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TGMC - a modular control pre-amplifier
TGMC - a modular control pre-amplifier
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Old 18th April 2017, 11:53 AM   #1
Bigun is offline Bigun  Canada
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TGMC - a modular control pre-amplifier
Default TGMC - a modular control pre-amplifier

A thread to capture some thinking and ongoing exploits to design and build a pre-amp for my TGM line of DIY amplifiers. Somewhat in the 'British' style ? - inspired by the Naim control amplifiers and modularity of those from DNM. A modular approach meaning a box where I'll put two or more boards over time to capture the functionality I find useful and to allow easy performance upgrades.

Where to start

(I'll update this first post as reqd.)

Edit:

Board #1 - is a complete phono amplifier. It incorporates a shunt regulator, a discrete phono amplifier, an output buffer and a mute relay. fyi - can be a stand-alone dual-mono MM phono amplifier (with an +/- 36V external power supply).

Board #2 - is a complete line-amplifier. It incorporates a shunt regulator, a discrete amplifier/buffer and a mute relay. fyi - can be a stand-alone dual-mono line amplifier (with an +/- 36V external power supply).

Board #3 - is possibly EQ
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Last edited by Bigun; 18th May 2017 at 01:26 AM.
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Old 18th April 2017, 12:05 PM   #2
nigel pearson is offline nigel pearson  United Kingdom
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If I get a moment I will start a few ideas. No right or wrong. Just ideas. Beat me to it guys.

One idea I had was make the output good enough to drive headphones also.
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Old 18th April 2017, 12:46 PM   #3
nigel pearson is offline nigel pearson  United Kingdom
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One idea that comes to mind is the power supply from the Naim NAP140 clone could be made to work as a +/- 15 V output that comes down a cable like a true Naim. Then one might have an add on PSU, A NAPCAPS if you like. It could be the old NAP140 transformer if it gets upgraded to a 500VA type.

One could do what Michael Creek did and use a separate recitifier and caps from the same winding to keep it cleaner whilst cheap to do.

I will leave the PSU design to others. LM317/337 are good choices if the voltage in reduced a bit ( a series zener might be enough , never tried it ). If 50V off load 20V needs to be lost. The 317/337 allows very low noise very cheaply. The one I built with 317 was almost the best I ever saw if the setting resistor filtered. -135 db below 12V if that is how we say these things. Naim used the big 317's

One could build a Pi filter before the LM317/337. Why not? If so I doubt it needs Michael Creek's idea.
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Old 18th April 2017, 01:12 PM   #4
Bigun is offline Bigun  Canada
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TGMC - a modular control pre-amplifier
Why limit ourselves to +/-15V?

....all my power amplifiers can provide at least +/-42V raw d.c. and if we allow for some voltage head-room for regulating then we could give ourselves +/-36V. This opens up some possibilities for greater linearity of some circuits, allows somebody to suggest the use of a cathode follower somewhere and it can still be regulated down further (capacitance multiplier anybody ?)
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Old 18th April 2017, 02:00 PM   #5
Mark Johnson is offline Mark Johnson  United States
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TGMC - a modular control pre-amplifier
Good old John Curl uses a cascade of two voltage regulators in series. The first one (the "upstream" regulator) is a cheap IC regulator whose only job is to reduce 2xMains ripple by 60-90dB. The second one (the "downstream" regulator) is more elaborate, with a larger parts-count budget. Its job is to provide wide bandwidth, low output impedance, low noise, and plentiful RFI filtering. But the one thing it doesn't have to worry about, is Line Regulation. The upstream regulator has already taken care of that.

Personally I favor using a low dropout IC for the upstream regulator, such as the LM2941. It's available in both thru-hole and surface mount packaging, with a dropout voltage of 500mV at 1 ampere (!). Linear Technology has some other LDOs with even more impressive specs.
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Old 18th April 2017, 02:14 PM   #6
bimo is offline bimo  Indonesia
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Quote:
Originally Posted by Bigun View Post
Why limit ourselves to +/-15V?
I agree.

I was try sim AKSA 55's style amplifier with PSU 77VDC for VAS and LTP and use 35VDC for output. I got very impressive result.

Why don't try pre-amp using higher voltage than 15VDC? With 2Vrms - 3Vrms output signal, THD will be very low.
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Old 18th April 2017, 02:50 PM   #7
Bigun is offline Bigun  Canada
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TGMC - a modular control pre-amplifier
Quote:
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...amplifier with PSU 77VDC for VAS and LTP and use 35VDC for output. I got very impressive result.
That's exactly what I'm doing with the power supply for my TGM11 amplifier. I have the pcb's in-hand, 2oz copper double sided, but no time or parts to build them yet. We are thinking the same things my friend.

Quote:
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Why don't try pre-amp using higher voltage than 15VDC? With 2Vrms - 3Vrms output signal, THD will be very low.
We want choices. Pre-amps have to have control knobs on the front. We want linearity when we want linearity. And when the source is a crap CD or teeny-bop radio station we need something to help us enjoy the music without being distracted by it's poor quality.
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Old 18th April 2017, 02:52 PM   #8
Bigun is offline Bigun  Canada
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TGMC - a modular control pre-amplifier
Quote:
Originally Posted by Mark Johnson View Post
Good old John Curl uses a cascade of two voltage regulators in series. The first one (the "upstream" regulator) is a cheap IC regulator whose only job is to reduce 2xMains ripple by 60-90dB. The second one (the "downstream" regulator) is more elaborate...

Personally I favor using a low dropout IC for the upstream regulator,
Excellent idea
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Old 18th April 2017, 03:26 PM   #9
dedegogo is offline dedegogo  Indonesia
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Why limit ourselves to +/-15V?
We dont need high gain and voltage swing in a pream. We need excellent noise and linearity. We should start with choosing the perfect transistor. Many good candidates have low Vce max. Many transistors sound better with low Vce. No need to limit ourselves with certain rail voltage, even tho it will be easier to make perfect power supply with low voltage.
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Old 18th April 2017, 04:55 PM   #10
Bigun is offline Bigun  Canada
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TGMC - a modular control pre-amplifier
We don't have to use high voltage, like you say, some choice transistors will demand some limits are placed.

I'm sure Naim were limited to +24V because they wanted a simple starting point. Put a regulator on the existing +34V rail of a NAP140 and you have to allow voltage drop etc. We're not going to clone a complete Naim system, but borrow the bits and ideas we like.

So we'll stay flexible, proceed as if we can use anything in the range of 0V to +/-36V because all this will be available in the chassis. We can use different voltages for different circuits if we want. We will regulate each circuit separately if we want too


And if we need current, we got that too because we're pulling power off the main power amp supply. Of course it all ends up as heat but if we need half an amp then half an amp is what we can have.
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Last edited by Bigun; 18th April 2017 at 04:58 PM.
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