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Old 21st March 2002, 11:19 PM   #1
claudio is offline claudio  Italy
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Default 2 class A SE amplifiers: need suggestion!

Hello everyone,
I would like to get some advise from you about 2 class A SE amplifiers:

1) the POWER FOLLOWER by A. Ciuffoli http://www.geocities.com/ResearchTri...llower_99c.gif

2) the HYBRID AMP by Generoso Cozza http://digilander.iol.it/essentialaudio/hybrid_home.htm

The PF is an easy to build circuit (Harry Haller said nice things on it); my concern is about the abcence of gain (actually it looses something) and should be preceded by voltage gain stage:so it's impossible to connect it directly to a Cd player, it needs a pre-amplifier with an output not lower than 10Vrms .Does it mean that if I use a pre-ampli with "only" 8 Vrms output, the PF will not work at all or just the PF will not be able to reach the maximum output power (30 Wrms @ 6 Ohm load) ?

The HA is a two-stage, DC coupled, single-ended, Class-A amp, capable to deliver around 30W in an 8ohms load or 15W in a 4ohms load, using a tube in the first stage; a 1V input signal will drive the amp full power. So, the output level of a typical CD player is sufficient to drive the HYBRID amp. My concern is about the topology: it is similar to the the Zen output stage, so it will not sound at his best with difficult loads , like loudspeakers with low impedance or with impedance module above 45°.

I appreciate your useful opinion.
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Old 22nd March 2002, 01:25 AM   #2
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Although I've never heard the Power Follower, I have heard many good things about it. I've never heard the Hybrid Amp either, but glanced quickly at the schematic--looks okay. Granted my usual reservations about current sources and current mirrors, it seems fairly clean.
The one reservation I have about the Hybrid Amp is that 35V rails are hardly optimal for a 6922. The poor thing doesn't even wake up at that kind of voltage. Distortion is likely to be higher than necessary.
Driving the Power Follower with less than 10V will not hurt anything at all. It will not develop full power though. It isn't difficult at all to drop a front end on a circuit like that--you could use a BOZ or BOSOZ as a dedicated front end, for instance. Hint: Don't think of the amps/preamps as all-or-nothing stand alone circuits, think of them as building blocks. Mix and match them to your heart's content.

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Old 22nd March 2002, 02:14 AM   #3
lohk is offline lohk  Europe
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Have a look at http://sound.westhost.com/project83.htm

This is another MOSFET Power Follower, which is probably even easier to build - and has a matching preamp too !

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Old 22nd March 2002, 11:13 AM   #4
argo is offline argo  Estonia
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GRollins: Isn't the BOZ delivering only 2,5 Volts?

Claudio: You can consider building a dedicated tube pre amp or integrated voltage stage like common cathode + cathode follower or SRPP circuit.
Also have you looked at JLH Class-A variations at The Class-A Amplifier Site http://www.gmweb.btinternet.co.uk? It doesn’t require preamp and will deliver up to 20W

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Old 22nd March 2002, 12:01 PM   #5
claudio is offline claudio  Italy
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about the Hybrid Amp 6922 low voltage rails, I think Mr. Cozza used the tubes low voltage operation topology (Erno Borbely, Glass Audio vol.10, num.1, 1998).
About "amp/preamp as all or nothing", I am already building a pre ,a simple cascode already discussed here: 3 preamp to choose from
In summary, this pre as an output impedance of about 1 Ohm, so it will match perfectly the PF; just its output "not too high" to drive the PF
is my concern. As you can see, I am following an idea in choosing the 2 blocks: single stage, easy load, simplicity.

thanks for the suggestion, but I prefer to decide between the 2 schematics I presented.
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Old 22nd March 2002, 12:12 PM   #6
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Default another MOSFET Power Follower

Make the +40 volt rail ground and power circuit from -40 volts. That is one of the most important features for the A. Ciuffoli design. The power supply is loaded by a constant current source instead of the series combo of speaker and follower. Much lower effective power supply impedance. A no brainer.....

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Old 22nd March 2002, 01:30 PM   #7
tvi is offline tvi  Australia
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With regard to the Hybrid Amp you might want to read the email reply at TubeCAD Journal about this design <a href="http://www.tubecad.com/march2001/page20.html"><b>LINK</b></a>, he does a redesign including feeding the ECC88 with +105Vdc.

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Old 22nd March 2002, 02:05 PM   #8
rickpt is offline rickpt  Portugal
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look at this design. here is the link: http://www.raunvis.hi.is/~matti/hifi.htm

if you need more power just increase voltage.

dont build the hybrid amp. I´ve tried to build it and had a lot of problems. the amp keept blowing up the output mosfets. i've tried to change the output transistors (irfp240) and i had no luck. so i cannot recomend it. i've tried almost every thing to make it work.


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Old 22nd March 2002, 07:18 PM   #9
claudio is offline claudio  Italy
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Thanks James for the enlightning article on TubeCad Journal; and thanks Ricardo for sharing your experience with the Hybrid Amp.

So now I think I will go on with the A.Ciuffoli Amp. About it, what do you think in changing the virtual battery power supply with a more common well filtered power supply?
For sure the virtual battery PS is cheaper to build and seems to work great: so why it's so uncommon to find it used in DIY projects?

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Old 22nd March 2002, 07:45 PM   #10
paulb is offline paulb  Canada
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It is quite common and normally goes by the name of capacitance multiplier. See the project article at www.sound.au.com for more details.
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