converting tape loop to mono switch

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stonedeaf

converting tape loop to mono switch
« on: 10 Jan 2017, 04:53 pm »
one of my oldest audio buddies ( I started selling him records in 1968) -has a functional gripe with his phono equipped BP-26. As is obvious if you look at the front panel switches  - the phono selectors substitute for the stereo/mono switch in this version.This guy owns the largest private record collection I am aware of - including several thousand LP's (and 78's) that were never released in any stereo format. He's irritated that Bryston hasn't given him a mono switch. Over the years - i have "given" several record collectors mono switches by simply taking two y-plugs ( one with single male RCA to two RCA males -second with a single RCA female to 2 RCA males) -these then are plugged in from tape in to tape out - when you switch from source to tape -tape is blended mono. I've done this a few times over the years - the only issue is does the tape loop have buffers on it's output ? This is probably a Mike question - but can you think of any reason why this wouldn't work on any of your more recent pre-amps ? 

mix4fix

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Re: converting tape loop to mono switch
« Reply #1 on: 10 Jan 2017, 05:23 pm »
Do not combine line audio without any isolation. And it doesn't matter if you haven't had any problems.

http://www.mobileaudio.com/rac-faq/rac-faq_5.html#SEC106

"Creating a mono signal is often necessary when you are powering a subwoofer by bridging the amplifier. Many people do not realize that bridging an amplifier does not always provide a mono signal -- many amplifiers will simply use only one input channel, which means that the subwoofer won't be receiving the full signal.

Some amplifiers have a switch that will allow you to combine the left and right channels into a mono signal. Some signal processors and head units provide a subwoofer-out channel that can be switched between stereo and mono.

If you don't have this feature on any of your equipment, you will need to provide a mono signal to the amplifier. The common thought is to use a Y-adapter to "combine" the left and right channels. However, by using a Y-adapter, you are actually summing the line voltages and directly shorting the left and right channels at the head unit, which could cause problems.

The correct way to create a mono signal is to cut off the ends of the RCA cables, combine the signal grounds (the outer shield), and then use a 1 kOhm (1/4 watt, 5% tolerance) resistor to each of the center conductors. Solder and insulate the resistors so that you don't short them prematurely, and then connect the two resistors together. Connect the summed signal ground to the shield of the new RCA plug, and the summed center conductor to the center pin of the RCA plug."

Basically, your center pins need to be isolated by those resisters. I made a box long time ago using Radio Shack project box and RCA jack panel so I didn't need cut cables and it was easy to build. After you have a true mono signal, you can split it to dual mono.

stonedeaf

Re: converting tape loop to mono switch
« Reply #2 on: 10 Jan 2017, 06:48 pm »
Question really is - are the tape outs buffered (isolated) on the newer Bryston Pre-amps ? Car stereo head units -can't say - never tried it on those ?

mix4fix

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Re: converting tape loop to mono switch
« Reply #3 on: 11 Jan 2017, 08:23 am »
It's all line audio.

stonedeaf

Re: converting tape loop to mono switch
« Reply #4 on: 11 Jan 2017, 03:44 pm »
if you look at the schematic for the tape output on a BP-26 (same on my BP-25) -there's a couple of resistors and a small value capacitor going to ground right before the RCA jack output - there's also a capacitor in series with the output of the discrete opamp ahead of the output. Zobel network ? Buffer ?
BTW- we always note the  20 year warranty (and the quality of the assembly and parts that make this  a reasonable business practice ) - but how many other manufacturers today publish the schematics for their current products ? Used to be common practice -heck - some electronics came with a schematic as part of the owner's manual - not today. Another aspect of what makes Bryston exceptional.