Powering a standalone solid state headphone amp

[bjosephs]

The monitor controller I built recently lacks headphone amp. I'd always planned to make little standalone box that would take in a balanced line level signal and drive phones. Something small I could velcro under my desk with just enough access for a volume knob. The OPA1688 app notes are simple enough but I wondering how to give it a small, quiet bipolar supply. I think an internal transformer and mains voltage is a bit overkill for one active device device so I've settled on a wall wart of some kind. The choices are:

1. An AC wall wart that I can half wave rectify for positive and negative rail, regulate and filter. I'm worried about proximity and hum.
2. A DC/DC converter that takes in ~9-12v and makes +/-12. CUI has a PDQ2 series with a 100kHz switching frequency.
3. I guess I could also try to get a DC wall wart with a rather high output (like 24-36VDC) regulate that down and use virtual earth. Can headphones float at 1/2V or would I need to AC couple the outputs?

I'm curious what other people have done.

 

[k brown]

'Twere I, I'd power it with three rechargeable 9v batteries, split with virtual earth.

 

[ruffrecords]

The first question is what is the impedance and sensitivity of the phones you are going to use? If they are 600 ohms then you definitely need plus/minus 15V supplies but if they are 30 or 60 ohms then plus/minus 6V should be plenty unless they are incredibly insensitive.

Cheers

Ian

 

[abbey road d enfer]

An AC wall wart that I can half wave rectify for positive and negative rail, regulate and filter. I'm worried about proximity and hum.

This shouldn't be an issue with any decent amp. The amp should be distant enough from the wall wart to make this a non-issue.

A DC/DC converter that takes in ~9-12v and makes +/-12. CUI has a PDQ2 series with a 100kHz switching frequency.

It's a possibility, but where will the 12V be supplied from? Back to square one...

I guess I could also try to get a DC wall wart with a rather high output (like 24-36VDC) regulate that down and use virtual earth. Can headphones float at 1/2V or would I need to AC couple the outputs?
I'm curious what other people have done.

Wall warts are floating, so it's your virtual earth that will be used as reference.
Your headphones will float at VE, which in turn should be grounded at the input.
You don't need to worry about DC offset.

Plus everything Ian says.

 

[MicUlli]

I would also vote for a solution with rechargeable batteries..
BR MicUlli

 

[bjosephs]

The first question is what is the impedance and sensitivity of the phones you are going to use? If they are 600 ohms then you definitely need plus/minus 15V supplies but if they are 30 or 60 ohms then plus/minus 6V should be plenty unless they are incredibly insensitive.

Cheers

Ian

Plan? I’d prefer to just make the amp flexible enough to handle any reasonable set of phones. I have 80 ohm and 32 ohm now, have had 600 ohm in then past and may again in the future.

Edit: let me amend that. While I’d prefer flexibility I’m OK with not being able to maximize high impedance phones in the face of some other tradeoff. For example, option 3 probably limits me to <24v of total headroom since that seems to be the largest linear DC wall wart I can find. But option 1 or 2 can easily supply +/-15 or even 18.

 

[bjosephs]

This shouldn't be an issue with any decent amp. The amp should be distant enough from the wall wart to make this a non-issue.

My concern was rectifier currents which would be in the box.

It's a possibility, but where will the 12V be supplied from? Back to square one...

I’m not trying to avoid a wall wart. I’m settled on using one. The question for me is whether to use an AC one to feed a linear supply or or a DC one and try an SMPS.

Wall warts are floating, so it's your virtual earth that will be used as reference.
Your headphones will float at VE, which in turn should be grounded at the input.
You don't need to worry about DC offset.

This is great intel, thank you.

Plus everything Ian says.

Always!

 

[abbey road d enfer]

My concern was rectifier currents which would be in the box.

Managing rectifier currents only requires thoroughly analyzing their circulation and isolating them from sensitive paths. It takes some experience or trial -and-error, but it's perfectly feasible.

I’m not trying to avoid a wall wart. I’m settled on using one. The question for me is whether to use an AC one to feed a linear supply or or a DC one and try an SMPS.

The answer is not really a technical one. It depends on availability, cost, reliability, ease of replacement the day it fails (it's bound to happen).
I would choose a DC - that would definitely eliminate the concerns with rectifier current, but still require analysis of circulation - unipolar type ( I guess they will be still available in the next century) and a grounded half-voltage. For low-Z earpieces, a guitar pedal 9V supply should be enough for 250mW into 32 ohms.
Make sure it's adequately rated. IMO 200mA is a minimum. Anyway most today are smps, with ratings of 0.5-1A. Just beware that some enter into hiccup mode when the load is minimal. The usual RoT is that the minimal drw should be at least 20% of nominal, which is somewhat difficult with a class B amp. You may have to add some things that draw current in all circumstances. Why not a ring of LED's?

 

[bjosephs]

The answer is not really a technical one. It depends on availability, cost, reliability, ease of replacement the day it fails (it's bound to happen).

This is a good point that I hadn’t considered.

I would choose a DC - that would definitely eliminate the concerns with rectifier current, but still require analysis of circulation - unipolar type ( I guess they will be still available in the next century) and a grounded half-voltage. For low-Z earpieces, a guitar pedal 9V supply should be enough for 250mW into 32 ohms.

Now that I know I can float the headphones at the output and only have to AC couple the inputs, a unipolar supply is more attractive. This unit is pretty cheap:

https://www.mouser.com/datasheet/2/410/WDU24-500-1351038.pdf

It’s around 30V unloaded and 24V at 500mA. I’m thinking my max current draw won’t exceed a quarter of that at ear-splitting wattage into 32 ohm headphones so there should be some headroom for an LDO and still maintain 24V supply. Not quite enough to max out 600 ohm phones maybe but very simple and potentially very quiet and a small in box footprint.

 

[abbey road d enfer]

It’s around 30V unloaded and 24V at 500mA. I’m thinking my max current draw won’t exceed a quarter of that at ear-splitting wattage into 32 ohm headphones so there should be some headroom for an LDO and still maintain 24V supply.

No need for an LDO. A pedestrian LM317 should work rather well. With a 24V rail, you can get 100mW into 600 ohms.

 

[bjosephs]

No need for an LDO. A pedestrian LM317 should work rather well. With a 24V rail, you can get 100mW into 600 ohms.

Agree but with some amount of supply drop under load, a 5% tolerance from Triad, and line voltage variation, I feel safer just planning around an LDO. I think LT3080 is a good choose in either the Q package or TO220.

Edit: never mind on the LT3080. It claims a 350mV dropout but needs 1.2V more on control than the output which functionally makes that the dropout in my application.

 

[MidnightArrakis]

The monitor controller I built recently lacks a headphone amp.
I'm curious as to what other people have done.

>> Something like this???.....








https://angryaudio.com/headphonegizmo/

>> Of course.....I know, I know.....ya wanna build yer own!!!

Get crackin'!!!.....

/

 

[bjosephs]

>> Something like this???.....


View attachment 130700



View attachment 130705

I mean, that is pretty much exactly what I wanted to make. It took a while to find someone selling one and at $230 it’s right at the edge of the “eh maybe I’ll just make my own and only lose $100 more than I could have bought it for” territory. If nothing else it’s an excellent implementation precedent. Thanks.