Input impedance of G1176 with OEP-TX (1+1:6.45+6.45)

Hi,

I just realized the input impedance of a
'1176-MnatsPCB-OEP-inp-TX-version' is
getting quite low.

Please correct me if I'm mistaken.
The suggested OEP is 1+1:6.45+6.45 OEP-TX (A262A3E).

Using this TX with the present 10k input-level pot would
give an input impedance of around 240 Ohms
(& skipping what's in // further down after the 10k-pot).

I should check the Mnats-PCB wiring if both the prim & sec
windings are in series each, but I thought they were.

If the sec windings are put in // then we'd get around 960 Ohms,
still low but OK.
Maybe they are already, I must check.

Anyone had any problems here ?


Thanks,

Peter

Using a 1:6.45 input transformer for a FET-compressor like 1176 seems wrong.

Use the 1+1:2+2 transformer in stead, wired as 1:1.

Same pin-out.

MJakob E.

Thanks Jakob for pointing out.

I didn't give the input-TX much thought & just started ordering...
- OK, so I'll get me another input-TX and use the 5532-frontend
meanwhile.
The 1+1:6.45+6.45 will find a use when I find time
for your G9 or for the mic-pre circuits Kev nicely suggested.

Use the 1+1:2+2 transformer in stead, wired as 1:1.

Click to expand...

OK, so that's the A262A2E again (as also used as output-TX but
there not as 1:1), thanks for the suggestion.

Thanks,

Peter

Isn´t the original 1176 input transformer wired as 2:1, 600:150 ?

I think its an O-12 and its wired 500/600 to 200....

I think thst if you use an OEP wired as 1:12.9, you probably can drive the 1176 with a mic. At least a condenser close micing, I guess... Or a really hot condenser that can spit out 1v or so... Not a ribbon, off coarse.

Can´t say if the results would really be nice, thought. One must try to know. Note that you would probably have to change or add one or two resistors to do it. Probably just the transformer termination. I don´t have the circuit in front of me right now. Gotta check later.

[quote author="rafafredd"]Isn´t the original 1176 input transformer wired as 2:1, 600:150 ?[/quote]

The original has no input transformer at all!

there were several revisions of the 1176, and the rev a thru rev f used a utc o-12, wired as a 500 to 200ohm stepdown, as the input transformer. in subsequent revisions, the input circuit used an opamp for debalancing.

it is important to note that in the original urei units, the t-pad attenuator was before the transformer. of the current reproductions, the ua unit has replaced the t-pad and o-12 with a cinemag transformer first, feeding into a more standard attenuator afterwards. the purple audio mc76/77 still use a t-pad into an o-12.

ed

from Rafael:

I think thst if you use an OEP wired as 1:12.9, you probably can drive the 1176 with a mic. At least a condenser close micing, I guess... Or a really hot condenser that can spit out 1v or so... Not a ribbon, off coarse.

Click to expand...

Since I have that OEP A262A3E on order anyway I'll be wiring it in a separate box untill it'll find a definitive place in another project (& trying to keep the sec-wiring short). Could add a DPDT for switching the primaries in series or in // (giving 1:6.45 & 1:12.9).

So it'll be easy to see how it does in front of the (G)1176-5532-input circuit.
Once both are finished I'm curious to see what it does with say a 421/441/57 in front of gtr/bss-amps. And that with & without limiting action.

Bye,

Peter

also from Rafael:

Can´t say if the results would really be nice, thought. One must try to know. Note that you would probably have to change or add one or two resistors to do it. Probably just the transformer termination. I don´t have the circuit in front of me right now. Gotta check later.

Click to expand...

I figure if there's still more gain required the 5532-circuit (if that's still used) could switch a few resistors indeed. Or were you thinking about other resistors ?

http://www.gyraf.dk/gy_pd/1176/1176sch.gif

Bye,

Peter

I would just use the stepup transformer feeding the FET directly. No opamps needed. I would wait for inputs from guys here before doing it, as I´m not really sure about what I´m talking about...

I would just use the stepup transformer feeding the FET directly. No opamps needed. I would wait for inputs from guys here before doing it, as I´m not really sure about what I´m talking about...

Click to expand...

Can try that, coming from TX-sec entering on the top of the INPUT-pot.
I'll be stuffing the 5532-section anyway, just cheap parts there, and then I'll skip it :grin:

Bye,

Peter

No. That´s not what I´m talking about. For using mics directly I think you should wire the secondary directly to the 27k resistor, before the fet. Also, you should remove the 270R resistor from the secnary of the input transformer and put something like 220k there. Also, you should try a lower value for the R27. A very low value, lke 150R. That should work, but let´s wait and see if someone can jump in and confirm my thinking. I´m not sure t will work, as you will have a very low current signal in there... So, don´t waste your solder-time with just my opinions yet. :wink:

from Rafael:

No. That´s not what I´m talking about. For using mics directly I think you should wire the secondary directly to the 27k resistor, before the fet.

Click to expand...

Right yes, that was the idea - but wait, you mean skipping the 10k (input-)pot as well ? We need that one :wink:

Also, you should remove the 270R resistor from the secnary of the input transformer and put something like 220k there.

Click to expand...

The 270 Ohm resistor is not present in the linked schematic but I remember it being present in the TX-input versions. Can imagine a higher value here yes, as in 'G9-style' high-ohmic loading of the A262A3E.

Also, you should try a lower value for the R27. A very low value, lke 150R. That should work, but let´s wait and see if someone can jump in and confirm my thinking. I´m not sure t will work, as you will have a very low current signal in there... So, don´t waste your solder-time with just my opinions yet.

Click to expand...

R27 seems to be the output-pot ? What's the thought here ? Just curious.

Thanks for the ideas / bye,

Peter

Sorry... I mean the 27k resistor, not R27. You should check the transformer input version. That´s what I have been looking at...

Sorry... I mean the 27k resistor, not R27. You should check the transformer input version. That´s what I have been looking at...

Click to expand...

Sorry for asking, could have been realizing that myself. I'll have a look at those earlier Revs.

Peter

Using a 1:6.45 input transformer for a FET-compressor like 1176 seems wrong.

Use the 1+1:2+2 transformer in stead, wired as 1:1.

Same pin-out.

Click to expand...

I've tried this out using the OEP A262A2E, paralleling the output windings. I get a very low output that lacks low frequencies - very much like the sound of driving terminal 1 of the input potentiometer directly with an unbalanced line level signal.

What have I done wrong?

I've tried this out using the OEP A262A2E, paralleling the output windings. I get a very low output that lacks low frequencies - very much like the sound of driving terminal 1 of the input potentiometer directly with an unbalanced line level signal.

Click to expand...

Hmm, could there be something else wrong ?

- I mean, why could it be that you'd get bad signal when driving the pot with an unbal line level signal ? (assuming an opamp with 100 or less Ohms output impedance)

Without having built it yet - looking at the schematic one would say it'd be working fine with unbal driving of that pot.

So I would have thought too. But if I put the opamp circuit back in it works fine. Must be doing something stupid...again.

Sure enough I was doing something stupid...like pulling the wrong wire out of the board and feeding the signal into it. Must have been the fumes from the floor varnish (I'm refinishing the kitchen floor over the holidays). Nah, I'm just dumb

Jakob's suggestion works fine and gives us the option of using the OEP A262A2E as the LINE input transformer. I'll get the next batch of boards made with the modified configuration and post some images of how to modify the earlier boards to use the OEP as a line input.

I looked through some notes and discovered who originally suggested the OEP-as-microphone-input idea. Would be nice if they would explain how they used the other OEP, what microphone they used etc. I also found the post where another lab member had built it with the other OEP transformer and didn't like the sound. Hopefully the A262A2E will get tested by better ears and judged accordingly...