DC offset
 

Hi All,

I am trying to design a simple, no feedback direct coupled (except for the input) Class A amplifier (please see the attached schematic). Some of you will like the design, some won't. It's not intended to be clever or original in any way, merely a tailored design to produce a sound I think I'll like. It uses a brute force approach to audio and very old principles - A HV 60mA single ended input stage which will easily drive those lateral output fets, in turn chosen because I can almost get by with no part matching. Anyway, I digress.

As you will see I have made a rather basic discreet DC servo. It works well for this purpose. The one problem I am having us that it is not totally benign and is acting as a feedback path, though not as much as you might think.
This may not surprise some of you, nor myself. I know it is not as nice as an opamp based circuit.

Anyway, what I would like to know is:

1) Given that I have no problem with an amplifier having up 100mV of offset, do I need a servo at all? Would a simple precision voltage reference bias for the front fet suffice? (I know, I should have 0mV, but I honestly don't care about the affect of 100mV of DC on a hundred watt speaker).

2) Any ideas on how to make the servo more benign? A larger filter cap does only so much. I have tried giving the servo a heap of "miller" style capacitance to try to numb its HF response. It's a nice theory, but the whole circuit ends up becoming a VLF oscillator. I have also tried filtering the input to the servo with a CRC filter and also the base drive of Q2 , both mods also turned the amp into a VLF oscillator.

Anyway, I look forward to your thoughts.

Greg.

I've used this kind of servo on my TGM3 and it worked like a charm - without it I was always wondering if the dc-offset had wandered off to a nasty value. Designed well it can be benign - I haven't looked at your design in detail but what is needed is to ensure the operating frequency of the dc-servo is really low. I don't think you have it quite right yet - it needs to be configured as an 'integrator' - I think you have a capacitor missing somewhere ?

The downside was that you get a significant start-up event whilst the servo settles down - in my case the speaker cone would move in and out a fair bit on power-up and then again a few seconds later when the servo came up.

Bigun,

Please have a look at the schematic and tell me where my "missing capacitor" is. I'd really like to know!

Download Gootee's simulation files.

Test your circuit using the simulator.

Any idea where I might find these files? Did a quick search to no avail.

I have simulated this circuit extensively and I know very well that it is creating feedback at all audio frequencies.

What I am asking for is a way of improving it. Any ideas how I might make it more benign?

Integrators have a tendency to bounce back and forth. This may not be a big problem in audio but in video it's bad. Instead of an integrator an OTA performs a similar task but without the oscillations. This chip works

Special Function Amplifier - Transconductance Amplifier - OPA860 - TI.com

G²

Hi Swordfishy, have a look at the schematic of the luxman C120 preamp to see how they implement a 1 transistor servo.
Regards.

Hi Greg

I like John Curl's approach to servos, which is to firstly get the DC offset as low as possible without the servo, so that when the servo is connected it has very little work to do. Then the servo can be very loosely coupled to the main circuit, to minimize the feedback in the audible range.

In your circuit you have two gain stages in the feedback loop (Q3 and Q2) and neither of them have degeneration, so loop gain is high and the feedback signal is also unnecessarily distorted.

I'd recommend adding emitter resistors to both to reduce the gain and improve linearity. There's a bit of detail missing in your schematic near Q3. That 0.672V voltage source looks like you might be using a long tailed pair?

Another thing I'd be a little uncomfortable with is the tight coupling between Q2's collector and the input. Three things to note about that:
a) Almost the entire input signal voltage is on Q2's collector.
b) That puts a nonlinear load on the input signal. (OK, it's a small effect but if we can avoid it, why not?)
c) The servo's control range is far greater than needed. If it weren't for the zeners, it could set U1's Vgs anywhere from 0V to 25V.

So anyway, I'd try to decouple that. If I come up with any bright ideas, I'll post them later. (I already came up with a couple of dumb ideas - you know the ones where you get half way through working out the details before realizing it's a complete ****-up because you forgot something important)

Cheers - Godfrey

p.s. Congrats on the FETZILLA project!
(Which reminds me, I think I forgot to reply to a PM at the time - sorry about that:o)

insert
gootee
dc servo
and it reports 14 results.
No.4 is what I was recommending.
If you have not read Gootee's post on the DC servo thread then you should never post a question on DC servos.
If you have forgotten some of the content on Gootee's DC servo webpage then go back and read it again.

Hi, thanks all for your responses:

AndrewT: Found it. Gootee's circuit looks very good, using an opamp based low pass filter. However it is a little more complicated than I was hoping for given the simplicity of the amplifier itself! If all else fails I'll investigate further. Definitely looks like a good circuit though.

Stratus46: Looks like a very impressive chip, though I'm not sure how I would go about using it. Going to read the datasheet a little more. I don't suppose you have a schematic of an example circuit with it anywhere? I'd rather stay away from surface mount devices but would use it if necessary.

SRH: Thanks for the response, I finally found a legible schematic but it's going to take me a while to work my way through it and even longer to figure out how the servo works.

Godfrey, long time no speak! Thanks for the encouragement. Fetzilla was a great learning experience and I am much better off now than I was then, but still pretty ignorant! I have built quite a few amplifiers now though with literally 50 or so incarnations of fetzilla with different input, vas and output stages (bjts/fets/etc).

Anyway, thanks for the servo ideas. You're right, the transistors could use some degeneration. I did actually have some degeneration in there at one point but removed it in the name of simplicity without really thinking of the consequences. In fact if I could linearise the servo, the feedback aspect might not be such an issue. The real problem is the VLF resonance that the servo seems to impose on the amplifier. There's a definite 80hz or so element in all the simulated FFTs I do, though it almost falls into the noise floor. Looking back, I didn't make this issue clear in my first post. The feedback is also certainly a problem, but I guess if I degenerate the transistors and it might almost be a non issue.

Hmm, it might be possible to reduce the amplitude of the oscillation if, as you say, I tighten up the intrinsic operating point of the input fet and reduce the ability of the servo to control the input voltage... Need to fiddle a bit.

The reason I have it the way it is is because, as you would be aware, the servo can only pull the input voltage down (output voltage up), so the gate of the fet has to tend to want to be up rather than down. So as a result I set the bias string to make sure of this. But you are right, it could certainly be tightened up to give the servo less control and reduce the upwards swing at the input gate.

I was also a little nervous about having the input directly coupled to the servo and I look forward to hearing any decoupling ideas you might have. This is why I used a fairly high impedance bias string for the input and between the servo and fet gate - trying to get as many ohms between the servo and the input as possible!

I have also tried adding a decoupling cap to the top of R34, to try to prevent rapid changes in the current through the bias string. This basically turned the amplifier into a VLF rail to rail oscillator.


Oh, and the voltage reference I was going use for the servo was going to be a simple LED and pot, an arrangement which worked very well with fetzilla. Though a LTP would work equally well I guess. I have actually experimented with a LTP based servo, but it was prone to the oscillating problems too. I'm happy to hear of a better servo arrangement using a LTP if you come up with one.

I'm off to keep fiddling!