New amplifier...possibly..

[danielwritesbac]

Originally Posted by AndrewT

When buying components that cost pennies and are likely to be useful in future projects, try to take advantage of 25off, or 50off, or 100off, discounts. This saves enormously on future p/p costs.

When I find a low priced component really great or very useful, I'll just up and suddenly buy a hundred of whatever it is, so its not terribly hard to find later. The special trouble is 1/2w carbon resistors as there are never enough of those. Every time I've tried to back-fill all of the missing values, the online shopping would manage to mistakenly log out before I could buy them (I should have made inventory before shopping). Lately, with the Circlophone project, a 1w zener variety pack was useful and several excess transistors made it possible to HFE match, and that turned out to be terribly easy to do, which was a surprise.

[fooboo]

Hello there

Ordinarily I do over order. I do this with the regular parts I need for the voice mods I do which certainly pays dividends when an item has gone out of stock on next orders and I still have a few to bridge the gap.

In this instance I have been so focused I haven't done so. This may well be something I regret as I am getting the distinct feeling that building your own audio equipment could be somewhat addictive

Here's an image of a circuit I've cobbled together. It uses a bog standard 555 circuit as a bistable driver using a momentary switch. This is connected to a relay driving circuit, that according to the text, reduces it's current/voltage to the 'holding' level to reduce consumption and possible heat build up over prolonged periods.

This could be, as Andrew suggested, driven from a small transformer to control any 'extras' as well as control the main supply.

regards

Foo.....Still tweaking my PCB's

[fooboo]

Me again

More a PS than anything useful to add. The circuit above has a resistor/capacitor delay on the reset pin of the 555 to prevent the relay being switched during the circuits initial power up.

regards

Foo

[danielwritesbac]

Originally Posted by AndrewT

I don't know why National show 2r2 for the output Zobel. I have found that 10r works and 5r works.

The datasheet "resistance" value is in the range of 6 to 9 ohms. NatSemi is inferring using a typical lossy polyester dip cap for 2r2 to 2r7 series to 3r9 to 7r loss (ESR). It is the least expensive cap typical for output zobel and therefore it is expected of production amplifiers. Mass market manufacturers don't purchase polypro caps for outut zobel. Unfortunately the NatSemi datasheet is a bit misleading in some spots.

What is the smallest cap value that you would consider using for output zobel?

[aditya]

hey daniel how are you...

[fooboo]

Hello all

Well the spectrol 2K trimhave arrived today.

The day after a slightly disastrous OS install. Basically I didn't spot the install was flagged to my backup drive..............

Despite the loss of the editable lay outs I at least have all the info and pictures I posted here to work from.

So there will be a slight delay as I sort out this 'senior moment'.

regards

Foo

[fooboo]

Hello all

Happily I found some 'earlier' files of the PCB's on my netbook. Good enough to use to bring the Patterns back to what I had.

That said I became aware of some errors on the regulator PCB with regards to a couple of components.

I have now corrected and updated the both the rectifier and regulator. The layout is actually better I think. See the latest attached images. The rectifier is still being fitted under the regulator,Trace sides facing, to save space and wiring.

Imagine, if you will, that the line that separates the two circuits is a fold line. You can see that the power pads will line up as will the four support holes in the corners making for a neat set up.

Hopefully back on track now. Ignoring the four months 'work' I have lost...

regards

Foo

[fooboo]

Hi Chaps

The last item has arrived at last. My 160VA 30/30 Toroidal Transformer

I've never seen one in the flesh before now and was mildly surprised at the weight. Tis a thing of beauty

I have now finalised the PCB patterns for the regulator and rectifier and just need to get them printed off along with the amplifier pattern.

In the meantime I have been contemplating building my own speakers to go with the amp.

Currently I am very enamoured of the 'Cyburg's Needles'. The design being relatively simple with a small footprint. They are also referenced by a chap who is using a LM3875 based amp to drive them.

A pair of Visaton FRS8 2004 full range drivers are used by the same.

I've draw up a 1:1 scale sketchup of one.

Any thoughts on speakers?

regards

Foo

[danielwritesbac]

Originally Posted by fooboo

Any thoughts on speakers?

It is doable.
Lenard Audio - Education - Amplifiers.
Lenard Audio - Education - Guitar Valve Amps.
An amplifier chip that is usually a bit forwards, if not sometimes loud mids, along with a speaker type a bit forwards, if not sometimes loud mids. . . can be used together if the amplifier is adapted to current drive. Some full range type speakers work well with modern amplifiers, but that is not the norm. So, if you get nothing but a shout, try current drive.
Alternatively, there is a passive notch/bsc type filter favored by lowther owners. I tried it on my LM3886 and it worked perfectly well. Other thoughts about full range speakers. . . with superbly high efficiency and with x-max maybe at 1mm, that kind of speaker doesn't need 60 watts.

[danielwritesbac]

Other thoughts about that speaker. . .
With an extremely efficient 8 ohm woofer and a 1st order series crossover, you'd be able to direct the beating into the woofer. You'd be able to put a resistor and a little inductor series to the full range driver to simulate current drive. And you'd be able to use an economical electrostatic helper tweeter to maximize entertainment value. The type of 3-way speaker that utilizes a full range for the majority of the audio band, is my favorite. With this type, avoiding crossover filter noise means selecting a woofer with a tiny/smaller LE figure and then the low crossover point finishes the job of hiding the crossover noise. To say it another way, the higher the woofer LE figure (higher=bad), the lower the crossover point will need to be to hide the crossover noise. Fortunately a real full range as a midrange makes it easy to do, so that it isn't impossible to select an efficient woofer. Many prosound speakers are made with the wideband style, aka a full range with helper woofer and helper tweeter.