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Old 26th June 2013, 08:51 AM   #721
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Default VSSA T03

Quote:
Originally Posted by ivanlukic View Post
It seems to me that all TH boards in this thread could probably work without gate resisitors. Only Borys's pcb with two pairs of HexFets must have gate resisitors. Gate resistors are necessary with multiple Fet output pairs. If you remember the generic Hitachi schematic that I sent to you (with one pair of output transistors), there was no gate resistors. All pcbs featured in this thread are compact, with short pcb tracks and good layout with topographic placement of components that go from input to output. Therefore there should be no problems in using one output pair without gate resistors. When you repair the boards used for "antenna experiment" you can replace gate resistors with jumpers and see if bandwidth is more extended compared with resistor version and if stability is exemplary.
Hi Ivanlukic
I have made ​​these small changes and there are reasons that have been made them by LC.
Really not need gate resistors Only 100pf colector to ground .Comp cap 5,6pf but when pull out 100pf and no comp cap i have oscilation with 1K and 47 Ω but with 2,2k and 100Ω no osc at all .All these with bd 139 and 140 .
I made some changes to the board Jason and working very stable
on 47 Ω 120mv to other 121mv
on 470 Ω 870 mv to to other 875 mv
1,6V total on gate without gate resistor
only 100 pf col to ground 10pf comp- CCS 2N5462 - 1k and 47Ω- offset 1-3mv positive.
I hope to not be a problem by Μr Shaan and Jason
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Last edited by nikosokey; 26th June 2013 at 09:18 AM.
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Old 26th June 2013, 10:40 AM   #722
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Default VSSA T03

Quote:
Originally Posted by nikosokey View Post
Hi Ivanlukic
I have made ​​these small changes and there are reasons that have been made them by LC.
Really not need gate resistors Only 100pf colector to ground .Comp cap 5,6pf but when pull out 100pf and no comp cap i have oscilation with 1K and 47 Ω but with 2,2k and 100Ω no osc at all .All these with bd 139 and 140 .
I made some changes to the board Jason and working very stable
on 47 Ω 120mv to other 121mv
on 470 Ω 870 mv to to other 875 mv
1,6V total on gate without gate resistor
only 100 pf col to ground 10pf comp- CCS 2N5462 - 1k and 47Ω- offset 1-3mv positive.
I hope to not be a problem by Μr Shaan and Jason
Small change on CCS
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Last edited by nikosokey; 26th June 2013 at 10:51 AM.
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Old 26th June 2013, 01:06 PM   #723
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Quote:
Originally Posted by nikosokey View Post
Small change on CCS
Oh No...
No CCS please
Shaan will not happy with it

read this:
Quote:
Originally Posted by shaan View Post
Hi fellow diy-ers.

Important:- Visitors/posters are welcome to modify any of the layouts according to their need and/or add totally new designs. However, if you want to use CCS or Multilayer Layout or Parallel capacitors in the feedback network or Resistive bias spreader then please post your VSSA experiment and results in the original VSSA thread. Link >Here<. This thread's topic is single layer printed circuit board designs based on the VSSA schematic without the above mentioned accessories, shown in one of the pictures attached below. Thanks for your co-operation.

the VSSA thread is moved, not in solid state anymore
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Old 26th June 2013, 01:37 PM   #724
wiljj78 is offline wiljj78  Philippines
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Quote:
Originally Posted by Tyimo View Post
I think your connection between the BC550 and BD140 is wrong. It should be Collector to Base, it isn't it?
Greets:
Tyimo
Thanks for the comments, PCB base on Shaan layout.
.... please clarify.
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File Type: jpg VSSA Ver.1.1 Blue.JPG (336.0 KB, 318 views)
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Old 26th June 2013, 02:14 PM   #725
PMI is offline PMI  United States
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Quote:
Originally Posted by Tyimo View Post
Hi!

Why do you need input cap? In the original VSSA schematic there isn't any.

Tyimo
Reason is the same as any amplifier circuit. You need it mainly to block any DC offset in the input signal. DC at the input would appear at the output amplified by the gain of the amplifier. In most cases this is a bad, for the speakers and for other reasons.

Some amp/preamp combinations work direct coupled, but usually that assumes good control of the grounds and good offset calibration in the entire signal chain.
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Old 26th June 2013, 02:40 PM   #726
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Could it be that there is error on Shaan's pcb? It seems to me that Tyimo is right. Usually the error is on pcb and not on the schematic. I checked PMI's pcb, compared to the schematic, and to me everything seems fine.
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Old 26th June 2013, 03:23 PM   #727
PMI is offline PMI  United States
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Quote:
Originally Posted by ivanlukic View Post
It seems to me that all TH boards in this thread could probably work without gate resisitors.
Agreed, but I can't think of a way to prove that it will work with any component combinations we have been discussing here. For example, I really look forward to JKuetemann's finding on those driver transistors, and building a set of boards with those...

Quote:
Originally Posted by ivanlukic View Post
... All pcbs featured in this thread are compact, with short pcb tracks and good layout with topographic placement of components that go from input to output...
There would actually be a significant benefit to removing those resistors completely on my board. There is a tradeoff that had to be made: shortest track v. single signal layer v. best component placement... something to be discussed if other people are still interested in another rev of my board.

Quote:
Originally Posted by ivanlukic View Post
... When you repair the boards used for "antenna experiment" you can replace gate resistors with jumpers and see if bandwidth is more extended compared with resistor version and if stability is exemplary.
I am waiting for parts from Tayda in Thailand (I automatically just clicked on the cheapest shipping option, now I wish I had the parts in hand... )

Full circle - as you know, I don't mind experimenting... it is sort-of part of my hobby, if I burn up a board or two, so be it, I have spares to listen to. Even though this may be a stable circuit with any reasonable care, we should also say that it does not take too much to make it oscillate, and the result could be some damaged components.
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Old 26th June 2013, 03:48 PM   #728
naf is online now naf  Indonesia
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i like this thread! very inspirative
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Old 26th June 2013, 03:52 PM   #729
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Quote:
Originally Posted by nikosokey View Post
Hi Ivanlukic
I have made ​​these small changes and there are reasons that have been made them by LC.
Really not need gate resistors Only 100pf colector to ground .Comp cap 5,6pf but when pull out 100pf and no comp cap i have oscilation with 1K and 47 Ω but with 2,2k and 100Ω no osc at all .All these with bd 139 and 140 .
I made some changes to the board Jason and working very stable
on 47 Ω 120mv to other 121mv
on 470 Ω 870 mv to to other 875 mv
1,6V total on gate without gate resistor
only 100 pf col to ground 10pf comp- CCS 2N5462 - 1k and 47Ω- offset 1-3mv positive.
I hope to not be a problem by Μr Shaan and Jason
nikosokey,

This is very useful information concerning TH version of VSSA.
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Old 26th June 2013, 04:02 PM   #730
PMI is offline PMI  United States
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Quote:
Originally Posted by nikosokey View Post
Really not need gate resistors Only 100pf colector to ground .Comp cap 5,6pf but when pull out 100pf and no comp cap i have oscilation with 1K and 47 Ω but with 2,2k and 100Ω no osc at all .All these with bd 139 and 140.
Very nice!, and thanks for posting your results.

However, I am not sure I agree with the conclusion. My logic is this: If using 1K in the feedback can cause oscillation with fairly slow drivers (BD139/140), and if everything else was the same, then we should be a bit careful when using other, faster drivers.

If eliminating the gate resistor means adding a 100pf cap, I am not sure what is gained. If you want to leave out the compensation caps completely, and at the same time have to add 100pF elsewhere, and at the cost of a more complicated layout, is there really an advantage?

On my board, a compensation cap on the driver transistors has almost zero impact on the layout. Adding a 100 pF close to the collector means bringing a new ground track into the middle of the board, using either a jumper, or a very long track. Clearly this makes the layout more complex, even if the gate resistor is eliminated. It also makes it more difficult to allow for a larger footprint for VAS heatsinks, which may be necessary for anyone who wants to raise the current maximum supply voltage.

....

I would like to have a board that follows Shaan's basic idea to have a VSSA that anyone can build with parts available almost anywhere, etch by himself, or use a professionally made one like I do. I think this is what makes Shaan's approach worthwhile.

If I wanted to make a VSSA just for myself, I would use CRD's, two-sided layout, supply split at the VAS stage and regulated, Exicon or Alfets, etc. I still think about a design for a double-die Exicon... . But, how many people would want to build one???
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