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Old 24th September 2010, 11:36 AM   #1
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Default My 35 watt single-ended amp

Hello

After some schematics essay and chating with a good friend, I've redone and simplified a single-ended amp using a standard LTP input type topology with a boot strap and also a phase lead at the VAS.

Single-ended amp compared to push-pull amp do have two big advantages, there is no cross-over distortions and no drivers/output transistors switching noises.

But to work correctly a single-ended amp need to work in class A, so it's mean a very high bias current and lot of heat. This is because the current is limited by the current source Q3 and if you have to low current you will not reach enough voltage for the output transistor and it will clip before reaching the max power that the rails voltage should permit.

But single-ended amp can sound excellent with a superb sound-stage.

So, for those who want to try this type of amp, here is my 35 watt single-ended amp, in my amp I've done a current source of about 2.7 amp, that should be quite enough for a load down to 7 ohm.

I should say that it's an amp for experiments diyer.

Since the bias are fixed by the current source, there is no need for a VBE multiplier.

You will need a really very BIG heat-sink.

Don't try to get more output power by raising the rails voltage, at higher power you will end-up with an horrible clipping sounds.

The total gain of that amp are arround 37x

It would sound better if you matched the two LTP input transistors for HFE and VBE.

You need to use a low cob and fast speed for Q5 the vas transistor, at least 120 mhz with a cob lower than 10 pF.

Here is the schematic.

Bye

Gaetan
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Last edited by gaetan8888; 24th September 2010 at 11:46 AM.
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Old 24th September 2010, 11:41 AM   #2
Salas is offline Salas  Greece
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Nice. 130W dissipation??? You built that? Having pictures?
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Old 24th September 2010, 11:44 AM   #3
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Hello Salas

Yes I know, you can heat your house with this amp

No, I did not build it, I have few other amps projects running and few prototypes to do and a friend amp pcb to populate and do listening.

And I should say that it's an amp for experiments diyer.

Bye

Gaetan

Last edited by gaetan8888; 24th September 2010 at 11:50 AM.
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Old 24th September 2010, 11:50 AM   #4
AKSA is offline AKSA  Australia
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Nice circuit, Gaetan. I think the sound of these amps is amazing.

I once built a water cooled SE amp with tube front end which ran at 9A and 70V - my wonderful daughter failed to turn on the water one day when she wanted to play it to her friends, you can imagine what happened.........

I think of this amp as the Manhattan Project!!

Enjoy the music, Gaetan, these things sound fantastic.

Hugh
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Old 24th September 2010, 11:54 AM   #5
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Hello Hugh

Water cool, that was your Glass Harmony amp.

I've done my SE amp at only 35 watt, so guys can try it without having to use a giant heat sink, but it still need a very big heat sink.

Bye

Gaetan

Last edited by gaetan8888; 24th September 2010 at 12:02 PM.
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Old 24th September 2010, 02:12 PM   #6
lineup is offline lineup  Sweden
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Quote:
Originally Posted by gaetan8888 View Post
The total gain of that amp are arround 37x
Most people would not need that much gain.
The rule is to have no more gain than absolutely needed.
If we have sources with low signal output voltage, we build us a small nice preamplifier.
Electrically there is nothing that will improve with higher gain.
A bunch things in fact will be worse.
Now, people have different sources. So, some may want some extra gain. I know.

With supply 24 Volt, you can count on max 22 Vpeak out.
1 Vrms in is a good sensitivity for most modern sources.
This is 1.414 Vpeak
I need for my CD-player: 22/1.414 = Gain x15.5

R21 = 47k .... divided with 14.5 = 3.24k for R24
We can make R24 like 3.3k or 3k

The good thing is we can reduce C12 (33uF) electrolytic.
We can make it 10 uF, and still have same low freq roll-off.
Which means we can start use film capacitors.
Either one 10uF, or parallell two 4.7uF, which gives C12 = 9.4uF


Thanks for publishing you nice idea.
It is straight forward and Class A is still the King of Performance!
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Old 24th September 2010, 02:32 PM   #7
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by gaetan8888 View Post
here is my 35 watt single-ended amp, in my amp I've done a current source of about 2.7 amp, that should be quite enough for a load down to 7 ohm.
35W into 7r0 requires 3.16Apk and 22.1Vpk.

2.7A bias is not enough for a 7ohm load.
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Old 24th September 2010, 05:29 PM   #8
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i have built amps nearly identical to this design a couple of years ago. these types do sound better than any ab amp but dealing with the heat can be a big issue. i used an aleph type current source which worked nicely and allowed a lower idle current.
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Old 24th September 2010, 08:17 PM   #9
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Quote:
Originally Posted by lineup View Post
Most people would not need that much gain.
The rule is to have no more gain than absolutely needed.
If we have sources with low signal output voltage, we build us a small nice preamplifier.
Electrically there is nothing that will improve with higher gain.
A bunch things in fact will be worse.
Now, people have different sources. So, some may want some extra gain. I know.

Thanks for publishing you nice idea.
It is straight forward and Class A is still the King of Performance!
Hello

I've experimented myself that the less global negative feedback you have the better the sound are, it would not allways be big differences but there is a little one for most amps topologies. So I alway give less global negative feedback in my amps, and yes it's give more gain.

Most of the times I was fixing amps gain at arround 30 db, but now I go a bit higher and wen you reduce the global negative feedback you can reduce a bit the value of the cdom cap.

Thank

Bye

Gaetan
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Old 25th September 2010, 09:11 AM   #10
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by gaetan8888 View Post
So I alway give less global negative feedback in my amps, and yes it's give more gain.

Most of the times I was fixing amps gain at arround 30 db, but now I go a bit higher and wen you reduce the global negative feedback
This is precisely why JLH set many of his amplifiers to what in modern terms are very high gains.
Leach also alludes to this for better accuracy/sound quality.
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