Changing Input Transformers

[spaceludwig]

Hello,

I would like some assistance in changing the input transformers on my mixing board.

Below are some pictures of the original GA8027(Right) and the transformers I want to replace them with GA80071(left)

Due to the size difference I will probably have to lie the new transformer down on the PCB side of the board as the individual strips are too close together to put them standing up like the original ones were (left in picture)

My questions primarily concern the difference in ratio between the transformers and how this might affect the change. Below are the schematics of the two units in their respective circuits:

GA8027

GA80071

The GA80071 is supposed to be 600:600. Will I need to change the value of any of the resistors or caps at the transformer output? I'm assuming that since the GA80071 is 1:1 that the signal might simply be a little lower. Is this correct?

Also, would anyone know which wires are the primary and secondary?  I suspect that the green and blue are secondary just like in the original. These transformers were from the line-in section of a PM2000 desk which, having no phantom power, I assume is why the center taps are "neutralized". IS this correct? or would I need to connect the black and blue secondary windings like is done with the GA8027 (see pic below)?   

Since I have  just finished recapping my board I would like to make these changes before I stick all the input channels back in. They are not modular and therefore a pain-in-the-ass to remove and put back in.

Thanks in advance for any and all help you can provide, it is mucho appreciado.

Cheers!

 

[spaceludwig]

I took some measurements with a multimeter. Between white & yellow wires = 247 Ω, between green & blue = 93 Ω

Has anyone reading racked a PM1000 or 2000 unit? The transformers should be the same, perhaps you could help.

 

[spaceludwig]

Investigating a little further revealed some more details. The transformers came in a block like this:

The underside was wired like this:

So the white and yellow wires from the transformer (top side of the PCB) must be secondary as the brown cable (bottom side) it is patched to has only two leads, white and clear, while the other yellow cable is of the balanced variety, so three leads: white, red and clear which must be coming from the XLR terminal - and patched to the blue and green transformer leads.

Provided I am consistent in wiring the same leads to the same pads on the PCB board of my input channels does it matter if the order of the leads are inversed? i.e. if the blue and green wires of the primary leads are swapped?

 

[pucho812]

why are you swapping out the ga8027? does it have a short?  looking at the schematic the original transformer is a 600:3k, which means 600 primaries and 3k secondaries, a step up, 1:2.  If your still thinking about using the alternate transformer.  This will help you find the info you seek. You need a DMM

Testing audio transformers

1 Test equipment required
o Multimeter – analog or digital
o Audio signal generator – fixed frequency or variable
o Variable resistors, 1off 25k to 50k and 1off 10 Ohm to 500 Ohm
o Connecting leads
o Audio voltmeter- preferably calibrated in dBm, sensitive down to –60dBm
2 Identify windings
o Use the multimeter to identify continuity between the various connections.
o For an output transformer, the high resistance winding, single ended or center-tapped is the primary
o For a microphone, line or an inter-stage transformer, the higher resistance winding is usually the secondary
o For centre-tapped windings, the two halves may not be exactly the same resistance, especially on E+I cored transformers
o There may also be a screen winding, which may or may not be connected to the frame.
3 Measure dc resistance
Use the multimeter to measure the dc resistance of each winding; write this down.
4 Measure impedance
4.1 Output transformers
1. Connect the 100 Ohm variable resistor across the low resistance winding; initially, leave one wire open;
2. Connect the 25k variable resistor in series with the high resistance winding and the output of the signal generator; set the resistance to 0 initially;
3. Set the signal generator to 1kHz and near maximum output, which may be say, 20V rms
4. Use the audio Voltmeter to measure the input and output Voltages; write these down
5. Calculate the turns ratio = VIN/VOUT
6. Connect the 100 Ohm resistor in circuit
7. Reduce this resistor till the output Voltage falls 3dB
8. Increase the 25k resistor till the output Voltage falls another 3dB
9. Without disturbing the settings of the variable resistors, disconnect one lead from each
10. Use the multimeter to measure the settings of these two resistors – these are the impedances of the primary and secondary – you may need to replace the 100 Ohm variable resistor with a lower value to get an accurate measure
11. Compare the turns ratio with the impedance ratio – do they agree? Allow for the dc resistance of the windings
4.2 Microphone, line and inter-stage transformers
Follow the procedure for output transformers, except:
1. Connect the 25k Ohm variable resistor across the high resistance winding; initially, leave one wire open;
2. Connect the 100 Ohm variable resistor in series with the low resistance winding and the output of the signal generator; set the resistance to 0 initially;
3. Set the signal generator to 1kHz and low output, which may be say, 20mV rms
Warning: do not exceed 20mV rms for microphone transformers
4. As above
5. As above
6. Connect the 25k Ohm resistor in circuit – it may have to be a higher resistance;
7. As above
8. Increase the 100 Ohm resistor till the output Voltage falls another 3dB
9. As above
10. As above
11. As above

5 Measure frequency response
Note: this section can only be done with a variable frequency signal generator.
1. Reconnect the variable resistors without changing their settings
2. Connect the audio Voltmeter across the secondary
3. Reduce the frequency till the output falls another 3db compared with steps 8 above; this is the lower 3dB point
4. Increase the frequency till the output falls another 3db compared with steps 8 above; this is the upper 3dB point
5. Write this information down – perhaps attach it to the transformer.

6 Measure power response
If you want to measure power response of output and inter-stage transformers, you will need a very good quality power amplifier, high power non-inductive load resistors and a CRO.
1. In place of step 1in section 4.1 above, use a variable high power load resistor to achieve the value found in step 10
2. In place of step 2 in section 4.1 above, use a high power variable resistor to achieve the value found in step 10
3. Connect the audio Voltmeter across the primary;
4. Set the input power to about 1W
5. Connect the CRO across the secondary. Set the signal generator to 1kHz
6. Increase the power input until the image on the CRT just starts to flatten – this is probably 3dB over the rated power of the transformer
7. Reduce the power input
8. Change the input frequency to the lower 3dB point identified in step 3 of section 5 above
9. repeat step 6 at this lower frequency – the low frequency 3dB point may be at a much higher frequency than the lower 3dB point at low power [section 5] – it depends on how powerful the accountant was with respect to the design engineer in choosing how much core of what quality to use.

 

[spaceludwig]

Thanks for the elaborate response. Unfortunately I don't have a 25k or 100 ohm variable resistor. However I have a signal generator, scope and multimeter. I'm quite confident I've figured out which are the primary and secondary leads. Though it would be very interesting to find out the high and low frequency 3db roll-off point of both transformers.

 

[abechap024]

don't cause more work for yourself. They should be pretty much drop in I would say. Both are terminated with a 4.7k resistor, thats the reason I say you can probably interchange them without too many problems.

And they are about the same ratio too.

 

[spaceludwig]

OK, thanks for the advice.

A related but slightly different question: what is a cheap, light, malleable, and readily available material I can use to wrap the cans in so they don't short anything when I attach them to the solder side of the PCB board. I'm thinking perhaps rubber, like dish washing gloves, or perhaps silicone, the one that comes in caulking tubes?

 

[gemini86]

Just a piece of electrical tape will do.

 

[abbey road d enfer]

OK, thanks for the advice.

A related but slightly different question: what is a cheap, light, malleable, and readily available material I can use to wrap the cans in so they don't short anything when I attach them to the solder side of the PCB board. I'm thinking perhaps rubber, like dish washing gloves, or perhaps silicone, the one that comes in caulking tubes?

I would say the best is using the industry standard.
http://fr.mouser.com/ProductDetail/3M-Electronic-Specialty/FP301-1-1-2-BLACK-48-BK/?qs=sGAEpiMZZMutDkoVpcgZZpLqojjdU7YHyr0bZJdZOhA%3d

 

[abbey road d enfer]

But still, I don't understand why you want to replace the xfmrs. It seems like you will lose 6dB gain AND about 6dB S/N ratio. What do you expect?

 

[spaceludwig]

I've read that these transformers are a significant upgrade from the ones found stock on this mixing board. I am using the console only as an output mix/summing bus and to add a little color. Since I found the transformers relatively cheap I figured why not try it out. I would think it's not worth the effort if the GA80071 sounds no different from the GA8027 or if they were much more expensive.

Having said all of that I will a/b two channels of the stock against replacement. I just want to make sure that when I do I've wired everything correctly and haven't shorted anything.

 

[CZ101]

AFAIK the GA80071 is not 600:600.

The Tamura GA80080 (from the PM1k), however, is 600:600 (line output).

I'm 99% sure the GA80071 has a 300:2.7k impedance ratio.

 

[s2udio]

YoU would be better off changing all those 10% jap resistors for Q 1% mf (thats a challenge  ).........leave the xformers where they are !
Just a my 2 cents !

 

[spaceludwig]

AFAIK the GA80071 is not 600:600.

The Tamura GA80080 (from the PM1k), however, is 600:600 (line output).

I'm 99% sure the GA80071 has a 300:2.7k impedance ratio.

Yes, they are not 600:600

 

[spaceludwig]

YoU would be better off changing all those 10% jap resistors for Q 1% mf (thats a challenge  ).........leave the xformers where they are !
Just a my 2 cents !

I think you might have misinterpreted what I wrote, I took the transformers in the picture to replace the stock ones I had in my console.