transformer ID + IC upgrade in Ela Ljud Sweden AB 602 mixer – help appreciated

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ask

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Feb 16, 2006
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Copenhagen, Denmark
I've had an old (late 70ies?) mixer tucked away in the attic until now, it's a model 602 from the small company Ela Ljud Sweden AB – of which very litte information can be found online. It's very service friendly though, and to my pleasant surprise it has Sennheiser TM003 mic input transformers (1:15). The output transformers are marked ST-22, does anyone have a clue as to what they are?

Also, it's somewhat of an IC opamp exercise, though sadly mostly using AD301 ICs – slew rate limiting on a chip, as far as I can deduct...  Could someone suggest IC upgrades, or point me to something in that direction? AD301 seems to have low current draw (2-3 mA) but with an external 35W PSU and just 15 301's total I'm guessing a more power hungry IC won't be a problem(?).

 

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> AD301 ICs – slew rate limiting on a chip, as far as I can deduct...

No, '301 CAN be a very fast chip. Way ahead of its time.

However-- I see 22pFd compensation cap. Which makes it hardly any faster than '741.

At 1:15 ratio, you need a LOW input current opamp.

I'd strongly suggest TL071. It is likely to be stable in an unsophisticated layout like this, where "better" opamps may just howl MHz. Remove the 22pFd, the '071 has that inside.

That output transformer looks like the transistor radio series. A 600:600 in that line may have OK freq response but can't take large bass level. However ST-22 appears to be 8K:2KCT. Dunno what they do with that.
 
Ask,

Ela Ljud AB, owned by Bengt Hellberg was an old friend of mine, (no longer with us) and started building these mixers early 1970s.

I have not seen them be equipped with AD301 earlier, normally used 741 and 748 as standard op-amp's

Normally, the output was unbalanced, but there were many special versions, so it may be one of those you have.

I will look if I have more info when I'm at my workshop tomorrow.

Btw,
do you have a picture on the mixer outside ?

--Bo
 
> AD301 earlier, normally used 741 and 748

(As you probably know) These are all the "same" chip.

The '01 and the 74X use slightly different details to avoid each others' design, but clearly the 748 is meant to "be" a 301 in all but brand. The 741 is a 301/748 type with the external compensation cap moved inside at 30pFd.

The way this board has comp-cap tacked on suggests it was planned for 741. 741 eventually became VERY available. But in the first year it may have been in short supply, while 301/748 went un-sold. The quick fix is to add small caps to 301/748.

30pFd compensates to full unity gain. 22pFd works for gain down to 1.36. The mike preamps probably don't go down that low. Inverting stages have another +1 so can be run to unity gain with 22p. Mix amps will have gain-functions of 4, 8, 12 according to their inputs, 22p is ample.
 
Ask,
I did not have much more info on the Ela Ljud mixer, but saw that they were built with the uA-741 op from the beginning, and had Sennheiser TM-003 or Beyer BV-35553 mic input transformer, both with 1:15 ratio.
But later they changed the mic transformer to Beyer BV-35600 that had 1:10 ratio, and was more suited to a solid-state circuit.

During those first production year, was the console's front panel light gray and had dark gray sides, but the later series had black front panel and red or orange sides.

The last few years was this mixer updated with later op-amps as TL071 and better component selection, and also assembled and manufactured by another Swedish company called Xelex.

Regarding the replacement of the op-amps.
Certainly there is tremendous good audio op amps today to choose from, but I would still suggest that you use the old workhorse TL071, it is reliable, safe and calm, accepts most PCB layouts and long cable runs without oscillation and other complications.


PRR,
Sure, LM301/AD301 was a similar op-amp to 741/748, but better, for example, 10 times faster slew rate, if I do not remember wrong.

Slightly off topic, but when we're talking about old op-amps.......
I remember those first op I came in contact with around 1969, was Amelco 805 and 811, the latter was not easy to get stable, almost impossible.
Later, I used uA741 and 748 which were easier to deal with, but sounded hard and lisped in the treble at high gain.
But then came the rescue when Raytheon launched 4558 and 4136, when it began to sound acceptable. (almost)
Then it went fast, did not take long until Texas TL0xx and National LFxxx series came on the market, and of course the Philips TDA1034 which then became Signetics NE5534.
It is interesting and fun to have been there from the beginning, ok, I missed the very first op generation before the UA-709, but was pretty close, anyway.

Best from
Bo
 
> LM301/AD301 was a similar op-amp to 741/748, but better, for example, 10 times faster slew rate,

The "special" point of the '301 is that YOU could pick the compensation cap to suit your needs.

In round numbers (it has been many years):

30pFd gave 1MHz GBW, 1V/uS slew, and stable at unity-gain
3pFd gave 10MHz GBW, 10V/uS slew, and stable at gain-of-10-up

'741 is like '301 but with 30pFd built-in.

'4558 is '741-like but slightly better. IIRC slew twice that of '741. If you also keep levels down, you lose the "lisp".

Older mike preamps "never" ran low gain. The first Gately mixers had similar input transformers (ratio too high for BJT input), '301/748 with 4.7pFd comp caps, around 6V/uS slew which is really ample for input stages. (It had a lot of output gain, so we didn't run the preamps hot.)

There is a variant "feed-forward" compensation on the '301 datasheet, incredible GBW and slew, but only useful for inverting connection.

 

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