implement a 1:1 transformer into a circuit

Is it possible to add an input 1:1 (600/600 or 10k/10k) transformer to a, lets say Green Pre? Just to add some flavour..?

Yes. On the input youd probably want a step up, 1:2 for instance. On the output 1:1 600:600 should be fine.

I cant remember the last time I used a mic pre without a transformer somewhere in it!



M@

I'm not that familiar with the Green Pre, but make sure the output stage can drive a transformer... if not, you'll fry some components.

[quote author="mattmoogus"]On the input youd probably want a step up, 1:2 for instance. [/quote]

Would this not run the risk of quartering the input impedance?

How about using the 1:1 transformer in place of the differential amplifier (IC2a) in Green Pre?

[quote author="kubi"]If your driver is too week, you would just loose some bottom end, but you wouldn't kill any more components then you would by terminating the circuit's output with the same load directly without a transformer.[/quote]
Exactly, that was my point. Make sure the Green Pre can drive the transformer of choice... ie don't choose an API 2503 for the output since I doubt it can drive a 75 ohm load. If it can drive 600 ohm load, then I suspect everything will be ok. I was just hoping to prevent someone from slapping any trafo on the output.

The reasons for using a step up input (1:2) is because it would probably give you better noise performance than a 1:1, I dont know of any 1:1 mic input transformers and as the purpose of this exercise is to get more 'sound' a step up would be more appropriate. Im not sure what the green pres input Z is set to but if its 2-3k like many similar designs then a 1:2 should give you around 600ohms in, perfect. But I dont know, I havnt seen the circuit.

Unless you use a step up transformer on the output (no reason to here) the output opamp should be able to drive the reflected impedance (almost certainly 10k in this case) down to 600ohms ok. Its a 5532/4 on the output isnt it?

Maybe you should just leave the green pre as it is and build a simple transformer pre instead. Then you have both sounds available!


M@

Yes, step up is usually done to improve the input noise figure. But 600 ohms input impedance is not really "perfect." Most mic manufactuers recommend at least 1k. Lower input impedance can result in gain loss, bass loss, and higher distortion. There's a paper on the Neumann website that explains how and why a preamp with low input impedance reduces max SPL. The effect is different for different mics. Some mic output stages work more or less distortion free up to a certain point, others gradually increase distortion when they have to drive a low load impedance.

Technically perfect maybe not (although thats still arguable - what mic cant drive 600ohms? certainly the new Neumanns can with ease) but were talking about more 'sound' here and from my experience 600ohm input mic pres usually sound better. There are always exceptions (ribbons!) but 80% of the time, loaded sounds better to me.


M@

[quote author="mattmoogus"]what mic cant drive 600ohms?[/quote]

Well, some higher impedance dynamics and ribbons might sound a bit crap. A 600 Ohm mic would only drop half of its voltage across which would counter-act the benefit of the extra 6dBs you'd get from a 1:2, plus it's likely you'd encounter noticeable bass loss.

600 ohms is okay for most mics, but I really see no benefit in low input impedances. As rodabod says, there's some gain loss that counteracts the initial idea of having more gain. And then there are some mics that don't like low impedances. Quite a few dynamics are higher impedance than 200 ohms and even some condensers perform worse than spec'd when faced with low impeance inputs. I have yet to find a situation when low impedance sounds better than normal/high impedance.

Concerning bass loss: it dosen't only happen to dynamics and ribbons but also to condensers. Many condenser mics have outputs coupled via electrolytics. Because of space limitations those are often 22-47u. When you lower the input impedance of the preamp the high pass cutoff frequency moves toward the audible range. (Don't forget the electrolytics in the pre, which are in series with the mic's electrolytics).

Most 1:1 's are pretty transparent, it's not til you hit about 5:1 or 10:1 do you get any "flavor".
Unless you are talkng about driving a transformer past it's db rating or biasing the core with dc current.
But how would you overdrive an input with a mic?

BTW, found out that the UTC A-10 is really only good to 10 db, not 16 like they advertise.

Aslo, you can use 1-2 and 5-6 to gat a 20 ohm input imp, good for moving coil, this secret courtesy of Doc Hoyer.

BTW, I found out that a mic transformer, like on the SM 57, wants to see a very low imp/resistance pri. This is because the moving coil puts out Cyrrent, not Voltage.
It's the transformer's job to turn this current into a voltage.
So, if modding a 57, go for an input tranny with only a few ohms dcr.

CJ, what's the important factor on the SM57 - primary DCR resitance or reflected preamp impedance?

Well, this is the transformer in the mic body, not the mic pre input iron.

I measured one or two ohms on the stock 57 iron.
I think the inductance figures are in that 57 mod thread, wherever that is.

Okay, that makes sense. It's probably not so different from the ribbon situation, except that the level from the 57 capsule isn't quite as low and the capsule's DCR isn't quite as low as a ribbon's.

Btw: I wonder what the SM7 transformer looks like. For all I know it uses the same capsule as the SM57.