Transformers in series?
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NewYorkDave
Well-known member
It'd be helpful if you'd tell us why you want to do this...
rafafredd
Well-known member
I have done soem tests with very tiny transformers and it has been working nicely. Sometimes you want it when you have low inductance and small transformers, and needs more inductance for good frequency response and also more leve handling. I did not tried it as mic input, but as line output. Three small very low inductance tyransformers worked nicely as a line output transformers. Three in series!
I think this thread belongs to the drawing board.
I think this thread belongs to the drawing board.
Family Hoof
Well-known member
I use two JLM 1:4 transformers with the primaries in series and secondaries in parallel for a 1:2 mic input. Joe Malone recommended it. I can use the input center taps or change the series-parallel relationships for variable input impedance. I'm going to do listening tests this weekend and will report back with the results.
> series them in this fashion
No. As drawn, when you are not connected to any primary tap on T2, the secondary of T2 is "dead" and high-Z, so output will be very poor.
Also the marked impedances are all wrong, but that's probably unimportant.
Are you adding this in front of a diff-input amp? Then use one 600/150:600/150 transformer with all windings in series as an auto-transformer. If the whole string goes to the mike-amp, then you can get voltage ratios of 1:4, 1:2, 3:4 (1:1.33), and 1:1. Impedance ratios about 1:16, 1:4, 1:2, and 1:1. If the load is a typical 2K mike input, then the mike sees 125, 500, 1K, and 2K ohms. If the mike is 125Ω, then the actual change of signal level over the whole range of taps is not quite 6dB.
More complex affairs are possible.
No. As drawn, when you are not connected to any primary tap on T2, the secondary of T2 is "dead" and high-Z, so output will be very poor.
Also the marked impedances are all wrong, but that's probably unimportant.
Are you adding this in front of a diff-input amp? Then use one 600/150:600/150 transformer with all windings in series as an auto-transformer. If the whole string goes to the mike-amp, then you can get voltage ratios of 1:4, 1:2, 3:4 (1:1.33), and 1:1. Impedance ratios about 1:16, 1:4, 1:2, and 1:1. If the load is a typical 2K mike input, then the mike sees 125, 500, 1K, and 2K ohms. If the mike is 125Ω, then the actual change of signal level over the whole range of taps is not quite 6dB.
More complex affairs are possible.
> you are wiring the primaries in series and not using the secondaries.
I'm using every winding on a 150/600:150/600 transformer. They are all about the same; primary or secondary is mostly a name-convention.
> I'm not familiar with an auto transformer
Something new to learn:
http://en.wikipedia.org/wiki/Autotransformer
I'm using every winding on a 150/600:150/600 transformer. They are all about the same; primary or secondary is mostly a name-convention.
> I'm not familiar with an auto transformer
Something new to learn:
http://en.wikipedia.org/wiki/Autotransformer
Family Hoof
Well-known member
Hmm.. Nice info, Thanks PRR! So I supose that is how Groove Tubes Vipre and Focusrite ISA 430 do it.
> So I supose that is how Groove Tubes Vipre and Focusrite ISA 430 do it.
I would presume no such thing, not until I opened one up (fat chance).
If impedance matters, and is good, the most obvious thing is to put a 1K pot (wired as rheostat) across the mike jack, and turn it down until the sound changes.
I would presume no such thing, not until I opened one up (fat chance).
If impedance matters, and is good, the most obvious thing is to put a 1K pot (wired as rheostat) across the mike jack, and turn it down until the sound changes.
TomWaterman
Well-known member
Hi - good info PRR!
If anyone listened to the interviews posted by Lynn Fuston, George Massenburg highlighted the fact that he knows of only one preamp in manufacture that uses the 'right' method to create variable input impedance - an Autotransformer.
I believe that to be the GrooveTubes Vipre.
I doubt the focusrite does this for some reason....but that just hearsay and I haven't seen one.
Cheers Tom
If anyone listened to the interviews posted by Lynn Fuston, George Massenburg highlighted the fact that he knows of only one preamp in manufacture that uses the 'right' method to create variable input impedance - an Autotransformer.
I believe that to be the GrooveTubes Vipre.
I doubt the focusrite does this for some reason....but that just hearsay and I haven't seen one.
Cheers Tom
volta
Well-known member
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Does a resisotor have to be in series with the rheostat to keep from shorting pins 2 and 3 of the mic input?
Also,this circuit would be in parallell with the input transformer.
2k(or whatever Rs value you use to get impedance) in parallell with 150 would be slighlty lower than 150 ohms wouldn't it?
Not much differrence..... :?
I'm probably not taking in to account that the output impedance of the entire circuit(pre amp) has an effect on input impedance.... I think? :?
NewYorkDave
Well-known member
Volta,
That last circuit you posted would work fine. I would suggest 5K for the pot and 150 ohms for the fixed resistor.
Assuming that the mic pre has an input impedance of around 2K (which is pretty average), switching in the circuit would vary the impedance seen by the mic from 1.4K down to 140 ohms.
That last circuit you posted would work fine. I would suggest 5K for the pot and 150 ohms for the fixed resistor.
Assuming that the mic pre has an input impedance of around 2K (which is pretty average), switching in the circuit would vary the impedance seen by the mic from 1.4K down to 140 ohms.
> Does a resisotor have to be in series with the rheostat to keep from shorting pins 2 and 3 of the mic input?
If the operator stupidly turns all the way and asks why it went dead, put a few ohms in there. I'm not sure what the point is, but play with the pot, find the lowest setting that is "useful", measure the pot, and stick that much fixed resistance in.
> this circuit would be in parallell with the input transformer. 2k(or whatever Rs value you use to get impedance) in parallel with 150 would be slighlty lower than 150 ohms wouldn't it?
I assume we want to LOAD the mike. You are right, a 2K input preamp with a maxed-out 1K pot gives 660Ω load on the mike; you show a switch to go back to the "unloaded" condition which is a wise plan.
The mike amp transformer may have been designed for 100-300Ω sources. Cranking the pot to a low-R setting will make the transformer see a lower Z source. That will typically "improve bass response" (actually reduce midrange more than bass), but might increase the size of the top-resonance in a high-Z secondary.
But hey. Changing the mike loading from the customary and generally-specified 1K-2K down to something lower either does nothing but reduce level, or it sucks-out the impedance peaks in the mike screwing with the frequency curve. If it also screws the transformer curve, that may be bad or good.
Mostly, I don't see that this loading does anything that couldn't be done better with good EQ. It is in effect an EQ locked into the mike design, not related to the sounds in front of it. On ribbon mikes, it may actually damp resonances and improve transient errors that could not be exactly EQed-out; dynamics don't have that much mechano-electrical coupling, and loading a condensor can't affect the diaphragm motion (you can only strain the output amp and load its transformer).
If the operator stupidly turns all the way and asks why it went dead, put a few ohms in there. I'm not sure what the point is, but play with the pot, find the lowest setting that is "useful", measure the pot, and stick that much fixed resistance in.
> this circuit would be in parallell with the input transformer. 2k(or whatever Rs value you use to get impedance) in parallel with 150 would be slighlty lower than 150 ohms wouldn't it?
I assume we want to LOAD the mike. You are right, a 2K input preamp with a maxed-out 1K pot gives 660Ω load on the mike; you show a switch to go back to the "unloaded" condition which is a wise plan.
The mike amp transformer may have been designed for 100-300Ω sources. Cranking the pot to a low-R setting will make the transformer see a lower Z source. That will typically "improve bass response" (actually reduce midrange more than bass), but might increase the size of the top-resonance in a high-Z secondary.
But hey. Changing the mike loading from the customary and generally-specified 1K-2K down to something lower either does nothing but reduce level, or it sucks-out the impedance peaks in the mike screwing with the frequency curve. If it also screws the transformer curve, that may be bad or good.
Mostly, I don't see that this loading does anything that couldn't be done better with good EQ. It is in effect an EQ locked into the mike design, not related to the sounds in front of it. On ribbon mikes, it may actually damp resonances and improve transient errors that could not be exactly EQed-out; dynamics don't have that much mechano-electrical coupling, and loading a condensor can't affect the diaphragm motion (you can only strain the output amp and load its transformer).
Family Hoof
Well-known member
PRR, you rock! :thumb: :sam:
