Class A from scrap

brig001 · Post by **brig001** » Sat Jan 08, 2011 9:05 am

I was playing with a pair of PA speakers and needed a clean amp for testing, so had a rummage in the garage and found enough bits to make this. It simulates quite well, sounds good and doubles up as a room heater while giving a nice 3/4 Watt per channel. Anyone any ideas on how to improve this design or other comments? All components are cheap and nasty unless specified otherwise, all electrolytics are bypassed and it runs on a 15V laptop power supply.

brig001 · Post by **brig001** » Sat Jan 08, 2011 10:11 am

Hmm, the picture has gone...
Is this better?

Post by **Nick** » Sat Jan 08, 2011 10:29 am

Maybe replace R1 with a CCS?

Mike H · Post by **Mike H** » Sat Jan 08, 2011 1:43 pm

Mmmm!

Hmm, the picture has gone...

No attachments visible until >5 posts? It's showing now

brig001 · Post by **brig001** » Sat Jan 08, 2011 4:51 pm

Thanks Nick. Had thought about that, but couldn't find enough parts - might have to buy something

. This is for the speakers you heard at Colin's - mainly for testing, but the sound is nice enough to keep. Will try simulating it and see if it makes any difference to distortion (when I can get Tina working again...) - already around 0.1% 2nd harmonic, and nothing else really to speak of higher up.

Thanks Mike, it seems to appear when I log in - I assume this is normal.

Brian.

Mike H · Post by **Mike H** » Sat Jan 08, 2011 11:27 pm

Ah! Can't see attachments if not logged in. That figures

brig001 · Post by **brig001** » Wed Jan 12, 2011 8:14 am

Has anyone any thoughts on output capacitors?
I don't know what make is in/how old they are etc., but thought about these as a low cost option:
http://cpc.farnell.com/panasonic/eca1ca ... dp/CA05734
4700uF is probably a better size anyway than 2200uF.
They are currently bypassed by some 0.1uF ceramics, but I don't think they'll be doing anything really, so thought about some 1uF polyester ones I have lying around. Would these help?

Thanks, Brian.

Post by **Nick** » Wed Jan 12, 2011 8:45 am

Worth a try.

Mike H · Post by **Mike H** » Wed Jan 12, 2011 1:17 pm

Ah I was going to suggest maybe Panasonic.

Is the polyester "audio grade" ? If not I wouldn't. Nor ceramic either. The pana's will probably be quite adequate by themselves. Higher working Voltage should be beneficial as well if poss. 50 - 63V?

Post by **Nick** » Wed Jan 12, 2011 2:10 pm

Higher working Voltage should be beneficial as well if poss. 50 - 63V?

Why? I would expect a higher DCR with higher voltage.

Mike H · Post by **Mike H** » Wed Jan 12, 2011 3:16 pm

That's interesting, 'cause I read it ought to be lower on the basis that the ripple current rating tends to be higher as well for higher Voltage ones

Post by **Nick** » Wed Jan 12, 2011 5:27 pm

You may be right, I will check some numbers. I assumed that a thicher oxide coating is needed for higher voltage which leads to a higher ESR.

Just checked, yep, you are right, ESR falls with increasing capacitance and increasing voltage.

colin.hepburn · Post by **colin.hepburn** » Wed Jan 12, 2011 6:11 pm

Nick wrote:You may be right, I will check some numbers. I assumed that a thicher oxide coating is needed for higher voltage which leads to a higher ESR.

Just checked, yep, you are right, ESR falls with increasing capacitance and increasing voltage.

Ho that's good to know when I come to recap the pioneer SA2600

brig001 · Post by **brig001** » Wed Jan 12, 2011 8:26 pm

Thanks Nick and Colin. They only sell audio grade (ECA series) up to 35V @ 4700uF. General purpose (ECO series) up to 100V with lower ESR, but probably stick with the ECA ones. I also didn't know that ESR reduced with voltage rating, so learnt something there. The polyesters are bog standard, so I'll give those a miss too.

Thanks, Brian.

Mike H · Post by **Mike H** » Wed Jan 12, 2011 8:56 pm

Try both? ECA & ECO? May not be much in it, if discernible

Nick wrote:Just checked, yep, you are right, ESR falls with increasing capacitance and increasing voltage.

Oh good I didn't imagine it

Hope so cos is what I've tended to be doing

A few years back there was a good web page on the net comparing capacitor types, from the point of view of the materials used and what they do to AC signals, but seems to have disappeared, or at least my link stopped working.

It included test rig circuits for finding out how the output differs from the input, and the graphs. What I remember about ceramic disc was it had a non-linear relationship with frequency, so tends to do odd things with an audio signal, and the 'high K' types were worst.

The page was headed with a photo of a butler holding out a silver tray in one hand on which was arranged a selection of different capacitors.

No really...

audio-talk

Class A from scrap

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