New Posts New Posts RSS Feed - Phono Preamp Project
  FAQ FAQ  Forum Search   Events   Register Register  Login Login

Welcome to the Graham Slee Audio Products Owners Forum

 

Open to all owners plus those contemplating the purchase of a Graham Slee HiFi System Components audio product and wishing to use this forum's loaner program: join here (Rules on posting can be found here)

This website along with trade marks Graham Slee and HiFi System Components are owned by Cadman Enterprises Ltd

Phono Preamp Project
 Post Reply Post Reply Page  123 58>
Author
  Topic Search Topic Search  Topic Options Topic Options
Graham Slee View Drop Down
Admin Group
Admin Group
Avatar
Retired

Joined: 11 Jan 2008
Location: South Yorkshire
Status: Offline
Points: 16298 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Topic: Phono Preamp Project
    Posted: 01 Dec 2022 at 10:09pm
Originally posted by OneyedK OneyedK wrote:

Thank you for taking the time to explain.  My brain hasn’t fully processed it yet :/

And I better take a break.  I stared at fig.17 for several minutes until I figured out you meant fig.19 ;)

Well done for spotting my deliberate mistake Wink
That none should be able to buy or sell without a smartphone and the knowledge in how to use apps
Back to Top
OneyedK View Drop Down
New Member
New Member
Avatar

Joined: 25 Dec 2012
Location: Mechelen/Belgiu
Status: Offline
Points: 15 		Post Options Post Options   Thanks (0) Thanks(0)   Quote OneyedK Quote  Post ReplyReply Direct Link To This Post Posted: 01 Dec 2022 at 10:04pm
Thank you for taking the time to explain.  My brain hasn’t fully processed it yet :/

And I better take a break.  I stared at fig.17 for several minutes until I figured out you meant fig.19 ;)
Back to Top
Fatmangolf View Drop Down
Moderator Group
Moderator Group
Avatar

Joined: 23 Dec 2009
Location: Middlesbrough
Status: Offline
Points: 8998 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Fatmangolf Quote  Post ReplyReply Direct Link To This Post Posted: 30 Nov 2022 at 9:46pm
Many thanks Graham.Handshake
Jon

Open mind and ears whilst owning GSP Genera, Accession M, Accession MC, Elevator EXP, Solo ULDE, Proprius amps, Cusat50 cables, Lautus digital cable, Spatia cables and links, and a Majestic DAC.
Back to Top
Graham Slee View Drop Down
Admin Group
Admin Group
Avatar
Retired

Joined: 11 Jan 2008
Location: South Yorkshire
Status: Offline
Points: 16298 		Post Options Post Options   Thanks (1) Thanks(1)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 30 Nov 2022 at 9:30pm
You'd need to study power amplifiers for quite some time to see how an op-amp works, and then study op-amps for a long time to see how linear voltage regulators work.


I don't know what the internal gain of a 78XXXX op-amp is, but take a look at https://www.farnell.com/datasheets/1692726.pdf and fig 15.


Note that the regulator impedance is rising from about 2kHz, and then look at the OP27 and see the voltage gain at 2kHz. It's around 72dB.

That's about 4000.

Now at 500mA a pass transistor should have an emitter resistance of 0.025/0.5 = 0.05 ohms.

But in fig 15 it's 0.008 ohms at frequencies below 2kHz, and 0.05/0.008 = 625, not 4000.

Straight away we can see the regulator's open loop gain is around 1/6th of a reasonable audio op-amp, i.e. not as good, but then again the op-amp has a large voltage supply and the regulator only has the 2 - 3 volts dropout voltage to work with.

So, with something like a gain of 625 that's fine up to 2kHz, but with increasing frequency, the open-loop gain falls away (again look at the way it falls away with the OP27).

That means at higher frequencies it has less negative feedback to bring the output resistance down.

And there you see its resistance rising above 2kHz, so we want a capacitor to take over to keep that a.c. resistance low, so the output resistance isn't divided into the load, losing voltage.

And there they place a 1uF tant which curtails its rising impedance before it reaches 1MHz.

So, it has a ropey op-amp dealing with its output impedance, but what can you expect for a 2- 3 volt supply (its dropout voltage).

But then again we only need it to satisfy to 20kHz for audio. Do we? No, the op-amp in the phono stage has a power bandwidth of 4MHz. We'd like the power supply to not go floppy until way after then.

But now we come to the current used by this Genera. It is only about 12.5mA ave. so 0.025V / 0.0125A = 2 ohms .

And the 625 loop gain can get that down to 0.003, which is great, but with such little current demand, the op-amp output stage beta has fallen, so it won't have such high gain, and it could be 0.01 ohms - who knows?

What do we actually need? What we need is a clean supply - a hum free supply - and the regulator's doing that. And the signal load is going to be ? That's the next stage the phono preamp is driving, and as far as IEC61938 goes, that is 2 x 22k in parallel (L and R channels) and will 2 ohms upset that by much?

Well 2 ohms is 1/5500th or 75dB down. No, it's not going to upset it.

So we might as well do without an IC voltage regulator and use a Zener referenced pass transistor.

But for the dropout voltage, and that makes the IC voltage regulator the winner, and also it's just one component, and it's not expensive, so we use it.

Now, if we put a capacitor straight across the output of an amplifier it's shorted, so its NFB loop cannot operate, and so it has no transient response, and the 78XXXX has no transient response. But placing a 2.2 ohm resistor in series with the capacitor, it has some transient response.

Now, if we make the 470uF smaller, say 1uF, then it probably has all its possible transient response.

So what about making that 470uF have the same reactance as 2 ohms? What value will it need to be at the 2kHz break frequency?

C = 1/ 2pi f R, = 1/ 2pi 2000 . 0.000002 = 40uF.

But that's a reactance, and so the capacitive impedance is really 0.707 . 2 = 1.414 ohms.

So add that 2.2 ohms series resistor an in total that's 3.614 ohms so the circuit's power supply "rejection" isn't 1/5500th, but around 1/3000th.

So perhaps we leave it at 470uF?

Now, op-amp PSRR is claimed to be really good, but it's a voltage ratio, but what about the current? The voltage regulator fixes voltage (EMF) but current flows where the load says.

I hope I didn't lose anybody with my reply, but to do this sort of thing justice I'd need to write a book that looked at all parts of the circuit, because we can't think of power supplies as being a separate item.
That none should be able to buy or sell without a smartphone and the knowledge in how to use apps
Back to Top
Fatmangolf View Drop Down
Moderator Group
Moderator Group
Avatar

Joined: 23 Dec 2009
Location: Middlesbrough
Status: Offline
Points: 8998 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Fatmangolf Quote  Post ReplyReply Direct Link To This Post Posted: 30 Nov 2022 at 8:36pm
Agreed about Graham's explanations being very good. When I built my Genera and read about the need for the resistor after the regulator it made sense. I've included it every since. I'll wait for the 470uF calc...
Jon

Open mind and ears whilst owning GSP Genera, Accession M, Accession MC, Elevator EXP, Solo ULDE, Proprius amps, Cusat50 cables, Lautus digital cable, Spatia cables and links, and a Majestic DAC.
Back to Top
OneyedK View Drop Down
New Member
New Member
Avatar

Joined: 25 Dec 2012
Location: Mechelen/Belgiu
Status: Offline
Points: 15 		Post Options Post Options   Thanks (1) Thanks(1)   Quote OneyedK Quote  Post ReplyReply Direct Link To This Post Posted: 30 Nov 2022 at 6:42pm
It's been a while (about 36 years) since I had the workings of the 78xx series regulators explained to me by my teacher.

Your way of explaining is a lot more revealing.  Thank you!

If you don't mind me asking...
By increasing/decreasing the capacitance, can we alter the transiënt response in a meaningful way?
Or, more direct, how did you end up with 470uF?

On a sidenote, the Genera has been powered on for about 70 hours (around 30 hours of music) and
it keeps getting better...
Back to Top
Graham Slee View Drop Down
Admin Group
Admin Group
Avatar
Retired

Joined: 11 Jan 2008
Location: South Yorkshire
Status: Offline
Points: 16298 		Post Options Post Options   Thanks (2) Thanks(2)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 30 Nov 2022 at 10:11am
All series regulators like the 78XXXX and even linear adjustables, are pass transistors on the end of a 2 or 3 volt supply op-amp. The reference pin on a fixed regulator isn't part of the supply - it's just reference current.

If it was a pass transistor alone, the output resistance would change depending on load. That being proven by the thermal voltage (25mV at 20C).

The op-amp in the voltage regulator provides negative feedback control over the pass transistor's output impedance to keep it constant, but within limitations of gain (which isn't much).

So what is the slew rate of the op-amp? Who knows? But that doesn't matter because we know some value exists and that's all we need to know.

Therefore, there will be a small time lag between current demand and correction, as there is with all NFB.

So what does that make the voltage regulator? It makes it a voltage source (some say a perfect Thevenin voltage source to show how clever they are), in series with a gyrator, which can be thought of as an inductor. And that's because inductors store energy and then reluctantly give it up (reluctance).

So the voltage regulator has inductance even though there is no inductance anywhere in it.

And an inductance into a capacitance gives?

A simple tuned network, or tank circuit (because it tanks).

So what do they suggest?

On all the old data sheets it was a 10uF capacitor across the regulator's output. And that brought down output impedance where the op-amp starts running out of beta.

But it also causes oscillations.

What damps oscillations? Resistance.

So the load on the voltage regulator was the preamp via a 2R2 resistor, and at that junction, a 2R2 resistor in series with a 470uF capacitor. The series 2R2 looks to be an error on my part, but the above has shown that the 2R2 in series with the 470uF capacitor will damp any tendency for the voltage regulator to try and oscillate.

We could go further and ask if the 470uF capacitor is too big? If the voltage regulator can do its job without oscillations then is anything required at all?

Maybe all it needs is something in the order of 10uF? But it will still need that 2R2 in series with it.

This is as the title says, a project. It is not the be all and end all of all audio science. Nothing man makes is perfect. We are on a mission to search out what makes things sound good, and we use the science of what we know to be correct in searching those things out.

It is a never ending road. Only one knows the ultimate answer, but we are not God.
That none should be able to buy or sell without a smartphone and the knowledge in how to use apps
Back to Top
 Post Reply Post Reply Page  123 58>
  Share Topic   

Forum Jump Forum Permissions View Drop Down

Forum Software by Web Wiz Forums® version 12.01
Copyright ©2001-2018 Web Wiz Ltd.

This page was generated in 0.188 seconds.