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Old 5th August 2011, 01:23 PM   #31
AndrewT is offline AndrewT  Scotland
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I have seen SQ comparisons for BC5xx vs 2sa LTP pairs.
I don't think that the parameters (hFE & Cob) you have quoted are responsible for these SQ differences.
I have not heard the difference, but most of my amps use BC5xxc at the inputs. I have only one amp, that I am aware of, that has 992 or 1845 at the input.
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Old 5th August 2011, 10:14 PM   #32
alex mm is offline alex mm  Europe
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Default PCB ready rev 1.0

...... of course I have layout the first PCB not sleeping yet
Alex.
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Old 5th August 2011, 10:52 PM   #33
Bigun is offline Bigun  Canada
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ooooh ! that's a nice pcb
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Old 5th August 2011, 11:18 PM   #34
Salas is offline Salas  Greece
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R1K under J162 better be 1K2 and paralleled with a 10K trimmer for offset control?
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Old 6th August 2011, 01:29 AM   #35
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Quote:
Originally Posted by Samuel Jayaraj View Post
Lazy Cat, thank you for your response. I think that the use of 2SC1845/2SA992 would make the amp more 'transparent' compared to BC550C/560C due to lower Cob of the former. They also exhibit higher gain. Noise should be comparable or better.

I have started with a PCB design and put the BF transistors on the ouput pair heatsink. Would it help in thermally coupling the input pair?

I take it that there is no need for a Zobel network or Output inductor, even with reactive loads. Thanks again. Wonderful design. Should soon be able to report on its sonic qualities.
I looked over specs of 2SC1845/2SA992 and they are very promising for the purpose, only, as you probably know it, try to find as close as possible matched pair regarding hFE. Because of the simplicity of the design (not so many parts) you can easily put some extra effort to choose the best parts for the job, for instance 1% metal-film resistors, DALE, Vishay, low ESR caps Vishay, Panasonic ... When will come to the listening tests you can also try to check the BC input pair among other possibilities.

Input pair must be well thermally coupled to keep bias/offset thermall unsymetry at minimum. The best way is to glue the flat surfaces, press them firmly together to minimum the distance, wrap Al foil ribbon around them and finish with heat shrink tube to surround the pair completely.

BF pair is not so critical regarding temperature and need no extra heatsinks, power dissipation ranges from 0,09 W to 0,12 W per transistor in the worst case, so no need to mount them on the main heatsink. Also capacitance from collector to the ground is kept at minimum if we leave them alone on the PCB.

As the amp is very fast the lenghts of the connections on a PCB should be short as possible to ensure good stability through the bandwidth. But that would be no problem cause count of the material is really small.

RC serial filter to GND and possible inductor from the output can be provided at the output connectors if needed.
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Old 6th August 2011, 02:06 AM   #36
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QUOTE=Salas;2662170]R1K under J162 better be 1K2 and paralleled with a 10K trimmer for offset control?[/QUOTE]

Yes we should put some offset control to the circuit, but the resistor changed to 2 k/Muti Turn trimmer should be according to sch attached. In this way we eqalize the currents through the input pair and balance them properly to get zero DC at output.

Also 100 ohm gate stopper resistors are there on the sch to provide some gate capacitance compensation, also to balance their values to fine tune different mosfets to VAS and to make any sound experimenting easy.
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Old 6th August 2011, 02:20 AM   #37
Salas is offline Salas  Greece
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An offset control should be on a final PCB IMHO. Now Alex can see the recommended way. Thanks. Are the Zeners well filtered with 100uF by the way? Takes either say 100R series resistor with each Zener to make it easier for the caps to filter more noise or bigger straight lytics to really quiet them down in my experience. Maybe he can add on final PCB too?
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Old 6th August 2011, 02:42 AM   #38
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Quote:
Originally Posted by alex mm View Post
...... of course I have layout the first PCB not sleeping yet
Alex.
What a smart PCB layout, very nice indeed. Thanks Alex.

As you can see some small differences took place on the sch so the board will change a little:
- gate stopper resistors added
- 2 k/MT trimmer for offset control
- two LEDs for rails power control
- fuses added
- input pair should be together with flats
- a little more space-room around BF's in a case they are vertically positioned

Please, one more 6,3 mm faston GND connection should be provided on PCB. Also the GND connection to the input should be routed directly from PWS GND faston point on the PCB to the input, where you connect 10 k together with two 100 ohm feedback resistors.

So far enough changes but Alex please stick to the approximately the same layout like it is now. Thanks again.

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Old 6th August 2011, 03:16 AM   #39
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Quote:
Originally Posted by Salas View Post
Are the Zeners well filtered with 100uF by the way? Takes either say 100R series resistor with each Zener to make it easier for the caps to filter more noise or bigger straight lytics to really quiet them down in my experience.
Feedback loop DC supply current through LED and 1k is app. 20mA. 100Hz ripple filter is formed by 1K/100uF parts who's time constant is app. 63ms if the capacitor would be ideal. There is a possibility to experiment and alter its value to max. 1000uF/16V low ESR cap to surpress the ripple to uV region on +/- 15V DC potentials, otherwise turn on phase would be just to long. So observation with scope can define the right value of these critical caps.
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Old 6th August 2011, 10:00 AM   #40
juma is offline juma  Serbia
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OK, since the schematic has been changed, let's change the "more legible one" too. Also, I propose to use the 1W parts for R3 & R18 since they dissipate about 400mW (20V * 20mA).
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