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 Post subject: Preamp
PostPosted: Sep Sun 13, 2020 9:01 pm 
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Location: Warner Robins, GA
I've got an issue that I need a good solution for.

I installed a sound card in my computer yesterday that has balanced inputs and outputs and no way of controlling the level.

The issue is that unbalanced consumer line level signals aren't quite high enough of a level.

So I either need to convert the unbalanced to balanced or increase the unbalanced signal level.

I do not want transformers, although I have used EDCOR WSM and XSM series transformers to go from unbalanced to balanced and vice versa and they did not adversely affect the sound. Reason is because I have no data on the FM tuner I am using (Curtis Mathes HT-315) so I don't know the recommended output impedance and I don't want to go modifying it as I don't want to risk negatively affecting the sound quality or making it to where it will not work properly fed straight to an amplifier.

The easy way is to get a small mixer, but I just want something real simple with minimal active devices.

Here's a couple pictures showing the issue.

Normal

Attachment:
20200913_154311.jpg
20200913_154311.jpg [ 4 MiB | Viewed 562 times ]


Adjusted for a more proper signal level.

Attachment:
20200913_154325.jpg
20200913_154325.jpg [ 4 MiB | Viewed 562 times ]


Is there some sort of plug and play option or would I need to build something?


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 Post subject: Re: Preamp
PostPosted: Sep Mon 14, 2020 1:36 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 18039
Location: Utica, NY 13502 (USA)
You can build a pair of these:

http://www.fivefish.net/diy/balanced/default.htm

Or buy a pair of these:

https://www.ebay.com/itm/NE5532-Unbalan ... 3373281425

Dave


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 Post subject: Re: Preamp
PostPosted: Sep Mon 14, 2020 2:10 pm 
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Location: Warner Robins, GA
What is the input impedance of that circuit?

Given I don't know the tuner's output impedance I need the input impedance to be around 100K.

Suppose it would be easy enough to get two more of those chips wired as buffers.

I may have a look at the tuner and see if I can trace where the audio output is connected to. Perhaps I can then get some sort of idea as to the output impedance.

My guess would be anywhere from 10K to 50K.

I do know the balanced input of the sound card is 20K though.

So I'm thinking that may be 10K on the + input and 10K on the - input.

The tuner seems to work ok feeding that.


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 Post subject: Re: Preamp
PostPosted: Sep Tue 15, 2020 12:23 am 
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Location: Utica, NY 13502 (USA)
After looking at the DIY circuit, I think I see a few errors. The unbalanced input signal ground isn't grounded. The input signal "hot" goes to the + input on both op amps. Maybe I'm missing something but I can't see how this could produce a balanced output. I could be wrong but I think this falls in the category of "never trust everything you see on the internet".

I've built unbalanced-to-balanced circuits in the past but that was quite a while ago. I'll try to find what I built using the NE5532 IC. Seemed a lot simpler than what shows in the link.

Dave


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 Post subject: Re: Preamp
PostPosted: Sep Tue 15, 2020 1:57 am 
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Location: Warner Robins, GA
Think it was oldwirebender who once said a lot of unbalanced to balanced circuits use two OP-AMPs set up for unity gain where the non-inverting OP-AMP drives the inverting OP-AMP which provides the necessary higher input impedance and also a balanced output.

There's also a DRV-134 fully differential OP-AMP with a gain of two and a DRV-35 which is the same only a gain of one, but they both have a lower input impedance. Plus I can only find boards on eBay for the DRV-134 and I don't need the gain.


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 Post subject: Re: Preamp
PostPosted: Sep Tue 15, 2020 2:30 am 
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Location: Utica, NY 13502 (USA)
Here is a circuit that is similar to what I remember building:

Attachment:
Unbalanced-to Balanced Circuit.jpg
Unbalanced-to Balanced Circuit.jpg [ 218.46 KiB | Viewed 483 times ]


R1 determines input impedance.
R2 determines gain.
R4 + R7 determines output impedance.

Requires a bipolar power supply as shown but I've modified a 24 volt wall-wart with 2 resistors and 2 caps to provide +12 and -12 volts to make it work.

It is my understanding that "consumer" level is -10 dbm (unbalanced) and "professional" level is +4 dbm across the balanced line

Dave


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 Post subject: Re: Preamp
PostPosted: Sep Tue 15, 2020 11:34 am 
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Location: Warner Robins, GA
Cool I may just have to build that.


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 Post subject: Re: Preamp
PostPosted: Sep Mon 21, 2020 8:22 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Warner Robins, GA
I found a temporary solution.

I have an older pro audio USB sound card with balanced outputs that doesn't seem to work so well under Windows 10 on the laptop I tried it on and I didn't need it for my two other computers as they have pro audio sound cards in them.

So I connected it to a USB power supply and put that between the tuner and sound card input.

Works like a charm.

May or may not be permanent.

Haven't decided yet.

EDIT:

Seems silly to use the sound card as a preamp so I tried connecting the sound card to my computer and it didn't seem to work so well with the other sound card installed.


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