Custom Desktop Audio Box Project - Page 1
Forum section: electronics / beginners

author: paulca
author_rank: Super Contributor
timestamp: February 06, 2018, 12:48:43 pm
content: Currently I have my desktop audio set up, kinda maker style. it's a collection of parts, hooked together. A PCM2707 USB DAC, a classic 47 headphone amp and a bunch of 3.5mm Y splitters etc.I would like to bring it all together into one box and put a few extra inputs onto it. I was putting it off as USB Dacs tend to be SMD only. I'm all ready about to bite that off and chew it for something else, so I can't use it as a blocker anymore.So here would be what I would be after building and I would love your suggestions and input on choosing components. Note I am not an audiophool. I will be happy with 44.1 or 48khz output and 192 to 256 encoded data.Inputs:* USB DAC input* Aux Input 3.5mm jack/RCA phono (these can on the same input)* Bluetooth inputProcessing:Pre-amp, Gain booster with gain control - I'm tired of not having that little bit of extra gain from some quiet sources.Outputs:* Headphone amp w/ single 3.5mm jack output* RCA/3.5mm x2Starting with the DAC, is there much difference between the various Burr Brown/TI PCM270* DACs? Are the TDA chips any different? I'm assuming the datasheet typical application circuit would suffice? The headphone amp and pre-amp I can just copy the 47 Amplifier schematic but I might need to work out the impedance between them so I don't just lose all that gain. I might also need to check the current limiting on the main headphone amp and pair up two op amps on the output, though I think the 47 already does this.On the multiple outputs, should I bother with unity gain op amps to balance the impedance or leave it as it? The 3.5mm jack/RCA will be output from the pre-amp stage.The multiple inputs. Is it possible to have them duplex in that they are all active at the same time? Maybe with a mute toggle or gain control on each instead of a mutually exclusive selector? I suppose this could be done on the pre-amp, in a stereo pair of opamps on each input with a gain control on each input rather than a single on the output.Finally, am I mad attempting this or should I consider an off the shelf solution that meets most of my needs such as some of the micro stacking components?

author: paulca
author_rank: Super Contributor
timestamp: February 06, 2018, 02:18:17 pm
content: So the above is very broad, but I've started to do some research. I will hopefully be able to produce a high level block diagram this evening.In the meantime I have a more specific question around mixing multiple varied impedance inputs.If I google "Audio mixer circuit" I see the vast majority of circuits have a pot directly on the input signal itself. The output then passes through an op-amp, finally they are combined into a pre-amp op amp.I would have thought a better approach would be to put the input into the op amp and control the level with the opamp gain by putting the pot in the feedback loop.I decided the NE5532P should do for all stages, it's cheap and audio capable and I haven't found anything wrong with how it sounds in the examples I have. I might revise this for the main headphone output depending on it's current limiting feature or pair or triple them up. I ordered a bunch from RS to avoid fakes.The USB Dac seems easy enough and the TI PCM2900C seems fine for my purposes and adds an pair of ADCs for recording if I choose to use it. The typical application circuit seems easy enough.The bluetooth element I need to research, as it involves RF I hope I can find an integrated solution, but will probably end up with a daughter board with Bluetooth 4.0 and audio DAC.

author: LeoTech
author_rank: Regular Contributor
timestamp: February 06, 2018, 02:26:23 pm
content: First of all, you are not mad at all for trying this!The project might take some time and cost, but hey, if it is worth it?In terms of controlling the levels of the individual inputs: I would do it on the preamp stage as well, but first rule of audio, do NOT use gain as volume control! It will cause unwanted distortion and unreliable control.To control the volume of the inputs I would use a passive volume control after the preamp stage by using a potentiometer.Output stage: If you plan to connect the output of the preamps to both the RCA jacks and to the headphone amp, then you should definitely buffer the RCA output.See attached picture for basic topology - as I would do it.

author: paulca
author_rank: Super Contributor
timestamp: February 06, 2018, 02:40:51 pm
content: Quote from: LeoTech on February 06, 2018, 02:26:23 pmThanks. That answers that. So I can either boost and then trim down with a pot or trim down and then boost. I think I agree with you that boost then attenuate sounds like a less noisy solution.The pre-amp probably only needs x2 gain, but my db theory is too old to remember if to get 2x actual do I need more that 2x gain on the pre-amp or not.

author: LeoTech
author_rank: Regular Contributor
timestamp: February 06, 2018, 02:46:08 pm
content: Your welcome,The gain of 2x on the pre-amp seems about right, or you could set a couple of fixed values and a switch, if necessary.And if you want a gain of 2x then the op-amp should have a gain of 2x, but if you mean a gain 2 dB then you would need a gain of approximately 1,26x.

author: paulca
author_rank: Super Contributor
timestamp: February 06, 2018, 02:49:25 pm
content: Different question.Virtual grounds on the outputs. I have found the headphone amp I have (the classic 47 design) leaves it's 6V ground on the output. Not only that but it pulls the input from the DAC up to 6V. While I know this shouldn't be an issue, the USB DAC seems not to care and the headphones don't care, it causes issues with popping and brown outs when connecting to other devices until the ground stablise.Is this just something I learn to live with or should I consider a real dual supply DC/DC converter IC?

author: LeoTech
author_rank: Regular Contributor
timestamp: February 06, 2018, 02:56:33 pm
content: Well, this is the start of audiophoolery.............if done wrong!The internet is full of discussions about virtual ground vs dual supply. Just make a Google search for it.From what I have gathered it seems like that dual supply offers a lot of advantages compared to virtual ground, both in noise, audio quality and general design, but I haven't gotten around to test it for myself - yet.The general practise is, as long as you can use a dual supply, do it! Virtual ground 'should' only be used on portable battery operated devices.

author: tooki
author_rank: Super Contributor
timestamp: February 13, 2018, 03:30:23 pm
content: FYI, sounds like you’re working on something similar to a project I’ve been working on (well, it’s been on ice for a bit, I need to get back into it again!!!)Seehttps://www.eevblog.com/forum/projects/audio-mixer-crosstalk/

author: Audioguru
author_rank: Super Contributor
timestamp: February 13, 2018, 05:58:08 pm
content: A gain of only 2 is barely noticeable. If you have a low signal level then a gain of 10 is what you need.A volume control is usually logarithmic so its settings match the logarithmic response of our hearing to loudness.Then if the volume control is set to halfway it cuts the input signal to 1/10th. Then with a gain of 10 times a normal input level produces a normal output level. Then a low input signal can be boosted up to 10 times and sound twice as loud or a high signal can be turned down to any lower level.