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Old 30th September 2010, 08:16 AM   #21
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Hello Hugh

It's hard find japanese numbers transistors here arround, I will look for a similar low cob high Ft low current US number transistor.

Is that was because of a better sound that you go back to single transistor VAS ?

I was using buffered VAS because that in simulations it was giving lower distortions, maby I should go back to single transistor VAS.

Thank

Bye

Gaetan

Last edited by gaetan8888; 30th September 2010 at 08:19 AM.
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Old 30th September 2010, 10:09 AM   #22
AKSA is offline AKSA  Australia
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Gaetan,

Try MPSA15.

A buffered VAS gives more refined sound, less coloration (mostly H2 and H3, warmer).

Cheers,

Hugh
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Old 30th September 2010, 11:41 AM   #23
Bonsai is offline Bonsai  Taiwan
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" . . . I've found the beta enhanced VAS (image C) harder to stabilise with low value cdom"

Why Gaeten? was it local parasitic issues as opposed to loop stability. I would think the beta enhanced CE VAS would be quite easy to stabilize. if you worry about the effect on bandwidth or SR, you can always up the LTP current.
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Old 30th September 2010, 09:21 PM   #24
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Hello

I've seen few amps using beta enhanced VAS, in the forum, where the guys need to use up to 100 pf cdom to stabilise the amp.

Maby the beta enhanced VAS are more sensitive to pcb layout or to the speed of the output stage.

Bye

Gaetan

Last edited by gaetan8888; 30th September 2010 at 09:25 PM.
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Old 30th September 2010, 10:19 PM   #25
AKSA is offline AKSA  Australia
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Gaetan,

I was wrong; should be MPSA18, not 15, my apologies.

With beta enhanced VAS you have two lag compensation choices; from emitter of buffer to VAS base, OR, from collect of VAS to VAS base. The former includes the buffer in the loop, which is is probably of little advantage since voltage gain of the buffer is unity and phase shift very small. But since the buffer output has very low impedance you'd better be sure the input stage can handle the charge slew for Cdom. The second, solely VAS comp technique is Self's preferred option, and it does result in lower values, I've verified this empirically.

For a VAS with active load and emitter to rail, gain set is maximum, and depends largely upon beta. Therefore loop gain is much increased with this configuration, and understandably compensation must pull this back to unity loop gain by the HF pole frequency. But if the VAS can now be run at lower currents, and thus higher impedances, a small Cdom should be fine, typically 30-40pF regardless of buffer current, seldom more.

Hugh
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Last edited by AKSA; 30th September 2010 at 10:24 PM.
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Old 1st October 2010, 04:07 AM   #26
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Hello Hugh

Very good, I have some MPSA18 in my parts bin.

Thank you

Bye

Gaetan
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Old 1st October 2010, 08:24 AM   #27
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Hello

Looking at the 2009 edition of D. Self amp book, there is a beta enhance vas using a hf compensation cap (C1), any one have try that C1 with this type of vas ?

Btw, in his text Self suggested a quite big cdom cap of 100pf for the beta enhance vas, even without a phase lead cap, a 100pf cdom cap are much bigger than needed.

Here is the image of the beta enhance vas with hf compensation cap C1.

Thank

Bye

Gaetan
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File Type: jpg VAS beta enhanced with C1 hf compensation.jpg (54.6 KB, 204 views)

Last edited by gaetan8888; 1st October 2010 at 08:40 AM.
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Old 1st October 2010, 10:05 AM   #28
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by AKSA View Post
With beta enhanced VAS you have two lag compensation choices; from emitter of buffer to VAS base, OR, from collect of VAS to VAS base.
comparing this to post27, I am confused.
Is Q1 the buffer and Q2 the VAS? Q2 is the device sending output to the next stage and I have interpreted Q2 as the VAS.
Q1 buffers the VAS input.

But, could your description have swapped VAS to Q1 and Q2 is a buffer on the output? But Q2 cannot be a buffer it is a common emitter stage.
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Old 1st October 2010, 12:27 PM   #29
Bonsai is offline Bonsai  Taiwan
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The pic above is the classic 'beta enhanced' VAS. Q1 is the 'buffer' and Q2 the VAS proper. Again, 'buffer' is not really what Q1 is - its there for beta enhancement to raise the VAS stage loop gain. I have looked at overall amplifer loop gain with and without beta enhancement in the VAS stage and the difference is about 12dB (simulated, but its quite accurate enough in this case) so Hugh's comments above are spot on in this regard. As to why DS seems to require 100pf, I cannot understand. I'm getting away with 33pf and 10mA tail current (yes, I like to run it HOT and fast !!). Then again, my LTP degen resistors are 150 Ohms, so loop gain is quite low.
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Old 1st October 2010, 12:27 PM   #30
AKSA is offline AKSA  Australia
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Andrew,

Yes, I can see why, my bad......!

If Q1 is buffer, and Q2 VAS, then the two choices are collector of Q2 to base of Q2, OR, collector of Q2 to base of Q1.

Chris,

Self mentions 'constant gm LTP degeneration' in his early nineties papers, and this is clearly what you refer to. If you increase stage current in the LTP enough, you can add degeneration, in your case to straighten the normally S shaped transfer function, and keep gm of the first stage unchanged over low stage current, zero degen. This trick means that you keep the same loop gain, but you widen the linear operational region of the LTP, so that the error correction remains more accurate for a given loop gain. The additional loop gain you pick up using an EF to drive the VAS comes at little expense, possibly a bit of H2 and H3 because the EF is single ended, not much more, and thus reduces further the spray of artefacts beyond H4.

This is not so easy to follow, I haven't expressed it particularly well, but you get the idea. The weak point of the LTP is non-linearity of the LTP at high diff input - that is, high output from the amp - and this results in signal compression and odd order distortion.

My apologies,

Hugh
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Last edited by AKSA; 1st October 2010 at 12:34 PM.
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