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Old 9th March 2003, 04:07 AM   #1
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Default Simple cheap 2A3 stereo amp

Hello all,

I just put out a site with a schematic for a 2A3 stereo amp and its power supply. It is simple and cheap, inasmuch as it only uses one transformer for all power needs.

It can be found here:

http://gabevee.tripod.com/2a3diy.html

Try it out!

Have fun!!

Gabe
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Old 9th March 2003, 04:34 AM   #2
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Ah, a no-nonsense amplifier, with some practical solutions to otherwise somewhat complicated problems.

Tim
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Old 9th March 2003, 04:57 AM   #3
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Default The Stereo SE...

Hi Gabevee!

The only sugestion that i can make...is that the shared catode resistor comum to the 2 chanels will introduce same crosstalk and consequent intermodulation in the low frequences...

In my opinion a resistor and the capacitor in each catode will be a improvement for such a small cost as a resistor and a capacitor!
Of corse in this last case the resistor must be doubled in value!

In a push pul amp it could be allright as a shared catode resistor in each chanel

Thanks for sharing!

Jorge

PS..IMHO of course!

Another drawback is if one tube fail or is geting weak the other will be overbiased!
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Old 9th March 2003, 05:10 AM   #4
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Tube_Dude,

You would be surprised about the cross talk. I thought so too, but I have built several different types of single ended amps with a common cathode for both channels. There is no cross talk because the bypass capacitor takes care of that.

If there is cross talk, the actual effect will be that of a sort of imaging enhancement, since any audio that gets past the bypass capacitor will be inverted from the other channel. So if anything, a wider soundstage will be the result.

The thing I do not think you are aware of also is that I need to have a common cathode resistor in order to use only one power supply for both 2A3s. To separate the cathode resistors means using two separate 2.5 volt power supplies for the filament of the 2A3s. More cost. No budget.

Thanks for the input!

Gabe
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Old 9th March 2003, 05:17 AM   #5
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Quote:
If there is cross talk, the actual effect will be that of a sort of imaging enhancement, since any audio that gets past the bypass capacitor will be inverted from the other channel. So if anything, a wider soundstage will be the result.
Well audio is for to guive satisfaction ...you are happy...who i'm i for disturbe...

good listening!

regards

Jorge
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Old 9th March 2003, 05:53 AM   #6
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Default The 2A3 Circuit

Since it uses a series filament scheme, I think it would be a good idea to use a non-polarized capacitor for the cathode bypass.

Another issue is the filiment supply. Depending on the filiment filter capacitor, the filiment voltage could rise to 7 volts. (3.5 volts per tube) At 3.5 volts, the 2A3 won't last.
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Old 9th March 2003, 08:53 AM   #7
SHiFTY is offline SHiFTY  New Zealand
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Wow cool circuit- simple, logical and looks easy to construct. I will definitely give it a go at some stage, I am just in the process of building a 2a3 amp at the moment, if I am not happy with it I will try your circuit. BTW What is the technical name when the ouput stage is driven from the cathode of the previous stage?

I am curious as to your reason for using DC on the 2A3 filaments- some people say there is a huge difference in sound and AC is better? Is there any difference?
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Old 9th March 2003, 09:03 AM   #8
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Quote:
Originally posted by Gabevee
You would be surprised about the cross talk. I thought so too, but I have built several different types of single ended amps with a common cathode for both channels. There is no cross talk because the bypass capacitor takes care of that.
I have seen comments a couple times on the JoeList where a common cathode R was preferred in a simple amplifier.

dave
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Old 9th March 2003, 11:17 AM   #9
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Gabe

Do you have a suggested PS transformer as well as a output transformer?



Joe
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Old 9th March 2003, 11:27 AM   #10
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It's an interesting circuit and one I'll be trying out in the future, although in my case I'd use 6B4s and a power supply with multiple 6.3v heater windings, simply because I have these available.

Otherwise, one option for using 6A3s or 6B4s might be to cannibalise old CB power supplies to provide the LT. These supplies, at least in the UK, are often cheap second-hand from the classifieds, junk shops and even eBay, going for as little as £5 ($7.50) and these could provide the innards for stabilised heater supplies for series connected 6B4 tubes (a little fiddling to reduce 13.8v out to 12.6v). This would certainly help keep the amp on a budget, even if it means an extra tranny on the chassis.
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