Print

Print


Radio stations are detected at low level inputs by the stray inductance and stray capacitance [and of course the diode] at the base-emitter junction or equivalent.  This L and C resonate at some frequency often exactly at the frequency of a local broadcasting station.  One of the most effective ways of stopping this kind of interference is to insert a low value resistor, say 470 ohms, in series with the input leads and as close as you can get it to the base emitter junction of the input transistor.  This louses up the "Q" of the resonant circuit and reduces the sensitivity of the circuit to RF frequencies.  And of course shielding!  Ellis is spot on about the common mode rejection; even dual matched oscilloscope probes used in differential mode aren't very good at high audio frequencies.

Ron Roscoe

-----Original Message-----
From: Association for Recorded Sound Discussion List [mailto:[log in to unmask]] On Behalf Of Ellis Burman
Sent: Thursday, June 19, 2014 7:52 PM
To: [log in to unmask]
Subject: Re: [ARSCLIST] Upgrading Audio Systems

Re: Balanced pre-amps eliminating RF interference.

I think there is a common misconception that balanced audio circuits help eliminate RF interference.  Common mode rejection of typical balanced audio input circuits quickly falls apart above a few hundred kHz.  At typical radio frequencies, the circuit looks more like a bunch of diodes, detecting any RF that enters, whether common mode or not.

If there is any improvement, I think it is due to better design and RF shielding of professional balanced devices, not common mode rejection at MHz frequencies.

Ellis


On Thu, Jun 19, 2014 at 12:24 PM, Robert Cham <[log in to unmask]> wrote:

> Take a close look at the sine wave distortion of your UPS.  Many are 
> not sine waves at all. Computers may not care, but analog electronics do.
>
> Responding to another thread, back in the '80s, Straight Wire Audio 
> made a balanced phono preamp. It required rewiring your tonearm, but 
> it was a godsend for radio stations whose studios were in high RF environments.
>
> Think about it, there are two wires for each channel on a phono pickup.
>  One channels low is generally grounded.  that;s usually a strap that 
> can be removed.  Then you can feed it into a balanced mic preamp, and 
> all you have to deal with is the RIAA curve.
>
> Bob Cham
>
>
>
>  Do people ever use dual-conversion online UPS's for the purpose of
>> power-line isolation for audio equipment?
>>
>> The power is constantly going to the battery and running off the 
>> battery being reconverted back to AC.  Thus isolating the equipment 
>> from any fluctuations from the power mains.
>>
>> It's what I use for my computer room servers, and so far (knock on
>> silicon) I haven't lost a power supply yet.
>>
>> Depending on the size they can produce some heat and noise (from the 
>> fans cooling the rectifiers, etc) but if someone's considering 
>> rewiring an entire house/studio perhaps an online UPS would work as 
>> well for less investment, maybe in a nearby closet, etc.?  Not an 
>> electrical engineer so can't really compare the power they're 
>> outputting to the other options mentioned, but seems like it could be 
>> a reasonable alternative if power-line isolation is a serious concern.
>>
>> Arthur Gaer
>> [log in to unmask]
>>
>> Senior Systems Manager
>> Harvard University
>> Department of Mathematics
>> Science Center
>> 1 Oxford Street
>> Cambridge, MA 02138
>>
>>
>> On Jun 19, 2014, at 11:19 AM, Tom Fine <[log in to unmask]>
>> wrote:
>>
>>   I'm with Lou about powerline hash. You guys with troubles might 
>> want to
>>> swap out some of the "high end" components and see if they are 
>>> faulty designs at high prices. In the professional audio world 
>>> (where good gear doesn't come all that cheap, as a matter of fact), 
>>> proper shielding and grounding are a given and otherwise the gear 
>>> acquires a bad reputation. A properly grounded and shielded piece of 
>>> equipment should reject powerline and RFI noise completely. I also 
>>> question whether some "high end" cables are the problem? I never 
>>> noticed any technical tests with the "reviews" of the fancy wires, 
>>> and manufacturer claims are mostly rooted in hooey rather than science, with any semblence of scientific proof rarely provided.
>>>
>>>  Bottom line, a modern audio system shouldn't need a crazy dedicated 
>>> power plant unless there are regular fluctuations in power line 
>>> frequency and regular spikes and dips in voltage. I'm not 
>>> underestimating the incompetence of power companies, these situations are definitely possible.
>>> It's also possible to have insurmountable hash, for instance in a 
>>> space shared with an industrial welding company. But it's not a common problem.
>>>
>>>  -- Tom Fine
>>>
>>>  ----- Original Message ----- From: "Lou Judson" <[log in to unmask]>
>>>  To: <[log in to unmask]>
>>>  Sent: Thursday, June 19, 2014 10:10 AM
>>>  Subject: Re: [ARSCLIST] Upgrading Audio Systems
>>>
>>>
>>>   Ya know, when I sit in my van listening to music, I use the car
>>>> stereo, not the garage hifi. Besides, how can you use the garage 
>>>> stereo when it is raining and the windows are up? And if it is 
>>>> raining in your garage, close the washer lid.
>>>>
>>>>
>>>>  But what would I know. My entire system including computers runs 
>>>> on a single outlet, and never a noise I don't want.
>>>>  <L>
>>>>
>>>>  On Jun 19, 2014, at 7:11 AM, Shai Drori wrote:
>>>>
>>>>   You guys are way off. You need to have your repair man make the 
>>>> spin
>>>>> cycle go only counter clock wise in the northern hemisphere. 
>>>>> Gheez, do I have to teach you everything?
>>>>>  Shai
>>>>>  �(tm)���� 19/06/14 1:05 PM, -���� Carl Pultz:
>>>>>
>>>>>   I've tried fabric softener to enhance liquidity, but I prefer 
>>>>> the
>>>>>> real thing - EL-84s. Seriously, I have a home-brew tube amp for a 
>>>>>> system in the garage. It's nice to have reference to old-school 
>>>>>> hifi sound while having Benchmark/Bryston/Raal-Accuton modernity 
>>>>>> too. Reinforces what the guys were saying on the Headroom thread.
>>>>>>
>>>>>  >>>>
>>
>>>  On 06/18/2014 10:21 PM, Frank Strauss wrote:
>>>>>>
>>>>>>>  And with an extension cord snaked to the basement, the computer
>>>>>>>> reaches
>>>>>>>>  another dedicated line. In the laundry room. I came to this in 
>>>>>>>> stages,  having done the single circuit to minimize differences 
>>>>>>>> in chassis potential  idea for ages. That may be the ticket for 
>>>>>>>> tube systems. On my SS gear, this  sounds much better, 
>>>>>>>> particularly with the computer plug change.
>>>>>>>> Only
>>>>>>>>  problem with that is the soundstage gets unstable during the 
>>>>>>>> spin-cycle.
>>>>>>>>
>>>>>>>
>>>>>>>  �Hi Carl-Have you thought of trying different detergents?�  I 
>>>>>>> can't play an  lp during the spin cycle.  I'm trying to get my 
>>>>>>> wife to take the clothes  down to the stream and beat them with 
>>>>>>> a rock.
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>
>>>>>  --
>>>>>  Cheers
>>>>>  Shai Drori
>>>>>  Timeless Recordings
>>>>>  [log in to unmask]
>>>>>  ����"�
>>>>>  ~��"�¯�
>>>>>  ����"��~��¯��"ӯ"�~����"�����"����ү����8-35����
>>>>>
>>>>
>>>>
>> !DSPAM:639,53a33245225291644332558!
>>
>


--
Ellis
[log in to unmask]
818-846-5525