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Old 1st October 2013, 09:24 AM   #2511
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Quote:
Originally Posted by Lazy Cat View Post
but I don't like diamond input stage since it brings more problems than solutions in reality. Low THD itself means nothing otherwise tube amps would not exist on the market any more.
Agree on all those points. More than this, diamond input stage sort of kill the CFA soul and character: feedback in the emitter of the first input device...expansive transients behavior... Diamond bring evils peaks and oscillations at high frequencies or during brutal edges in CFAs... Is-it your experience too ?

In itself, the common emitter/ common base combination in the single input stage, because the ultra low collector level of the signal/feedback difference don't produce disagreeable or even noticeable HD.

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Huge change is bias current in the frontend.
Cleverly solved by the use of CSS ? CFAs are very special animals Ask more feeling and experience for good designs. Humble attitude and a deep experience of their savage character. :-)
In the CFA thread, i noticed the approach was very VFA kind of...looking for HD instead of square waves and slew rate behavior etc...
That's why i'm very interested to build the Dadod's schematic and compare with my poorer measuring own CFA, or VSSA, just to confirm what i presume...
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Last edited by Esperado; 1st October 2013 at 09:35 AM.
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Old 1st October 2013, 09:28 AM   #2512
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Originally Posted by sonnya View Post
Maybe a little of topic.

But what problems does it bring?
Sound reproduction is not so good as with one part modulation only, sorry, tested.

VSSA showed thermal drift +/-1 mV of the output DC from cold start to normal operation without any extra method or DC servo and that is extraordinary achievement.

VAS bias current drifts from 12-15 mA but that has no influence to the output stage bias current since we have shunt bias regulator, independent to VAS bias current fluctuations.
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Old 1st October 2013, 10:10 AM   #2513
sonnya is offline sonnya  Denmark
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Quote:
Originally Posted by Esperado View Post
Agree on all those points. More than this, diamond input stage sort of kill the CFA soul and character: feedback in the emitter of the first input device...expansive transients behavior... Diamond bring evils peaks and oscillations at high frequencies or during brutal edges in CFAs... Is-it your experience too ?

In itself, the common emitter/ common base combination in the single input stage, because the ultra low collector level of the signal/feedback difference don't produce disagreeable or even noticeable HD.

Cleverly solved by the use of CSS ? CFAs are very special animals Ask more feeling and experience for good designs. Humble attitude and a deep experience of their savage character. :-)
In the CFA thread, i noticed the approach was very VFA kind of...looking for HD instead of square waves and slew rate behavior etc...
That's why i'm very interested to build the Dadod's schematic and compare with my poorer measuring own CFA, or VSSA, just to confirm what i presume...
The ccs does not Solve it unless it is thermally coupled and constructed to compensate for the bias drift.

Regarding peaks. If it done right you do not have a peaking.

Andrej, i am not talking dc offset. I know that it works beautifully In the vssa. I am talking general bias drift In frontend and vas stage.

If the diamond buffer is constructed correctly, i am In the believe that it will play up to the same level as the vssa frontend.

I know it is your thread, but Maybe there should be a shootout test In another thread.
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Old 1st October 2013, 03:16 PM   #2514
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Resuming my (limited) personal experience:
Unless you can find an active device 10 times faster than the ones you used in a 3 stages CFA, any added pole in signal path and specially in the input or output stages will bring more evils than improvements.
The main one is HQ peak at the open loop Fc. (Have a look in the CFA thread)
Kind of desperate, but always confirmed till now.
After that, you begin to play with compensations to calm down Mr Nyquist, and your tiger is transformed in a cat.
The difficult part is to bring more open loop gain with those 3 stages only topology.
The simple and clever L.C. idea was to add a AC gain factor to the input stage, keeping its distortion low enough, to reach this goal of high feedback ratio.
Quote:
Originally Posted by sonnya View Post
The ccs does not Solve it unless it is thermally coupled and constructed to compensate for the bias drift.
I don't understand: CCS provide Constant current (fixed bias current). Its thermal behavior will be the main factor.
Of course, there is a little part of the current provided by the CCS witch flows in the feedback bridge in the VSSA topology. So any change in the transconductance of the input tranies, due to the temps changes, will keep a little influence, changing the ratio of the CCS current flowing in them. Not an issue omho.
Could be addressed by fixing the CCS collector side ?
L.C. said 12 to 15mv variation in the VAS. Cumulative effect of temp changes in input and VAS.
Any problem if the VAS lowest current is high enough to provide enough current to the gates capacitances at HF ?
The Output quiescent current, witch is the main issue, is fixed by the super Zener, witch is thermally very stable by design. Only remain the transconductance changes in power devices due to their temp variations. Here too not high enough to worry thanks to laterals.

Yes, L.C had took care of those problems despite the very simple VSSA schematic. Reason of this "wow" effect when i first looked at it, and my admiration to the designer :-)

.
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Old 1st October 2013, 05:44 PM   #2515
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Originally Posted by Lazy Cat View Post
Each resistor-diode pair shorten with one wire. By listening tests was confirmed more grip and better bass and even more resolution in mid-high region.

The 100 Hz ripple noticable at the output was the same as before (barely noticable), since we use CCS for the input stage.

Hi Lazy Cat,

This shorting of these resistors and diodes is recommended when used with the SMPS (Hypex) power supplies? Is the voltage adjustment procedure the same?

Thanks,

Mark

Last edited by DuffyDawg; 1st October 2013 at 05:47 PM. Reason: Forgot the reply.
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Old 1st October 2013, 05:51 PM   #2516
sonnya is offline sonnya  Denmark
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See the attachments.

You can clearly see the change in bias current through the collector with respect to temperature.

http://www.onsemi.com/pub/Collateral/BC546-D.PDF

Look at figure 1 and figure 6 wich shows the current gain and CURRENT-GAIN - BANDWIDTH PRODUCT. You can clearly see that you will move up and down the curve.

Frontend of the diamond buffer can be seen as a source, it is not part of the feedback loop. With an Re of 10R and bias of 2mA you will have an source with an impedance of ~20 - 25R for upper and the lower transistor. This is better than driving next stage directly from a source through a cable.

But i think we can continue this in the CFA thread.
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File Type: jpg vsa frontend.jpg (203.0 KB, 372 views)
File Type: jpg vssa frontend.jpg (216.0 KB, 367 views)
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Old 1st October 2013, 08:16 PM   #2517
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Collector DC current of VSSA input Vs temp from 15° (i don't listen music when it is cold like that) to 60° (air inside my amp is not as hot in the hottest summer's days):
Variation is 5.5%.
Attached Images
File Type: gif temp.gif (9.1 KB, 341 views)
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Last edited by Esperado; 1st October 2013 at 08:18 PM.
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Old 1st October 2013, 08:24 PM   #2518
sonnya is offline sonnya  Denmark
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A bit more change than i would expect from the VSSA. But that is ok.

I did some simulation on 40 degrees step:

VSA 728uA
VSSA or TSSA for that matter: 72.8uA
Diamond buffer: below 32uA. But matching voltage across the transistor will bring it even more down.
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Old 1st October 2013, 08:50 PM   #2519
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To give an idea of consequences, according to simulations, this 50¨temp variation makes vary the VSSA open loop gain by ~2db.
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Old 1st October 2013, 10:44 PM   #2520
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Each resistor-diode pair shorten with one wire. By listening tests was confirmed more grip and better bass and even more resolution in mid-high region. .....
Hi Andrej Is this the amending to be done, or is there more meant in that statement from post #2245 qoute "VSSA improvement will be detaily described after all VSSA modules will be shipped out", just curious if D5/D6/R19/R20 are the baby for that improvement, or more is published when shipping is done.

About this D5/D6/R19/R20 shortening do you recommend one to make sound listening test with original setup first, and then do the shortenings with sound listening test to compare. Or is it recommended to jump to shortening setup righting the R19/R20 fast pulse problem in same amend.
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