goldenGeek

Transformer newbie / how to hook up transformer
« on: January 29, 2010, 12:41:08 AM »
Hello
I am about to rack some mic-pres that I have around and I have some questions regarding transformers/balancing. The mic-pres are mixer-stripes that only has unbalanced out, so how do I go at this? Do I just hook the outputs up to the primary on the transformer and the secondary to the XLR-outs on my case? Or should there be introduced some other components in the signal path as well?

As for the transformers - I have several more around, so I'm curious if someone has ideas for other projects to use these. They are Sowter 3612 which came from the group outputs on an old Midas desk. Brian Sowter told me that this transformer has a 30 OHM CT primary and that the secondary is tapped at 200 and 30 ohms. So - what does CT primary actually mean? And should I use 30 or 200 ohms on the secondary for my mic-pre rack-project?

Edit: I suddenly recalled that I have asked about these Sowters before, but I got no response back then. Sorry for "double-posting".
« Last Edit: January 29, 2010, 12:45:43 AM by goldenGeek »


Re: Transformer newbie / how to hook up transformer
« Reply #1 on: January 29, 2010, 01:02:21 AM »
Brian Sowter told me that this transformer has a 30 OHM CT primary and that the secondary is tapped at 200 and 30 ohms. So - what does CT primary actually mean? And should I use 30 or 200 ohms on the secondary for my mic-pre rack-project?

Hmmm.  Think about the primary...  if something is 30 Ohms in total, what would you think the impedance would be on each side of the primary's outside taps to the CT?

Do you have a schem?  or do you know the output impedance?  it sounds like the wrong xfmr, but there is not enough info.

There used to be a good xfmr tutorial, maybe try the META.

CJ

Re: Transformer newbie / how to hook up transformer
« Reply #2 on: January 29, 2010, 02:40:04 AM »
i get different numbers at the Sowter vintage archive page.

i think you have the right idea on the hookup, but what kind of channel strips?

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goldenGeek

Re: Transformer newbie / how to hook up transformer
« Reply #3 on: January 29, 2010, 04:51:40 AM »
I was thinking of using them on a couple of Midas Pro04 strips (from the same console these transformers were taken from) - I have racked two already, but then I included the bus/group-channels in which the transformers originally were sitting.

Here is the exact reply I got from Sowter:
This transformer has a 30 ohm CT primary. The secondary is 600 ohms tapped at 200 and 30 ohms. You can check the windings with a resistance meter.  The dc resistance will be 5%  to 10% of the nominal impedance.

goldenGeek

Re: Transformer newbie / how to hook up transformer
« Reply #4 on: January 31, 2010, 11:44:59 AM »
So, I guess I'll simplify the question... Lets say you have an unbalanced channel strip that you want to stick a trafo on the output of to balance and to add some "mojo" or whatever you call the sound :-) Do you simply put a trafo with the right resistance and config directly on the output or does it need additional components in the path before the trafo?

Mark Burnley

Re: Transformer newbie / how to hook up transformer
« Reply #5 on: January 31, 2010, 01:36:27 PM »
Well,

The answer is -it depends on lots of factors.

Most channel strips that are "raw"- i.e. not installed in a desk work at "internal" levels. This can vary depending on the make/type of desk. As a rule, most lower-to-mid spec desks have an unbalanced output to the stereo mix bus (presuming your channel has a pan and fader control) and will operate at anything from -6dB w.r.t 0VU (+4dBu) to anything lower, depending on the architecture of the original desk.

So the first thing to do is to hook it up, apply an input signal to the mic/line input, set the gain and fader controls to "0dB" to allow (hopefully) 0dB signal gain through the channel, make sure any EQ or filters/pads are "out", and then measure the signal voltage at the bus (or group!) output at the edge connector.

Check the schematic to see where the mix "build out" resistors are- depending on the channel strip, the output of the fader/pan buffer will hit the mix-bus buffer, and this will have a resistor which acts as the input to the mix bus. If there is a resistor here, replace it with a link to optimise output signal and to ensure good response (this normally hits the mix-bus rail, that then goes to the mix-bus buffer/amplifier) Sometimes this will be located on a motherboard, but often on the channel strip.

Now, you should have a "direct" output from the channel. This is usually an op-amp stage which is decoupled with an output capacitor. Depending on the circuit and the internal level of the strip, it may have enough level to provide a useful output (even though it is unbalanced)

An output transformer can be used to make a balanced output by connecting the primary "high side" of the transformer to the unbalanced signal output, and the 0V (Audio Gnd) of the channel to the "low side" of the transformer primary. The secondary terminals then become the +Ve and -Ve ("hot" and "cold") balanced audio output (XLR pins 2 and 3). The Audio Ground for the balanced output can be the chassis ground (or channel 0V if this is connected internally- be careful with your grounding, and watch out for "digital ground" which often is used for switching/relays/LED's in the channel and is inherently noisy.)

To be honest, don't get too tangled up with the impedance of the transformer you use- with an output transformer for a solid-state application, it's the "Voltage Ratio" (i.e. direct turns-ratio) that is more important. If the channel strip is working at -6dB relative to output line level, a 1:2 (stepup by +6dB) transformer will do. Make sure as well that it is an actual output transformer too- an output transformer is specially designed to handle the possible higher output currents required. Using an e.g. 10k:10k input transformer will compromise your output drive! Also, be aware that the mix bus buffer/driver stage you're tapping this signal from is not optimised for driving a line output to the outside world! You may be able to modify the final op-amp stage, but be careful here- it's best to have a look at the schematic if you have any doubts.

If your channel strip has a much lower bus output, you'll need to add a signal-booster stage (just an op-amp stage) plus a line-driver output- e.g. transformerless DRV134 IC, or an op-amp + transistor buffer (as discussed here countless times) driving your transformer. Either way, watch out for your global signal polarity- i.e. you may need to flip the transfomer windings to get +ve signal polarity from mic/line input to line output. This is because you don't know what happened after the  mix-bus amp- it may have gone through patchbay insert buffers, or output routing, and don't presume there weren't subsequent inverting stages!

Best thing to do is just hook a transformer up to your channel and see what sort of levels you get. You may get mixed results, but experimenting is the best part of these projects!

Mark

 

« Last Edit: January 31, 2010, 01:38:56 PM by Mark Burnley »
O_O tape is life O_O

goldenGeek

Re: Transformer newbie / how to hook up transformer
« Reply #6 on: January 31, 2010, 02:17:31 PM »
Thank you! This was a spot on and absolutely fantastic answer to my question  :)

Mark Burnley

Re: Transformer newbie / how to hook up transformer
« Reply #7 on: January 31, 2010, 02:54:37 PM »
No problem.

Don't forget that you can find the turns ratio (i.e. step-up/step-down ratio) if you know the impedance ratio (ac impedance ratio of transformer- not DC resistance!)

If for example impedance ratio is stated as-

primary= 150R
secondary = 600R

impedance ratio = 1:4 (from 600/150 = 4)

turns ratio is square root of impedance ratio-

Square root of 1:4 = 1:2

Therefore- turns ratio of 150R:600R transformer is 1:2

Or a stepup of +6dB

(...or step-down of -6dB if you reverse primary and secondary connections)

Mark
O_O tape is life O_O

Re: Transformer newbie / how to hook up transformer
« Reply #8 on: January 31, 2010, 03:35:34 PM »
Quote
(...or step-down of -6dB if you reverse primary and secondary connections)

it might be stupid what I'm gonna ask but since you can reverse primary and secondary why there are both input and output xfmrs available?
I'm asking cause I'm actually looking to buy 600:150 input xfmrs and 150:600 ouput xfmrs at the moment.  :)
So, in other words 'is 1:2 and 2:1 the same thing depending on how you connect the primary and secondary?

thanks
w.


Mark Burnley

Re: Transformer newbie / how to hook up transformer
« Reply #9 on: January 31, 2010, 04:05:18 PM »
It's to do with the expected signal levels and power transfer across the magnetic circuit- a line input is usually a high impedance input (bridging of 10k or more) and so very little power (read- magnetc flux energy) is required to faithfully transmit the audio signal from primary to secondary.

A line output should be able to deliver full(-ish) undistorted level into a "traditional" 600R line termination without too much trouble. Even in the Modern World of >10k input impedances, a connecting cable can be seen as a complex load of R, L and C (overall seen as an impedance Z) and so the output stage has to be able to deliver power (i.e. a more substantial current at a stated voltage level- P=I*V) This requires a greater flux linkage path, and more efficient windings in primary and secondary, plus the ability of the core to handle the greater flux energy without saturating.

A mic transformer is different again- they are designed for much smaller signal levels, and have better CMRR and RFI/magnetic field rejection and screening (and smaller core size due to lower operating flux levels)

There are general purpose transformers which have specs which allow them to be used in input or output locations, but they are not optimised for either purpose.

Have a read of this Jensen PDF

...and this Mmuch larger 3MB Bill Whitlock PDF

Good reading...

Mark
O_O tape is life O_O


Re: Transformer newbie / how to hook up transformer
« Reply #10 on: January 31, 2010, 04:24:29 PM »
great! thanks  :)


Tubefreak

Re: Transformer newbie / how to hook up transformer
« Reply #11 on: February 04, 2010, 03:40:46 PM »
I've hooked up several trannies and they all seem to have a slight high pass filter. Something like -2dB at 40Hz. They were run straight from and to Lynx soundcard XLR connections. The ones I've tried are oa LL1539, T1442, VTB9045, VTB9057 and A-39. Do I need to add some resistors to eliminate the high pass filter effect? If so, what would be some rough guidelines?

Thanx,
Maarten

Mark Burnley

Re: Transformer newbie / how to hook up transformer
« Reply #12 on: February 04, 2010, 05:16:37 PM »
Have you swept out just a straight XLR cable connected from input to output?

Do a sweep of your soundcard so you can "normalise" your results by seeing what the frequency response of your measuring system is!

This will give you a better idea of what is going on. A bit like a "tare" function on weighing scales- it sets up the measuring system to see what errors are to be expected.

Always test the tester before examining results!

BTW, what was your test level?

Mark

O_O tape is life O_O

Tubefreak

Re: Transformer newbie / how to hook up transformer
« Reply #13 on: February 06, 2010, 11:33:41 AM »
thanx for the reply.

Did a straight in/out test and it's flat from 20Hz to 20kHz. Tests were done at 4dB and -10dB at 44.1 kHz with no significant differences. The high pass affect is something I notice with almost all of the transformers. Some have a slight boost around 15kHz.

According to Lynx it's outputs are 100 Ohm and inputs are 24k Ohm.

Hope this helps to identify the cause, or even better attributes to finding a solution.

Thanx,
Maarten

Rybow

Re: Transformer newbie / how to hook up transformer
« Reply #14 on: February 06, 2010, 02:56:33 PM »
I hope its alright if I just ask a couple of quick questions on transformers in this thread. I am a newbie when it comes to transformers as well, and I thought it would be better to ask in an existing thread than to start a new one. Sorry if I am hijacking.

I have the hook up all sorted, but I am concerned about placement. Is there a place inside the chassis where an output transformer should not be mounted? I know its good to keep it away from the toroid, but what if its close to the XLR's, will it add noise? I am using an OEP a262a2c, and the circuit is an LA 4.

Now onto another question thats been eating me. I am building a dual mono version of the LA4, and I will be trying to make each channel as different as possible. One of the things I am looking at is going with an output transformer that is not recommended on the BOM. I know I need an output transformer that will give 600:600, but are there any other specs that are important for me to look at?

One last thing. Switchable output transformers. From what I have read, all I would need to do is wire the transformers to a switch. Is this correct? No additional components? Is it safe to switch them when the unit is on?

Thanks in advance for the help!
« Last Edit: February 06, 2010, 05:02:31 PM by Insomniaclown »

substitute

Re: Transformer newbie / how to hook up transformer
« Reply #15 on: February 07, 2010, 08:06:35 AM »
Wow Mark that belongs in the transformer meta! 

Quote
Did a straight in/out test and it's flat from 20Hz to 20kHz. Tests were done at 4dB and -10dB at 44.1 kHz with no significant differences. The high pass affect is something I notice with almost all of the transformers. Some have a slight boost around 15kHz.

You can use a zoble network to passively tame the high boost, but that wouldn't do anything for the low cut.  Since you're getting that on a number of xfos from differnt manufacturers that are all high quality maybe thats not unusual and not something you should worry about correcting.

Quote
I have the hook up all sorted, but I am concerned about placement. Is there a place inside the chassis where an output transformer should not be mounted? I know its good to keep it away from the toroid, but what if its close to the XLR's, will it add noise?

Just keep it away from the PT, and any high voltage leads.

Quote
I know I need an output transformer that will give 600:600, but are there any other specs that are important for me to look at?

One last thing. Switchable output transformers. From what I have read, all I would need to do is wire the transformers to a switch. Is this correct? No additional components? Is it safe to switch them when the unit is on?

Any 600:600 will do, I recall luny saying the circuit seemed adaptable to using different xfos.  I don't know how drastic of a difference changing the output xfo would make, but if you mess around you might stumble across something you really like.  People consider the OEP's to be pretty colored, maybe you could use audiox's balancing amp instead.

Rybow

Re: Transformer newbie / how to hook up transformer
« Reply #16 on: February 07, 2010, 01:43:34 PM »
Thanks for all the advice! I am not too experienced with different transformers, so I wanted to make sure I could just plug in a 600:600 transformer to see whats what. Thanks again!

Re: Transformer newbie / how to hook up transformer
« Reply #17 on: February 07, 2010, 01:48:18 PM »
Can someone please tell me what CT stands for in some transformers?
ie 1:2CT

thanks
w.

edit: doesn't mean Center Tapped, does it?  :)

Mark Burnley

Re: Transformer newbie / how to hook up transformer
« Reply #18 on: February 07, 2010, 03:16:16 PM »
Yep,

CT = Centre Tap

...note that this means the mathematical "centre"- i.e. the tapping connection is at the electrical centre (with respect to the winding ratio- no. of turns from Start and Finish of winding) of the winding, and not the physical centre pin of the transformer body/casing.

Often the "centre pin" is an electrostatic ground/screen or just a "dummy" pin to allow mechanical stability on the PCB.

Mark

O_O tape is life O_O


 

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