Op amp replacement, tantalum caps and slew rates

[ebartlet]

Hi everyone,

A couple of questions:

Slew rate: Is slew rate cumulative? (I think I know the answer...) I mean, after going through several op amps, does this slew error add up?

The bottom line is that I am mostly interested in Olympic/Helios and EMI stuff from the late 60's or early 70's. I like the sound of the music as much as the music. What are the slew rates of opamps used during this era? (I realize that there are many other factors besides slew rates that contribute the sound of anything) Apart from the opamps in a tape deck, does tape itself have slew rate error?

A while back another poster stated that some studer and otari decks are full of 4558's which have a slew rate of 1.7V/us. People don't seem to mind the sound of these, but rechip their consoles all the time.


I have been considering rechipping a tapco c12 mixer. It is full of 4558's. I would like to get closer to the sound metioned above. I know that the best thing to do is plug and play to see what I like, but before I start, I would like to know what op amps (if any) might get me closer without PS ugrades or RF problems.

I would also like to experiment with tantalum caps, I have read that I may like the sound, but understand that they need to be biased properly. What kind of bias, and can I just measure across and exsisting electrolytic? I am guessing that this reading would change with a tant in place. Here is the shemo for an input channel, all IC's are 4558.

http://www.ericbartlett.com/images/tpchannel.GIF

Thanks

 

[Svart]

As for replacing any opamp IC, you need to know how much max current the PS supplies and try to figure how much the board actually consumes with the 4558 IC's. you'll of course pick another IC to replace the 4558, but take care to compare the cumulative current needs of the IC's you will be using to the PS max output.. I don't know the pinouts of the 4558 off hand but see if something like the 5532 will work. i'm pretty sure that the "better" the opamp IC, the more current it draws so BB/AD parts might be out of the question without PS mods. Most ICs are pretty stable without too many RF problems, but again the faster/better IC's will be more likely to cause problems without decoupling mods and PS mods.

as for slew rate being cumulative.. I would think so to a degree but, I would think that the system as a whole would only be good as it's weakest link. You could have a whole path full of BB parts but go through one 4558 and pretty much ruin anything you preserved using better parts. so i guess the answer depends on the configuration.

as for tantalum caps, I would rather reach for good 'lytics and polypropylene/polyester/teflon/polystyrene/polycarbonate etc before tantalums in the signal path. I don't know much about tantalum caps but i believe there is more to getting them to work correctly than just measuring. for something like this you will be better off spending money on other types of good caps for easy replacement.

just my .02$

hope this helps!
:guinness:

 

[clintrubber]

The only piece of gear I have that uses tants in the signal path is a Trident comp/lim (CB9146 I thought). I don't seem to recall any special biasing precautions.

 

[gyraf]

Yes and no..

Slewrate limitations dosen't add up - it is simply a maximum speed value, limited by your slowest building block.

But unwanted side-effects due to asking too much from a limited slewrate block will off course be cumulative..

For tantalums, the "biasing" issue is simply that you prefer to have a predictable-polarity DC voltage the right way across them, as they hate being polarized wrong.

Jakob E.

 

[ebartlet]

[quote author="gyraf"]
For tantalums, the "biasing" issue is simply that you prefer to have a predictable-polarity DC voltage the right way across them, as they hate being polarized wrong.

Jakob E.[/quote]

So the bias on a coupling cap needs to be greater than a voltage swing in the opposite direction...

 

[NewYorkDave]

I don't think the Helios stuff used op-amps at all, did it? The schematics I've seen are of cascaded feedback pairs or triples, if I recall correctly, and not op-amp topologies. (And I'm using "op-amp" to mean an operational amplifier circuit topology, whether discrete transistor or IC).

 

[ebartlet]

[quote author="NewYorkDave"]I don't think the Helios stuff used op-amps at all, did it? The schematics I've seen are of cascaded feedback pairs or triples, if I recall correctly, and not op-amp topologies.[/quote]

I wouldn't know. Were there other well known consoles that used an arrangement more like the Helios design vs conventional op-amp topologies?

 

[PRR]

You are not going to get a Tapco to sound like the previous generation of discrete design; even I could hear that change.

Tapco is an especially bad example because it was the bleeding edge of a revolution: hyper-feedback and variable gain (instead of variable loss).

I'm not real clear-eared, but after hearing my Tapco against the rather lame preamps in my Ampex, I never used the Tapco.

And no: it isn't just slew or caps. You are almost asking why a Stratocaster does not play like a Martin. Very different architecture.

The Tapco might be an excellent "transparent" board after replacing all transformers, all chips, and most caps. But that's rather silly.

It would be period-correct for 1980 punk.

Tantalum bias is just making sure they never see reverse voltage. Most tants will hold voltage over an audio cycle, so you don't need to know the peak swing, only that the idle DC voltage is NOT reversed. We often use Aluminum electrolytics in spots that get "no" DC bias, which with errors sometimes means a few tenth-volts of reverse bias. Al will stand this for a while, Tant won't.

 

[ebartlet]

[quote author="PRR"]Tapco is an especially bad example because it was the bleeding edge of a revolution: hyper-feedback and variable gain (instead of variable loss). [/quote]

What your saying is that older boards used fixed gain stages and variable attenuators?

 

[NewYorkDave]

Yes. Until about 1970, that was the usual way to go.

It was typical to employ several of one or two different amp types in a console, with the amps at a fixed gain (usually 40dB) and, when less gain was needed, pads were used.

 

[ebartlet]

[quote author="NewYorkDave"]Yes. Until about 1970, that was the usual way to go.

It was typical to employ several of one or two different amp types in a console, with the amps at a fixed gain (usually 40dB) and, when less gain was needed, pads were used.[/quote]

Why did things change? Lots of feedback=less amp stages=less parts?

Would their be anything gained by going to a fixed gain variable loss topology?

I originaly wanted to build a mixer from scratch. I figured if I could find an exsisting mixer which was externally what I wanted, it would save metal work, labeling, and provide some of parts needed for the project. Each channel has its on PCB and plenty of room to work, very modifyable, Unlike the mackie 1604 I will be getting rid of, SMT nightmare. The tapco was very cheap and met my needs for a platform to build on.

 

[CJ]

Speaking of Helios, I saw these at the AES show. I didn't know someone was still making them. Don't know which company this is. Anyone?

 

[soundguy]

there are now like three different people which have come out with "official" helois stufff in the last two years or so.

dave

 

[ebartlet]

[quote author="cjenrick"]Speaking of Helios, I saw these at the AES show[/quote]

Drool...

[quote author="buttachunk"]
the Helios stuff gets its sound from its transformer and its biasing-- a very unconventional design has a very unconventional sound.
[/quote]

What is unique about its biasing? And why does it sound good to us?

 

[audiocraft]

Speaking of Helios, I saw these at the AES show. I didn't know someone was still making them. Don't know which company this is. Anyone?

Maybe these guys http://www.helios-audio.com


Luis

 

[ebartlet]

[quote author="buttachunk"]
about your tapco-- i've redone small mixers from the ground-up before. is the PSU on a seperate circuit board from the rest of the mixer circuits ?[/quote]

yes

 

[ebartlet]

What I like about this unit is that although it could be considered a "portable" mixer (its a 25 year old tapco, not a new one), its not small either and everything is modular. Each channel has its own pcb.

Will the 5532 drop right in without any change in surounding components?

I have heard people speak against using sockets for IC's in audio circuits other than for testing purposes. Have you experienced any long term problems with leaving them in?

I'm not looking for a transparent board. Of course I'm not sure I want to leave the board as-is either. I would like something that is pleasingly dirty, ala 1970 or as close as I can considering what I'm working with.

 

[dayvel]

About 20 years ago, I replaced all the opamps in my little Tapco mixer. It was a real pain cuz it used CA4136 ICs, which are 4 opamp chips with a non-standard pinout. I had to use wire-wrap sockets to make adapters for all the chips. It was not fun.

 

[ebartlet]

[quote author="dayvel"]About 20 years ago, I replaced all the opamps in my little Tapco mixer. It was a real pain cuz it used CA4136 ICs, which are 4 opamp chips with a non-standard pinout. I had to use wire-wrap sockets to make adapters for all the chips. It was not fun.[/quote]

I was scared about getting at first because I heard it had TO-5 (round can) LM301 opamps. Fortunately everthing is 8 pin DIP.

 

[ebartlet]

[quote author="buttachunk"]just took another look at your schemos,


not sure if anyone's mentioned this yet, but replace C3 (the one connected to R6) with higher value (1000uf-4700uf 6.3v), and you can replace C5, C6, and C10 with same value.

if you want tantalum in there, go for C1 (does that say 10uf?), and C11 / C18 (22uf/10v)[/quote]

Thanks for all of your help so far.

Thats a big jump from 100uf to 1000-4700uf. How does this effect things?

With the tants, should I replace these with the stock value?

Would the tants in the EQ circuit have any effect on the sound with the EQ at 0?

BTW, I recorded a few different sound sources through this board, and plan to do the same when I am done modifying it. I'll post clips for comparison when I'm done.