Noisy Teletronix LA-3

[Basspumper]

I've got a pair of lovely LA3's but they are noisy. I've heard they can be modified so there is less noise. Can anyone help me with that?
I also have pairs of Telefunken V372, V374a and V676a that need to be recapped. Any suggestions (and schematics) ?

Many Thanx,

Peter

 

[Gert]

Peter,where did you find those baby's?you could use
other low-noise transistors like BC550C or BC560C.
Gert :thumb:

 

[Helsing]

Use XICON electrolytics to recap those Tele Modules. Got that tip from Oliver Archut at TAB-Funkenwerks who found them to to have similar characteristics to the originals. They are very inexpensive. You can get them at Mouser.

Jon Erickson
www.helsingaudio.com

 

[gyraf]

[quote author="Basspumper"] I've heard they can be modified so there is less noise. [/quote]

Where did you hear that?
Are you sure you you're not confusing them with LA4?

Jakob E.

 

[Guest]

I believe you're thinking of the Ed Evans mod(often thought of as the Bob Clearmountain mod), I'll see if I have some details around here somewhere.

Zach

 

[Gus]

Xicon electros??? realy? I don't like them at all. I rate Xicons with IC elecros.

 

[Guest]

Peter-

So I now recall that my documentation on the mod is actually written on one of my LA-3A's, but I can't get to it as they are in use right now...but I have a PDF that has the mod in it, with some other suggestions from Ed, but have no place to post it. If you want, PM me your email and I can send it to you.

Zach

 

[jrmintz]

[quote author="SPG"]I believe you're thinking of the Ed Evans mod(often thought of as the Bob Clearmountain mod), I'll see if I have some details around here somewhere.

Zach[/quote]

That wasn't a noise mod, was it? It had to do with moving the switches around and changing the high frequency control to a switch, or so I recall. I had one I sold and I really regret it now.

 

[soundguy]

I know from my experience that a component that sounds good in one circuit doesnt sound so good in others, but those xicon caps are ******* horrible. Ive never tried them in a LA3A, so I should probably not comment, but I would really be shocked if those things wound up being the "best" selection given proper listening comparisons to other caps. I used to use those all the time when I was starting out building stuff because they were cheap, and well, they are cheap. To *really* spin this out, what the **** is the point of reference for what a circuit sounded like 35 years ago? This is a different debate for sure, I'll leave it at that...

dave

 

[Guest]

That wasn't a noise mod, was it?

There was a noise mod, as well as all the other stuff with moving switches around etc. Hope you got a good price for that sucker, they just seem to keep going up!

Zach

 

[SSLtech]

There was a gain mod... involved changing the value of a single resistor, I believe. That should reduce noise, but only if you find yourself operating the gain and peak reduction controls a long way from 'nominal' settings.

Otherwise I don't know of any low noise mod. Noise is essentially (at its simplest reduction) a function of gain in any circuit, so if the gain structure isn't a long way from optimal at the moment there won't be much you can do. If however you find you have too much gain, then you can do a lot to improve the noise with a simple mod, but your recall settings from before the mod will be useless.

Keith

 

[Helsing]

[quote author="soundguy"] those xicon caps are ******* horrible....what the **** is the point of reference for what a circuit sounded like 35 years ago?

dave[/quote]

Just making a suggestion based on my experience. Easy does it bud.

Jon

 

[NewYorkDave]

Oh, Dave just has strong opinions on caps. Don't take it personally :guinness:

 

[Kev]

Yep, there was a simple gain mod.

It was spelled out at John K's forum at one stage. Could be part of John's old forum pre software changes.

From what I remember it was more about backing off the pad at the front door so you didn't have to gain it up so much to get back to normal levels.
Didn't change the ultimate noise floor but did move your normal operating level up and away from the noise floor.

yes
... at the cost of headroom BUT in standard form there is a bucket of it in the LA3 ... too much ??? :roll:

 

[squib]

i've done a gain mod, that improves the signal to noise without shifting the operating points too much. My notes are a bit hazy, but it involves removing the pad at the front end and placing a pair of 4K7 resistors in series with each leg of the input transformer ( this has the additional benefit of raising the input impedance from the original 600R ). The makeup / output amp has an original gain of 32dB, this can be lowered to 21dB by replacing the 220K feedback resistor to 56K, thus the noise floor is lowered by 11dB.
Regarding caps, the only electolytics in the signal path are the 400uF amplifer ouput cap and the 10uf ac coupling the shunt side of the feedback network. I'd use Bipolars here.

One interesting thing about the ouput transformer, is that it has a separate primary winding, that simply has the main DC rail ( ~ 24V ) coupled through it via a 2.4K series resistor. Why???

 

[NewYorkDave]

Well, I'd have a better idea if I could find an LA3 schematic online! :?

But, as a blind guess: perhaps it's to balance out DC current in the other half of the primary winding?

EDIT: OK, I just found a schematic
http://www.waltzingbear.com/Schematics/Urei/LA_3A.htm

My "blind guess" was wrong. Honestly, it's not immediately obvious to me why they're doing that.

 

[soundguy]

[quote author="NewYorkDave"]Oh, Dave just has strong opinions on caps. Don't take it personally :guinness:[/quote]



Ha, I have strong opinions on everything. Nothing worth doing is worth having a weak cup of tea stance on...

dave

 

[Basspumper]

[quote author="gyraf"][quote author="Basspumper"] I've heard they can be modified so there is less noise. [/quote]

Where did you hear that?
Are you sure you you're not confusing them with LA4?

Jakob E.[/quote]

No I'm very shure they are LA3's and the mod I'm talking about is the Ed Evans mod (thanx SPG !!).

 

[gyraf]

The supply voltage through the output transformer could be either bootstrapping the powersupply (I dont think so..) or a wierd way of cancelling ripple on the supply line..?

 

[Gus]

I just looked at it and the following come to mind.

1) increase C5 maybe you could make it 2 470uf in parallel it is used to make a somewhat constant current sink for Q4( the drive for the outputs).

2) make R8,9,10,11,5 and 6 metal film if they are not now.

3) disconnect R20 and/or the winding that has DC in it. I don't understand why DC is in the winding. I will have to think about it some more (Jakob noted the DC in the winding in the above post)

4)CR7 can be bypassed with a cap to filter the Zener noise that goes to the base of the pass transistor and on into the power rail C12 does filter it a little, This will also act as a cap multipler, ~= Hfe X cap value. Try a 100uf at 35V to start. I would also increase C10 to about 4,700uf. Low ESR electro caps

I don't think changing the input pair will make any difference because 2n5089 are low noise.