Trident 80 non-inverting Mic Pre : gain?

[RFSiesta]

Yes!
We did rework the mic pres on our Trident 80 to non-inverting configuration.
For this the resistor R101(22k) is connected to ground instead of the transformer secondary, the non-inverting input of IC13 is connected to secondary instead and terminated with 100k to ground. This increases (and stabilizes) input impedance, which is good - the original inverting stage decreases Z with increasing gain.
Changed ICs to opa134, compensated the second stage with 10pF between out and in- for stability.
Sounds LOTS better! Clean, brilliant, tight, still with a character we wanted to preserve.

What's strange to me is that gain at zero position increased to approximately +12dB instead of 0dB.
This can be a problem with some really loud signals like kick drum or trumpet...
I now try to find a way to get the correct 0dB gain at position zero, if possible without affecting maximum gain.
I calculated (and tried) with changing R101 and R100(normally 22k), but didn't find a solution yet.
Any ideas?

Thanks!

EDIT: Attachment now works. Was a javascript issue...

 

[RuudNL]

It seems the schematic is missing. (Anyway: I can't see it!)

 

[ruffrecords]

It seems the schematic is missing. (Anyway: I can't see it!)

Same here. I have had this with several posts recently. Not sure if it is a side effect of the site upgrade. Sometimes you can see it  by opening the image in  another tab but not this time.

Cheers

Ian

 

[pucho812]

I got a 404 error when I try to open the schematic....

 

[rock soderstrom]

Same here. I have had this with several posts recently. Not sure if it is a side effect of the site upgrade. Sometimes you can see it  by opening the image in  another tab but not this time.

Cheers

Ian

Same here!

 

[PRR]

> rework the mic pres ...to non-inverting configuration.
> What's strange to me is that gain at zero position increased to approximately +12dB instead of 0dB.

Why is that strange?

What is the minimum gain of the Inverting configuration? Infinite loss.

What is the minimum gain of the Non-Inverting configuration? Unity!

It appears you have 1:4 (12dB) gain in a transformer.

And yes: 404.

 

[RFSiesta]

Well: I don't know how to correct this, it's just a jpeg.
Will try later.
Yes: The transformer is 1:4, of course the gain of 12dB is the transformer. What's strange is that noone has mentioned this, while the mod is widely known and even Trident seemed to have a later version with non-inverting preamp - and there's a Lundahl tranny version that's been built for the swedish broadcasting, I've read somewhere that it also has +12dB at zero.
Bo Hansen mentioned there would be a way to reduce it, I guess it must be a pad, then

- try to upload the pdf instead of jpg... hope this helps

 

[ruffrecords]

Well: I don't know how to correct this, it's just a jpeg.
Will try later.

Instead of linking to the image you should try just attaching it. See the options at the bottom of the posting box.

Cheers

Ian

 

[RFSiesta]

Instead of linking to the image you should try just attaching it. See the options at the bottom of the posting box.

Cheers

Ian

I uploaded both of them! Strange...
I might have a solution anyway that's better than a pad in front of the pre: Just change values in the second (inverting!) stage. Hope this doesn't affect sound or stability.
Let's have another try with the attachment:

 

[radardoug]

If you change the gain of the following stage then the first stage will still overload. Hmm, you need to think it through.
Also since you have reversed phase at the front of the pre, did you reverse the wiring on the transformer to bring it back to correct phase?

 

[pucho812]

The trident reissue rack mount channel strips of the 80B use a lundhal 1538 wired as a 1:5.... different then the console strips...

your increase in level could be handled by  simple paid on a switch... I would go with Bo hansel's suggestion on that.

 

[PRR]

> Hmm, you need to think

Without thinking, or knowing the pot values, I plotted this extreme possibility. You really can't manage this much gain-range on a single knob. The minimum gain is below unity (including iron) but inputs approaching 2V risk clipping the first stage.

 

[PRR]

Also some op-amps have poorer distortion when input pins swing, as in non-inverting use. When the input pins are servoed to virtual ground, the distortion may be less. Some of the older resistor-tail opamps would just freak-out for large common-mode swing.

EDIT- as you have replaced the opamps with non-antique type, this may not be a big concern.

 

[ruffrecords]

If you want to preserve polarity you could do the same mod to the second stage.

Cheers

Ian

 

[pucho812]

my trident notes have vr2 as a 100K lin...

 

[RFSiesta]

Thanks for the comments!
Yes, of course we restored polarity by also inverting the transformer inputs.
I want to avoid using a pad switch, it's mechanically hard to do on this board since there's not enough space left.
Changing the second stage would indeed handle the problem, I think, because I expect to have +-17V headroom per chip (OPA134 in my case), and only <2V on the converters. The converter clipping is the issue here.

Even though we have gain of 4 after the trannies already, I don't expect to ever have mic signals with output peaks over 3Vpp (->12Vpp at IC1), which would still result in no overload. If this should ever be the case, I could pad the mic with an L-pad adapter between mic and cable already.
Rail voltage is 17.5V in our board.

Yes: the dual pots are 100k, all resistors in the network are 22k. We think of changing R99 to 47k and R98 to 11k. Maybe only the latter, or we loose a little maximum gain... The OPA134 has much lower output Z and is able to drive smaller load resistances than a 071 does. There's already a 10p cap between out an -in of IC12, in case of instability I could raise this value to 15 or 22pF.

 

[pucho812]

Yes face plate real estate is a premium.  On our tridents at the backroom we had to have a phantom power switch on the face plate.  I forget  what size we used but if IIRC the switch was as small as we could find.

 

[RFSiesta]

Yes face plate real estate is a premium.  On our tridents at the backroom we had to have a phantom power switch on the face plate.  I forget  what size we used but if IIRC the switch was as small as we could find.

We built an EQ pre/post switch into it, making it possible to record from insert sends and mix from insert returns simultaneously, with- or without having the EQ changes on the recording. The small push button from ck is located close to the locut and just mounting them took us almost weeks... and drilling the holes without wearing the surface wasn't easy, too!

Still - any opinions on the preamp second stage issue? Still wondering if someone has knowledge of modified 80B preamps and how the gain problem is adressed.
It's one of the modifications that Jim William seems to recommend for this desk.
Anyway - I will try out changing R98 to half the value and test, then double R99. Check if there is still enough maximum gain then.

 

[pucho812]

I would  love  hear about the pre-post switch. I recall our tridents at the back room had a 3 position switch  for inserts which did  pre, post and bypass. The reason for the bypass was  with the desk  left stock, the inserts only work because they normal through the patch bay, and that for each channel the insert send goes down all that wire,  to the send jack, normals through the patchbay  and  then back down the length of wire back into the channel.  The tech at the time thought it was necessary, But for the life of me I do not have those mod notes.

 

[guy_4]

Pucho wrote :
"I would  love  hear about the pre-post switch "
Me too !   
Thanks
Guy