[isophase]

Hi all,
I'm about to finish wiring the dual mic pre from Elliot Sound Project 66.
i have two spare EOP A262 A2E output transformers to use and thought using them for this project.
Is this circuit capable of driving this output Xformer correctly? and what is the best configuration?
600+600ohms in series as primary, and 150+150ohms in series as output/secondary?
Also is it worth it adding this output transformer to this circuit? any advantages apart from floating output and i guess little harmonic distortion mojo?
Thank you

[isophase]

EOP A262 A2E data attached for reference

[PRR]

.

[ruffrecords]

This transformer is only suitable if driven by a very low source impedance. Notice each primary winding has an inductance of 125mH so if you wire them in series you get 500mH. The impedance of 500mH at 20Hz is only 63 ohms so the NE5532, which is designed to drive a 600 ohm load, will not be happy with this.

OEP make many excellent transformers at keen prices but for power output you will need one with a much larger core. Try something in their K30 range.

Cheers

Ian

[isophase]

Thank you Ian & PRR,

I checked on OEP site and find it a bit confusing.
This transformer is described as 600ohm C.T. primary to 2400ohm C.T. secondary (see attachement) but the data sheet states 150+150 primary and 600+600 secondary :-/
Doesn't resistance added in series?
You say 125 mH + 125 mH in series gives 500mH? or is it 250mH?
I think i will never understand transformers..

How about if i reverse the transformer and use only on winding (600ohm) as primary ?
Will the THD figures be better?

Also if i want to add a bypass switch around the output transformer, should the output XLR connector have pin 1 & 3 shorted when in bypass mode?
Will there be a decrease in level when the transformer is inserted in the circuit? is it negligible?

Thank you!

[squarewave]

I checked on OEP site and find it a bit confusing.
This transformer is described as 600ohm C.T. primary to 2400ohm C.T. secondary (see attachement) but the data sheet states 150+150 primary and 600+600 secondary :-/
Doesn't resistance added in series?

Yes, but a transformer is like a lens in that it just reflects the load on the other end. So if you have 300 ohms source on the primary, the secondary will look like 1200. But if you have 600 ohms on the primary, that is ok too and so it looks like 2400 on the secondary. But if you put 10k on the primary, it's not necessarily going to be 40k on the secondary because the frequency response will start to get wonky. You'll get peaking at HF and, as Ian pointed out, the impedance is capped at low frequencies because the inductance is a little low.

You say 125 mH + 125 mH in series gives 500mH? or is it 250mH?

I think i will never understand transformers..

No you're right it is 250mH.

Actually it is 500mH because of mutual inductance as Ian says below (the inductors share the core so it's 4x and not just 2x).

How about if i reverse the transformer and use only on winding (600ohm) as primary ?
Will the THD figures be better?

Yes, you could flip it around and get a 1200:300. But then you're stepping down 4x or 12dB 2x or 6dB which is slightly counter productive for a microphone amplifier.

Also if i want to add a bypass switch around the output transformer, should the output XLR connector have pin 1 & 3 shorted when in bypass mode?

It's not really a great idea to leave the transfomer hanging on the OA and then also drive the line directly. You should use a two pole switch so that you can redirect the OA output to a coupling cap w/ drain resistor into 68R or so in series to pin 2 and then the other pole to switch pin 3 to a 68R to analog ground (this is called an "impedance balanced" output and will help reject ground noise). Pin 1 should be connected to the chassis which should not be connected to analog ground at that point. Ideally analog ground and chassis ground should be connected at one point at the power supply filter cap ground (or wherever the chassis connects to the PS ground).

[ruffrecords]

Thank you Ian & PRR,

I checked on OEP site and find it a bit confusing.
This transformer is described as 600ohm C.T. primary to 2400ohm C.T. secondary (see attachement) but the data sheet states 150+150 primary and 600+600 secondary :-/
Doesn't resistance added in series?
You say 125 mH + 125 mH in series gives 500mH? or is it 250mH?
I think i will never understand transformers..

Transformers a really simple in principle. They simply transformer voltages, currents and impedances according to their turns ratio. They have no instrinsic impedance of their own so when a manufacturer says this winding is 600 ohm it really means this transformer will work best at this imedacnce.

Inductors in series add. So two  125mH inductors connected in series will be 250mH. BUT, in a transformer they are not two separate inductors, they are on the same core. In this case the inductance is proportional to the turns squared. SO two winding is series have double the turns and hence four times the inductance.

Cheers

Ian

[mhelin]

Yes, you could flip it around and get a 1200:300. But then you're stepping down 4x or 12dB which is somewhat counter productive for a microphone amplifier.

Isn't it 1+1:2+2 = 2:4 = 1:2 or 2:1, same as sqrt(1200/300), reversed (both pri and sec in series), so only 6 db loss? That would be still quite okay.

[abbey road d enfer]

Isn't it 1+1:2+2 = 2:4 = 1:2 or 2:1,

In terms of voltage, it's correct, but in terms of impedance 1+1:2+2 is equivalent to 4:16
150+150=>600 and 600+600=>2400

[isophase]

Merci!
I will wire the transformer on one channel only and compare with the other see if i like it.
Will start with 1:1 configuration as suggested by PRR and experirement a bit with the ratios to see how it sounds.
6 or 12dB loss should be ok, as planning to use it mainly to record rock music (drums, amplifiers...)

What is the output impedance of this mic preamp? is it set by R16/100ohms resistor?
Is it possible to lower this value to 50 or 30 ohms in order to lower the output impedance of the circuit?
is it better to use an TL071 or NE5532 if i decide to use output transformer?

If i decide not to use the output transformer, is it really important to implement "impedance balance" resistors on the output? or should i simply connect output from R16 to XLR pin 2 and connect pin 3&1 to chassis?

Thank you!



 

[moamps]

In terms of voltage, it's correct, but in terms of impedance 1+1:2+2 is equivalent to 4:16
150+150=>600 and 600+600=>2400

The datasheet says 150+150:600+600.

[abbey road d enfer]

The datasheet says 150+150:600+600.

Yes, that means that, with primaries and secondaries wired in series, the "nominal" impedance is 600:2400, but if wired as PRR recommends, it becomes 600:600.

[moamps]

Yes, that means that, with primaries and secondaries wired in series, the "nominal" impedance is 600:2400, but if wired as PRR recommends, it becomes 600:600.

So, is it or not "150+150=>600 and 600+600=>2400" as you already said?

[abbey road d enfer]

This transformer is only suitable if driven by a very low source impedance. Notice each primary winding has an inductance of 125mH so if you wire them in series you get 500mH. The impedance of 500mH at 20Hz is only 63 ohms so the NE5532, which is designed to drive a 600 ohm load, will not be happy with this.

It may be compounded by the 100R resistor in series with the output; IIRC, the 5532 can drive 150r loads, so it would work, albeit with safety margin. In addition, the xfmr would be driven by 100r, which, as you mentioned, is not favourable to low distortion.
Driving xfmrs with SS circuitry is always an exercise in compromise. The need for minimization of DC in the primary imposes taking measures by either inserting a capacitor (evil ), a resistor (that increases THD and minimizes headroom), or a DC servo (purists sneer at them because not minimalist).

[abbey road d enfer]

So, is it or not "150+150=>600 and 600+600=>2400" as you already said?

Yes it is. It seemed to me your post #10 expressed disagreement over my post #8.

[moamps]

Yes it is. It seemed to me your post #10 expressed disagreement over my post #8.

It is 150+150:600+600, not 150+150:600 because there are two secondaries.
It can be wired as 1:4, 1:16, 1:1 and vice versa (impedance transformation), and in some not critical cases it can be also used as
1:9, 1:25, 1: 2.25 and vice versa

[abbey road d enfer]

It is 150+150:600+600, not 150+150:600 because there are two secondaries.

I never wrote that; check post #8. When I wrote 150+150=>600, the notation => means "results in" or "induces", not "equal to". That's what I learned at school many years ago.

[squarewave]

Isn't it 1+1:2+2 = 2:4 = 1:2 or 2:1, same as sqrt(1200/300), reversed (both pri and sec in series), so only 6 db loss? That would be still quite okay.

Right. Sorry. I'm really messing up this thread.

The OEP A262A2E is 1+1:2+2 which indeed equates to 1:2 so if you flip it around 2:1 that's only a -6dB loss.

So if the output load is say 600 then the driver will see 2400 ohms and 1H inductance. That would work fine. If the output is going into a ADC, -6dB is probably not a horrible idea anyway.

If the output is feeding more analog gear, then maybe it's not such a good idea because you won't be exercising the full dynamic range of your gear which equals more noise.

[moamps]

I never wrote that; check post #8. When I wrote 150+150=>600, the notation => means "results in" or "induces", not "equal to". That's what I learned at school many years ago.

It is clear now what you like to say. Thanks.
I know what => notation means, and what confused me is that this can be suitably used as "translates to"  also, where
150+150=>600+600 is correct, IMO.

[abbey road d enfer]

what confused me is that this can be suitably used as "translates to" 

That's the problem; abbreviations and symbols are often context-dependant.
For example, => has another meaning in Java script.

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