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Old 14th January 2008, 04:47 PM   #361
gl is offline gl  United States
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Hi Ian,

Thank you for the thank you. I return the thanks to you for all of the encouragement and contributions you have provided over the years. I am very pleased to hear that your amps are up and providing excellent sound. I look forward to seeing your project thread!

Hi Dave,

Thank you for the update on the sound of your system. There's nothing that provides encouragement more than hearing about someone else's success - except maybe your own!

No, I have not heard the new PassLabs AX100.5. I am still working on building a JFET front end for the existing AX100. The trick there is to come up with a design that uses obtainable parts. A major goal for me is to create an upgrade that others could implement.

I don't think that doubling up on transformers will necessarily solve the hum and buzz problems. Up to this point I have never built a power amp using toroids so I can't comment on the related noise issues even in a general way. And I see that others already have. There are a lot of threads on the forum on this subject.

Regards,
Graeme
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Old 15th January 2008, 12:11 AM   #362
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Quote:
Have you ever tried doubling the trannies to get quieter sound? I have to think of something, I called Plitron and Avel Lindberg and to buy quiet trannies is 4 times the cost and neither will guareentee quiet products.
I had the same problem with my Mini-A. I now use a Plitron at 4x the VA needed and have had good success with no mechanical hum.

JJ
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Old 17th January 2008, 10:35 PM   #363
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Graeme,

I think I have a handle on the parts.

If you can elaborate here on the implementation or send me a pm on what you are proposing.

I would like to compare the standard 9610 to the Jfet front end in the X aleph 100.

The 9610's sounds quite impressive and in direct comparison with the stock factory X250.5 there ain't a lot in it. The X aleph 100 mono blocks are perhaps tonally richer and have a deeper/broader sound stage.

I therefore wonder if a Jfet front end is worth all the fuss.

Macka
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Old 18th January 2008, 09:37 AM   #364
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Hi Macka,

it is worth all the fuss!

And most of the fuss is done allready. If you know how it is quite simple to change the front end.

William
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Old 18th January 2008, 05:05 PM   #365
gl is offline gl  United States
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Hi Ian,

OK. Here is a schematic of what I'm doing. I publish it with reluctance because I'm still in the middle of the fiddle as it were. It was my intention to start a new thread for this mod entitled "AX100J 100W... etc. etc.". I didn't want the project to be posted deep in middle of this thread. So I'm hoping this schematic will pass by largely unnoticed. Only a handful of people seem to be viewing the thread anymore so I may get my wish.


Here's a detailed description of the experiment:

1) I have left the AX100 as unchanged as possible. That means that the operating voltages and bias currents have been preserved as much as possible. All of the changes have been to the input diff pair and the input and feedback networks.

2) I have substituted two 2SJ109BL JFET pairs in place of the original 9610's. Each pair is divided between the plus and minus sides. I wanted to maintain the 20ma overall front end bias and the original CCS. The J109 has about one fourth the transconductance of the 9610 (@10ma). I knew that one J109 wasn't quite enough from watching the efforts of others and I'm hoping that two pairs will do the deed.

For all those people that are scouring the world for these parts it must seem like a really flagrant case of conspicuous consumption to be paralleling these parts. I apologize to you. It should be possible to implement this circuit with matched J74BLs.

3) The input JFETs have been cascoded. There are two reasons for this. First, it limits the power dissipation to roughly 40mW per transistor by limiting the the voltage across the JFETs. Some people like to run their JFETs really hot. I prefer to avoid the blazing Balkan barbeque effect (I guess it's because I don't want to choke on the smoke). Second, the cascodes reduce the tendency toward current hogging in the paralleled JFETs.

4) I have increased the input network resistors to 22.1K from 10K because these are the values I've seen in the PL manuals that have been publicly (if temporarily) released. (i.e. the X2 and X5). The same goes for the MacMillan resistors. The main feedback resistor is scaled up to 475K. But these are just starting points for experimentation. I can easily experiment with lower values by tack soldering resistors onto these parts.

5) I have removed the input capacitors to make more room on the board. The new board has to match the form factor of the old one. I will try tacking the capacitors back in at the end of the project just to see how the performance and sound are affected.

6) The input protection diodes are removed and so are the gate resistors on the diff pairs. One of the benefits of JFETs over MOSFETs.


Anyway that's it for now. If you spot any issues please point them out. The same goes if you think of something worth trying.

I agree with William wholeheartedly by the way. I've built (and bought) lots of audio gear over the years and IMO the pieces with the JFET inputs sounded best. There is just a level of detail and timbrel rightness that I don't hear with bipolar or MOSFET inputs. This is nothing more than one mans opinion. But just look at the enormous trouble people like Nelson Pass, John Curl, and Charles Hansen go to to obtain and use the special Toshiba JFETs they use on the inputs of their products. There is something there.

Regards,
Graeme

Here is the schematic. Ignore the notes in the lower right hand corner. I have not updated them yet.
Attached Files
File Type: pdf ax100j-00.pdf (97.2 KB, 830 views)
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Old 18th January 2008, 06:18 PM   #366
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The amplifier with parallel JFETs is definitely better.
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Old 18th January 2008, 07:22 PM   #367
EUVL is offline EUVL  Europe
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At the risk of being accused of suggesting the use of obsolete parts, why not use a pair of cascoded LU1014s at 100mA bias each for the diff pair ? You can lower the drain resistor to 39R, and still get enough gain, as the transconductance of the LU1014 at 100mA is about 1-2 S.

Or maybe one can ever get the LU1014 to work in triode mode for yet lower current, say 50mA ?


Patrick
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Old 18th January 2008, 07:29 PM   #368
gl is offline gl  United States
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It's a good idea in theory. But the Lovoltech parts are N-ch not P.

Graeme
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Old 18th January 2008, 10:36 PM   #369
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Hi Graeme,

Thankyou for posting in such detail.

As always well thought out!

On point 3 cascoding will help with the issue of additional input capacitance with parrallel pairs.

I will give you design modification a tryout.

To do this I will make a small daughter board (from vero board) probably using the 2SJ74 as the pin out with vero board may be earier to arrange. The 2SJ109 will require less matching but as we know they are few and far between and expensive.

If it is preferred to move this discussion to another thread by all means do so.

I agree a new board is in order but a mini daughter board may not be a bad idea for those who have built their amps.

Send me a message and I try and organise parts for you to try.

Macka
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Old 18th January 2008, 11:12 PM   #370
gl is offline gl  United States
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Hi Ian,

My original board is Vero board so making a complete new one is the best plan. It keeps the amps out of service for the minimum amount of time while switching over (or back).

Thank you for the offer of parts but I already have everything I need. I stocked up several years ago on JFETs.

The project has been making little progress since last summer but your posts have caused things to lurch back into motion. I take Nelsons comment above as very positive and encouraging.

One thing I didn't mention above is that I am changing to the tighter version of the diff pair CCS in order to make the absolute offset adjustment pot smoother and easier to deal with.

If you get there before I do please post your results! I'm at least three or four weeks away from light up.

Graeme
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