Headphone Amp front end design.  - Page 1
Forum section: electronics / projects

author: Jbliss
author_rank: Regular Contributor
timestamp: October 28, 2018, 03:15:38 am
content: Hi All,I am designing a headphone amp with two line level balanced stereo inputs.I need to design a small front end active summing mixer that sums the two stereo signals to a Left and Right bus which then feed the headphone amp section of the circuit.I would like the ability to adjust the volume of the two signals, so I can create a mix of the two stereo inputs.My question is what would be the best way to attenuate ? I have realised it is quite difficult to attenuate a balanced stereo signal precisely with a potentiometer. Here are some of the options I have thought ofOption 1;Attempt to attenuate Stereo Balanced signals with 4 Gang Potentiometer.Option 2;Use a line Receiver (INA2134) to convert to a single needed signal then attenuateOption 3;Look at the use of digital pots (Would prefer not to due to more PS rails and micro)Let me know that you think, all of these options could be terrible ideasreally appreciate any help.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: October 28, 2018, 03:57:09 am
content: I'd go with attenuation after the input stage. If you have enough drive, you can use a fairly low value attenuator, thereby reducing thermal noise. I'd go with a stepped attenuator, as good pots are expensive and hard to find.

author: Jbliss
author_rank: Regular Contributor
timestamp: October 28, 2018, 04:06:44 am
content: Hi Conrad,Thanks for your quick reply, Silly question but what part of the input stage are you referring to ?

author: Jbliss
author_rank: Regular Contributor
timestamp: October 28, 2018, 04:22:07 am
content: Option 4;Using line receivers at input convert to single ended signal then use a VCA (THAT 2181) to adjust volume.

author: Richard Crowley
author_rank: Super Contributor
timestamp: October 28, 2018, 05:37:13 am
content: Option 1 - 4 Gang PotentiometerThis seems like the most unlikely. Acquiring/making a 4-gang pot (or DIY stepped attenuator) with decent tracking will be fiddly, expensive and time-consuming. unnlikely to have a positve cost/benefit ratio unless there is some other requirement (like precision cablibration,etc.) that you did not mention.Option 2 - Line receiver and conventional potsThis seems like the simplest, most straightforward, and highest cost/benefit ratio. It would certainly be my first choice.Option 3 - Digital potsMaybe. Seems more fiddly to design/construct and operate than Opt.2 Not clear what benefit it would have over Opt.2 This (and ALL) the options need some kind of balanced to unbalanced scheme.Option 4 - Line receivers and VCASame disadvantages as Opt.3I believe @Conrad Hoffman was recommending Opt.2

author: Jbliss
author_rank: Regular Contributor
timestamp: October 28, 2018, 05:58:09 am
content: Hi @Richard Crowley,Thank you for that great breakdown !Agreed option 2 does look to be the winner.However I would still need to use double ganged pots for the stereo volume control for each of the inputs.I am worried about the accuracy especially at high attenuation levels. I suppose The only way to be sure is to do a test which I will.Are there any other ways to overcome this?

author: technix
author_rank: Super Contributor
timestamp: October 28, 2018, 06:29:33 am
content: Quote from: Jbliss on October 28, 2018, 03:15:38 am4-gang pot = $$$ and fiddly to match. Unless you have a good reason to I would generally avoid it.Quote from: Jbliss on October 28, 2018, 03:15:38 amThis is much better as a solution really, and it allows you to use cheaper 2-gang pots.Quote from: Jbliss on October 28, 2018, 03:15:38 amIf you are going digital, why not just use differential-input ADC and a small DSP with built-in DAC? This way you get to do much more than a mixer while not costing too much than a pair of good digipots.The small DSP I am talking about is ADAU1701, which 1) can boot itself from an external I2C EEPROM so no micro needed, 2) has internal 1.8V Vcore regulator except the power pass element so you just supply a MMBT8550 PNP transistor and the rest run on 3.3V and, 3) has built-in single-ended DAC for headphone driver amplifiers. The ADC would be something like CS5364 that has four native differential inputs for your two balanced stereo inputs.

author: Richard Crowley
author_rank: Super Contributor
timestamp: October 28, 2018, 08:55:25 am
content: Quote from: Jbliss on October 28, 2018, 05:58:09 amMillions of these stereo, audio-taper pots are used every year in consumer devices cheap and expensive.Modern components are matched good enough that there are rarely any such complaints.What makes your circuit so special.If you are really worried about it, buy 100 of them and select the best matched.

author: voltsandjolts
author_rank: Unknown
timestamp: October 28, 2018, 09:33:10 am
content: They are definetly be better matched than my ears are!

author: Jbliss
author_rank: Regular Contributor
timestamp: October 28, 2018, 09:49:42 am
content: Quote from: Richard Crowley on October 28, 2018, 08:55:25 amYou are 100% Correct, I was under impression the majority of manufactures where doing a different way, the more I read the majority of devices use basic pots. I am probably over complicating the issue.Many Thanks !

author: Jbliss
author_rank: Regular Contributor
timestamp: October 31, 2018, 12:39:16 pm
content: Hi AllThanks for all the help so far, here is my first go at the Schematic of my headphone Amplifier Design.Would really appreciate some feedback on where I have possibly gone wrong. (Still learning)ThanksJB

author: SiliconWizard
author_rank: Super Contributor
timestamp: October 31, 2018, 05:25:25 pm
content: First off - your schematic is rather neatly laid out. Maybe I would have flipped (mirrored) the XLR symbols so as to avoid crossing the + and - lines, but that's a detail.I was first a bit wary of the 39.2 ohm series resistors at the heaphone outputs, but apparently this is what they recommend on the example applications in the TPA6120A2's datasheet. Still think that may be a bit on the high side, dissipating unnecessary power and giving too much attenuation for lowish impedance headphones (such as 16 ohm which they claim this chip supports). This will certainly work, but you may want to test this and maybe lower their value a little depending on the headphones you're going to use.Another point is about the separate grounds. I don't know how you're going to do the PCB layout, but this split ground thing is a recurrent topic of discussion here. I personally would just use two big ground planes (on each side, going for a 2-layer board) and avoid splitting the grounds and joining them in one point, which is usually a bad idea in spite of still being regularly suggested. Obviously others may have a different take on this.Lastly, I would have used the opamps (op1611) as non-inverting summing amplifiers instead of inverting. Not that it will matter a whole lot, but you would avoid an unnecessary phase reversal and possibly a marginally higher noise (although it would probably again not matter here). It would load the volume pots less as well. Just a thought.Other than that, I haven't seen anything problematic so far.

author: Jbliss
author_rank: Regular Contributor
timestamp: October 31, 2018, 11:58:40 pm
content: Quote from: SiliconWizard on October 31, 2018, 05:25:25 pmThank you! Great idea regarding the OP1611 as non inverting!On the topic of grounding I was basing the grounding system off thishttps://www.rane.com/note151.htmlI suppose my idea was to run a trace from C3, C23, C33, C43 back to there respective Pin 1's. The pin 1's would be tied to chassis Ground at a star point where both grounds would meet. This could be a terrible idea? any opinions on the best way to do this ?Thanks AgainJB

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 01, 2018, 03:02:29 am
content: Quote from: SiliconWizard on October 31, 2018, 05:25:25 pmThe power into a pair of headphones at usual listening volume is well below -20dBm. In fact, it can often be helpful to increase the resistance to get the usable SNR up, especially when using mass market parts.

author: Richard Crowley
author_rank: Super Contributor
timestamp: November 01, 2018, 05:25:56 am
content: Quote from: SiliconWizard on October 31, 2018, 05:25:25 pmOr, he could simply un-cross the XLR + and - lines

author: SiliconWizard
author_rank: Super Contributor
timestamp: November 01, 2018, 05:02:55 pm
content: Quote from: NiHaoMike on November 01, 2018, 03:02:29 amDidn't I say that it would essentially depend on the headphones impedance you use?With 16 ohm impedance headphones, which are common in the higher-end range of headphones, it's almost guaranteed that the max sound level with those series resistors' value will be way too low.They are just giving values that are OK for an average use and that would limit the output power to safe levels for average headphones.Given that the op's project rather looks like an "high-end" headphones amplifier (XLR inputs, metal chassis, ...), he's bound to use high-end headphones with it IMO. So I'm just pointing this out.High-end headphones amplifier can deliver up to 1W RMS of power to adequately drive some high-end headphones.

author: Jbliss
author_rank: Regular Contributor
timestamp: November 04, 2018, 02:30:49 am
content: Quote from: Jbliss on October 31, 2018, 11:58:40 pmAnyone have any thoughts on my plan for the grounding system ? Starting to layout board would be good to get some opinions ?

author: technix
author_rank: Super Contributor
timestamp: November 05, 2018, 08:10:54 am
content: Quote from: Jbliss on November 04, 2018, 02:30:49 amIf your board have 4 layers or more, having a continuous ground plane in one of the middle layers can be a better idea than star grounding, and it puts less stress on the layout.

author: Jbliss
author_rank: Regular Contributor
timestamp: November 05, 2018, 12:02:13 pm
content: There is a ground plane, this is just for the seperate chassis ground.