true.

The input and output coils of reverb tanks are inductors, so the impedance rises with frequency right across the audio band. The numbers quoted in the published data are for 1kHz only PLUS the normal input power level is less than 10 milliwatts.

The 8ohm input types cannot be driven direct from an audio amp, it would massively over power them. Output impedance values vary from a few hundred ohms to a few thousand at 1kHz.


By directly, he means that they use the same power amp that drives the speakers
in order to drive the reverb tank. There _is_ a resistive attenuator in there.

It's all part of an attempt to keep the total parts count as low as
possible for cost containment. It's a problem since of course the
level into the tank changes as you adjust the total amp gain.
--scott
--
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Scott Dorsey wrote:

-------------------



There seem to be two common configurations, one with an input impedance
of around 10 ohms that can be driven by the speaker output of an amplifier,
and the other with an input impedance of around 2.5 k Ohms, which can be
driven by practically anything. They all have a pretty low output impedance
so they can drive anything.


** Not quite true.

The input and output coils of reverb tanks are inductors, so the impedance rises with frequency right across the audio band. The numbers quoted in the published data are for 1kHz only PLUS the normal input power level is less than 10 milliwatts.

The 8ohm input types cannot be driven direct from an audio amp, it would massively over power them. Output impedance values vary from a few hundred ohms to a few thousand at 1kHz.



By directly, he means that they use the same power amp that drives
the speakers in order to drive the reverb tank. There _is_ a resistive
attenuator in there.



** The word "directly" has a very clear meaning.

It means without an intermediate.



It's all part of an attempt to keep the total parts count as low as
possible for cost containment.


** Nonsense.

No guitar amp made uses its own output to drive the reverb tank.

There is always a independent, low power drive amp.




..... Phil
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Les Cargill[_4_] Les Cargill[_4_] is offline
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Scott Dorsey wrote:
Phil Allison wrote:
Mike Rivers wrote:

There seem to be two common configurations, one with an input
impedance of around 10 ohms that can be driven by the speaker
output of an amplifier, and the other with an input impedance of
around 2.5 k Ohms, which can be driven by practically anything.
They all have a pretty low output impedance so they can drive
anything.


** Not quite true.

The input and output coils of reverb tanks are inductors, so the
impedance rises with frequency right across the audio band. The
numbers quoted in the published data are for 1kHz only PLUS the
normal input power level is less than 10 milliwatts.

The 8ohm input types cannot be driven direct from an audio amp, it
would massively over power them. Output impedance values vary from
a few hundred ohms to a few thousand at 1kHz.


By directly, he means that they use the same power amp that drives
the speakers in order to drive the reverb tank. There _is_ a
resistive attenuator in there.


Most[1] guitar amps have a drop somewhere about the input of the power
amp/PI with a separate driver. Having the power amp itself drive it
would be a feedback loop.

Having said that... that might be something completely awesome to play
with

[1] many have a digital reverb now, driven as if in an F/X loop.

The Fender AB763 schem has "1/2 7205" where the reverb return is - the
12AX7 to the left of that is the drive. They're all PDFs; a pile of 'em
he
https://schematicheaven.net/fender.html

It's all part of an attempt to keep the total parts count as low as
possible for cost containment. It's a problem since of course the
level into the tank changes as you adjust the total amp gain.
--scott


--
Les Cargill


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Les Cargill wrote:

--------------------


There seem to be two common configurations, one with an input
impedance of around 10 ohms that can be driven by the speaker
output of an amplifier, and the other with an input impedance of
around 2.5 k Ohms, which can be driven by practically anything.
They all have a pretty low output impedance so they can drive
anything.


** Not quite true.

The input and output coils of reverb tanks are inductors, so the
impedance rises with frequency right across the audio band. The
numbers quoted in the published data are for 1kHz only PLUS the
normal input power level is less than 10 milliwatts.

The 8ohm input types cannot be driven direct from an audio amp, it
would massively over power them. Output impedance values vary from
a few hundred ohms to a few thousand at 1kHz.


By directly, he means that they use the same power amp that drives
the speakers in order to drive the reverb tank. There _is_ a
resistive attenuator in there.


Most[1] guitar amps have a drop somewhere about the input of the power
amp/PI with a separate driver. Having the power amp itself drive it
would be a feedback loop.


** Exactly, reverb units need an independent drive amp - which is often just an op-amp.

I know of onlyy one guitar amp where speaker signal was tapped off and fed to a reverb tank. However that amp had TWO power channels and the output of the reverb connected to the input of the spare channel.


..... Phil



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On Wed, 10 May 2017 20:54:32 -0700 (PDT), Phil Allison
wrote:

Les Cargill wrote:

--------------------


There seem to be two common configurations, one with an input
impedance of around 10 ohms that can be driven by the speaker
output of an amplifier, and the other with an input impedance of
around 2.5 k Ohms, which can be driven by practically anything.
They all have a pretty low output impedance so they can drive
anything.


** Not quite true.

The input and output coils of reverb tanks are inductors, so the
impedance rises with frequency right across the audio band. The
numbers quoted in the published data are for 1kHz only PLUS the
normal input power level is less than 10 milliwatts.

The 8ohm input types cannot be driven direct from an audio amp, it
would massively over power them. Output impedance values vary from
a few hundred ohms to a few thousand at 1kHz.

By directly, he means that they use the same power amp that drives
the speakers in order to drive the reverb tank. There _is_ a
resistive attenuator in there.


Most[1] guitar amps have a drop somewhere about the input of the power
amp/PI with a separate driver. Having the power amp itself drive it
would be a feedback loop.


** Exactly, reverb units need an independent drive amp - which is often just an op-amp.

I know of onlyy one guitar amp where speaker signal was tapped off and fed to a reverb tank. However that amp had TWO power channels and the output of the reverb connected to the input of the spare channel.


.... Phil


Same here. My Fender Twin Reverb is probably typical of how it is
done. The reverb tank is fed as the anode load by a pair of triodes in
parallel running at fairly low current. the output is again picked up
by another triode.

http://www.soundthoughts.co.uk/look/twin_reverb.png

The DC bias helps to linearise the feed.

d

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Don Pearce wrote:

--------------------




** Exactly, reverb units need an independent drive amp - which is often
just an op-amp.

I know of onlyy one guitar amp where speaker signal was tapped off and
fed to a reverb tank. However that amp had TWO power channels and the
output of the reverb connected to the input of the spare channel.




Same here. My Fender Twin Reverb is probably typical of how it is
done. The reverb tank is fed as the anode load by a pair of triodes in
parallel running at fairly low current. the output is again picked up
by another triode.

http://www.soundthoughts.co.uk/look/twin_reverb.png

The DC bias helps to linearise the feed.


** The plate load for the parallel triodes ( a 12AT7) is a small audio transformer rated at 15kohms to 8 ohms.

There is no DC in the reverb drive coil.



..... Phil


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On Wed, 10 May 2017 23:58:41 -0700 (PDT), Phil Allison
wrote:

Don Pearce wrote:

--------------------




** Exactly, reverb units need an independent drive amp - which is often
just an op-amp.

I know of onlyy one guitar amp where speaker signal was tapped off and
fed to a reverb tank. However that amp had TWO power channels and the
output of the reverb connected to the input of the spare channel.




Same here. My Fender Twin Reverb is probably typical of how it is
done. The reverb tank is fed as the anode load by a pair of triodes in
parallel running at fairly low current. the output is again picked up
by another triode.

http://www.soundthoughts.co.uk/look/twin_reverb.png

The DC bias helps to linearise the feed.


** The plate load for the parallel triodes ( a 12AT7) is a small audio transformer rated at 15kohms to 8 ohms.

There is no DC in the reverb drive coil.



.... Phil


Well spotted. Quite right

d

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Having the power amp itself drive it
would be a feedback loop.



intentional feedback is very common in reverb systems
consider a tape delay reverb

m



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wrote:

-----------------

Having the power amp itself drive it
would be a feedback loop.



intentional feedback is very common in reverb systems
consider a tape delay reverb


** Different animals.

Feedback adds more echoes to a tape delay but would just make a reverb tank howl at one note. It is never used.

..... Phil


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Phil Allison wrote:
wrote:

-----------------

Having the power amp itself drive it
would be a feedback loop.



intentional feedback is very common in reverb systems
consider a tape delay reverb


** Different animals.

Feedback adds more echoes to a tape delay but would just make a reverb
tank howl at one note. It is never used.

.... Phil


Like any other feedback circuit, you just have to ensure that the loop gain
is less than one at all frequencies where the phase shift through the loop
is 0°. So just keep the loop gain less than one (ie: feedback is lower
level than the original signal) and it should all come out in the wash.

Not to say that I've seen this, but it shouldn't be difficult to achieve
with a spring reverb.

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On Fri, 12 May 2017 08:40:09 +0000 (UTC), Ralph Barone
wrote:

Phil Allison wrote:
wrote:

-----------------

Having the power amp itself drive it
would be a feedback loop.


intentional feedback is very common in reverb systems
consider a tape delay reverb


** Different animals.

Feedback adds more echoes to a tape delay but would just make a reverb
tank howl at one note. It is never used.

.... Phil


Like any other feedback circuit, you just have to ensure that the loop gain
is less than one at all frequencies where the phase shift through the loop
is 0°. So just keep the loop gain less than one (ie: feedback is lower
level than the original signal) and it should all come out in the wash.

Not to say that I've seen this, but it shouldn't be difficult to achieve
with a spring reverb.


You are joking, right? The phase shift through a spring is many, many
wavelengths resulting ain a fast-rotating phasor that can never be
controlled. You can run feedback round a spring to extend its time,
but its loop gain must be kept well below unity. The higher the loop
gain gets, the more that the frequencies that happen to come through
in phase will be emphasized - a comb filter effect. Eventually
whichever one is biggest and closest to zero degrees will howl.

Feedback loops are best kept well away from springs - or plates.

d

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Ralph Barone wrote:
Like any other feedback circuit, you just have to ensure that the loop gain
is less than one at all frequencies where the phase shift through the loop
is 0°. So just keep the loop gain less than one (ie: feedback is lower
level than the original signal) and it should all come out in the wash.


Right, and a lot of depends on how long the delay of the reverb is. The
Accutronics one out of the Hammond has a 33ms delay between input and output.
So if you put it into a positive feedback loop, it would howl at about 30 Hz.
And it's easy to just highpass the system so that is out of band.

It's not really all that unusual for people to use spring tanks in a feedback
loop in order to get a longer reverberation times. With some guitar amps,
you can in fact make them self-sustaining so with the gain up they just ring
at one note forever, and that note gets increasingly distorted as they go on.

Not to say that I've seen this, but it shouldn't be difficult to achieve
with a spring reverb.


The king of this technique was the LT Sound Microplate Reverb which managed
a very smooth and not-clangy reverb with a very long reverb time using
fairly inexpensive spring tanks.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."


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On Friday, May 12, 2017 at 9:25:22 AM UTC-4, Scott Dorsey wrote:
Ralph Barone wrote:
Like any other feedback circuit, you just have to ensure that the loop gain
is less than one at all frequencies where the phase shift through the loop
is 0°. So just keep the loop gain less than one (ie: feedback is lower
level than the original signal) and it should all come out in the wash.


Right, and a lot of depends on how long the delay of the reverb is. The
Accutronics one out of the Hammond has a 33ms delay between input and output.
So if you put it into a positive feedback loop, it would howl at about 30 Hz.
And it's easy to just highpass the system so that is out of band.

It's not really all that unusual for people to use spring tanks in a feedback
loop in order to get a longer reverberation times. With some guitar amps,
you can in fact make them self-sustaining so with the gain up they just ring
at one note forever, and that note gets increasingly distorted as they go on.

Not to say that I've seen this, but it shouldn't be difficult to achieve
with a spring reverb.


The king of this technique was the LT Sound Microplate Reverb which managed
a very smooth and not-clangy reverb with a very long reverb time using
fairly inexpensive spring tanks.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."


I agree that I have never SEEN a guitar amp with the springs driven by the power amp, but if it was done, it could work.

It would have the same limitations that any other reverb system with feedback has..... and that is as the other poster said, the gain around the loop has to be 1.

The tape loop or bucket brigade or any other reverb system has same limitation. With a tape loop, if the feedback is 1 the tape loop will create an ever increasing echo that becomes a continuous feedback sound.

A spring reverb with feedback could work as long as the feedback gain is 1 which is the same limitation all the other revers have.

To avoid a flame war...I agree that I have never seen it done in practice.

m



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Phil Allison[_4_] Phil Allison[_4_] is offline
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Default Help modding a circuit: MultiVox MXD-5

Scott Dorsey wrote:
-----------------------

Ralph Barone
Like any other feedback circuit, you just have to ensure that the loop gain
is less than one at all frequencies where the phase shift through the loop
is 0°. So just keep the loop gain less than one (ie: feedback is lower
level than the original signal) and it should all come out in the wash.


Right, and a lot of depends on how long the delay of the reverb is.

The
Accutronics one out of the Hammond has a 33ms delay between input and output.
So if you put it into a positive feedback loop, it would howl at about 30 Hz.
And it's easy to just highpass the system so that is out of band.


** Simply not true !!!

As in a room, the distance between speaker and microphone has little effect on feedback frequencies. The time of travel is irrelevant.

Reverb tanks simulate reverberant rooms, complete with resonant peaks and dips every few Hz over their working frequency range.

Operation in a feedback loop produces oscillation at one or more of the peak frequencies.


....... Phil


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Phil Allison[_4_] Phil Allison[_4_] is offline
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Default Help modding a circuit: MultiVox MXD-5

wrote:

-----------------------



I agree that I have never SEEN a guitar amp with the springs driven
by the power amp, but if it was done, it could work.


** Only if you have a second amp.


It would have the same limitations that any other reverb system with
feedback has..... and that is as the other poster said, the gain
around the loop has to be 1.


** It has the same characteristics as a mic, room & speaker combination.


The tape loop or bucket brigade or any other reverb system has
same limitation.


** Not the SAME limitiation.

The number of echoes can me hugely magnified by feedback.


To avoid a flame war...I agree that I have never seen it done in practice.


** Nobody has, cos it doesn't do anything useful.



...... Phil

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