[sdiy] Buzzing reverb

[harrybissell]

Hi steve

multiple post answers:

1) Measuring "impedance".   Impedance is basically 'AC Resistance'... often
for audio its measured at a frequency of 1KHz (roughly midband in the Human
hearing by octaves...).   Ohmmeters measure DC resistance... so things that are
like
coils (whose DC resistance would ideally be infinitely low...) measure much
LOWER
than the actual impedance (at some higher frequency).  Capacitors (whose DC
resistance is ideally infinitely high) usually measure 'open circuit' to an
ohmmeter.

There is an old trick of switching the meter leads back and forth, to charge and
discharge a cap... this works best with old analog meters and larger caps (like
lytics)
Some of you young punks (you KNOW who you are...) have meters that will read
capacitance.

That said... 200 ohms DC for both input and output is probably fine unless you
suspect it bad for another reason (like no worky at all...)  I think you are OK.

2) Reverb tanks usually will not become MORE noise sensitive... but other noise
sources may have increased.  First... if you are not used to spring reverbs you
might
be expecting a 'digital quality' noise floor, which it will NEVER have.  (otoh
if you
compare to a BBD its probably REALLY good :^)  Second... reverb tanks are
ALWAYS succeptable to external magnetic fields, so moving the tank is a fair
method.

You can also try to shield either the power transformer, or the reverb tank.
Against magnetic fields, your chief weapons will be either Mu-metal (a nickel
cobalt alloy)
that is 'pervious' to magnetic flux.  Usually you'd say 'impervious' but
mu-metal
is VERY good at intercepting magnetic flux and directing it in a path AROUND the

thing you are shielding.  It is available in very thin foils. Often it must be
heat treated if
bent (and you will need to bend it)... always it is quite expensive.  Next
weapon is IRON.  Good stuff, but heavy as sin, and really hard to work with.
Next would be a steel box around the tank.

Another weapon is distance, move away from the source of interference. Or change

the orientation (as you discovered already).

Its possible that the POWER TRANSFORMER you have is bad.  If a transfomer has a
gap in the core (old, loose varnish)... or if it is being overloaded... or if it
has an asymmetrical load (more current on one half-cycle than another)... it
will allow flux to
escape from the core. This could get into the tank at a much higher rate than
the original design.

Does it run hot (a clue to transformer overload or saturation) ???

You might replace the transformer with a toroidal core transformer... these have
a
MUCH lower radiated flux.  They will also saturate more easily so you need to
study
the supply carefully to see if it is OK first.

The easy way to fight here is at the transformer (source of the hum)... but this
will
not help if you rack the unit next to some other known offender (can anybody say

MATRIX 1000 ???) that has transformers that are leaky (of magnetic flux).

It sounds from your posts like you are on the right track, keep going !!!

H^) harry

Steve Begin wrote:

> It seems like if i move the tank somewhere else or even turn it around
> inside the enclosure the buzzing is reduced drastically.
> I got it really cheap but it's been buzzy right from the start, and it would
> seem kind of crazy if it was just built this way.
> Is there anything that could cause a spring reverb to become more sensitive
> to noise from the power transformer?  (it looks like it's probably over 20
> years old)
> If I can't find any other solution I'll probably just mount the tank
> externally, but I'm curious as to how it got this way.
> steve
>
> ----- Original Message -----
> From: "Bob Weigel" <sounddoctorin at imt.net>
> To: "Steve Begin" <trypannon at hotmail.com>
> Sent: Saturday, April 16, 2005 7:52 PM
> Subject: Re: [sdiy] Buzzing reverb
>
> > sure.  That means the tank is probably ok..barring any broken springs
> > inside. Possibly then the problem is in the amp that picks up the signal
> > from it. -Bob
> >
> > Steve Begin wrote:
> >
> >> Forgive me for asking, but how would you measure these impedances?  if i
> >> have the spring tank unplugged and i measure across the rca connector i
> >> get 200 ohms on both input and output.  Does this mean anything?
> >> -steve
> >>
> >>
> >> ----- Original Message ----- From: "Bob Weigel" <sounddoctorin at imt.net>
> >> To: <synth-diy at dropmix.xs4all.nl>
> >> Sent: Friday, April 15, 2005 9:59 PM
> >> Subject: Re: [sdiy] Buzzing reverb
> >>
> >>
> >>>
> >>> Yeah that's what I tried to say..not sure if my mail got through. Reverb
> >>> cans OFTEN have problems with coil connections to leads (they fail in
> >>> mexico to use strip-x or equiv to pull the enamel off the wires it
> >>> appears..and they just solder and say a prayer.)  check again the
> >>> impedences to the can. Input depending on can should be between almost
> >>> nothing and a couple hundred ohms.  Output 20 to maybe 800. Most common
> >>> ones I see in that kind of thing measure a DC impedence of 57 ohms on
> >>> the in and 200 or thereabouts on the out.  If no connect on output
> >>> you'll be amplifying an antenna. -Bob
> >>> harrybissell wrote:
> >>>
> >>>> oooh... my bad
> >>>>
> >>>> I missed a clue.  If the Parametric EQ works without hum, but he reverb
> >>>> DOES hum... its is very likely NOT a cap in the power supply !!!
> >>>> Broken
> >>>> wire, or EMI.  Or it could be a bad connection at, or insdie the reverb
> >>>> tank itself. They usually use RCA connectors, famous for having
> >>>> corrosion
> >>>> etc...
> >>>>
> >>>> H^) harry
> >>>>
> >>>> jays at aracnet.com wrote:
> >>>>
> >>>>
> >>>>> You haven't moved it next to something that has a lot of EMI have you?
> >>>>>
> >>>>> Reverbs are great for picking up AC hum from computer monitors and
> >>>>> other units power supplies.
> >>>>>
> >>>>> Like Harry said, either bad caps or bad ground would be my guess after
> >>>>> EMI. Usually linear power supplies aren't that bad to deal with.
> >>>>>
> >>>>> Jay S.
> >>>>>
> >>>>> Steve Begin wrote:
> >>>>>
> >>>>>
> >>>>>> I have an Intersound PRV-1 Spring Reverb / Parametric EQ, and when
> >>>>>> the reve=
> >>>>>> rb section is switched on, there's a really loud buzzing sound added
> >>>>>> to the=
> >>>>>> output.
> >>>>>> It does this regardless of whether or not there's an input present,
> >>>>>> and the=
> >>>>>> buzz would appear to be 60Hz.
> >>>>>>
> >>>>>> Would this mean it's likely one of the filter caps somewhere is dead?
> >>>>>> If s=
> >>>>>> o is there anything anybody can tell me about how I should go about
> >>>>>> locatin=
> >>>>>> g the offending capacitor?
> >>>>>>
> >>>>>> Thanks,
> >>>>>> Steve=
> >>>>>>
> >>>>
> >>>>
> >>>>
> >>>>
> >>>
> >>
> >>
> >