Preparing to mod Yamaha PM-430 mixer

[shot]

Hi all!

I got this beautiful Yamaha PM-430 mixer for dirt cheap. It's 8 channel mixer with Tamura xformers on input into TA-7136 opamps. Simple low/high shelf and two aux sends (pre and post).
I've tried it and it sounds very nice.
Except from cleaning faceplate, knobs, some isopropyl on pots, I plan to modify it for 8 channel recording.
The thing is that is very compact with it's own carrying case and it is perfect for me when going out of studio to record drums. That way I could finally avoid those uninteresting audio interface mic pres (at least first 8 of them).

I've attached excerpt from the schematic in this post so you can follow my train of thought.

1. Direct outs
Am I okay to assume that it would be enough to use impedance balancing here. For transformers there's no space and they would break out the bank in my case.  I plan to tap off signal from the 10uF cap after the second opamp since at this point level seems good.
Is it okay to use 100R on signal to pin 2 and also 100R from pin3 to ground? Or should I go with lower resistor value?

2. Would it be enough to use SPST switch for EQ bypass? If I just leave EQ to be always on, but use this switch to bypass signal in parallel from one 10uF cap after first opamp to next 10uF cap after TR4/TR5? Or should I use DPDT to completely isolate EQ section (disconnect both input and output signal of the EQ)?

3. I plan on replacing TA7136  opamps on this input module. I was thinking NE5534... any other suggested opamps?

4. Any reason I should replace those five transistors? I went to cross reference them with BC550B and they might be similar. Does anyone know anything about those 2SC1681 ? Can they be the noisy part that I should ditch out? Candidates?

5. If I add phantom on input xlrs, I would probably mount additional DB25 connector to still have possibility to run line level signals into channels (because they distort lovely and could be useful in the mix situation). Is it enough to put 10uF BP cap on each hot and cold signal line to prevent phantom traveling along line input? Or should I use 47uF or higher? Bipolar or regular polarized lytic?

6. Of course, I plan to recap all the electrolytics. But should I also replace tantalum caps? Do they tend to age also? Remember that this is 1974 mixer...

And of course, feel free to throw other suggestions what to do!
I still haven't decided if I need output section. It's so tempting to disable it and use those six discrete opamps in a separate preamp build! Or at least take out four of them and leave two on the main mix bus to still retain mixing capability.

Sorry for bombarding with too many questions in one post!



Luka

 

[joaquins]

I wouldn't bypass the eq but null the effect, using a DPDT and disconnecting the 4.7µF and 0.022µF from the wipers and maybe connecting them to ground. This way you change nothing but the filtering, the other way you might be changing something else in the sound and bring other problems. If you don't want for A/B eq comparison you might be ok with the SPDT alone.

  If you like the sound as it is I wouldn't start swapping semiconductors just because, they might be responsable for what you like from it, or even worse than loosing that, having other issues because of the difference they have. Real world Op Amps are non ideal 5 terminal devices, swapping them one for another just 'cause it's treating them as ideal 3 terminal devices.

  Note that the TA7136 was the original opamp in the boss DS1 used way beyond the linear range to get the desired distortion, it might have some non linear behavior somebody liked enough to choose for it and sell several thousand units.

  Changing the electros is good practice, keep the same values wherever is possible. Same thing, those dubious electros might help the sound you like from it, but not designed in nor bring any troubles newer (wetter) ones. Tants aren't so problematic, leave them in for now.

Impedance balanced output is fine, I like it a lot from the design point of view.

For the phantom, go for the 47µF caps, polarized, negative to the line inputs, maybe add some pulldown resistors, so the caps are always loaded the correct amount.

JS

 

[Humner]

I'm very familiar with the PM-430

You can see my mods here - https://groupdiy.com/index.php?topic=59851.0

1. I didn't implement direct outs on my 430, but what you propose sounds like a good start.

2. I think I used DPDT for my EQ bypass, but the other pole was going to be for indication light, so SP should be fine. Check the schematic for the PM700 as the master section has EQ bypass as a stock feature and will show you exactly how to wire it up.

3. I used OPA134 - but when I think about it probably not really necessary to swap out the opamps. Maybe do a few and compare. You need to do up custom boards to change to SIP and remember that the - and + opamp inputs on the TA7136 is switched if you do!

4. I have used BC550(for NPN) and BC560 for the single PNP without issue in that style of EQ. Unfortunately didn't make any comparisons, but its very quiet with the BC transistors so no harm none the less.

5. I converted mine to purely summing duties, so no phantom, I will let others answer that one for you.

6. I used Panasonic SU(bipolar) caps for the electrolytic's, however I can't remember seeing any tantalum caps.

I would keep the master section and salvage the other opamps - good idea, that's what I did and I used the spare DOA's in a ACA-BO for my 430.

Let me know if you have any other questions, happy to try and help out further.

 

[shot]

I wouldn't bypass the eq but null the effect, using a DPDT and disconnecting the 4.7µF and 0.022µF from the wipers and maybe connecting them to ground. This way you change nothing but the filtering, the other way you might be changing something else in the sound and bring other problems. If you don't want for A/B eq comparison you might be ok with the SPDT alone.

This is interesting idea! I will surely try that! At first I can do that even without installing switch, but just by clipping alligator clips to ground the wipers, just for test to hear how it sounds! Thanks for that!

  If you like the sound as it is I wouldn't start swapping semiconductors just because, they might be responsable for what you like from it, or even worse than loosing that, having other issues because of the difference they have. Real world Op Amps are non ideal 5 terminal devices, swapping them one for another just 'cause it's treating them as ideal 3 terminal devices.

  Note that the TA7136 was the original opamp in the boss DS1 used way beyond the linear range to get the desired distortion, it might have some non linear behavior somebody liked enough to choose for it and sell several thousand units.

I should add that when I tested to hear how this mixer sounds I did like it's sound, but it has more than obvious noise in the background. Went on to google those opamps and seems that many ppl agree that this is too noisy in today's standards. So they definitely go out and I'd swap them for something else.
And man, when I read that original DS-1 used those ICs, my eyes got shiny and I know that I'll be more than tempted to veroboard one or two DS-1 distortions just for fun when those opamps leave my mixer! Not that I need Boss distortion, but this is more like addiction.

  Changing the electros is good practice, keep the same values wherever is possible. Same thing, those dubious electros might help the sound you like from it, but not designed in nor bring any troubles newer (wetter) ones. Tants aren't so problematic, leave them in for now.

I don't want to change values of any cap. Just replace them with newer ones.
I found affordable Nichicon ES series Muse bipolar caps that I'll order.

Impedance balanced output is fine, I like it a lot from the design point of view.

So you think it's okay using 100R in this situation? Or should I go with lower value for any reason?

For the phantom, go for the 47µF caps, polarized, negative to the line inputs, maybe add some pulldown resistors, so the caps are always loaded the correct amount.

Ok got it!
Pulldown with 10k value resistor?
Are they necessary? It's going to be crowded with just electro caps, and I'm worried that it'll be wiring nightmare if I add resistors also, since I was thinking doing it directly on DB25 connector. It would be smart to etch utility pcb for those caps and resistors... I hate etching...



Luka

 

[shot]

I'm very familiar with the PM-430

You can see my mods here - https://groupdiy.com/index.php?topic=59851.0

Wow! You did huge job on this!
As much as I love fitting ACA summing, but I don't think I would need it.
But it's tempting... huh! Maybe in the end...
And I absolutely LOVE the blue faceplate on channels! Looks very classy!

2. I think I used DPDT for my EQ bypass, but the other pole was going to be for indication light, so SP should be fine. Check the schematic for the PM700 as the master section has EQ bypass as a stock feature and will show you exactly how to wire it up.

Thanks for the tip! I'll dig out that schematic and check!

3. I used OPA134 - but when I think about it probably not really necessary to swap out the opamps. Maybe do a few and compare. You need to do up custom boards to change to SIP and remember that the - and + opamp inputs on the TA7136 is switched if you do!

I was going to go with NE5534 but now I'm more towards trying OPA. Somewhere I read that OPA604 also work well in this circuit. I have lots of SOIC to DIL adapter boards and I'll improvise something. I will put sockets instead of original SIP opamp and try to do some swapping. I guess listening to different opamps is the only way in this situation.
But I must say that I'm very tempted to try OPA604 and increase the supply voltage! This mixer has +/-22v rails for discrete opamps used on output, and OPA604 can swing with that. That would possibly open up more headroom (and more headroom is always cool when recording drums!). Maybe I'll try that someday!

4. I have used BC550(for NPN) and BC560 for the single PNP without issue in that style of EQ. Unfortunately didn't make any comparisons, but its very quiet with the BC transistors so no harm none the less.

I will probably try to swap for those transistors on one channel. If it'll be any good then I'll do other channels.

6. I used Panasonic SU(bipolar) caps for the electrolytic's, however I can't remember seeing any tantalum caps.

I think I saw at least one tantalum cap near the input section. Didn't at that time read it's value to locate it in schematic.
But if they don't age then I'll leave it be.

I would keep the master section and salvage the other opamps - good idea, that's what I did and I used the spare DOA's in a ACA-BO for my 430.

It is impressive what you did!
At the end, after I do recap and direct outs, I'll think about doing the same.

Let me know if you have any other questions, happy to try and help out further.

Yes I will! Thank you!

I have to travel out of town in few days and after I get back home I'll order caps and opamps to start working on this mixer!



Luka

 

[joaquins]

This is interesting idea! I will surely try that! At first I can do that even without installing switch, but just by clipping alligator clips to ground the wipers, just for test to hear how it sounds! Thanks for that!

I don't want to change values of any cap. Just replace them with newer ones.
I found affordable Nichicon ES series Muse bipolar caps that I'll order.

So you think it's okay using 100R in this situation? Or should I go with lower value for any reason?

Ok got it!
Pulldown with 10k value resistor?
Are they necessary? It's going to be crowded with just electro caps, and I'm worried that it'll be wiring nightmare if I add resistors also, since I was thinking doing it directly on DB25 connector. It would be smart to etch utility pcb for those caps and resistors... I hate etching...



Luka

  Not grounding the wipers but the caps, you need to cut the connection between the wiper and the cap, then is optional as good practice and easy enough to ground those caps, not the wipers.

  About the 100Ω, it depends on the amp driving capability, it doesn't make sense to put 22Ω if the amp can't drive anything under 2k but is a shame to put 100Ω if the amp can drive 150Ω loads. They are there for stability issues when driving capacitive loads, Bill W likes to add am inductor in parallel with that resistor to archive both, low impedance  at audio freqs and decoupling for capacitive loads. It was something around 33Ω and 20µH IIRC but that's for the beefy 990 DOA

  You don't need etching but might save a lot of time, specially if using PCB mounting connectors! They aren't really necessary but may save you from loud noises when plugging stuff in.

  I found a few of those opamps in an old electronics shop back when I was in full stompbox fever, I expend an afternoon with a guy in the shop and he would bring the buckets of opamps for me to select the JRC4550 from the load of 4550, they had a lot of germanium stuff and some other interesting NOS for stompbox nuts. I might still have some stock from that time.

JS