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α24 with M³ preamp built log&Q's

alpha24 (preamp/buffer, voltage DAC analog output stage, balanced ↔ unbalanced converter)

α24 with M³ preamp built log&Q's

Postby Pieterh » November 28th, 2019, 8:39 am

Hi Ti, others, I started building a preamp that matches with my Beta 24 power amp (see: beta24-pieterhs-build-t3748.html ;) ). I intend to use this thread to share the progress of my preamp built and for questions.

Outline of my pre-amp: to reduce built time I bought a ready made 'third party' Maya R2R volume controller/ 4 input selector, with display and with constant 10K output impedance. After the volume controller, I use ALPHA 24 (not soldered yet), powered by SIGMA 25 / 26 with SIGMA 78 / 79 (all ready soldered). I bought a relatively large chassis, the Modushop slimline U3 (120mm height/4.7").

As I have plenty of space left inside the chassis, I started thinking about integrating the M3 headphone amplifier in my preamp with variable gain. I thought about having a gain of 2 for bal/unbal for the A24. This suits the Beta 24 and M3, right? Then, the M3 having a variable gain control range between 2 and 17. Does this work? Any second thoughts about these gains in this situation?

I thought about connecting the unbalanced output of A24 through a toggle switch (4P3T; ON-OFF-ON) on the back panel to: or the M3 or the unbalanced RCA output connectors. Is this a good idea ? I thought about buying this switch: https://www.tme.eu/nl/en/details/ic4201 ... -switches/

Do you maybe have other ideas to switch the A24 unbalanced outputs to the M3 or RCA output connectors with a button or turning knob on the front panel (i don't want the toggle switch on my front panel :shock: )? I have been looking into a rotary switch that I could operate from the front panel with a small knob but I am not sure that a 4P3T rotary switch (https://docs.rs-online.com/6690/0900766b8152d5ba.pdf ) is what I need? Do you know about that?
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Re: α24 with M³ preamp built log&Q's

Postby amb » November 28th, 2019, 12:06 pm

Pieterh wrote:As I have plenty of space left inside the chassis, I started thinking about integrating the M3 headphone amplifier in my preamp with variable gain. I thought about having a gain of 2 for bal/unbal for the A24. This suits the Beta 24 and M3, right? Then, the M3 having a variable gain control range between 2 and 17. Does this work? Any second thoughts about these gains in this situation?

That's quite a wide gain control range, but why not, it would suit many different headphones, even the most inefficient.

I thought about connecting the unbalanced output of A24 through a toggle switch (4P3T; ON-OFF-ON) on the back panel to: or the M3 or the unbalanced RCA output connectors. Is this a good idea ? I thought about buying this switch: https://www.tme.eu/nl/en/details/ic4201 ... -switches/

Why do you need a 4 pole switch? A DPDT should be sufficient for this purpose... Do you even need a switch at all? Maybe connect to both simultaneously, and if you want only one or the other to be playing, you can either unplug headphones, or turn off whatever the externally-connected device is (another power amp?). It's one less switch to worry about.
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Re: α24 with M³ preamp built log&Q's

Postby Pieterh » February 15th, 2020, 11:18 pm

Building your audio equipment is much fun. Sometimes you have to get a bit creative and find yourself proper tools to get opamps or other smd parts soldered precisely. Here I found a useful tool from my 'wifes hair department':

20200120_160603_4_kl.jpg

Here is how I used it:
20200119_105100_1_kl.jpg

20200120_143223_3_kl.jpg
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Re: α24 with M³ preamp built log&Q's

Postby amb » February 15th, 2020, 11:22 pm

That’s very creative! Great for smaller PCBs.

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Re: α24 with M³ preamp built log&Q's

Postby Pieterh » February 15th, 2020, 11:27 pm

20200120_160710_5_kl.jpg

20200215_094652_7_kl.jpg

20200215_094700_8_kl.jpg
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Re: α24 with M³ preamp built log&Q's

Postby Pieterh » February 15th, 2020, 11:32 pm

Here is one finished alpha 24:
20200215_094803_9_kl.jpg


Another set of Alpha 24 is 'alive'. Both measure well, highest dc offset around 5 mV, lowest around 1 mV:
20200215_093707_10_kl.jpg

The Alpha 24 have a gain of 2 for balanced and one for unbalanced. No low pass filter installed.
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Last edited by Pieterh on February 16th, 2020, 12:34 am, edited 1 time in total.

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Re: α24 with M³ preamp built log&Q's

Postby amb » February 15th, 2020, 11:36 pm

Nice work! :)
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Re: α24 with M³ preamp built log&Q's

Postby Pieterh » February 21st, 2020, 7:34 am

My friend Schaeffer delivered my front and back panels, so the construction works have lift off. The front panel (10mm) and back panel (3mm) are from Modushop. I am really pleased by the solutions I have found to accommodate a head phone jacket 1/4 inch (6.3 mm) and a tactile switch. I didn't want a jacket with a big nut on the front side. And both the jacket and a switch cap also had to look similar, as I wanted my front panel to be as symmetrical as possible.

So the question (for this first time preamp builder) was how to attach such a jacket on the back side of the front panel? Although my solution is quite straight forward, I did not found clear photo examples on such a construction on the internet. So lets have a clear look. I used the Cliff CL 12345 jacket with gold-plated contacts. The standoffs are 20 mm.

Jack 1.jpg

Jack 5_hor 2.jpg

Jack 3.jpg
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Last edited by Pieterh on February 21st, 2020, 10:01 am, edited 2 times in total.

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Re: α24 with M³ preamp built log&Q's

Postby Pieterh » February 21st, 2020, 7:36 am

Jack 4.jpg

Jack 2_hor 2.jpg

The jack is fixed.
Jack 6.jpg
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Last edited by Pieterh on February 21st, 2020, 10:00 am, edited 2 times in total.

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Re: α24 with M³ preamp built log&Q's

Postby Pieterh » February 21st, 2020, 7:39 am

The final result
Jack 7.jpg

Jack 8.jpg
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