Question about different operating levels and xfmrs.

[warpie]

ok then, I'm trying to understand a bit more the whole xfmr theory/practise/etc. I'm reading posts and links that I found here but a question was generated by reading the following info:

Operating Line level was 0dB = 0.447 volts. This is a legacy of EMI's earlier consoles which worked at an operating impedance of 200 ohms, as opposed to the US operating impedance of 600 ohms. 0dB Line level in a 600 ohm system is 0.775 volts (1 milliwatt) with the usual operating level being +4 dBm which is 1.228 volts. The TG operating level is 0 dB into 200 ohms, which is 0.447 volts. This is 8.8 dB lower than +4 dBm (usually rounded up to a nominal 9 dB).

Item 1 above makes it slightly awkward to use these consoles with modern recording equipment operating at +4 dBm line levels. Although most tape machines have sufficient gain in hand to allow for adjustment, other equipment sometimes needs a +9 dB buffer amp on the send and a simple resistive attenuator on the return.

Ref: http://www.mercenary.com/emitgconsole.html


So, the question:
Let's say I own an EMI desk (I wish  ;D) or any other old equipment with operating level 0 dB into 200 ohms and want to match the impedance with modern equipment (0 dB into 600 ohms).

>Is it correct to assume that a 1:3 input xfmr will match the impedance from 600ohms to 200ohms and then another output 200:600 xfmr will bring the impedance back to 600ohms?

>If my above assumption is correct,  should I expect that the xfmrs will 'solve' the -9db loss problem?

>is it what explained from mercenary as "...other equipment sometimes needs a +9 dB buffer amp on the send and a simple resistive attenuator on the return." ?

>What's the solution for using older with modern equipment? Do all the studios modify their old gear?

Sorry for the storm of questions but I'm trying to understand some of the benefits of using tranformers (apart from the sound coloration and the better SNR) 

thansk
w.

 

[doubleroger]

I think 200:600 is not 1:3 as it seems.
150:600 is actually 1:2, IIRC. There is a square somewhere in the formula.
1:3 with 200 ohms primary would be 200:1600.

So the rest of your theory is wrong I'm afraid.

>Is it correct to assume that a 1:3 input xfmr will match the impedance from 600ohms to 200ohms and then another output 200:600 xfmr will bring the impedance back to 600ohms?
Yes with the right ratio transformers. A pad on the output is a more cost effective solution though.

 

[abbey road d enfer]

ok then, I'm trying to understand a bit more the whole xfmr theory/practise/etc. I'm reading posts and links that I found here but a question was generated by reading the following info:
Operating Line level was 0dB = 0.447 volts. This is a legacy of EMI's earlier consoles which worked at an operating impedance of 200 ohms, as opposed to the US operating impedance of 600 ohms. 0dB Line level in a 600 ohm system is 0.775 volts (1 milliwatt) with the usual operating level being +4 dBm which is 1.228 volts. The TG operating level is 0 dB into 200 ohms, which is 0.447 volts. This is 8.8 dB lower than +4 dBm (usually rounded up to a nominal 9 dB).
Item 1 above makes it slightly awkward to use these consoles with modern recording equipment operating at +4 dBm line levels. Although most tape machines have sufficient gain in hand to allow for adjustment, other equipment sometimes needs a +9 dB buffer amp on the send and a simple resistive attenuator on the return.
So, the question:
Let's say I own an EMI desk or any other old equipment with operating level 0 dB into 200 ohms and want to match the impedance with modern equipment (0 dB into 600 ohms).
>Is it correct to assume that a 1:3 input xfmr will match the impedance from 600ohms to 200ohms and then another output 200:600 xfmr will bring the impedance back to 600ohms?

The correct xfmr would be a 1:1.732 turns ratio. The impedance ratio is the square of the voltage (turns) ratio.

>If my above assumption is correct,  should I expect that the xfmrs will 'solve' the -9db loss problem?

No, because the TG operates at 1mW and the "+4dBm" standard is 2.5mW (1.228sq/600). You would still be off by 4dB.

>is it what explained from mercenary as "...other equipment sometimes needs a +9 dB buffer amp on the send and a simple resistive attenuator on the return." ?

Yes

>What's the solution for using older with modern equipment? Do all the studios modify their old gear?

Depends on what's easier. Some gear actually accepts operating with non-standard level (you mentioned tape recorders, compressors offer some versatility too), other units are not too difficult to modify, and some cannot so one has to install active level adaptors.

...the benefits of using tranformers (apart from the sound coloration and the better SNR) 

IMO I don't think there's any other significant advantage in using transformers. Some claim their ability to avoid RF interference, but it can be done without a xfmr, and, although most transformers offer rather good CMRR, most of the modern transformerless circuits excel too in this regard. Better SNR: only in mic preamps (or some esoteric task such as elevating the impedance of a sync head ot a MC turntable cartridge). A good xfmrless mic pre has a noise factor < 1dB. But I agree, if you want a tube mic pre, you need a xfmr.

 

[Mark Burnley]

Also,

In my experience a good audio transformer is incomparable at breaking ground loops in installations. You can do as many different combinations of screen-dropping at the send and receive of a piece of equipment, but galvanic isolation using an audio transformer will always work- especially in large buildings across different mains supplies (e.g. in large studios, broadcast, theatre applications etc.)

I always keep a few in my tool box!

Mark

 

[lassoharp]

So, the question:
Let's say I own an EMI desk (I wish  Grin) or any other old equipment with operating level 0 dB into 200 ohms and want to match the impedance with modern equipment (0 dB into 600 ohms).

>Is it correct to assume that a 1:3 input xfmr will match the impedance from 600ohms to 200ohms and then another output 200:600 xfmr will bring the impedance back to 600ohms?

With most modern equipment you'll most likely be dealing with input impedances of 5-10K or higher.  You don't need to 'match' here.

You can use a matching transformer such as a UTC A-20 to match a 600ohm line to a 200ohm line provided the loads on either side are in fact 600 and 200ohms. This can be done with a resistor network as well. There is a certain amount of inherent loss of signal level that will occur with either type in this particular case.  This would work in the latter case under the same conditions, roughly making up the signal lost from initially stepping down. But as said, for typical situations in a modern studio you don't need to 'match' in this way. 

The big issue you have to watch when using older gear with modern stuff is terminating the output transformer with the correct resistive impedance.  200 expects 200, 600 expects 600, etc. So, use the correct value resistor to terminate, then use a 1:1 type for sonic 'flavor'.

 

[abbey road d enfer]

Matching impedances is one thing; it ensures proper frequency response.
But the issue of getting the level right when using a nominal 0.447V output to drive a nominal 1.228V input may only be solved by adding an active stage.

 

[warpie]

I know I'm late here but things have been a bit crazy at work lately
anyway....

I was looking around for some more EMI desks info and found this (handbook) if anyone wants to take a look.

http://vintageking.com/EMI-24-Channel-TGI-Console-used

Some pretty interesting info  there and I think it says somewhere about the 1:1.732 xfmr that also abbey road d enfer
mentioned (thanks for all the info btw)

However, I'm not sure if I have understood everything  :

For example, if you buy a racked EMI tone control (EQ), it should have a 1.732:1 input transformer (600:200),
a 1:1.732 (200:600) output transformer and the signal will still be 4db less than modern equipment, unless you add an active stage... right??  

and if that's the case I suppose you prefer to have 4db loss, since adding an active stage would probably 'alter' the sound.
Does all the above make any sense?  

 

[abbey road d enfer]

I was looking around for some more EMI desks info and found this (handbook) if anyone wants to take a look.
http://vintageking.com/EMI-24-Channel-TGI-Console-used

The link is kaput...

For example, if you buy a racked EMI tone control (EQ), it should have a 1.732:1 input transformer (600:200), a 1:1.732 (200:600) output transformer and the signal will still be 4db less than modern equipment, unless you add an active stage... right?? 

No, because, the system will be unity gain; when you feed it with +4 dBu, the EQ will be "overloaded" by 4dB (which it should normally withstand), and the final output will be +4dBu.

 

[Biasrocks]

The link is kaput...

Try this one

http://vintageking.com/site/Pdf_specifications/EMI_Recording-Console-Handbook%20Mk2.doc

Mark

 

[abbey road d enfer]

Thanks for the link, Bias.
Too bad there are no schemos, only text.
But everything is in there: "The output transformer with which it is used has a step up ratio of 1:1.78 thereby yielding the desired overall gain of 10dB.
The Amplifier U which is used to drive a V.U. Meter requires and input at a level of –10dBV...
...At the secondary of the transformer the output impedance is about 200 this being mostly the resistance of the transformer.