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Old 23rd March 2012, 01:32 PM   #11
AndrewT is offline AndrewT  Scotland
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Use his simulation to establish the operation of your DC servo.
I am NOT telling you to adopt his nor any one else's servo.
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Old 23rd March 2012, 11:35 PM   #12
godfrey is offline godfrey  South Africa
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Thinking about this a bit more.....

PSRR is a bitch. The input signal goes directly to a voltage gain stage that's referenced to -75V, so any ripple on the -75V rail will appear at the output, multiplied by the full gain of the amp.

Switch-on thump could be a nightmare too, since the input coupling cap has to charge up to about 70V before the amp can begin to stabilize.

It would be a lot easier if you had an input stage referenced to ground, with it's output driving the voltage gain stage. Only problem is the design starts looking a bit normal then.

Anyway, heavy filtering of the -75V line can get the hum low enough. Maybe CRCRC filtering with big enough time constants that the voltage rises sloooowly at switch-on to avoid a nasty thump.
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Old 24th March 2012, 02:35 AM   #13
godfrey is offline godfrey  South Africa
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Hi Greg

Here's a slight reworking of your servo. With the values shown, frequency response rolls off below 1.5Hz due to the input cap, and again at 0.5Hz due to the servo. Looking at it the other way around, the feedback factor reaches unity at 0.5Hz, is -40dB at 10Hz, and -60dB at 30Hz. In theory.

The high supply voltage makes choosing Q1 a bit difficult. MPSA92 has lousy current gain. BC556 is probably OK, but close to it's max voltage. Maybe BC560C plus cascode would be better?

The trimpot should allow very accurate trimming of DC offset. If it fails open circuit, the two resistors across it get the voltage there close enough that the servo can still sort out the bias fairly accurately.

If you're not too fussed about accuracy, you could leave out the trimmer, R8 and R9, and just use one zener of about 7.5V.

One thing I'm not totally comfortable with is the 100uF electrolytic cap. Hopefully leakage and ESR won't be too much of a problem. The nice thing about opamp based servos is you can use plastic film caps.

Cheers - Godfrey

p.s. Sorry for any typos. Tired, need to sleep now.
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Old 24th March 2012, 08:10 AM   #14
godfrey is offline godfrey  South Africa
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On second thoughts, the servos shown above will misbehave badly because they can't cope with low supply voltages. It's probably a better idea to use something like the circuit below, which works OK over a wide range of supply voltages, down to about +-5V or so.

One possible improvement: Ideally the voltage across zener D2 should be matched to the Vgs of the MOSFET, so it might be better to replace the zener with a trimmable Vbe multiplier. If you do that, you probably don't want or need the other trimmer.

p.s. Note the klutzy transistor choices in the LTP.
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Old 24th March 2012, 09:48 AM   #15
GregH2 is offline GregH2  Australia
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Thanks for putting all the time in Godfrey!

I like what you've done, essentially turning the input low pass filter into an even lower pass filter for the servo by using a super high resistance. I toyed with that idea but I got nervous about having such a high impedance bias network. Not sure why that bothered me, guess I've just never seen it done! Completely makes sense though.

Very good!

My only question is the purpose of D2 in your first schematic. Is that to ensure Q2 is on at turn on?

Can I ask if you have tried the circuit in a turn on situation with an input signal applied? I came up with a few interesting servo ideas that worked well until I did a 0v startup with a 1kHz 1Vp-p input signal i.e. the effect of turning on the amp when the preamp is already attached with music playing. I got terrible rail to rail oscillations. I think your circuit should be fine though. I didn't understand what was going on before but I think you have addressed the problems that were causing it.

As for PSRR, yes it's a real problem and I was thinking of smoothing the HV rails using a ridiculous CRCRCRC filter. Need to think a little more about the actual values, but I was thinking of something like 100R and 1000uF, off the top of my head.

The other thing I wanted to think about, if I were to go the CRCxxx route is the effect of such a high impedance supply on the distortion of the input stage. Anyway, something to think about.

I was also thinking of things like a huge zener string, a basic shunt regulator, a feedback regulator and also a few other ideas. Always keen to hear your thoughts.

I like to think of this thing as a back to basics brute force amplifier which basically (hopefully) uses terrible inefficiency to perform well with only one voltage gain stage and no global feedback. It certainly sims surprisingly well considering the circuit (I know simulation results don't mean much, but its relative performance compared to some more complicated designs is not bad). So the simpler the power supply solution the better I think.

I have stuck with a singleton input because I love the way they sound. All that H2. I have also used a high rail voltage and current for the input stage to keep it well away from clipping and hopefully increase linearity. I'll probably drop the voltage a bit to suit the kind of capacitors I can get for a reasonable price.

I have elected to use lateral output fets, which from my fetzilla experience I know can make a pretty decent 20kHz square wave with only 5mA of drive current. 60mA in the input stage should do the job OK I think, though the input fet will need to be on the main sink. The lateral gets can probably get by with no part matching too.

Anyway, gotta run but thanks so much for taking the time to draw up a circuit and think about it!
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Old 25th March 2012, 07:56 AM   #16
GregH2 is offline GregH2  Australia
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Actually, I've been having a think and a quick chat with a friend, and it would seem there's no real point to making this circuit direct coupled anyway...hmmm. Might as well make it single rail and capacitor couple it.
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Old 25th March 2012, 09:37 AM   #17
godfrey is offline godfrey  South Africa
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Quote:
Originally Posted by swordfishy View Post
My only question is the purpose of D2 in your first schematic.
Mostly to make the mental arithmetic easier. There may have been some other reasons I thought were good at the time, I forget.

D2, Q2, R5 and R6 form a "mirror with gain" i.e. Q2's collector current is directly proportional to Q1's collector current, without a huge DC offset. This means the voltage across R4 is directly proportional to the voltage across R1.

Somewhere along the line, I had a useful insight:
a) The voltage across R1 is 1V (due to LED biasing).
b) I want 35V across R4 to give the servo maximum range.
c) Therefore the DC voltage gain of the servo from base of Q1 to collector of Q2 is going to be 35, no matter what current gain is chosen for Q2 or what idling currents are chosen.

That gets rid of a lot of uncertainty and makes life much easier. Without the diode, everything affects everything else.

Quote:
Originally Posted by swordfishy View Post
Can I ask if you have tried the circuit in a turn on situation with an input signal applied?...
Nope, I didn't try any start up/shut down stuff, but I'm surprised you got oscillation. What sort of frequency was it? I would expect the output to be stuck hard to the negative rail for a few moments while the input cap is charging.

Quote:
Originally Posted by swordfishy View Post
I like to think of this thing as a back to basics brute force amplifier....
I was seeing it as a clash between minimalism and the quest for maximum open-loop linearity. One of the things I love about this forum is you get to see, and play with, all different kinds of design philosophy. The high rail voltage and heavy degeneration should indeed give good linearity.

Quote:
Originally Posted by swordfishy View Post
I have stuck with a singleton input because I love the way they sound. All that H2....
I think a nice compromise would be to add an LTP input stage with heavy degeneration, so it has little (if any) gain and negligible distortion. That way, distortion will still be dominated by the MOSFET voltage gain stage, there's no need for a separate servo, and no worries about switch on/off thumps.

Quote:
Originally Posted by swordfishy View Post
...it would seem there's no real point to making this circuit direct coupled anyway...hmmm. Might as well make it single rail and capacitor couple it.
I'm not really a fan of that approach, and prefer no coupling cap and no thumps when switching on or off.

If you do decide to go that route, I'd suggest putting a coupling cap between the voltage gain stage and the buffer as well, so you can still run the VGS at high voltage.
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Old 26th March 2012, 11:42 AM   #18
GregH2 is offline GregH2  Australia
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Thanks for the explanation Godfrey, I get it now.

Actually I just revisited my simulations and the oscillations were only there for some of the more elaborate versions I came up with.

Your idea of using a heavily degenerated LTP isn't a bad one, though I hate the thought of adding another gain stage, no matter how benign! I have no problem with a big turn on thump provided it's not damaging. Going to play around with a few simulations over the coming days and see what I come up with. Do you have any thoughts on the HV rail smoothing? I am also tempted to try an opamp servo though I'm still yet to come up with a circuit for this amplifier.
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Old 26th March 2012, 12:37 PM   #19
godfrey is offline godfrey  South Africa
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Quote:
Originally Posted by swordfishy View Post
Do you have any thoughts on the HV rail smoothing?
What you mentioned earlier sounds OK. The output impedance of the RC(etc) network will be quite low and reasonably linear in the audio band, so I don't think it will cause distortion. OTOH, it might cause some cross-talk depending on the amp circuit, so having the last RC separate for each channel might be a good idea.

Something else to consider is a cap multiplier. They can give a very long time constant and good ripple rejection, without using huge caps. They also have very low output impedance but it's very nonlinear, so a bit of a mixed bag.

I'm thinking of something like shown below, but maybe that's overkill.

btw, It occurred to me you don't need a separate transformer for the high voltage rails. You can use a diode-capacitor voltage multiplier fed from the transformer that powers the output stage. Should save some cash that way.
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Old 26th March 2012, 01:29 PM   #20
GregH2 is offline GregH2  Australia
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Wow, you whip these things up in an instant!

Ah yes, a cap multiplier. Nice circuit, those things still intrigue me. I must build one at some point, in fact it could be a good choice for this amplifier. I wonder how a fet based version would work, in keeping with the overall theme of this design?

What I have been thinking though is this. Correct me if my reasoning is wrong here as we're on the edge of my understanding.

1) One big disadvantage of a single rail amplifier, normally, is that the output cap is outside the feedback loop, unlike the rail caps in a direct coupled amplifier. Feedback normally helps correct any power supply capacitor induced distortion. Since this amplifier has no global or interstage feedback we can pretty much ignore that one and assume that having a capacitor in the output is going to have the same sonic penalty (if any) as capacitors in the negative rail of a dual rail version. So this particular aspect is perhaps not as significant as one might expect...I think.

2) Running a single rail, with a HV input stage means an interstage blocking cap is needed. Not so good...but I do often think the supposed sonic penalty of capacitors is overstated. Plus a good friend tells me that it is alleviated somewhat by placing a high potential across the capacitor, as would be the case here.

3) By running a single rail, I could use two off the shelf transformers (one HV and one LV) with dual windings and have a dedicated winding for each channel and almost full dual mono construction. This lightens the requirements for preventing the crosstalk you mentioned above.

4). As we discussed months ago, a single rail can be set up in a way which keeps the current through the output stage more or less constant, reducing supply ripple and distortion.

5) A single rail with a coupling cap provides inherent DC protection and removes the need for any servos.

Soooo, as much as it is tempting to always go more complicated, the single rail version is looking pretty promising. The only problem is that turn on thump, which I need to go model I think.

Only having one HV supply per channel would also give me a little more motivation to try something more exotic in the power supply (cap multiplier).

It's funny. I really hated the idea of a cap coupled amplifier which is why I came up with the servo in the first place. Now it seems I have talked myself into it!
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