Input impedance

Kyrk

Member
2019-07-03 5:28 pm
Athens
I have a rotel RA-840BX4 amplifier with an imput impedance of 20kohms. I want to connect a digital source to it like a laptop/smartphone. Every time i try it the amplifier, besides playing on a lower volume, after a couple minuets of playback time it suddenly plays extremely distorted sound. I recon that my devices have a very low output impedance, designed to drive headphones. What are my options? I could use my pc that can output 250ohm but that's just not practical. is there some short of relative inexpensive DAC to get the job done?
 

Kyrk

Member
2019-07-03 5:28 pm
Athens
It is good for a source to have very low output impedance. That is the norm, and it helps to transfer the signal with little loss.

So, whatever is wrong with your system, that is not it.

Jan

But my amp/speakers play just fine if i connect them with my tuner, cd player,cassette player or turntable. Its only if i use the 3.5mm to rca cable to connect my smartphoen to the amp that this happens. i have tried connect it to all the inputs of the amp(av/aux/tuner/cd)
 
According to the circuit diagram, all inputs (CD, Tuner, AV/AUX1, AV/AUX2, Tape1, Tape2) are fed to the 50k volume pot, then capacitor coupled to a NE5532 opamp. I rather suspect a broken interconnect cable, or an incorrectly wired one. The 4-pole jack on the smartphones are wired differently than a 3-pole TRS. Try with a 3-pole TRS to RCA L/R cable.
 

Kyrk

Member
2019-07-03 5:28 pm
Athens
According to the circuit diagram, all inputs (CD, Tuner, AV/AUX1, AV/AUX2, Tape1, Tape2) are fed to the 50k volume pot, then capacitor coupled to a NE5532 opamp. I rather suspect a broken interconnect cable, or an incorrectly wired one. The 4-pole jack on the smartphones are wired differently than a 3-pole TRS. Try with a 3-pole TRS to RCA L/R cable.

But this is what I have. A 3 pole trs(3.5mm jack?) to rca l/r. Where did you find the circuit diagram of the amp. I can't find them anywhere. It could be the amp, it used to play really badly and often loosing one of the channels but I fixed that with just some contact cleaner in the Pots
 
I found some of these devices have a simple BJT in class A, or the like, with the phones as load. If you don't close DC for this BJT, large distortions are expected, as a consequence of the lack of DC currents. Perhaps it starts OK while some caps get charged/discharged, when reach the final value, it is insufficient to the stage do the job OK.
 
Could it be that the amp is being upset by the unfiltered Class D switching pulses coming from the smartphone? If so you need a low pass filter.

I think you hit it -- the impedance of the Apple iPhone earbuds is 23 ohms and a filter is unnecessary, but looking into 20K or 50K Zin for the receiver is a different matter. Even some tens of microHenries in series will knock down the switching pulses.

(Those switching pulses can also cause "un-intended" rectification in BJT integrated circuits -- something Analog Devices discusses in one of their ap-notes.)
 

Kyrk

Member
2019-07-03 5:28 pm
Athens
I think you hit it -- the impedance of the Apple iPhone earbuds is 23 ohms and a filter is unnecessary, but looking into 20K or 50K Zin for the receiver is a different matter. Even some tens of microHenries in series will knock down the switching pulses.

(Those switching pulses can also cause "un-intended" rectification in BJT integrated circuits -- something Analog Devices discusses in one of their ap-notes.)

Oh I see, you are referring to the low pass filter that normally exist in class d amp boards that filters the very high frequencies (above 20khz)?? Something like this doesnt exist after the smartphone's amp. Is there a realistic way to asset this? would i need to build a passive low pass filter and place it in between the amp and the smartphone. if so, do you know some short of a guide or some schematics i could follow?
 
Every Class D amp should have a filter, whatever the load, if it wants to be a good citizen and not spray RF muck around. Unfortunately leaving your rubbish around for others to cope with is the new normal so most low power Class D omit the filter. Some get around FCC or EU EMC regs by doing spread-spectrum - the muck is spread more thinly but over a wider spectrum.

I am sure someone somewhere will have designed a filter for this task. Google?
 
Every Class D amp should have a filter, whatever the load, if it wants to be a good citizen and not spray RF muck around. Unfortunately leaving your rubbish around for others to cope with is the new normal so most low power Class D omit the filter. Some get around FCC or EU EMC regs by doing spread-spectrum - the muck is spread more thinly but over a wider spectrum.

I am sure someone somewhere will have designed a filter for this task. Google?


Its normal to omit filters when the amp is physically next to the speaker, there's too little wire to radiate enough power to care about (or rather its on the same scale as the emission from all the digital traces). Of course tight layout is a prerequisite for this.


For sending external to the device there needs to be filtering as long leads can be efficient radiators at the frequencies involved.


10µH in series, 0.47µF shunt might work into 8 ohms.
 

iperv

Member
2008-01-13 3:26 am
Italy
Or,
you could try this:
take a 3.5mm stereo splitter connector and connect your earpads AND your 3.5mm to rca cable to the amp's input.
It should work.

Edot: i mean: the splitter has a 3.5mm stereo input and 2 3.5mm stereo output.
Connect one output to your earpads and the other to the amplifier with your cable (i think it's a 3.5mm stereo to rca cable).
 
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