Making Neve Style Input Transformer for 1073/1290 preamp

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im not being funny but i dont seem to be getting very accurate results?
my DMM is a fairly good one and i set it to AC volts it jumps around a bit but steadily it does seem to climb in volts?
i try and set as close as i can to 0.775v but one time it was 0.742-0.744v and then when i put it as per your picture it was 0.782-0.785v.
but i notice if it takes me 30 seconds to rig back up from testing the signal gen voltage to wiring up as per picture the voltage has gone up from what i first tested!!! i know its only a few volts but thats what im saying here, this doesnt seem very scientific?
could you be really clear with this please, and what is the calculation?

regards

Spence.
 
AC ma?  is it an autoscale meter or does it have ranges?

if it has ranges, switch to the low range,

maybe there is so much permeabillity that the current is in the micro-amps and your meter is not sensitive enuff,

 
could anyone help with a way of relay switching the input transformers so i could use them back to front as Line and Mic?
I think i may need 3 relays but im not exactly sure of how to wire this up?
anyone have a schematic?

regards

Spence.
 
Thanks PRR! Brill

Do you happen to know where I should be using screened leads on the schematic, as I'm finding it a little confusing the way it's written on the original.
My left channel seems fine but there is definitely a bit of hum on my right channel, probably gnd loop. Just need to make sure I do the gnd and screening correctly.

Regards

Spence.
 
12 Henries is about right for a mic input, figure a 150 ohm mic, you want ten times the input  from the transformer at the lowest freq, 12 H at 20 Hz is 1500 ohms so there you go.

can't vouch for the hum problems,  not there,
 
right ive changed gnd connections to correct and all working really good, no hum, big sound!!!! and i mean BIG SOUND!!!!
now to build some more!!!!
thanks everyone for your help.

regards

Spence.
 
Spencerleehorton said:
the only way i can think of off hand is using 4 relays to reverse the transformer? is there a easier way?
If you are just going to use a toggle switch to change input configurations, they make 4PDT toggle switches. No relay necessary.

Wire per PRR's carefully drafted schematic.

Gene
 

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