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Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

DIY Full Featured Preamp
DIY Full Featured Preamp
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Old 30th September 2017, 04:07 PM   #1
jwilhelm is offline jwilhelm  Canada
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Join Date: Jun 2014
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Default DIY Full Featured Preamp

With VZaichenko's help I've been working on a modular design full featured preamp with the design goal being a very good quality preamp DIY option for experienced builders. The initial design vision is to include single ended and balanced analog inputs, optical and coaxial SPDIF inputs, optional USB input, single ended and balanced output an optional headphone amp. I'm looking at optional FM tuners, Sirius/XM, Bluetooth ect. Control will be open source Arduino based with options for control over Bluetooth, RF, Wifi, IR (any format really). A 2U chassis is the target size for the whole project.

I have a couple prototype boards running so far. The analog design is almost complete. The input board has a couple errors and I need to add some more internal inputs, so it will see at least one more spin. Volume control is done with a PGA2310.

The output buffer board is working very well. It has an onboard Jung regulator designed by Ostripper to clean up the analog input supplies. The output buffers are one of Valery's current drive designs. It has RCA outputs as well as a THAT1646 differential converters for XLR out

The SPDIF selector board is designed to stack on top of the analog section. I have a first round SPDIF board operating, but I want to add extra inputs for I2S selections. This board also acts as a back-plane for the DACS. Mono PCM1794 DACs with output I-V converters and buffers designed by Valery will stack on this back-plane. These boards contain power supply pre-regulators and regulators locally for all their circuits as needed.

I've designed all digital controls to accept multiple voltages to allow for control from more modern 3.3V microcontrollers.

I have power supplies designed to fit in a separate chamber/Faraday cage to the side of the preamp. The analog supply is a simple unregulated supply with CRC design, the Jung regulator on the output board feeds all analog power to the rest of the preamp. The digital supplies is a lot more involved. It has a control transformer that powers all switching and controls with an onboard microcontroller controlling the analog supply and monitoring it's voltage. It has 12V, 5V and 3V3 outputs that are switched by onboard relays for any digital control needs. It also has a second unregulated supply for the DAC and SPDIF boards.

A lot of hours (and dollars) have already gone into this design and there are a lot more to go. I haven't done any formal measurements on anything yet, but the input and output buffers do a wonderful job, not adding anything or taking anything away from the sound. Output noise is under 25uV, so it's dead silent. Ostripper's Jung Regulator design is delivering wonderful clean DC with ripple well under 500uV from no load up to 1A+. The SPDIF switch is performing perfectly. The board outputs I2S signal to the DAC boards with no issues. DAC designs have had a few bumps but are coming along well now.
Attached Images
File Type: png Input Selector.PNG (198.3 KB, 1077 views)
File Type: png Output Buffer.PNG (208.9 KB, 1065 views)
File Type: png SPDIF Switch.PNG (152.5 KB, 1040 views)
File Type: jpg Input selector.jpg (212.0 KB, 1057 views)
File Type: jpg Output Buffer.jpg (190.8 KB, 1042 views)
File Type: png DAC.PNG (313.9 KB, 484 views)
Attached Files
File Type: pdf DipTrace Schematic - Inputs and Volume Control V1.3.dch.pdf (135.1 KB, 245 views)
File Type: pdf DipTrace Schematic - Output Buffer and Regulator.dch.pdf (167.7 KB, 189 views)
File Type: pdf DipTrace Schematic - SPDF Switch.dch.pdf (168.8 KB, 109 views)
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Old 1st October 2017, 04:28 PM   #2
thimios is online now thimios  Greece
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Oh my god!
Advanced diy.
Big project is in the middle of the way,Jeff i like this,i'm here if i can help.
Good luck.
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Old 1st October 2017, 04:43 PM   #3
jwilhelm is offline jwilhelm  Canada
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Join Date: Jun 2014
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Your testing assistance is always welcome. I'll send you some samples to build and test once they are in final form.
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Old 1st October 2017, 04:45 PM   #4
thimios is online now thimios  Greece
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Quote:
Originally Posted by jwilhelm View Post
Your testing assistance is always welcome. I'll send you some samples to build and test once they are in final form.
Wonderful news!!!
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Old 1st October 2017, 05:48 PM   #5
aparatusonitus is offline aparatusonitus  Croatia
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Hi jwilhelm,

Since you use bjt input opamp/buffer without match input impedances and no means for output dc offset control, how do you plan to control it? Btw, I like the basic principe.
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Old 1st October 2017, 05:59 PM   #6
jwilhelm is offline jwilhelm  Canada
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The input buffer is fet input. There is a bit of DC on the output, but it's AC coupled to the PGA2310, so it's not an issue anyway. The output buffer is trimmed by R320/420. This is the same input as our VHex+ amps use. It's very stable, only drifting a couple mV from cold to warm operation.
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Old 1st October 2017, 07:02 PM   #7
jwilhelm is offline jwilhelm  Canada
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The supplies are assembled and ready for testing. The layout is working out well. Power entry is short and sweet!
Attached Images
File Type: jpg 20171001_144853.jpg (133.7 KB, 297 views)
File Type: jpg 20171001_145237.jpg (215.5 KB, 296 views)
Attached Files
File Type: pdf DipTrace Schematic - Analog Supply.pdf (18.7 KB, 94 views)
File Type: pdf DipTrace Schematic - Untitled.pdf (107.6 KB, 84 views)
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Old 7th October 2017, 01:08 PM   #8
rsavas is offline rsavas  Canada
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Hi Jeff,

I can help or offer up my AM/FM tuner design if you want. You might recall I designed a portable radio/media player a few years back now, built 2 protos that have been running this long, so the design seems to be very reliable. The radio was based on a Elektor mag article, project 100126-DSP Radio. This std FM no IBOC(HD)
A user in Germany re-wrote a good portion of the Si4735 code with enhanced RBDS(RDS) code, it filled the mega168. I ported/integrated the enhanced RBDS sw to my xmega platform.
The portable uses some Bourns encoders and a 4x40 LCD display. I use the bottom row as menus for encoders. There is the Si4735, PCM2902B USB codec, BU9458 USB/SD mem stick mp3 file reader as sources for audio.
Sirius/XM I have no idea about. Bluetooth control I have seen a few implementations of, if you want to stream audio over bluetooth I have no idea about.

One option could be a Alps motorized volume pot instead of the PGA2310/20. There is "C" code available for this feature in Elektor project, 071135(Motorized volume control). I have it compiling in Atmel studio 7 which is the dev platform that I suggest or BASCOM-AVR, as it is simpler to understand for most new coders.

One issue is that the code for Si4735 radio chip is it is written in BASCOM-AVR. It is a lot of code, it runs on either a mega or xmega. I suggest to use the xmega which is the mcu I used in the portable due to fact that it the most featured 8-bit mcu that Atmel makes and has the largest flash mem available. You do not want to use a mcu that limits your coding size if at all possible.
So I have a complete radio/media player which is essentially a complete stereo receiver if you want to run with it. Let me know.
I will take a look at your design and offer suggestions if you would like.

Cheers
Rick
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Old 7th October 2017, 02:05 PM   #9
rsavas is offline rsavas  Canada
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Jeff,

I looked at the design a bit. From my design experience I can suggest that you need to step back and work on/consider the user interface(s) and control sections. I think that you are putting the horse in front of the cart. I had a similar issue with my portable design where I did not think the whole design through before committing to h/w first.
I know it is exciting to get H/W in your hands, but pls consider the big picture first.
Have some doc like a block diagram is a great way to have an design overview so that you can best figure out design partitioning and have other do a review.
I can see an issue like putting the mcu on the PS assy, where is your user interface/display functions being put?

Stay away from 5V control, it is legacy = PITA, most I2C and other control I/F's chips are only 3V3! You are not using Arduino H/W so stay away from it. Arduino is intended for using there H/W and adds a level of abstraction to coding, so inexperienced "C" coders do not have to fully understand register level coding access. Thus there is no reason to use Arduino, stick with either Atmel Studio 7 for "C" coding or BASCOM-AVR, which is what I suggest if you want others to help coding unless they have enough "C" experience.

Where is the turn on/off audio muting being done?
I will offer my portable documentation to use as an example if it is of use to you, if I can ever get it to load, if not I can email it to you. I have your email, so I will just send it to you that way.

Last edited by rsavas; 7th October 2017 at 02:28 PM.
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Old 7th October 2017, 02:32 PM   #10
jwilhelm is offline jwilhelm  Canada
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Hi Rick.

I'd definitely like to have a look at your tuner design. I've already been working on front end design. I'm trying to leave the hardware ready for anything though. I've done a lot of work with Bryan Levin with his remote control design and want to go with something like that. Check out his videos from Burning Amp Festival last year. Cool stuff!
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