Raindirk Preamps, too dark and undetailed. Change TBA231 IC to NE5532?

[Ted Krotkiewski]

Hi friends.
I have a lovely old Raindirk Mini Mixer from 1973 (See attached Pic), wich sounds fantastic when using the line in and it´s inductor EQ´s.
The Mic preamp, sounds a bit on the darker side and lack a bit of detail for some sources. The dark and thick tone of the desk is the reason I fell in love with it in the first place. However, sometimes, tracking a whole band, I would like to have the clarity and detail on some sources.
For example, I also have 2 Raindirk Channelstrips from a Series 3 desk (from Olympic studios), wich sounds a bit more open and detailed while still feeling dark and woody in a very nice way if that makes sense?
Both the Series 3 and my Mini Mixer was manufactured during the same time period.

* The Mini Mixer uses an old IC called TBA231 (UA739). that I suspect is the culprit here. Line input also uses the same IC, but when feeding it with already high-end material I love the sound it imparts.
* The Series 3 uses the TDA 1034 (Equivalent to NE5532 from what I know)
(see attached schematics below)

So I suppose my 1st question is, do you think I will notice any major difference in swapping the old TBA231 Op-amp to a new NE5532???

If Yes (or maybe) on the above Q:
The next question is about the common oscillation issue that might occur when updating to new Opamps.
There is a super interesting article from Eddie Ciletti who once cerviced an almost identical Mini mixer, where he talks about oscillation when replacing the old IC. Its really an enjoyable read in general! Read it HERE

In the article he says the following:
One obstacle to upgrading op amps is that their extended high-frequency response at high gain settings can emphasize other shortcomings in the design, like the circuit board layout or power distribution. Troubleshooting Mystery One’s asymmetrical headroom issues revealed oscillation when cranking the gain and/or boosting the HF EQ. The typical approach to solving oscillation is adding power supply bypass capacitors (0.1 uF to 10 uF, and sometimes both) at the IC sockets, which reduced the problem but didn’t eliminate it.

So in this case of me adding a NE5532 at the preamp section, where should I add the Cap? at the + rail? on both? or where?

Again, I really REALLY enjoy the sound of this little console, So I just want wee more "Resolution" in the mic preamp on 4 channels or so.

I have also thought about the alternative of building 5 channels of Helios 2128 mic preamp for 5 channels, so that I can toggle between different flavors... But thats for another day,... and thread.
The Series 3 mixer replaced the Helios mixers at Olympic back in the day.

If you have any ideas, pleas just throw them at me.

Thank you for your time and help!

 

[Voyager10]

Describing something as 'dark' or lacking detail suggests that the high-frequency response is limited. Personally I'd try to measure it before heating up the soldering iron.

Are you able to do this? The simplest thing would be to use a phone with a signal generator app, feeding in to a DI box, through the desk and then into an audio interface line input. You set the gain for some fixed level (e.g. -10dBFS) using a 1KHz tone, then look at the level for say 10KHz and 15KHz. If it starts dropping below the -10dBFS level, there could be an issue.

(Obviously there could be problems with the phone, DI box or audio interface, so you might need to repeat the process with another desk, or with the DI going into the interface directly).

 

[k brown]

Hi friends.
I have a lovely old Raindirk Mini Mixer from 1973 (See attached Pic), wich sounds fantastic when using the line in and it´s inductor EQ´s.
The Mic preamp, sounds a bit on the darker side and lack a bit of detail for some sources. The dark and thick tone of the desk is the reason I fell in love with it in the first place. However, sometimes, tracking a whole band, I would like to have the clarity and detail on some sources.
For example, I also have 2 Raindirk Channelstrips from a Series 3 desk (from Olympic studios), wich sounds a bit more open and detailed while still feeling dark and woody in a very nice way if that makes sense?
Both the Series 3 and my Mini Mixer was manufactured during the same time period.

* The Mini Mixer uses an old IC called TBA231 (UA739). that I suspect is the culprit here. Line input also uses the same IC, but when feeding it with already high-end material I love the sound it imparts.
* The Series 3 uses the TDA 1034 (Equivalent to NE5532 from what I know)
(see attached schematics below)

So I suppose my 1st question is, do you think I will notice any major difference in swapping the old TBA231 Op-amp to a new NE5532???

If Yes (or maybe) on the above Q:
The next question is about the common oscillation issue that might occur when updating to new Opamps.
There is a super interesting article from Eddie Ciletti who once cerviced an almost identical Mini mixer, where he talks about oscillation when replacing the old IC. Its really an enjoyable read in general! Read it HERE

In the article he says the following:
One obstacle to upgrading op amps is that their extended high-frequency response at high gain settings can emphasize other shortcomings in the design, like the circuit board layout or power distribution. Troubleshooting Mystery One’s asymmetrical headroom issues revealed oscillation when cranking the gain and/or boosting the HF EQ. The typical approach to solving oscillation is adding power supply bypass capacitors (0.1 uF to 10 uF, and sometimes both) at the IC sockets, which reduced the problem but didn’t eliminate it.

So in this case of me adding a NE5532 at the preamp section, where should I add the Cap? at the + rail? on both? or where?

Again, I really REALLY enjoy the sound of this little console, So I just want wee more "Resolution" in the mic preamp on 4 channels or so.

I have also thought about the alternative of building 5 channels of Helios 2128 mic preamp for 5 channels, so that I can toggle between different flavors... But thats for another day,... and thread.
The Series 3 mixer replaced the Helios mixers at Olympic back in the day.

If you have any ideas, pleas just throw them at me.

Thank you for your time and help!

If your description of the sound is fairly accurate, I'd expect the particular op amp used to be least reason for it.

 

[ccaudle]

do you think I will notice any major difference in swapping the old TBA231 Op-amp to a new NE5532

The packages are different, you can't just swap one for the other. uA749 is in a 14 pin package with external compensation (the resistor and capacitor connected between pins 3 and 4), while a NE5532 is an 8-pin package, internally compensated so it doesn't need the additional pins to connect external compensation components.

 

[Voyager10]

If your description of the sound is fairly accurate, I'd expect the particular op amp used to be least reason for it.

Yes, I'd tend to agree. Only if there is a HF rolloff which varies with the gain setting would you suspect the GBW figure might be insufficient.

I'd take a look at that 12K resistor across the input transformer secondary. If the tx is giving 10dB voltage gain its impedance ratio will be 1:10, so the load presented to the mic will be 1K or so, which is low-ish compared to many preamps. I can imagine ribbon or tube mics sounding different here.