Altec 1578A octal mic-pre plug-in ** now with circuit **

[clintrubber]

Hi,

Got me a pair of the TX-less Altec 1578A octal mic-pre's
and I'm looking for more info, especially the circuit-topology.

Has anyone opened or traced these yet ?
Is the circuit in any way related to the 1588A, 1588B or 1588C plug-ins ?
(apart from the no TX for the 1578A)

So far I've found '1588-alike specs (apart from the TX):

The pinouts are as follows: 7 and 8: Input 1 and 6: Output 5: +12V power Pin 6 is also the ground connection for the power
The other specs are as follows: source: 150 ohms useable: 30 to 20,000 ohms load: 600 ohms gain: 33.5 db output: +8dbm power: 12VDC, 13 ma

I found some info here from tablebeast in this thread:

[quote author="tablebeast"]AS for a transformerless Altec pre module, maybe the 1578A is what the previous poster was thinking about. I have never had one, but I have a document for the 1581A modular mixer line amp/eq and it states the 1578A's frequency response is something like from 20 to 40k! I can't find that document right now, but I remember those numbers[/quote]
Can't locate any 1581A-info either I'm afraid...

Any links or info appreciated!

Thanks :thumb:

Peter

 

[ciminosound]

I read on one of the boards it was the same circuit but transformerless. Meant for high-Z mics. I have info SOMEWHERE! I'll keep diggin'.

http://alteclansingunofficial.nlenet.net/publications/techletters/TL_146.pdf

 

[emrr]

From 1592A manual:

1578A
source impedance nominal 150 ohm; usable 30-20K ohm
+/- 0.5 db 20-20K
+/- 1.5 db 15-50K
EIN -122 dbm

1588A
transformer isolated input with 90 db shielding
source impedance 150 / 250 ohm
+/- 1 db 30-30K
EIN -122 dbm
Physically and electrically interchangeable with the 1578A


From the 1591A manual:

use 1588A for balanced mic input and 1578A for unbalanced mic input.

That last one's funny to me!

 

[clintrubber]

[quote author="ciminosound"]I read on one of the boards it was the same circuit but transformerless. Meant for high-Z mics. I have info SOMEWHERE! I'll keep diggin'.

http://alteclansingunofficial.nlenet.net/publications/techletters/TL_146.pdf[/quote]
Thanks for responding. Had seen that pdf - intriguing the 'high' is made into 'low' (the handwritten addition)!

A simple calculation says that the TX in the 1588B is around 1:3 in ratio.
OK, I have some of those small Beyer TXs around: 1578A + Beyer-TX = 1588B (w.r.t. functionality).

Regards,

Peter

 

[clintrubber]

Thanks Doug for adding :thumb:
Couldn't find 1592A-info & the 1592B-info I found for downloading doesn't mention the 1578A (anymore).

[quote author="emrr"]From 1592A manual:

1578A

1588A[/quote]
Since both are stated to do 33.5dB of gain I expect then that the 1578A might have a smaller value of the internal resistor to pin#2 (the TX-gain of the 1588A/B is estimated to contribute 10dB).

From the 1591A manual:

use 1588A for balanced mic input and 1578A for unbalanced mic input.

That last one's funny to me!

You mean you wouldn't want to use such a thing as an unbalanced mic ?

Regards,

Peter

 

[emrr]

[quote author="clintrubber"]

[quote author="emrr"]

use 1588A for balanced mic input and 1578A for unbalanced mic input.

That last one's funny to me![/quote]
You mean you wouldn't want to use such a thing as an unbalanced mic ?

Regards,

Peter[/quote]


No, that it infers the 1588A is unsuitable for unbalanced input, when it would work just fine with correct Z.

 

[clintrubber]

[quote author="emrr"]No, that it infers the 1588A is unsuitable for unbalanced input, when it would work just fine with correct Z.[/quote]
I see what you mean :thumb:

 

[clintrubber]

[quote author="ciminosound"]I read on one of the boards it was the same circuit but transformerless. Meant for high-Z mics.[/quote]
A few 1578A's arrived and I had little effort opening one.

Haven't done tracing yet but for now I can tell the circuits are not the same:
the 1588B uses 2*NPN & 1+PNP but the three BJTs in the 1578A are all NPN (AL2712).
No TX for the 1578A indeed, but there is a small coil.

Stay tuned (for those interested), tracing will follow.

I have info SOMEWHERE! I'll keep diggin'.

Interested in anything you could find, thanks.


Bye,

Peter

 

[clintrubber]

[quote author="clintrubber"]Haven't done tracing yet but for now I can tell the circuits are not the same:
the 1588B uses 2*NPN & 1+PNP but the three BJTs in the 1578A are all NPN (AL2712).
No TX for the 1578A indeed, but there is a small coil.

Stay tuned (for those interested), tracing will follow.[/quote]

This is the circuit, unchecked but there's not that much that can go wrong with this topology:

Topology not anything new of course. It made me think of this:

So note that unlike the 1588B etc the 1578A doesn't have any pad-connection to change the closed loop gain. Could easily be added of course, but for smaller gains it may require redimensioning of the compensation.

W.r.t. the transistors, it's three times the AL2712 which is a 2N1712 or a 2N339X sub.
The red dots for Q2,3 indicate a hFE range of 175-225 but the green dot on Q1 isn't accounted for. I assume it's a higher range.
Is there a 'standard' for the use of colours for hFE-ranges ?
IIRIC we've discussed this before and it was some random use...


Bye,

Peter

 

[NewYorkDave]

What you traced makes sense; it's a pretty typical transistor "ring of three" circuit. The little coil is to keep RF out of the B-E junction of the input transistor.

You seem to be on a bit of a kick with these Altec plug-ins. Maybe I should sell you my pile of 1588Bs :green:

 

[clintrubber]

[quote author="NewYorkDave"]What you traced makes sense; it's a pretty typical transistor "ring of three" circuit. The little coil is to keep RF out of the B-E junction of the input transistor. [/quote]
Yes, the use of that coil was a bit disappointing :wink: When I openend the can and saw the coil I was expecting that perhaps some 'single ended 990-DOA coil tricks' had been used, but no.

You seem to be on a bit of a kick with these Altec plug-ins.

Yep, it's this weeks craze. Next week it'll be something else

The 'masterplan' with this 'old junk' is to have some stuff with a bunch of octal sockets in which you can plug & test various tastes of circuits.
The 1578A could use a TX, but even those Beyers don't fit too well. But unbalanced in has its use as well.

Maybe I should sell you my pile of 1588Bs :green:

Let's not talk about your 1588B's, your price may be nicer than eBay and then I don't want to know :wink: Skipwave sold me one for a nice price though, but of course I had to have a few more....

Bye,

Peter

 

[clintrubber]

[quote author="NewYorkDave"]What you traced makes sense; it's a pretty typical transistor "ring of three" circuit.[/quote]
And FWIW, I just realized it's in essence the same topology as this one:

I had never really had a look yet at what was actually going on in those BA283-sections.

Bye,

Peter

 

[solder_city]

youre gonna keep finding that circuit in all kinds of places. it was used a lot!

 

[clintrubber]

[quote author="solder_city"]youre gonna keep finding that circuit in all kinds of places. it was used a lot![/quote]
And for a reason as it seems, it looks like three BJTs decently spent :wink:

FWIW, I did some simulations (with other devices, but with the given 1578A topology) and the optimal source resistance came exactly on 2kOhms.

Hmm, that's a value one would expect for the 1588B - which assumingly has a 1:3 TX. So that would make sense: 2k/(3*3) ~= 200 Ohms.

But not for the TX-less 1578A, for 200 Ohm sources the NF-penalty would be 10 dB then.

As I said, FWIW, since other devices may influence things. The 2k OSI was striking though, given the 1:3 stepup-TX of the 1588B.

Regards,

Peter

 

[Whoops]

Very nice info,
please post the docs in the Altec page in the Technical Documents section of this forum, here:

https://groupdiy.com/threads/altec.44774/page-2