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Old 20th February 2011, 12:13 AM   #1
Bonsai is online now Bonsai  Taiwan
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Default Universal Buffer/HP Amplifer

I've posted a piece on a class A buffer on my website below. Although intended primarily as a line buffer to drive heavy loads, it will drive a pair of 32 Ohms headphones to 3V in class A. Distortion is pretty low at 3-5ppm.

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Old 14th March 2014, 10:52 PM   #2
Dr_EM is offline Dr_EM  United Kingdom
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Really surprised this has no responses! I saw this design on your site a while ago and now that I'm looking for a headphone amplifier for part of my DSP crossover unit I thought I'd look to see what feedback had been left on here.

It's a really elegant design, quite simple, doesn't require too many parts (or expensive/rare ones) and offers great performance. I went ahead and built a prototype!

Click the image to open in full size.

There's a few changes as I built using parts I have around. Op-amp is OPA227, Class-A biasing transistor is an MPSA18, R3 is only 4.7R, C2 is 150pF. Still, it worked first time with no issues apparent on my old 12MHz scope.

I only have 1 channel here so am running the left and right cans in parallel, a tough 16R load in fact! Still, bass is very deep and controlled, frequency response in general is the best I've heard from these, admittedly cheap, headphones.

I would like to get closer to 0R output impedance, and you say it is possible by just restricting the bandwidth? Should C1 be increased as well as C2? Perhaps R3 can come down to around 1R and R7 be replaced with an inductor+resistor parallel combination?

Also, it seems this design should be somewhat short circuit proof? Since max current is 300mA, provided the short isn't for too long and heatsinking is reasonable.

Anyhow, thanks for the design and I look forward to adding it to my next PCB layout alongside balanced line drivers. The plan is to go predominantly SMD.
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Old 15th March 2014, 08:16 PM   #3
Dr_EM is offline Dr_EM  United Kingdom
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Well, it's quite easy to convert to zero Ohm output impedance but whether it's a good idea or not I am unsure. We lose the current clamp and so a short on the output will blow the output device. In terms of stability however it seems just fine using this inductor+resistor combination commonly seen on power amplifiers. The inductor is 25 turns of 22SWG copper, air core, 12mm diameter, DCR is approx 50mOhms.

You can see the response to a 100kHz square wave with the output inductor and with it shorted. Load is 34Ohms resistive plus 10nF parallel capacitance. With the inductor in place I see no issues with stability even at the op-amp output and feedback (-) points.

Perhaps an over-current circuit can be designed which has minimal impact on output impedance? I will be using an SSR at the output anyhow for start-up mute, so anything which could detect excess current and trigger this would be neat.
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File Type: png Bonsai HP zero Ohm.png (42.4 KB, 625 views)
File Type: jpg Bonsai HP inductor.jpg (72.7 KB, 610 views)
File Type: jpg Bonsai HP without inductor.jpg (64.3 KB, 589 views)
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Old 15th March 2014, 10:07 PM   #4
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An RL seems a bit overkill for a circuit like this (I would've tried with just a Zobel first), but if you already have the parts and don't feel like sacrificing some performance for stability...

For short-circuit protection, you could insert some resistance in series with the collector of Q1. 10 ohms or so, depending on how much voltage swing you think you can afford to lose. The emitter follower will do its best not to let the output notice.

BTW, I would probably prefer a push-pull buffer, simply because it'll deliver twice as much output current in Class A for a given quiescent current (and can be pushed into AB if need be). It does have a somewhat higher parts count, of course.
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Old 17th June 2014, 02:30 PM   #5
Bonsai is online now Bonsai  Taiwan
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I just noticed this now. Nice job Dr_EM.

How does if sound?

Here's the link to the original article for those that have not found it on my website

http://hifisonix.com/wordpress/wp-co...uffer-V1.0.pdf
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Last edited by Bonsai; 17th June 2014 at 02:33 PM.
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Old 22nd June 2014, 07:55 PM   #6
Dr_EM is offline Dr_EM  United Kingdom
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I ended up implementing the design exactly as in your application since I decided the short circuit protection was really worthwhile in practice!

My implementation is nearly all SMD in fact and quite compact, but of course needs sizable heatsinking. It sounds flawless, never heard my headphones sound as good as with this but they are only quite cheap, Sennheiser HD205.

Bit of a poor photo but you can see it along the edge of this 8 channel buffer board, permanently connected to channels 7+8. Heatsink is just temporarily attached for lining up, of course all the devices shall be screwed in place later! The Class-A biasing transistor is now an SMT BC847 and op-amp is a humble OPA2134, OPA2227 also worked fine but the price went up before I ordered.

Thanks for the design!
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File Type: jpg Headphone amp section.jpg (396.8 KB, 200 views)
File Type: png OctoBuff PCB.png (184.3 KB, 171 views)
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Old 25th June 2014, 10:31 PM   #7
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Looks good - I'm glad you like the sound! If your heatsink and power supply can stand it, you run it at even more standing current - that will give more headroom.
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Old 26th June 2014, 02:34 AM   #8
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Novice question, can the circuit in figure 1 of the pdf be used as the final build, ( if I don't plan on driving more than 600 ohms ), or would it be better to use figure 13? I was confused over the use of the LM4562 dual op-amp as opposed to the LT1115 single. I guess the LT1115 was for simulation. Does it take two Lm4562's to make a stereo board? Thanks.
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Old 26th June 2014, 09:59 AM   #9
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Of course you can use just one dual opamp for a stereo amp - the second half of the LM4562 as shown in Fig. 13 does absolutely nothing.

Fig. 13 is closer to real life otherwise. You'll just need to read the build notes carefully and decide which of the options you need or would like to have configurable. I assume you won't need output loading options, for example.
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Old 28th June 2014, 02:33 AM   #10
Ripcord is offline Ripcord  United States
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Well, I thought I would try to make a small pcb for this amp. Please point out any of the obvious mistakes. Honestly, there were a couple of things I was unsure of as far as the different configurations. I used figure 13 of the pdf. What I was going after, for my use anyway, was a headphone amp to drive ~40 ohm headphones. If I need to start over so be it, it's good practice. Thanks.
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File Type: jpg Bonsai Universal Class A Amp.jpg (129.7 KB, 158 views)
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File Type: pdf DipTrace PCB - Bonsai Universal Class A Buffer HPA.pdf (23.1 KB, 27 views)
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