1073 type input stage caps...why??

[HenryL]

The 1073 has 180pF across the mic input TX and 2n2 across the line input TX.

As far as I can tell the only effect of these is to make overshoot/ringing worse.

I suppose Neve had a good reason, but what then?

/Henry

 

[user 37518]

The 1073 has 180pF across the mic input TX and 2n2 across the line input TX.

As far as I can tell the only effect of these is to make overshoot/ringing worse.

I suppose Neve had a good reason, but what then?

/Henry

Don't expect everyone to know this from memory, can you post a schematic. Usually, low valued caps have to do with stability, RF filtering, or similar.

 

[HenryL]

Don't expect everyone to know this from memory, can you post a schematic. Usually, low valued caps have to do with stability, RF filtering, or similar.

Of course. (It seems so many people round here eat 1073's for breakfast each day that I thought many would be familiar, so I kept it simple

 

[user 37518]

Those are probably some sort of Zobel networks to tailor the response of the transformer, it doesn't necessarily makes ringing worse. If you have one nearby, you can look on a scope with and without the cap. Transformers have resistance, capacitance, and obviously, inductance, so adding a cap would change the response, not necessarily making it "more ringy".

Years ago, I used to experimentally add capacitors to the secondaries of transformers whilst applying a square wave to the primary. Combinations of capacitors, or capacitors with resistors, obtained in many instances very good results.

 

[Rob Flinn]

Are you testing the ringing with the transformer connected to the rest of the circuit ? In my view you should be looking at the ringing on the output transformer with a sensible load connected to it. i.e not just a meter or scope probe.

 

[user 37518]

Are you testing the ringing with the transformer connected to the rest of the circuit ? In my view you should be looking at the ringing on the output transformer with a sensible load connected to it. i.e not just a meter or scope probe.

I guess that a better question to ask is if he has measured any ringing or if he is just assuming it rings from looking at the schematic.

 

[HenryL]

Those are probably some sort of Zobel networks to tailor the response of the transformer, it doesn't necessarily makes ringing worse. If you have one nearby, you can look on a scope with and without the cap. Transformers have resistance, capacitance, and obviously, inductance, so adding a cap would change the response, not necessarily making it "more ringy".

Years ago, I used to experimentally add capacitors to the secondaries of transformers whilst applying a square wave to the primary. Combinations of capacitors, or capacitors with resistors, obtained in many instances very good results.

Yes that's exactly what I've been doing, with a Neve clone preamp and a new transformer that is supposed to be a good substitute for the one used by Neve. My conclusion was that in all cases having these capacitors straight across the capacitor made the ringing significantly worse. So I wonder why NEve put them there - I'm obviously missing something!

I tend to loosely think of the zobel with a series resistor and capacitor as a kind of frequency selective damping, with the damping performed by the resistance. Putting a capacitor straight across the TX without any additional damping seems very unintuitive to me. Sure I can see that as the reactance drops with frequency that will tend to a 'short circuit' for high enough frequencies, so it should kill those, but putting an inductor together with a capacitor like that looks like a surefire way to make something that will resonate!? (Edit: I do appreciate that as you say the TX has resistance, interwinding capacitance and so on, and the interactions are more complex - but I don't expect the TX I'm using to be radically different to the neve original so was expecting TX-differences to be a minor component of the behaviour I"m seeing)

I have been posting a more detailed thread about my actual experiments trying to get some help with my specific project (without much response so far), so I was just trying to ask the general question here to try to get some different angle/insight .

Cheers

 

[HenryL]

Are you testing the ringing with the transformer connected to the rest of the circuit ? In my view you should be looking at the ringing on the output transformer with a sensible load connected to it. i.e not just a meter or scope probe.

Absolutely. Yes that's what I've been doing. There's more detail about how/what I"ve been doing experimentally over in this thread Neve-ish modded GoldenAge-pre73 w.Carnhill inp TX damping results/advice? . I've been largely talking to myself there though I reached my own conclusions in the end, but as it seemed to fly in the face of the Neve design I'm trying to sanity check it!

thanks

 

[HenryL]

I guess that a better question to ask is if he has measured any ringing or if he is just assuming it rings from looking at the schematic.

Hi again, yes it's based on measurements (see response to Rob Flinn). I will add some more to my project/experiments thread as I tested the line input similarly and came to the conclusion it also looked a lot better without the 2n2 cap.

 

[johnR]

The amount of ringing is affected by the output impedance of the signal source. Neve probably chose those capacitor values because they worked with the expected signal sources at the time. Some modern mics have a very low output impedance so the input transformer may be underdamped.

I've used quite a few original Marinair, St Ives and Carnhill transformers, but I've always tailored the damping to suit the expected operating conditions (including use with modern equipment). I usually determine Zobel network RC values experimentally to give the cleanest square wave response over a reasonable range of source and/or load impedances. This approach works better for me than blindly copying the original circuits.

 

[johnR]

Another thing: materials used in modern transformer reissues are not necessarily the same as they were 50 years ago. There may be differences in dielectric constant of insulation etc which could affect resonance behaviour. I wouldn't assume they were identical without checking with test equipment.

 

[HenryL]

@johnR - Thanks. Yes I appreciate that transformer construction and characteristics vary of course. It just seems such a blatant [apparently]unwanted effect that it seems to go beyond small[?] variations in the tx, but I don't have enough experience in this area to have a reliable view on it.

I usually determine Zobel network RC values experimentally to give the cleanest square wave response over a reasonable range of source and/or load impedances. This approach works better for me than blindly copying the original circuits.

Yes that's what I was doing.

I did all my mic experiments with 155 ohms generator impedance which is in the general area of many of my mics, and mics in general. I also did spot checks at various points reducing the source impedance to ~20 ohms but it didn't make any difference to the observed ringing. For the line input I used a source impedance of ~100 ohms as I have some devices with line outs with that spec, this is perhaps a little high into an input that seems to be designed for 600 ohms, but again spot checks at lower impedance didn't show anything different.

Another aspect which reinforced my puzzlement was that I found the same phenomenon in a non-TX-replacement scenario: in the preamp I've been experimenting with I did not replace the line input transformer, just the mic tx, but I found exactly the same there ie. removing the 2n2 cap on the line TX greatly reduced the ringing. So my thinking was "well they deliberately put in bespoke non-Neve cap and loading components for their own mic TX so presumably their own line input TX circuit was equally deliberate" thus should reflect the up-to-date characteristics of their chosen line tx. (Other interpretations are available..!)

 

[HenryL]

I see the 'EZ1290' seems to just assume the 180p on the mic input TX with the Carnhill 9045 (and with various other brands of TX if I saw that right) but I didn't manage to find anything so far about damping/ringing optimisation.

 

[emrr]

That’s a part that shouldn’t be copied with different transformers until proven.

 

[metalockpick]

Very good iron (term used here as a pseudonym for "Transformers") needs little correction by way of Zobel network.
Lundahl typically requires very little compared to old school, steel laminate xfrmrs.

 

[waltzingbear]

Terminology
A zobel network is a R and C in series.
a single C or R across the secondary is capacitive or resistive termination.

Cheers

 

[HenryL]

That’s a part that shouldn’t be copied with different transformers until proven.

That was my own starting point, and practical approach - it just seemed so often copied I thought I must be missing something!

 

[HenryL]

Very good iron (term used here as a pseudonym for "Transformers") needs little correction by way of Zobel network.
Lundahl typically requires very little compared to old school, steel laminate xfrmrs.

That's an interesting comment - the kind of present day experience I was hoping to find out about - thx.

 

[HenryL]

Terminology
A zobel network is a R and C in series.
a single C or R across the secondary is capacitive or resistive termination.

Cheers

This is the kind of distinction I'm interested in understanding better! As mentioned in a comment I made somewhere up above (#7), I think of a 'zobel' as a capacitor in series with a resistor as you say, but find it hard to see why you'd have zero ohms of resistance in it, creating what you call a capacitive termination. If the (terminating) capacitor across the TXout was much smaller I could see it might help reduce RF ingress and keep out of the way of the audio band, for example, but the 180p in the [modernised]Neve circuit seem to create ringing at much lower frequencies.

When/why would a design typically employ a capacitive termination like this vs a zobel (or resistive...) ?

 

[HenryL]

Are you testing the ringing with the transformer connected to the rest of the circuit ? In my view you should be looking at the ringing on the output transformer with a sensible load connected to it. i.e not just a meter or scope probe.

Sorry Rob, I just scanned past your comment again and noticed that I misread it previously. I read it as "on the output of the transformer with a sensible load" but in fact you said "on the output transformer..." - in case that wasn't a typo then I should add that yes I did that too. I found that a 600ohm load on the line output transformer killed the majority of spikes and twitches on the leading edges of the square wave test signal, the worse ones still broke through a little, and some funny little distortions of the leading corner occurred in a few cases.

This points to another aspect of this whole thing: how important is it to get rid of overshoots etc at the input stage if they don't make it out of the exit in any case? Ok, so there's a common sense angle that it would be better not to be amplifying a load of crap you don't want, and a "well can you hear the difference" angle, but I wonder if there's any compelling theoretical reason to stop it on the way in rather than on the way out, causes other misbehaviours of the circuit for example - I don't see it, but ?

 

[emrr]

That was my own starting point, and practical approach - it just seemed so often copied I thought I must be missing something!

“Often copied” is an ongoing problem in electronics, commercial and DIY. Lots of stuff made and sold with little understanding of fundamentals. Always question!

 

[metalockpick]

The whole point of this exercise is better sound. If you've got ringing happening up high, you can either have transformer saturation or modulation that affects the audible sound.
If you don't have a test laboratory at your disposal, that's okay.Try a few components In and out of circuit. Alter a few values.
If you hear a difference, then they're most certainly is something going on.

Whether or not you like it is another matter.

Winding techniques, core size, insulation materials and lamination materials all affect the sound and performance of a transformer. There are laminations with high nickel content (much more expensive) that can (all else equal) give superior frequency and distortion performance over plain or grain-oriented steel in audio transformers.
With extended bandwidth, the need to correct for various issues is reduced.

That said, cheaper materials cause different kinds of harmonic distortion which can be pleasing and sought after. Transformer ringing is a sonic feature in some designs.

 

[HenryL]

Thanks all for the input. I guess I conclude:
- There seems to be no particular known basic reason for the 180p in the Neve, other than ringing/damping, that I've missed.
- More modern transformers may have better characteristics and require less compensation to achieve a theoretically desirable behaviour.

As to the matter of taste/subjective appeal of one sound over another, with/out ringing and/or other distortions...I completely agree and appreciate!

With a single unit of this type reliable A/B comparisons are a bit more tricky to arrange and time consuming so I'm going to put the preamp back into use with all its ringing caps removed and live with it for a while.