Is a mini-Aleph using BF862 possible?

[njepitt]

I have recently been looking for a circuit for another class A amp to build, due to a couple of ideas I've been pondering;

1. I have built two or three "Le Monstres", with some success, as discussed in a thread on the Solid State forum, where I mentioned my interest in using modern equivalents for the (mostly non-availabl;e) original components, perhaps using SMD components, at least for the jfet stage. This idea seems to fail due to the non-availability of p-channel jfets in SMD format. I can get the 2n5457/2n5460 series, but these seem to be a poor replacement for the 2SK170/2SJ74 originally used.

2. I am interested in the Mini-A, or (in principle) any other cut-down Pass amp - the F5 I have in the living room sounds terrific, and I would like to find something around 8-10W (about the size of the original Monstre...) for my (much smaller) office.

3. I am interested in trying out something really single-ended like the Aleph-J, rather than the symmetric designs I've done so far.

4. The BF862 is clearly very highly regarded by people here, so it seems a good candidate, but doesn't have any p-channel equivalent, which rules it out for a "Monstre" redesign.

Putting all these thoughts together, I am wondering if anyone knows of a schematic for a 8-10W class A amp, single-ended, using maybe BF862s in a long-tailed pair (or something) at the input, probably mosfets in the output stage. Failing that, or something close, is anyone interested in playing around with the design of such a circuit?

Of course, if nothing turns up I may end up doing a Mini-A, which seems quite appealing, but the idea of a BF862 input stage appeals also. I should add I have read the BF862 preamp thread and am considering one of those also, but that's a little different. (And intended for the living room, not the office, anyhow...)

Thanks in advance for any input.

Cheers

Nigel

[Bigun]

Originally Posted by njepitt

Putting all these thoughts together, I am wondering if anyone knows of a schematic for a 8-10W class A amp, single-ended, using maybe BF862s in a long-tailed pair (or something) at the input, probably mosfets in the output stage. Failing that, or something close, is anyone interested in playing around with the design of such a circuit?

Nelson did a take on the JLH amp, I think he called it the PLH. Look up Nelson's paper on that, it's an interesting build option that allows you to adjust it from SE to PP (kind of).

[juma]

Originally Posted by njepitt

....

3. I am interested in trying out something really single-ended like the Aleph-J, rather than the symmetric designs I've done so far.

4. The BF862 is clearly very highly regarded....

Hi,
here you got Mini-A-J reconfigured to use BF862 as input LTP. Output devices are Fairchild's P-channel MOSFETs (you can use IRFP9240 too but they have a certain anomaly that produces a bit more distorsion).
If you are familiar with Aleph concept there is nothing really new here, just watch PS polarity

[njepitt]

Originally Posted by juma

Hi,
here you got Mini-A-J reconfigured to use BF862 as input LTP. Output devices are Fairchild's P-channel MOSFETs (you can use IRFP9240 too but they have a certain anomaly that produces a bit more distorsion).
If you are familiar with Aleph concept there is nothing really new here, just watch PS polarity

juma:

Bingo! That looks exactly the sort of thing I was hoping for. A very quick glance suggests it's like the Aleph J with all polarities switched (to accomodate the BF862, I presume) and with only one output pair. Did you already have this schematic figured out? Have you built it, by any chance?
Did I miss discussion of this somewhere?

I already have some questions:

1. Any idea of output power? Doesn't have to be very precise but I'll need some kind of estimation of heatsink size, for instance. An estimate like "about Le Monstre size" or "about F4 size" is good enough.

2. Any idea of bias current through the mosfets? My guess would be about 0.5A, but that is VERY unscientific. I hope it doesn't have to be biased as high as the F5 or something. Reading the Aleph J article closely would probably help - any other suggestions?

3. I gather the FQA12P20 is discontinued - I may still be able to get a few, but I wouldn't bet on it. Apart from standard IRFP9240s, any other suggested substitutes?

Bigun: I'll take a look at the PLH, but I think juma's suggestion is what I'm looking for. Thanks anyway.

Cheers

Nigel

[juma]

Originally Posted by njepitt

juma:
...I already have some questions:....

There is nothing to figure out about this schematic - it is Mini Aleph-J with reverse JFET's, MOSFET's and PS polarities to accommodate (as you noticed) N-channel JFETs on input.

Answers:
1. It is SE circuit (it can work in class A only - and somewhat more over it, thanks to the Aleph CCS) and its' power output is limited by load value. For 16 ohms speakers output voltage sets the limit; for 4 ohms speakers the limit is imposed by output current, for 8 ohms it's both. To find exact power figure, find Aleph power spreadsheet (wuffwaff made them I think) in old threads. Amp built by this schematic will produce about 8W at 16 ohms speaker, 14W at 8 ohms speaker; 18W at 6 ohms speaker and 13.5W at 4 ohms speaker

2. As it is drawn on the schematic bias current through output pair is 1.2A~1.3A - it is predominantly set by value of R16 (0R5 5W)

3. Any similar MOSFET can be used, it's just those from IR that have that anomaly.

Your questions indicate that you are not very familiar with Aleph amps, so read the old threads, Aleph service manuals and articles on Zen 2., 3. and 4. to learn more about Aleph CCS.

Have fun !

[juma]

2 things more:
R12 (56 R) should be replaced by 100 ohms pot - to set the DC offset,
Polarity of C3 (220uF) should be reversed,

[Dennis Hui]

It looks like you can still find some FQA12P20:

http://uk.farnell.com/jsp/displayPro...iew&CMP=e-b1ef

http://www.stkcheck.com/evs/fsc2/fsc...&part=FQA12P20

Cheers,
Dennis

[njepitt]

Thanks guys! I'm going to get busy reading and planning, and will let you know how things go.

Cheers

Nigel

[njepitt]

So I've been reading up on the Zen documentation and Aleph threads here, as juma recommended. I don't think I've "grokked" it all yet, but I'm getting there slowly, and will keep reading. Meanwhile I'm starting planning and component collection.

Originally Posted by juma

There is nothing to figure out about this schematic - it is Mini Aleph-J with reverse JFET's, MOSFET's and PS polarities to accommodate (as you noticed) N-channel JFETs on input.

I've searched here, and can't find any specific reference to "Mini Aleph-J", although it's like looking for a needle in a haystack when you remember just how much stuff there is on "Aleph-J" and "Mini-Aleph"... Did you have a specific schematic/thread/discussion in mind, or were you referring just to stuff in "Mini" jfet-fronted versions in general? I'd be grateful for any specific reference if you have one. (I realise it would be for p-channel jfets in the LTP, but it should be part of my reading if it's there.)

Answers:
1. It is SE circuit (it can work in class A only - and somewhat more over it, thanks to the Aleph CCS) and its' power output is limited by load value. For 16 ohms speakers output voltage sets the limit; for 4 ohms speakers the limit is imposed by output current, for 8 ohms it's both. To find exact power figure, find Aleph power spreadsheet (wuffwaff made them I think) in old threads. Amp built by this schematic will produce about 8W at 16 ohms speaker, 14W at 8 ohms speaker; 18W at 6 ohms speaker and 13.5W at 4 ohms speaker

2. As it is drawn on the schematic bias current through output pair is 1.2A~1.3A - it is predominantly set by value of R16 (0R5 5W)

That's plenty of power for what I have in mind - even smaller would be enough. I don't need a more precise number. This bias appears to be a little lower than the standard Aleph J, with half the number of mosfets, so is it reasonable to guess a little less than half the heatsinking is necessary? I have a nice heatsink which is 30cm x 20 cm with 38mm fins; in fact, except for the fins being 38mm instead of 48, all other dimensions match a 20cm length of Conrad MF30. Oh, and anodized black. Anyone like to guess as to whether it'll be enough? My rough calculations suggest yes, but not by a big margin.... (I don't have a mountable power resistor to measure it, and can't get hold of one easily...)

3. Any similar MOSFET can be used, it's just those from IR that have that anomaly.

The Fairchild part definitely has to be imported - I previously got two from Farnell-Newark in Sao Paulo for my F5, but they don't have any now, and if the part is now out of production they won't be getting any more. Importing to Brazil is a slow and expensive, so I'd like to see about alternatives. My local place has IRFP9140, but this seems a little different from the datasheet, and in any event, I think I read somewhere here that all the IR P-channel mosfets have the same problem (???). Does anyone have a diferent suggestion? I'd consider a lower power device if anyone has one in mind, since 8-10W into 8 ohms, say, would be sufficient. If no-one has other suggestions I'll just have to import.

On a related question, does the upper mosfet (in the current source - Q6 in your schematic) have to be the same as the lower one? In fact, it seems to me that the extra distortion in the IR devices is only important in the gain device, right? (I ask because I happen to have two IRFP9240s in my part box that I ended up not using in the F5... If I can track down two better devices for Q3 then problem solved...)

juma:
On a different topic, I read up on the adaptor boards you used in the BF862 preamp (which I also plan having a go at) and am wondering about "adapting" this idea here. Of course, there's no reason not to do a full PCB, I guess, but I like using proto board, and it has the advantage that you can fiddle with things more easily later, so I was thinking of putting the first stage (input and LTP) on a small PCB, and maybe the circuitry around Q1 and Q4 (which is just a voltage regulator circuit, right?). The idea would be to do the PCB with SMD components, as far as possible, and do the high-current (and high-power) stuff around the mosfets through-the-hole and point-to=point. The datasheet for the MJE243 says it's a 4A device, which suggests heatsinking and so forth, but how many of the resistors and things around it need to be higher power rated? SMD resistors at 1/8 W are probably OK for the LTP stuff, I'd guess, but maybe the ones around Q1 and Q4 have to be a little heavier duty? Any thoughts?

Your questions indicate that you are not very familiar with Aleph amps, so read the old threads, Aleph service manuals and articles on Zen 2., 3. and 4. to learn more about Aleph CCS.

Have fun !

I'm working on it! And fun is the main point for me... And as always, thanks for any help.

Cheers

Nigel

[Nelson Pass]

Originally Posted by Dennis Hui

It looks like you can still find some FQA12P20:

Last call is at the end of the year, so they can still be ordered,
although some distributors stop dealing with parts the
moment the notice is issued. We already had a bunch of
them on order and had to insist on the contracted delivery.