A 'Neve'er ending story (with pics)

[matta]

[quote author="CJ"]What have I been saying for the last zillion?
Good work!
But now you will alwyas have a rats nest of pwr supplies on the carpet.
Me at least!
:thumb:

But yes, there is just no way to get the same results, you would need ten tons of mu.
\[/quote]


Hey CJ,

Yes KUDOS to you!

It was actually you who gave me the idea of mounting the torrid on the rear of the case, much like alot of older tube gear, and let me say it more than HALVED the hum I was getting.

Even now at 50-60dB's of Gain I can BARELY hear it above the noise floor and for what I'm doing it is more than acceptable.

Distant from the circuit seems to be the ONLY SUREFIRE way of ridding the hum, I know that Joe Malone also advocates this.

I won't build another preamp with an in the box power trafo, it is not worth the headache and I would rather live with the rats nest than the incessant hum.

Cheers

Matt

 

[khstudio]

when loaded with a 150R resistor.

Does the 150 ohm resistor just go from pin 2 to 3?


Kevin

 

[matta]

[quote author="khstudio"]

when loaded with a 150R resistor.

Does the 150 ohm resistor just go from pin 2 to 3?


Kevin[/quote]


Hi Kevin,

You got it! The reason you do this is to mimic a source-impedance on the mic pre and can then listen to noise, hum, buzzes that are inherent in the circuit without picking up sounds from a mic, which might mask the circuit noises.

Cheers

Matt

 

[clintrubber]

[quote author="matta"]You got it! The reason you do this is to mimic a load on the mic pre [/quote]

Got confused a bit by this at first. No nitpicking is intended, but I understand the 150 Ohm is not a load for the micpre but actually presenting a source-impedance to the pre-amp (i.s.o. the mic), right ?

Regards,

Peter

 

[rodabod]

[quote author="clintrubber"]
Got confused a bit by this at first. No nitpicking is intended, but I understand the 150 Ohm is not a load for the micpre but actually presenting a source-impedance to the pre-amp (i.s.o. the mic), right ?
[/quote]

Yes, you should really call it a source impedance.

 

[matta]

Thanks Guys,

I've corrected my terminology. I guess I've always called it a 'dummy load' in the sense that the resistor mimics a mic, or rather 'loads' it with some kind of impedance, but can be misconstrued.

Unless I have totaled missed the plot and understanding over the years.

Cheers

Matt

 

[Greg]

Whenever I test for noise and hum, I usually just short the inputs. Perhaps I'll try with a source impedence and see if there is a difference.

 

[clintrubber]

[quote author="matta"]Thanks Guys,

I've corrected my terminology. I guess I've always called it a 'dummy load' in the sense that the resistor mimics a mic, or rather 'loads' it with some kind of impedance, but can be misconstrued.

Unless I have totaled missed the plot and understanding over the years.

Cheers

Matt[/quote]
Cool !

'Load' would imply that 'the loaded thing' (the pre-amp in this case) has to work harder when the load is made more severe (less Ohms). But that isn't the case here; the amount of sweat as produced by the pre-amp is unrelated to the source-impdance presented to it.

(OK, higher noise as coming from a higher source-impedance results in the pre-amp making more output-swing, but let's skip that, still very small signals. And note that the relation would also be inverse: more Ohms ('less severe loading' in the original terminology) give more output signal so some more sweat from the pre-amp).

Bye !

Peter

 

[rodabod]

[quote author="Greg"]Whenever I test for noise and hum, I usually just short the inputs. Perhaps I'll try with a source impedence and see if there is a difference.[/quote]

Yeah, it's good to do this as you'll appreciated the noise incurred for instance by noise current pulling through the source impedance which effectively forms noise voltage (depending on how you look at it!)

Peter, yes, the term "load" suggests something having to do work, and the source impedance is not causing the preamp work here as of course, the source impedance is not being driven with any current. It's the other way round!

 

[khstudio]

Very good info.

While we're on the subject of correct testing... how many of you guys do the polarity check of the power trannys secondary to Ground/chassis?

Found in this thread:
http://www.groupdiy.com/index.php?topic=7126

You might try phasing your pwr transformer.
Be careful with this:

1) Unhook the Earth Ground wire from the power chord, if you are using one.
2) Connect a Hi-Z AC voltmeter between Earth Ground and your Neve chassis.
3) Connect an AC test cord to the pwr transformer. Plug in the line cord and note the voltmeter reading.
4) Reverse the polarity of the test chord and repeat step 3.
5) One of the two polarities will provide a lower voltage reading. This is the preferred polarity for that transformer. Mark the primary leads of that transformer accordingly.

The benifits obtained by this "primary polarization" are twofold:

1) Improved reliability and saftey.
2) Improved sound may also result.

CJ

This makes me want to pull ALL my DIY stuff apart & check.

Kevin

 

[khstudio]

Hey Matt,

I finished my 1272 but I'm having a hum/hoise problem :sad:

http://www.groupdiy.com/index.php?topic=19648

I'm using the center tap on the ACDC (24-0-24) so I get 48v & +24v, the -24v section is not loaded.

I'm woundering if the center taped input from the toroidal to the ACDC is adding to my problem. I think I can wire it in parallel & use the TRIPLER on the ACDC like you did.

Check out the thread if you can... I've been trying to sort it out... even tried a 24v 1000ma wall wart - noise was lower but the hum was still there.


Thanks,
Kevin

 

[3nity]

Hey Matta!
By having the toroidal externally means he is always on and on the front panel you only switch the secondaries?
Can you give me some hints??

 

[rodabod]

By having the toroidal externally means he is always on and on the front panel you only switch the secondaries?
Can you give me some hints??

I'm sure Matt will answer this, but I just thought I'd point out that switching the secondaries only is not a great idea. Switch mains primary, otherwise it will always be powered.

 

[3nity]

Thanks Rodabod.
I know that leaving a toroidal on without a load its not the best too...
but the connectors he is using are 3 prong so i was wondering how??

 

[matta]

Hey 3nity, yip, as Roddy shared I am switching the primaries, otherwise the trano would always be on. The 3 prong is a standard IEC. Grounded at the rack case. There is a small SPST switch switching Live in the rear of the unit. The front panel switch is a dummy switch.

Cheers

Matt

 

[3nity]

So the power switch its on the transformer casing?
I tought you were doing something differently...

Thanks.

 

[matta]

So the power switch its on the transformer casing?
I tought you were doing something differently...

Thanks.

The only thing different is the trafo is on the outside of the case rather than in, providing some isolation from the EMF given off by the torrid... which has substantial lowere the noise floor of the pres.

Cheers

Matt