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Old 29th November 2004, 08:46 AM   #1
Rox is offline Rox  Spain
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Default neve input transformer

Hi:

im triyng to make some neve mic input preamplifiers and would like to ask some questions here:

actually i have 10 mic input transformes (10468 , vt 22670 , T1454 are equivalent mic input transformers).

it works fine with mic input but would like to have line input as well, then i could conect bass, acustic guitar,... sinth..... but the problem is that i have not the line input transformer.
Actually i know there are thiferent transformers to mach the output impedance of the previous "thing" (other pre or instrument...). Look at the image atached and check the diferent aplications: primary series/paralell secondary serries/paralel....
for each line input, mic input and something intermediate input.
first of them is line iput, last is mic input (mines).

I thought i could use my mic input transformer as line input if i use it reverse way, i mean conect the line to the secondary known side and the primary known side would be the terminals to conect to the input point of the preamplifier. Check the image, lok at the db gain 10468 series series, +6db that would be -6db if i use it reverse that is exactly as the first one line input 31267 series series.

is it posible?, somebody has done it allready?, it is practical or the sound get undesirable? thanks
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Old 30th November 2004, 06:57 AM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
by line input do you mean no voltage change?
If so then there is a connection on the table showing equal impedance. Equal impedance means equal voltage in & out.
It looks like the balanced transformer you have has a 2 :1 turns ratio & by combining the turns you can preset it to give 4:1 2:1 &1:1.
I would not reverse the transformer. It will reduce your output voltage below your source voltage & make noise more of a problem. I would guess that your transformer will work in reverse without loss of transformer performance but I still stand by my previous statement.
Be wary of the hi impedance on the output, you should try to close couple it to the following input stage to reduce all capacitance to a minimum & thereby maintain freq response.
Can I assume you are using balanced inputs?
regards Andrew T.
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Old 30th November 2004, 08:28 AM   #3
Rox is offline Rox  Spain
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one thing is the gain on the transformer (db) and other is the impedance match.

Line and mic input are thifferent imedance sources. They must be adjusted to insert to a same point. Thats what the input transformers do. If you see, on te table i posted, there is a gain of -6 db on the series/series conbination of the line input transformer. And in the mic input transforers there is +6 db on the series/series conbination. Then if i input line on a mic input transformer, it will gain 6 db and it will clip the next stage (i gues the transformer could get cliped itself too). I need to get down the line level on the input transformer, then i thougt i can reverse the mic input transformer, it is +6 db the right way, it should be
-6db the reverse way then. (thats exactly what line transformers do, it wont match the impedances perfectly since it is 4,8K istead of 10K line input inpedance (see table). But i guess it is a god solution, just wonder if somebody did it).

I guess the series/paralell/series/parallen diferent conbinations are to match the closest impedance of the mic/line source we have since not all of them are same impedance. hi/low impedance you now.

and yes, i use balanced sources, thats the main use of the input transformers, it is for unbalancing purposes mostly.
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Old 30th November 2004, 09:01 AM   #4
moamps is offline moamps  Croatia
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Quote:
Originally posted by Rox
........ it is a god solution, just wonder if somebody did it.....
Hi,

It works, but this isn't an optimal solution (two transformers in series).IMO
Beter solution is using two separate interchangeable inputs; one mic input with passive transformers gain about 6 to 12dB and low input impedance thru XLR input connector and another line input (with transformers attenuation for impedance matching purpose eventually) thru jack connector.

Regards,
Milan
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Old 30th November 2004, 09:09 AM   #5
Rox is offline Rox  Spain
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sorry, it should be my english that confuses all of you.

i am not using 2 transformers in series. Each transformer, has diferent pins, so you can use the primary series or paralell and secondary series/parallel as well. It is one transformer but depending how you conect, it has diferent gain (ratios X:Y) and different impedances as well.

please have a look to the image on the first post. There are 3 input transformers, (line in, something intermediate in, mic in).

All transformers i have are MIC IN. But i would like to input line, bass, sinth.... but do not have line input non mid input transformers. Then, if you check the table, if you REVERSE the mic input ransformer (last one) you have somethin very smilar to line input transformer.

Once again, did somebody try it?
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Old 30th November 2004, 09:25 AM   #6
moamps is offline moamps  Croatia
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Hi,
microphone transformers have usually smaller headrom-saturation point (ca +6 to 9dBV) than line transformers. Reversing primary and secondary connection will works if your signal is below this limit.
I use sometimes this configuration for connecting a guitar pickup in mic input on PA console (passive DI-BOX).

Regards,
Milan
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