Open Collector opamps?

I have just bought a used tape deck, that uses some (to me atleast) very strange opamps. They have open collector outputs and are rated for currents op to 70mA. Here's a schematic from the datasheet:



Why would opamps like that be used in audio circuits? The class A outputs (if I'm not mistaking :grin?

Best regards,

Mikkel C. Simonsen

They didn't have good PNPs on IC processes yet.

But the same circuit includes TL082 op.amps with PNPs in the outputs. So I guess they must have had some other reason for using the open-collector types also.

Best regards,

Mikkel C. Simonsen

In what context are the opamps used in?

It looks to me like some sort of current output....maybe driving the tape heads?

thats what i was thinking, one side of tape head to Ucc+, one side to open collector output. but a failure of the opamp would fry the tape head, so its not something i would do.

They are used all over the place. The playback amp, line input, mic input, meter driver etc. In both the mic and playback amp inputs the op.amps are preceeded by a discreet transistor stage though.

The only places the "normal" TL082 types are used are in the output stages of the recording amp and the headphone amp. I thought you needed something more powerfull to drive headphones... :grin:

Best regards,

Mikkel C. Simonsen

These are east german opamps. I guess this tape is from RFT. They are designed for mixed analog/digital applications. It's easy to connect open collector outputs together as a NOR gate.
In east germany you first had to look which parts are available then you can build something with this parts. I guess that's the reason they were used in this circuit.

[quote author="Michael Krusch"]I guess this tape is from RFT.[/quote]
Yes: VEB Stern - Radio Berlin Betrieb des VEB Kombinat Rundfunk und Fernsehen

In east germany you first had to look which parts are available then you can build something with this parts. I guess that's the reason they were used in this circuit.

Click to expand...

I know that also. But RFT did make standard op.amps like TL082 and TL084 (and they are also used as I mentioned). So I still think the engineers that designed it had a reason to use strange rather than normal op.amps - but I guess that's hard to find out now...

Given the amount of parts used, and the quality, I don't think saving money was top priority BTW. They use reed relays for signal switching rather than the switch chips also made by RFT for instance.

Best regards,

Mikkel C. Simonsen

As I recall (scratches head and looks skyward) opamps were designed to functional building blocks in analog computers. Adders, multipliers, log etc. These operations were chained together to provide the solution to some function (and that predated digital computers). At the time analog computers were much faster then the digital machines of their day (digital was just starting in that era)

Open collector outputs gave you significant design flexability. You could take the maximum value of more than one function, you could sum more than one value

I remember that we had a hand me down analog computer in school that I played with.

Regards

[quote author="mcs"]....that uses some (to me atleast) very strange opamps....[/quote]

Hi,
B761 is an opamp from the 70's, IMO copy of Siemens TAA761. You may check TAA761, 762 on Google if you need more info.
Regards,
Milan