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Old 3rd June 2009, 02:59 AM   #311
Bigun is offline Bigun  Canada
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Quote:
Originally posted by ostripper
Just an idea, gareth. I did this with a blameless (attached) , biased both the VAS cascode and the CCS together from rail to rail. It should rock with the TGM. (super PSRR).
OS
Well, all these comments got me thinking.

I still got to thinking about improving the VAS (reducing distortion), it's well trodden ground so here goes...

As I understand, there are a few things for me to look at:

1) Cascode: I poked my nose into D.Self on the web to look at the Cascoded VAS. It looks like a nice step forward. There is the question of buffering it from the non-linear load of the output but even without, it's still effective in reducing distortion. What I've learned is that you acknowledge the inherent non-linearity of the VAS device and try to reduce the distortion by increasing the local negative feedback. So we do this by increasing the effective collector resistance. The additional cascode device pins the VAS collector voltage.

2/ PSRR: We get noise off the -ve rail (I saw this with my X-talk simulations). So we can try to isolate the VAS emitter off the -ve rail. One option is a constant voltage source (ahaha, I get to put a device below the VAS afterall !) which we can implement with an LED and BJT.

3/ Differential Drive: This one is a bit more tricky. I found this in D. Self. The VAS is driven in a quasi-differential fashion. The collector from the input device of the LTP goes to the base as usual, but we also take a wire from the feedback device collector and apply this to the VAS emitter. This also drives up the gain. It also seems not to require a buffer which the Cascode method recommends. This sounds interesting because it may be possible to do this without adding any more active devices to TGM1 (the example in D.Self adds an extra device though) and this appeals to me.

Quote:
Originally posted by nigel Music has a vast dynamic range and the transients are easily clipped if care is not taken.
Hi Nigel,

That's an interesting comment, I have neglected to look at the clipping behaviour. Right now I have the feedback set up to produce fairly low gain, a 1V input should give me a 15V output so I am limiting things.
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Old 3rd June 2009, 04:30 AM   #312
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Quote:
Bigun - Perhaps 24V rails with big music into big speakers at party volume levels are not ideal bedfellows.
Definitely , 40V would be perfect for deep bass at moderate levels on medium speakers (8" 2-ways).
I see you have been reading the "bible" (D. self). , one drawback to the cascode and the CVS is that you lose voltage swing (it will run out of steam sooner) ,but you will have less THD and PSRR.

The differential drive sounds interesting , with the low rails of the TGM , a mod like this would not be as dangerous an exercise as with my 77v- 10+a rails. If you read to the end of self's VAS chapter ,you also see the balanced VAS , he says it has too much complexity ??

Another thing to consider is that reducing distortion by the above methods is comparable to actually building a full blown blameless. You will alter the character (sound) of the bootstrap. I built a few blameless's , I think their sound is flat and sterile (here I am in agreement with carlos).
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Old 3rd June 2009, 03:50 PM   #313
Bigun is offline Bigun  Canada
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I'm not planning to go down the route of a balanced VAS, it's digging a bigger hole in terms of complexity. Also it it would probably lead to making a more symmetrical design overall and this isn't the direction I have taken with TGM. Furthermore, a fully balanced VAS seems a well travelled route whereas the quasi-differential looks like a path less travelled and that makes it more interesting for me. My next step in this regard is to look at some simulations; this will help me understand this quasi-balanced approach before I go tinkering with the hardware. And definitely it is a goal to retain the sonic character of the bootstrap.
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Old 5th June 2009, 12:42 AM   #314
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Update:

Following advice I am now also changing capacitors (having already changed some resistors) to see how things can be optimized for TGM1.

I have focussed on C5 (compensation at VAS). I have to say right off the bat that I found the changes hard to quantify, hard to be sure of myself because each change took some minutes and my brain-ears were losing their memory between. I used my Bryston on L channel, TGM1 on R channel to compare and try to hold a consistent course.

stock: 68p - so far I like this sound, not as precise/open as the Bryston, bass is less firm but mids and highs are nice

change to: 22pF - didn't like this, bass sounded less but probably because highs were more (Carlos warned us of how changes in trebble can trick our perception of bass). It sounded somehow flat, not engaging, like a bedside radio (well not that bad actually !)

change to 44pF - the magic came back, the amp sounded more open, bass still good and trebles still smooth, it's as if the amplifier found where it's comfortable.

change to 112pF - I didn't like this, the magic was less again.

Conclusion - I have left it with 44pF and listened for a longer time. It seems improved but how much I can't quantify as my ears simply struggle with the changes. With Chesky CD (christy baron) it is simply very satisfying (Zinfandel?).

I am getting very impatient to add the 2nd channel...but have to fix the hum first.
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Old 5th June 2009, 01:12 AM   #315
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My experience of the VAS capacitor was that too little C allowed oscilation on the output and too much made the amp lose higher frequencies.

If your losing bass then look at the input filter if there is one.
Also look at the capacitor in the feedback divider chain and consider increasing its value.
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Old 5th June 2009, 01:22 AM   #316
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Gareth,
that capacitor is a nuisance and should not be used to tune the sound. Also, there`s nothing magic about the bootstrap.
Quote:
I don't want to isolate the VAS from the sonic benefits of the bootstrap
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Old 5th June 2009, 01:25 AM   #317
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Hi Nigel (you're up late !),

Indeed, I don't have particularly generous caps in those locations. The HT project has a sub so the amp doesn't have to deliver below 80Hz. However, the thing is, it's not how low the bass goes but how much impact it has (some people call it 'slam' ?). It isn't a problem as such but I would like to learn what aspects of the design impact 'slam' rather than simply the bass extension.
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Old 5th June 2009, 01:30 AM   #318
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Quote:
Originally posted by Bigun
It isn't a problem as such but I would like to learn what aspects of the design impact 'slam' rather than simply the bass extension.

Power supply!

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Old 5th June 2009, 01:33 AM   #319
Bigun is offline Bigun  Canada
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Quote:
Originally posted by Lumba Ogir
Gareth,
that capacitor is a nuisance and should not be used to tune the sound. Also, there`s nothing magic about the bootstrap.
Lumba, good to see you here !

I remember you have a dislike for this capacitor but to be honest, it didn't seem to sound as nice when I dropped the value of it. At the end, it's always there inside the transistor anyhow.

What would you adjust to optimize the sound of TGM1 ?
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Old 5th June 2009, 02:21 AM   #320
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Gareth,
to get better bass (and overall) performance, the power supply capacitance that needs to be increased (not the rail voltage). The capacitor bank is to be counted in hundred thousands of microfarads.
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