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Old 17th June 2003, 09:31 AM   #11
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Quote:
Originally posted by JOE DIRT®
Correct me also if I am wrong I prefer multiple caps and have them bypassed with smaller caps to keep the impedance to ground lower...thats just how I have done my design approach..I find the smaller caps more rugged for power supplies

DIRT®

Hey JOE,

Have you done your GC?
Have you finished unpacking?
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Old 26th June 2003, 06:47 AM   #12
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Hey~! I guess that means the ESR is lowered when you parellel caps right?? any one tried low ESR caps?? How does it sound...I'm waiting for the chip samples to come...then I will be ready to wack a**(self-censoring)...what kind of rectifiers will you guys recommend...other than standard bridge rectifiers...should be easily available as well...RS Components or Farnell will work for me...will 25V DUAL separate secondaries work well for the LM3875 in this instance?? Just need some ideas...and it's pretty hefty at 4.45A EACH...meaning I can get +-35V at at least 4 Amps?? I got the potted toiodal transformer from RS last time and it has been lying there for a long time...just tested it today and it seems to work well...so any comments and suggestions?? I will probably use the wiring diagram from here
http://www.decdun.fsnet.co.uk/gaincloneFAQ.html
I was wondering if I need to put a .22 Ohm resistor on the amp's output (pin 3) to the speaker...and do I have to add a decopling cap (0.1uF or 1uF) from pin 1 to pin 4?? If so what value would you guys recommend?? Thanks in advance.
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Old 26th June 2003, 09:15 AM   #13
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still setting up the Lab Carlos!!..LOL....I did find a new toroid for my GC from a local company called ILP in Toronto....its a dual 18 volt 3.33Amp (416-667-9914) for you TO guys....I dont know how it will compare to a plitron but when I did buy it the price was right.
I dont go to my fabrication shop often as it is 2 hours away so when I get the time to build a chassis I will....all you guys have me hooked building this now...I am going to be using a Dallas 6 channel 1806 digital pot IC for input gain to put a twist on things as I dont just want to copy all you guys.....the more I unpack the more goodies I find for this project so this should be interesting

Cheers!!The DIRT®
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Old 26th June 2003, 09:20 AM   #14
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Quote:
Originally posted by mikelm


I think that the ripple current would be an average figure and normally quoted at quite a high temperature so the 10 amp pulses may not be that great a problem especially if ( in the real world ) the caps don't get too hot.

cheers

mike
Indeed, the original post said "2.5A RMS ripple". The 10 Amp pulses will remain below the RMS spec, but it should also be less than the PEAK value. Unless you are happy with a very short cap life, make sure the ripple and peak current specs are OK.

Also, note that if you increase the capacitance, the pulse current will also increase. That is one of the reasons that if you use very large capacitances in power supplies like 50.000 or 100.000 uF, you need a large xformer just to keep up with the caps. It then seems that a smaller xformer ruins the sound, but in fact it is the excessive capacitance, and to neutralize that you need to buy a more expensive xformer....


Jan Didden
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Old 26th June 2003, 12:53 PM   #15
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Quote:
Originally posted by JOE DIRT®
still setting up the Lab Carlos!!..LOL....I did find a new toroid for my GC from a local company called ILP in Toronto....its a dual 18 volt 3.33Amp (416-667-9914) for you TO guys....I dont know how it will compare to a plitron but when I did buy it the price was right.
Cheers!!The DIRT®

I don't know whow much you guys pay for your Plitrons or other transformers, but as I can't find decent ones in the market here, I always order them to a factory here in Portugal, as my specifications.

Two examples: my 2x24V/8A toroid, for my LM3875-based GC cost me 45 Euros (including P&P).
I ordered another one for my father (for a LM3886-based GC), and this beasty 2x22v 12A toroid (528VA! ) cost him 55 Euros (including P&P).
And they are very well built.
They even send the fixing disc and a screw.
And I can order anything I whant, they make it and send it in one week.

JOE, I think 3.33A is a little weak, but if it was really cheap...
One of those per channel is OK.
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Old 26th June 2003, 01:27 PM   #16
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Hi Carlos,

I susbscribe to the 'you can have too much VA' school of thought having read a good article by Hugh Dean (AKSA). However, I'm not sure what 'too much' may be for a GC as it seems to break many of the rules of amplifier building.

My GC's are currently running on 80VA per channel and sound great. They also sound great with a single supply of 120VA! And I know of somebody building GCs with just 50VA per channel!

Just a thought!
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Old 26th June 2003, 02:05 PM   #17
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Quote:
Originally posted by Nuuk
Hi Carlos,

I susbscribe to the 'you can have too much VA' school of thought having read a good article by Hugh Dean (AKSA). However, I'm not sure what 'too much' may be for a GC as it seems to break many of the rules of amplifier building.

My GC's are currently running on 80VA per channel and sound great. They also sound great with a single supply of 120VA! And I know of somebody building GCs with just 50VA per channel!

Just a thought!
Well, one thing is to use one transformer you already have, or found in your neighbour's trash can.
Other thing is to go and buy one transformer if you need one.
I asked that factory for a quote on two tranformers before I ordered mine:

1 - 2x24V/6A
2 - 2x24V/8A

Do you know what was the difference in price between the 6A and the 8A toroids?
3 Euros.
I ordered the 8A one.
I'm not saying you need all this current.
For me it was a question of peace of mind and price-performance ratio.
As you see, It wouldn't be much cheaper ordering a smaller transformer, it really had to be much smaller.

I always felt that the most important thing in an audio device is a good PSU, and more specifically a good transformer.
Most brands always cut on the transformers to reduce production prices, and then put some more capacitance.
Completely wrong.
As I'm doing the things myself, I like to give good food to my amps.
These chips work great with 1000uf capacitance, but please don't use too small transformers.
Connect that to 4ohm speakers and take the volume up and you'll have a huge voltage drop.
Not for me.

I'm talking power amps.
In preamps I use any small transformer that has the voltage I need.
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Old 26th June 2003, 04:13 PM   #18
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Quote from Hugh Dean on the Printed Electronics FAQ page

"We haven't ourselves trialled the Plitron; in Australia we have our own manufacturers, and a lot of imported toroidals as well. Actually, the larger toroids are not quite as good as the smaller, 160VA (or thereabouts). The power supply considerations are essentially conflicting; you want a transformer with good regulation, so the voltage sag on heavy signals will be quickly restored. This means a larger transformer, like a 300VA. However, many of the current demands are rather transient, very quick; so the inductance of the primary and secondary is important, since it prevents current flowing rapidly. A smaller transformer might have worse regulation, but this is essentially a constant current parameter; the transient voltage regulation is arguably something better. We are using 160VA transformers with amorphous cores, and they perform outrageously well. The bass and image depth is really so much more remarkable than ONE 300VA quality toroid. I am inclined to think the best sound doesn't really matter too much in terms of transformer; what is important is the fact that the supplies are separate. The filter capacitors do make a difference, but the onboard, PCB capacitors make a difference too, and you'd probably be better playing with them; (they are much cheaper than altering the main filter capacitors)."

So maybe the answer lies somewhere in between our solutions, ie use two smaller transformers instead of one but go for 160VA, not 80! Now if only I was wealthy!
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Old 27th June 2003, 11:17 AM   #19
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Nuuk,

Their conclusions were basically that two smaller transformers are better than one bigger transformer.
This is nothing new.
My toroid is 384VA.
I would be better with two 180~200VA toroids, but that would be more expensive.
One toroid for each channel improves soundstage.
Two 160VA toroids for a stereo GC seams to me right on the mark.
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Old 27th June 2003, 01:05 PM   #20
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Quote:
Two 160VA toroids for a stereo GC seams to me right on the mark.
Hi Carlos. Yes that's what I would call the optimum transformer specification for the GCs. Now if only all international debates could be concluded so gracefully


I just priced up the cost of 2 x 160VA against one 300VA from Maplins and the extra cost is 10UKP. Is that too much to pay for a bigger soundstage?
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