SA2022 - Modular High End Analog Precision Preamp (with headphone amp, digital controlled relay volume, monitor output, IR remote, SE and bal. I/O)

[astx]

A long journey developing a high end analog preamplifier is going to come to an end. The parts mostly materialized on my table 😉 .
Many pcb's/components have been tested and are ready to be put into a nice case in the style of SA20XX amplifiers.
For me it was really time to develop a preamp - I have built so many SA20XX power amplifiers - but a preamplifier was still missing - till now!
Volume can be set from -63.5dB to +12dB in 0.5 dB steps (precision +/-1%) and because it is relay switched, the distortion is ultra low. No analog volume control chip can have such good THD+N values. The (stereo!) zero crossing circuit and firmware switches click and pop free.
The preamplifier firmware can be updated in the field via the project integrated usb serial console.
With firmware from 2024.11.30 nearly every infrared remote control with extended NEC protocol can be used to control the preamp. Included in the project is a pic16f690 based remote control if you want to do even this DIY.

Have fun, Toni

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A) Preamplifier:

• Latest valid schematic of 2x17V power supply post 117
• Valid schematics in post 2
• pcb's are available via: astx (at) astx.at

B) Remote Control for Preamplifier:

• Latest valid schematics in post 2
• pcb's are available via: astx (at) astx.at

INDEX / UPDATES

• 2022.07.05: SA20XX Amplifier Series which need a preamplifier
• 2022.07.04: Case design - front and back view - rendered
• 2022.07.22: Some measurements
• 2022.07.22: Knob background using lightblue transparent acryl, final front design
• 2024.01.11: New module: MM/MC Phono preamplifier. Based on the famous D. Self MM/MC preamplifier series.
• 2024.02.11: Moving magnet phono preamplifier works out of the box. Moving coil hat module unfortunately has hefty oscillations. Need a revision update.
• 2024.10.17: Moving coil hat module rev. 1.2 does the job now without oscillations. Perfect sound. Schematic attached.
• 2024.10.22: New module: SA2024 preamp USB DAC PCM1794A. Schematic attached.
• 2024.11.01: SA2024 Preamplifier USB DAC module PCM1794A in action

USERS BUILD:

• 2022.07.05: My prototype build

NOTE: (c) 2021 - 2024 - astx.at
Only for personal and non commercial DIY usage! May contain copyrighted and/or patented information from third party!

 

[astx]

The baseboard includes 6 slots for stereo input channels. It can be populated with different input card variants. Currently a universal SE/balanced stereo input card with selectable gain from -15/+10dB is available. A MM and MC phono input card is in development - pcb only - the design will be from D. Self.
2 slots are for tone control
2 slots are for relay based volume control -63.5dB to +12dB in exactly 0.5dB steps (+/-1%)
2 slots are for the high end diamond buffer headphone amp.
Included is a monitor output - this can be used e.g. as "tape record" output
To program the PIC18F46K22 controller has been a long running project. First ideas are from 2006 ...
Of course such a preamplifer project needs also a homebrew remote control based on NEC protocol too. This is a perfect job for an old PIC16F690.
For you nerds: some PCBs may be available at the end of the project!

Attached are all schematics. Note the copyright: only DIY/private use!

Have fun, Toni

 

[astx]

From left to right:
First powering, headphone amp 1, 2, remote control, machining knobs, machining top cover, prototype got a 3D printed ABS case - works!

 

[astx]

Soldered 4x44 resistors to get the 5k linear poti for the tone control.
23 positions are selectable.

 

[astx]

SA2022 Universal input module:
left card assemled as balanced input, right card as SE input.

 

[astx]

These PCB's have been designed during the development process...

 

[astx]

... and these small shields ...

 

[TNT]

Lovely design, perfectly executed. Impressive. The lack of attention is I suppose a sign of that the architecture of the audio system is changing and the era of the "pre-amp" is over or at least, sleeping. Skills!!!

//

 

[PMA]

Very nice design and nice job!!

 

[astx]

Lovely design, perfectly executed. Impressive. The lack of attention is I suppose a sign of that the architecture of the audio system is changing and the era of the "pre-amp" is over or at least, sleeping. Skills!!!

//

Thx for the nice words! You are right and not: Vinyl and other audio devices from the 80ies have their time for a comeback. Currently I'm reviving a tangential vinyl player from revox with MC pickup. One of the next input modules will be a MM/MC Phono RIAA card.
My Tapedeck from 1981 is working again after replacing the v-belt.
Another input card will be a XMOS/PCM1794A combo...
So it was time for a analog high end preamp with the possibility to connect old and new devices with modern comfort: IR remote control but also the possibility to integrate it in the house automation (USB port accepts also remote control commands).
Don't forget the fun it makes, to develop this M.H.E.P.P.

Have fun, Toni

 

[diyralf]

Very nice ideas. Does the zero crossing detection actually work well? What quality do the 23-position rotary switch have? Are they soft when turning or does it bump? I've thought of such a solution as well. Relays don't work because you need too many of them.

 

[astx]

The zero cross detection (2 separate channels for le/ri) works very well. the interrupt is only enabled for one shot when a volume change is needed. the logic waits a configurable ms maximum till a hard volume switch will occur. the default is 50ms - a full period of a 20 Hz wave. The small relays are fast and need about 1ms to switch. The logic takes care of the special cases where a volume change goes from e.g. 0b0111111 to 0b1000000 to avoid clicks. During my test I have never heard any click or thumb on volume change.
The tone control rotary switches are also click and pop free.

BR, Toni

 

[maxw]

Really nice work, love the modularity 💙

 

[astx]

The planned case design based on modushop parts:

 

[astx]

Today some parts arrived.

Bottom stainless steel 2.5mm.
The Backside is constructed of 3 metal sheets:

• sandwich outer in 1.5mm Aluminium see post #13
• sandwich middle stainless steel 1.5mm
• sandwich inner stainless steel 1.5mm fixates the input/output shield pcb:s from post #7

 

[thimios]

Today some parts arrived.

Bottom stainless steel 2.5mm.
The Backside is constructed of 3 metal sheets:

• sandwich outer in 1.5mm Aluminium see post #13
• sandwich middle stainless steel 1.5mm
• sandwich inner stainless steel 1.5mm fixates the input/output shield pcb:s from post #7

View attachment 1069422
View attachment 1069423

Very interesting and very professional preamplifier !!!
A complete solution & very high level.
Have you calculated the approximate cost?

 

[astx]

Thx, unfortunately the costs are also "high end" for the prototype. 😳

A groupbuy would massive lower the costs ...

 

[thimios]

Indeed, the prototype is a painful story even if we ignore the cost of time!
Something like this,noone expect to be cheap...

 

[astx]

Today IO shields arrived...
Four different types:

• RCA
• RCA Phono with MM/MC switch
• mini XLR female
• mini XLR male

 

[diyralf]

Are mini xlr useful? Why not normal xlr?