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Old 26th June 2013, 03:03 PM   #731
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Quote:
Originally Posted by PMI View Post
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.

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.
Gate resistors should be used by diy-ers that do not like to experiment too much, that want their pcb to work immediately without any problems, from the first power-up. This is simple preventive measure.
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Old 26th June 2013, 03:20 PM   #732
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Originally Posted by wiljj78 View Post
Thanks for the comments, PCB base on Shaan layout.
.... please clarify.
Hi wiljj78,

I think Tyimo is right (in post #724). While the BD139 and BD140 pinout labels are correct, they appear to have their bases and emitters reversed on your layout. (That is, they connect to each others destinations).
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currentflow

Last edited by currentflow; 26th June 2013 at 03:27 PM. Reason: For clarity!
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Old 26th June 2013, 03:20 PM   #733
wiljj78 is offline wiljj78  Philippines
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Originally Posted by ivanlukic View Post
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.
You're right ivan! i found it thanks...
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Old 26th June 2013, 03:23 PM   #734
wiljj78 is offline wiljj78  Philippines
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Quote:
Originally Posted by currentflow View Post
Hi wiljj78,

I think Tyimo is right (in post #724). While the BD139 and BD140 pinout labels are correct, they appear to have their bases and emitters reversed on your layout.
Yeep! BD139/140 have to rotate 180 degree, thanks guys....
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Old 26th June 2013, 03:31 PM   #735
Tyimo is offline Tyimo  Hungary
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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.
Ofcourse! But with DC free output ( capacitor coupled DAC or preamp) it needn't.

Greets.
Tyimo
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Old 26th June 2013, 03:39 PM   #736
PMI is offline PMI  United States
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Originally Posted by Tyimo View Post
Ofcourse! But with DC free output ( capacitor coupled DAC or preamp) it needn't.

Greets.
Tyimo
My guess is, LC left it out to save space. The through-hole boards are bigger to start with, so leaving it in is the more correct approach, because putting a jumper across the leads (if you don't want to use it), is easier than an off board cap with flying leads.

edit:

See this post in the VSSA thread about what can happen even with "DC free direct coupled signals".

VSSA Lateral MosFet Amplifier

Don't get me wrong, I am absolutely in favor of modifying just about anything... amplifiers, cars, coffee pots... but I have also learned rule #1 of basic modding... don't do this with your DD (daily driver)!

Last edited by PMI; 26th June 2013 at 03:51 PM.
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Old 26th June 2013, 04:42 PM   #737
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Originally Posted by PMI View Post
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 I understand nikosokey finds, with BD pair, all we need for stability is 2K2 feedback resisitor and 100R gain setting resistor.

In that case we do not need neither of the three:

1. gate resistors
2. VAS compensation cap
3. colector to ground cap

which in fact means unlimited choice of VAS transistors from ubiquitous BD pair to something better.

That is consistent with some of Shaan's comments on stability of the circuit and with theory of current feedback amps. With CFB amps there is no constant gain/bandwidth product. Bandwidth is not dependent on ratio of feedback and gain setting resisitors but on value of feedback resistor. This is very straightforward way of limiting bandwidth and influencing stability. Why should we be limited to 1K/47R values? Lower values improve noise performance of the ciruit but are not beneficial for stability. I've seen even lower values in circuit similar to VSSA (820R/16R) but also with some additional compensations.
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Old 26th June 2013, 04:51 PM   #738
Tyimo is offline Tyimo  Hungary
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My guess is, LC left it out to save space. The through-hole boards are bigger to start with, so leaving it in is the more correct approach, because putting a jumper across the leads (if you don't want to use it), is easier than an off board cap with flying leads.
O.K. I see.

Is there any rule of thumb for PSU sizing for Class AB amp?

Greets:
Tyimo
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Old 26th June 2013, 06:13 PM   #739
PMI is offline PMI  United States
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Originally Posted by Tyimo View Post
O.K. I see.

Is there any rule of thumb for PSU sizing for Class AB amp?

Greets:
Tyimo
Not really. There are various considerations. The type of load, 8 ohm or 4 ohm, peak power v. continuous power, and the amount of acceptable noise and ripple.

I originally sized my power supply for low-to-moderate power into 8 ohms.

250 VA transformer 25V AC, rms, dual secondary
Schottky diode rectifier w. minimal heatsink
3x 4700uF per rail, in CLCRC passive filter configuration
+/-35V DC nominal, 37V max rail voltage
3A fast blow glass fuses in power supply rails

Since then I decided to use a power supply with an active filter, capacitance multiplier type, from a circuit posted by MrEvil here in the power supply section. I did that because I gradually realized that this amplifier needs much cleaner power supply rails for best performance than I first thought, especially without CCS.

I posted some addition info about that here, but not tested yet:

Linear Capacitance Multiplier Supply
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Old 26th June 2013, 06:29 PM   #740
PMI is offline PMI  United States
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Here is what my power supply looks like now, compared to a 3D model of the new design:
Attached Images
File Type: jpg P3A_PS_detail.jpg (273.2 KB, 304 views)
File Type: jpg VSSA_CMPS2.jpg (127.6 KB, 286 views)
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