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 Post subject: Re: Balanced audio mod
PostPosted: Aug Thu 29, 2019 10:38 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 20495
Location: Warner Robins, GA
The transformer arrived today.

Feeding a stereo signal into it i got less output than feeding a signal into the box.

So I connected the balanced output to the input of the box and the output is now something like around 100mV higher than just feeding the box.

So I wired the transformer output to feed the other transformer.

That works but it is lower than the output I get with the radio as a source.

So I'm left with no choice but to put the radio's OP-AMP back to unity gain and add a TL-082 to the input of the plate amp and give it a gain of two or three.

I can maybe then use the full primary winding of the balanced/ unbalanced to balanced/unbalanced transformer.

I'll then feed a signal to the stereo to mono transformer and the full primary of the unbalanced to balanced transformer to see how close they are in output.

If they are close enough I'll get a DPDT switch and a dual gang 10K audio taper pot so that I don't have one transformer feeding another unless it's best left as is to keep from having to have as much gain in the OP-AMP.

Adding the OP-AMP is easy given the plate amp has +/- 12 volt supplies.

Just need to wire up two sections as non-inverting with a gain of two or three and use a couple coupling caps on the input.

The output will go to where the balanced in jack originally connected.

The only issue there is that by adding gain to the plate amp it also will amplify any hum picked up by the transformers. Shouldn't be a problem so long as the box is kept away from the power transformer of the plate amp.

Suppose I could measure the voltages and replace the transformer with a torroidal transformer if so desired.


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 Post subject: Re: Balanced audio mod
PostPosted: Aug Sat 31, 2019 3:17 am 
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Joined: Apr Thu 14, 2016 8:25 pm
Posts: 467
Location: pensacola fl
try one of these to make mono from stereo. they come in various impedences like 600 ohm. you will find these all over ebay .

https://www.ebay.com/itm/RDL-STD-10K-Di ... SwnUlakNup


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 Post subject: Re: Balanced audio mod
PostPosted: Aug Sat 31, 2019 6:16 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 20495
Location: Warner Robins, GA
Is it a transformer, resistors or active circuit?



EDIT:

I have an older version of a Planet Audio PEQ15 5 Band Pre-Amp Equalizer

https://www.planetaudio.com/product/car ... dio-peq15/

Given it is stereo I can use it as a mono device on the balanced line between the box and amp.

That will give me both a step up in signal and some form of EQ.

It has a subwoofer output, but I need to verify whether or not it is actually a two way crossover or just a subwoofer output with the main output being full frequency.

I also need to check for phase shift.

EDIT 2:

That EQ does increease the signal a bit.

I did notice that my bluetooth adapter tends to distort the sinewave some under around 32 Hz in much the same way an audio transformer does when fed a frequency lower than what it can properly pass.

I connected the phone to the control box directly to be sure it wasn't an issue with the box and the sinewave was clean down to 20Hz.

So I'll need to replace the bluetooth receiver with a better one that can operate off USB. Plus the one I'm currently using is a bit finicky sometimes.

EDIT 3:

I got everything working right.

When I reduced the level of the radio input to where it and the aux input displayed a sinewave of the same amplitude with the level controls full CW the levels weren't so equal at lower settings of the level controls with the aux input of the box being lower by a bit.

Not sure why though.

I found a 30.4K in series with the mono line in reduced its level enough so that the audio from the radio's aux in and the box's aux in were basically equal when playing music.

I noticed that with the radio's line in, the audio from my bluetooth receiver was slightly distorted at 400Hz and a 7.297K resistor to ground after 10K line input resistors solved the problem.

Overall it seems to work much better.

I noticed the box's aux in seems to have more control at the lower end of its level control and the radio input has more control at the upper end of its level control.


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