Output Capacitors on the Neve 1073

[user 125886]

Hey everybody

I'm about to purchase some electrolytic capacitors for the output section of a 1073. I have looked at many neve clones, and have noticed that the caps on the original schematic were two 470uf caps in parallel. Then, years later, Neve increased these to two 670uf caps in parallel. Then, I read, on this very site, that Geoff Tanner said that the noise of the 1073 is directly related to the quality, and the size of the capacitance on the output. I guess that's true seeing that most clones now have values of 2200uF - 4400uF.

Here's my question. I have been seeing the adds for "Audio Grade Capacitors". Should I go with these audio grade caps? Or should I just go for specs like / dissipation rate / low ESR / high ripple current? My instincts tell me this "audio grade" stuff is a bunch of silliness. Maybe I should just put standard general purpose Nichicon caps thru the whole amp.

Also, is 2200uF enough capacitance? Or should I go for 4400uF, like the ez1290?

Any suggestions?

 

[abbey road d enfer]

Hey everybody

I'm about to purchase some electrolytic capacitors for the output section of a 1073. I have looked at many neve clones, and have noticed that the caps on the original schematic were two 470uf caps in parallel. Then, years later, Neve increased these to two 670uf caps in parallel. Then, I read, on this very site, that Geoff Tanner said that the noise of the 1073 is directly related to the quality, and the size of the capacitance on the output. I guess that's true seeing that most clones now have values of 2200uF - 4400uF.

It must be taken with some distance.
The output stage of the 1073 is such that any residual noise coming from the power rail is directly imposed on the output. The only thing that filters the PSU noise is the RC circuit constituted of the 12 ohm resistor and the two capacitors. Of course, the higher the capacitance the better. It also helps reducing x-talk between units that draw on the same PSU.
But it's not a substitute for a good clean PSU.

Here's my question. I have been seeing the adds for "Audio Grade Capacitors". Should I go with these audio grade caps? Or should I just go for specs like / dissipation rate / low ESR / high ripple current?

Yes.

My instincts tell me this "audio grade" stuff is a bunch of silliness.

Not totally, but the prices are often overinflated.

Maybe I should just put standard general purpose Nichicon caps thru the whole amp.

Low ESR is important in this position, as well as using the highest possible temperature rating.

Also, is 2200uF enough capacitance? Or should I go for 4400uF, like the ez1290?

It can do no harm.

 

[ruffrecords]

I think it is important to rememeber that the 1073 was nothing more than a channel amplifier used a module in fairly large mixers. As such, it was important to ensure that, not only was the module quiet, but also that it kept itself to itself and did not influence or be influenced by adjacent modules or other modules fed from the common supply. The capacitors you are referring to are decoupling capacitors and the really big ones are in the output stage where they act as local energy stores thereby reducing the changes in current draw from the power supply. This significantly reduces crosstalk between modules.

If you are just using a 1073 in isolation as a mic pre then some of these considerations really do not apply so there is no real benefit in increasing capacitance values or in using exotic types.

Cheers

Ian

 

[soapfoot]

To piggyback on this question:

Would it be worthwhile to experiment with bipolar electrolytic caps in this application? Per Cyril Bateman's 2002 experimental methodology in Electronics World, bipolar electrolytics, when they fit, were the lowest-distortion electrolytic coupling capacitor alternative.

Of course in a Neve 1073 we may not necessarily want lower distortion...

 

[ruffrecords]

Bipolar caps are really only useful in split rail supply deigns where the dc level at the signal inputs and outputs is close to 0V but you never quite know which side it is going to be. The Neve design is a single rail design which means there will always be a dc voltage across the caps and you always know which end is more positive than the other so there is no need for bipolar caps.. Electrolytic capacitors, if properly sized for the application, will produce negligible distortion. I suggest you invest in a copy of Doug Self's excellent book "Small Signal Audio Design"

Cheers

Ian

 

[abbey road d enfer]

The aforementioned capacitors are not coupling capacitors, they are rail bypass capacitors.
It can be argued that they are in the signal path, but as they are permanently submitted to a single polarity voltage, the benefits of bipolar electrolytics do not apply.

 

[Matt Syson]

Of course electrolytic capacitors need to 'leak' a SMALL amount of DC which is simply the way that the electrolyte is 'formed' into an insulating layer. the expected 'leakage' is quoted on capacitor data sheets, usually a couple of microamps as a 'base level but more according to the capacitance value and voltage. When used in a single rail unit like the 1073 the capacitors have a constant, correct polarity DC bias applied so can be expected to live a long life. Bipolars are a bit of a 'cheat' in that once built and in operation, the 'reforming process' will become asymmetrical I suspect.
As Ian (sorry, Abbey road) said the 470 uF caps are effectively in the signal path. Having 2 in parallel 'automatically' reduces the ESR which may be one of the considerations made at the time the circuit was designed as decent low ESR capacitors were a bit thin on the ground. Also 2 smaller physical size caps were easier to mount. One of the biggest bugbears in single supply rail operation is that the 'ground' is reference for both signal and power.hence the Neve circuits having multiple 'ground' pins on the subcards and module connectors. The use of 'half inch' diameter copper ground reference bars was another attempt to minimise crosstalk and contamination of the wanted signal paths.
Matt S

 

[abbey road d enfer]

As Ian (sorry, Abbey road) said the 470 uF caps are effectively in the signal path.

You don't have to be sorry. I believe I may not have been clear that rail bypass caps are in the signal path, however, being submitted to a voltage that does not change polarity, they produce a different type of distortion than non biased capacitors.

 

[FIX]

One thing that is a little un-nerving now days are surface mount caps, as they now say "AUDIO" instead of "BI-POLAR". I still measure any offset in a circuit and make sure any cap, polar or not, is configured that direction.

 

[abbey road d enfer]

? How do you orient a non-polar cap? I take you're not talking about connecting the external armature to the less radiating node.