Low Distortion Buffer - Page 1
Forum section: electronics / projects

author: nickm
author_rank: Regular Contributor
timestamp: February 08, 2014, 10:08:36 pm
content: I have a distortion analyzer and I want to drive low impedance loads at low voltage with its generator. The generator output has an output impedance of 50 ohms so I want to add a unity gain buffer stage. The buffer needs to have distortion low enough so it doesn't add to the THD+N floor. What I've tried so far is just a textbook amplifier which works well at no load, but as soon as it's loaded the distortion goes up. It seems the dominant source is crossover distortion. I don't have as much experience in linear amps so I'm hoping one of you has suggestions for improvement.The opamp is an LME49720.I've tried 3904/6 and IRFZ24/34 for output devices. Both performed well under no load, but with 16 ohms the distortion was bad (3% and 0.25% at 100mV respsectively).I've got them powered from two nine volt batteries. 2x1000uF caps + 2x0.1uF caps for bypass.The gain should be around unity.

author: dannyf
author_rank: Super Contributor
timestamp: February 08, 2014, 10:19:37 pm
content: If your application allows, a simpler emitter follower should do. Alternatively, get rid of the resistors / diodes and put a small resistor (330ohm for example) in between the transistors' b/e junctions.Haven't read the mosfet's datasheet but it is unlikely a normal opamp can drive those things at audio frequencies.

author: Marco
author_rank: Super Contributor
timestamp: February 08, 2014, 10:30:20 pm
content: More bias current? Try two diode drops and an emitter resistor to set the bias like on page 3 here :http://www.ti.com/lit/an/snoa600b/snoa600b.pdf

author: dannyf
author_rank: Super Contributor
timestamp: February 09, 2014, 01:04:51 am
content: You may also consider a composite amplifier, like an opamp + a diamond buffer (either discrete or an IC). Many CFB amplifiers use diamond buffers as their output stages so you can use a VFB opamp + a CFB opamp for that.I have used AD815 as power amps on 8ohm speakers, for example.

author: Co6aka
author_rank: Unknown
timestamp: February 09, 2014, 02:04:09 am
content: Maybe BUF634T or OPA547T would do what you want.

author: Marco
author_rank: Super Contributor
timestamp: February 09, 2014, 03:06:45 am
content: Or LME49600 to stay in the theme of the TI audio components.

author: oldway
author_rank: Super Contributor
timestamp: February 09, 2014, 05:45:10 pm
content: How low are your low impedance loads?For higher current in the load, you don't need a buffer: you can rise the output voltage of your generator to a higher value than 100mV.You have to measure the voltage across the load and ajust the voltage to read 100mV.If you need a buffer, a NE5534 can drive a load lower than 50R at 100mV (without need of adding transistors) with a distortion of 0.004% in full audio range but output current is limited to 38mA peak.

author: wiss
author_rank: Frequent Contributor
timestamp: February 09, 2014, 06:27:49 pm
content: How low THD? How much power?A 5532 with a class A buffer will probably work ok.

author: nickm
author_rank: Regular Contributor
timestamp: February 09, 2014, 09:49:31 pm
content: Thanks for the replys. I've managed to cut the distortion in half by increasing the bias current. I'll have to try the other suggestions. I want to drive headphones so at least 16ohms but the amount of power is less than 100mW. The floor noise of the of the analyzer is 0.02% at 100mV so it would be nice if it was less than that.

author: wiss
author_rank: Frequent Contributor
timestamp: February 09, 2014, 09:58:43 pm
content: Quote from: nickm on February 09, 2014, 09:49:31 pmA NE5532 with a class A stage inside the feedback-loop, you might need to add a compensation-cap over the op.QuoteYou might even get away with using a tl071...btw, build two channels with the 5532 and you have a ~10 ppm THD headphone-amp!

author: tom66
author_rank: Super Contributor
timestamp: February 09, 2014, 10:03:29 pm
content: Have used LME49600 before, it's really simple. You should add an op-amp in the loop to keep the output stable with temperature and supply voltage. Distortion figure lower than the 5th zero past the decimal point.

author: blackdog
author_rank: Frequent Contributor
timestamp: February 10, 2014, 10:06:18 am
content: Hi,Use one 5532a per channel in parallel, it wil blow your eardrums uit of your nose with low distortion en very cheapUse a good capacitor direct over the powersupply pins.Kind regarts,Blackdog

author: dannyf
author_rank: Super Contributor
timestamp: February 10, 2014, 12:22:51 pm
content: -100mW. The floor noise of the of the analyzer is 0.02% at 100mV-given how much speakers and headphones distort, I would encourage you to focus somewhere else for improvements.I would focus on high open loop linearity / low global feedback and speed.ad815 and other cfb amps - tpa6102 for examples - are amazing headphone amps.a discrete jlh1969 with to92 or to126 devices would be great too - though its the figures aren't that great.

author: nickm
author_rank: Regular Contributor
timestamp: February 15, 2014, 05:43:39 am
content: Well I tried the LME49600 and it works great. Its down at the floor noise all the way until clipping. I tried 3ohm load and it was perfect into that (Although the distortion hit the cliff at a much earlier point than the higher impedance loads. After messing with this I think the CMOY folks are a little crazy trying to drive headphones with opamps. They must be at least 1% distortion through all amplitudes.

author: Marco
author_rank: Super Contributor
timestamp: February 15, 2014, 06:27:17 am
content: To be fair, your BJT output stage added a ton of crossover distortion.

author: wiss
author_rank: Frequent Contributor
timestamp: February 15, 2014, 09:39:13 am
content: The bjt-stage has to be within a feedback loop!

author: dannyf
author_rank: Super Contributor
timestamp: February 15, 2014, 10:39:45 am
content: QuoteYou are comparing a buffer with a poorly designed opamp + buffer. Not fair at all.

author: Marco
author_rank: Super Contributor
timestamp: February 15, 2014, 01:34:11 pm
content: Quote from: wiss on February 15, 2014, 09:39:13 amIt was, he just used labels instead of a wire for the feedback loop ... which does not improve intelligibility of the schematic in this case.