1176 noise floor

[alexc]

Low noise gain staging across many pieces is an art!

I'm a lot more harsh with my builds now days - and have broken down a half a dozen units to repurpose ...  because they are too noisy at idle. Half of them 1176 based  ;D ;D

Ass a result, and happily,  I'm getting better at builds - my later ones are holding up well at my current 'threshold' of noise (before being benched).

The 'lounge room' studio is my 'proving ground'  ..  if they survive a few weeks here, then they are ready to go to the studio.

I can now get my  guitar + pedals into a chain of diy builds with low enough noise to pass the test 

But it did take quite a few years  of diy...  and I still have an api+pultec+1176j 3RU on my bench to 'review and optimise'  ;D

 

[Spencerleehorton]

Tried out a different preamp the other day and got a lovely sound using the g1176 and the la-2a.
Really low noise and the controls on the g1176 were about 11 o'clock on input and output.
The preamp is a Neve 1290, so I'm going to check what's going on with the api-312 channels as something isn't right with them if all is good with this Neve.
Could anyone suggest a good way of testing for noise on all my units ? I have a scope, multimeter and a laptop, is it possible to use any of these?

 

[Tubetec]

I agree with Alec on gain stageing being especially important with the 76 ,you have an attenuator right out in the front end pre transformer,the input gain and threshold are one and the same ,if your input levels are on the low side  ,having to increase output volume will certainly increase noise . The other thing is if your really hammering your signal with an analog compressor chances of pumping and breathing effects becoming audible are greater, it just part of the physics of the circuitry. I guess its a bit of a balancing act between the effect you require vs audible artifacts that result from squashing things down .

 

[Tubetec]

I use a suite of tools from www.ymec.com for measurements ,its not exactly cutting edge its quite a few years old now ,but it does what it does fairly well .It will run on almost any pc too so its a handy way to repurpose an old laptop as a dedicated  test unit.
.Works with internal sound cards or with an external interface.

 

[alexc]

You really need to use an audio analyzer app to measure noise at the low levels you would get from an 1176.  At levels like -70dBu and lower, only an analyzer will do.

I like to use Room Eq Wizard 'cause it's so very awesome, especially so because it can use proper asio drivers that come with decent audio interfaces. And it's free 

At the cheap end, I use m-audio 1814fw which is a firewire - it's huge performance for so few $ on ebuy

It does suffer from unbalanced analog inputs, which is a pain at times, but for transformer output units like the 1176, it's good to go with the appropriate debalancing cable  (and remember the +6dB factor!)

Here's an example of a simple noisefloor test  .....

 

[Spencerleehorton]

thats great stuff, thank you.

I have just opened up the stereo 1176 (i think its called a 1178?) and noticed i have a couple of Sowter 4257 transformers in there on the output! this may be the problem?

specs say 300 / 30k at 1:1.29

i think the OEP ones are meant to be  600 / 150 at 2:1 so i will try somethings else in here and probably put the Sowters on the input stepping down, rather then step up.

downloading the room eq wizard and will setup tomorrow along with changing the transformers.

 

[Spencerleehorton]

Yes,

the issues i was having on the stereo 1176 were due to the transformers, i've moved the Sowters to the input txf and used a couple of others for the time being as output transformers, one channel sounds great, the other needs work.
I have a couple more OEP txf coming which will sort it, then i'll setup correctly.
Already the functioning channel sounds much better.

 

[Spencerleehorton]

put the new  output transformers in today and had to replace BD139 transistor as was getting horrible sinewave, now have perfect signal on each channel and sounds great, i can hardly believe i was using them like they were before!!! what a difference!!

 

[alexc]

Happiness truly *is* having a great DI with  1176 and LA2A styled diy working right!

Heaven is with a really good gate  ....  and some low power SE amp monitoring 

 

[Tubetec]

The 76 is an amazing piece of kit alright ,the class a discrete is hard to beat ,in a different league to any kind of digital compression Ive ever heard. I read one time the 76 and La-2 combo was a favourite of Motown studios during the golden age, course they added tape saturation too for a little extra sparkle . Im tempted by the warm audio 76 seems good value for money ,works out alot cheaper in the states than in europe though, might try and get someone to bring me back one and avoid getting raped for 25% import duty.

 

[alexc]

It would be very interesting to try some of the more recent takes of the classics - I think the Warm Audio units are crazy appealing.

What's not to like ? Cinemag traffos work  great and I do think that the modern production values can achieve stonking performance when done with some inspiration.

I'd bet that those modern commercial units *are* a bit quieter than my own builds   

I had some pretty good results with Mnats revs  'j' and 'e',  however .

I did a stereo pair in a 2ru of the 'j ' with oep iron all round, then a single 2ru with the 'e' and Ed Anderson iron.

*

They are both very cool - I use the single  'e' mostly,  and inline with a tube unit like an la2a nearly all the time.

But the oep 'j' unit did come out pretty good, better than my earlier attempts in the 'very old days' of this venerable group D DIY.    I have it with some chineve 1081 style preamps and some diy 525 eqs.

Works super well for pretty much everything. I have it set up for kik+snare mics as well as some DAW sends and 'countryman' DIs.

*

The best performing limiter I've done is probably the 'drip 175' - with the fixed op traffo, sowter all round. 

It just has the edge on 'quiet' and a good ways along the way to 'elegant'. Pure expensive class!

That big Sowter interstage traffo is about the best transformer I've come across. It's so good in that critical role, you don't even  want to test it.  :'(    The traffos it is model'd on don't come close.

One day, I'll do Sowter traffos  with some 300b tubes  ;D

The original ua175 was supposedly ante-related to the 1176  8)

One can only imagine how amazing mr Putnam's studios would have sounded