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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).
Possibly an optional tube buffer.
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).
Possibly an optional tube buffer.
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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.
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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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 ?)
....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 ?)
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.
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.
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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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.
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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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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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.
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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Dave Hafler ,en.wikipedia.org/wiki/David_Hafler. designed a really nice sounding pre-amp with all the necessary knobs. I built one in 1978 and it served my needs still today - I won't part with it. Had many amps but never needed to change the pre-amp.
I have thought of upgrading the PCB at one time or another and this might just be that opportunity.
I have thought of upgrading the PCB at one time or another and this might just be that opportunity.
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Yes Sir, there will most definitely be an RIAA. I already have 3 designs in LTSpice
This will be a single box pre-amp with power drawn externally. The power-amp will be in a separate box. Like Naim, I will arrange to draw the power from the power-amp but you can, if wanted, build a separate dedicated power supply. My plan here is to design the pre-amp so that it will not be necessary to seek a dedicated external power supply beyond what can be tapped off the power amp in order to deliver top-notch results. Of course, if your power amp doesn't have a place to tap off the power then a separate box will be required.
Hey if you would work with us on this thread that would be fantastic - maybe better to build new, leave the one you have as a monument to past endeavors, avoid risk of damaging it and also gives you a platform to compare against in listening tests.
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Scope of project
I'm open for suggestions if anybody else is interested in building alongside me, but my personal objectives in absence of input are:
a) the box should also act as an input selector (phono, line-1, line-2) and volume control without power, be able to work as a passive preamp, for more flexibility in my hobby space. So the input selector should be mechanical switch(es) and the volume control a mechanical potentiometer or DACT-style attenuator
b) the physical size of enclosure to match up with my power-amps
c) I'm aiming to include 4 functions which might be defeat-able from the front panel:
i) phono amplifier board (MM/MC ? maybe with variable loading)
ii) gain stage board (modest gain)
iii) filter board (h.f. cut, maybe more)
iv) output buffer to drive cable and power-amp input (with mute function)
I have no plans to add a headphone amplifier.
Whatdya think ? - have I kept it simple enough ?
I'm open for suggestions if anybody else is interested in building alongside me, but my personal objectives in absence of input are:
a) the box should also act as an input selector (phono, line-1, line-2) and volume control without power, be able to work as a passive preamp, for more flexibility in my hobby space. So the input selector should be mechanical switch(es) and the volume control a mechanical potentiometer or DACT-style attenuator
b) the physical size of enclosure to match up with my power-amps
c) I'm aiming to include 4 functions which might be defeat-able from the front panel:
i) phono amplifier board (MM/MC ? maybe with variable loading)
ii) gain stage board (modest gain)
iii) filter board (h.f. cut, maybe more)
iv) output buffer to drive cable and power-amp input (with mute function)
I have no plans to add a headphone amplifier.
Whatdya think ? - have I kept it simple enough ?
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The Hafler has a gain stage of 6dB and a dual differential design, it has high noise immunity and PSRR. The RIAA section is also dual differential, and it was the first time I saw someone use electrolytic caps connected back to back making it "non" polar. The original RIAA section is for moving magnet, but a very low noise moving coil I have built in the mid eighties is available too. It was originally published in Elector Summer Circuits of August 1982
I will post the gain stage later for comment and if it is suitable. I have to capture the drawings from paper to PC and will take a few hours.
I will post the gain stage later for comment and if it is suitable. I have to capture the drawings from paper to PC and will take a few hours.
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You could and make it part of the PSU cascade. +/- 24V should be good and +/- 15V for the phono. Even a simple zener and pass transistor is good for the +/- 24V. That would give OK results from the NAP140 clone PSU as a starter.
LTP. VAS . VAS CCS. BC327-40 337-40 outputs ( +/- 20 V ). The CCS could be a bootstrap. The same card could do phono and output buffer.
Bart HRK | How to design Discrete Operational Amplifier - Design Guide - Theory - Bart HRK
Ian said about relay switching in the TGM10 thread. Not a bad idea. I would have 3 contacts to also switch a common ground. Make before brake is best. Alas if a Lorlin swich that would be 4 inputs only. I do exactly that as I find the Lorlin switch better than some and cheap. I then have an expansion box on one input for the less critical inputs. I switch a common earth/ground as it reduces hum especailly wiith TV devices.
The output buffer should have HF roll off. It could be a simple passive filter at the output. I don't like active for that use unless inverting. If CD goes direct to the volume control best it is non inverting and perhaps a bit of gain. If the gain of other stages need to be higher why not switch the gain to the buffer as it will keep the Naim style simplicity. Even the phono could include this. Thus make CD the prime gain setting.
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