[GreenKeys] Far OT: audio inputs

Tue Feb 17 07:32:47 EST 2009

Good Morning Gerry.  Don't tell me you are broadcaster?

AL Murray
Director of Engineering
Wisconsin Public Station WRFW FM
University of Wisconsin-River Falls
410 South Third Street
River Falls,  Wi 54022
715-425-3887

Gerry Block wrote:
> Oops..
>
>  In my response I said '0' level at 1.23vrms... I meant .775vrms... 
>
> Gerry
>
> -----Original Message-----
> From: greenkeys-bounces at mailman.qth.net
> [mailto:greenkeys-bounces at mailman.qth.net] On Behalf Of Gerry Block
> Sent: Monday, February 16, 2009 10:06 AM
> To: gil at baudot.net; eugene at hertzmail.com
> Cc: greenkeys at mailman.qth.net
> Subject: Re: [GreenKeys] Far OT: audio inputs
>
> Gil,
>
>   I wanted to add a couple of small facts which might interest the net.  As
> far as transformers go, in professional use they were always pretty good -
> in fact the debate still rages over transformers vs direct.  If you want an
> idea of how good transformers can be, see as an example
>   http://www.jensentransformers.com/datashts/11ssp6m.pdf
>   They have inherently good common mode rejection ratio (can reject hum in
> balanced configuration) whereas with opamps you can get it good but only
> using very expensive matched components.  Its easier to count the turns on
> the transformer.
>   Transformers are often preferred in an RF environment as they are
> essentially immune from RF fields where opamp inputs are not necessarily
> unless they have good rf filters.  My guess is the inputs on the Harris unit
> are probably filtered as it is RF equipment.
>   And, as you indicated, almost required in long distance transmissions that
> are not digital.
>
>   Regarding line level, I probably can add to the confusion.  If you take
> out a voltmeter, +4dBm and +4dBu at '0' level read the same - 1.23vRMS.
> What happened a bunch of years ago is someone said "zero reference is 1mw
> into 600 ohms and we don't use 600 ohms anymore..." (this was the 'm' in
> dBm) and so they kept the same reference voltage level and changed the
> suffix to dBu (I'll guess: the 'u' means 'unloaded').  There is also a dBv
> where the zero reference is 1vRMS.  So for all practical purposes dBm and
> dBu are the same if you are using a voltmeter to measure the level, which is
> what everybody does.  A Vu meter is really a voltmeter with special
> ballistics and scale.
>
>   I believe the reason for '+4' had to do with the use of the VU meters
> themselves.   These meters had a rather low impedance and also had a
> rectifier in them.  With an actual 600 Ohm source (i.e. a transformer output
> from a vacuum tube amplifier) they would a) load the line slightly and b)
> could cause not just measurable but audible distortion.  You could easily
> hear it listening to a tone from an oscillator.  So a 3600 ohm resistor was
> added in series with the meter to reduce the load on the 600 ohm line to
> something around 6000 ohms (10:1 rule of thumb).  This also had the nice
> side effect that if you used a small potentiometer instead of the 3600 ohm
> resistor, in a large facility you could calibrate each Vu meter to a precise
> accuracy.  That is why you will see many Vu meter panels from the old days
> with little brackets and small AB pots on the back of them.
>
>   The standard for 'professional' audio equipment was a maximum (before
> onset of clipping) output of +24dBm, which for a signal registering '0' on
> the Vu meter gave 20db of headroom before clipping for signal peaks.
> Plenty.
>
>   In some instances, for instance with old remote equipment, they often had
> the ability to dial in a zero reference level of something other than +4..
> for instance I have a Collins 12Z 'remote mixer' which lets you set the zero
> reference on the Vu meter to +4 - or +6, +8, +10 etc... This was because
> these remote systems were used to drive actual telephone leased lines set up
> for the remote broadcasts, and they might need to drive a higher signal
> level (at the expense of some potential distortion on the louder signals) to
> get rid of crosstalk and noise at the receiving end.
>
>   Over the years to make the professional systems more perfect (on paper)
> transformers were removed from audio systems and Vu meter isolation amps
> (using opamps) were built, finally replaced by electronic Vu meters..
>
>   Not that this had anything good to do with the sound quality, which has
> continued, mostly, to get worse and worse.  This has resulted in a robust
> 'retro movement' back to vacuum tubes and transformers, a marketers dream
> but which misses the point.  The problem wasn't the equipment, it was lack
> of training.  That is another story.
>
> Regards
> Gerry Block
> Ad6MC
>
>
>
> -----Original Message-----
> From: greenkeys-bounces at mailman.qth.net
> [mailto:greenkeys-bounces at mailman.qth.net] On Behalf Of gil at baudot.net
> Sent: Sunday, February 15, 2009 10:54 PM
> To: eugene at hertzmail.com
> Cc: greenkeys at mailman.qth.net
> Subject: Re: [GreenKeys] Far OT: audio inputs
>
> Hey Eugene:
>
> I am curious to see posts about this as well.  I don't have your
> answers, but I can add a couple of things.  I always wondered where the
> 600-ohm standard evolved from -- anybody?  It seems too high to be the
> characteristic impedance of a cable;  I expect that it is a practical
> value for transformer design, but undoubtably there will be some other
> reason.  Bell telephone perhaps?
>
> As for the inputs on your gear:  a transformer input allows you to
> connect an audio signal and keep both sides electrically isolated from
> your chassis -- a balanced signal input with neither side referenced to
> ground.  This is used to prevent a ground loop between systems, which
> can result in a hum superimposed on the audio.  I'm not sure what was
> generally done with the center tap in those applications, but I presume
> you could leave it open, or tie it to ground (via a short, resistor, or
> capacitor; whichever helped).  I'd like to hear from folks who used this
> type of gear, with their experiences.  A 600-ohm transformer input is
> driven from a 600-ohm source (active, or transformer output), not unlike
> 50-ohm antenna lines or 75-ohm video signals (except that those coax
> lines are unbalanced).  Your differential-amplifier inputs sound like
> they are also balanced inputs, but I can't suggest why one would be
> better than the transformer input on that gear of yours.
>
> I can add a bit of information related to unbalanced and balanced audio
> equipment, and used in consumer and pro-audio applications:
>
> A ground-loop hum may be noticed when an unbalanced-audio output from a
> consumer audio source (eg: cd player)  is connected to
> amplifiers/speakers in a distant room. A line-level coaxial cable is
> connecting the source and the amps, which are on different circuit
> breakers. These ground-loop problems are caused by different ground
> potentials (perhaps only
> millivolts rms) of the two AC power circuits feeding the two systems
> (source/amplifier). When you connect these systems
> together with an unbalanced line-level coaxial cable, the outer shield
> of the cable connects to each chassis, and hence to the two AC grounds,
> resulting in a small AC current flow in the ground shield. In an
> unbalanced signal connection, this AC
> current flow in the shield becomes an audible signal: the 60-cycle hum
> you hear (and harmonics: 120-Hz...) In Europe and other places that use
> 50-Hz power you are, of course, hearing 50-Hz, 100-Hz...).
>
> You can convert an unbalanced signal to balanced, and this is just what
> is done with pro-audio gear, where you see the three-pin XLR connectors
> on everything.  Transformers can be used in this application, but
> generally don't have enough bandwidth or linearity for a high-fidelity
> application, so active systems are generally used (transformers are just
> fine for voice use).  An active system can also use a specific
> source/termination impedance (like 600-ohms), or can be a
> low-Z-source/hi-Z-termination design (which has less loss).
>
> Balanced audio cables are usually twisted pair, either unshielded (like
> 100-ohm CAT-5 cable), or shielded twisted pair, in which case the shield
> is usually connected only to the ground at the receiving end of the
> cable.
>
> Unbalanced and balanced audio systems use different references for their
> nominal volume level:
> The nominal level for unbalanced signals (eg: consumer cd-player) is -10
> dBV. The unit dBV, is a voltage level with respect to 1 Vrms. A nominal
> -10 dBV signal is 0.316 Vrms from center pin to ground (-10 =
> 20log(0.316/1)).  The nominal level for balanced (pro-audio) signals is
> +4 dBu. The unit dBu, is a voltage level with respect to 0.775 Vrms. A
> nominal +4 dBu signal is 1.23 Vrms from + to - terminals (+4 =
> 20log(1.23/0.775)).  Why 0.775V ref?  I have no idea.
>
> An older reference is dBm, which is 1 mW into 600-ohms -- this came from
> days when balanced signals used 600-ohm transformer coupling, a lossy
> conversion (which also limits bass response). Sometimes dBm is used in
> specs of balanced systems
> that are not 600-ohm impedance -- this is incorrect. Transformer-coupled
> systems use 600-ohm output and input impedances, for maximum power
> transfer over these balanced links.  Most electronic balanced audio
> systems today use a low output impedance and a high input impedance, for
> minimum voltage loss, and amplify/attenuate as needed for the
> conversion, that is, -10 dBV is amplified to +4 dBu, and vice-versa.
> This is not a 14-dB difference -- since there are different reference
> levels, there is electrically just an 11.8 dB difference.  
>
> It is easy to use opamps to build balanced drivers and receivers for
> audio;  one I did drives well over 1000-feet of CAT-5 with virtually no
> distortion or noise.  
>
> Buy you may not need to convert signals.  You PC output is unbalanced
> (the computer ground is the shield of the coax) and typically sees a
> load of perhaps 5K or greater -- if you connect this to a 600-ohm
> transformer input, you will get the benefit of the balanced input since
> the transformer's input side is floating.  The PC needs to be able to
> drive the 600-ohm impedance, which it likely can, or you could insert
> some resistance in the line to lighten the load on the PC's driver. 
> Whether there will be a compatible signal level is another story, but I
> would guess that you would be able to adjust the PC output to a decent
> volume to drive your gear.  If the PC cannot drive a 600-ohm
> transformer, you will need to insert a small preamp, but again, it does
> not necessarily need to have a balanced output if you are connecting to
> a floating transformer. 
>
> Your microphones are likely a different matter, and one that I can't
> comment on -- but I presume that some sort of preamp will be needed to
> drive a 600-ohm input.
>
> FWIW,
>
> gil
>
>
> greenkeys moderator
> gil at baudot.net
> www.baudot.net
> Vaux Electronics:  480-354-5556
>
>
>
>   
>> -------- Original Message --------
>> Subject: [GreenKeys] Far OT: audio inputs
>> From: <eugene at hertzmail.com>
>> Date: Sun, February 15, 2009 5:41 pm
>> To: <greenkeys at mailman.qth.net>
>>
>> I have an exciter (rf-1310) that has several audio inputs. Any audio
>> input can be selected for any type of mode (two different inputs are
>> selected for ISB mode).
>>
>> Two of the inputs are transformer input with 3 connections. 2 outside
>> and one center tap connection.
>>
>> Three of the inputs have differential amplifier inputs (op amps?) with
>> only two connections (basically inputs to the inverted and non-inverted
>> inputs to the op amp)
>>
>> All inputs seem identical in terms of expected input levels (0dbm
>> nominal into 600 ohms). So why would I choose one type of input over the
>> other? Whats the benefit/difference/proper application?
>>
>> The inputs I am interested in using are 
>>
>> a)       PC audio out 
>>
>> b)       A Sure 55S dynamic mic with High (100K ohms) Medium (150-250K
>> ohms) and Low (30-50) impedence selection 
>>
>> c)       A D104 mic non-amplified
>>
>> Do I need an amplifier beteween the mics and the 600 ohm 0dbm inputs of
>> this exciter? 
>>
>> So what do I need to know about audio inputs? 
>>
>> Thanks
>>
>> Eugene
>>     
>
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