Randy Slone "Fig 11.6" amp, modded: will work? - Page 5 - diyAudio
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Old 29th December 2006, 03:10 PM   #41
AndrewT is offline AndrewT  Scotland
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Hi,
the location of the fuses whether on the PCB or in the lines feeding the PCB or at the smoothing caps makes no difference to the level of protection offered to the downstream end.

The FETs must never see more than 15V across the gate source leads, otherwise they fail instantly.

Similarly if you look at the gate drain voltage (Vds) v drain current (Id) you see a limiting current for each Vgs.
If you select a suitable Vgs limit you can just about set a Current limit on the FET. It is this Vgs v Id that is the main short term protection for the FET. Continued current draw at this level then breaks the fuse.

Omit the Zener and the FET blows before the fuse.

You also need the Zener to catch any spikes that come into the gate.
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Old 29th December 2006, 03:33 PM   #42
tcpip is offline tcpip  India
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Added the two diodes in the supply lines to isolate the input and VAS from the OPS. Check D2 and D3.
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Old 30th December 2006, 12:22 AM   #43
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Quote:
Originally posted by tcpip

You mean I cut the negative rail between the bottom of R22 and the bottom of R26, and insert this resistor?

On this subject, I was wondering about the entire idea of doing additional decoupling of both rails between the OPS and the VAS. Is this a good idea? So few amps seem to do it.

If I imagine that the input power supply flows into the circuit near the OPS and then flows from right to left towards the VAS and then towards the input, then the maximum surges of current are in the OPS, and the VAS and input stages get the side-effects of this. Is it a good idea to cut each supply rail between the OPS and VAS, and put in an RC there?

Taking the idea one step forward, since the OPS will occasionally sink huge amounts of current, maybe there will be momentary current flow depletion for the VAS and input stages. So, to prevent this, maybe we could fit diodes (1N4001?) before the RC filters. That way, the supply current would flow in and charge the cap, but would never flow out.

What do you think? And if yes, then what values of RC make sense? We can estimate the current draw of the VAS and input stages by looking at the CCS resistors I guess.
I think that you are absolutely right in every aspect here. Choose rail resistor so that VAS and input stage do not "lose" too much volts. If your input stage and VAS consume 8mA or so, than with 330R in rail they will lose only 2.5V. If it is too much for you choose lower value.
Caps as big as possible


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Old 30th December 2006, 12:26 AM   #44
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Default Re: Managed to add OPS drain resistors

Quote:
Originally posted by tcpip

You can see the four power resistors. I've added one vertically (R34). This one will almost certainly have to be fitted on the underside of the PCB. (This of course implies that I'll need that much empty space below the PCB in my chassis.) The other three may either be fitted on the top surface, raised by a quarter inch, or on the bottom, as is found convenient. The inductor (marked "L1+R") will be fitted on top, raised by half an inch or even more above the PCB.

What do you think?
Power resistors on drains servo no purpose, waste of space on PCB in my opinion.

Andrew
I think, that 'ground' from output stage decoupling should go back to star point, maybe I don't understand your question...
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Old 30th December 2006, 08:08 AM   #45
MiiB is offline MiiB  Denmark
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Nice work. TCPIP
Is it by any chnce possible that you could uplod the schematics and Brd files for eagle CAD. I have two rails of BUZ lateral fets and this amp seems well suited to put them to Work

Thanx

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Old 30th December 2006, 10:36 AM   #46
AndrewT is offline AndrewT  Scotland
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Default Re: Re: Managed to add OPS drain resistors

Hi Dark,
Quote:
Q.When output device bypasses are added, do they return to the ground side of the load?
What if the ground side of the load does not return to the PCB?
Quote:
Originally posted by darkfenriz
I think, that 'ground' from output stage decoupling should go back to star point, maybe I don't understand your question...
You understand perfectly.
the bypass has to connect from the supply side of the active device to the ground side of the load to complete the circuit for "hi speed current".
The problem is that the distance between these two bypass connection points is very large.
The distance will increase the inductance in the connecting links and that inductance defeats the capacitance and the attenuating ability of the bypass.
It seems to me that the problem is not solved.
Is there another place to return the bypass that will "do the job" other than the ground side of the load?
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Old 31st December 2006, 01:58 AM   #47
tcpip is offline tcpip  India
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Quote:
Originally posted by darkfenriz
I think that you are absolutely right in every aspect here. Choose rail resistor so that VAS and input stage do not "lose" too much volts. If your input stage and VAS consume 8mA or so, than with 330R in rail they will lose only 2.5V. If it is too much for you choose lower value.
Caps as big as possible
Can I then just assume that R1 drops 0.6V and R14 drops another 0.6V, and use those to calculate the currents for the input and VAS respectively?

BTW, why does it matter even if the R that I add drops 2.5V? Don't these amps run happily with a fairly wide range of rail voltages (as in +/-10%)? This amp, as given in Slone's book, is rated for +/-55V supply. Should a drop of 2-3V matter?

Secondly, will it be effective without the resistor? What about just using a diode and caps?
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Old 31st December 2006, 02:58 AM   #48
tcpip is offline tcpip  India
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Quote:
Originally posted by MiiB
Nice work. TCPIP
Is it by any chnce possible that you could uplod the schematics and Brd files for eagle CAD. I have two rails of BUZ lateral fets and this amp seems well suited to put them to Work
I'll certainly pass it on, but please give it a few more days to stabilise it. I'm making changes almost every day now.
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Old 31st December 2006, 03:01 AM   #49
tcpip is offline tcpip  India
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Default v5: with diodes and moved decoupling caps

I've managed to move the first set of decoupling caps (C11 and C12) close to the OPS, and there are additional decoupling caps (C3+C14, C4+C15) "after" the supply-rail diodes for the input and VAS. These are "near" the input section.

Click the image to open in full size.

Click the image to open in full size.

I'll now have to think of finding space for adding resistors "after" the supply-rail diodes.

The supply-rail diodes are marked as 1N4148 in the schematic. Please don't pay attention to that... I'll use 1N400x series devices. I just don't have appropriate packages in the Eagle library for 1N400x. And of course, cap values and resistor values are all up for discussion... they don't affect the PCB layout usually, so I'm going ahead with the layout. For instance, should C3 and C4 be 220uF or 1000uF, or something else?
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Old 31st December 2006, 03:22 AM   #50
jaycee is offline jaycee  United Kingdom
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There's loads of packages for diodes in the "diode" library, don't just look at the devices that specify a type.

There is a multiple package device in the "diode" library called "DIODE-" which has lots of packages. DO-27 size is probably most appropriate for flyback diodes.
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