600 Ohm inputs

[Brian Roth]

Looking at THAT data sheets:

http://www.thatcorp.com/datashts/THAT_1606-1646_Datasheet.pdf

... with the 1206 (for a "net" of unity gain):

http://www.thatcorp.com/1200-series_High_CMRR_Balanced_Line_Receiver_ICs.shtml

...I see the parts are available in DIP8....which makes DIY easier for Olde Pharts <g> such as myself.....

Lemme see what Mouser stocks.....

Best,

Bri

 

[Brian Roth]

You can probably satisfy the most common interface problems from active balanced drivers with a simple dip switch, so the end user worst case may have to try a couple options until they get a good result. The circuitry can be made reasonably bulletproof so it will tolerate shorts or opens where they shouldn't be, without failure.

JR

John, In A Perfect World <g>,  the end user should NOT be required to fuss with DIP switches!  Hence, my "quest".

IOW...just poke-in the patch....

Sooooooooooo, I am thinking along of the lines of a universal "input fixer-upper"....

Best,

Bri

 

[emrr]

There's a lot of interesting bench work in this thread:

http://www.proaudiodesignforum.com/forum/php/viewtopic.php?f=6&t=464

 

[JohnRoberts]

You can probably satisfy the most common interface problems from active balanced drivers with a simple dip switch, so the end user worst case may have to try a couple options until they get a good result. The circuitry can be made reasonably bulletproof so it will tolerate shorts or opens where they shouldn't be, without failure.

JR

John, In A Perfect World <g>,  the end user should NOT be required to fuss with DIP switches!  Hence, my "quest".

IOW...just poke-in the patch....

Sooooooooooo, I am thinking along of the lines of a universal "input fixer-upper"....

Best,

Bri

My bad.. I was thinking in terms of outputs. a good input stage will generally work despite pretty much all attempts to mis-wire or mess with it.
=====
Only on the output side, can you encounter random opens or shorts in places that would be nice to avoid. While you can pretty much make a solution that works for most cases, I would personally like to avoid making custom cables for dumb product interfaces. I've encountered both completely open and dead shorts on various + or - audio inputs... you can work around the dead short, but the floating open sometimes causes level errors and weirdness, even with transformer outputs (unless the transformer has a shorted centertap, and even transformers don't like dead shorts, unless you use large build out R's

So perhaps an over sized,  center tap grounded, transformer output, with 600 ohm series buildouts,,,, too much cost and effort IMO, I'd add the dip switch, or make the user re-wire the dumb ass offending oddball gear (the right solution). 

of course opinions vary... but the universal input side is much easier, just provide a differential input... most problem you will see with that being mis-wired is a gain error.

JR

 

[Brian Roth]

Egads...here I go again....


Seeking a "buffer'' to  pull a signal in a tooob device....requiring perhaps 1 Meg load on the
input, yet ability to drive (yikes!) anything in the patch bay...

Apologies.....

Bri

 

[AUDIO FREQ]

It says on the data sheet that the LME49740 can "easily drive 600 ohm load". But it seems they are all sold out. Anyone know of any good alternative 4 channel op amps that can drive 600 load? I want to add some transformers on the input and output of the ADA8000. I have some tamura transformers sitting here. Would like to put them to use

 

[ccaudle]

It says on the data sheet that the LME49740 can "easily drive 600 ohm load". But it seems they are all sold out. Anyone know of any good alternative 4 channel op amps that can drive 600 load? I want to add some transformers on the input and output of the ADA8000. I have some tamura transformers sitting here. Would like to put them to use

Do you have any equipment with a 600 Ohm input impedance? That would be very unusual unless you have either antique equipment, or equipment which slavishly duplicates designs from mid-1960's or earlier.
Transformers reflect the impedance from one winding to the other, so do not be mislead into thinking that a transformer designed for 600 Ohm systems always implies a 600 Ohm load on the driving side.

Do you have an existing footprint that you have to use? If not you could use two 2-channel op-amps, might have a wider selection.

 

[Walter66]

Transformers reflect the impedance from one winding to the other, so do not be mislead into thinking that a transformer designed for 600 Ohm systems always implies a 600 Ohm load on the driving side.

Audio Signal transformers are frequency optimised for a given input and output load. In this case 600 Ohm, the old telephone line standard for amplifiers.
If loaded or terminated with other impedances unsuitable, they may mirror the same ratio, but can degrade in audio measurements. Am I right or wrong on this topic?

 

[user 37518]

Audio Signal transformers are frequency optimised for a given input and output load. In this case 600 Ohm, the old telephone line standard for amplifiers.
If loaded or terminated with other impedances unsuitable, they may mirror the same ratio, but can degrade in audio measurements. Am I right or wrong on this topic?

Right, the winding inductance makes the most difference

 

[AUDIO FREQ]

Just wanted to add an output transformer that's 600:600 to a microphone preamp that has an op amp output.

 

[user 37518]

Just wanted to add an output transformer that's 600:600 to a microphone preamp that has an op amp output.

Shouldn't be a problem in most cases, if the op-amp has decent driving capabilities (e.g., Ne5532) it should be fine. You can try the negative resistance approach to cancel the transformer primary resistance and thus reduce transformer distortion.

I don't know of many quad BJT op-amps with specs similar to the LME49740. The LME4562 is essentially the same as the LME49740 but a dual rather than a quad version. The MC33079 is a quad BJT op-amp, but not as high-end as the LME49740, still, I think it is pretty good. IIRC, gyraf (Jakob) uses it to drive transformers and he likes it. The OPA4134 is also a quad, but it is a FET rather than BJT op-amp. It could work depending on your requirements.

 

[AUDIO FREQ]

Shouldn't be a problem in most cases, if the op-amp has decent driving capabilities (e.g., Ne5532) it should be fine. You can try the negative resistance approach to cancel the transformer primary resistance and thus reduce transformer distortion.

I don't know of many quad BJT op-amps with specs similar to the LME49740. The LME4562 is essentially the same as the LME49740 but a dual rather than a quad version. The MC33079 is a quad BJT op-amp, but not as high-end as the LME49740, still, I think it is pretty good. IIRC, gyraf (Jakob) uses it to drive transformers and he likes it. The OPA4134 is also a quad, but it is a FET rather than BJT op-amp. It could work depending on your requirements.

Any of those are better than the TL074 I am assuming?

 

[user 37518]

Any of those are better than the TL074 I am assuming?

The TL074 is a JFET-input op-amp from the 70s, and its driving capabilities are not rated for 600 ohm loads.

 

[AUDIO FREQ]

The TL074 is a JFET-input op-amp from the 70s, and its driving capabilities are not rated for 600 ohm loads.

I'll definitely be swapping the TL074C for the MC33079 then. Thank you.

 

[My3gger]

I'll definitely be swapping the TL074C for the MC33079 then. Thank you.

Compare current draw, possible caps for stability and other specifications of new op amps before replacing the old ones.

 

[user 37518]

I'll definitely be swapping the TL074C for the MC33079 then. Thank you.

No, it doesn't work like that, the TL074C is a FET op-amp, if it is being used in a circuit designed for it you can't just swap it willy-nilly with a BJT op-amp like the MC33079, since the impedances are probably optimized to be used with a FET op-amp, and the result might be detrimental in terms of noise, offset, stability, and other things. The design goals should dictate which op-amp you use, one size does not fit all (if it is only to drive the transformer, the MC33079 should be fine)

 

[nsalis]

And as also previously discussed there are two standards... "pro" or wannbe pro, have driven 600 ohm terminations as long as I can recall... While many value products, don't anticipate encountering Low input Z gear.

There is a huge difference between being 600 ohm capable at the output, and expecting the rest of the world to be 600 ohm capable to drive your input.

FWIW the Tangent board that Brian is ranting about, is perhaps a design flaw. I was designing studio gear around the same time, and my gear often used discrete transistor buffers on the major output lines so I could drive parallel 600 ohm loads. Unbalanced TRS inserts are another story. There was little expectation that unbalanced inserts would drive long lines or 600 ohms, while there were lots of bump boxes around back then to interface all the bedroom recording gear's -10dBV level up to normal studio level. These bump boxes have pretty much gone away with the low end recording gear, but some are still available. http://www.peavey.com/assets/literature/manuals/ia10_4.pdf Note: from looking at the picture, this has been repackaged since the original version I designed back in the '80s, so I have no idea what the internal circuitry looks like. The data sheet does not say that it drives 600 ohms (my old one did) so caveat emptor.

I suspect a premium version if that old stereo bump box, using THAT chips could satisfy this niche market need.

JR

Small correction to your link information. The correct link is:

https://assets.peavey.com/literature/manuals/ia10_4.pdf
or

https://tinyurl.com/2ddj2jpw

 

[nsalis]

OK, then hack them to be essentially unity gain....

Bri

Not necessary. Connect the RCA Out jacks to the RCA In jacks and you have a unity gain buffer amplifier with XLR in and out.

 

[nsalis]

Egads...here I go again....


Seeking a "buffer'' to pull a signal in a tooob device....requiring perhaps 1 Meg load on the
input, yet ability to drive (yikes!) anything in the patch bay...

Apologies.....

Bri

Or like JohnRoberts put it: "A universal High input Z, low output Z, low drive Z black box could be a nice fixer circuit."

I had also wished for such a device. Surprisingly, there doesn't seem to be one on the market. The closest is perhaps the Sonifex RB-Li2.

https://www.sonifex.co.uk/redbox/rbli2_ld.shtml
Its input impedance is missing from the datasheets, but Sonifex told me it's 100k ohms.

This is high enough to work with any conceivable line level (4 dBu) or consumer level (-10 dBV) source. Your requirement that the input impedance should be 1M ohms could only be justified if you wanted to use old consumer (not professional) tube equipment, e.g. an FM tuner or reel-to-reel recorder, as a source. Such devices sometimes usually have high-impedance outputs and sometimes expect input impedances of the downstream devices of at least 500 kOhm at the output.

 

[ccaudle]

Just wanted to add an output transformer that's 600:600 to a microphone preamp that has an op amp output.

Don't you have schematics for ADA8000? This would be a lot easier to discuss with specifics.
This post has the schematics for the I/O circuitry of the ADA8000:
Behringer ADA8000 I/O circuit schematics

 

[ccaudle]

Seeing if I can attach here to make easier to discuss.

 

[ccaudle]

The first pair of amps in the quad are the diff-to-SE and image filters for the DAC, so you can't just swap bipolar amps into there without evaluating the effect on the filter components. Might work, but would need to evaluate.

You can probably just put the input transformer at the input of the mic-amp, but you would need to change the routing of the phantom power of course if you need to use that.

What equipment are you driving? If it has a high impedance input you could probably just stick the output transformers on the existing output, you don't need devices capable of driving 600 Ohms if you aren't driving a device with 600 Ohm input.

 

[AUDIO FREQ]

Thank you for the posts, but my post here isn't about the ADA8000. But very informative thank you.

 

[ccaudle]

I want to add some transformers on the input and output of the ADA8000.

but my post here isn't about the ADA8000.

Very confusing response.