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Ranchu32 20th March 2014 09:02 AM

My Blameless
 
1 Attachment(s)
Hi all

I've been interested in the Linn/Blameless topology for a while and wanted to try my hand "designing" one of these.

I've got some spare DX Blame MkIII amplifier board from the last group buy and thought I'd have a go at selecting my own component values that will result in a "reference design" that is entirely conventional in every respect. The schematic is based on what I've learnt from reading Cordell and Self, and what I've seen from the DIY projects here - notably the DX designs from Carlos and Kevin's recent LTT4 project.

The MkIII boards support:
- LTP input stage with currrent mirror and cascoded CCS
- EF2 output stage

I know this will bore seasoned members here; however, I would really appreciate your advice in fine tuning the C and R values.

thanks

mcd99 20th March 2014 09:07 AM

Am I being blind? but I can't see any compensation around the VAS.

Ranchu32 20th March 2014 09:11 AM

Fixed :-)

nannoo 20th March 2014 09:15 AM

Looks like a good thread waiting to happen, there's a lot to be said for a reference, conventional design

still4given 20th March 2014 09:54 AM

Hi Christian,

Watching with great interest. Be sure to check out the end of the of the MKIII thread to see how to fix the lifted ground error on those boards. Wishing you great success with this.

Blessings, Terry

Ranchu32 20th March 2014 10:29 AM

Thanks Terry - will do. I remember following your progress in the Builders Thead several months ago - I've got in my bookmarks to follow up on.

AndrewT 20th March 2014 11:44 AM

The Input 1uF DC blocker is in the wrong place.
It must go before the 47k.

Or move the 47k to after the 1uF.

R16 is tapped into the wrong end of R14.
The collector of Q10 drives the base stopper.

Reduce R13 to <=1k5
You have too much Vbias available with 2k2. Depending on actual Vbe of the transistors, you might find you can bias the output using just 1k0 or 1k1 for R13

R17 is far too high.
Try 1k0 to 2k0.

Add an output Zobel.

R10 is fed from the -ve rail, try increasing it's value substantially, or move it to Power Ground.

mcd99 20th March 2014 12:06 PM

Would like to add, I would use BC5x0c transistors where there is very low Vce.

TonyTecson 20th March 2014 12:09 PM

yes, the base of the left hand side of the ltp needs to be properly dc biased,
otherwise huge offset voltage at the output....

Ranchu32 20th March 2014 09:36 PM

1 Attachment(s)
Thanks everyone - I will make the changes to the schematic tonight.

The boards I'm using can accomodate one or two diodes strings (or resistors) on the rails between the driver and front end. However most designs I've seen here and in books have nothing. Would there be benefit in using them?

Also there are a couple of extra locations for power supply decoupling (HF). Again in most of the Linn/Blameless designs they aren't used.


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