Neve 10468 HI/LO Switch level difference?

Hi All,

In have mad a few 10468 Mic Input transformers for my Neve 1290 preamps and have seen i can use the 10468 for both line and Mic if i use a relay, haven't done this yet but would appreciate any drawings of how to use the relay to this please.

My question is this, i have rigged up the input transformer with a DPDT switch so i can either have the transformer in series or parallel, the only thing is the level doesn't seem to be that different when switching between the two?

I might implement the DPDT switch with the API-312 pad configuration, just wanted to check that the series/parallel switch version i am trying works or does work that well?

whats your thoughts?

regards

Spence.

Depends on the impedance of your source and the load on the transformer secondary. If your source impedance is low (50 ohms or so) then you should see a significant level difference (as much as 6dB). If your source impedance is relatively high (say 200 ohms) this will be significantly loaded in the 300 ohm position so there will be less change in level.

All this assumes the secondary is loaded with 4K8.

Cheers

Ian

Hi Ian,

when you say the secondary is loaded with 4.8k do you mean i should have a 4.8k resistor on the secondary?
I really do need to learn more about all this loading etc, I am taking an output spdif from my interface (96i/o) and into my dat machine, im then sending an audio out into patch bay then patching back into interface on channels 13/14.
Im assuming this is line level?

If so i need to use the 2:1 config so it see's 10k down to 2.5k right?

Ive got the 10468 wired as mic 1:4 (600/10k)

Im happy to learn!!!

Transformers do what they say on the tin - they transform voltages, currents and impedances. Voltages are transformed by the the turns ratio and impedances are transformed by the turns ratio squared.  The HI setting of the 10468 gives a turns ratio of 1:2. The secondary is normally loaded with 4K8 so if you look into the primary you will 'see' this 4K8 transformed by 2 squared down to 1200 ohms. If you parallel the primaries (LO setting) the turns ratio becomes 1:4 so the 4K8 gets transformed to 300 ohms.

Conversely, if you connect a 150 ohm mic to the primary in the HI setting it looks like 600 ohms at the secondary. This is loaded by the 4K8 across the secondary so there is little loss.

Note: the 10468 is intended as a low level input transformer. With the primaries paralleled the turns ratio is 1:4. With the secondary loaded with 4K8 the primary 'looks like' 300 ohms. It is not intended to be fed line level signals so it is possible in your experiment that there was some transformer saturation.

Cheers

Ian

I have heard that the 10468 can be wired with a relay to give both line level and mic level from one transformer,  i don't know how to wire this yet?
Any ideas?

Or could it be done with a lorlin? 4PDT?

Basically I wanted to add 48v, polarity, pad and line/mic switch to input.
I like the design of the api-312 front end but don't know if it will work well on a neve?
The pad on the api uses 2 x 470R and one 120R.

So to explain how I have it setup at the moment, I have the output producing 10k into the transformer which is seeing 600/10k (1:4) or when I switch it to parallel of PRI it see's 600/4.8k, 1200/10k.
So the mismatch of 10k to 600R is why I'm not hearing any drop?
I will switch round the transformer so it connected as line level.
If your able to figure out how to switch them with a lorlin and do one of your wonderful drawing that would be fantastic.

Spencerleehorton said:

I have heard that the 10468 can be wired with a relay to give both line level and mic level from one transformer,  i don't know how to wire this yet?
Any ideas?

Or could it be done with a lorlin? 4PDT?

Click to expand...

I have heard this but as far as I am aware it was never done at Neve. Like I said, the 10468 is a low level transformer. If you strap it 1:4 and swap input and output you get 4:1. If you load the new secondary with 600 ohms, the primary now 'looks like' 10K and it is a bit like the 31267 which is a 10K:600 transformer designed for line level inputs (up to +26dBu). However, the 10468 is not designed for line level inputs and will probably distort badly if used this way. There was a scheme at Neve to use a 10468 instead of a 31267 for line input but it involved adding a pad at the input of the 10468 to drop the level.

Cheers

Ian

I don't understand the bit where you say 'the output is producing 10K'.

As I mentioned, the 10468 usually has its secondary loaded with 4K8.. It is normally wired 1:2 so the input looks like 1200 ohms - this is ideal for mics of up to 200 ohms impedance. It is usual for an input pad to try to maintain the 1200 input impedance. Notice the 2x470 + 120 ohms of the API totals 1060 ohms (I use 2x680 + 150 = 1510 because my mic inputs are designed to look like 1500 ohms). Both of these will create a 20dB pad.

For a line input you need a higher input impedance (about 10K) and a bigger voltage drop in your pad to avoid overloading the transformer.

Cheers

Ian

I'm meaning from my dat machine output, da-30 output unbalanced is 430R.
I use this output for summing back into my interface.
So now I don't really understand what I need ?

Could you suggest something which would be better for my setup please Ian?

Ok I've drawn out a chart now and I see that 430R falls into line level 100R-600R is line level right?

I have heard the 10468 works well as line level txf?
I'm not getting my head around this!
Really want to suss this all out!
Doesn't help working till 4am!

So to get my head around this if we think about a mic which is 200R is put into the input of the neve, the neve sees 200R and the 10468 in 1:2 config sees 200R on pri so produces 800R on sec?

Do I have this correct?

I thought it was the 31267 that could be flipped for mic use

shabtek said:

I thought it was the 31267 that could be flipped for mic use

Click to expand...

That is right it can be.

Cheers

Ian

Spencerleehorton said:

I'm meaning from my dat machine output, da-30 output unbalanced is 430R.
I use this output for summing back into my interface.
So now I don't really understand what I need ?

Click to expand...

OK, level and impedance are two different things. Line level is around 0dBu or about 775mV. DATs typically output up to 2V so they are considered line level. Line level output typically have an output impedance in the range 75 ohms to 600 ohms. To accommodate all this without signal loss, and to allow one output to feed more than one input, it is normal; practice to make line level inputs present a 10K load.

So you really need a 10K line level input for your DAT to work properly. Let's say you use your 10468 as intended wired 1200:4800 feedng a mic preamp. You need to add a pad at the input to drop the level so the transformer does not overlaod and to raise the input impedance to about 10. So a couple of 4K& input resistors in the pad will raise the input impedance to nearly 10K. The mic input works best wit a 150 ohm source so use that value as the theird resistor in the pad. The attenuation will be 150/(150+4700+4700) = 36dB. The 10468 has 6dB of gain so the input to the mic pre is  nominally -30dBu for a 0dBu input. Set the mic pre gain to 30dB and you should be good to go.

Cheers

Ian

thank you Ian, that is very helpful indeed.

So as per the API-312 pad config where i had 2 x 430R connected to middle pin and end pin i would replae these with 4k resistors?
and would i still keep the 120R resistor on the top pins?

also should i still use the 2.2nf cap on the secondary as per the neve schematic?

right, well i didn't have any 4k resistors but i thought 4.7k is close enough.
Ive put them from middle legs on DPDT switch to end and put 120R on top legs, dont know if this is right but i get a nice drop in volume know when switching between the two and the signal sounds lovely.
I have done a comparison between feeding the signal through another 1290 preamp which has mic input txf and it red lights on the desk way before the level is up near the line level channel.
Its also looks like about 6db on the meter of the desk that the difference is between the two.

Am i in the ball park?

Here's a link to how I do a 20dB mic pad and phantom and mic/line input switching:

https://drive.google.com/open?id=0B_n67A1hN3qtVFNYd3FiT0RNVzA

Notice I use 3K3 resistors for the line in because I do not need so much attenuation. You can just use 4K7 instead.

Cheers

Iab

Thank for all your help Ian really appreciate it, I almost understand it all now!
Have wound another 10468 so will try out both channels now.

My other input question would be about instrument level, I do want a dedicated input for my bass guitar.
Is the line level ok at just 10k, I've read instrument impedance is 20k?
Yes I have several di boxes but just want to totally understand all my inputs.