I read in sci.electronics.design that wrote (in
.com) about 'Reducing
hiss by changing op-amps ?', on Wed, 23 Mar 2005:
Hmmm...... what value resistors are in your potential divider ?

75K in series with both the live and the shield and 560 Ohms across
them. These figures (and configuration) were arrived at with the help
of the folks over in rec.audio.pro.

You are attenuating the mixer output by nearly 50 dB and then amplifying
up again. This is not good. I suppose you are feeding into the
low-impedance balanced input of the Carlsbro. You would do better to
feed into the high impedance unbalanced input, provided you
interconnecting cable is not more than about 2 m long (to minimise
interference pickup, not anything to do with frequency response. You
need a different attenuator pad: 22 kohms in series with the live,
nothing in series with the shield and 1 kohm across them.

Using a mic input is a really bad idea. Performance is limited by the
noise figure of the " PA head's " own mic amps.
It is easy to show using theory that this will *always* be worse than
just using the PA head's mic amps on their own ! No matter how good
the mixer is.

I know, I know... :-( We're not gigging at the moment (band reshuffle
!) and have spent a bit on other gear recently, but a separate PA amp
is next on my shopping list. I can finally run in stereo then ! (I
just want to hear my stereo chorus pedal through the PA :-) )

I really don't think the desk is to blame.

Perhaps it's not. I will do some more testing on Friday. I'm only
really basing my accusations of the desk on the fact that when we plug
into the PA head directly (mics and guitars), there's almost no hiss.
Bit of mains hum (diabolical electrics in the place we practice in !),
but no hiss. When we bring the mixer into play and plug into that,
there's a noticeable increase in hiss from the PA speakers when we're
not playing. I'm also presuming that this hiss will be 'polluting' the
sound when we are, - I can't hear it then, but I know it's there! We
have nice guitars and good mic's and I want it as clean as I can get it
:-)


Doesn't the PA head have a line level - ish input ( like a stereo
input for example ) ? Or an insert point. That would produce far
better results.

Yes, but none that I can route via the internal reverb of the PA head.
I need to use two mic inputs on the PA head, one with a bit of reverb
for the vocals & rhythm guitar, and the other one dry (I have a better
quality reverb in an effects pedal) for my guitar. I can't see any way
of routing a line level input on the PA head through it's internal
reverb - that would solve everything if I could !

http://homepage.ntlworld.com/anengin...7%20Manual.pdf
(though sadly it's not a schematic).

The Carlsbro doesn't have a proper line-level input, only an 'effects
return'.

Incidentally, the mixing desk has it's own 'reverb'. Possibly the
worst I have ever heard though ! It's a simple delay, more an echo
than a reverb (unless it's faulty and not supposed to sound like that
!). Once again, it's stuff I plan to sort when funds are available - a
decent outboard reverb unit - eventually.

Is it electronic or electromechanical? If the latter, it will certainly
have been damaged. Sometimes they can be repaired.


--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

You are attenuating the mixer output by nearly 50 dB and then
amplifying up again. This is not good. I suppose you are
feeding into the low-impedance balanced input of the Carlsbro.
You would do better to feed into the high impedance unbalanced input

I opted for the low impedance mic input on the basis of advice in
rec.audio.pro where it was suggested to me to be wary of using the high
impedance instrument input as "sometimes these also contain fixed EQ to
sort-of simulate the tone shaping of a guitar amp; ie, a midrange
notch."

I therefore figured it was easier to just go with the mic inputs to
avoid this potential problem.

Is it electronic or electromechanical? If the latter, it will
certainly have been damaged. Sometimes they can be repaired.

Electronic. I'm guessing it's based around IC706 /707 on the power
board ( http://homepage.ntlworld.com/anengin..._Schematic.pdf ).
Does this look like a very basic delay circuit, or do you think it
should sound like a proper reverb ?

wrote:

You are attenuating the mixer output by nearly 50 dB and then
amplifying up again. This is not good. I suppose you are
feeding into the low-impedance balanced input of the Carlsbro.
You would do better to feed into the high impedance unbalanced input

I opted for the low impedance mic input on the basis of advice in
rec.audio.pro where it was suggested to me to be wary of using the high
impedance instrument input as "sometimes these also contain fixed EQ to
sort-of simulate the tone shaping of a guitar amp; ie, a midrange
notch."

That sounds unlikely to me. Why not try it and see ?

Also your current pad is presenting too high an impedance to the mic input
which will worsen noise anyway. I'd suggest no more than 100 ohms at that
end and no more than 40dB of attenuation ( maybe less ).

Try 100 ohms and 10k in place of your current values. You shouldn't have a
resistor in series with the shield in case you used one btw.

Wire as follows.
Mixer jack shield to pin 3 of mic XLR.
Mixer jack tip to 10k series resistor mounted in the XLR - then to pin 2.
100 ohm resistor between pins 2 and 3 of the XLR.

You don't need to make any connection to pin 1 of the XLR. This will
prevent hum loops.

I doubt it'll overload the PA head's mic amp if you substitute 3k3 for the
10k above.


I therefore figured it was easier to just go with the mic inputs to
avoid this potential problem.

Is it electronic or electromechanical? If the latter, it will
certainly have been damaged. Sometimes they can be repaired.

Electronic. I'm guessing it's based around IC706 /707 on the power
board ( http://homepage.ntlworld.com/anengin..._Schematic.pdf ).
Does this look like a very basic delay circuit, or do you think it
should sound like a proper reverb ?

IC 708 is a 'bucket brigade' analogue delay line and IC709 is a compander
chip ( no doubt to attempt to improve the s/n ratio ) IIRC.

In short - it's not a reverb.


Graham

I opted for the low impedance mic input on the basis of advice in
rec.audio.pro where it was suggested to me to be wary of using the
high impedance instrument input as "sometimes these also contain
fixed EQ to sort-of simulate the tone shaping of a guitar amp;
ie, a midrange notch."

That sounds unlikely to me. Why not try it and see ?

OK - but I'm going to have to change the values of the resistors - the
low impedance input is ten times more sensitive than the high impedance
one. (low = 6K8 ohms - 3.5mV, high = 68K ohms 35.0mV).

What would you recommend here ? The specs for the mixer output say
it's 600 Ohm, 1.0 V and I'd be going into a 68K Ohm, 35.0mV input on
the PA.


Also your current pad is presenting too high an impedance to the mic
input which will worsen noise anyway. I'd suggest no more than 100
ohms at that end and no more than 40dB of attenuation ( maybe less ).

I arrived at the figures I'm using on the basis of reading somewhere
that the resistor across the tip and shield should be ten times (or
less) the input impedance of the unit you are driving. In my case, 6K8
Ohms. My 560 Ohm resistor would therefore be about 1/12 of that. I
will gladly give your advice priority over this as if you design this
stuff then you obviously know what you're talking about ! :-)

Try 100 ohms and 10k in place of your current values. You shouldn't
have a resistor in series with the shield in case you used one btw.

I am. This was suggested to me in rec.audio.pro as per :-

.....since the mic inputs are balanced, you may want
something like this:


tip--------- 120K ---+----XLR 2
|
1000
|
sleeve---+-- 120K ---+----XLR 3
|
+----------------XLR 1


The two 120k's are basically in series with your signal to add up to
something near your 270 k unbalanced attenuator. "


OK... so what I need to do now is to make up some more attenuators, a
pair for the XLR low impedance input option using 10K & 100 Ohm
resistors and another pair for the 1/4" high impedance input option
using whatever values I'm hoping you can suggest ! :-)


In short - it's not a reverb.

Ah well, it was worth a shot, just in case !


Incidentally Graham, I don't know if you remember, but we exchanged a
few posts a while back about a Studiomaster 400 Powerpack with a blown
main transformer. Just fyi, I never did manage to locate any suitable
replacement, so it's going in the bin on the weekend !

I read in sci.electronics.design that wrote (in
.com) about 'Reducing
hiss by changing op-amps ?', on Thu, 24 Mar 2005:

Electronic. I'm guessing it's based around IC706 /707 on the power
board ( http://homepage.ntlworld.com/anengin..._Schematic.pdf ).
Does this look like a very basic delay circuit, or do you think it
should sound like a proper reverb ?

I can't tell; it looks very complicated.
--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

I doubt it'll overload the PA head's mic amp if you substitute 3k3
for the 10k above.

I'm somewhat confused here... if I use 10K and 100R then I am going to
get 10mV (max) output for a 1V max input. If I use 3K3 then the output
will be 29mV. In both cases, surely this will overdrive the PA mic
input, which is expecting just 3.7mV max ???

wrote:

I doubt it'll overload the PA head's mic amp if you substitute 3k3
for the 10k above.

I'm somewhat confused here... if I use 10K and 100R then I am going to
get 10mV (max) output for a 1V max input. If I use 3K3 then the output
will be 29mV. In both cases, surely this will overdrive the PA mic
input, which is expecting just 3.7mV max ???

I don't know where you got the 3.7mV max info. Sounds wrong to me.

You may be interested to know that a low impedance mic can easily produce
up to 1 *Volt* of signal when used close up and loud !

I recently designed a new unit with a fixed gain mic input and we allowed
for an input signal of up to 200mV as a reasonable compromise between
sensitivity and likely overload in its intended useage..


Graham

wrote:

I opted for the low impedance mic input on the basis of advice in
rec.audio.pro where it was suggested to me to be wary of using the
high impedance instrument input as "sometimes these also contain
fixed EQ to sort-of simulate the tone shaping of a guitar amp;
ie, a midrange notch."

That sounds unlikely to me. Why not try it and see ?

OK - but I'm going to have to change the values of the resistors - the
low impedance input is ten times more sensitive than the high impedance
one. (low = 6K8 ohms - 3.5mV, high = 68K ohms 35.0mV).

Ahhh - in that case the line input looks like an internally padded down'
version of the mic input.

I wouldn't worry too much about which input you use in that case.

What would you recommend here ? The specs for the mixer output say
it's 600 Ohm, 1.0 V and I'd be going into a 68K Ohm, 35.0mV input on
the PA.

Let's stick with the mic input for now. It's balanced which is good for
rejecting hum loop type problems.

I suspect the sensitivity figures are 'nominal' - probably actually the
*minimum* voltage needed to produce max output.


Also your current pad is presenting too high an impedance to the mic
input which will worsen noise anyway. I'd suggest no more than 100
ohms at that end and no more than 40dB of attenuation ( maybe less ).

I arrived at the figures I'm using on the basis of reading somewhere
that the resistor across the tip and shield should be ten times (or
less) the input impedance of the unit you are driving.

That's good practice.

In my case, 6K8
Ohms. My 560 Ohm resistor would therefore be about 1/12 of that. I
will gladly give your advice priority over this as if you design this
stuff then you obviously know what you're talking about ! :-)

The lower the source impedance - the lower the thermal noise contribution
will be. Low impedance mics typically vary in impedance from 150 - 200 ohms
( most 'pro' mics ) to around 600 ohms ( far eastern rubbish and cheap
'copies' ).

Typical professional mixers have a mic input impedance of 2 kohms. But I
digress.

I recommend the use of 100 ohms for ease of doing the calculations and
lower thermal noise. ;-)


Try 100 ohms and 10k in place of your current values. You shouldn't
have a resistor in series with the shield in case you used one btw.

I am. This was suggested to me in rec.audio.pro as per :-

.....since the mic inputs are balanced, you may want
something like this:


tip--------- 120K ---+----XLR 2
|
1000
|
sleeve---+-- 120K ---+----XLR 3
|
+----------------XLR 1


The two 120k's are basically in series with your signal to add up to
something near your 270 k unbalanced attenuator. "

I can understand why someone might suggest that arrangement but I don't
happen to agree - lol !


OK... so what I need to do now is to make up some more attenuators, a
pair for the XLR low impedance input option using 10K & 100 Ohm
resistors and another pair for the 1/4" high impedance input option
using whatever values I'm hoping you can suggest ! :-)

I suggest sticking with the XLR input in view of the info you gave me.


In short - it's not a reverb.

Ah well, it was worth a shot, just in case !

Incidentally Graham, I don't know if you remember, but we exchanged a
few posts a while back about a Studiomaster 400 Powerpack with a blown
main transformer. Just fyi, I never did manage to locate any suitable
replacement, so it's going in the bin on the weekend !

Erk !

Any chance of holding fire on that ?

If you're simply going to junk it - I'll give you a few bob for it and
arrange collection if that's good with you.

It would actually be quite handy to me for various reasons.


Graham

I read in sci.electronics.design that Pooh Bear
wrote (in
) about 'Reducing hiss by changing
op-amps ?', on Fri, 25 Mar 2005:

I don't know where you got the 3.7mV max info. Sounds wrong to me.

From the spec. And it presumably means the input required to get 0dBu or
some other reference level out with **all gain controls at max**, which
is not a realistic operating condition.

What matters in this case in the maximum source e.m.f of the mic input,
and the spec doesn't say.
--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

Erk !

Any chance of holding fire on that ?
Graham

Graham, did you get my email regarding this ?

Cheers,

Kev.