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In case anyone cares or wonders what happened in a future search of the AA archives.
I was able to try the experiment using my SL1200 and two Cambridge 651P's along with some custom adapter cables I made.
I just couldn't get rid of the hum wired up this way. It wasn't especially bad hum but I wouldn't say running like this bought me anything though. I really can't see any point in leaving it like this now that I tried it.
I tried and tried getting the hum to go away Even despite my adapter cable build keeping everything STP in the cabling as much as possible only utilizing short as possible stubby STP + XLR to coax + RCA y cable converter adapter cables right at the connectors on the back of the phono pres.
I feel like if there was any advantage to running like this I would have seen it. I'm abandoning the concept now.
Follow Ups:
-Thats one of the 'things' about balanced!
The easy way to do it of course is to run balanced from your tone arm to input transformers, which then convert to single-ended operation for your phono section. Any SUT and any transformer can do this.
To get two mono single-ended phono sections to behave as a balanced phono section, about the only way I can think of involves an input transformer to do the phase inversion.
IMO though, you would have been better off to get an actual balanced phono section, as it would work properly and would not have sent you down the rabbit hole.
Thank you for your insightful posts!
In hindsight it seems so obvious to me and I feel so foolish now...
I made the mistake of tying each of the preamp signal ground to pin 1 instead of leaving it floating.
I don't know what I was thinking about.
As your previous post suggests this is not how balanced is done!!!
I need to rerun the experiment.
I'm especially feeling the pain since right after I posted yesterday I thought the experiment was over so I decided to switch my SL1200 back from shielded twisted pair cables to coax cables. Anybody who knows how much work it is to pull apart the SL1200 chassis to change wires knows how much fun I'll be having doing it all again to rerun the experiment. Grr!!! probably somewhere around a half an hour just to swap out tonearm cables probably. Grr. I like to do things the hard way sometimes.
Maybe I'll get back to it next weekend.
You don't need to rewire anything on the Technics SL1200. All you have to do is construct a different cable to go from the arm to the preamp.
The specs for doing this is in the owner's manual for our preamps which is a free download- but the important thing is that you don't allow the barrel connection of the RCA to come in contact with the shield of the cable, which ties to the ground post on the 'table, and to pin 1 of the XLR input on the preamp side (which is also ground).
I probably mis-spoke. What you recommended is in fact all I've been doing, ie swapping out the interface cable that solders to the little circuit board inside the TT. I had previously had some shielded twisted pair installed for the experiment then put some coax on there.Just a little whining. No show stoppers.
I swap between twisted pair and coax depending which preamp I'm playing with at the moment. I didn't mention earlier I also have a full balanced preamp I'm playing with. The problem with this particular balanced phono stage is the noise. It's self noise, at equivalent gain, is around 5-10dB higher than where the hum of the 651p's are sitting which is also 5-10dB above their self noise. In other words the balanced performance of the balanced preamp is offset by it's crummy electronics design. Though it has no apparent hum visible in the FFT scans I've captured from it, it's wideband noise is pretty massive.
Edits: 05/31/17
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Yes, the noise is present with cart connected. I also terminate unused inputs with shorts.
I like to do my noise testing and troubleshooting with the equipment hooked up as I plan to use them.
The coils in my moving magnet cartridge seem to pick up a bit more noise than when I just use shorting plugs but not too bad really.
and ultimately built myself a balanced phono section. As far as I know, it was the first in the world. The preamps we make were the first balanced preamps for home audio. Seems normal now, but back then they were kinda weird.
Balanced is mostly a Sales gimmick in Audio shops.
Unless in a Very RF noisy industrial studio setup there is Small (agruably Zero) advantage to Balanced.
Also tricky to get balanced carts :-)
First, balanced operation has the advantage of reducing the artifact of the cable, regardless of length. If you've ever had to audition a single-ended cable to find one that sounds right, you might then understand what a blessing this can be.
This means if you do it right, no need for a boutique cable. Audio shops don't like that, so there you are.
The second is that all magnetic phono cartridges are balanced sources. Don't believe me? Go ahead and switch the phase on one channel of your setup, and see what happens. If you did this with a true single ended source (like a tuner or single-ended CD player) the result would be a loud buzz. With a cartridge all you did is invert the phase with no other consequence. Balanced sources do not employ a center tap- no need for matched resistors to simulate that as implied elsewhere on this thread. Balanced operation ignores ground, so what you really want is what the cartridge really is- a floating source.
Here's a handy page on balanced operation:
"no need for matched resistors to simulate that"
Wouldn't you need that center point voltage to use as a reference for things like RIAA filter reference if, for example, you were doing a full active riaa stage?
I just meant to point out that even with the coils true midpoint buried in the middle of the cartridge coil it isn't impossible to know the voltage, or at least come very close.
is what you are looking for.
In the case of the RIAA network, it can be done differentially so you don't need dual matched resistors there either (although you do want them to be correct for the right curve of course).
For example in our preamp the EQ network calls for dual 110K resistors in one location but they can be replaced by a single 220K resistor. In this way if one half of the balanced circuit does not perform as well as the other half, the EQ is unaffected.
The prior example is also a bit of myth-busting about the idea that you need twice as many parts as single ended; obviously you don't; our EQ network has the same number of parts as a single-ended embodiment, despite being balanced differential.
I came up with another balanced circuit puzzle.A front end circuit I'm playing with is using a servo in order to eliminate ultra low frequency so I can DC couple it.
Is there a better way to derive a servo reference point than the two matched resistor method?
Edits: 06/01/17
What the ' two matched resistor method ' is. Do you have a schematic?
As a tip, if using a servo, I think you will find that 2 poles in the servo's operation will work a lot better than one!
My servo attempts to bring the preamps output offset to the same potential as the midpoint of the cartridge coil by stripping out the low frequency from the output and (negative) feeding it back.
All I meant by "two matched resistor method" is by placing two matched resistors in series across the coil outputs, the midpoint between the resistors may be usable as the servo reference.
Unless there is some better way to find the cartridge midpoint than using two "matched" resistors which will never actually be matched in the real world....I'd love to find a better way but have not come up with anything.
Sorry that was unclear.
I'm at work now. I can hack up a schematic when I get home and post it.
My servo circuit uses more than two poles. It is based on a weird circuit I found on the internet in an old LT app note and mixed in a bit of my own special sauce but all seems to simulate nicely. I get very deep audio band rejection but takes maybe a bit too long to settle. I will need to fine tune it when I get hardware in hand, ie this is all simulation at this point.
Just use ground as a reference for the servo, so the DC output is 0V.
As far as the input, each preamp side will have its own resistor to ground so that the preamp can function; that value should be 23.5K so that the load on the cartridge is 47K.
Thank you for your replies. They are very much appreciated. I had some ideas about things to try but it sure instills confidence to talk to someone who has tried some of this stuff.The plan all along has been to make it easy to try either ground referenced or floating referenced servos via jumper setting.
My desire to leave the servos referenced to a floating , non ground node was triggered by these fully balanced/differential op amps, apparently targeted at the audio market, such as the OPA1632.
These parts have integrated servos to set output common mode voltage. The app notes suggest common usage being tying the Vocm pin to the ADC reference. To my mind this implies there are must be audio market ADC's out there using external non ground voltage references.
My concern is that if I wanted to use one of these ADC's and I had my front end servos referenced to ground I may exceed the common mode range of my of my output stage under certain large signal swing circumstances.
My thinking was that floating front end servo references or tying it to my own version of a Vocm reference pin and an output stage that allows setting output common mode voltage (similar to OPA1632 et al) gets me around that problem.
Edits: 06/03/17 06/03/17 06/03/17 06/03/17
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Well, building may be putting it just a bit strong. I just sit around using gigawatt/hours simulating then changing my mind and then do that some more.
I think I might be getting close...but I've said that before and then it evolves.
At the moment I'm fairly excited about some breakthroughs I'm having on the bass processing circuitry I'd like to include.
It's always something.
Who knows, maybe someday I'll start laying out a PCB. haha
Zoooo.. a daydreamer speculating on what if.. and a man with a horse in the race .. hoping to make 10s of $.
Not the most enlightening of threads.
And yet you read to the end.....lol
I would guess the general life misery level must be pretty high for someone as apparently clueless to feel the need to comment in such a worthless manner.
I hope things turn around for you soon.
"Suddenly, I'm not half the man I used to be. 'Cause now I'm an amputee" J. Lennon
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"Unless in a Very RF noisy industrial studio setup there is Small (agruably Zero) advantage to Balanced."If it kills the hum, and other emi, what's not to love?
"Also tricky to get balanced carts :-)"Really? Just because the electrical midpoint is tricky to access does not mean it doesn't exist.
For example what is wrong with trying to exactly split the difference with a couple of highly precision matched resistors?
Edits: 05/30/17
Sorry about the small details. You can really zoom in on the picture though.This the system with noise floor with preamps running, turntable attached via adapter cable, arm sitting in rest position, motor shut off, system gain set so a reference signal from my denon test disk sits around -10-12dB. The 651Ps got to 2X Millenia HV 35P's which feed a MyteK Stero192 ADC.
It's not as if noise is bad like this. It just doesn't buy me any performance over running two 651P's in single ended configuration into the mic premap intrument input. It's an identical picture, but I can eliminate these crummy wye cables I cobbled together.
Noise wise these 651'ps rock. It's just that this so called balanced configuration isn't buying me anything I haven't been able to get to get from them via simpler configurations.
Edits: 05/28/17
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