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Old 16th April 2009, 02:12 AM   #121
Bigun is offline Bigun  Canada
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I am within a few hours of giving up with toner transfer. It's clear that the professional TTP paper from Pulsar is NOT suitable for iron-on, it really needs the controlled pressure of the laminator they sell.

I tried the suggestion of glossy magazine paper. On a small sample, it worked pretty good in terms of transfer, first time. On the full size pcb I didn't get 100% coverage on two tries at it. I suspect the iron temperature needs tweaking up - I had to turn it down for the Pulsar paper. The magazine paper tends to jam in my printer too - it's a bit slippery. The coverage of toner from my printer doesn't look brilliant - they design them not to use up too much toner and there are no controls in the print driver. This begs the question of whether it will work even if I can get it to transfer.

I'm out of time today so I'll have to try again tomorrow. I have 'made' about two dozen boards so far !

Still, the delays allowed a small update to the pcb layout.
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Old 16th April 2009, 02:51 AM   #122
Bigun is offline Bigun  Canada
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And here's what I was referring to as an update to the base version:
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File Type: gif tgm1.6.gif (34.3 KB, 447 views)
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Old 16th April 2009, 03:49 AM   #123
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Gareth,
you know, I like your basic view on things and I`m pretty sure that some sunny day you will design and construct good sounding amplifiers.
An active current source for the LTP (without a current mirror) does not mean constant current through the devices but a heavier emitter/source load causes decreased functional accuracy and increased distortion. However, a current mirror and a VAS CCS, without further measures taken, would raise the open-loop gain, lowering THD but also sound quality.
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It is supposed to have a front-end that is not designed for measured excellence.
Amplifiers should not be designed for measured excellence.
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Old 16th April 2009, 01:20 PM   #124
Bigun is offline Bigun  Canada
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Thanks Lumba, I appreciate the confidence.

I'm still playing catch-up, just think how old the AKSA design is (and it was based on even older designs as starting point), and I haven't yet built my pseudo-clone of it yet.

Things change too fast these days, I already feel that this discrete power amp hobby is today's vacuum tube design. ClassD chip amps will be the only thing we can find soon.

Chasing very low distortion is a lot of work. It's hard for the hobbyist to measure the results because listening is so subjective and most of us don't have the equipment needed.

Perhaps there is some merit in global feedback based on a microphone in the listening room sampling the output our ears receive rather than the electrical signal that the speaker binding post receives?
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Old 17th April 2009, 03:28 AM   #125
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H...OK?
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Old 17th April 2009, 03:30 AM   #126
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Gareth,
things do not change fast, almost everything worth a mention was invented and known in the tube era.
Class D is unsuitable for high quality audio.
You should not care about achieving low measured distortion, just care about the sound you hear. These are loosely correlated.
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Old 18th April 2009, 01:21 AM   #127
Bigun is offline Bigun  Canada
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Quote:
Originally posted by Lumba Ogir
H...OK?
Lumba ?



PCB
___

Well, I am able to report that having given up on Pulsar paper I have had more luck with some magazine paper (a big thanks to MJL for that one). The BBQ adverts seem to work well, so too the car adverts.

I'm not completely satisfied with the results, but I no longer have the patience to rub down some copper clad board so into the etch bath it went, just half an hour ago...
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File Type: jpg dscn1885f.jpg (80.3 KB, 500 views)
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Old 18th April 2009, 01:51 AM   #128
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Looks good Gareth
Did you get the rest of your parts? What's the model # of that transformer?

The photo paper at Staples is this stuff. Available at every store, I'd expect.
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Old 18th April 2009, 04:29 AM   #129
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Unfortunately I have a few parts still on backorder. They send me a piece at a time, like a ransom demand.

The transformer is something a bit risky, it's not very beefy, isn't a 'full torroid' so the EMI might try to frighten the amplifier. You can check it out at DigiKey: 595-1346-ND
But I like that these cousins of EI design reject mains HF a bit better, the split bobin gives it low capacitance between windings and the core losses help further. Sounds good in principle, but of course I just wanted cheap and small !

I did update the pcb layout (see attached) with the additional resistor to the LTP. It's an experiment, a last minute crazy thought. It will give me another tool to affect the LTP distortion in addition to the current mismatch. Between them I should be able to wreak some havoc.

My home computer doesn't allow me to do much with Spice, but I have access to spice at work now, so some lunch-hour investigations may help me understand more.
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File Type: jpg tgmlayout1f.jpg (90.2 KB, 483 views)
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Old 23rd April 2009, 12:07 AM   #130
Bigun is offline Bigun  Canada
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Wow, it's nice to have LTspice to play with, no wonder some people never build anything

So I tested out my amplifier and it works fine. So back to the idea of manipulating the H2 content by unbalancing the LTP (after all, that was my excuse for not using current mirrors).

I am simulating TGM version1, with the simply resistor biassed LTP pair. I'm running it 'cool' with only 0.58mA through the combined LTP. I tweaked the biass resistor to give me this value since it's close to the current that flows through the AKSA LTP (my role model). Picking an E12 value gave me a lucky coincidence that the current balance was pretty good. The input bjt sees 288.054 uA, the feedback bjt sees 288.254 uA. So the currents at dc operating point are balanced to around 0.07%. I obtained an FFT of the output as shown in the attachment top trace.

Then I unbalanced the LTP by reducing the biass resistor and pushing the total LTP current up to 1.1mA (still pretty low). The two current flows are now 288uA and 809uA respectively, which is a huge imbalance. I checked the FFT of the output and saw the lower trace in the attachment.

What gives ? - the harmonic content is not really changed that much, did I miss something here. H2 is at -70dB, H3 at -60dB. If anything H2 is a bit lower when the LTP is unbalanced. I had expected an increase in H2 with the imbalance based on some reading on the web.

Sorry about the image quality, files size limits are the cause...
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File Type: jpg ltp1_2 copy.jpg (65.3 KB, 400 views)
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