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Getting started on a modular headphones amp
Getting started on a modular headphones amp
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Old 23rd January 2020, 09:29 AM   #1
00940 is offline 00940  Belgium
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Default Getting started on a modular headphones amp

I'm making some progress in the design of my next project. The idea is to build something I can fit into a galaxy gx247 (1U, 170*230mm) and which is spread around a few sub sections I can easily debug/replace/upgrade if needs be.

Sources of inspiration mostly are Self's book on small audio designs and some Hypex app notes.

The sub sections I'm planning to start with are:
- AC-DC board,
- input board,
- volume control board,
- headphones amp.

The overall design will follow like this:

Click the image to open in full size.

AC-DC board

There really isn't much to say about it. It's just a 90*70mm adapter board for the recom 10 or 20W ac-dc dual output converters. Added on board are an IEC, fuse, connection for a mains switch and an output CLC filter. Special care has been taken for the mains clearance (8mm minimum to secondary, 4mm to PE, 4mm between primaries).

No further particular regulation is intended but I could later add a regulator board if the need is making itself clear.

Click the image to open in full size.

Input board


Basically it's a classical inamp. I don't particularly need multiple inputs but what I wanted is the possibility to have either balanced or SE input with minimum hassle. So I'll have combo xlr jacks, to minimize the space taken. Using relays to switch between the xlr and 1/4 jacks allows me to also switch the gain at the same time. It's at unity for balanced input, and at 6dB for SE, to account for the usual difference in levels.

I'll use opa1642 for the input buffers. They have a very strong EMI rejection ability, so it allows me to keep the external input filter minimal. For the output, the lm4562 is well suited.

At 60*70mm, it's compact enough that I could have two of these boards in the box if I ever wanted dual inputs.

Click the image to open in full size.

Click the image to open in full size.

Volume control

The volume control board is a bit more complicated than necessary, a gain stage has been added so that it can be used as a stand alone preamp if needed in another project. The board dimensions stay reasonable, at 50*50mm. The pot used is the typical 17mm Bourns or Alpha. The first opamp after the wiper should be an opa1642, the inverting opamp afterwards a lm4562.

Getting started on a modular headphones amp-volschem-png

Getting started on a modular headphones amp-volpcb-png

Headphones amp

To come, simple composite lm4562-opa1656.
Attached Images
File Type: png overall.PNG (34.8 KB, 231 views)
File Type: png smpspcb.png (39.3 KB, 184 views)
File Type: png receiverschem.png (27.1 KB, 192 views)
File Type: png receiverpcb.png (47.6 KB, 194 views)
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Last edited by 00940; 24th January 2020 at 08:49 PM.
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Old 23rd January 2020, 10:04 AM   #2
avtech23 is online now avtech23  Australia
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Getting started on a modular headphones amp
Looks like an interesting project.
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The ever growing project list.. :NAP140 Clone, NAD 3020i & 3225PE, JLH1969, TDA2050, Fender 5F2A Princeton, NE5532 Pre, SHPA, B1 Korg, T2. : VSQC Phunk Style, Retro 50W, Filmosound 601, LM4780. : Apex AX-14, TGM-10, TDA7265 (Last in line)...maybe
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Old 24th January 2020, 08:21 PM   #3
00940 is offline 00940  Belgium
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PCB and schematic of the volume control added.
Attached Images
File Type: png volschem.png (15.2 KB, 157 views)
File Type: png volpcb.png (41.4 KB, 163 views)
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Old 24th January 2020, 11:31 PM   #4
sgrossklass is offline sgrossklass  Germany
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An inverting volume control wasn't exactly a hit with owners of the Musical Fidelity A1 in the long run... it's like a loupe for scratchy pots. You have been warned.

(Also, the measly 220n coupling cap may negatively impact noise performance in the bass.)
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Old 24th January 2020, 11:47 PM   #5
00940 is offline 00940  Belgium
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Wasn't the problem with the A1 that the wiper carried DC as the pot was used in a trimmer fashion ? The classical Baxandall as used here doesn't suffer of this, due to the cap.

I can increase quite easily the cap's value, there's room enough.
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Old 25th January 2020, 01:46 PM   #6
00940 is offline 00940  Belgium
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I just ran a sim in ltspice. The following graph shows the cap at 100n, 220n, 470n, 1u, 22u. The point of diminishing return seems to me 470n.

A 470n/100VAC wima MKP2 can fit with some minor shuffling around, at 8.5*75mm, 5mm spacing.
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