Balanced Inputs for Likes of PIMETA Amplifier

http://www.jensen-transformers.com/datashts/10kbd.pdf

May be the easiest solution but a pair of them will be a little spendy.

You may wish to consider Twisted Pear Audio's Ventus headphone amp which has balanced inputs and single ended outputs. If you needed volume control on the amp, then this would need 4-gang attenuator (pot/stepped/etc) which can be pricey.

Or could use their Ballsie balanced to single ended converter into any single ended amp input. This would also mean that you could throw just a 2-gang attenuator between the ballsie and the amp, which will save you bucks and give you a much greater choice of attenuators.

I'm by no means suggesting you have to use TPA stuff, just they were the name that sprung to mind when thinking diy and balanced - as basically all of their stuff is balanced.

Also you're mistaken about the levels of consumer stuff being -10dBV. This is some standard that specifies this, but manufacturers of hifi gear don't pay any attention to it. The most common level of modern consumer hifi sources is ~2Vrms which is higher than pro levels.

Quote:

Originally Posted by MisterX 

http://www.jensen-transformers.com/datashts/10kbd.pdf

 

May be the easiest solution but a pair of them will be a little spendy.

Rather less spendy if you use a pair of CineMag CMLI-15/15B's.

http://cinemag.biz/line_input/CMLI-15-15B.pdf

se

Is the schematic linked what you're referring to when you say "Tangent recommends a Texas Instruments INA121 chip"?

http://tangentsoft.net/audio/tpm/misc/tpm11-sch.pdf

Quote:

Originally Posted by epithet 

DingoSmuggler brings up a great point that leads me to a question.  Assuming I design the "unbalancing circuit" into the PIMETA or other headphone amp, could I then use the potentiometer after unbalancing the signal, but before the input to the "stock/standard" unbalanced PIMETA?  

 

I also wasn't aware of Twisted Pear.  I just took a quick glance and will look further soon.  Interesting.  How is the sound quality compared to PIMETAs and such?

 

I am surprised to hear you say that about operating levels.  I am just pedantically quoting what I've heard for years and years.  But I certainly do see much weaker signals when I input consumer gear into pro audio inputs.  I have empirically seen it. But it's rare to go the other way--outputting pro gear into consumer gear inputs.  Maybe I'll try it soon for fun to see if it clips.  But anyway, we're probably both right--some is "truly" -10db and some is higher.

 

Steve/MisterX, How big do such transformers tend to be?  I sadly have no idea about their size.

 

Thanks.

 

Grant

99.995% of all studio gear operates as you describe - balanced cables, convert balanced to single ended as soon as it enters the box, do all "stuff" single ended. In the case of studio gear at signal level, the SE signal is then converted back to balanced to avoid noise pickup on its trip to the next box. Dont worry about that in an amp. 

Balanced outputs only matter on power amps with minimal global feedback, and in signal level outputs/inputs. Power amp circuits with lots of global feedback dont benefit from it as much as simpler stuff. Dont worry about it. 

Will the amp clip... I doubt it. The pot is before the first gain stage which makes the input voltage basically meaningless, unlike some newer designs. I'd be more concerned that the amp will have too much gain. Make sure that your chosen op-amp is stable at unity gain. This is actually a perfect application for a 10K:600ohm transformer, if you use a transformer for balanced-SE conversion. 

The cinemag transformers mentioned above are fairly small. 2 could just about fit in the plastic canister 35mm film comes in. Exact dimensions are on the cinemag website. Cinemag, and a few other people make 10K:600ohm transformers in a similar case size.

Quote:

Originally Posted by epithet 

I am surprised to hear you say that about operating levels.  I am just pedantically quoting what I've heard for years and years.  But I certainly do see much weaker signals when I input consumer gear into pro audio inputs.  I have empirically seen it. But it's rare to go the other way--outputting pro gear into consumer gear inputs.  Maybe I'll try it soon for fun to see if it clips.  But anyway, we're probably both right--some is "truly" -10db and some is higher.

The -10 and +4 values you quote are for 'Line' level I/O, that is, inputs and outputs that are specifically marked as 'Line In' or 'Line Out'. These normally have an output level which is unaffected by the source device's volume control. The 'standard' (not very well adhered to) for volume-controlled outputs is 2v rms max. I think that's a 'Red Book' recommendation.

The -10 is dBV. The +4 is dBu. These are obviously not directly comparable, 0dBV is one milliwatt in 1k (1v rms), 0dBu is 1mW in 600R (0.7746v rms). This is designed to prevent audio professionals (objectivists) from communicating with consumer audiophiles (subjectivists) and vice versa.

w

Oh, dBm's are used by radio amateurs and such, we don't want any of THAT here...

Quote:

Originally Posted by epithet 

I'm not.  That's one reason that in the original post I said I'm not interested in balanced output.

 

:facepalm: Thats what I get for skimming. 

Regarding the transformer:

You got most of it. You will probably need to add something though.

Depending on what value of pot you have and which transformers you get you will want a resistor in parallel with the WHOLE pot, so that the load on the secondary of the transformer is within design tolerances. 

As an example, with a 50Kohm pot, and the cinemag CMLI-15/15b transformers you will need a 22Kohm resistor in parallel with the pot to get a ~15Kohm load for the transformer.