Audio buffer amplifier

[cumesoftware]

I'm planning to make a discrete audio buffer to interface the LM1036 audio control and a TDA2030 amplifier. The LM1036 cannot drive 16Ohm or 32Ohm headphones by itself, so an audio buffer is required at the output. If no headphones are connected, the buffer will drive a TDA2030 power amp.

I was planning to do this with discrete components, since DIP packaged audio buffers are very rare. I was planning to use a simple common drain amplifier with only one FET per channel, but I don't know what resistor value should I use.

Any ideas?

[nigelwright7557]

Originally posted by cumesoftware
I'm planning to make a discrete audio buffer to interface the LM1036 audio control and a TDA2030 amplifier. The LM1036 cannot drive 16Ohm or 32Ohm headphones by itself, so an audio buffer is required at the output. If no headphones are connected, the buffer will drive a TDA2030 power amp.

I was planning to do this with discrete components, since DIP packaged audio buffers are very rare. I was planning to use a simple common drain amplifier with only one FET per channel, but I don't know what resistor value should I use.

Any ideas?

How about 32ohms to match the headphones ?

There are numerous ways to drive headphones, I have even done it cheap with op-amps with a resistor in series with the output. But this was a low tech solution.

You could use a class b driver if you arent bothered about cross over distortion, this would use less current than a class A solution.

What about the LM380 audio driver ?

[unclejed613]

another approach you might consider is paralleled op amps. you split the input signal 4 ways, have each opamp in a quad opamp connected for unity gain. and sum the 4 outputs through 120 ohm resistors. for stereo you need 2 quad opamps

[lineup]

Originally posted by unclejed613
another approach you might consider is paralleled op amps. you split the input signal 4 ways, have each opamp in a quad opamp connected for unity gain. and sum the 4 outputs through 120 ohm resistors. for stereo you need 2 quad opamps

Yeah.
I have seen for example OPA2134, unity gain stable
being used this way.
Parallell both halves in a dual OP-amp, To drive HeadPhones.
The output current in OPA2134, for example,
is most times more than enough to drive 16-32 Ohms H-Phones.

Something like 30-40 mA can be delivered, from EACH (making Upper limit like 60-80 mA for one pair)
and you should not need use higher Voltage supply, than maybe +-12 VDC.

There are several other OP-Amps with good output current.
When operating at gain = 1, they will drive any modern 16-32 Ohm Headphones.

Because, usually, these headphones need no more than ~ 0.4 - 0.8 Vrms
to make you Half-Deaf .. on both ears

[nigelwright7557]

Originally posted by lineup



Because, usually, these headphones need no more than ~ 0.4 - 0.8 Vrms
to make you Half-Deaf .. on both ears

I used a couple of op amps to drive the headphones on my disco and it was easily loud enough even with the disco running at 200WRMS. In fact I ran one record deck on one channel and the other on the other channel. This meant I could hear what was playing and what was being cued at the same time.

[cumesoftware]

I've considered using the OPA2134, but then I saw that the output voltage might vary with the load, because of that output resistor. Wouldn't that be a problem?

I also considered using the LM4880, but the THD is too high for an intermediate stage. Only the power stages driving the amps should have a THD of that order of magnitude. So I had to bury this last idea.

Back to the OPA2134. Will it drive 16Ohm? Some headphones have that impedance. I don't want to resort to paralleling op-amps, to save space. Is there other solution? I see that they can supply 35mA, which seems to be more than enough. The problem is that some designs have that nasty resistor at the output to avoid RF interference. Can I take it out in case of unity gain?

http://www.headwize.com/projects/cmoy2_prj.htm
http://www.elecfree.com/electronic/p...ier-by-opa134/

[lineup]

Originally posted by cumesoftware
I've considered using the OPA2134, but then I saw that the output voltage might vary with the load, because of that required output resistor. Wouldn't that be a problem?

I also considered using the LM4880, but the THD is too high for an intermediate stage. Only the power stages driving the amps should have a THD of that order of magnitude. So I had to bury this last idea.

Back to the OPA2134. Will it drive 16Ohm? Some headphones have that impedance.

1. Not a problem.
At least in the several projects done this.

2. Don't know.
But any circuit or chip, that had high THD at Gain=1 (buffer only, no voltage gain)
shouldnt be used in Audio.

3. Yes.
Pure math says so.
I said this: 0.4 Vrms in to 16 ohm will make you half deef, on both ears.

400 mVrms = 566 mVpeak
566/16 ohm = 35 mA peak = 25 mA average (RMS)

Output of one ( 1/2 ) OPA2134 is something like 30-40 mA.

[cumesoftware]

Sorry, I was editing:

The problem is that some designs have that nasty resistor at the output to avoid RF interference. Can I take it out in case of unity gain?

Here are the designs I saw. The R5 position varies. I would prefer the later, so R5 doesn't atenuate with the headphones:
http://www.headwize.com/projects/cmoy2_prj.htm
http://www.elecfree.com/electronic/...fier-by-opa134/

[nigelwright7557]

Originally posted by cumesoftware
Sorry, I was editing:

The problem is that some designs have that nasty resistor at the output to avoid RF interference. Can I take it out in case of unity gain?

Here are the designs I saw. The R5 position varies. I would prefer the later, so R5 doesn't atenuate with the headphones:
http://www.headwize.com/projects/cmoy2_prj.htm
http://www.elecfree.com/electronic/...fier-by-opa134/

I would leave the resistor in otherwise you could end up with oscilation driving an inductive load.

Power amps use a Zobel network to counter the speaker inductance.

[lineup]

Originally posted by cumesoftware

I'm planning to make a discrete audio buffer
----
I was planning to do this with discrete components, since DIP packaged audio buffers are very rare.

I was planning to use a simple common drain amplifier with only one FET per channel, but I don't know what resistor value should I use.

Any ideas?

if you want to do it discrete, with JFET and/or lownoise transistors,
which is certainly more FUN than doing Op-Amps 'dead chips',

then I suggest:

1. http://www.diyaudio.com/forums/search.php
Use search words like: buffer, discrete, jfet
I have read and posted in maybe 50 topics at this forum.
Where people have built Hi-Fi Buffers.
Both for HeadPhones and Line-amplifier
and using both JFET, lownoise bipolar and MOSFET for their designs.


2. Wait until Nelson Pass publish his series of buffers / preamps.
He has talked a lot of this. He is doing these articles right now.
We are many that await
for The One of Several diyAudio Masters new article works.

Coming Soon:
http://www.firstwatt.com/watts_new.htm

The B1 Buffer Preamp will be available in a matter of weeks. It is configured as a “passive” selector and volume control, but with a no-feedback JFET buffer that offers ultra low distortion and noise with ultra wide bandwidth. If passive preamps have not floated your boat in the past, you need to check this one out; it resolves several interface issues that have limited the popularity of passives.

The B2 Biamp Buffer. You say you tried bi-amping but didn’t get sound you wanted? The B2 uses two pairs of the buffers found in the B1 but configured for splitting the output of the preamp with variable relative gain to two sets of power amplifiers. This allows not only isolated amplifier impedance buffering, but adjusts the gain levels for different amplifiers. Even for identical amplifier channels it also allows the subtle adjustment required to get exactly the top/bottom balance you need.

B3 Preamp. Maybe you’d like a B1 but you need some gain.

The B4 Crossover / Equalizer has been designed around the needs of full range speakers (Lowther, Fostex, and all the newer entries). It has circuitry for equalizing to the needs of full range drivers and for adding a woofer.