Simple unbalancing and balancing line amp

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thomasdf

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Hello
I have been looking around but haven't found much. I am looking to build a simple stereo unbalancing line amp and a simple stereo balancing line amp to use before and after my DBX118 in a balanced environnement. The idea is also to be able to play with the gain a bit, like this : input amp would mostly need attenuation or very little level boost, and output amp would mostly need amplification to provide makeup for extreme comp settings.
Since the comp is already pretty colored soundwise, I'm looking for something clean.
Anybody know some schematics ?
I could also use a board like this and tap some gain pots ?
 

ccaudle

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I have been looking around but haven't found much. I am looking to build a simple stereo unbalancing line amp and a simple stereo balancing line amp to use before and after my DBX118 in a balanced environnement.

These would be the best starting points:
SparkFun That 1206 board
SparkFun That 1646 board

If you click on the "Documents" tab there are schematics and Eagle PCB files.
 

abbey road d enfer

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Anybody know some schematics ?
This subject has been discussed many times here. Look for something with SSM2142, That 1646
I could also use a board like this and tap some gain pots ?
From what I can see, this is only a double balancer that needs to be driven by a very low impedance source, so if you want a potentiometer you must add a buffer in-between.
The Sparkfun 1646 has exactly the same issue; for good CMRR, it must be driven from an active buffer.
This particularity of several monolithic balanced line drivers is often neglected by "designers" (read PCB drafters).
There is another model that uses the DRV134
that is more tolerant, however the gain is only 6dB. It may or may not suit you.
 
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thomasdf

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You are right !

Regarding the "bump boxes" it seems that a pair of JLM Dingo might do the trick with RV2 replaced by a stereo 20K pot. I just can't find how much gain it is able to put out.



edit : some of you may think that its a lot of work for such a cheap compressor, but I realised I'm really in love with this unit on my mixbus for some projects... instant vibe and harmonic excitement :)
 

Gold

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If you aren’t planning on using discrete op amps I’d go for the Dingo Pup. It’s the same circuit on a smaller and less expensive PCB.

The output stage can be set up for any gain you want. I think it will work unity gain but I haven’t tried that.

The input is a simple four resistor single op amp balanced input. Not super great CMRR.
 

abbey road d enfer

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Regarding the "bump boxes" it seems that a pair of JLM Dingo might do the trick with RV2 replaced by a stereo 20K pot. I just can't find how much gain it is able to put out.

:)
This circuit uses the dreaded non-floating output arrangement that prevents using it correctly with unbalanced inputs. For unbalanced connection, you have to disconnect the - output, which makes it essentially an unbalanced output.
Check for "Tascam problem".
 
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Matt Syson

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The 'simple'diff amp 4 resistor input can be pimped to better CMR simply by fitting closer tolerance resistors or by buying ordinary 1% and matching them yourself using a decent resolution multimeter. You are looking for 'matches' (in quads) rather than absolute accuracy. A huge percentage of the professional audio industry manages with simple diff inputs like this and the output balance is also not really too critical. Besides this very few balanced inputs and outputs are really that good over a wide frequency range as it is easy to 'unbalance' at high frequencies at which point any form of amplifier stage will struggle to maintain CMR board layout and internal wiring from input connectors to a board can become significant. Balancing (and it's interference rejecting properties) while a great concept is actually pretty difficult to achieve as a complete 'system' using real parts.
 

thomasdf

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Erf, I am a bit lost now. Dingo Pup sounds pretty good ... if I use a 1:1 transformer on the output, will it solve the "dreaded non-floating output arrangement" problem ? I still have to figure out completely what this is, I'm learning every day 🤓
 

abbey road d enfer

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Besides this very few balanced inputs and outputs are really that good over a wide frequency range as it is easy to 'unbalance' at high frequencies at which point any form of amplifier stage will struggle to maintain CMR board layout and internal wiring from input connectors to a board can become significant. Balancing (and it's interference rejecting properties) while a great concept is actually pretty difficult to achieve as a complete 'system' using real parts.
That's why I often insist on qualifying balanced connections. Proper balance requires action on both the driving and receiving ends, as well as the cable in between.
 

thomasdf

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Yes it would, but is the overcomplicated circuit necessary for driving a xfmr?
If you're willing to use a xfmr, you just need a simple buffer to drive it.
Regarding the input, I just need a clean unbalancing and maybe some buffering in order to feed the comp a clean signal. As for the output, I need a clean balancing and makeup gain (20db tops) in some situations. I have a bunch of transformers lying around ... maybe 4 transformers for a DBX118 is a bit overkill, but I don't mind using a pair on the output if it makes interfacing easier.

Edit : Aphex 124 looks like what I need, but the gain adjustement is +/- 6dB ... I some situations I will need more output gain. Here's the schemo. To my eyes, the output looks similar to JLM Dingo Pup's output.
 

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abbey road d enfer

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Aphex 124 looks like what I need, but the gain adjustement is +/- 6dB ...
This is not a major issue. I think it's not SMD, so it shouldn't be too difficult to fix this.
To my eyes, the output looks similar to JLM Dingo Pup's output.
No. It's different; it is the well-known cross-coupled arrangement that works correctly with unbalanced receivers as well as balanced.
 

thomasdf

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Well Aphex 124 is pretty hard to find second hand 220V, and I don't want to spend that much money for a new one ... so if I decide that it's the perfect circuit and that the max gain can be tweaked easily, I think I'll just clone the circuit and etch my own boards !
 

Gold

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This circuit uses the dreaded non-floating output arrangement that prevents using it correctly with unbalanced inputs. For unbalanced connection, you have to disconnect the - output, which makes it essentially an unbalanced output.
Check for "Tascam problem".
I was under the impression that this was for a small home setup. The only time this arrangement is a problem is when the negative side of the signal line is shorted to ground. That scenario can accidentally happen in a patchbay. If there is a patchbay it’s a bad idea. If the only way it will ever be connected is balanced it’s arguably a better solution than cross coupled .
 

abbey road d enfer

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Well Aphex 124 is pretty hard to find second hand 220V, and I don't want to spend that much money for a new one ... so if I decide that it's the perfect circuit and that the max gain can be tweaked easily, I think I'll just clone the circuit and etch my own boards !
Another possibility is finding a second-hand SCV Universal Buffer. It's a card system. There are dual input cards (electronically balanced or xfmr-balanced), dual output cards (xfmr balanced) and combined input/output cards.
I sold tons of them in the 80' and 90's. Almost unused these days, you can find them for the price of iron.
 

inkster

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One of the reasons I keep an old adat around is that it has 8 channels of -10 to +4 I/O stages...you do have to have an elco cable
 
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