Need help calculating current of Yamaha 80100 DOA
« on: January 13, 2019, 12:40:02 PM »
Hi folks,

I guess I need some hand holding here. I'm trying to calculate the current draw from this 80100 opamp (and it's mic pre brother 80200 btw, but can't find the exact schemo for that one, but I understand it's a 1 transistor difference)

Anyway, I really like the sound of these GarAM10 opamps that CAPI sells. According to Silent Arts, here is their current draw:

+/-24V Idle = +/- 3,5mA
+/-24V +26dBu into 600R = +/- 15mA

+/-16V Idle = +/- 2,9mA
+/-16V +23dBu into 600R = +/- 11mA

I'm trying to figure out if I need more power if I replace (lots) of the 80100's and 80200's in my console with the Gar AM10's.

Here is a link to the 80100 DOA schematic:

I don't understand how to calculate. All I'm seeing on datasheets when referencing the transistors is max current ratings. For example:

2SA672 (E) (There are four of these per opamp)
Max collector current: .2A

2SC1213A (C,D) (X1)
Collector current: 500mA

2SC1775 (X1)
Collector current 50mA

2SC1624 (O,Y) (X1)
Max collector current: 1A
hfe1: DC current 150mA
hfe2: DC current 500mA

2SA814 (O,Y) (X1)
Max Collector current 1A

All these max collector current ratings. They're not that useful correct? How do I calculate
current at +/-24VDC in order to compare power draw for these opamps?


Re: Need help calculating current of Yamaha 80100 DOA
« Reply #1 on: January 13, 2019, 01:44:11 PM »
Why don't you just measure your 80100 / 80200?


Re: Need help calculating current of Yamaha 80100 DOA
« Reply #2 on: January 13, 2019, 06:21:02 PM »
> Why don't you just measure your 80100 / 80200?

That would give the right answer, quick, sure.

The "Max" ratings would normally be MUCH more than the circuit runs. Say I build a bridge with beams which will *break* at 180 pounds (my weight). I would not want to cross it. If I gained a pound, or sneezed, I'd fall in the stream. We normally design bridges and houses to break at 5X of a worst-case load-- a solid mass of people (party or panic). (*)

Simple circuit analysis will give the static consumption. Tr4 is biased with 0.6V across a 680 Ohm emitter resistor, call it 1mA. TR5 is 0.6V on 120r so 5mA. The 56K bleeds <1mA to D4D5. The output stage.... well, you can't "know" without device data which is not published. It can be solved if you could know the current density in D6-9 and Tr7-8, which comes down to the sizes of the dies. However for thumb-estimation assume the Designer aimed for 15mV-30mV across each emitter resistor. 1mA-2mA here.

But you are gonna put a load on it and jack-up the signal. A loudspeaker amp with 8r load driven full-Sine "acts like" 48 Ohms on the power supply. So 600r acts-like 3,600 Ohms on the power supply. At 48V total this is 13mA. Add <10mA for all the rest, 10mA to 23mA depending on signal.

Unless you do a BBC-Test with ALL channels at MAX, it is unlikely all channels will suck the full 23mA. In fact for unclipped speech/music the dynamic current won't average over 10%. So <10mA idle plus <2mA dynamic average, 12mA budget.

If load is "worse" than 600r..... that mess-of-diodes actually limits the current to 80mA in each Tr7 Tr8. Being push-pull, the average supply current for Full Square can not be over 40mA (plus idle current). That's clearly abnormal operation, and in dead-short the transistors dissipate nearly 1W, so may melt soon.

(*) I said we take large margin of safety on bridges and floors. This amplifier allows 80mA in a 1A (1,000mA) part 2SC1624. That's 12:1 which is a little more generous than house floor design. 10:1 is often advised for "overhead lifting", where danger is high and loads very uncertain. While electronics designers are known for lower FoS than structural engineers, here the low-low cost of an "oversized part" made large FoS cheap.
« Last Edit: January 13, 2019, 06:26:02 PM by PRR »


Re: Need help calculating current of Yamaha 80100 DOA
« Reply #3 on: February 10, 2019, 01:26:45 PM »
PRR, man thanks. That's some good teaching. Took me awhile to attempt to digest, and a lot applies to my whole console. Bigger picture. Thanks a bunch.

I just went ahead and cut some signal traces on the pcb to insert the meter.

But I probably had 20 channels plus all the master channels (PM2K)  powered up, but idle. I then ran a 1k signal into the channel I was measuring, which has 1 80200 and 1 80100. Measuring each opamp both idle and with 1K maxed into super distortion territory where the current actually starts to fall off. This is by the way, while IN the mixer (Yamaha PM2000) , loaded by the master bus.

80200 idle generally around 3mA per rail (various test subjects)
80200 maxed out around 6-8mA per rail
80100 idle 8-12mA (various test subjects)
80100 maxed around 19-21mA


Re: Need help calculating current of Yamaha 80100 DOA
« Reply #4 on: February 10, 2019, 03:47:20 PM »
While this is a subtle secondary effect, increasing the current consumption of every op amp inside a console chassis will increase the internal heat load a linear amount. Two times the current is two times the heat to dissipate.

Not an issue until it is....

John Roberts
Tune it, or don't play it...


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