Simple unbalancing and balancing line amp

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thomasdf

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And if you are going to do that, might as well get one with buffers so that the CMRR is better, like INA1650 or INA1651.
Hard to have more simplicity than one device.
Wow ! I had never heard of that chip, it looks like a gift from heaven ! That's exactly what I need in a single component package ! And it's even stereo. Thanks !

For the input, yes, why not, but I wouldn't recommend that for output.
For the output I meant a pair of 1:1 Edcor right after that single 5532. It seems simple, space isn't a issue, but I don't want to use 4 transformers on such "low end" device. This sounds like a good in between solution.
I just need to figure out how to make gain changes with a pot in that circuit.

Feels like I'm getting close, thanks !
 

FIX

Paul Wolff
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The 2520 is not like any op-amp, though I remember someone from Signetics saying that they tried to make a chip out of it (5534) but it wasn't stable. Many op-amps say they are the same but aren't. Their is one that is, and it's spelled backwards... It's a high gain design that if modeled on spice is latched to the negative rail, in real life it always has negative offset... The slew rate (in several configurations) is only 2.5V/usec, but with a 1:2 transformer, that doubles...
 

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Paul Wolff
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I went for simplicity : I took out my "Small Signal Audio Design" book by Douglas Self, and got interested in these : what's your opinion ?


Input :

View attachment 97341


Output, 2 options :
View attachment 97340 View attachment 97342
In both option, I'll see how I can make gain variation, i'll study the 5532 datasheet. 2nd option is almost too simple to be true, it def has my vote for the moment :)
I used to use servo DC offset amps in the API stuff, they aren't worth it unless you have a lot of offset. The design will actually put DC on the input if you don't have caps, so you might as well have a cap on the output. They do seem to effect transients sometimes. The slower you make them, the longer the console swings back and fourth until they settle.

I use the THAT 1256 on the input, which sounds really nice, and 2) 1206 with +/- reversed on the second one for balanced outputs. Yes, if you short one side you lose half, but they sound better than the 1646 output thing, which I've never liked.

No record ever failed because someone picked the wrong input stage...
 

thomasdf

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Hey Paul, thanks for chiming in !

I am confused again now. Are you saying the THAT 1256 is preferable to the TI INA1650 for the input ?
I see the 1256 has -6dB gain, I assume this is to compensate for the unbalancing of the signal, to keep an overall unity gain "feeling" ?

Also, if I understand correctly, you recommend that I use a THAT 1206 for the output. I've read the datasheet and I feel like I won't be able to play with the gain of the chip and have, say, 20dB gain to compensante for the my unit's gain reduction. If my assumption is correct, then I should be stick with the Douglas Self design using the 5532 + transformer, right ?

No record ever failed because someone picked the wrong input stage...
Haha, true !
 

JohnRoberts

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I used to use servo DC offset amps in the API stuff, they aren't worth it unless you have a lot of offset. The design will actually put DC on the input if you don't have caps, so you might as well have a cap on the output.
the benefit of servo amps is the ability to use well behaved small film caps , in place of less well behaved large electrolytics.
They do seem to effect transients sometimes.
Not if properly executed. A good servo topology involves a passive LPF, so extreme HF is scrubbed harmlessly.
The slower you make them, the longer the console swings back and fourth until they settle.
not if they are well damped and stable..
I use the THAT 1256 on the input, which sounds really nice, and 2) 1206 with +/- reversed on the second one for balanced outputs. Yes, if you short one side you lose half, but they sound better than the 1646 output thing, which I've never liked.

No record ever failed because someone picked the wrong input stage...
Indeed....

JR
 

ccaudle

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Are you saying the THAT 1256 is preferable to the TI INA1650 for the input ?

That 1256 is just a diff-amp with trimmed resistors, same as INA134. Any kind of diff-amp based input is going to be very sensitive to any imbalances on the drive side impedance. The INA1650 will have better CMRR with slightly unbalanced drive. The That InGenius parts (120x family) will have even better CMRR robustness, at the cost of a couple of extra components, maybe slightly higher price.

I'm fine with soldering some SMD, but OK

Don't let the fact that Abbey's eyes are aging out of being able to work on TSSOP dissuade you. ;)
I have to use strong magnifiers myself, but can still solder them for the time being.

I see the 1256 has -6dB gain

Most of the parts come in variants with 0dB, -3dB, or -6dB. Pick the one that works for your design based on the gain structure that you need.

I feel like I won't be able to play with the gain of the chip and have, say, 20dB gain to compensante for the my unit's gain reduction

For any of the integrated balanced line drivers you will need an additional op-amp stage if you want adjustable gain. You should have an additional op-amp as a buffer anyway if your input signal is not being driven directly by an op-amp output. It isn't clear if you are going to tap into the internal circuitry, or just connect this to the existing output signal. If the latter, outputs usually have a build out resistor, so you would need to buffer that before driving any of the integrated differential output devices.

Of course just using impedance balanced output with asymmetric drive is a valid choice as well. That hasn't really come up, but has been discussed plenty of times on other threads. With 12V power supplies you should be able to get to about +18 dBu to almost +20 dBu depending on the output amps you use. You never really defined your level requirements, so not clear whether that is enough for you or not.
 

thomasdf

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Thanks. I don’t want to modify the DBX118 too much, so I’ll just use its stocks RCA input and output.
Before the unit’s input : I want a simple unbalancing stage with 0dB gain.
After the unit’s output : I want to be able to add gain from 0dB to about 20dB, and then balancing. +18dBu is plenty enough
 

abbey road d enfer

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I'm fine with soldering some SMD, but OK. Do you suggest I go with the THAT 1256 Paul is talking about ?
The 125x series is a downgraded version of the 124x. I've never used the 125x, always the 124x. The only significant difference is the nominal CMRR is 50dB instead of 90. It may seem a lot, but in a studio environment, typically 40dB is enough.
 
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