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D.I.Y. amp

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Re: D.I.Y. amp

Postby Folderol » Sat Aug 01, 2020 1:12 pm

I'm almost sorry this saga is drawing to a close. It's been a very enjoyable (protracted) learning experience. Once the new front panel is ready, I'll have to move one internal partition, as a couple of controls are in a slightly different place.

I agree the placement of those sub controls is less than ideal. However, it's rather a case of "If I be goin' there I be-n't start from here."
When I started this project I had very little planned. Also, until it was well under way I was using stuff I already had, like the rather large heatsinks (which determined the size of the case) and the power transistors. The very idea of having the sub channel only occurred to me when I realised I had enough space on the heatsink, and enough transistors!

The way the layout works now, the front panel is the only practical place. There is no free space on the back. Almost all the mechanical work was done where I used to work, so I don't now have the engineering facilities for creating a 'pocket' behind a door, and besides, a lot of stuff close to the front would then have to be repositioned. The only other possibility might be to create a tray above the preamp board, which would be very tight for space and again require engineering. Access would then be by unscrewing and sliding back the top perforated cover.
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Re: D.I.Y. amp

Postby Hugh Robjohns » Sat Aug 01, 2020 2:30 pm

Fair enough, just a thought.
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Re: D.I.Y. amp

Postby ef37a » Sun Aug 02, 2020 9:07 am

Cracking project Will, be lovely when it is all done.

The fact that you can reduce iq to around 30mA tell me your 'evolved' circuit is in fact a form of CFP.
Did you also say the current was stable? Another feature of the configuration as OP and driver transistors are locked in a local feedback loop.

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Re: D.I.Y. amp

Postby Folderol » Sun Aug 02, 2020 3:44 pm

Well as it was previously it was sort-of stable. From switch on to running temperature there was a 50% current increase, that was quite rapid. Once there it didn't seem to change much whatever you did.
I don't really know what the situation is now as I don't have loads that can take full power for any length of time, and the music levels I can run into speakers (without the neighbours coming round with pitchforks) barely make any difference to the heatsink temperature, so there's no measurable increase in current :think:
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Re: D.I.Y. amp

Postby ef37a » Sun Aug 02, 2020 5:17 pm

An electric kettle makes a good load in a pinch.

Or 55W car headlamps, about 5 Ohms.

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Re: D.I.Y. amp

Postby Folderol » Mon Aug 03, 2020 2:11 pm

Interesting idea, but neither my kettle nor toaster would be much use, they both come out at about 55 ohms. I'd never risk a filament lamp of any type, they change their resistance dramatically with applied current, and I want to test the amp - not torture it!
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Re: D.I.Y. amp

Postby ef37a » Mon Aug 03, 2020 3:02 pm

Folderol wrote:Interesting idea, but neither my kettle nor toaster would be much use, they both come out at about 55 ohms. I'd never risk a filament lamp of any type, they change their resistance dramatically with applied current, and I want to test the amp - not torture it!

It should not mind a varying load if you just want to run it for temperature tests? Not, of course for distortion analysis!

What does the impedance curve of your intended speakers look like?

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Re: D.I.Y. amp

Postby Folderol » Tue Aug 04, 2020 4:46 pm

Well, I just had the bright idea of Checking RS, and they do 8 ohm 50W metal clad resistors for just over £6 which is the right value and much cheaper than lamps!

The amp is currently running into a pair of Monitor1 Mk2 nearfields. These are in fact 4 ohm, whereas the amp is characterised for 8 ohm. However, The amp manages quite well on them.
I can't find any impedance graph for the speakers, but the DC measurement is 3.9 ohm, which suggests they are pretty well managed.
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