isolating balanced and unbalanced equipment - help requested

[Jeremy Taylor]

I have hum inserted into the signal path when I place a processor between mic and
mixer, and I'm not sure exactly what to do about it, though I have a few ideas, I'm
not sure what is the best practice.

I have a couple of mics feeding into a simple mixer/amp (with no chan inserts). I
cannot push the levels on the channels very far before the mics feedback. It gives
adequate level, but more headroom is desirable. It is not practical to relocate
either the speakers or the microphones.

I have a Behringer DSP1100 Feedback Destroyer at my disposal. Unfortunately, while
the rest of the equipment uses balanced lines and XLRs, this DSP1100 has 1/4" jacks.

I mistakenly thought that they were TRS/balanced jacks, based on the PDF manual I
read that I obtained from Behringer's website, and used a couple of balanced patch
leads with XLRs at one end and 1/4" TRS jacks at the other. When connected up to the
system, there was a distinct and unacceptable level of hum.

At this point I double checked everything, and it was at this point that I realised
that while the back of the DSP100 had screenprinted notes regarding the Tip (signal)
and Sleeve (ground symbol) there was nothing about anything for Ring. I interpret
that as meaning this unit has unbalanced inputs/outputs only.

I suppose I have several options I could try.
(i) I could use cables that have XLR at one end and TS 1/4" jacks at the other, with
the cold signal line shorted to the shield/sleeve.
(ii) Between the DSP1100 and the mixer, I could put a DI box. What about the other
side, from the mic to the DSP1100? another DI box, passive, running backwards? Does
that work/How well?
(iii) Some other sort of line isolator(s) between the units (both sides?)
(iv) Something like the balance line drivers/receivers at Rod Elliott's website
http://sound.westhost.com/project51.htm and http://sound.westhost.com/project87.htm
but using the receiver between the mic and DSP1100 and the driver between the
DSP1100 and the mixer?

Advice on best practice (for making do with this equipment that is) would be very
much appreciated.

(Yes, a viable alternative is to trade the DSP1100 in on a DSP1124P with XLRs. I'll
go down that track if there is not a simple cheap solution with the equipment I have
to hand, if the fix will cost more than upgrade).

Thank you all in advance...
Regards
Jeremy Taylor

 

[Phil Allison]

"Jeremy Taylor" <jeremy.taylor@tait.co.nz> wrote in message
news:3F275988.905229F3@tait.co.nz...

I have hum inserted into the signal path when I place a processor between
mic and
mixer, and I'm not sure exactly what to do about it, though I have a few
ideas, I'm
not sure what is the best practice.

I have a couple of mics feeding into a simple mixer/amp (with no chan
inserts). I
cannot push the levels on the channels very far before the mics feedback.
It gives
adequate level, but more headroom is desirable. It is not practical to
relocate
either the speakers or the microphones.

I have a Behringer DSP1100 Feedback Destroyer at my disposal.
Unfortunately, while
the rest of the equipment uses balanced lines and XLRs, this DSP1100 has
1/4" jacks.

I mistakenly thought that they were TRS/balanced jacks, based on the PDF
manual I
read that I obtained from Behringer's website, and used a couple of
balanced patch
leads with XLRs at one end and 1/4" TRS jacks at the other. When connected
up to the
system, there was a distinct and unacceptable level of hum.

(snip)


** Domestic hi-fi systems are totally unbalanced - and do not hum - so
can be done.

Is the DSP1100 powered by a plug pack or a 3 core lead ?


................. Phil

 

[Russ]

"Jeremy Taylor" <jeremy.taylor@tait.co.nz> wrote in message
news:3F275988.905229F3@tait.co.nz...

I have hum inserted into the signal path when I place a processor between
mic and
mixer, and I'm not sure exactly what to do about it, though I have a few
ideas, I'm
not sure what is the best practice.

I have a couple of mics feeding into a simple mixer/amp (with no chan
inserts). I
cannot push the levels on the channels very far before the mics feedback.
It gives
adequate level, but more headroom is desirable. It is not practical to
relocate
either the speakers or the microphones.

I have a Behringer DSP1100 Feedback Destroyer at my disposal.
Unfortunately, while
the rest of the equipment uses balanced lines and XLRs, this DSP1100 has
1/4" jacks.

I mistakenly thought that they were TRS/balanced jacks, based on the PDF
manual I
read that I obtained from Behringer's website, and used a couple of
balanced patch
leads with XLRs at one end and 1/4" TRS jacks at the other. When connected
up to the
system, there was a distinct and unacceptable level of hum.

At this point I double checked everything, and it was at this point that I
realised
that while the back of the DSP100 had screenprinted notes regarding the
Tip (signal)
and Sleeve (ground symbol) there was nothing about anything for Ring. I
interpret
that as meaning this unit has unbalanced inputs/outputs only.

I suppose I have several options I could try.
(i) I could use cables that have XLR at one end and TS 1/4" jacks at the
other, with
the cold signal line shorted to the shield/sleeve.
(ii) Between the DSP1100 and the mixer, I could put a DI box. What about
the other
side, from the mic to the DSP1100? another DI box, passive, running
backwards? Does
that work/How well?

I've saw a DSP1100 a while back, and I'm pretty sure it had balanced
connections, but yours might be a different incarnation. One main thing -
congenitally it is placed after the desk and before the amps - in the same
way that you'd use a set of graphic equalisers to dip the offending
frequencies, the DSP1100 is a bunch of parametrics with very tight Q's, with
the ability to function in a fixed mode - used when the mics are static, or
auto, where the mics are moving around.

It sounds like you are putting it on specific microphones before they enter
the desk, which it isn't designed for - particularly if there is phantom
power coming from the desk. It is designed to work with -10 to +4dB levels
(otherwise known respectively as domestic and pro "line" levels) and
therefore needs to go on the output of the desk. If the unit is indeed
unbalanced and the output of the desk is balanced, I'd be careful about
tying either of the hot or cold lines to earth - if it is an old desk with
output transformers, you can do this, but if it has electronic outputs,
tying one line down could overload the output buffer. Either let one line
float, use a matching transformer, or a balanced->unbalanced box. If you are
having earth loop problems downstream, you may need to re-balance the signal
before the amps so you can lift the earth.

As Newsy mentioned, you should make sure you've done the basics first - you
may have phase buttons on each channel of your desk - often this can reduce
feedback on specific channels, as can using the parametrics on the channel,
provided it has a narrow enough Q.

Russ.

 

[Rob Judd]

Jeremy,

Modify the Behringer to add balanced inputs. That's what I'd do.

Rob


Jeremy Taylor wrote:

I have hum inserted into the signal path when I place a processor between mic and
mixer, and I'm not sure exactly what to do about it, though I have a few ideas, I'm
not sure what is the best practice.

I have a couple of mics feeding into a simple mixer/amp (with no chan inserts). I
cannot push the levels on the channels very far before the mics feedback. It gives
adequate level, but more headroom is desirable. It is not practical to relocate
either the speakers or the microphones.

I have a Behringer DSP1100 Feedback Destroyer at my disposal. Unfortunately, while
the rest of the equipment uses balanced lines and XLRs, this DSP1100 has 1/4" jacks.

I mistakenly thought that they were TRS/balanced jacks, based on the PDF manual I
read that I obtained from Behringer's website, and used a couple of balanced patch
leads with XLRs at one end and 1/4" TRS jacks at the other. When connected up to the
system, there was a distinct and unacceptable level of hum.

At this point I double checked everything, and it was at this point that I realised
that while the back of the DSP100 had screenprinted notes regarding the Tip (signal)
and Sleeve (ground symbol) there was nothing about anything for Ring. I interpret
that as meaning this unit has unbalanced inputs/outputs only.

I suppose I have several options I could try.
(i) I could use cables that have XLR at one end and TS 1/4" jacks at the other, with
the cold signal line shorted to the shield/sleeve.
(ii) Between the DSP1100 and the mixer, I could put a DI box. What about the other
side, from the mic to the DSP1100? another DI box, passive, running backwards? Does
that work/How well?
(iii) Some other sort of line isolator(s) between the units (both sides?)
(iv) Something like the balance line drivers/receivers at Rod Elliott's website
http://sound.westhost.com/project51.htm and http://sound.westhost.com/project87.htm
but using the receiver between the mic and DSP1100 and the driver between the
DSP1100 and the mixer?

Advice on best practice (for making do with this equipment that is) would be very
much appreciated.

(Yes, a viable alternative is to trade the DSP1100 in on a DSP1124P with XLRs. I'll
go down that track if there is not a simple cheap solution with the equipment I have
to hand, if the fix will cost more than upgrade).

Thank you all in advance...
Regards
Jeremy Taylor

 

[Jeremy Taylor]

Phil Allison wrote:

"Jeremy Taylor" <jeremy.taylor@tait.co.nz> wrote in message
news:3F275988.905229F3@tait.co.nz...

I have hum inserted into the signal path when I place a processor between
mic and
mixer, and I'm not sure exactly what to do about it, though I have a few
ideas, I'm
not sure what is the best practice.


(snip)

** Domestic hi-fi systems are totally unbalanced - and do not hum - so
can be done.

Is the DSP1100 powered by a plug pack or a 3 core lead ?

It is powered by a 3-core lead.

I am unsure what you meant by your first statement. Perhaps I can clarify - I am not
speaking of a domestic hi-fi system by a low-end public address system.

Regards
Jeremy

 

[Jeremy Taylor]

Newsy wrote:

"Jeremy Taylor" <jeremy.taylor@tait.co.nz> wrote in message
news:3F275988.905229F3@tait.co.nz...
I have hum inserted into the signal path when I place a processor between
mic and
mixer, and I'm not sure exactly what to do about it, though I have a few
ideas, I'm
not sure what is the best practice.

I have a couple of mics feeding into a simple mixer/amp (with no chan
inserts). I
cannot push the levels on the channels very far before the mics feedback.
It gives
adequate level, but more headroom is desirable. It is not practical to
relocate
either the speakers or the microphones.

There may be other solutions - why cannot you get acceptable gain before
feedback?

I can get ok gain before feedback, but occasionally with the movement of a mic near
a ceiling mounted speaker will cause some feedback, and an extra 3dB of headroom
even would make all the different to getting occasional feedback and never getting
any.

Speakers behind mics?

In ceiling, flush mounted. Mics occasionally walked/moved under them.

Mics omni directional rather than cardioid?

Cardiod, but sometimes the mic is pointing in the general direction of the speakers.

Change speakers for ones with more directional output?

Not practical in this situation, I'm afraid.

Put in some isolating bal/unbal boxes (I have some Ebtech hum eliminators if
you ae interested)

Yeah, this is the sort of thing that I am thinking - any other recommendations of
other such gear that I might source locally (NZ) that you would consider ok?

What is the exact operating circumstance?

church hall, ceiling mounted speakers overhead over auditorium, mobile mics. Mixer
is a pretty simple PA mixer with no insert jacks (which is where I would like to
stick the processor in) and integrated amplifier (unfortunately cannot fit the
DSP1100 between the mixer and the amp.)

The best advice is to do proper placement and choice of equipment and
modification of room acoustics.

Yes, you're right - the DSP1100 is basically not the best choice of equipment to go
with the other gear there already. The rest of the gear is basically doing an
adequate (not perfect) job; I guess the choices are to find some other appropriate
equipment that will fit into the existing system ok; or talk about replacing the
existing equipment - which would of course be more expense, and needs to be
justified against the low severity of the feedback problem. And unfortunately that
also counts out the remodelling that would be needed with changing the existing
installation of speakers etc...

Thank you for you help.
Jeremy

 

[Jeremy Taylor]

Rob Judd wrote:

Jeremy,

Modify the Behringer to add balanced inputs. That's what I'd do.

Rob

Probably not a bad idea either, Rob! Considering the DSP1100 units came in a variety of
flavours (I've seen XLR versions myself), the board probably has the capacity for a
variety of connectors, the selection of which would be probably a purely mechanical thing
when the unit is constructed. I can't imagine they would produce seperate board designs
for each variant...

Wouldn't tackle the job myself, but I'd get a electronics tech to look at it. However,
other advice I have received here has pointed out that the DSP1100 probably wants a
stronger signal than mic level - will want either -10dB or +4dB (it is switch selectable).
So I don't think I am going to solve the feedback problem with the current equipment to
hand anyway. It was a reasonably minor problem anyway...

Thanks
Jeremy

 

[Jeremy Taylor]

Phil Allison wrote:

It is powered by a 3-core lead.

** Probably, if you disconnect the ground on that lead the hum will
stop.

Yes, I expected some sort of isolation or ground lift would eliminate the hum.

I am unsure what you meant by your first statement. Perhaps I can
clarify - I am not
speaking of a domestic hi-fi system by a low-end public address system.

** There is basically no difference, both are systems of audio
components. Balancing is not essential to eliminating hum - but correct
earthing is - as is proven by unbalanced domestic hi-fi.

Right, I see what you mean now - I think you are absolutely correct.

The main problem I have been experiencing with the hum has really been caused by
trying to put a piece of equipment where it really didn't belong; elsewhere it has
been pointed out to me that the unit will require signal levels higher than mic
level anyway, so I've basically being trying to tackle the problem from the wrong
point. One can't expect things to work right if you try to break the rules!

Thank you for your assistance

Jeremy

 

[Phil Allison]

"Jeremy Taylor" <jeremy.taylor@tait.co.nz> wrote in message
news:3F2A20EA.D7183062@tait.co.nz...

Phil Allison wrote:

It is powered by a 3-core lead.

** Probably, if you disconnect the ground on that lead the hum will
stop.


Yes, I expected some sort of isolation or ground lift would eliminate the
hum.


I am unsure what you meant by your first statement. Perhaps I can
clarify - I am not
speaking of a domestic hi-fi system by a low-end public address
system.

** There is basically no difference, both are systems of audio
components. Balancing is not essential to eliminating hum - but
correct
earthing is - as is proven by unbalanced domestic hi-fi.


Right, I see what you mean now - I think you are absolutely correct.

The main problem I have been experiencing with the hum has really been
caused by
trying to put a piece of equipment where it really didn't belong;
elsewhere it has
been pointed out to me that the unit will require signal levels higher
than mic
level anyway, so I've basically being trying to tackle the problem from
the wrong
point. One can't expect things to work right if you try to break the
rules!

** If you search the ESP site again you will find a nice, low-noise
balanced mic pre-amp by me.

PCBs in pairs are now available for a modest price too.



............. Phil

 

[Jeremy Taylor]

Phil Allison wrote:

** If you search the ESP site again you will find a nice, low-noise
balanced mic pre-amp by me.

PCBs in pairs are now available for a modest price too.

Ah, now that would bring the signal level up to what we want... I'll look into that,
thanks!

 

[Jeremy Taylor]

Jeremy Taylor wrote:

Hm, this is something I had not thought of til you pointed it out. A birdie told me
(well a salesman in a sound store) a while back that it was possible to put the
DSP1100's little brother the Shark DSP100 (?) between a mic and a mixer, and I made
an *assumption* the same was true of the DSP1100. Pretty poor assumption, seeing as
I don't even know if what I was told about the Shark was true!

It is true - the Behringer Shark DSP110 has a mic preamp in it. It was just a stupid
assumption on my part that the DSP1100 would also be able to fit between the mic and
mixer.

JT

 

[Newsy]

"Jeremy Taylor" <jeremy.taylor@tait.co.nz> wrote in message
news:3F2A1F7E.F689DAFB@tait.co.nz...

Rob Judd wrote:

Jeremy,

Modify the Behringer to add balanced inputs. That's what I'd do.

Rob

Probably not a bad idea either, Rob! Considering the DSP1100 units came in
a variety of
flavours (I've seen XLR versions myself), the board probably has the
capacity for a
variety of connectors, the selection of which would be probably a purely
mechanical thing
when the unit is constructed. I can't imagine they would produce seperate
board designs
for each variant...

Wouldn't tackle the job myself, but I'd get a electronics tech to look at
it. However,
other advice I have received here has pointed out that the DSP1100
probably wants a
stronger signal than mic level - will want either -10dB or +4dB (it is
switch selectable).
So I don't think I am going to solve the feedback problem with the current
equipment to
hand anyway. It was a reasonably minor problem anyway...

You must have a preamp between the mics and the Behringer - consider a cheap
microphone preamp. Then put the output of the Behringer into the
mixer/amplifier. It then won't matter if the output from the Behringer is
balanced or not. Just balancing the inputs is not going to help.


Rod

Thanks
Jeremy

 

[Newsy]

"Jeremy Taylor" <jeremy.taylor@tait.co.nz> wrote in message
news:3F2A1942.7FAE087D@tait.co.nz...

Newsy wrote:

"Jeremy Taylor" <jeremy.taylor@tait.co.nz> wrote in message
news:3F275988.905229F3@tait.co.nz...
I have hum inserted into the signal path when I place a processor
between
mic and
mixer, and I'm not sure exactly what to do about it, though I have a
few
ideas, I'm
not sure what is the best practice.

I have a couple of mics feeding into a simple mixer/amp (with no chan
inserts). I
cannot push the levels on the channels very far before the mics
feedback.
It gives
adequate level, but more headroom is desirable. It is not practical to
relocate
either the speakers or the microphones.

There may be other solutions - why cannot you get acceptable gain before
feedback?

I can get ok gain before feedback, but occasionally with the movement of a
mic near
a ceiling mounted speaker will cause some feedback, and an extra 3dB of
headroom
even would make all the different to getting occasional feedback and never
getting
any.

Is it possible to disconnect the speakers that make feedback occur?
Sometimes these are just the ones at the front of the auditorium.

Speakers behind mics?

In ceiling, flush mounted. Mics occasionally walked/moved under them.


Mics omni directional rather than cardioid?

Cardiod, but sometimes the mic is pointing in the general direction of the
speakers.


Change speakers for ones with more directional output?

Not practical in this situation, I'm afraid.


Put in some isolating bal/unbal boxes (I have some Ebtech hum
eliminators if
you ae interested)

Yeah, this is the sort of thing that I am thinking - any other
recommendations of
other such gear that I might source locally (NZ) that you would consider
ok?


What is the exact operating circumstance?

church hall, ceiling mounted speakers overhead over auditorium, mobile
mics. Mixer
is a pretty simple PA mixer with no insert jacks (which is where I would
like to
stick the processor in) and integrated amplifier (unfortunately cannot fit
the
DSP1100 between the mixer and the amp.)

As I said in another post - get another mixer/preamp and connect thus:
mics - new mixer/preamp - Behringer - amplifier - speakers. Look in Crap
Converters for something reasonable but cheap.

The best advice is to do proper placement and choice of equipment and
modification of room acoustics.

Yes, you're right - the DSP1100 is basically not the best choice of
equipment to go
with the other gear there already. The rest of the gear is basically doing
an
adequate (not perfect) job; I guess the choices are to find some other
appropriate
equipment that will fit into the existing system ok; or talk about
replacing the
existing equipment - which would of course be more expense, and needs to
be
justified against the low severity of the feedback problem. And
unfortunately that
also counts out the remodelling that would be needed with changing the
existing
installation of speakers etc...

The other suggestion is to suggest to the mic users to not go underneath the
speakers. Maybe map a path on the floor that is "between" the speakers.

Rod

Thank you for you help.
Jeremy

 

[Russ]

"Jeremy Taylor" <jeremy.taylor@tait.co.nz> wrote in message
news:3F2A1E4F.3C2BDDD5@tait.co.nz...

I've saw a DSP1100 a while back, and I'm pretty sure it had balanced
connections, but yours might be a different incarnation. One main
thing -

Yeah, I have seen one with XLRs also. I think there are several flavours.


congenitally it is placed after the desk and before the amps - in the
same
way that you'd use a set of graphic equalisers to dip the offending

Yes, unfortunately this is an integrated PA mixer/amp and I cannot put it
between
the mixer and the amp. It also is unfortunate enough not to have an insert
on the
channels so I cannot put the processor on a Channel Insert.

The integrated mixer/amp thing is a bit of a killer. Behringer do a bunch of
really cheap but high quality mixers, but I imagine a new amp would put the
price up a fair bit.

What model mixer is it? There may be a work-around.

Russ.