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Old 11th July 2007, 12:35 AM   #1
Jaime is offline Jaime  Uruguay
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Default Hybrid amplifier A2 class

It is an High End amplifier?
It have a wide band?
The power is 50w using 2 x 6A3 !!!

http://www.electra-print.com/pushpull_a2.php

Best regards Jaime
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Old 11th July 2007, 02:24 AM   #2
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"It is an High End amplifier?"

Doesn't look like it to me. The performance is determined largely by that chip amp up front. These are mediocre performers useful for a quick 'n' dirty solution where fidelity isn't all that important.

Secondly, Class AB2 and cathode bias do not mix. You need a stable source of fixed bias if you're going Class AB2 and you want sound quality above what you'd get from any PA amp.

The 6A3 has a Pd= 15W. You're not gonna get 50W from a pair of those, at least not for very long.

I'd take a pass on this one.
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Old 11th July 2007, 02:44 AM   #3
anatech is offline anatech  Canada
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Sorry Jaime,
I have to agree with Miles.

Hey, I saw a couple schematics that didn't use that chip! He must have a drawer full of TDA2030's to use up.

Quote:
The power is 50w using 2 x 6A3 !!!
You really have to be careful with claims like that. Output power is not the most important consideration with a tube amp. This one may put out that power - possibly - maybe. IT would be here for a semi good time, not a long time.

Quote:
It have a wide band?
Errr, depends on the transformers mostly. I am doubtful.

Quote:
It is an High End amplifier?
I have trouble with that too. I'll bet they could sell it as high end in some shops. It has TOOOBS!

Yeah, skip this and look for something more conventional.

-Chris
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Old 11th July 2007, 03:19 AM   #4
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Link is down for now but VTL used to claim similar ratings. 50 watts from a pair of 807s triode strapped
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Old 11th July 2007, 03:52 AM   #5
anatech is offline anatech  Canada
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Yes they did,
They did it at HIGH voltages. At least they attempted 50 W with a 25 W plate type tube (EL34). I didn't see the 807 deal, but it makes sense given the voltages they liked to run.

-Chris
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Old 11th July 2007, 07:09 AM   #6
corne is offline corne  Netherlands
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There's no way two 6A3 tubes in A2 PP will give you 50W output power.
The maximum efficiency for a class A2 amp is 50% so the theoretical max. power output is 30Watt.
Even running two 300B tubes A2 the power output is restricted to 40W.
If you want 50W from two 6A3's you'll need to run it class B2 requiring fixed bias.

Quote:
Miles Prower wrote:
The performance is determined largely by that chip amp up front.
It's a big factor but even worse is the way the grid current is supplied.
The transformer is in series with the 380ohm cathode resistor. For a large part this amp will run class1 (no grid current) so the transformer sees a load of 20k. As it goes into grid current the load changes to 380ohm and due to the grid current a significant portion of the signal is lost in the cathode resistor.
The amplification for class1 and class2 is different, this will give a very nasty distortion at the class1 to class2 transition


Quote:
It is an High End amplifier?.
No not even close.


Corne
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Old 11th July 2007, 11:13 AM   #7
corne is offline corne  Netherlands
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Quote:
The maximum efficiency for a class A2 amp is 50% so the theoretical max. power output is 30Watt.
That should be 15Watt not 30Watt.

Corne
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Old 11th July 2007, 11:41 AM   #8
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found this at Electraprint describing A2 and his use of op-amps -
http://www.electra-print.com/techblog.php?blogid=2
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Old 11th July 2007, 01:28 PM   #9
corne is offline corne  Netherlands
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I've read the Electra-print article on A2.

This is my opinion:
It has bits of truth, alternative explanations to what is really going on (shifting bias), a solution to a problem (shifting bias) but fail to tell the real solution (don't use a coupling capacitor in A2), compares two completely different things (chips drivers with FB and tube drivers without FB) and leaves out key issues (effective driver impedance at the output tube grid).

In my experience it's possible to design a good tube A2 driver.
Use a high transconductance Cathode Follower (low output impedance) directly coupled (no bias shift) to the output tube grid and a driver with local FB from the CF output.
This will ensure low output impedance and therefore the driver signal doesn't get distorted when the output tube draws grid current. And don't use cathode resistors when using A2.

Corne

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Old 27th May 2012, 12:21 AM   #10
HeyBill is offline HeyBill  United States
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Hello,

I built a pair of 6A3 A2 monoblocks last year using the ElectraPrint parts. Took me a few years to get it done. It's a very strong amplifier with good bandwidth and stage. The chip front end was a bit of a bugger to get right. I used a TDA development board from RS as a starter. I don't have the equipment to make serious measurements. A very tiny amount of hum disappears after the units heat up. I also used Jack's matching transformers and a DACT 10K volume control. Very nice.

This is my second amp. The other one being a K-12 modified in the usual ways. Got help from Bruce on the K-12.
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