Aleph-X: High-Power Version

[Rob Dingen]

Hello

I posted this one in the original Aleph-X thread but nobody cares so I think to post it here
What do you think from the idea to build the mini XA and ad a bunch of source followers behind it to get enough power and if you wish less heat

Rob

[GRollins]

I'm not clear as to what you mean by mini XA.
If you mean the Mini-A, then yes, you can add more devices if you want. The wattage output will be limited by the rail voltages (and bias), however, not the number of devices. Given the size of the amp as drawn, there's not much point in using more than two pairs of output devices unless you're using really small devices or intend to drive really low impedance loads. Besides, if you go to more than two pairs, you might as well build an Aleph 3.
If you mean the Aleph-X, then again the answer is yes, you can add output devices. That is--at least in part--what this thread is about. Like the Mini-A, ultimately the power will be limited by the rails and bias current.
In either case, increasing the number of output devices will not (by itself) decrease the amount of heat the amp puts out at idle, although you can back off on the bias each device sees and the individual MOSFETs will run a bit cooler.
Adding followers isn't exactly what these amps are about. They're adaptations of Nelson Pass's Aleph current source circuit. As such, you need to view the pairs of output devices as a functional unit--to be added as a set. In the Aleph-X, you'll need to add the same number of output pairs to each side. The original Alephs (including the Mini-A, which I set up to be compatible with the 'real' Alephs) can take one set at a time.
If you want more power than the Mini-A offers, why not build one of the real Alephs? Either the Aleph 3 or the Volksamp 30 offer 30W/ch. As for the Aleph-X...well, it's kinda off by its lonesome. I jumped the gun on availability of official schematics. I've left a trail of bread crumbs for those who want to take a whack at rolling their own, but there's not a full product line, so to speak, just the original 40W version and I'm chipping in a bit on a 100W here in this thread.

Grey

[Blitz]

Ok, To give this threat some new life: Here you find a choke-input-configurationn for the previously posted 100W at 6 ohm comfiguration with 19,5V rail voltage and 6,3 A. Obviously you you lower the 26 V of the transformer to 24V-25 V if you would use Schottkys.

The configuration is by purpose a choke-input which in my experience sounds better and here you see as well why: The peak current for the transformer and the bridge rectifier is with 8A very low compared to a pi-filter. If you want to go with a pi-filter, tha simulate it your self with psu-designer from www.duncanamps.com.

Best Regards

[Lucas_G]

Hi Folks,

I have been following this thread for a while, but want to ask one thing.

In the normal Aleph-X thread there have been some designs of PCBs by members.

My questions are:

Which board by whom is under production?

Can one still order these PCBs?

Which board would be suitable to use for an Aleph-X high power version?

Any info on this appreciated!

Regards,

Lucas.

[Ian Macmillan]

I see there are still few comments on the actual circuit posted by Blitz so here are a few more of my own.

I'm unsure of the purpose of resistors R6 and R14, presumably these are intended to promote current sharing between the output MOSFETs. I don't think I've seen these in other variations of the Aleph-X and I'm not sure whether they are really required. Including them with have the undersirable effect of reducing the open loop gain. The additional voltage drop will also require the input diff pair current to be increased or R23 increased in value.

Having run a quick simulation, R16, R18, R27 and R30 seem a little large. Using the values suggested will result in a high frequency rolloff starting at 10KHz. These resistors and the value of R23 in conjunction with the gate capacitance of Q2 have the biggest influence on high frequency response.

The power dissipation in Q6 will be quite large (about 312mW) so this may well require a hear sink if a ZVP3306 is used.

I am attaching the circuit I simulated. This is running at the suggested bias (about 1A per output device) and with an input diff pair tail current of 25mA. I've reduced the feedback and input resistors to give a better high frequency response but with the same gain. Not that I would use the LM329 in preference to the zener shown but I do not have a model for the former. I've also reduced the value of R19, R29 to 10k as suggested elsewhere in the Aleph-X thread.

Thoughts?

Ian.

[Lucas_G]

Originally posted by Blitz
Ok, To give this threat some new life: Here you find a choke-input-configurationn for the previously posted 100W at 6 ohm comfiguration with 19,5V rail voltage and 6,3 A. Obviously you you lower the 26 V of the transformer to 24V-25 V if you would use Schottkys.

The configuration is by purpose a choke-input which in my experience sounds better and here you see as well why: The peak current for the transformer and the bridge rectifier is with 8A very low compared to a pi-filter. If you want to go with a pi-filter, tha simulate it your self with psu-designer from www.duncanamps.com.

Best Regards

With such a choke-input you are giving a way a lot of the initial voltage, aren't you?

Doesn't it mean that a lot of heat is dissipated by the first choke?

In what way does such an arrangement sound better?

Is the huge value of the input-choke neccesary?

Thanks for the info!

Regards,

Lucas

[Blitz]

Ian,

Thanks for your comments. Let me see, just to get it right:

I understand that you mean the 220K as feedback-resistors (R16 & R30) which are a bit high and the input-resistor of 22K which are R18 & R28 (not R27) right ? In the meantime I have read some comments from people who have built it and that 10K sounds indeed better as it gives more resolution than 22K, so I have changed that. OK, the 220 K resistor should decrease the feedback, but I can change it back to 100K as well. Why did you increased the caps across it ? I believe everbody reported better squarewave with 5pf instead of 10 pf or 18pf.

Are you shure you meant R6 and R14 ? Just to be sure that we mean the same resistor: Which values do they have ? They have been always around...

A 100nF across the zener would be as well not bad I guess. Could you check component values with using a BZX79-6,8 as this lower voltage is what the lm329 would deliver (6,9V) ?

In the Aleph-X threat Nelson gave some hints how the front-end could be improved and pointed to the usage of jfets to get noise down. What are your thoughts on this ? Would it not even sound better to use jfets instead of mosfets in the input ?

Potential candidates would be the P-complements of the well known 2sk179 and 2sk389 (dual type) which stand up to 20mA. Please find the datasheets of the complements here:

http://www.schuro.de/Daten/Japanhalbleiter/2SJ74.pdf

http://www.schuro.de/Daten/Japanhalbleiter/2SJ109.pdf

Some very experienced guys like Mr. Borberly ( www.borbelyaudio.com ) anyhow point out that jfets are the best sounding devices, so combining them with Nelson's / Grey's circuit, well wouldn't this be a Super-XA ?

I used paralleled 2sk170V to get super-quiet current-sources in my preamp as well and sounds great.

Ian, Do you have some Models on these devices to see how much needs to be changed in order to ustilize them in the current source and the differential pair ?

Best Regards

[Blitz]

Lucas,

I see that we posted in parallel. To answer your questions:

With such a choke-input you are giving a way a lot of the initial voltage, aren't you?
Yes, but nothing is for free. In a pi-filter you "get" more voltage, but you increase the current rating of the transformer as well.


Doesn't it mean that a lot of heat is dissipated by the first choke?
No, this is not lost energy as we transformer voltage and current only differently than a pi-filter. But the choke should be of good quality not to generate any vibrations.

In what way does such an arrangement sound better?
Instead of up to 45A current peaks, you have only 8 A peak in normal operations, so much less stress and HF-garbage coming from the diodes and transformer. Less technical sounding, less harsh. Less solid-state-sounding.

Is the huge value of the input-choke neccesary?
Yes, maybe you can get a bit down, but I wanted to be on the safe side. Here are dozens of threats how to calculate the min. inductance. Search for choke-input.

Best Regards

[Bricolo]

Originally posted by Ian Macmillan
I see there are still few comments on the actual circuit posted by Blitz so here are a few more of my own.

I'm unsure of the purpose of resistors R6 and R14, presumably these are intended to promote current sharing between the output MOSFETs. I don't think I've seen these in other variations of the Aleph-X and I'm not sure whether they are really required. Including them with have the undersirable effect of reducing the open loop gain. The additional voltage drop will also require the input diff pair current to be increased or R23 increased in value.

Having run a quick simulation, R16, R18, R27 and R30 seem a little large. Using the values suggested will result in a high frequency rolloff starting at 10KHz. These resistors and the value of R23 in conjunction with the gate capacitance of Q2 have the biggest influence on high frequency response.

The power dissipation in Q6 will be quite large (about 312mW) so this may well require a hear sink if a ZVP3306 is used.

I am attaching the circuit I simulated. This is running at the suggested bias (about 1A per output device) and with an input diff pair tail current of 25mA. I've reduced the feedback and input resistors to give a better high frequency response but with the same gain. Not that I would use the LM329 in preference to the zener shown but I do not have a model for the former. I've also reduced the value of R19, R29 to 10k as suggested elsewhere in the Aleph-X thread.

Thoughts?

Ian.

what is I1, concretely?

[Ian Macmillan]

Blitz,

We seem to be in agreement regarding feedback and input resistors. I used 18pf across the feedback resistors because this gives the flatest frequency response according to the simulator (which may or may not be the case in practice). If others have measured a real life circuit and found 5pf to be better than that is the value to go for.

The resistors I was questioning are marked as R6 and R41 in your schematic and are 0.47R (or 0.68R in my version). I don't think I've seen these used in the gain stage although obviously they are required as part of the Aleph current source.

Sorry, didn't mean to omit the cap across the zener.

Not sure about the use of JFETs for the front end in this application. They are better in low noise circuits for sure, but in this application they have less transconductance and hence may lead to a higher overall distortion figure. Also the maximum disspitation of 2SK389 is about 200mW so operation at the required current is not likely to be possible without paralleling several. Lower current means larger values for R23 which will lead to a poorer high frequency response. I'll try playing around with these on the simulator but I'm not that hopeful.

Bricolo,

I'd be happy to give you a concrete value for I1 if you tell me to which current this corresponds.

Ian.