Vintage Synth Repair

Hello,

Does anybody know if a LF411a is a 
safe substitute for a LM301?

I have read an article that claims
the LF411a can be subsituted for the
LM301 in Arp 2600s.

TIA,

Ethan

--- In vintagesynthrepair@yahoogroups.com, "ethanzer0 
<ethanzer0@y...>" <ethanzer0@y...> wrote:
> Hello,
> 
> Does anybody know if a LF411a is a 
> safe substitute for a LM301?
> 
> I have read an article that claims
> the LF411a can be subsituted for the
> LM301 in Arp 2600s.
> 
> TIA,
> 
> Ethan


These chips are very different: The 301 is a bipolar opamp
that needs external compensation (and which can be optimized
for speed, depending on the voltage gain in the circuit).
The 411 is a low offset BiFet amp (low offset for a cheap Fet
input, that is ...) with internal compensation.
Nothing could be farther from being a drop in replacement.
But of course there are a lot of circuits where you _could_
use both chips, especially if you remove the 301's external
copensation cap(s). OTOH, if you have any cap in your 301
circuit with a value other than 30 ... 33pF (or if you even
have two compensation caps for feedforward compensation),
don't think about replacing it with a 411.

Why would you even want to use a 411 instead of a 301?

If memory serves, the ARP has LM308's somewhere - now here
it might make more sense to replace them with LF411 ...

JH.

--- In vintagesynthrepair@yahoogroups.com, "dark_november2000 
<jhaible@t...>" <jhaible@t...> wrote:

> 
> Why would you even want to use a 411 instead of a 301?
> 

According to the analogue heaven 
archives, the audio path in the
Arp 2600 can be upgraded by 
substituting the 411 for the 301.
However, it does require cutting
pins 5 and 8 on the 411.

I am trying to determine if this
is a wise mod or not.

Thanks for your input.

Ethan

Depends on what you mean with "upgraded".

If you like how it works and how it sounds, I'd
just leave it as it is.

If, however, you want to improove certain parameters,
you can certainly do this. Like using a low noise
opamp if your module is too noisy, or using a low
offset opamp if you think you have too much offset,
or using a FET input opamp if you have too much drift ...

JH.

> > Why would you even want to use a 411 instead of a 301?
> > 
> 
> According to the analogue heaven 
> archives, the audio path in the
> Arp 2600 can be upgraded by 
> substituting the 411 for the 301.
> However, it does require cutting
> pins 5 and 8 on the 411.
> 
> I am trying to determine if this
> is a wise mod or not.
> 
> Thanks for your input.
> 
> Ethan

--- In vintagesynthrepair@yahoogroups.com, "dark_november2000 
> If, however, you want to improove certain parameters,
> you can certainly do this. Like using a low noise

The mod I was reading about suggests
the  411 as a low noise replacement
for the 301.  I admit my 2600 is a bit
noisy, most notably in the mixer and
reverb section.

Thanks again!

Ethan

> The mod I was reading about suggests
> the  411 as a low noise replacement
> for the 301.  I admit my 2600 is a bit
> noisy, most notably in the mixer and
> reverb section.

The LF411 isn't exactly a low noise opamp.
Typical noise voltage is 25nV/SQRT(Hz).

The LM301 has a typical noise voltage of ca.
15nV/SQRT(Hz), which actually makes it
_better_ than the LF411.

Both are not exactly low noise opamps. For
comparison, a decent mic amplifier chip
like the SSM2017 has less than 1nV/SQRT(Hz).
Of course this will _not_ fit in here; just
to show the difference. There are opamps with
similarly low noise, whouch _would_ fit in,
but don't ask me for part numbers.

Now this is only part of the sory, though.
If you have a high impedance circuit (large
resistors in your circuit), BiFet Opmaps
like the LF411 might be less noisy, because they
have very low noise *current*. (This adds
to the noise voltage, depending on the feedback
resistors around your opamp.) So depending on the
circuit, you might even get a little noise
improovement with a 411 instead of a 301.
BUT then I'd really suggest a low noise BiFet
Amp like the TL071 (if you want an inexpensive
replacement). This has a noise voltage similar to
the LM301, but - like the LF411 - does not contribute
much noise current.
If you want to go a little further, try one of the
plenty modern opamps offered from Analog Devices
or TI (formerly Burr Brown) which have noise voltage
in the range of a few nV/SQRT(Hz). Expect to pay a few
dollars, though, and check if you like the sound.
Replacing an opamp with a better one will not automatically
lead to a better sound - it might, or it might not. 
You have to try it with your own ears. I'd try a
cheap TL071 first. (needs no external compensation
capacitor, so remove this if you swap the opamps.)

JH.