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Old 28th February 2013, 12:59 AM   #11
The Dastardly Dad of Three
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Just a small update on things. Thanks to the suggestions on-list and off-list, I've removed C1, C3 and C4. Yes, the dreaded 'bypass' caps are gone (well for the moment).
The input fet (M4) has had a 1k resistor added in line with the gate, and I've increased the gate stoppers for the Jfets to 470R for now. I kept the -ve rails as far away from the outputs as I could and the amp is now very stable. Square wave testing is also clean with the larger stoppers. HF performance is not yet optimal (you can see some small rolloff on the leading edges).

Overall I'm pretty amazed how well such a simple circuit does - with the basic config proven, I'll go ahead and do a PCB for further work to take place... if anyone with PCB experience wants to take a stab that would be most welcome else its a transparency sheet, layout tape and marker pens for me (dates me a bit i know!)....
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Old 28th February 2013, 02:13 AM   #12
The Dastardly Dad of Three
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30v pk-pk square wave at 1khz ... any comments other than the HF rolloff ?
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Old 28th February 2013, 02:16 AM   #13
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Maybe you want to trim your scope probes.

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Old 5th March 2013, 05:24 AM   #14
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Default I like it...if was BJTs i would give a try

Thank you.

Carlos
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Last edited by destroyer X; 5th March 2013 at 05:29 AM.
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Old 5th March 2013, 07:30 AM   #15
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Carlos, wish my BJ skills could accommodate hahaha maybe some day )
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Old 5th June 2013, 03:09 AM   #16
The Dastardly Dad of Three
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ok time for a small update and a question. I have pretty much finished with this configuration and now have moved to using two feedback networks instead of one - this was important as it gave the opportunity to reduce/eliminate the large 100R resistor from the source of the LTP. Otherwise you end up locked into having to use the same values in the virtual earth network and the source resistors. You can see the dilemma...

Anyway, with balanced fb loops we get rid of this interdependency (and create a few more but manageable ones) and I am now pretty happy with the outcome.

One of the things I am trying now is to find current production fets for the input LTP so that anyone trying to make this doesnt have to deal with unobtainable parts.

I have tried SJ74s, SJ313s and they both work well (the latter is pretty excellent imo) but are nla. I've tried most of the usual IRF 96xx IRF, 95xx suspects but they didnt do so well. IRF9Z24 worked well. Small devices tend not to have much Gm so that is why I am looking at To-220 types generally.

BJts I have tried here didn't work so well for me - measured distortion was great, sounded a bit leaden in my setup and other issues due to base current that I'm not eager to deal with.

Basically I need a current p channel fet with highish Gm > 0.5S or higher at low currents (3-10ma) with low/flat capacitance - impossible ask ? Anyone any ideas ?
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Old 7th July 2013, 10:29 AM   #17
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Default Sic Fets

I was playing around with some different input/output fet combos and will post results later when properly digested. Some of the outputs that i tried were the new (and expensive) Sics from Cree and Rohm. I measured them on a curve tracer and also put a trace of our workhorse IRF240 for comparisons.

I bought 4 of each - unsorted other than being in the same order. So the general observation based on a very limited sample size is that the Crees have very low transconductance and relatively widely dispersed Vgs and Gm. The exact device was CMF10120.

The Rohms were SCT2080KE and measured quite ok IMO. Dispersion was fairly good and the reason you see only three traces is that there was a pair that measured almost identically. Gm was a bit better than the crees but still nothing compared to the 240s.

These sorts of comparisons are not the whole enchilada of course but I will be measuring these in-situ in the amp and we can compare the results in a working amp and see how they get on.

Graphs attached - first is the Gm and second are the traces. You will notice the 'outlier' cree as the second trace from the left. The umarked ones are the SCTs.
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File Type: jpg SIc-fets.jpg (122.5 KB, 473 views)
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Old 2nd August 2013, 01:16 PM   #18
The Dastardly Dad of Three
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Default Start of mini-article - clever chappies pls skip

Ok so I've been meaning to write up the notes I had compiled on this, so before pressing on with the latest practical results and things, lets take a look at some of the building blocks that I used for this amp.

We'll start with the circlotron output stage first and then work backwards to the input stage. Almost all of the circ material is covered well in at least two other places so I am only going to provide a potted version so that the final circuit is easier to follow. The two places ? M. Rotarchers article and the really excellent Tubecad website.

So what is this type of stage and why do we want it ? The attraction is that it uses identical polarity parts for push-pull action that is required for us to swing electricity through the loudspeaker load.

Why the interest in identical polarity parts ? Well, what would be perfect would be output devices which amplify AC signals identically but differ only in their polarity. Unfortunately these things don't quite exist at this time so why not just use the same devices

There are only very few ways of getting these like-polarity devices in push-pull to also deliver voltage gain. The really excellent F6 shows one approach and the circlotron is another. The transformer coupled output stages similar to those used for tubes would be a third. I am saving that for my next amp

This attached schematic shows what the basic output stage looks like.

[Next post:- how it works]
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File Type: png Basic-Circ.png (4.2 KB, 389 views)

Last edited by kasey197; 2nd August 2013 at 01:31 PM. Reason: clarity - this is going to happen a lot
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Old 2nd August 2013, 01:54 PM   #19
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Let's start with the bottom fet. The first part of the sinewave we can see in Input-2 is going LOWER in amplitude and as a result, the Vgs of the bottom fet starts to drop. This makes it conducts less. As a consequence, the voltage at L2 moves further away from the negative rail of V2 and now becomes more POSITIVE with respect to the arbitary ('virtual') ground we created at the junction of R2 and R3.

At the same time, at the top fet, the inverted signal is going HIGHER in amplitude (turn the monitor upside down if you dont believe me) so the Vgs of the top fet starts to rise. This makes the top fet conducts more, dragging L1 closer to the negative rail of V1 - causing L1 to go more NEGATIVE wrt ground. So we have L2 going more +ve at the same time that L1 is going more -ve and current starts to flow through the load ! There we have it: a pertubation at the input causes a bigger one at the output.

How much bigger ? By two times the transconductance of the fet multiplied by the resistance of the load. So if we used a Fet with a Gm of 2 siemens and an 8 ohm load, we would get a stage gain of 32x.

In many ways you can think of this as being two single-ended amplifiers sitting one on top of the other. See the zen ones - they work the same way - we have just rearranged the voltage rails and as a consequence, gotten rid of the current sources.

Like in the zen articles also, we now have to find a way to drive these big beastly fets and set up the bias voltages we want at the same time. M Rotarchers Amazing Fet Circlotron shows us one very cool way of doing this - we will try another.
Oh, we also need to figure how to do the phase splitting so that we can use our lovely phono-stage too in addition to balanced inputs.

[Next post;- the genesis of the input stage]

Last edited by kasey197; 2nd August 2013 at 02:01 PM. Reason: usual
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Old 4th August 2013, 10:07 AM   #20
The Dastardly Dad of Three
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Default Input input

In thinking about an option for driving the fets, it seemed reasonable to try and adapt something that already worked well. So I turned to the Long Tailed Pair design used by NP for many many of his successful amps in the 90s and noughties.

He tasked it to drive OP stage FETs directly so that was a good start - but the problem was obvious - i needed to 'split' the negative rails since the source references of the op stage were not conjoined as it is for his designs.

At that point I was not yet spice conversant and so many failed experiments ensued....

Some dead-ends [and dead fets] later, I wondered if it would work if i just cut along X (heh) and added another voltage source

[edit: see figure 15 and text in NP's DIY Opamp article http://www.firstwatt.com/pdf/art_diy_opamp.pdf to understand the constant current source bits M5, R17, 18 and D1]
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Last edited by kasey197; 4th August 2013 at 10:07 AM. Reason: drat
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