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Old 25th January 2014, 12:03 AM   #91
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Default Module set-up

Hi guys,

For those who have received or will be receiving my boards here's some information on calibration / setup. Please note that it is possible set the adjusters in such a way as to cause problems (as in minimum resistance for any one, combination of two, or all three); this layout was not intended to be fool-proof. Perhaps a future revision may be more forgiving in this regard. Be certain of your starting points with the adjustments and there should be no issues. It is helpful to have at least two multimeters available if possible, one to monitor VAS current and another to monitor DC offset.

The two trimmers for the CCSs VR1 and VR3 will have two effects, first is to control VAS current and second to null out DC offset. They are interactive, you will be going between the adjusters to bring the modules into adjustment. The trimmer VR2 is just for setting quiescent bias in the OPS.

1) Pre-set the CCS trimmers VR1 and VR3 to 270Ω as a starting point.

2) Set the bias trimmer VR2 to maximum resistance so as to yield minimum bias current.

3) Insert an ammeter into the +V supply to measure total module current draw. As an alternative you can measure voltage drop across resistors placed in both of the +/-V supply lines. I'd suggest perhaps 10Ω in each supply line and measure across one of them with a volt meter.

4) Power up with a limited supply that outputs your target supply voltage and verify there is no fault; some DC offset is likely to be present since we haven't yet adjusted anything but it should be no more than a few tenths of a volt.

5) Turn the bias trimmer to set up some bias current though you won't need full bias just yet. Perhaps 30mA to 50mA total module draw to start. Having some bias seems to make DC offset adjustment easier.

6) Measure across either R13 or R14 (10Ω in the VAS emitter circuit) to determine VAS current, every 10mV will represent 1mA. The target range will be anywhere from 12mA to 16mA, so we want to achieve a voltage reading from 120mV to 160mV.

7) Monitor DC offset at the output.

8) Adjust VR1 and VR3 such that you achieve VAS current in the desired range and the lowest possible DC offset at the same time. Clockwise should reduce current, counter-clockwise should increase it.

9) Place module on a normal power supply. Adjust bias current to the desired level with VR2, I suggest a total module current anywhere from 120mA to 160mA would be appropriate. Recheck VAS current and DC offset, adjust if necessary.

10) Allow module to reach equilibrium and as a final check ensure DC offset , VAS current and total module current are within tolerances.

Then enjoy the music. Hopefully this helps give builders a trouble-free set-up.
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Old 25th January 2014, 12:11 AM   #92
Sheldon is offline Sheldon  United States
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Quote:
Originally Posted by jkuetemann View Post
Just a reminder that it would be really nice to get feedback when your packages arrive. So far:

still4given - 2 - Delivered
ivanlukic - 2 - Shipped
quan - 2 - Shipped
triplej - 2 - Shipped
baswamin - 2 - Shipped
potepuh - 2 - Shipped
spind - 2 - Delivered
brianco - 2 - Shipped
Sheldon - 2 - Shipped
Got it. Have never had a shipment lost, and I'm not young. Sheldon
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Old 25th January 2014, 03:44 AM   #93
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Originally Posted by ivanlukic View Post
Evidence is accumulating in Serbian diy forums that two bipolar transisitor CCS for the VSSA input does not work well with this particular topology. Simulations show that transistor + LED work much better. Removal of two transistor CCS was suggested.
Any chance of more info? One possibility is thermal stability. As you mention simulations I'm excluding sonics. Also over on the CFA / VFA thread there is suggestions that a CCS and a mirror is better for PSRR and thermal drift. All things that can be experimented with on this board IMHO.
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Old 25th January 2014, 06:45 AM   #94
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Originally Posted by triplej View Post
Any chance of more info? One possibility is thermal stability. As you mention simulations I'm excluding sonics. Also over on the CFA / VFA thread there is suggestions that a CCS and a mirror is better for PSRR and thermal drift. All things that can be experimented with on this board IMHO.
I got this information in PM. As soon as I receive more info, I shall post.
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Old 25th January 2014, 06:58 AM   #95
BYRTT is offline BYRTT  Denmark
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Quote:
Originally Posted by ivanlukic View Post
Evidence is accumulating in Serbian diy forums that two bipolar transisitor CCS for the VSSA input does not work well with this particular topology. Simulations show that transistor + LED work much better. Removal of two transistor CCS was suggested.
Quote:
Originally Posted by jkuetemann View Post
What would the technical reasoning be? The 2Q CCS should be better than 1Q+LED even though that arrangement in itself isn't bad either.

As a thought, it would not be difficult to sub in an LED in place of one of the transistors and adjust the resistance to get the current at the correct level should one desire it.
Quote:
Originally Posted by triplej View Post
Any chance of more info? One possibility is thermal stability. As you mention simulations I'm excluding sonics. Also over on the CFA / VFA thread there is suggestions that a CCS and a mirror is better for PSRR and thermal drift. All things that can be experimented with on this board IMHO.
As a fact seems LC use for his high power VSSA (FET+LED). First picture (First One - mosfet amplifier module).
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Old 25th January 2014, 10:31 AM   #96
brianco is offline brianco  Ireland
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PCBs arrived safe and sound about 10 mins ago. Look to be superb!

Many thanks.
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Old 25th January 2014, 12:26 PM   #97
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Default Wow, first overseas arrived.

A few more confirmed...

still4given - 2 - Delivered
ivanlukic - 2 - Shipped
quan - 2 - Shipped
triplej - 2 - Shipped
baswamin - 2 - Shipped
potepuh - 2 - Shipped
spind - 2 - Delivered
brianco - 2 - Delivered
Sheldon - 2 - Delivered
__________________
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Old 25th January 2014, 04:26 PM   #98
egra is offline egra  Hungary
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I think two questions arise about the ccs:

1. If we assume a really good PSU, does VSSA really need any kind of ccs to the optimal sonic performance? Shaan's answer is clear no. He used ccs in an experimental version of PeeCeeBee (not mentioned what type used) and the result described as "something strange" or "clinical" His opinion is that vssa sings perfect without ccs, and the music appears to be free of any limitation. Lazy Cat has totally opposite experience: the difference is like "day and night" benefit of ccs.

2. If yes it is not so clear to me, what is the correlation between the applied ccs and the sonic quality. Theoretically 2 Q's ccs is better than any other in respect of thermal stability, accuracy of FB, lower THD, etc.(see Rod Elliott's article e.g.) and very close the ideal constant current source though some builders found that bjt+led work better with this topology as ivanlukic mentioned above. How these collerate to the listening impressions. Has anyone experience with other type ccs, like TL431?

Thanks
egra

Last edited by egra; 25th January 2014 at 04:29 PM.
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Old 25th January 2014, 04:34 PM   #99
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I had done the basic 'Shaan' style PeeCeeBee and liked it very much. I was looking to explore different avenues with the VSSA 'core' and this represents a start towards my evolution of the concept.

As far as the CCS goes, the simulations on my end don't seem to show much, if any, difference between a 2Q CCS and 1Q+LED, but I'm no expert either.

I'm also following ostripper's work as well and see some good things going on there that might be worth looking at incorporating into a future revision.
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Old 25th January 2014, 05:33 PM   #100
egra is offline egra  Hungary
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Thank You Jason.

I'm currently working on two other projects, once I have completed them I will try a VSSA variant and yours looks fine to build.

Regards
egra
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