Balancing output of a consumer EQ

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craigmorris74

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I tried searching the forum for this, but didn't find exactly what I was looking for.

I have an old Realistic EQ that I like the sound of on certain sources.  I've attached the schematic.  According to the manual, it has 10 ohm output impedance.

I was wondering if I could use a THAT1646 chip to balance the output?

Thanks,
Craig
 

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craigmorris74

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JohnRoberts said:
It would be easier (and cheaper) to impedance balance it with a resistor.

JR

Thanks for your reply, John.  I should have mentioned this in my post abpve, but I'd also like to compensate for the 6dB of signal loss.
 

ruffrecords

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craigmorris74 said:
Thanks for your reply, John.  I should have mentioned this in my post abpve, but I'd also like to compensate for the 6dB of signal loss.
Personally I do not think it is worth the effort. There is no loss really. In practice the balanced out circuit has 6dB of gain.And anyway, the EQ has a 43V power rail so I expect it is quite capable of outputting a healthy level without needing any additional gain.

Cheers

Ian
 

craigmorris74

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ruffrecords said:
Personally I do not think it is worth the effort. There is no loss really. In practice the balanced out circuit has 6dB of gain.And anyway, the EQ has a 43V power rail so I expect it is quite capable of outputting a healthy level without needing any additional gain.

Cheers

Ian
I would agree Ian, but I'm often inserting it in hybrid  mixing setup into a summing mixer, where I don't have any gain.  This throws off the balance of the mix I'm working on.
 

craigmorris74

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scott2000 said:
These were brought up earlier. I can't remember exactly the thread... but it had some other cool things too...

https://www.sparkfun.com/products/14003
That looks perfect.  Would it work fine with the device I was asking about?
 

Bo Deadly

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THAT1646 is dual supply and it doesn't provide gain.

Try adding a 3k3 to the base of TR105/TR106 (the ones connected to R115/R116) to a 10uF capacitor to ground. Meaning base of transistor -> 3k3 -> 10u -> ground. That should give you 6dB of gain.

But because it's discrete, I'm not 100% that will work. There are others here that are better at figuring out that sort of thing but just try it with little test clips if you have them and verify that it works and it doesn't screw something up.

Then make it impedance balanced like JR said. Impedance balanced out is actually very effective at cancelling ground hum. Lots of pro gear uses impedance balanced outputs. But the output impedance is definitely not 10 ohms. It's probably a few 100 ohms. You would need to measure it by running a 1kHz tone into it and measuring the AC on the output. Adjust the source for say 1VAC on the output unloaded. Then add a resistor load starting with maybe 220 ohms or so and read the voltage again. Change the load resistor until you get exactly 0.5VAC on the output. The value of that resistor is the output impedance. Then just put a resistor of that value between the output ground and the analog ground of the circuit. Then you'll have a decent impedance balanced output. Unless the output impedance is more than 500 ohms or so. That would be unfortunate but it would work fine as long as what it's driving is 10K or so

That's a nice circuit though. Simple. I like it. It's a distortion machine with inductor EQ. Groovy.
 

john12ax7

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Are you getting a loss in level when you insert now? The 1646 will give you +6 dB but it also wants to be driven from a low impedance.  The stated 10 ohm is rather optimistic given it is driven from a transistor collector.
 

TwentyTrees

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Slightly off-topic of me, but many thanks Squarewave for the walkthrough on how to impedance-balance outputs - I will be referring back to this frequently!


squarewave said:
Then make it impedance balanced like JR said...
 

moamps

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craigmorris74 said:
I tried searching the forum for this, but didn't find exactly what I was looking for.

In the spirit of this design I would suggest adding an inverted buffer consisting of two transistors that can be modified to be powered by 43V.
https://sound-au.com/no-opamps.htm
 

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Bo Deadly

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TwentyTrees said:
Slightly off-topic of me, but many thanks Squarewave for the walkthrough on how to impedance-balance outputs - I will be referring back to this frequently!
Actually the procedure I described is a little different in this particular case because the output impedance of the amplifier is high. Normally when you want to make something impedance balanced, it's output is already very low (like a fraction of an ohm) and so you would not need to measure the output Z and you would add TWO small resistors - one in series with the output and one from the output ground to analog ground.

Like this:

03chefRg0xQgTLA9QA8onLwZspMwZ6-29f37_RQtvoQYolrU0DWxy2RcnF32xD4YJQitGhqzWN2jVDBPvramXKv15Mrk1meTud7PCEPtpwo


In fact, the more I think about it, the best thing to do in this particular scenario would be to just add a dual op amp and do proper impedance balanced outs like the above dia. Then you can add +6dB gain. Or use a trimmer and make it adjustable. Even though it's dual supply, you can make a simple voltage divider virtual ground and run it from single supply. That way you can use a really low current op amp that won't suck too much juice from the supply. Something like TL072 for example uses 3mA. But RC4580 uses 6mA and it's got a bit more drive capability. Using two THAT1646 would use 10mA but it would be double the drive current since it's driving both lines. Make a little general purpose PCB through OSH park for $10 US.
 

JohnRoberts

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We need to clarify what the problem is that the OP is working to resolve?

You can impedance balance a singled ended output with a few cents worth of resistors, the only active circuitry involved is a differential input in the receiving gear's input stage.

OTOH if the OP want more level adding an active differential output (that could be as simple as a single op amp will give you +6dB level (again ASSuming a proper differential receiver stage). This active differential output stage will also increase your noise floor the same +6dB.

Balanced I/O is not the only difference in more professional gear so perhaps temper your expectations.

Good luck..

JR
 

craigmorris74

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JohnRoberts said:
We need to clarify what the problem is that the OP is working to resolve?

Thanks for all the suggestions thus far.  I'm looking into all of them.

I take 8 stereo stems out of the computer and mix them to stereo in a passive mixer with API 312 style makeup gain.

When needed, I insert EQ and or compression between the computer and mixer to process the stems.  If I do that with the Realistic EQ, that stem loses 6dB, and throws off whatever balance I had.  Of course, I could just start with it in the signal path, but frequently I have a rough mix going in the DAW before it gets sent out to the hardware.

Also, when I suggested using  the THAT1646, I intended to put six or so of them in their own box with their own +/-16V power supply.
 

Newmarket

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abbey road d enfer said:
What loss? This circuit is unity gain. If you experience loss, it means you have a wrong connection.
Can you detail how you connect it (input and output)?

Possibly (?) the output from the 'computer' is active balanced - so equal and opposite signal on each leg - but Realistic EQ unbalanced / single ended output reading only one leg  to '0V' ???
 

abbey road d enfer

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Newmarket said:
Possibly (?) the output from the 'computer' is active balanced - so equal and opposite signal on each leg - but Realistic EQ unbalanced / single ended output reading only one leg  to '0V' ???
Yes, that's a possibility. In that case, balancing the output does not change anything.
 

ruffrecords

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If your interface is electronically balanced then it may be that you are losing 6dB at the input to the unbalanced EQ. To recover it you could fit a 10K:10K isolating transformer at the EQ input. Then you could simply impedance balance the output without losing any signal level.

Cheers

Ian
 
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