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Old 12th June 2015, 05:34 PM   #1
Zaaphod is offline Zaaphod  United States
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Default Headphone amp series current limiter resistor

I have a small headphone amp made by pyle which uses RC4580 OP amps for it's amplification. It has very good sound quality, however, it does not actually amplify anything at all, in fact, it attenuates slightly. I took it apart, figuring I didn't have much to lose since it only cost me $14 figuring I could maybe figure out why it wasn't amplifying. What I found was, it actually was amplifying quite a bit actually, however, they put a 470ohm resistor in series with each output which ended up cutting down the output to less than what the original signal was. just for fun, I shorted out one of these 470ohm resistors with my known load applied and it sounded great with plenty of amplification. I was thinking... why did they put these here to hinder it's performance? My conclusion: they must have been getting a lot of returns from people hooking up 4 ohm or 8 ohm speakers to the outputs instead of headphones and burning out the chips... so series current limiters was a cheap, quick and easy way to idiot proof the design.. or perhaps to prevent the chips from burning out if a headphone cord shorted out...

This amplifier uses a 12V single supply running through a reverse polarity protection diode, giving it a working power supply of 11.3V. The RC4580 Dual op amps it uses have an absolute maximum output current rating of 50mA. the output of the op amps goes through the 470 ohm resistors, then through the headphones, then the common lead of the two headphone elements go through a 100uF capacitor to ground.

I'm wondering if I could safely reduce the resistance of these 470 resistors by soldering another resistor on top of them to get better performance to my headphones but still provide some protection in case of a short circuit.. Any idea how low I could safely go with these resistors?
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Old 12th June 2015, 06:31 PM   #2
sreten is online now sreten  United Kingdom
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Hi,

Change the 470R to 100R, that will give you near the standard 120R*
output impedance and pretty much provide sensible current limits.

rgds, sreten.

* A much ignored standard nowadays, typically ignored for
low voltage devices, but typical of amplifier headphones out.
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Old 12th June 2015, 06:43 PM   #3
Zaaphod is offline Zaaphod  United States
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thank you for the quick response! I'll give it a try
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Old 13th June 2015, 04:47 PM   #4
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RC4580? Cheapskates - that one does not nearly have as much power as the NJM4580. Then again, with 470R series resistors it wouldn't have mattered. Not sure how much lower than 100 ohms you can go with that type. Headphone cables can easily have 1 nF of parallel capacitance, and capacitive load driving abilities vary widely among opamps. Always depends on layout/decoupling, too.
What sort of headphones do you have? Ones that are not impedance-critical may be quite happy at 100 ohms already, and some extra series resistance can actually be quite beneficial to distortion performance. This happens when load impedance becomes about as large as or smaller than the part's internal output impedance (which usually ranges anywhere from 20 to 75 ohms), because then it starts to bog down open-loop gain. With less OLG comes more distortion, which otherwise is mainly dependent on output current levels.

From what you write the circuit might be a (thrifty) cMoy variation, which is a well-documented design that could serve as a reference point here. Does one leg of the feedback network go to that 100 cap, too? That one may need some bumping up now, as it becomes more critical with output series resistors reduced and would end up limiting output power in the bass. I suggest you take some time to draw out the schematic of the whole thing, it can't really be too complex. It has to be a single-supply opamp amplifier circuit after all, either noninverting or inverting.

Ultimately I'd be shooting for an NJM4556 and a gain of maybe 3, depending on what you need. The '4556 can be used down to essentially zero output series resistance and is about the most powerful cheap opamp you can get; it was originally developed as (surprise!) a headphone driver. If for some reason you should be unable to obtain one, NJM4580 or NE5532 aren't bad either and aren't typically calling for more than 10-47 ohms.
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Old 14th June 2015, 05:53 PM   #5
Zaaphod is offline Zaaphod  United States
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I took some time and drew up a schematic of the entire device and attached it here. I'm using this with relatively cheap heaphones, Koss UR10s. My purpose for it is to use my Galaxy Tab for the input source playing, and plug in 3 headphones to it and have good volume on each of the 3 pairs. I got a 12V Li-ION battery pack rated 6000mA-H to power the headphone amp. My intention is to have 3 users viewing movies on a long flight while each listening to headphones through this device. I know from experience, that the headphone output on my tablet with one of these headphones attached is not really high enough to hear over the noise of the plane, so I'm comparing headphones on the tablet alone to headphones though the amp, and I would like it to be able to go quite noticeably louder through the amp.

With the 470ohm resistor temporarily shorted, it's defiantly loud enough, but I don't want it to burn out 1/2 way though the flight either. I am happy with the sound quality of it, even with the 470 ohm resistors shorted, it still sounds good to me. I've downloaded the datasheet for the 4580 and it states maximum output current is 50mA, I'm not clear if that's per channel or total though. I should also note, that the datasheet I got was for Texas Instruments RC4580, but I'm not positive thats whats on this board. the ICs are marked:

4580
C203G
JRC



I thought I could change the gain on the preamp buffer they have in front of the 4 separate amps, but that would be hard to do since I would have to cut the zero ohm negative feedback loop, and that connection is made under the chip, and I don't care to remove a surface mount IC to get to it.

I just want to see if I can improve this a bit by changing resistors and such, preferably by soldering other resistors in parallel instead of removing any resistors. I could just get a better headphone amp, but this one was cheap and I won't be using it much other than on vacations, so I figure it would be worth a try to modify it to suit my needs.
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File Type: jpg headphone.JPG (72.5 KB, 89 views)

Last edited by Zaaphod; 14th June 2015 at 05:58 PM.
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Old 14th June 2015, 06:32 PM   #6
Zaaphod is offline Zaaphod  United States
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I found the box my headphones came in, it says they are 32 Ohms and frequency response is 60Hz - 20KHz

Last edited by Zaaphod; 14th June 2015 at 06:49 PM.
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Old 14th June 2015, 08:31 PM   #7
Zaaphod is offline Zaaphod  United States
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After more searching, I found the JRC on the op amps is the New Japan Radio Co. Designation, making them probably NJM4580 op amps. The datasheet for these does not specify a maximum output current. I've attached a screen shot showing the internal equivalent circuit, notice there are resistors between the output transistors and the output pin of the device. These seem to me would act as current limiters, and I'm wondering if I really need any external current limiting resistors at all with these ICs?
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File Type: jpg 4580 internal.JPG (105.5 KB, 81 views)
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Old 15th June 2015, 01:01 PM   #8
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NJM4580s are good for about half as much current as 4556s, about 80 mA max if memory serves. Still reasonably beefy by opamp standards. Output series resistors are not so much for short-circuit protection but rather for keeping the circuit out of oscillation in the face of capacitive loading.

The circuit is a little fancier than I thought, there even is an input buffer. BTW, I don't think the 47 goes in there like that, or at least I would expect it to be in parallel to the lower 22k rather than going over to the LED. Otherwise power supply rejection would suffer.

Koss UR10s are closed headphones of a nominal 32 ohm impedance. It's no surprise you didn't get any amplification out of the circuit, maximum gain is -10 (+20 dB, inverting) and you lose 24 dB due to the high output resistance. Closed cans tend not to be too impedance-critical. Sensitivity is given as 94 dB SPL, presumably re: 1 mW, so that should be 109 dB SPL / 1 Vrms. So even when shooting for a conservative 110 dB max, you'd be getting along fine with 1.1 Vrms, and for 103 dB even 0.5 Vrms would do.

Based on that, 100 ohms would work just fine. I'd probably go with something in the 47-100 ohm range. Don't overdo it or you may end up with audible hum, depending on how good the power supply is. The 100F would have to be bumped up considerably, preferably 330-470F (10 V minimum, good quality Panasonic, Nichicon or similar with lowish ESR) if you can fit one. The RC time constant can be calculated with R = 1/2 (Rout + Rload). I hope power-on pop noise doesn't become too much of an issue.

100n input coupling caps strike me as a bit on the low side, too.

It's not the luckiest concept in the world. Inverting amps with volume control in the feedback loop are prone to scratchy pots long-term, as MF A1 owners should be able to attest. And putting the coupling cap in the common return, I don't know. Sure most bass is center-panned, but still it's not totally the same as two separate coupling caps. For UR10s though, a slightly tweaked version should work splendidly.

Last edited by sgrossklass; 15th June 2015 at 01:20 PM.
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Old 15th June 2015, 02:27 PM   #9
Zaaphod is offline Zaaphod  United States
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You are correct about the 47uF cap.. it does go to ground, making it in parallel with the lower 22K resistor.. it is VERY close to that led resistor, but it doesn't touch, there must be a trace on the top of the board that I can't see.. I verified it with my meter, and it does go to ground, not the LED. I've corrected the drawing here just for anyone who comes across this thread later.

Finding a 10v cap in the 330uF to 470uf range that will fit in the space allowed is going to be challenging. I have only 0.219" between the output jacks where the caps need to go... maybe I can sneak two 220uF caps in parallel in the space available. I found 220uF 10v Nichicon caps.
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Old 16th June 2015, 05:12 PM   #10
Zaaphod is offline Zaaphod  United States
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I happened to have some 100ohm resistors, so I decided to try soldering them in parallel to the existing 470ohm resistos, giving me 82ohms. That made a huge difference, and I am not getting any noticeable distortion even with it all the way up. I tried putting a 330uF cap in parallel with the 100uF cap, just hanging down under the board, it would never fit in the box.. but just wanted to see. I didn't really notice any difference at all.. so as difficult as it is to replace them, I think I'll leave them alone for now.

Thanks for the advice here! It's actually amplifying quite a bit now and should be loud enough to hear on a plane. I've played it for about 4 hours and everything seems to be working great.
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