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Old 19th July 2008, 05:35 PM   #1671
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Hi Graeme, hi Ian,

thanks a lot for your answers!

Some sentences from me again:

1) Graeme I don’t know if I have understood you right. Do you mean that the 10k resistors to the rails are directly connected to the outputs or like I have drawn them down only for AC (with an divider between them bridged with elcaps) ? Do you or Ian tested the shown circuit and got enough stability without the additional FE output ground resistors? When it is so then something works wrong in my amp.....!? I find the variation of the absolute offset to high without them..........grrrrrrrrr........

2) Shure, should be no problem to change the source resistor of the JFETs and try it. One reason for more current is for example the F4 or F5 with 6mA, also a B1 with something between 5 and 10mA. Other reason are the heat sinks for the shifters on the UGS module. There would be no need for them by a consumption of about 300mW (with +-50V rail voltage 600mW) per device. I would try it out, but I have no distortion analyser to find the best value. Feel I have to increase the JFET bias to 5,5 – 6mA for a level shifter bias of about 25mA.

3) There are only two main types which are used for the X-Amps. The older circuit which you can find in the X5 manual, and a much more simpler circuit used in the newer amps. They consist of a relay, resistor in series with its coil, a switching transistor with base resistor , a rectifier (don’t know if the XA.5 is using it) to rectify the external 12V (think for safety when 12V is reverse poled connected) with the minus pole connected with a resistor to ground and plus pole connected to the base resistor, and the front panel switch. I only can find one solution for the switch: In parallel to the collector – emitter. This workes in the ways written above.

4) OK, I should buy one per monoblock and when it is not enough I can add a second. My target power is 100W @ 8Ohm classA, 200W @ 8Ohm classAB .....
800W @ 2Ohm classAB should be possible for a short time with a 1000VA transformer (think it can shortly handle 1500VA)
I would take 2x25V secondaries. Is that enough for an peak output voltage of +/- 56V ?

Curious is that the used Plitron LONO transformers in all Passlabs amps are not the LONO types available on the Plitron web page. The offered there are much bigger for a given power rating. Does anyone know what a special type in the amps is used?

PS: Thanks Graeme for your nice words!  I hate written exams....

Regards Dirk
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Old 20th July 2008, 10:05 AM   #1672
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Default CCS working?

Hi Guys,

I am building an Aleph X. In the last steps of the process, progress grinded to a halt. I blew a range of precious matched mosfets, trying to figure out what was wrong.
Now, two months later, I traced one of the problems back to some broken capacitors (C3). My circuits started to oscillate as soon as the circuit started changing from voltage regulating (larger load) to current regulating (smaller load).

I thought I understood how everything worked (newbie attitude….) and started checking all separate parts of the circuit one by one. One part is still giving me trouble, the same CCS. I made my own PCB’s, so I am not 100% sure whether or not I have a problem.

What I wanted to do is test whether or not the CCS is working as it should be. I am testing a part of the circuit, just the CCS. The complete setup is in the sketch. During the test, only Rload is being changed. The power supply is CLCRC, so it will drop when more amps are used. That is why I included the Vtotal.

Measurements:
R8_________Vtotal______Vload______Rload_____=> Iload
0,321V_____20,05V_____2,835V_____2,977_____0,925A
0,307V_____20,09V_____1,791V_____2,078_____0,862A
0,288V_____20,15V_____0,875V_____1,162_____0,753A

(measurements after warming up. Retries a day later gave the same measurements.)

Conclusions: when I change the resistor load 256%, the current will change 23%.
1st question Is this a normal deviation?

Another problem. When I double Vtotal, the circuit shuts down. To get it conducting again, I have to change VR1/R11. I have the feeling that I will have the same problem when the Aleph X will have a full voltage swing.
2nd question Is this correct?

Thanks for all feedback!
Attached Images
File Type: jpg ccs.jpg (44.9 KB, 874 views)
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Old 20th July 2008, 11:35 AM   #1673
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Quote:
Originally posted by noisefree

3) There are only two main types which are used for the X-Amps. The older circuit which you can find in the X5 manual, and a much more simpler circuit used in the newer amps. They consist of a relay, resistor in series with its coil, a switching transistor with base resistor , a rectifier (don’t know if the XA.5 is using it) to rectify the external 12V (think for safety when 12V is reverse poled connected) with the minus pole connected with a resistor to ground and plus pole connected to the base resistor, and the front panel switch. I only can find one solution for the switch: In parallel to the collector – emitter. This workes in the ways written above.
Hi Dirk,

I don't understand your proposed switching circuit. If I have interpreted your description correctly the front panel switch would have to be a permanent (rather than momentary) type and would also override the external signal. I have to admit that I can't come up with a circuit that would work with so few parts. Do you think that Nelson perhaps uses a latching type relay? This might provide some additional options although I am not very familiar with these parts.

Ian.
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Old 20th July 2008, 02:20 PM   #1674
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Hallo,

quite sure that Mr. Pass is always using this relay type. Most time the same parts...
....and this part seems to be a favorite of him! I could imagine that he has stocked thousands...

I don’t wanne say that the circuit could’t be donne in many other ways.
It is more that I am a bit confused about the word “override” in the manuals.
The extern switch only works with leaving the intern switch open.
Perhaps I have overlooked a possibility where to connect this switch...

Dirk

The circuit has to look like this:
Attached Files
File Type: pdf switching circuit.pdf (85.4 KB, 261 views)
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Old 22nd July 2008, 06:52 PM   #1675
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Hi Dirk,

Thanks for posting the circuit of your proposed switching circuit. I agree that it will work even though I'm not sure it has quite the behaviour I might like. The problem is that the switch will have to be a permanent type, i.e. one that switches on with one push and off with another, rather than a momentary action pushbutton. Nothing wrong with this in some ways except that:

1. Momentary action pushbuttons are more easily obtainable in the required physical shape.
2. Using a permanent type (I am sure this is not the correct terminology but hopefully you will understand), will result in the external "switch" working only when the front panel switch is in the off position.
3. The front panel switch cannot turn off the amplifier if the external control is on
4. The amplifier will not default to standby when switched on unless the front panel switch happens to be in the off position.

Maybe none of this matters and it could be that the current PassLabs amps do work like this but it seems a little odd to me.

As an aside, I think I can see other possibilities for the location of the switch in your circuit but none that would improve the characteristics. For example, switching +ve to the transistor base (via the resistor) would also work in much the same way.

BTW, what is the purpose of the resistor in series with the relay coil? Is it to drop the voltage?

Ian.
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Old 25th July 2008, 03:13 PM   #1676
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Hi Ian,

thanks for your reply!

I agree with you in point 2) – 4). The behave of such a constellation is a bit unattractive.
It is not so important how you implement such a simple circuit. It was more like a puzzle for me to find a circuit with this parts which offers the right operation. This was the reason why I asked for the behave of the front panel switch in the commercial amps.
Hoped to get an answer from Mr. Pass or a X-amp owner...

Perhaps someone can tell me....

And yes Ian, the series resistor is there to drop down the voltage to the right working value.

I will come up with a more important question for me in 15 minutes. Need a bit time for phrasing it...

Regards Dirk
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Old 25th July 2008, 03:45 PM   #1677
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Hello again,

I know there was a discussion about the influence of the virtual ground to ground resistor some posts before. But sometimes it is difficult for me to understand such explanations in english.
So I give it a try to resume how I have understood the function of this resistor and hope to get some answers if it is the right interpretation:

The resistor acts in this way that it reduces the input signal for the same factor as it enlarges the gain. The result is that there is - because of the enlarged gain - lesser open loop gain left over for the feedback.
I will give you the following example with an op-amp with 60dB open-loop-gain (in the audio frequency range):
When I add a 15k input resistor to its negativ input and a 300k feedback resistor between its output and its negative input then the closed loop gain is set to 26dB. This results in 34dB (60dB – 26dB) applied feedback.
Now I add a 15k virtual ground to ground resistor and the following will happen:
The input signal is attunated with -6dB (15k / (15k + 15k) = 0,5) and the amplifier amplifies with 32dB (300k / (15k||15k) = 40). The result is an overall input to output gain of 26dB (-6dB + 32dB) but only with 28dB (60dB – 32dB) feedback.
Is that the right description?

When it is, I would like to know if this “trick” has some negative effects when I use it more extreme?: input R = 15k, feedback R = 300k, virtgndgnd R = 1k, open-loop = 60dB. Theoretically this would result in only 10dB feedback! Practically I will make some measurements tomorrow...

Regards Dirk
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Old 25th July 2008, 03:53 PM   #1678
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Hi Dirk,

I don't know whether this is right for sure but it matches my own reasoning. I've also tried this on the simulator which seems to support our reasoning. Chef did a more formal mathematical analysis in a previous post but unfortunately the paper was only in French which didn't help me a lot.

I don't see any particular disadvantage to using a low value of resistor, consistent with the amount of negative feedback you wish to apply. I beleive this is precisely what Nelson does to "throw away" some gain.

Interestingly I was planning on using a value around 1.5K for this resistor myself but I haven't tried it in practice yet.

I look forward to hearing about your results

Ian.
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Old 25th July 2008, 06:44 PM   #1679
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It seems that there is generally an "optimal" feedback figure
for sonic performance of any given circuit. With the resistors
to ground, you can trim that value.

Also you must consider the range of possible source impedances,
including the reactance at high frequencies, and resistance to
ground at the Gates allows some delimiting.
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Old 28th July 2008, 06:27 PM   #1680
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Thanks a lot for your answer!

Have done a lot of measurements with different virtual gnd to gnd resistors and FE output to gnd resistors at the weekend.
Will come up with some results, but need some time to try other things out.
Till then I have the following question:
When I meassure the open loop output impedance of one follower pair biased with 250mA, can I emanate from that the open loop output impedance of ten paralleled output pairs biased with 2,5A overall is than a tenth (both cases under the same supply voltage)?

Dirk
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