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 Post subject: Balanced audio mod
PostPosted: Jun Mon 24, 2019 3:47 am 
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Location: Warner Robins, GA
I have a Sylvania RM-300X radio which I modded to where the external speaker jack is a 1/4" stereo jack which feeds a Dayton Audio PMA250 250W PA Module with Mixer through its line input.

https://www.daytonaudio.com/index.php/l ... mixer.html

The radio is such that I can use either an unbalanced cable to connect the radio's output to the amp line input with an 8 ohm load wired into the cable or so I can use a balanced to unbalanced 1/4" cable with the balanced end having the tip connected to the ring so the internal speaker will be on.

The amp drives an Argos column speaker which was rebuilt into a HI-FI speaker.

At the moment I only use it in my bedroom, but there may be instances in the future where I might want to use it for DJing a small function.

That brings me to the following question.

The line input on the amp has an electrically balanced input. I don't know what the impedance is and the specs and manual don't say, but I am assuming it may be 10K unbalanced 20K balanced.

I don't quite like feeding a speaker level output into a line level input due to the chance of possible damage to the plate amp.

So I'm thinking of using an Edcor transformer https://www.edcorusa.com/xsm_series which is a 15K balanced/unbalanced to 15K balanced/unbalanced transformer.

That would do the unbalanced to balanced conversion.

I will then add a level control to the box so that I can set the level to where the full undistorted output of the radio will make the plate amp put out its full undistorted power.

I'll add a switch to the box that will select either the radio's speaker or an 8 ohm load.

That way the radio's amp will see a proper load while the plate amp will get a balanced input.

The reason for this is there may be times when the plate amp is a distance away from the radio and the balanced line would mean I could run a longer cable without picking up interference plus it would ensure there's no chance of a ground loop.

Is that a good idea or should I use it as is or is there a better way to do it?


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 Post subject: Re: Balanced audio mod
PostPosted: Jun Thu 27, 2019 4:30 pm 
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Location: Roanoke, VA
Tube Radio wrote:
The line input on the amp has an electrically balanced input. I don't know what the impedance is and the specs and manual don't say, but I am assuming it may be 10K unbalanced 20K balanced.

Balanced audio lines of that vintage would typically be used with a 600 Ohm input.

Dale H. Cook, Radio Contract Engineer, Roanoke/Lynchburg, VA
https://plymouthcolony.net/starcityeng/index.html


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 Post subject: Re: Balanced audio mod
PostPosted: Jun Thu 27, 2019 4:54 pm 
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The amp is the Dayton Audio plate amp I linked to.


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 Post subject: Re: Balanced audio mod
PostPosted: Jun Sat 29, 2019 3:23 pm 
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I thought you were working on something old. You would need to contact the manufacturer, examine the schematic, or measure the input impedance yourself. I could measure it myself with my GR 1603A Z-Y bridge.

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 Post subject: Re: Balanced audio mod
PostPosted: Jun Sat 29, 2019 3:58 pm 
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no I'm connecting the speaker output of something old to the balanced line level input of something new.

I've found that a lot of balanced inputs are usually 20K balanced 10K unbalanced so perhaps I'll just use the 15K transformer and use a non-inductive 8 ohm resistor across the primary so the radio sees a proper load.

I could use a push pull outout transformer in reverse, but I don't have one handy and it would cost a bit.

I may also get an 8 ohm dummy load so I can set the attenuation I will need for the transformer to where full undistorted output of the radio produces full ubdistorted output of the amp.

Here's a thought.

Given the Sylvania uses a transformer with a split secondary to drive the push pull output transistor stage (good for around 10 watts of audio) I wonder if I could simply cap couple the secondary to a balanced 1/4" jack and feed that straight to the amp's balanced input?


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 Post subject: Re: Balanced audio mod
PostPosted: Jun Sat 29, 2019 4:52 pm 
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Hi all. You are on the right track with the dummy load resistor and transformer, however a few points here. If the source is a tube type with an output transformer then the proper load is needed to protect the output stage as well as match it to your load. If the source is a solid state amplifier it depends on the output stage type. If it uses an output transformer then by all means load it as you would if it were tube type. If it is a complementary symmetry or quasi complementary type then it is not that critical since the true output impedance of these is way below 8 ohms. It is not uncommon to load these with a higher resistance of say 47 ohms since this will give the output stage a dc load without making the stage deliver a larger than needed output power only to turn it into heat. The output transistors will also draw less current on audio (not dc) so they will not heat up driving a dummy load. Several pro audio companies used this strategy. The reason I bring up the dc is this. Some transistor power amplifiers have a negative feedback resistor from the output terminal back to the earlier stages of the amp say to a transistor base. If this were to float the bias would be off and the amp not be happy. A load of 47 ohms compared to the feedback resistor will be very small so the bias is fine in this case. The Gates radio company did this. Now the 15k 1:1 transformer is a good idea for many reasons as long as it meets the needs of response etc. It will not be carrying real power and the following amp should be happy as well since all the balanced line inputs today are high impedance. The one drawback to this is if the output stage has a high noise floor you may hear the hiss. Your mention of a push-pull output transformer could be dangerous to the second amp as if it were a typical tube type transformer then if you wired it 8 ohm to source and primary as secondary to amp then the voltage step up could be substantial and well as much as 100 volts and probably more would go into the amp input designed for only 2 volts or so max. Remember rule number 1, Do not let the magic smoke out!


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 Post subject: Re: Balanced audio mod
PostPosted: Jun Sat 29, 2019 5:16 pm 
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Agreed plus the transformer would be nearly $100 and quite heavy. Definitely not what I need.

I do like the idea of the larger resistor for the load.

There is feedback from the speaker side of the output coupling cap to earlier in the amp.

Here's a photo of the amp showing just the driver and output stage. It has a gain stage, bass and treble controls then a volume control followed by another stage with the emitter connected to the base of the driver. I don't really like the gain stage before the volume pot as it is easy to overload the circuit when feeding an external input to the radio (added a switch on the back of the radio to select external input or AM audio) so I have to watch the level of whatever source I use.

The amp is quite quiet.

Attachment:
RM-300Xaudiooutput.JPG
RM-300Xaudiooutput.JPG [ 254.28 KiB | Viewed 2003 times ]


For the transformer I'm thinking of getting a 10K trimpot or panel mount pot with locking nut to adjust the signal going into the transformer so it isn't too high for the plate amp.

The radio itself is built like a tank and is high quality.

I could even pick up a lot of the more local FM stations fairly decently with no static at all with just the twinlead running from the main board to the antenna screws on the rear panel.


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 Post subject: Re: Balanced audio mod
PostPosted: Jun Sat 29, 2019 5:25 pm 
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Location: pensacola fl
Those 2N301 series germanium transistors can produce 8 watts easy so if it were me I would use a load of around 47 ohms 2 watts non inductive like carbon resistor and then place your 10k pot in paralell with it before the transformer to keep the secondary line balanced for maximum common mode noise rejection. Placing the pot after the transformer could up set this and that would undo the whole reason for a balanced line in the first place.


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 Post subject: Re: Balanced audio mod
PostPosted: Jun Sat 29, 2019 6:00 pm 
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Yes the pot would go before the transformer.

I'm picky about things like this given I run sound at church and at the skating rink I dj at and have seen the issues that can arise when improperly going between balanced and unbalanced.

EDIT:

Here's the circuit

Attachment:
Unbalanced to balanced box.png
Unbalanced to balanced box.png [ 5.02 KiB | Viewed 1946 times ]


With the monitor switch set to on (connects the radio's speaker and disconencts the 47 ohm load) and after figuring in the speaker's 4 ohm impedance I get a dc resistance of 3.967 ohms at full CCW on the level control and 3.891 ohms at full CW on the level control.

With the monitor switch set to off (disconencts the radio's speaker and conencts the 47 ohm load) I get a dc resistance of 45.5 ohms at full CCW on the level control and 37.1 ohms at full CW on the level control.

Those resistances should be ok given the impedance is what actually matters.


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 Post subject: Re: Balanced audio mod
PostPosted: Jul Tue 02, 2019 3:10 am 
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Well the circuit works as I hoped it would.

I added the level control as I thought the output would be twice the input, because I forgot that a signal put across the full primary will be the same across the full secondary and that because 1/2 the voltage is going to the hot terminal and 1/2 the voltage is going to the cold terminal of the balanced line input, the output to the speaker would be the exact same for a given input voltage.

I also noticed there is a slight level change between using the radio's internal speaker and the 47 ohm load. So I'm thinking of perhaps using an 8 ohm 20 watt non-inductive resistor instead of the 47 ohm resistor.

That would also be better given there's feedback from the + speaker output to the driver transistor emitter.

Here's the box.

Attachment:
20190701_222259.jpg
20190701_222259.jpg [ 487.08 KiB | Viewed 1913 times ]


What I'm thinking of doing is adding another switch and mono 1/4" jack to the box so I can select between the radio's speaker or an external speaker.


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 Post subject: Re: Balanced audio mod
PostPosted: Jul Sun 14, 2019 1:43 am 
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I've ran into an issue with the Sylvania.

Turns out that the amp is not flat from 20-16KHz

I replaced three coupling caps with larger value caps and it somewhat helped but didn't fully get the response flat to 20Hz.

There was a 470pF cap that once removed helped with the upper treble response, but it isn't fully flat.

Also when the treble control is completely removed that helps some. Will need to reduce the cap value.

There is a series cap and resistor in parallel with the emitter resistor for the first audio transformer
With that cap installed the amp has an increased response around 1KHz on up and as expected has a drop in frequency response starting around 10KHz.

I'm thinking of trying a smaller cap value to where it flattens out the response as I believe Miller capacitance is what's causing the drop in response above 10KHz. I may also look at changing the resistor value. The ultimate solution would be to get a trimpot and variable cap so I can dial in the desired response.

That would fix the treble response.

The bass response however I believe is in the driver transformer for part of it as the waveform is slightly distorted at 20Hz with an 8 ohm load.

So what I may do if it is possible is to take the signal fro the base of the driver transistor and either cap couple it to a jack on the back which then feeds the box provided a 10K load won't affect the circuit or I'll use a TL-082 OP-AMP to buffer the signal.

I would still keep the box setup to turn the internal speaker on and off though.

Also even though I've replaced coupling caps I still have a slight low frequency loss in the first stage and I dont know where it's coming from unless the filter cap supplying that stage with B+ is too small which can be confirmed by putting the scope across it and seeing if there's any waveform as I lower the frequency.


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 Post subject: Re: Balanced audio mod
PostPosted: Jul Thu 18, 2019 11:59 pm 
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I have a final update on the project.

Had to do something different for the signal to the plate amp so here's what I wound up doing.

Took the signal from earlier in the amp and buffered it with a TL-082 which then feeds a 10K pot through a capacitor that feeds the transformer which feeds the plate amp. A switch selects whether the radio or a stereo input feeds the plate amp.

The speaker switches turn the speaker on and off and selects the radio's speaker or an external speaker.

Attachment:
Unbalanced to balanced box.png
Unbalanced to balanced box.png [ 5.82 KiB | Viewed 1721 times ]


EDIT:

I did notice something though.

When feeding a 400Hz signal to the radio and viewing the output on a scope I noticed that as the treble control is rotated CCW the output decreases some then starts to increase only the output signal begins to lead the the input signal and at full CCW is the same amplitude as the input signal. For the test conditions I set the V/DIV to a value where I could adjust the amp's volume control to make the displayed signal match the displayed signal from the AF generator.

Not sure why that is happening at 400Hz and I don't know why the level would be reduced at all only to return to the same level it was at before and I also don't know why there would be a phase shift. Could it be a possible issue with the cap? It is one of those green .0027uF caps that goes between ground and a 500K pot.


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 Post subject: Re: Balanced audio mod
PostPosted: Aug Thu 15, 2019 2:20 pm 
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I did not like the resistors as they caused a reduction in signal so I decided to mod the box with an Edcor WSM 6400 stereo to mono coupler which I have yet to order.

Attachment:
Unbalanced to balanced box.png
Unbalanced to balanced box.png [ 10.2 KiB | Viewed 1534 times ]


At the moment I just have it feeding the existing unbalanced to balanced transformer given that transformer is providing a step up of the voltage, but if that doesn't sound good I can try using a DPDT switch on the balanced out jack and switch between the two transformer secondaries since both are a balanced output.

The transformer however does have a .5dB insertion loss which is more than likely less than what the resistors have so it should give a little more output.


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 Post subject: Re: Balanced audio mod
PostPosted: Aug Wed 21, 2019 6:49 am 
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Location: Powell River BC Canada
When dealing with the term balanced in audio, the term bridging comes into play.

A program line may be bridged by a balanced input to an amplifier or other device
that will transfer the signal without affecting the level of the source line.

This language haunts up legions of old radio geezers. Best let them lie. :D

Attachment:
Bridging.jpg
Bridging.jpg [ 86.08 KiB | Viewed 1408 times ]

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VE7ASO VE7ZSO
Amateur Radio Literacy Club. May we help you read better.
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 Post subject: Re: Balanced audio mod
PostPosted: Aug Wed 21, 2019 12:02 pm 
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So basically it is saying a high enough load impedance won't affect the signal voltage, right?

Only reason for the aux input on the box is so the plate amp can be operated without the radio if so desired.

I'm thinking that I will run into the issue of the signal from the aux in not being high enough to properly drive the plate amp without turning the plate amp's gain up real high.

What I really want to do is switch the plate amp between both balanced signals, but I have the schematic as it is because I think I'll need the voltage step up of the transformer I'm currently using.

Now I only guessed at the input impedance of the plate amp's balanced input which I based on what I've seen for some balanced inputs which is usually 10K unbalanced and 20K balanced since I could not find that spec in the manufacturers data.

I also want to move the level control to the balanced output. Reason being is the transformer tends to pick up hum from the plate amp's power transformer if I set the box too close to the plate amp.

Is it possible to use a single 20K variable resistor across the balanced output with the pot connected across the full secondary and one of the wires to the balanced out jack connected to the center terminal of the pot so that at full CCW the + and - output of the balanced jack is shorted together for aero volume?

Or do I need a 10K dual gang pot?


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 Post subject: Re: Balanced audio mod
PostPosted: Aug Wed 21, 2019 6:03 pm 
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It depend on the application, but but given a signal source of
specified level, a T pot will control level.

Nice box you have made. I would have lined it with mesh or copper foil and grounded transformer
shell.

Attachment:
Daven T Pot.JPG
Daven T Pot.JPG [ 14.12 KiB | Viewed 1390 times ]

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VE7ASO VE7ZSO
Amateur Radio Literacy Club. May we help you read better.
Steve Dow
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 Post subject: Re: Balanced audio mod
PostPosted: Aug Wed 21, 2019 7:16 pm 
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That was the first box.

The box everything is now in is metal.

Attachment:
box.jpg
box.jpg [ 167.35 KiB | Viewed 1379 times ]


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 Post subject: Re: Balanced audio mod
PostPosted: Aug Wed 21, 2019 11:34 pm 
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That looks wonderful !

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VE7ASO VE7ZSO
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Steve Dow
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 Post subject: Re: Balanced audio mod
PostPosted: Aug Thu 22, 2019 12:19 am 
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To avoid the potential for a ground loop the box is only grounded at the RCA input jack.

I may see if mouser has any 10K dual gang audio taper pots and use those to control the level provided the stereo to balanced mono transformer has enough output to drive the amp properly.

I could easily make the box active using a TL-082 and two resistors to sum the stereo input to mono and increase the gain of it, but that would require running B+ or an AC voltage to the box which could introduce its own set of problems plus the box would no longer operate without the radio unless I build a power supply box.

I did change things around to where the signal to the box comes from the input to the radio's volume control as I didn't like the amp being at near full volume in order for the control of the volume to be by the box.

The OP-AMP consists of a TL-082 wired as a non-inverting stage with a gain of three initially followed by an inverting amp with a gain of one which was necessary due to the first audio stage of the radio inverting the signal.

That just did give enough output so the plate amp with its controls set to near full would produce full output right before the radio's amp would distort when using the radio's volume control to control the radio and plate amp.

After taking the signal from the input to the volume control I had to reduce the OP-AMP to a gain of 2 and the plate amp controls don't have to be set as high.

The speaker switches permit me to turn the speaker on and off using the radio's speaker or an external speaker connected to the radio's amp.

The radio also still has a switched speaker jack so it can operate by itself if so desired.

The only reason I want the plate amp to operate without the radio is so I can still use it if the radio needs to be repaired or so that I don't require the radio if I am using the amp somewhere and am just using my smartphone for the source.

What I'll do is feed a 400Hz signal to the radio and the stereo to balanced mono transformer and measure the output and see how close they are to each other.

Now if I want gain I could maybe use two Edcor WSM-15K/15K transformers using 1/2 the primary of both then use a resistor network to combine a stereo balanced signal to mono balanced provided I have room for two transformers which I highly doubt I'll have room for. Already going to have trouble finding room for a second transformer.


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 Post subject: Re: Balanced audio mod
PostPosted: Aug Mon 26, 2019 2:52 pm 
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The transformer has shipped. When it arrives I'll do the test feeding the same signal to the radio and the transformer so I can see if the transformer can be used to directly drive the balanced input or if I will need to feed the transformer through the existing transformer.


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