Help understanding Si4735, amp, preamp, voltages, gain, ...

Hi,

I'd buy a Si4735 shield from sparkfun. I did some custom mods and added an XBee on it using a voltage divider(10k/10k) on xbee DIN. Also, GPIO wasn't work because a mistake from sparkfun guys. As said on http://arduino.cc/forum/index.php/topic,66554.0.html, I added a diode and it's working now. I can get volume, frequency, RDS, etc...

Now I want to learn how a preamp and a amplifier work to get a good sound from it.

I have an old NE5532 preamp board that I bought from ebay a long time ago(like this http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=110711886084#ht_2681wt_1114). I found that it has a gain of 3 times.

I plugged this preamp using 12-0-12 15VA transformer on shield and my iPod headphone. I can hear but the volume is low. I saw that R3 and R5 on board = 33k, they say that I can change this resistors to have a greater gain. I have a bunch of 1% resistor with different values. May I swap with higher or lower resistors to increase the gain? Will I be able to use only the preamp to drive the headphone?

Also, I'll make a gainclone(LM3886 chip amp) amplifier. It'll work fine?

Some Si4735 info:

| Parameter | Min | Typ | Max | Unit | | - | - | - | - | - | | Audio Output Voltage | 72 | 80 | 90 | mV RMS | | Audio Output L/R Imbalance | - | - | 1 | dB | | Audio Frequency Response Low | - | - | 30 | Hz | | Audio Frequency Response High | 15 | - | - | kHz | | Audio Stereo Separation | 32 | 42 | - | dB | | Audio Mono S/N | 55 | 63 | - | dB | | Audio Stereo S/N | - | 58 | - | dB | | Audio THD | - | 0.1 | 0.5 | % | | Audio Output Load Resistance | 10 | - | - | kOhm | | Audio Output Load Capacitance | - | - | 50 | pF |

Thanks,

Wagner Sartori Junior

Have a link to schematic of your pre-amp? It looks like NE5532 is too weak to drive headphones directly:

Output Short Circuit Current ISC ? 10 38 60 10 38 60 mA

The same with Si4735:

Audio Output Load Resistance 10 - - kOhm

You know resistance of headphones, correct?

You need audio amplifier.

Schematic attached.

Well, it’s chinese so, the values are wrong! I think my R5 is R31 on schematic and R8 is R8 and the value of this resistors on my board is 33k and it’s supposed to give a gain of 3 times. Do you have an idea of how the resistors is supposed to be to give me a gain of 10 times?

  1. If you connected Si4735 to this pre-amp input, than remove R14 and R18. 1 kohm is too low for 10 kohm output impedanse of Si4735. May be it would solve all of your problem.
  2. Gain is

edited:

R1 / R8 - left channel
R20 / R31 - right channel
decrease R8 and R31 to 10 kohms in order to get gain ~10. It’s true for input stage only. There is second stage on U2A ( U2B ), with gain less than 1.
3. What is resistance of the VR4A/B? I don’t think you can drive headphones by this amplifier even you turn VR4 at maximum, output power is limited by NE5532 to:
I^2 x R = 10 mA x 10 mA x 16 ohm = 0.0016 W
for worst case scenario, and
I^2 x R = 60 mA x 60 mA x 16 ohm = 0.0576 W
for best, (assuming headphones have 16 ohm impedance).

Thanks, for the clarifications, answers are inline.

Magician:

  1. If you connected Si4735 to this pre-amp input, than remove R14 and R18. 1 kohm is too low for 10 kohm output impedanse of Si4735. May be it would solve all of your problem.
  2. Gain is (R1 + R8)/R8 - left channel
    (R20 + R31)/R31 - right channel
    decrease R8 and R31 to 11 kohms in order to get gain ~10. It’s true for input stage only. There is second stage on U2A ( U2B ), with gain ~2.

I’m at work, I’ll give it a try at night.

Magician:
3. What is resistance of the VR4A/B? I don’t think you can drive headphones by this amplifier even you turn VR4 at maximum, output power is limited by NE5532 to:
I^2 x R = 10 mA x 10 mA x 16 ohm = 0.0016 W
for worst case scenario, and
I^2 x R = 60 mA x 60 mA x 16 ohm = 0.0576 W
for best, (assuming headphones have 16 ohm impedance).

All pot’s is 50k(bass, mid, treble, balance and volume) but I don’t know if it’s linear or logarithm. I don’t know the impedance of my headphones. It’s an original iPod headphone.

Sorry, there was a mistake in my calculus for gain, I edited a post. This is why it's 3: 100 k / 33 k = 3 Just to verify, you can look at "Inverting amplifier" on page: http://en.wikipedia.org/wiki/Operational_amplifier (too much tea in the morning :grin: )

Magician: 1. If you connected Si4735 to this pre-amp input, than remove R14 and R18. 1 kohm is too low for 10 kohm output impedanse of Si4735. May be it would solve all of your problem.

removing r14 and r18 introduce noise(much noise) on headphones! I'm using headphones for testing. changing the 33k with 10k resistors makes the sound loader. I'll add an amplifier on output of this preamp and see how it sounds.

thanks for the help...

Wagner Sartori Junior