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Old 8th December 2009, 04:20 AM   #521
AR2 is offline AR2  United States
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Quote:
Originally Posted by vitalica View Post
Please tell me , to Aleph JX what values are P1 and RP?
In alternate current source when using the J74 - C4, Z1, R8, R5 are used?
Which is scheme to connect the J74's?
Thanks!
I just re read the whole thread for the third time and realized that Rp value is not mentioned anywhere, nor is written on the schematic from Peter. Here is someone else that has the same question that I have, but never received the answer, at least not here on the thread. Somehow this seems to be the mysterious value, so if we post it here it will be the first time. If I am missing that post please correct me.

Thanx
AR2
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Old 8th December 2009, 05:40 AM   #522
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Quote:
Originally Posted by AR2 View Post
Hello Peter,

I am finishing assembly on my Aleph JX and I am kind of blanked out. For the CCS I am using j74. What is the value for Rp? P1 trimmer value is between 25 and 100 ohms, I believe based on EUVL's post - please confirm.
Next, I am not sure is what is going on with R5 - is this resistor used or not if I am using j74.
Since i am using j74 I know that I do not need to populate CCS on the right size of daughter board, and than in that case C4, Z1 are not used?
Please confirm,

Thank you very much
AR2
To refresh everybody's memory, I'm attaching proposed FE schematic for the X version.

The value of Rp/P1 was never discussed in depth, and since J74 as CCS was EUVL's idea, you need to contact him directly for any suggestions. In fact Rp is a buffer resistance for P1 and it's value will depend how much adjustment range you require there. You might not need Rp at all; I needed something to 'go over' a trace and thought that instead of a jumper additional resistor could be handy

I guess experimenting would be most recommended action here.

In case you are using J74 for CCS, Q2 is not needed and so all the other parts 'around' it, including R5.
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Last edited by Peter Daniel; 8th December 2009 at 05:59 AM.
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Old 8th December 2009, 05:47 AM   #523
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Quote:
Originally Posted by vitalica View Post
How do you, who are better resistors to Aleph JX : Holco or PRP?
I would suggest using Dale CMF50. They fit board nicely and sound pretty decent. I used them for F5 and didn't notice much difference when switching to Caddock/Vishays.

http://dkc3.digikey.com/PDF/C091/P1890.pdf
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Old 8th December 2009, 06:02 AM   #524
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Quote:
Originally Posted by Peter Daniel View Post
To refresh everybody's memory, I'm attaching proposed FE schematic for the X version.

The value of Rp/P1 was never discussed in depth, and since J74 as CCS was EUVL's idea, you need to contact him directly for any suggestions. In fact Rp is a buffer resistance for P1 and it's value will depend how much adjustment range you require there. You might not need Rp at all; I needed something to 'go over' a trace and thought that instead of a jumper additional resistor could be handy

I guess experimenting would be most recommended action here.

In case you are using J74 for CCS, Q2 is not needed and so all the other parts 'around' it, including R5.

Peter, thank you very much for the prompt response. I also contacted Euvil,
and here is his response sent to me. This will greatly round up your info and shed the light on the last part of I am expecting great project. Here is quote from Euvil's email:

" The exact value for the source resistor depends on the Idss. If the Idss is 8mA, the resistor value is 0.
I suggest you start with a JFET (J74) with Idss of around 10mA, and use a 100R trimpot to trim.
Once the amplifier is fully set, you should consider replacing the trimpot with fixed resistors. "

Thank you Peter and Patrick!
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Old 8th December 2009, 06:06 AM   #525
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That info helps me to when I finally get to finishing that amp, thanks.
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Old 8th December 2009, 09:37 PM   #526
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Default low voltage effects

I don't think this question has been fully answered yet and was wondering if someone could elaborate. Is there a performance penalty (i.e., loss of dynamics, lower wattage output, etc.) if the rail voltage supplying Peter Daniel's boards is lower than the specified +/- 24V? Is +/- 20V acceptable (the readings across power resistors is around .530 over .47Ohm, so current I believe is OK)? At what point is the voltage too low for the circuit to function properly? And what is too high (assuming you increase the voltage rating of power supply caps to say 35V, can you safely increase the voltage input to 30V, for example)? Thanks in advance for any/all insights.
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Old 9th December 2009, 02:36 AM   #527
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Originally Posted by algg View Post
I don't think this question has been fully answered yet and was wondering if someone could elaborate. Is there a performance penalty (i.e., loss of dynamics, lower wattage output, etc.) if the rail voltage supplying Peter Daniel's boards is lower than the specified +/- 24V? Is +/- 20V acceptable (the readings across power resistors is around .530 over .47Ohm, so current I believe is OK)? At what point is the voltage too low for the circuit to function properly? And what is too high (assuming you increase the voltage rating of power supply caps to say 35V, can you safely increase the voltage input to 30V, for example)? Thanks in advance for any/all insights.
My guess is that jfets are not going to like higher voltage. The lower voltage could be compensated with higher bias that flows through the output fets. I would say you do not want to go bellow 20V on the rails. My estimate is that you would not want to go above 26 - 28 V.
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Old 9th December 2009, 04:48 AM   #528
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You might actually be able to get away with a much lower trimpot value than 100 ohm (e.g. 50R or even 20R), but since I always recommend replacing trimpot with fixed resistors, it does little harm to start with 100R. The total dissipation on a 100R trimpot will be 6.4mW (assuming 8mA), which is nothing.

If however you are going to use +/-24V rails (or higher), the J74 will see (24Vx8mA) = ca. 200mW, which is quite a lot. So either you use a heatsink on the TO92 (recommended anyway), or (not exclusive OR) you use a resistor to drop the voltage seen by the JFET to 10V or so (meaning Rp =1.8k 0.5W, preferably 1W). Wire wound resistors will be just fine, since you want constant current anyway. So some inductance will actually help. You can also put a cap after Rp before trimpot to Ground for extra PSU filtering. I use 2200uF myself (only because I have lots of them).

If there is sufficient interest, I shall make a custom TO92 heatsink available, similar to my Dual JFET heatsink.

Toshiba Dual JFET Heatsink


Patrick

Last edited by EUVL; 9th December 2009 at 05:16 AM.
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Old 14th December 2009, 01:59 AM   #529
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As I am writing this report I am listening to Aleph JX. I am done with one channel, so that i could check if everything is OK.
Very nice sound and plenty of power. It nicely heats up the room, so that I do not need to use anything else for heating.
On Rp I used 1.2 Kohm instead of just short. Rg has to be more powerfull than 3W, for sure. I used what I had handy and that is 27 ohm 25 W, and that works well, no heat there. I had 33 ohm 3 W, there and they were getting pretty hot when turned on. The reason is most likely when for first time plugged, all the current was flowing through those two guys. I do not know how much that changes after the adjustment, didn't change it yet. I am little bit reluctant, based on how hot it was. So, my suggestion is - go with something much bigger for Rg and if you want change later once you have everything set up.
I was very careful in matching jFets, so my offset between the two is very well balanced. It would be really nice feature to add trimmer between the two jFet sources in order to align them if they are not well matched. It takes a while to fix the DC offset while the case is taking on heat and the temperature rises. With that I am talking about voltage between the outputs and ground. Voltage between the two outputs is very close to 0 so no worries there.
Thank you Nelson, Peter and Euvil!
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Old 14th December 2009, 03:43 AM   #530
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Power for Rg = Rail voltage square/Rg/2.
So if you use 24V rails and 33R, you need about 10W. If you use 100R you need 3W.
I use myself 100R Caddock MP930. I usually use a factor of 10 margin on rated power of resistors.

There is no reason to have trimmer at the JFET diff pair source, because it will only further reduce open loop gain and would not improve distortion cancellation. If only for differential DC offset adjust, it is easier to adjust the Aleph current sources. If the JFET match is not perfect, you will end up with a slightly different bias between the left and the right halves at the power MOSFETs, and hence slightly different heat dissipation. So get properly matched JFETs. They also give you better even harmonic cancellation as a bonus.

And BTW, my codename is EUVL, and not EUViL. The code EUVL has a specific meaning which is anything but evil.
You are of course most welcome to call me Patrick instead. I don't think there are many "Patrick"s here.


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Last edited by EUVL; 14th December 2009 at 03:49 AM.
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