Hi guys, this is my very first thread, please be kind! I'm asking your advices since I have an old mixing console with mic transformers per channels.
my first attempt was a 10 kohm resistor in series with signal path + a 470 ohm shunt after it, in parallel with the primary winding.This solution is good for the input impedance but then the signal feeding the transformer was almost killed needing to be followed by an absurd gain stage that would add a lot of noise and nonlinearity..
So i tried experimenting with resistors and a network that let me keep a good secondary signal level is a 1kohm resistor in series with the signal + 220 ohm shunt, in parallel with the primary winding.The transformer was used to show a 200 ohm input impedance and has a ratio of 1:10 and the secondary is loaded with 47 kohm
(but since I use it with an unbalanced connection the ratio should be halved if I'm not wrong).
Anyway I just wanted to know if this route may have sense since in this case my input impedance is quite low respect to normal line level input impedance (in my case is around 1,2 kohm).
By the way, do you thing it could be a better alternative swapping the secondary load, originally 47 kohm, with a 1 Mohm?
If I'm not wrong in this case the primary winding should exibit an impedance of 10 kohm without needing any network, may this be a better choice?
In the latter case, I'm ok to adjust the following preamp stage to avoid clipping since the voltage would be boosted of 5 times
(since I use it unbalanced).
Having a schemo would help understanding the problem.
This would indicate that the actual input impedance of the mixer is absurdly low...which I don't really believe. I would rather think you made a mistake with your connections.
That would result in an input impedance of about 500 ohms, which is quite low according to current standards, but may have been considered normal at the time.
No. This 6dB thing is an issue with some sub-par electronically balanced stages. Not with transformers. Have you connected the cold leg of the input to the cold leg of the source (which may be the "ground" in an unbalanced output)?
As long as you know it and use it only with low-Z sources, who cares?
Depends on what's there, a tube, a transistor, an opamp...?
That would be true only if the inductance of the xfmr's primary is high enough; if not, there may be a lack of bass.
No; the step-up ratio of the xfmr is fixed. The input impedance may change, but the step-up gain will not. I don't understand that comment; the ratio of a xfmr does not change with how it's connected.
The mic Input Impedance is super low since was planned to give 200 ohm impedance, quite old design. I modified the secondary loading it with 47kohm
Since the primary is center tapped
if I use it with an unbalanced connection the current I feed into it runs an halve of its windings and the ratio should step up in this case, am I wrong?
So, with your attenuator of 10k in series and 470r shunt, that would result in about 34 dB attenuation. That is not in accordance with your comment "the signal feeding the transformer was almost killed needing to be followed by an absurd gain "; a line level with 34 dB attenuation would result in about -30 dBu. The preamp gain should be about 30-35dB, which is not absurd. So one element of data is missing here. What are you using as line source?
That is quite unexpected! If you're using only one leg for signal, that may explain the extra loss.
True, but the impedance would be much lower, 50r instead of 200. So the attenuation would be 12 dB more.Really, a good schemo, with the description of your connections would help solving the mystery.
About my padding network would you sudgest to increase primary impedance just working on secondary loading or you think it may be good to keep the pad?
By the way I found a quite good balance using a 2,2kohm input resistor + a 68ohm shunt on primary and 47kohm + a 220 nF as Zobel on the secondary.
Lower cap values where uneffective and on my first squarewave test I had a really distorted wave (poor bass response).I guess the reason is that the transformer were handling more power than expected so the low freqs were struggling, the 68 ohm shunt resistor on primary helped a lot.
Source impedance can also affect secondary resonance. I have observed this in three 1:10 mic transformers by Sowter, Cinemag and Jensen. With a 150 ohm source their response is platform to 40KHz with a 150k secondary load. With a 50 ohm source there is a 4dB peak around 25KHz.CheersIan
These are two different subjects; LF response indeed is influenced by level and source impedance, and the Zobel network acts at HF. The 68 ohm resistor providdes the primary with a low source impedance, which helps with LF frequency response. The attenuation provided by the 68 ohms and whatever series resistance reduces the level, so it helps LF distortion.HF response is tamed by the 47k resistor, and the 220nF is useless, you may as well have a jumper there. This is brute force, as it loads the secondary all over the audio spectrum; a Zobel is more subtle, as it loads only in the frequency band where it's necessary, in order to compensate the resonance of the secondary.The 47k is probably ok, but you should decrease the capacitor, whilst observing the frequency response and optimizing the HF peak.
Well, my primary recommendation was to disconnect the center-tap from ground; have you done it? Any attempt at optimization is vain as long as it is not done, because it actually shorts the primary.
I tried to disconnect it but the the trafo didn't worked, I was able to just hear some tiny high freqs but nothing more.Anyway the original wiring configuration was:Primary (CT):- Black -> Signal (Hot)- Withe -> Phase (Cold)- Grey -> GroundSecondary:- Red -> Signal Transformed- Green -> GroundTo have output on the secondary I needed to have the Grey wire grounded.
If you disconnect the center tap, you have to connect the cold leg (white) to the ground of the incoming signal; the circuit needs to be closed. then you'll have 200 ohms input impedance, instead of 50 (or even less).
The withe was grounded.Maybe I had a failing contact, I'll try again this morning.