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Old 22nd September 2005, 07:34 AM   #1
Ipanema is offline Ipanema  Malaysia
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Default Blameless Amp layout

Having just browse through Douglas Self book, I'm just excited want to tried out his blamesless amp, which measured very well. I'm using MJE340 instead of BC550C as phase splitter where I would mount it on top of power transistor for better thermal tracking. Can this work?

Attached is my layout and pls give comment.

Thank you.

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Old 22nd September 2005, 07:47 AM   #2
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I didn't think there was any phase splitters in this design. Are you sure you understand the design? Perhaps you mean the Vbe multiplier (bis generator)? In which case yes it would be good to mount it in thermal contact with the output devices. I'm sure this part is very well covered in the book/article as he has tweaked the Vbe multiplier for certain thermal characteristics.
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Old 22nd September 2005, 07:54 AM   #3
AndrewT is online now AndrewT  Scotland
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Hi,
which blameless is that? The CFP or the EF ( I see no capacitor across the EF bases).
The thermal connection for the Vbe multiplier is completely different for the two types!!!!!
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Old 22nd September 2005, 08:52 AM   #4
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By the 200 ohm resistor joining the driver emitters it should be clear that it's the EF version. The actual Blameless amp is EF anyway.
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Old 22nd September 2005, 09:17 AM   #5
forr is offline forr  France
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A blameless amplifier can have either an EF or a CFP output
If EF, which is yours, the thermal sensing transistor should be sticked to the top of one of the output transistors. This is one of the biggest difference with almost all amplifiers which were commercialized before the publication of Self's works.
Do not forget the capacitor across the emitter resistance of the drivers.
All this is very clear in Douglas Self's book.

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Old 22nd September 2005, 09:33 AM   #6
Ipanema is offline Ipanema  Malaysia
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sorry for the confusion. It is indeed a bias generator. Always confuse with the name.

The blameless schematic can be found here (Figure 13).

http://www.dself.dsl.pipex.com/ampins/dipa/dipa.htm

I made a small modification by adding a low pass filter to the supply so that input stage and bias generator stage supply voltage will be more stable.

In the original schematic, the bias generator transistor is MPSA06 (TO92 package), which I find it to be harder to mount for thermal tracking on heatsink, so I change this to MJE340 (TO220) package for easier mounting on top of power transistor. Will this affect the circuit performance?

Attached is the schematic. Hope it is clearer now.

Pls also advice me on the PCB routing as I get lots of hum noise from my previous layout.



Thank you.
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Old 22nd September 2005, 09:34 AM   #7
Ipanema is offline Ipanema  Malaysia
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Schematic here.
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Old 22nd September 2005, 10:21 AM   #8
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Have you read the rest of the book on good layout practices? I think you need to revisit that bit as I can see you sharing decoupling and input ground traces.

I would also significantly reduce your low-pass filter resistors to maybe 100 ohms (from 680) each as an absolute maximum as they will be strangling the VAS stage.
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Old 22nd September 2005, 11:10 AM   #9
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Your schematics is a bit different form D. Self's blameless.

There is no CC input at the VAS.

Beware that the two pole compensation can lead to instabilities. Build your amp with only with a simple Miller compensation first (I abandon the two-pole compensantion the first time I tried it because of oscillations. My boards were from Signal Transfer, I do not think the recent ones include the two pole compensantion in the layout).

I glued the small thermal sensing transistor on top of one of the TO3 power transistor. Theorically, the use of the MJE340 should not change much, this is what is done with the CFP output stage, in thermal contact with one of the drivers.

Take acount of the other remarks of the thread about the decoupling, the schematics and the layout .

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Old 22nd September 2005, 12:00 PM   #10
MikeB is offline MikeB  Germany
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Hmm, i see 4 problems:
- 6ma at ~36v is quite much for a to92 transistor, with >200mw they might run quite hot. (q6,7,9)
- This kind of topology really needs an inputcap, or you get plenty of DC-offset !
- Ground for input and feedback is taken from 2 completely different points,
current ripple to the 470uf caps should cause severe hum !
- You are missing the output-zobel

The original blameless has a buffer between currentmirror and vas, you skipped it ?

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