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

My Borbely All-Fet Preamp with Subbu
My Borbely All-Fet Preamp with Subbu
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Old 28th February 2016, 12:12 AM   #11
Algar_emi is offline Algar_emi  Canada
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My Borbely All-Fet Preamp with Subbu
Quote:
Originally Posted by Algar_emi View Post
Here the supply PCB completed, preamp dual mono transformers, high current HEXFRED rectifiers and auxiliary 12V supply and Mute timer. This pcb is also use to supply the front blue power-on led.
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Old 28th February 2016, 08:23 AM   #12
generg is offline generg  Germany
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Hi Algar_emi,

incredible to combine such different elements in such a short time in a working audio unit.

Not to speak of the mechanical work.

Hats off!
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Old 28th February 2016, 10:41 AM   #13
merlin el mago is offline merlin el mago
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One million question: how it sounds?
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Old 28th February 2016, 11:01 AM   #14
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Algar_emi View Post
This the rear Input/Output & Mute PCB. It uses an Elma dual pole rotary switch for source selection, very short path to the input connectors using resistors legs wire connection (CFM55 Dale resistor legs were used for the connection). It also has the Tape selection flat-ribbon socket and the Output Mute relay.

I use CMC RCA connectors. The preamp is Single-Ended only but the chassis I have has provision for balanced XLR In/Out connector.
I decided to installed them for convenience only. The Balance input is 'Pseudo Balance Only' and has only pin2 (+) & gnd connected.
The Output is only Pin2 and Pin3-Gnd are shorted. They are ready to connect to a PassLabs amp for example.

When you use a input selector switch on the preamp back it is always a good thing to use a flexible shaft coupler between the switch and the front connecting rod. With DIY there is always some error between switch and front button axis, and the flex coupler take care of that. I got mine from Mouser, from Adafruit, Part 485-1176
You could use pin2 for signal hot and pin3 for signal return as the unbalanced output. (your back plate label states "Pin 3 N/C")

That leaves pin1 connected to chassis.
The outputs then become compatible with any/every conventionally wired XLR balanced impedance cable.

If you want, the Pin3 can be converted to balanced impedance using the passive circuit in Jensen an003 section 2.4 "a simple alternative".
This would be compatible with both balanced and unbalanced XLR cable to either balanced, or unbalanced inputs of the receiver.

The extra resistance of the "simple alternative" in the signal return line is of little consequence if you keep it's value low, anywhere from 10r to 51r would be OK.

A correctly wired "conventional" XLR balanced impedance cable should have Pin1 to shield/screen at both ends, Pin2 signal hot, Pin3 signal cold.
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Last edited by AndrewT; 28th February 2016 at 11:09 AM.
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Old 28th February 2016, 04:14 PM   #15
Algar_emi is offline Algar_emi  Canada
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Hi Andrew. Greats suggestions, I'll implement them for sure

Merlin, it sounds great

Thanks again
SB
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Old 28th February 2016, 05:02 PM   #16
Algar_emi is offline Algar_emi  Canada
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So Andrew it should looks like this for the Input/Output?

My preamp has no 100K from Out to ground at the output. Should I install one so it matched the pin3 side on the output?
Also my preamp uses a Servo, hence there is no coupling cap at the output.
And my preamp signal ground is not connected to chassis either.
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Old 28th February 2016, 05:07 PM   #17
androa76 is offline androa76  Slovenia
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Nice work...
Why 50 ohm Bnc instead 75ohm?
Enjoy in sound.
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Old 28th February 2016, 06:06 PM   #18
Algar_emi is offline Algar_emi  Canada
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Because I use mostly 50ohms coax cable, because it is a little DAC, and I reserved the full 75 ohms, coax & cable for my big DAC, and because the impedance mismatch that degrade the signal, and increase the jitter is removed a great deal by the excellent Wolfson receiver, not counting the excellent jitter tolerance of the ESS dac chip.

I found that the meticulous respect to standard of 75ohms of the SPDIF is more acute with older digital receiver, such as the Cirrus Logic ones.

I tried the 75ohms BNC, but I couldn't hear a difference with the Subbu, driven at 24/192Khz by my music server and an M2Tech USB converter. Anyway my USB converter is using a normal 50ohms BNC as well...

I have only 75ohms BNC on my top Mark Levinson DAC and Transport, and a few of my top DAC. The small ones still use 50ohms connectors.

Hence I used the 50ohms that I have more in stock.
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Old 28th February 2016, 06:30 PM   #19
merlin el mago is offline merlin el mago
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Quote:
Originally Posted by Algar_emi View Post
Hi Andrew. Greats suggestions, I'll implement them for sure

Merlin, it sounds great

Thanks again
SB
I'm glad you enjoy the preamp.

Cheers
Felipe
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Old 29th February 2016, 07:31 AM   #20
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Algar_emi View Post
So Andrew it should looks like this for the Input/Output?

My preamp has no 100K from Out to ground at the output. Should I install one so it matched the pin3 side on the output?
Also my preamp uses a Servo, hence there is no coupling cap at the output.
And my preamp signal ground is not connected to chassis either.
The 100k can have two uses.
To carry any capacitor leakage current to ground.
To define the signal to audio ground impedance.
For both uses the 100k value is not critical. It could be much higher or much lower. I think you should retain some value. Match them to better than 1% (<0.1% if you can).
The electrolytics defeat your DC input. You should consider whether your out/in works with/without and whether omitting them is better.
Read Jensen's note re decreasing the series resistor and increasing the cap value and going to bi-polar.

There is probably a Main Audio Ground to Chassis connection somewhere. Measure to check.
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Last edited by AndrewT; 29th February 2016 at 07:33 AM.
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