Altec mic pre

[toby Montana]

Can someone tell me the difference between a Altec 1588A, 1588B and the 1588C mic pre?

Thanks

Toby

 

[emrr]

Very slight revisions. Possible transistor changes. All will work in same position, though if replacing an A with a B or C, pins 2 and 6 of the mounting socket need to be strapped. Data books says 'most applicable Altec equipment of current (?) manufacture have pins 2 and 6 connected but certain older units may require its' addition'.

 

[solder_city]

1588a has no input transformer, the others do

 

[tablebeast]

The 1588A most certainly DOES have an input transformer. It is a Peerless transformer, model 4185, while the 1588B has the Peerless 4838. They sound slightly different, thought I like the 1588B better. The 1588C also has an input transformer but I forget its number, it much larger than the A or B's. 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 and thinking it was an incredible spec for a mic input transformer that would fit in one of those little Altec modules. If there was no transformer in those, that spec would make way more sense.

Jesse

 

[NewYorkDave]

Actually, a dinky input transformer can do 40K or even higher with no problem. It's getting flat response down to 20Hz that's the problem (for a small xfmr).

I'm glad that there's some "vintage market" interest growing around this old PA stuff (which was generally considered "dirty old junk" just a few years ago). Why do I care? Because I have some 1588Bs and Cs and some other Altec pieces I picked up for a song many years ago. See, I'm not a packrat, I'm an investor.:grin:

 

[rafafredd]

I´m a packrat. :green:

 

[sahib]

1588 C accepts phantom power.

We have about 20 off each of these.

Good investment indeed.

Will be passed on to my son.

 

[clintrubber]

[quote author="tablebeast"] while the 1588B has the Peerless 4838.[/quote]
Out of curiousity, any idea which ratio that 4838 transformer is ?

Could derive it from the circuit-gain & the total gain, but didn't do that yet.

Cheers,

Peter

 

[clintrubber]

[quote author="clintrubber"] but didn't do that yet. [/quote]

Done, TX-ratio will be around 1:3 for 10dB of voltage gain (in addition to the gain of the active part).

 

[clintrubber]

[quote author="tablebeast"]the 1588B has the Peerless 4838. They sound slightly different, thought I like the 1588B better. [/quote]

I have a question about the input impedance of the 1588B.

Since I'm thinking of a little bit of circuitry before a 1588B, I had a look at the load that the 1588B presents to preceeding stuff (usually a mic).
The stated input impedance of the 1588B is 200 Ohms, source impedance 150 .. 250.

specs

From a simple calculation of the active part of the circuit one can conclude the 'TX-gain' is 10dB, so a ratio of around 1:3.

circuit (thanks Dave :thumb: )

So the load that the TX-secondary sees for AC will be 3*3*200 = 1k8.
The load of the TX-secondary looks much higher than that though, by at least an order of magnitude.
(Q1 runs at 28 uA, Q2 at 0.6 mA, Q3 at 19 mA)

Should we conclude that the quoted input impedance it incorrect ? Is it the usual confusion about source vs loading impedance ?

It'd be good to know whether it's indeed 200 Ohms or way higher, since it'll save me current in the intended circuit before the 1588B.

I'll definitely measure the input-Z myself but don't have a 1588B here yet.

Thanks,

Peter

 

[emrr]

[quote author="clintrubber"]
The stated input impedance of the 1588B is 200 Ohms, source impedance 150 .. 250.

Should we conclude that the quoted input impedance it incorrect ? Is it the usual confusion about source vs loading impedance ?

It'd be good to know whether it's indeed 200 Ohms or way higher, since it'll save me current in the intended circuit before the 1588B.
[/quote]


at a glance I'd say it's a reference of intended source impedence only. Very few things were designed to match mic impedances by this era.

 

[clintrubber]

[quote author="emrr"][quote author="clintrubber"]
The stated input impedance of the 1588B is 200 Ohms, source impedance 150 .. 250.

Should we conclude that the quoted input impedance it incorrect ? Is it the usual confusion about source vs loading impedance ?

It'd be good to know whether it's indeed 200 Ohms or way higher, since it'll save me current in the intended circuit before the 1588B.
[/quote]


at a glance I'd say it's a reference of intended source impedence only. Very few things were designed to match mic impedances by this era.[/quote]
Thanks for responding! I expect a measurement will comfirm the higher input impedance.

Still a bit strange. Since both the input & source impedance were specified, one would assume they're each the correct figure and not two spec-items actually describing the same (the source impedance).

But it'll be OK like this; the higher input-Z means less current needed in the circuit in front of it.

Bye,

Peter

 

[skipwave]

Hey Peter,

You're all over this one! :wink:

I had noticed, too, that R3 is way too large an RLoad for this transformer. I assumed it was chosen to properly bias Q1. I think to eliminate or downsize it we would have to change the biasing method.

Also, how weird is it to see just a 47pF cap as the zobel network? Makes me wonder if this transformer has a serious rise in the high end.

 

[clintrubber]

[quote author="skipwave"]Hey Peter,

You're all over this one! :wink:[/quote]
Yeah, to my own surprise as well it turned out that a lot can be said about just three transistors and a lump of Peerless-iron :wink:

I had noticed, too, that R3 is way too large an RLoad for this transformer. I assumed it was chosen to properly bias Q1. I think to eliminate or downsize it we would have to change the biasing method.

For the audio signal R1 also joins, so the input impedance as seen after the TX is R1 // R3 // Q1-inp-imp. But that's still much higher than the 'expected' 1k8 as per the specsheet.

If we would decrease the input-Z imho we could best leave the operating point unchanged and put a resistor // to C3.

But that higher input-Z actually suits me.
And who knows, the present unmodded situation might better suit the TX (although the presence of C3 might indicate that the TX would rather have seen heavier loading ?)

Let's just declare the 1588B-specs to be in error about the input-Z and happily play on

Also, how weird is it to see just a 47pF cap as the zobel network? Makes me wonder if this transformer has a serious rise in the high end.

IIRIC some other TXs have that as well (certain versions of Neve mic-pre's ?) but I guess the name Zobel no longer applies. So indeed, it seems there's some correcting going on :wink:

Enjoy,

Peter

 

[Rybow]

Hello! I actually have a 3 channel altec unit with a VU meter for output. It contains 3 1588A mic pre modules set in 3 14799a mixer modules. Looks easy enough to mod for individual outputs. I am having trouble finding a datasheet for the 1588a though, so I have a bit of work to do to confirm the pin out. The 14799a mixer looks to just provide the connectivity for the 1588a's, and also gives a potentiometer for gain. Output is unbalanced.

I was going to use a cheap 24V ac/dc switching power supply to see if it works. By my calculations a 300Ω 0.5W (1W to be safe) resistor should give me 12V @ 0.039ma just to get these powered up. Not sure if they are worth it though. Could be interesting! If all goes well, then I'll build a proper regulated linear power supply. Probably set for a slightly higher voltage.

Anyone add a line amp to these to boost gain and balance the output?

 

[lassoharp]

Hello! I actually have a 3 channel altec unit with a VU meter for output. It contains 3 1588A mic pre modules set in 3 14799a mixer modules. Looks easy enough to mod for individual outputs. I am having trouble finding a datasheet for the 1588a though, so I have a bit of work to do to confirm the pin out. The 14799a mixer looks to just provide the connectivity for the 1588a's, and also gives a potentiometer for gain. Output is unbalanced.

I was going to use a cheap 24V ac/dc switching power supply to see if it works. By my calculations a 300Ω 0.5W (1W to be safe) resistor should give me 12V @ 0.039ma just to get these powered up. Not sure if they are worth it though. Could be interesting! If all goes well, then I'll build a proper regulated linear power supply. Probably set for a slightly higher voltage.

Anyone add a line amp to these to boost gain and balance the output?

I've still got several of the C modules which have specs and pin out schematic printed on the label.  Not sure if they were the same for the As & Bs.  Cs have the extra pin for phantom (#4).  May want to check the Altec Lansing heritage page.  Seems like they had some data sheets for these.

I've had a project on the shelf involving adding a simple single bottle tube stage to the output of these.  Should be an interesting sound.  The Altecs are fairly smooth sounding on their own.  Somewhat lo-fi relative to API, Neve etc but still useful. 

For PS requirements, the 1599 would be ideal starting point.  That mixer housed only the preamp modules (6 total) with no makeup amp.  Altec undersized the PT and it ran very hot. No problem with modern choices.  The 1592 PS would also cover these.

 

[Rybow]

I've still got several of the C modules which have specs and pin out schematic printed on the label.  Not sure if they were the same for the As & Bs.  Cs have the extra pin for phantom (#4).  May want to check the Altec Lansing heritage page.  Seems like they had some data sheets for these.

I've had a project on the shelf involving adding a simple single bottle tube stage to the output of these.  Should be an interesting sound.  The Altecs are fairly smooth sounding on their own.  Somewhat lo-fi relative to API, Neve etc but still useful. 

For PS requirements, the 1599 would be ideal starting point.  That mixer housed only the preamp modules (6 total) with no makeup amp.  Altec undersized the PT and it ran very hot. No problem with modern choices.  The 1592 PS would also cover these.

Lo fi is cool with me. I have hi fi covered, so these could add an intersting new colour.

The 1588A's don't have an extra pin for phantom, and they do have the pin out printed right on the can. I can never remember where pin 1 starts on an octal socket, so I'll be getting in there with the old continuity meter.

For the PS I was just going to throw in some volts to see if they are still functional before getting into building a proper PS. I have enough parts on hand to get a linear supply going that should work well.

Cool idea about a single tube stage line amp! That would be really interesting. I was thinking of doing 2 channels with a nice clean op amp something with balanced output. Maybe a 0/+10/+20 gain switch idea. On the 3rd I was thinking of trying to put my spare gar1731 to work with a transformer balanced output and variable gain. We shall see. This hinges on whether these modules still work or not. Anyway, ways down the road on that experiment. Gotta save money right now.

 

[lassoharp]

I can never remember where pin 1 starts on an octal socket, so I'll be getting in there with the old continuity meter.

For the male pins, looking straight at them from the top it's always clockwise with pin #1 starting at 1:00.

I'll post back here when I get a version of the hybrid thing prototyped.