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Is a mini-Aleph using BF862 possible?
Is a mini-Aleph using BF862 possible?
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Old 19th February 2012, 12:26 AM   #241
njepitt is offline njepitt  United Kingdom
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Hi pisesd,

Thanks for the response.

Quote:
Originally Posted by pidesd View Post
first of all, what i would do is maybe substitute only the mosfets, with best matching with what you have, and see i'd hear an improvement.
The idea is really a complete rebuild, for reasons explained earlier in the thread, so it's not really practical to just switch the mosfets.

Quote:
You could also build with pairs and not triplets to get better matching...?
Yes, I wondered about this, but the question still stands - how closely should they be matched in such a situation? Besides, this would imply reducing total bias current, right? At least if I keep the source resistors and hence current through each, at the values juma suggested above...

Quote:
or maybe you could build an f5 . Buzzforb matches the fets specifically for this amp
Actually I'm rebuilding my f5 as well, although I'm staying with the mosfets I've got. The plan here is really a new and improved version of the amp juma posted near the top of this thread.

So, can I repeat my question - how closely would one want to match the mosfets in a situation like this?

Cheers

Nigel
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Old 19th February 2012, 12:59 AM   #242
pidesd is offline pidesd  Canada
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some people matches to within 1% and some don't care at all. whether you ll hear a difference is totally unpredictable and depends on many variables. ie ear,systems,own tolerance,other parts matching,topology,etc. In the present case, i m not sure sure how the circuit will accept mismatch. So i suggest you make listening tests with what you have available first and then decide if you want to find better matched devices. you can even try with single 2sj313 (if you dont draw lot of power from speakers).

but if you absolutely an answer, i d say 10%

i m not sure how you are set up but leaving ''space'' for experimentation is never bad

Last edited by pidesd; 19th February 2012 at 01:07 AM.
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Old 19th February 2012, 01:08 AM   #243
buzzforb is offline buzzforb  United States
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Quote:
Originally Posted by njepitt View Post
Hi Guys,

I'm still collecting parts for my rebuild of this circuit. I've decided to rebuild the PSU also, probably in the chassis, and I brought back a nice toroidal transformer from a recent visit home to the UK, which at 250VA should be enough for a stereo build (I hope). I also bought some thin film SMD resistors and stuff for the front end around the BF862s.

I've been thinking about pisesd's question about matching. I bought exactly the 12 2SJ313s I needed, not having thought about matching too much, and not wanting to buy too many, since they aren't that cheap. I'm hoping I can match adequately within the 12 I have, but am wondering how closely the triplets should match. Anyone have ideas?

Cheers

Nigel
Your fets were matched to within .1V. That should be very adequate if spaced in proximity to each other on the HS. In my Juma F5 with this matching level, I was able to achieve offset of less than 1mV at output.
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Old 19th February 2012, 09:47 AM   #244
njepitt is offline njepitt  United Kingdom
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Quote:
Originally Posted by buzzforb View Post
Your fets were matched to within .1V. That should be very adequate if spaced in proximity to each other on the HS. In my Juma F5 with this matching level, I was able to achieve offset of less than 1mV at output.
Hi buzzforb,

Ah, my apologies. I had the impression that the 2SJ/2SK pairs were matched, for use in juma's f5 variant, but I didn't realise they were matched *between* the 2SJs. The fets are still in the package you sent; I just opened it up again to take a look and they're all clearly marked, of course....

Sorry to waste your time, guys..

Cheers

Nigel
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Old 24th November 2017, 12:13 AM   #245
njepitt is offline njepitt  United Kingdom
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Default Resurrecting an old project....

Hi Everyone,

A happy Thanksgiving to all in the US. (And anywhere else where you're celebrating the holiday...)

So I'm revisiting this project, after five years of periodically remembering I bought most of the parts - and then doing nothing about it. I doubt anyone remembers this thread after so long, so let me briefly review the story. Juma kindly posted a schematic (in response to my query) for a voltage-reversed Aleph using BF 862 on the inputs, and P-channel mosfets on the output. After a lot of help from generous people here I got the amp working, liked it, and at some point decided to rebuild it with better hardware, more attention to details, and so on. I also decided (again, with input and help from folks here) to use paralleled 2SJ313 on the outputs, with three pairs per channel.

I've spent a couple of evenings trying kicad fo the first time, and designing a pcb for the project. I hope I've incorporated everything I learned last time around with the point-to-point build. Although it seems unlikely anyone else will want to use the design, I tried to make it fit the universal mounting specs from this site, just in case.

I'd be grateful for any input on the design, or any other comments.

Best

Nigel
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File Type: png test-brd.png (374.1 KB, 170 views)
Attached Files
File Type: pdf schematic.pdf (41.9 KB, 40 views)
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Old 2nd December 2017, 11:13 PM   #246
njepitt is offline njepitt  United Kingdom
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Hi Everyone,

I thought I'd post a slightly altered board design. Reading over the thread again I thought I should make the heatsink pad on the gate of Q3 the same size (or close to it) as the one on Q1. This should help to keep them at close to the same temperature. It may not make much difference, but it just seems to be a little more careful, and costs nothing...

No-one had any comments on the last post - so perhaps no-one is reading this - but I'll keep posting as I continue with the build. (Probably slowly...) Next step is to send the boards off for fabrication - something I've never done before, but there's a first time for everything,as they say...

Nigel
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Old 3rd December 2017, 10:35 PM   #247
njepitt is offline njepitt  United Kingdom
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A small problem. The large-ish copper pads on Q1 and Q3 are meant to act as small heatsinks. Previously I etched the one-sided boards at home these were just bare copper, but I'm planning on sending this board in to be fabricated, and I presume they'll be covered in soldermask... I've never sent in boards to be fabricated before, and I can't find answers on their website, so...

1. Is soldermask thermally insulating? That is, if they *are* covered in green (or purple) lacquer, will they still radiate heat OK? (My uninformed guess is no...)

2. Is there some standard way to ask for these *not* to have soldermask on them? I'm using kicad, and having some trouble figuring it out, but maybe I missed something...

3. If the boards have soldermask on these pads and I need to get it off, is there some sensible way to do it without otherwise damaging the boards?

Thanks for any input here...

Nigel
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