AX100 100W Aleph-X Monoblocks

[steenoe]

Thanks for sharing with us, your findings Graeme. Very useful as I am in the process of adjusting my A-X's. I think I will solder in the 10k resistors now when the amps taken apart anyway. I wasnt quite aware that the absolute DC offset take so long time to settle. I was a bit put off to find 11V initially, thinking something was wrong, but I will try and let it cook for while longer.

Steen

[gl]

Hi Ian,

I agree regarding the 30 ohms. It's just that NP uses 100 ohms and I just didn't feel like wasting the current. 30 ohms is fairly significant wrt 8 ohms.

I am a bit surprised that so few people have responded my last post. It's been almost four years since the Aleph-X thread started and there have been at least four different board sets for sale. That means there must be a large number of amplifiers running out there. To my knowledge only Grey and one or two others have built amps without these MacMillan resistors. You would think this would be a topic of interest. Perhaps I should have posted this particular discussion on the Aleph-X builders thread. What do you think?

There is a mistake in my post. I said below 30Hz regarding the improved bass. I should have said below 45Hz.


Hi Steen,

Good to hear from you. Let me know what you hear when you try the different resistor values.

I found that when I changed from 4.7K to either 10K or 22K I had to go throgh a complete re-adjustment cycle on the absolute DC offset. In fact after I removed the 4.7K resistors and had no resistors in place I turned on the amp and read a solid 25V output. I realize now that I should have tried to do an adjustment and listening cycle with no MacMillan resitors in order to establish a real baseline. Maybe I'll go back and try that. For now I'm having too much fun with the music.

Regards,
Graeme

[macka]

Yeah well I recall it being discussed but I don't remember all the details and the conclusion on those magic resisters.

I am getting some new boards soon so I will definately try your modifications.

You could post a link in the builders thread..I am sure it would be appreciated!

macka

[wuffwaff]

Hi Graeme,

I also did some experimenting with the McMillan resistors but only went down. From 4k7 to 3k3, trying as low as 2k7.

When I did this I was mostly concentrating on absolute DC Offset and it´s behaviour over time and did no special listening tests with the different settings (I did listen but didn´t notice a difference)

At the moment the dc starts at 5-6V and goes down very rapidly to 1-2 volt and then to almost zero over the next hour.
I´ve used 78R for the output resistors (5 x 390R).

I will put in 10k in the near future and have a listen.

Thanks,

William

[CheffDeGaar]

Hi Graeme,

I just did as William. Most concentrated on offset, and didn't perform listening tests Too bad. Was just living happy with my 4K75, but you raised some doubt... I won't sleep well the few next nights Till I make some tests...

But I'd like to understand how the IMMR (Ian McMillan Resistors ) can be involved in the frequency response of the amp. Does anyone has a clue ? Is it a kind of bypass for the input capacitance of the input mosfets ? Assuming the whole amp is perfectly matched and symetrical, the IMMR nodal point is a virtual ground at AC.... Do I miss something ? Need to dig it a bit, but brother, can you spare a hint...

Among the other things I'd like to test is to try another input diff pair, such as Zetex mosfets (ZVP33xx). Really love these little thingies for their low capacitance. But need high currents to keep a low output Z of the first stage (low drain resistors, but 4V needed), so cascoding would be necessary here to lower dissipation. Has anyone experimented ?

So many things, so little time...

[Algar_emi]

Great amp. Thanks for sharing your final finding. What is the VA rating of your transfo?

[gl]

Thank you to everyone for posting. I have another correction. The key comments by NP including the "going to hell" one is on page 34 of the Aleph-X page. It seems to me that NP was trying to tell people (loudly) to use moderation with regard to the IMMR's but everyone was fixating on DC drift.

William,

I look forward to hearing what you think. I have always appreciated the advice and help you've given others here.


Cheff,

I didn't mean to disturb your sleep. The input is a virtual ground. The output node is very low impedance. The diff pair sources by comparison are a high impedance node set by the CCS. The IMMR's can be seen as being in parallel with this impedance and can lower it quite a bit. IMO whatever process is suppressing the absolute DC drift can reach up into the low bass region if too mach feedback is applied from the output. But I'm just speculating. And if you do any experiments please post the results.

I have a bag of ZVP3310's as well. Very nice parts. Even 6 volts at 10ma is only 60mw. That's fine for a mosfet that's rated for 600mw. Then use your existing 9610's as the cascode transistors. Set the cascode voltage with a 9V zener and you're in business. Let me know how it turns out. You really didn't need to sleep did you?


Algar_emi,

It's a Signal 88-8 which has an 88VAC secondary at 8A. So I'm figuring 704VA plus 10% because of the 108V and 128V primary taps. My guess would be a conservative 750VA.

Regards,
Graeme

[hifimaker]

Originally posted by CheffDeGaar
Hi Graeme,

I'd like to understand how the IMMR (Ian McMillan Resistors ) can be involved in the frequency response of the amp. Does anyone has a clue ? Is it a kind of bypass for the input capacitance of the input mosfets ? Assuming the whole amp is perfectly matched and symetrical, the IMMR nodal point is a virtual ground at AC.... Do I miss something ? Need to dig it a bit, but brother, can you spare a hint...

Where can a guy get more information regarding MacMillan resistors? Where can they be found?

Thanks!

-David

[steenoe]

I have a few questions about the startup/adjustment procedure. Hope you can help out. When measuring the voltage over the source resistors in my A-X's, I get a reading of about 0,6V initially but the value rises. When it get close to 1V I get concerned and turn off the amp. The source resistors are 0,47r so I guess I should be able to adjust for 0,47V to have 1 amp flowing through each fet? I have 12 fet's per monoblock, and 1000VA, 2x18V trafo secondaries which gives just over +/-24V at no load, when the amp is connected to the supply, the rails drops to about 19V, I think thats a lot of drop?. I increased the value of R14/R31 and R15/R32 to 1,5k and kept the trimpots VR1 and VR2 (set to minimum resistance). Will the voltage drop over the source resistors settle down over time as the absolute DC does?
There is, as I wrote in the post above approx 11V absolute DC offset and there is next to no relative DC at the outputs. Seemingly the amp works and the heatsinks holding the outputdevices works too, thats for sure. The voltage drop over the source resistors are the same, both at the output devices and the CCS devices.
Gate to Source voltage is Approx. 4,6V on all the devices.
Should I be concerned about the rising source voltage (as I am) or should I let the amp heat up for a longer period (which I didnt risk at this point)?
Steen

[CheffDeGaar]

Originally posted by gl
[I didn't mean to disturb your sleep. The input is a virtual ground. The output node is very low impedance. The diff pair sources by comparison are a high impedance node set by the CCS. The IMMR's can be seen as being in parallel with this impedance and can lower it quite a bit. IMO whatever process is suppressing the absolute DC drift can reach up into the low bass region if too mach feedback is applied from the output. But I'm just speculating. And if you do any experiments please post the results.

I have a bag of ZVP3310's as well. Very nice parts. Even 6 volts at 10ma is only 60mw. That's fine for a mosfet that's rated for 600mw. Then use your existing 9610's as the cascode transistors. Set the cascode voltage with a 9V zener and you're in business. Let me know how it turns out. You really didn't need to sleep did you?

Hi Graeme,

Well, I did have a very good night, in fact

WRT IMMRs and impedance, I sure share your opinions... An influence of a parasitic capacitance somewhere ? Will perform some tests, but this won't be right now...

I do not plan to use a mosfet as a cascode element (sorry ). I'd like to get a decent Vds on the input mosfets, just to keep the capacitance as low as possible. I'm aiming at about 10V on the mosfets (so about 100mW), and use a BJT for cascoding, so I won't "loose" 4V in the cascode. Need enough swing on the drain resistors voltage (theoretically 20mA x Rd, so about 8V), and as my rails are @ 21V, fits perfectly for a bjt here Any particular reason for using a mosfet here ?

With a friend of mine, we're designing a pcb for the AX trying to allow for all this kind of modifications : Various current sources for the input pair, plugable input diff pair (IRF, ZVP, 2SJ, etc...) and so on, just to have an easy beast of burden for all our tests . And of course, we will report our findings here.

Cheers