Yamaha PM-430 Voltage Divider Help

[rjb5191]

A little background. I'm modifying a Yamaha PM-430 with discrete opamps in place of the TA7136 IC's and they draw more current. As I add more opamps, the +/- 15V rails drop a little by little.

If you take a look at the power supply in the schematic on page 5 of the service manual (attached), you can see the voltage divider formed by the 47R and 3.3k fusible resistors to drop the supply voltage from +/- 22 to +/- 15. I've attempted to use online voltage divider calculators but they are not working for this application.

I've already experimented by replacing the 47R resistors with lower values. I've already gone down to 22R which has helped to raise the voltage back up but I know this is also reducing ripple reduction as part of the RC filter there.

So, my question is in regard to the 3.3K resistors to ground. These must be bleeding off current to ground but I'm guessing it is a small fraction of what is passing through the 47R, I just do not know how much!

How do I properly calculate this?

Will raising the 3.3K resistors have much effect? What about negative effects?

What is the most efficient way to design this section without having to resort to using active devices? I would like +/-17V output if possible,

 

[rjb5191]

Inserted image to make it clear what I'm referring to.

 

[Khron]

47 ohms and 220uF won't be doing THAT much ripple reduction in the first place.

http://www.learningaboutelectronics.com/Articles/Low-pass-filter-calculator.php
47R works out to about 15Hz (that's the -3dB point, and as a first-order filter, that'll keep going down at 6dB per octave). 22R would about double that, to ~30Hz. Double the caps (to 470uF) to end up with largely the same effect as stock.

 

[radardoug]

You would be better off using a couple of 15 volt 3 terminal regulators.

 

[rjb5191]

47 ohms and 220uF won't be doing THAT much ripple reduction in the first place.

http://www.learningaboutelectronics.com/Articles/Low-pass-filter-calculator.php
47R works out to about 15Hz (that's the -3dB point, and as a first-order filter, that'll keep going down at 6dB per octave). 22R would about double that, to ~30Hz. Double the caps (to 470uF) to end up with largely the same effect as stock.

Thank you. Yes, They are 1000 uf now as I've recapped the power supply. I'm mostly interested in the resistor values to optimize this part of the circuit to achieve +/- 17V at an estimated 300-350 mA current per rail.

 

[rjb5191]

You would be better off using a couple of 15 volt 3 terminal regulators.

Yes, I agree but the pcb layout does not easily allow this as far as I can see.

 

[JohnRoberts]

maybe try a few values and see what works?

JR

 

[Khron]

If anything, there's not much voltage dividing going on there. With no load, 22R and 3.3k drop the voltage to barely 99.3% of the 22v at its input. The voltage drop happens thus mostly due to the current draw.

Speaking of which, dropping 7v across the stock 47R works out to a draw of about 150mA, so if that 300-350mA figure is realistic, ~22R should be just the thing. Are those discrete opamps limited to +/-15v though?

 

[rjb5191]

If anything, there's not much voltage dividing going on there. With no load, 22R and 3.3k drop the voltage to barely 99.3% of the 22v at its input. The voltage drop happens thus mostly due to the current draw.

Speaking of which, dropping 7v across the stock 47R works out to a draw of about 150mA, so if that 300-350mA figure is realistic, ~22R should be just the thing. Are those discrete opamps limited to +/-15v though?

What is the point of the 3.3k to ground then? How far can I sensibly raise this value and what are the potential consequences of doing so? Why dump this current to ground at all?

The discrete opamps run up to +/- 24V volts. I'm hoping to run at +/- 17V since I think I like the sound at that voltage more than at lower voltages. The sound is a bit more stiff and punchy.

This does help me make sense of this a bit better because your 99.3% is basically what the calculators spit out. I just didn't realize that I needed to factor in the active current draw as well (seems obvious now). I have a 2 watt 15R that I can try. I suppose this will get me a bit closer.

 

[Khron]

3.3k's gonna dump a mere ~6mA or so to grund, so, to quote AvE over on Youtube, "f**k/all in a big ship". If anything, maybe to set a minimum load for the discrete +/-22v regulators (even though they may well not require it). Raising it will do pretty much nothing, in the grand scheme of things. 350mA draw from 17V is an equivalent resistance of about 48R. Whether you have 3.3k in parallel with that, or 1G, won't make any meaningful difference.

350mA through 15R works out to about 1.8W, so a bit "on the edge"...

 

[cLoudBase]

I have modded another of that vintage Yamaha desks & replaced all those opamps with modern ones. Joe Malone even designed & produced these great little kits for this very purpose. Super happy, highly recommended.

JLM Audio Shop

 

[FIX]

I would use regulators, as the divider will also bounce a little with varying signals.

 

[Admiral-dk]

According to the Schematic - those 47R are Fuses or problably more like Resistors that are meant to work as Fuses and Blow when you reach 145mA ....! They where very common (as in required by Law) in old TV sets here in Denmark - to protect against Fire.

We can only guess if that is to 'Protect' the Circuit before or after the Resistor - but my guess is the Transformer ....

Per

 

[JohnRoberts]

According to the Schematic - those 47R are Fuses or problably more like Resistors that are meant to work as Fuses and Blow when you reach 145mA ....! They where very common (as in required by Law) in old TV sets here in Denmark - to protect against Fire.

We can only guess if that is to 'Protect' the Circuit before or after the Resistor - but my guess is the Transformer ....

Per

yup those are called "flame proof" resistors and engineered to fail without getting hot enough to start a fire. I used a lots of them inside consoles to protect the PS rails on every strip. That way a bad strip would not take down the entire console.

JR

 

[Admiral-dk]

Thank you for reminding me about the correct name (slipped my mind) + a great example on a real Life application & why - JR

Per

 

[rjb5191]

3.3k's gonna dump a mere ~6mA or so to grund, so, to quote AvE over on Youtube, "f**k/all in a big ship". If anything, maybe to set a minimum load for the discrete +/-22v regulators (even though they may well not require it). Raising it will do pretty much nothing, in the grand scheme of things. 350mA draw from 17V is an equivalent resistance of about 48R. Whether you have 3.3k in parallel with that, or 1G, won't make any meaningful difference.

350mA through 15R works out to about 1.8W, so a bit "on the edge"...

Thanks, but I think your math is a bit off on the 48R calculation unless I'm missing something. The stock current through the resistor is listed in the schematic as 145 mA. The discrete opamps I'm installing will more than double that in the end.

R= V/I

V = 22-17 = 5V

I = .35A

R = 5/.35 = ~ 14.3R

I agree with you and it makes sense to me that there's essentially nothing to be gained by replacing the 3.3k

 

[rjb5191]

yup those are called "flame proof" resistors and engineered to fail without getting hot enough to start a fire. I used a lots of them inside consoles to protect the PS rails on every strip. That way a bad strip would not take down the entire console.

JR

Also known as fusible. Sansui hifi amps are famous for them drifting high in value.

 

[rjb5191]

I would use regulators, as the divider will also bounce a little with varying signals.

What type of performance do you think could be gained by installing a 3 terminal regulator? I "hear" that something like an lm7818 isn't that great and is somewhat noisy. If that's not true, I could consider cutting some traces, drilling the pcb, and kludging something together.

 

[Khron]

Thanks, but I think your math is a bit off on the 48R calculation unless I'm missing something. The stock current through the resistor is listed in the schematic as 145 mA. The discrete opamps I'm installing will more than double that in the end.

R= V/I

V = 22-17 = 5V

I = .35A

R = 5/.35 = ~ 14.3R

I agree with you and it makes sense to me that there's essentially nothing to be gained by replacing the 3.3k

I was talking more about the 2W rating of the 15 ohm resistors you were planning to use (P = R * I^2).

 

[rjb5191]

I was talking more about the 2W rating of the 15 ohm resistors you were planning to use (P = R * I^2).

O gotcha, I think these fusible resistors are ok to be run near their fuse rating. The stock resistors seemed to be even closer to their power rating in its original configuration.